Local Area High Speed Networks [1st ed] 9781578701131, 1578701139

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Loca l Ar e a H igh Spe e d N e t w or k s By Dr. Sidnie Feit

Publisher: New Riders Publishing Pub Dat e: June 19, 2000 I SBN: 1- 57870- 113- 9 Table of • Cont ent s • I ndex

Pages: 704 Slot s: 2

Copyright About t he Aut hor About t he Technical Rev iew ers Acknowledgm ent s Feedback I nform at ion I nt roduct ion Audience Organizat ion Part I : Et hernet , Fast Et hernet , and Gigabit Et hernet Part I I : Bridging, Sw it ching, and Rout ing Part I I I : Ot her old and New Technologies Part I V: Appendix es Part I : Et hernet , Fast Et hernet , and Gigabit Et hernet Chapt er 1. I nt roduct ion t o LANs The Birt h of Et hernet Token Ring Fibre Dist ribut ed Dat a I nt erface Fibre Channel and ATM Com m unicat ions Layering Model Basic LAN Com ponent s SNMP, Monit ors, and RMON St andards Body Overv iew Sum m ary Point s Chapt er 2. LAN MAC Addresses Universally Adm inist ered MAC Addresses Locally Adm inist ered MAC Addresses Broadcast and Group Addresses I ndividual/ Group and Universal/ Local Flag Bit s Et hernet Address Convent ions Token Ring Address Conv ent ions FDDI Address Convent ions Sum m ary Point s References Chapt er 3. Et hernet LAN St ruct ure Et hernet LAN Archit ect ure

Sum m ary Point s References Chapt er 4. The Classical Et hernet CSMA/ CD MAC Prot ocol Classic Et hernet Shared Bandw idt h LANs Et hernet MAC Fram es Tabulat ion of I m port ant Et hernet Param et ers Sum m ary Point s References Chapt er 5. Full- Duplex Et hernet Com m unicat ion Full- Duplex Archit ect ure Sum m ary Point s References Chapt er 6. The Et hernet 10Mbps Physical Lay er Baseband Et hernet on Thick Coaxial Cable ( 10BASE5) Baseband Et hernet on Thin Coax Cable ( 10BASE2) 10Mbps Tw ist ed- Pair Et hernet ( 10BASE- T) Building a Mixed Coax/ Twist ed- Pair Collision Dom ain 10Mbps on Fiber Opt ic Cable Sum m ary Point s References Chapt er 7. The Et hernet 100Mbps Physical Layer Coexist ence and Migrat ion w it h Aut o- Negot iat ion 100Mbps Et hernet on Tw ist ed- Pair Cabling 100BASE- FX and FDDI Media I ndependent I nt erface 100Mbps Collision Dom ain Diam et er 100VG- AnyLAN Sum m ary Point s References Chapt er 8. Gigabit Et hernet Archit ect ure Gigabit Configurat ions Full- Duplex Gigabit Et hernet Half- Duplex Gigabit Et hernet Sum m ary Point s References Chapt er 9. Gigabit Et hernet Physical Layer Feat ures of Gigabit Et hernet Physical Charact erist ics of Gigabit Et hernet 1000BASE- X Technology 1000BASE- T Technology 1000BASE- T Encoding Sum m ary Point s References

Chapt er 10. Tw ist ed- Pair Cabling St andards and Perform ance Requirem ent s Cabling St andards Bodies TI A/ EI A Cat egories Unshielded Tw ist ed- Pair Cabling Perform ance Param et ers Managing Tw ist ed- Pair Cabling Test ing Fiber Opt ic Cable Sum m ary Point s References Chapt er 11. Aut o- Negot iat ion Aut o- Negot iat ion for Tw ist ed- Pair I nt erfaces Aut o- Negot iat ion for 1000BASE- X I nt erfaces Sum m ary Point s References

Part I I : Bridging, Sw it ching, and Rout ing Chapt er 12. Et hernet Bridges and Layer 2 Sw it ches Main Bridge/ Sw it ch Funct ions Ot her Bridge Funct ions Transparent Bridge I nt ernals Layer 2 Sw it ch Archit ect ure Building Redundancy int o a LAN Handling Mult icast s Sum m ary Point s References Chapt er 13. The Spanning Tree Prot ocol LAN St ruct ure and SubLANs Prot ocol Overv iew Elem ent s of t he Prot ocol Prot ocol Messages Sum m ary Point s References Chapt er 14. Sw it ches and Mult icast Traffic Mult icast ing I GMP Snooping The GARP Mult icast Regist rat ion Prot ocol ( GMRP) Generic At t ribut e Regist rat ion Prot ocol Sum m ary Point s References Chapt er 15. Link Aggregat ion Using Link Aggregat ion Link Aggregat ion Concept s and Procedures Link Aggregat ion Param et ers The Link Aggregat ion Cont rol Prot ocol

Sum m ary Point s References Chapt er 16. VLANs and Fram e Priorit y Virt ual LAN Concept s I m plem ent ing VLANs Processing VLAN Fram es VLAN Prot ocol Elem ent s Assigning Priorit ies t o Fram es GARP VLAN Regist rat ion Prot ocol Det ails Sum m ary Point s References Chapt er 17. Source- Rout ing, Translat ional, and Wide Area Bridges Source- Rout ed Bridged LANs Translat ional Bridges Tunneling Token Ring Across Et hernet St ruct uring a LAN Around a High- Speed Et hernet or FDDI Back bone w it h Tagging Rem ot e Bridges Sum m ary Point s References Chapt er 18. Rout ing and Layer 2/ 3 Sw it ches Feat ures of Rout ing Rout ing Procedures Bridge/ Rout ers Layer 4 and Applicat ion- Layer Sw it ching Sum m ary Point s References

Part I I I : Ot her Old and New Technologies Chapt er 19. Token Ring and FDDI Overv iew Feat ures of Classic Half- Duplex Token Ring Classic Tok en Ring Prot ocol Elem ent s Dedicat ed Token Ring High Speed Tok en Ring Fiber Dist ribut ed Dat a I nt erface FDDI Topology FDDI Prot ocol Elem ent s FDDI Form at s Sum m ary Point s References Chapt er 20. ATM Overv iew ATM Concept s ATM Archit ect ure The ATM Cell Header Sum m ary Point s

References Chapt er 21. ATM LAN Em ulat ion Em ulat ed LAN Environm ent s LAN Em ulat ion Client s LAN Em ulat ion Serv ers ATM LANE Prot ocol Elem ent s LANE Version 2 Sum m ary Point s References Chapt er 22. Fibre Channel Feat ures of Fibre Channel Fibre Channel Equipm ent and Topology Fibre Channel Nam es and Addresses Fibre Channel Levels Fibre Channel Dat a Transfer Exam ples of FC- 4 Fibre Channel Use Arbit rat ed Loops Sum m ary Point s References

Part I V: Appendixes Appendix A. Physical Layer Encodings for Et hernet , Fibre Channel, FDDI , and Token Ring Code- Groups and Special Signals Et hernet 10BASE5, 10BASE2, and 10BASE- T Manchest er Encoding Et hernet FOI RL, 10BASE- FL, and 10BASE- FB 100BASE- X Et hernet , FDDI , and CDDI Et hernet 100BASE- T4 Et hernet 100BASE- T2 1000BASE- X and Fibre Channel 1000BASE- T Token Ring Sum m ary Point s References Appendix B. Tables Binary, Decim al, and Hexadecim al Charact ers 8B/ 6T Tables 8B/ 10B Translat ion Table References Appendix C. St andards Bodies and References Form al St andards Bodies Vendor Groups Appendix D. Acronym List Glossary

Num erals A B C D E F G H I –K L M N O P–Q R S T U V W–Z

I ndex

Copyright Copyright Information FI RST EDI TI ON: June, 2000 All right s reserved. No part of t his book m ay be reproduced or t ransm it t ed in any form or by any m eans, elect ronic or m echanical, including phot ocopying, recording, or by any inform at ion st orage and ret rieval syst em , wit hout writ t en perm ission from t he publisher, except for t he inclusion of brief quot at ions in a review. Copyright ( © ) 2000 by MTP Library of Congress Cat alog Card Num ber: 99- 66354 04 03 02 01 00 7 6 5 4 3 2 1 I nt erpret at ion of t he print ing code: The right m ost double- digit num ber is t he year of t he book's print ing; t he right m ost single- digit num ber is t he num ber of t he book's print ing. For exam ple, t he print ing code 00- 1 shows t hat t he first print ing of t he book occurred in 2000.

Print ed in t he Unit ed St at es of Am erica

Trademarks Acknowledgments All t erm s m ent ioned in t his book t hat are known t o be t radem arks or service m arks have been appropriat ely capit alized. MTP cannot at t est t o t he accuracy of t his inform at ion. Use of a t erm in t his book should not be regarded as affect ing t he validit y of any t radem ark or service m ark.

Warning and Disclaimer This book is designed t o provide inform at ion about local area high speed net works. Every effort has been m ade t o m ake t his book as com plet e and as accurat e as possible, but no warrant y or fit ness is im plied. The inform at ion is provided on an " as- is" basis. The aut hors and MTP shall have neit her liabilit y nor responsibilit y t o any person or ent it y wit h respect t o any loss or dam ages arising from t he inform at ion cont ained in t his book or from t he use of t he discs or program s t hat m ay accom pany it .

Dedication To m y husband, Walt er, who kept his pat ience and good hum or.

About the Author D r . Sidn ie Fe it is t he chief scient ist at t he St andish Group, a leading research consult ancy. An analyst , lect urer, and writ er wit h m ore t han 30 years of dat a com m unicat ions and inform at ion processing experience, she has designed, t est ed, and reviewed num erous com m unicat ions product s and assist ed in product rollout s. She also has developed t raining program s in com m unicat ions t echnologies for vendors and sem inar organizat ions. Dr. Feit , who received her B.A., M.A., and Ph.D. from Cornell Universit y, has writ t en m any t echnology m anuals and published books, art icles, and report s in t he areas of dat a com m unicat ions, t elecom m unicat ions, net work m anagem ent , I nt ernet servers, and I nt ernet applicat ions. Her ot her books include Wide Area High Speed Net works ( MTP, 1999) ; TCP/ I P: Archit ect ure, Prot ocols, and I m plem ent at ion, Signat ure Edit ion ( McGraw- Hill, 1998) ; and SNMP: A Guide t o Net work Managem ent ( McGraw- Hill, 1993) .

About the Technical Reviewers These reviewers cont ribut ed t heir considerable hands- on expert ise t o t he ent ire developm ent process for Local Area High Speed Net works. As t he book was being writ t en, t hese dedicat ed professionals reviewed all t he m at erial for t echnical cont ent ,

organizat ion, and flow. Their feedback was crit ical t o ensuring t hat Local Area High Speed Net works fit s our readers' need for t he highest - qualit y t echnical inform at ion. A.G. Ca r r ick has m ore t han a quart er cent ury of experience in inform at ion processing. His career spans t he spect rum of t he indust ry, including end- user organizat ions, hardware m anufact urers, soft ware publishers, t hird- part y m aint enance firm s, universit ies, and research and developm ent firm s. His career began wit h punch- card t abulat ing m achines and proceeded t hrough program m ing, syst em s analysis, syst em s engineering, net working, soft ware developm ent , MI S m anagem ent , and consult ing. He owned a soft ware developm ent firm for seven years, developing soft ware packages for federal, st at e, and local governm ent agencies; universit ies; and Fort une 1000 com panies; and sm aller firm s. He now develops soft ware packages, consult s about t he m anagem ent and use of com put ers and net works, and is a lect urer at t he Universit y of Texas at Arlingt on in t he Com put er Science and Engineering Depart m ent . Gil has writ t en art icles for professional j ournals and for nat ional com put er group m eet ings. He also has been an invit ed speaker on several occasions at com put er professional group chapt er m eet ings and regional com put er user group conferences. Gin a M ie szcza k is an int egrat ion t est engineer wit h 3Com Corporat ion. She has eight years of experience in t he net working/ I T indust ry, including posit ions in t he consult ing, ret ail, and t ravel indust ries. Her skills include net work and t elecom m unicat ions t roubleshoot ing, using prot ocol analyzers, providing t echnical support , and execut ing net work planning. Gina is a Cisco Cert ified Net work Associat e ( CCNA) wit h t echnical cert ificat ions, including Microsoft MCSE and MCP+ I , and she current ly is finishing 3Com 's Mast er of Net work Science in Rem ot e Access Solut ions. She also is current ly pursuing t he Cisco Cert ified Net work Professional Cert ificat ion.

Acknowledgments I wish t o t hank Lisa Thibault , t he developm ent edit or, who read t he full t ext and m ade lot s of good suggest ions. She was m y const ant em ail com panion, providing encouragem ent and helping m e over bum ps in t he road. Laura Loveall supervised product ion and rem ained good- nat ured and unfazed when chapt ers were reordered or figures were replaced. Karen Wachs, t he acquisit ions edit or, helped t o keep t he proj ect on t rack. I also wish t o express m y grat it ude t o t he t echnical reviewers, A.G. Carrick, Gina Mieszczak, Lennert Buyt enhek, and Murali Raj agopal, who cont ribut ed generously of t heir t im e and t heir knowledge. I especially wish t o t hank A.G. Carrick, who was an inexhaust ible source of real- world inform at ion, and Gina Mieszczak, who read t hrough t he whole t ext carefully and pinpoint ed num erous sect ions t hat needed clarificat ion. Net work Associat es provided current Sniffer Pro m onit or soft ware, and Tom Rice of Net work Associat es supplied m any int erest ing t races and insight s. Michael Em ert on of 3Com provided som e int erest ing t races of recent ly developed prot ocols.

Feedback Information At MTP, our goal is t o creat e in- dept h t echnical books of t he highest qualit y and value. Each book is craft ed wit h care and precision, undergoing rigorous developm ent t hat involves t he unique expert ise of m em bers from t he professional t echnical com m unit y. Readers' feedback is a nat ural cont inuat ion of t his process. I f you have any com m ent s regarding how we can im prove t he qualit y of t his book, or ot herwise alt er it t o bet t er suit your needs, you can cont act us at www.newriders.com / cont act . Please m ake sure t o include t he book t it le in your m essage and t he t en- digit I SBN num ber, which can be found on t he back cover above t he bar code. We great ly appreciat e your assist ance.

Introduction Local area net work capacit y has rocket ed upward due t o t he int roduct ion of Gigabit Et hernet links, cheap 100Mbps Et hernet adapt ers, and swit ches whose t hroughput is accelerat ed by ASI C hardware. More cost ly net work equipm ent based on fibre channel archit ect ure is producing even higher perform ance levels. Bet t er equipm ent and higher bandwidt hs have encouraged a t rend t oward building bigger LANs. However, a big LAN is prey t o bot t lenecks, congest ion, and broadcast st orm s. A proliferat ion of new t echnologies should help solve t hese problem s by cont rolling t raffic flows, prevent ing swit ch congest ion, and enabling parallel links t o be inst alled. Som e of t he new capabilit ies are ast onishing. For exam ple, t he GARP VLAN Regist rat ion Prot ocol ( GVRP) can assure t hat a LAN fram e addressed t o a part icular st at ion always is delivered t o t hat st at ion and t o no ot her. This book int roduces m odern LAN t echnologies, explains what t hey do, and describes how t hey work. More t han 250 figures are included t o present concept s in an easyt o- underst and m anner. The book includes chapt ers t hat follow t opics t o t heir full t echnical dept h. However, t he m at erial has been organized t o allow t he reader t o explore t he t opics t hat are of int erest and skip subj ect s and det ails t hat are not current ly needed.

Audience This book is for people who want t o underst and t he t echnologies t hat t hey use and who also want a reference t hat provides det ails down t o any dept h. You can benefit from t his book if you are responsible for planning, im plem ent ing, support ing, or t roubleshoot ing local area net works. Consult ant s, syst em s engineers, and sales engineers who design corporat e net works for client s also can benefit from t his inform at ion.

The book provides t he t echnical foundat ion for knowing how and where t o deploy m ult ilayer swit ches. I t also describes swit ch opt ions t hat net work planners and archit ect s can use t o m ake LANs m ore reliable, t o im prove perform ance, and t o sim plify ongoing m aint enance chores.

Organization The book is organized int o four part s. The first part describes classic Et hernet and t he new, high- speed versions of Et hernet . The second part describes Layer 2, 3, and 4 swit ches and applicat ion- layer swit ches. The t hird part covers Token Ring, FDDI , ATM, and fibre channel LANs. The fourt h part provides reference inform at ion.

Part I: Ethernet, Fast Ethernet, and Gigabit Ethernet Chapt er 1, " I nt roduct ion t o LANs," describes and cont rast s Et hernet , Token Ring, and FDDI LANs. I t present s som e general net working background, including descript ions of lower- layer net work funct ions, SNMP net work m anagem ent , and RMON. Chapt er 2, " LAN MAC Addresses," explains how LAN MAC addresses are adm inist ered, assigned, and used. I t explains t he bit - ordering discrepancies bet ween Et hernet , Token Ring, and FDDI MAC addressing. Chapt er 3, "Et hernet LAN St ruct ure," t racks t he evolut ion of Et hernet from CSMA/ CD coaxial cable LANs t o m odern full- duplex swit ched t wist ed- pair and fiber opt ic LANs. I t also int roduces t he role of rout ers. Chapt er 4, " The Classical Et hernet CSMA/ CD MAC Prot ocol," covers bot h classic and high- speed half- duplex Et hernet prot ocols. I t describes fram e form at s and displays t races t hat illust rat e t he ways t hat various t ypes of prot ocol dat a are encapsulat ed. Chapt er 5, " Full- Duplex Et hernet Com m unicat ion," focuses on full- duplex Et hernet prot ocols and operat ions, and present s t he full- duplex congest ion cont rol prot ocol. Chapt er 6, " The Et hernet 10Mbps Physical Layer," describes t he cables, connect ors, t ransceivers, hubs, swit ches, and t opology rest rict ions for 10Mbps Et hernet LANs. I t includes inform at ion needed t o int egrat e legacy LANs wit h high- speed LANs. Chapt er 7, " The Et hernet 100Mbps Physical Layer," covers all t he physical im plem ent at ions of 100Mbps Et hernet . I t provides inform at ion on t he cables, connect ors, hubs, swit ches, and dat a encodings t hat are used for each im plem ent at ion. I t present s a st raight forward explanat ion of half- duplex 100Mbps t opology rest rict ions and Class I and Class I I hubs. A sket ch of 100VG- AnyLAN is included as well. Chapt er 8, " Gigabit Et hernet Archit ect ure," describes Gigabit LAN configurat ions, param et ers, and m echanism s, including t he nonst andard Jum bo fram es. I t explains t he archit ect ure of buffered full- duplex repeat ers. Half- duplex Gigabit Et hernet also is present ed, along wit h an analysis of why it is not used.

Chapt er 9, " Gigabit Et hernet Physical Layer," covers t he physical im plem ent at ions of Gigabit Et hernet . I t provides inform at ion on t he m edia, connect ors, dat a encodings, and m ast er/ slave roles used for each im plem ent at ion. Chapt er 10, " Twist ed- Pair Cabling St andards and Perform ance Requirem ent s," is im port ant for net work planners and t roubleshoot ers. I t describes Cat egory 1- 7 cable and cabling layout s. This chapt er also provides det ailed explanat ions of t he t wist edpair perform ance param et ers used t o t est for Gigabit Et hernet qualificat ion and gives descript ions of t he t ools used t o t est cables. A brief descript ion of som e opt ical cable t est param et ers is included as well. Chapt er 11, " Aut o- Negot iat ion," describes t wo fam ilies of Aut o- Negot iat ion prot ocols. The first enables a pair of Et hernet UTP int erfaces t o negot iat e t heir best set of shared capabilit ies. The second is used by 1000BASE- X adapt ers.

Part II: Bridging, Switching, and Routing 2

Chapt er 12, " Et hernet Bridges and Layer 2 Swit ches," present s t he funct ions perform ed by Layer 2 swit ches. I t includes an in- dept h descript ion of t rans- parent bridge/ swit ch int ernals and perform ance charact erist ics. Chapt er 13, " The Spanning Tree Prot ocol," explains why a t ransparent ly bridged LAN m ust have a t ree st ruct ure and how t he I EEE 802.1D Spanning Tree Prot ocol reconfigures t he LAN t opology aft er a link or bridge failure. Chapt er 14, " Swit ches and Mult icast Traffic," describes m echanism s t hat can be used t o cont rol m ult icast t raffic in a bridged LAN. I t out lines how I GMP snooping and t he GARP Mult icast Regist rat ion Prot ocol work. Chapt er 15, " Link Aggregat ion," present s a prot ocol used t o m ake a set of links t hat connect t wo syst em s operat e like a single, aggregat ed link. Chapt er 16, " VLANs and Fram e Priorit y," is a m aj or chapt er t hat deals wit h virt ual LANs. I t describes t he different t ypes of VLANs t hat can be creat ed and gives t he purpose of each. This chapt er also present s t he processing st eps perform ed when a fram e passes t hrough a VLAN swit ch. VLAN prot ocols ( including GVRP) are discussed as well. Chapt er 17, " Source- Rout ing, Translat ional, and Wide Area Bridges," describes source- rout ing, t ranslat ional, and wide area bridges. I t explains how t ranslat ional swit ch product s overcom e t he problem s t hat arise when different t ypes of LANs are bridged t oget her. Chapt er 18, " Rout ing and Layer 2/ 3 Swit ches," put s rout ing and swit ching in perspect ive. I t walks t hrough a rout ing procedure t o highlight t he differences bet ween Layer 3 rout ing and Layer 2 swit ching. I t briefly discusses t he " rout e once, swit ch m any" philosophy support ed in t he em erging Mult iprot ocol Label Swit ching Archit ect ure ( MPLS) st andards. Layer 4 swit ching, applicat ion swit ching, and load balancing are covered as well.

Part III: Other old and New Technologies Chapt er 19, " Token Ring and FDDI overview," describes Token Ring and FDDI LANs. A discussion of t he classic Token Ring prot ocols is followed by a descript ion of t he newer Dedicat ed Token Ring prot ocols, which support full- duplex Token Ring com m unicat ion and High Speed Token Ring. FDDI t opology and prot ocols also are present ed. Chapt er 20, " ATM overview," lays t he groundwork for underst anding ATM LANs. I t describes ATM service cat egories, AAL5 fram es, and ATM cells. I nt erleaved cell t ransm ission and ATM Layer 1 swit ching also are discussed. Chapt er 21, " ATM LAN Em ulat ion," out lines t he LAN Em ulat ion ( LANE) prot ocols t hat are needed t o t urn ATM int o a LAN t echnology. Addressing, swit ching, LAN Em ulat ion servers, dat a fram es, cont rol fram es, and init ializat ion procedures are covered as well. Chapt er 22, " Fibre Channel," deals wit h fibre channel, t he t echnology t hat current ly offers t he highest LAN speeds. The chapt er describes fibre channel feat ures, applicat ions, and equipm ent . I t also covers fabric and arbit rat ed loop t opologies and classes of service. Maj or elem ent s of t he prot ocol fam ily are present ed, and t he encapsulat ion of ordinary user dat a and SCSI com m ands and dat a is described.

Part IV: Appendixes Part I V st art s wit h an appendix t hat originally was writ t en as a chapt er. Appendix A, " Physical Layer Encodings for Et hernet , Fibre Channel, FDDI , and Token Ring," provides det ailed descript ions of t he way t hat dat a is encoded and signaled ont o a m edium for Et hernet , fibre channel, FDDI , and Token Ring LANs. I t explains m echanism s such as 4B/ 5B encoding and MLT- 3 t ransm ission. The m at erial has been placed int o an appendix for easy reference from ot her chapt ers. Num erous t ables are included as well. Appendix B, " Tables," includes som e addit ional relat ed t ables. Part I V also includes acronym s, a glossary, and point ers t o st andards groups, consort ia, and ot her inform at ion resources.

Part I: Ethernet, Fast Ethernet, and Gigabit Ethernet 1 I nt roduct ion t o LANs 2 LAN MAC Addresses 3 Et hernet LAN St ruct ure 4 The Classical Et hernet CSMA/ CD MAC Prot ocol 5 Full- Duplex Et hernet Com m unicat ion 6 The Et hernet 10Mbps Physical Layer 7 The Et hernet 100Mbps Physical Layer 8 Gigabit Et hernet Archit ect ure

9 Gigabit Et hernet Physical Layer 10 Twist ed- Pair Cabling St andards and Perform ance Requirem ent s 11 Aut o- Negot iat ion

Chapter 1. Introduction to LANs A local area net work ( LAN) connect s a set of com put ers so t hat t hey can com m unicat e wit h one anot her direct ly. Once upon a t im e, t he t erm local was quit e appropriat e—early LANs spanned a lim it ed space. A LAN t ypically connect ed syst em s locat ed wit hin a single room or in an area confined wit hin one st ory of a building. Gradually, LANs snaked across m ult iple floors of a building. As t im e went by, new t echnologies m ade it possible for a LAN t o span a cam pus consist ing of several buildings, or even t o incorporat e sit es locat ed at great dist ances from one anot her.

The Birth of Ethernet A lot of experim ent at ion t ook place in t he early days of LANs, when a flock of net work equipm ent com panies m arket ed propriet ary LAN product s. Propriet ary t echnologies locked a cust om er int o using equipm ent t hat was m anufact ured by one vendor. A wat ershed event changed t he landscape. I n t he 1970s, Robert Met calfe and several of his colleagues at t he Xerox Palo Alt o Research Cent er ( PARC) invent ed and pat ent ed Et hernet . The invent ors of Et hernet t ook t he first st ep along t he road t o success in 1980, when t hey decided t o publish t he Et hernet specificat ion and m ake it available as an indust ry st andard. I n 1983, 10 m egabit per second ( Mbps) Et hernet becam e an I nst it ut e for Elect rical and Elect ronic Engineers ( I EEE) st andard. I t subsequent ly also won accept ance by t he Am erican Nat ional St andards I nst it ut e ( ANSI ) and t he I nt ernat ional Organizat ion for St andardizat ion ( I SO) . St andardizat ion at t ract ed bot h vendors and cust om ers t o Et hernet . A growing m arket , cust om er feedback, and vendor com pet it ion creat ed an environm ent t hat fost ered increasing funct ionalit y, im proved perform ance, and decreasing prices. The original 10Mbps Et hernet s ran on coaxial cable m edia. I n t he lat e 1980s, fiber opt ic im plem ent at ions were added. Anot her significant m ilest one was passed in 1990, when Et hernet over unshielded t wist ed- pair t elephone wire was st andardized. This gave an enorm ous boost t o t he Et hernet m arket . Developers t ackled im plem ent at ion of 100Mbps Et hernet in t he early 1990s. When t he 100Mbps t echnology was officially st andardized in 1995, product s already had been shipping for t wo years. I nst ead of rest ing on it s laurels, t he I EEE im m ediat ely form ed a subcom m it t ee t o work on 1000Mbps Et hernet . By 1998, t he st andard was com plet e, and product s hit t he m arket in 1999.

Today's Et hernet LANs operat e at 10Mbps, 100Mbps, and 1000Mbps. There are deskt op Et hernet int erfaces t hat can run at eit her 10 or 100Mbps and can be bought for t he price of a lunch. At t he t im e of writ ing, vendors are exploring t he possibilit y of im plem ent ing 10 gigabit per second ( Gbps, t he equivalent of 10,000Mbps) Et hernet .

Ethernet Characteristics Com m unicat ing using t he classical Et hernet prot ocol is like t rying t o be heard at a dinner part y. I f t here is a break in t he conversat ion, you can j um p in and say som et hing. I f t wo people t alk at once, however, t he t ransm ission fails. As m ore people j oin t he part y, it get s harder t o be heard. The classical prot ocol includes rules t hat m ake it very likely t hat , in t he long run, everyone will get a fair share of t he speaking t im e. However, t his prot ocol is based on a random select ion m et hod: There is no absolut e guarant ee t hat you will get a fair share of t he speaking t im e wit hin a given t im e period. St ill, Et hernet has com e a long way in t he last few years. A full- duplex version of t he prot ocol im plem ent ed wit h m odern Et hernet swit ching equipm ent enables all part icipant s t o t ransm it and receive at t he sam e t im e. I n ot her words, everyone at a swit ched Et hernet dinner part y can t alk ( and hear) at t he sam e t im e. This is called full- duplex LAN com m unicat ion.

Token Ring Researchers in I BM's Zurich research lab designed t he Token Ring LAN archit ect ure in t he early 1980s. I BM laid down a challenge t o Et hernet when it int roduced Token Ring product s in 1985. The Token Ring specificat ion was present ed t o t he I EEE, and Token Ring event ually j oined Et hernet as an I EEE, ANSI , and I SO st andard. Token Rings were craft ed for predict abilit y, fairness, and reliabilit y. Nat urally, t hese feat ures at t ract ed m any users. Alm ost all I BM m ainfram e shops used t he Token Ring t opology, t hinking t hat it would be around for a long t im e t o com e if I BM support ed it . Token Ring got an ext ra boost am ong I BM cust om ers as well: For m any years, I BM did not support Et hernet int erfaces in m ost of it s com put ers and net working devices. As t he nam e suggest s, syst em s in a Token Ring LAN are connect ed int o a ring. The act ual layout of a Token Ring LAN is organized very convenient ly. Groups of st at ions are connect ed t o concent rat ors t hat can be locat ed in t elephone wiring closet s. This m eans t hat a building can be st ar- wired for dat a in m uch t he sam e way t hat it is st ar- wired for t elephone service. Figure 1.1 illust rat es t he layout ; t he arrows indicat e t he pat h t hat dat a follows around t he ring.

Figu r e 1 .1 . Tok e n Rin g w ir in g.

A special m essage called a " t oken" is passed around t he ring. When a st at ion receives t he t oken, t he st at ion is allowed t o t ransm it dat a for a lim it ed am ount of t im e. Then it m ust pass t he t oken on t o t he next st at ion in t he ring. Every st at ion get s an equal opport unit y t o send dat a, and t here is a lim it t o t he am ount of t im e t hat can elapse before t hat opport unit y arises. Classical Token Rings operat e at 4Mbps and 16Mbps. The original 4Mbps version ran on shielded t wist ed- pair cable. Support for unshielded t wist ed- pair t elephone wire was added lat er. A 100Mbps version called High Speed Token Ring ( HSTR) was developed in t he lat e 1990s, and product s m ade t heir debut in 1998. The 100Mbps Token Ring is useful t o cust om ers who wish t o im prove t he perform ance of t heir exist ing Token Ring net works. However, t he int erfaces are expensive. A swit ched, full- duplex version of t he Token Ring prot ocol exist s for st at ions at t ached t o a swit ch. This m eans t hat , as wit h swit ched Et hernet , everyone can t alk at once. There is no need t o circulat e a t oken t o cont rol perm ission t o send. However, Token Ring swit ches are far m ore cost ly t han Et hernet swit ches. Furt herm ore, t he arrival of gigabit Et hernet —and t he likelihood of a fut ure 10Gbps version—has m ade m any users view Token Ring as a t echnology of t he past . Et hernet is t he cheap, scalable solut ion of t he present . Alt hough I BM cont inues t o support it s loyal Token Ring cust om ers, t he com pany now prom ot es Et hernet for all it s plat form s. Current ly, Token Ring t echnology is not winning new convert s, and m any Token Ring shops are gradually m igrat ing over t o Et hernet .

Fibre Distributed Data Interface At a surprisingly early dat e—1983—work st art ed on what was, at t hat t im e, a supernet work. A Fibre Dist ribut ed Dat a I nt erface ( FDDI ) net work operat es at 100Mbps.

The j ob of designing FDDI was undert aken by t he NSI X3T9.5 t ask group. Com ponent s of t he FDDI archit ect ure were described in a series of st andards docum ent s whose release was spread across a period of years last ing int o t he early 1990s. FDDI was designed wit h an eye on Token Ring. I t has a ring t opology, and, like Token Ring, a FDDI ring usually is built around a core ring of concent rat ors. The FDDI inform at ion t ransfer prot ocol is closely based on t he Token Ring inform at ion t ransfer prot ocol. However, t he j ob of physically set t ing up an FDDI LAN is a far m ore difficult t ask t han set t ing up a Token Ring LAN. I n addit ion, FDDI includes som e ext ra prot ocol elem ent s t hat are com plex and hard t o m ast er. FDDI oft en has been used as a backbone net work when high availabilit y is im port ant . A dual- ring t opology opt ionally can be used at t he core of an FDDI net work. This dual ring support s aut om at ic recovery from an event such as a broken cable. An FDDI net work can cover a pret t y large area. The m axim um t ot al pat h t hrough t he net work is lim it ed t o 200 kilom et ers ( km ) . However, if t he core of t he net work consist s of dual rings, t he circum ference of t he LAN is lim it ed t o 100km . As t he nam e suggest s, FDDI originally was designed t o run on fiber opt ic m edia. An ANSI t ask group lat er im plem ent ed 100Mbps t ransm ission over t wist ed- pair copper wire, an im port ant achievem ent . The copper version is called Copper Dist ribut ed Dat a I nt erface ( CDDI ) . Com ponent s t aken from FDDI and CDDI t ransm ission t echnology were reused in 100Mbps Et hernet as well as 100Mbps HSTR. FDDI provided an ideal backbone for int erconnect ing Et hernet or Token Ring LANs spread across a cam pus or spanning sit es separat ed by t ens of kilom et ers. The downside t o using an FDDI net work is t hat it is com plex and ext rem ely cost ly t o inst all. For t he m ost part , t he use has been lim it ed t o net work backbones. Swit ched full- duplex im plem ent at ions of FDDI are available, but t hey carry a big price t ag.

Fibre Channel and ATM A fibre channel LAN operat es at very high speeds and has a t opology t hat is ringbased, swit ch- based, or a com binat ion of bot h. The t echnology init ially was used t o build st orage area net works ( SANs) t hat enabled com put ers t o access and share disk or t ape st orage resources. I ncreasingly, fibre channel is used t o build LANs t hat int erconnect high- perform ance com put ers, st orage devices, and ot her peripheral devices. Asynchronous Transfer Mode ( ATM) init ially was int roduced as a wide area net working t echnology. A LAN capabilit y was added lat er. I t enabled ATM syst em s t o com m unicat e wit h one anot her—and wit h Et hernet or Token Ring syst em s across a local swit ch or across a wide area connect ion.

Fibre channel and ATM have t wo key feat ures in com m on: •

•

Bot h require a connect ion t o be set up ( like set t ing up a phone call) before a pair of syst em s can com m unicat e. Bot h support qualit y of service, m eaning t hat error and delay levels can be kept wit hin specified bounds.

Communications Layering Model I n t he early 1980s, t he I SO began t o publish a series of Open Syst em s I nt erconnect ( OSI ) docum ent s t hat defined st andards for dat a com m unicat ions. One of t hese described t he OSI layered m odel for dat a com m unicat ions. A sim pler m odel was used by t he I nt ernet Engineering Task Force ( I ETF) , whose com m it t ees produced t he TCP/ I P com m unicat ions st andards. Figure 1.2 shows t he t wo layered m odels.

Figu r e 1 .2 . Th e TCP/ I P a n d OSI com m u n ica t ion s m ode ls.

LAN Layering Model LAN t echnology operat es at Layers 1 and 2. The j ob of Layer 1, t he physical layer, is t o t ransm it 0s and 1s across a m edium . As indicat ed in Figure 1.3, LAN physicallayer st andards describe t he following:

Figu r e 1 .3 . LAN la ye r s.

• • •

The charact erist ics of a physical m edium The physical signals used t o represent digit al inform at ion Ot her physical specificat ions, such as t he connect or t ypes t hat canbe used, t he m axim um lengt h of a cable, and environm ent al const raint s ( such as t em perat ure ranges and freedom from elect rom agnet ic dist urbance)

At Layer 2, t he dat a link layer, 0s and 1s are organized int o unit s called dat a fram es. LAN dat a link layer st andards describe t he following: •

•

•

The overall form at of a dat a fram e and t he m eaning of each field t hat is included in t he fram e. The rules t hat m ust be followed t o win access t o t he m edium t o t ransm it a fram e. These rules define t he m edia access cont rol ( MAC) prot ocol. The form at and int erpret at ion of LAN addresses, which are called MAC addresses or physical addresses.

N ot e Layer 2 MAC addresses are local. They are m eaningful only wit hin one part icular LAN. These addresses m ake it possible t o deliver dat a from one syst em connect ed t o t he LAN t o anot her syst em on t he sam e LAN.

LAN Frames Fram e form at s used for Et hernet , Token Ring, and FDDI are discussed here because t hey have m any feat ures in com m on. Figure 1.4 shows t he general layout of an Et hernet , Token Ring, or FDDI fram e.

Figu r e 1 .4 . Ge n e r a l fr a m e la you t

Each fram e has a header t hat includes a dest inat ion and source MAC address. A fram e t hat carries dat a has an inform at ion field.

N ot e All Et hernet fram es carry dat a. Som e Token Ring and FDDI fram es are devot ed t o special prot ocol funct ions—for exam ple, passing t he t oken.

All LAN fram es end wit h a t railer t hat cont ains a fram e check sequence ( FCS) field used t o det ect t ransm ission errors. The FCS field cont ains t he result of a calculat ion ( called a cyclic redundancy check, or CRC) perform ed on t he rem aining bit s of t he fram e. The sam e calculat ion is carried out at t he dest inat ion st at ion; if t hese answers are not ident ical, t he fram e is discarded.

Basic LAN Components Som e basic t erm inology is needed before get t ing st art ed on t he in- dept h st udy of LAN t echnology and prot ocols. The subsect ions t hat follow describe t he m ost fundam ent al LAN com ponent s: st at ions, LAN m edia, net work int erface cards, and device drivers.

N ot e Chapt er 3, "Et hernet LAN St ruct ure," describes ot her essent ial net work equipm ent , including repeat ers, hubs, bridges, swit ches, and rout ers.

Stations and DTEs Syst em s t hat can originat e and receive fram es are called st at ions. St at ions include client and server com put ers t hat exchange dat a wit h one anot her, as well as rout ers t hat forward dat a out of a LAN and t ransm it dat a int o a LAN. A syst em t hat is a fram e originat or or receiver also is called dat a t erm inal equipm ent ( DTE) .

N ot e I t m ight seem st range t o classify a rout er as a st at ion or DTE. However, a rout er act s as a source or dest inat ion for Layer 2 fram e t ransm issions. When a fram e is sent t o a rout er, t he rout er rem oves t he fram e header and t railer and processes t he cont ent . I t t hen refram es t he cont ent and forwards t he new fram e away from it s source LAN and ont o a different part of t he net work.

LAN Media Many different physical m edia have been used for LAN com m unicat ions: coaxial cable, shielded t wist ed—pair cable, unshielded t wist ed—pair cable, and fiber opt ic cable all are in use t oday. I n addit ion, several wireless t echnologies have been int roduced ( infrared, frequency- hopping spread spect rum , and direct sequence spread spect rum ) .

Network Interface Card The point of connect ion bet ween a st at ion ( such as a com put er) and a LAN m edium is called a net work int erface. Net work int erface funct ions are perform ed by a net work int erface card ( NI C) t hat is inst alled in a st at ion expansion slot . Figure 1.5 cont ains a rough sket ch of a net work int erface card.

Figu r e 1 .5 . A n e t w or k int e r fa ce ca r d.

N ot e A NI C also is called an adapt er or adapt er card.

Each net work int erface card has a unique MAC address. Fram es t hat are sent t o a st at ion use t he NI C's MAC address t o ident ify t he dest inat ion. I n fact , t he MAC address oft en is referred t o as t he NI C address.

Device Drivers Each of t he m any devices used in a com put er—such as print ers, CD- ROMs, m onit or displays, and com m unicat ions hardware—is cont rolled by a soft ware program called a device driver. Net work- layer prot ocols send and receive dat a t hrough a net work int erface by int eract ing wit h t he device driver t hat cont rols t he syst em 's NI C. Figure 1.6 illust rat es t he relat ionship bet ween t he net work- layer prot ocols, t he device driver, and t he net work int erface card:

Figu r e 1 .6 . Role of t h e de vice dr ive r .

• •

A net work- layer prot ocol passes out going dat a t o t he device driver and receives dat a from t he device driver via a st andard set of applicat ion program int erface ( API ) calls. The device driver passes out going dat a t o a LAN int erface card. I t also accept s incom ing dat a from a LAN int erface card and passes it t o t he appropriat e net work prot ocol.

N ot e For Windows syst em s, t he Net work Driver I nt erface Specificat ion ( NDI S) defines t he program m ing int erface bet ween net work prot ocols and device driver program s. For Sun Microsyst em s com put ers, an API archit ect ure called STREAMS is used.

The device driver and t he NI C cooperat e in creat ing t he fram e headers and t railers t hat are wrapped around t he dat a t hat needs t o be t ransm it t ed. A different device driver is required for each NI C product because a vendor's device driver com m unicat es wit h it s NI C hardware in a propriet ary m anner. Suit able device driver soft ware m ust be supplied by t he NI C product vendor.

The benefit of using a st andard program m ing int erface bet ween net work prot ocols and a device driver is t hat t he net work prot ocols can funct ion wit h any NI C t hat is com pat ible wit h t he t ype of st at ion being used. You can replace t he current NI C wit h a new one wit hout m aking any change t o t he net work prot ocol soft ware. However, you m ust rem em ber t o inst all t he device driver soft ware t hat has been supplied wit h t he new NI C. Even if you bought t he new card from t he sam e vendor, it is very likely t hat t he old soft ware will not be capable of " t alking" t o t he new NI C hardware. Vendors are perpet ually m aking design changes t o t heir product s t o enhance perform ance and add new feat ures. These changes alm ost invariably require t he card's device driver soft ware t o be updat ed.

SNMP, Monitors, and RMON Many of t oday's LANs cont ain hundreds—or t housands—of syst em s. Net work m anagem ent facilit ies are needed t o configure, m onit or, and t roubleshoot a LAN. The Sim ple Net work Managem ent Prot ocol ( SNMP) is t he m ost widely im plem ent ed net work m anagem ent t echnology. The capabilit y t o part icipat e in SNMP is built int o virt ually every net worked device. A SNMP net work m anagem ent st at ion can com m unicat e wit h any device t hat has been SNMP- enabled. SNMP originally was creat ed t o int roduce net work m anagem ent funct ions int o t he I nt ernet . I t was designed by an I nt ernet Engineering Task Force ( I ETF) com m it t ee; t he I ETF organizat ion is responsible for developing I nt ernet st andards. Monit ors ( also called probes) are devices t hat can eavesdrop on LAN act ivit ies. A m onit or can be configured t o wat ch out for errors or t o provide an early warning t hat t rouble is brewing by report ing t hat som e crit ical t hreshold has been crossed. Monit ors also can capt ure and analyze prot ocol t raffic, a capabilit y t hat was used t o produce m any list ings t hat are displayed in t his book. Rem ot e m onit oring ( RMON) st andards enable an SNMP net work m anagem ent st at ion t o cooperat e wit h a net work m onit or. The m ain feat ures of SNMP and RMON are out lined in t he sect ions t hat follow.

SNMP Architecture SNMP follows a dat abase m odel. All devices cont ain inform at ion t hat a net work adm inist rat or would like t o see, including t he following: • • •

Configurat ion set t ings St at us inform at ion Perform ance st at ist ics

Wit h t he help of an SNMP net work m anagem ent st at ion, an adm inist rat or can read t his inform at ion and t hen updat e configurat ion or st at us set t ings. Figure 1.7 shows t he elem ent s of t he SNMP m odel. At t he request of applicat ions in t he m anagem ent st at ion, a SNMP m anager reads or updat es m anagem ent variables

at a rem ot e device by sending request s t o t he device's SNMP agent . The m anager com m unicat es wit h an agent using t he SNMP prot ocol.

Figu r e 1 .7 . The SN M P m ode l.

I f a significant event such as a reboot or a serious error occurs at a device, t he agent in t he device can report it using a m essage called a t rap. Device vendors and t hird- part y soft ware developers enhance t he usabilit y of a m anagem ent st at ion by writ ing applicat ions t hat display m anagem ent inform at ion in graphical or pict orial form . For exam ple, t he adm inist rat or m ight be shown a pict ure of a device and be able t o t roubleshoot or configure a specific com ponent by clicking on it .

M a n a ge m e n t I n for m a t ion Ba se

A collect ion of net work m anagem ent variables is called a Managem ent I nform at ion Base ( MI B) . Exam ples of MI B variables include t hese: • • •

The descript ion of a device The num ber of net work int erfaces in a device and t he t ype of each int erface Count s of incom ing or out going fram es

The st andardizat ion of MI B variables is an im port ant part of t he SNMP effort . A st andard MI B variable value has t he sam e form at and m eaning, independent of which vendor has built t he device.

M I B D ocu m e n t s Many docum ent s describing st andard MI B variables have been published as I ETF Request for Com m ent s ( RFC) docum ent s. These are freely available online at t he I ETF Web sit e ( ht t p: / / www.iet f.org) . I n addit ion, vendors have writ t en MI Bs t hat describe variables t hat are specific t o t heir own devices and t hat are not covered in t he st andards. A MI B docum ent includes a set of relat ed definit ions t hat are organized int o a unit called a MI B Module. These days, as soon as a new t echnology is int roduced, a group of expert s writ es a MI B Module for t he t echnology. A m aj or fact or in t he success of SNMP is t hat MI B docum ent s are writ t en in a form al language t hat can be underst ood by a m anagem ent st at ion. An adm inist rat or sim ply copies a MI B docum ent t o a m anagem ent stat ion's hard disk and ent ers a com m and t hat adds t he new MI B definit ions t o t he set of definit ions already underst ood by t he m anagem ent st at ion. The st at ion t hen is capable of reading and updat ing t he variables defined in t he docum ent . The usabilit y of a new MI B is enhanced by inst alling an applicat ion t hat aut om at ically gat hers t he MI B inform at ion and present s it in a succinct form .

RMON MIBs I n t he past , a m onit or was a st andalone device t hat had t o be accessed via it s at t ached keyboard and screen. A series of rem ot e m onit or MI B docum ent s ( RMON MI Bs) opened up com m unicat ion bet ween net work m anagem ent st at ions and m onit ors or probes placed in st rat egic locat ions wit hin t he net work. For exam ple, probes can be int egrat ed int o swit ches and rout ers. I nform at ion gat hered by m onit ors can be ret rieved, viewed, and archived at one or m ore m anagem ent st at ions. I n addit ion, m onit ors can spont aneously report problem sit uat ions t o m anagem ent st at ions. Wit h t he help of a net work m anagem ent st at ion applicat ion t hat provides a good user int erface t o RMON dat a, com m unicat ion wit h m onit ors becom es a powerful t ool.

SNMP Transports SNMP queries, updat es, responses, and t rap m essages can be carried bet ween syst em s using any convenient com m unicat ions prot ocol. Because SNMP was creat ed t o m eet I nt ernet needs, t he I nt ernet UDP prot ocol running on t op of I P is t he m ost popular t ransport . However, several ot her prot ocols are used t o carry SNMP m essages, including one t hat is designed for ATM net works.

Standards Body Overview Several im port ant st andards groups have been m ent ioned in t his chapt er: • •

•

•

I EEE publishes st andards relat ed t o elect ronic t echnologies. I t s 802 com m it t ee has produced a series of st andards docum ent s t hat describe LAN prot ocols and LAN physical t ransm ission m echanism s. ANSI is a coordinat ing organizat ion for dozens of specialized U.S. st andards organizat ions and t echnical com m it t ees. I SO publishes st andards t hat cover a wide range of int erest s. I nt ernat ional dat a com m unicat ions st andards are organized under it s I SO OSI endeavor. The I nt ernet Engineering Task Force ( I ETF) is responsible for TCP/ I P st andards. The SNMP st andards are m aint ained and published by t he I ETF.

Appendix C, " St andards Bodies and References," cont ains sum m ary descript ions of st andards groups and provides point ers t o t heir World Wide Web sit es. The first st eps int o t he world of LANs are now com plet e. The next chapt er will present a det ailed st udy of MAC addresses.

Summary Points •

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• •

• •

• • •

•

•

A LAN connect s a set of com put ers so t hat t hey can com m unicat e wit h one anot her direct ly. Et hernet , Token Ring, FDDI , fibre channel, and ATM are described in published st andards docum ent s. Classic Et hernet , Token Ring, and FDDI all were half- duplex prot ocols—t hey allowed only one st at ion t o t ransm it successfully at a given t im e. Current ly, swit ched " full- duplex" im plem ent at ions of Et hernet , Token Ring, and FDDI enable m ult iple syst em s t o com m unicat e concurrent ly. Price/ perform ance and an expanding set of usabilit y feat ures have m ade Et hernet t he dom inant LAN t echnology in use t oday. LAN t echnology operat es at t he physical and dat a link layers of t he OSI m odel. The j ob of Layer 1, t he physical layer, is t o t ransm it 0s and 1s across a m edium . At t he dat a link layer, 0s and 1s are organized int o fram es. A MAC address is associat ed wit h a net work int erface card ( NI C) . Every LAN fram e has a header t hat includes a dest inat ion and a source MAC address. All LAN fram es include a fram e check sequence ( FCS) field t hat is used t o det ect t ransm ission errors. Syst em s t hat can originat e and receive fram es are called st at ions or DTEs.

• • •

• •

•

Net work int erface funct ions are perform ed by a net work int erface card ( NI C) . Net work- layer prot ocols send and receive dat a by int eract ing wit h an int erm ediat e piece of soft ware called a device driver. The Sim ple Net work Managem ent Prot ocol ( SNMP) is support ed by alm ost all net work devices. Monit ors ( or probes) are devices t hat can eavesdrop on LAN act ivit ies. A Managem ent I nform at ion Base ( MI B) is a collect ion of net work m anagem ent variables. Rem ot e m onit oring ( RMON) MI Bs enable net work m anagem ent st at ions t o com m unicat e wit h net work m onit ors.

Chapter 2. LAN MAC Addresses I f a com put er want s t o com m unicat e wit h anot her syst em on it s LAN, it needs t o ident ify t he t arget syst em . Media access cont rol ( MAC) addresses ( also called physical addresses) are used t o ident ify LAN dest inat ions. Every net work int erface card ( NI C) t hat connect s t o a LAN m ust have a MAC address t hat ident ifies it uniquely on t hat LAN. Unplugging a com put er, m oving it , and plugging it int o a different LAN is a pret t y com m onplace event . Early on, it was decided t hat it would be a good idea t o assign a unique MAC address t o every NI C t hat is m anufact ured. That way, a card could be used anywhere wit hout any worry of running int o an address conflict . The I EEE has been given t he j ob of supervising t he assignm ent of unique addresses t o NI Cs. This chapt er describes how t he I EEE carries out t his t ask and explains how t he MAC address space has been divided up and used for different kinds of addresses. The chapt er also includes a discussion of t he order in which t he bit s are t ransm it t ed ont o a m edium for each fram e t ype. This t opic is m essy and can be t edious. However, it is needed in order t o underst and problem s t hat can arise when you int erconnect different t ypes of LANs int o a single LAN. This also enables you t o underst and one feat ure of t he VLAN headers t hat are discussed in Chapt er 16, " VLANs and Fram e Priorit y."

Universally Administered MAC Addresses As part of t he m anufact uring process, each Et hernet , Token Ring, or FDDI LAN adapt er card is configured wit h a MAC address. A LAN MAC address consist s of 48 bit s ( 6 byt es) . By convent ion, an address is writ t en as X', followed by six pairs of hexadecim al charact ers separat ed by dashes. ( The sym bol pat t ern X' st ands for " hexadecim al." ) For exam ple, t he address of t he adapt er in t he com put er on which t his book is being writ t en is: X'00- 60- 08- BD- 7C- 1E

N ot e

Appendix B, " Tables," cont ains a descript ion of t he way t hat an 8- bit binary st ring is convert ed t o hexadecim al not at ion. The appendix cont ains a t abulat ion of binary, decim al, and hexadecim al represent at ions of 4- bit quant it ies.

The I EEE carries out an adm inist rat ive funct ion t hat enables vendors t o assign globally unique addresses t o t heir adapt er product s. This works as follows: • •

A vendor subm it s a form t o t he I EEE and pays a regist rat ion fee. The I EEE assigns a 3- byt e address prefix, called an organizat ionally unique ident ifier ( OUI ) , t o t he applicant . For exam ple, t he address prefix X'00- 60- 08 shown previously is owned by 3Com . Xerox owns prefix X'00- 00- 00, I nt el owns X'00- 90- 27, and Xircom has X'00- 10- A4.

The applicat ion form and a public list of assigned OUI prefixes are available online at ht t p: / / st andards.ieee.org/ .

N ot e I n t he early days of Et hernet , all t he prefixes were adm inist ered by Xerox and were called block ident ifiers.

Aft er get t ing an OUI , a net work int erface card vendor has t he rem aining 3 address byt es ( 24 bit s) at it s disposal. This m eans t hat t he vendor can m anufact ure 224 ( 16,777,216) NI Cs wit h dist inct addresses appended t o it s prefix. These globally unique addresses are called universally adm inist ered MAC addresses. A vendor can obt ain an addit ional OUI prefix when alm ost all of it s current addresses have been used up. The I EEE provides a valuable service by overseeing MAC addresses. I f you use I EEE universal addresses, you can be sure t hat t he address of a new device added t o a LAN will not conflict wit h t he address of any ot her device on t he LAN.

N ot e There have been rum ored cases of m anufact uring errors t hat caused duplicat e MAC addresses t o be produced, but at worst , t hese are very rare event s. The duplicat es m ight be t he work of "pirat e" clone NI C vendors who pay no at t ent ion t o t he rules. I t is a good idea t o check t hat your NI Cs com e from a reput able source.

Locally Administered MAC Addresses When universally adm inist ered addresses are used on a LAN, t he MAC addresses look like random j um bles of hex charact ers. Som e LAN adm inist rat ors prefer t o reconfigure each adapt er at inst allat ion t im e and assign addresses t hat have a local int erpret at ion. For exam ple, each address could be split int o fields t hat ident ify t he

building, floor, wiring closet , and specific office in which t he syst em has been inst alled. Knowing t he locat ion of a m alfunct ioning card is helpful when t roubleshoot ing LAN problem s. The I EEE num bering archit ect ure t akes t his preference int o account . A bit in t he first address byt e is equal t o 0 in all universally adm inist ered addresses and m ust be 1 for locally adm inist ered addresses.

N ot e Using local addresses im poses an adm inist rat ive burden. Every card m ust be m anually configured wit h a MAC address t hat has a locally defined m eaning. I f t he MAC address corresponds t o a st at ion's locat ion, t his address m ust be changed when t he st at ion is m oved. I f a card fails, t he st aff person replacing t he card m ust rem em ber t o configure t he replacem ent correct ly when t he new one is inst alled. The benefit of using a universally adm inist ered unique address is t hat you can at t ach a syst em t o any LAN wit hout worrying about duplicat e MAC addresses.

Broadcast and Group Addresses An address t hat ident ifies a single syst em is called a unicast or individual address. A fram e sent t o a unicast MAC address is t arget ed at a single MAC addresses dest inat ion. A st at ion at t ached t o a m ult iaccess LAN has t he capabilit y t o direct a fram e t o every syst em on t he LAN. A broadcast address is used t o do t his. All MAC int erfaces absorb any fram e t hat is addressed t o t he broadcast address: X'FF- FF- FF- FF- FF- FF This address consist s of 48 1- bit s. Som et im es it is convenient t o send a fram e t o a select group of recipient syst em s. Group addresses ( also called m ult icast addresses) im plem ent t his feat ure. A bit in t he first address byt e is equal t o 0 for an individual address and 1 for a group dest inat ion address. To set up an adapt er so t hat it will recognize and absorb fram es sent t o a part icular group address, a higher- layer program passes a request t o t he device driver, which t hen not ifies t he adapt er t o add t he new address t o it s list . The NI C inFigure 2.1already has been configured t o accept fram es addressed t o Group- Address- 1 and Group- Address- 2, in addit ion t o it s own unicast address and t he broadcast address. The figure shows an applicat ion program asking t he device driver t o add Group- Address- 3 t o t he NI C. A lat er request could be used t o rem ove a group address from t he card.

Figu r e 2 .1 . Addr e sse s r e cogn ize d by a N I C.

Individual/Group and Universal/Local Flag Bits The t wo least significant bit s in t he first byt e of a MAC address are t he special flag bit s t hat ident ify whet her an address is: • •

I ndividual ( 0) or group ( 1) Universal ( 0) or local ( 1)

Figure 2.2 shows t he posit ion of t hese im port ant bit s as t hey appear when each byt e of a MAC address is writ t en in binary wit h it s m ost significant bit on t he left .

Figu r e 2 .2 . I n dividu a l/ gr ou p a n d u n ive r sa l/ loca l fla g bit s.

N ot e This is t he norm al m at hem at ical order in which t he bit s in a byt e are expressed. See Appendix B.

Ethernet Address Conventions The I EEE 802.3 com m it t ee specified t hat every byt e in an Et hernet fram e m ust be t ransm it t ed wit h t he least significant bit first .

N ot e This convent ion som et im es is called lit t le endian order. St andards do not specify t he order in which bit s and byt es are st ored wit hin a com put er. How- ever, a st andard does need t o spell out t he order in which bit s and byt es are t ransm it t ed. When com put ers com m unicat e wit h one anot her, each needs t o know what t o expect when bit s arrive across a m edium .

Not e t hat t ransm it t ing t he least significant bit first assures t hat t he first MAC address bit t hat goes ont o a wire is t he individual/ group bit and t he second is t he universal/ local bit . For exam ple, if X'C2 is t he first byt e of an address, t he first bit t hat is sent is a 0 ( individual address) , and t he second bit is a 1 ( local address) . The order of t ransm ission is indicat ed here by t he let t ers a- h. The bit m arked " a" is sent first . h g f e d c b a- > 1 1 0 0 0 0 1 0 = X'C2

Ethernet Multicast Addresses A large num ber of Et hernet MAC addresses—in fact , half of t hem —are m ult icast addresses. Mult icast addresses are used t o send a fram e t o a group of syst em s. Recall t hat t he flag t hat indicat es whet her an address is unicast or m ult icast is t he least significant bit in t he first byt e of an address. When an organizat ion regist ers and obt ains an OUI for unicast addresses, it also can use t hat OUI for m ult icast addresses. The organizat ion can define m ult icast addresses for any purpose t hat is deem ed convenient . The I EEE 802 com m it t ee owns unicast OUI X'00- 80- C2. This gives t he I EEE t he right t o define m ult icast addresses t hat st art wit h X'01- 80- C2. For exam ple, t ake a look at t hese addresses and definit ions: X'01- 80- C2- 00- 00- 00 Used as a group address for bridges t hat support t he spanning t ree prot ocol X'01- 80- C2- 00- 01- 10 Used as a group address for st at ions configured t o receive FDDI st at us report fram es Digit al Equipm ent Corporat ion ( DEC) owns unicast OUI X'08- 00- 2B, so it can define m ult icast addresses t hat st art wit h X'09- 00- 2B. DEC has defined m any specialpurpose m ult icast addresses. For exam ple, DEC uses X'09- 00- 2B- 00- 00- 0F as a local area t ransport ( LAT) m ult icast address. LAT is a t erm inal access prot ocol t hat is used in DEC net works. Not e t hat t he bit s in t he first byt e ( X'09) of t hese DEC addresses are as follows: 0 0 0 0 1 0 0 1 Least significant bit

Traces of Ethernet MAC Addresses Every MAC fram e has a header t hat contains t he dest inat ion and source MAC addresses. The address fields in List ings 2.1, 2.2, and 2.3 were t aken from fram es capt ured by a Windows NT Server 4.0 Net work Monit or ut ilit y. List ing 2.1 shows t he address fields in a fram e t hat has been sent from one LAN st at ion t o anot her.

List in g 2 .1 D e st in a t ion a n d Sou r ce Addr e sse s in a Un ica st Et h e r n e t Fr a m e H e a de r ETHERNET: Destination address : 0020AF3BD450 ETHERNET: .......0 = Individual address ETHERNET: ......0. = Universally administered address ETHERNET: Source address : 00A024A6EDE4 ETHERNET: .......0 = Individual address ETHERNET: .......0 =Universally administered address The first byt e of bot h t he dest inat ion address and t he source addresses in List ing 2.1 is X'00. I n binary: X'00 = 0000 000 0 Least significant bit

The least significant bit is 0 in bot h addresses in List ing 2.1, confirm ing t hat t hey are individual addresses. The prior bit indicat es whet her an address is universally adm inist ered ( 0) or locally adm inist ered ( 1) . Bot h of t he addresses in t he t race are universally adm inist ered. List ing 2.2 shows t he addresses for a fram e t hat was broadcast t o all syst em s on t he LAN. There is a m isleading st at em ent in t he t hird line of t he Microsoft prot ocol analyzer report . Because all bit s in t he broadcast address are 1, t he relevant universal/ local bit also is 1. However, t he broadcast address is an except ion t o t he rule st at ing t hat t his m eans t hat t he address is locally adm inist ered.

List in g 2 .2 D e st in a t ion a n d Sou r ce Addr e sse s in a Br oa dca st Et h e r n e t Fr a m e H e a de r ETHERNET: Destination address : FFFFFFFFFFFF ETHERNET: .......1 = Group address ETHERNET: ......1. = Locally administered address ETHERNET: Source address : 00A024A6EDE4 ETHERNET: .......0 = (Individual address)

N ot e A source address m ust be a unicast address. Aft er all, som e int erface sent t his fram e. The source address m ust ident ify which int erface it was.

List ing 2.3 shows addresses for a fram e wit h a m ult icast dest inat ion address. An adapt er will not absorb fram es sent t o a part icular m ult icast address unless it has been configured t o do so. A quick check of a list of st andard m ult icast addresses revealed t hat t he fram e in t he t race is a Hello m essage direct ed t o a group of rout ers on t he LAN. The originat ing rout er sends t his m essage t o announce t hat it is t here, alive and well. Each rout er on t he LAN has been configured so t hat it s LAN adapt er will accept fram es sent t o m ult icast address X'01- 00- 5E- 00- 00- 05.

List in g 2 .3 D e st in a t ion a n d Sou r ce Addr e sse s in a M u lt ica st Et h e r n e t Fr a m e H e a de r ETHERNET: Destination address : 01005E000005 ETHERNET: .......1 = Group address ETHERNET: ......0. = Universally administered address ETHERNET: Source address : 00A02456AB6F ETHERNET: .......0 = (Individual address)

N ot e The Token Ring and FDDI sect ions t hat follow are fairly t echnical. The reader m ay wish t o skip t hem now and com e back t o t hem lat er, if needed.

Token Ring Address Conventions The Token Ring prot ocol was invent ed by I BM. I BM had defined m any com m unicat ions prot ocols prior t o designing t he Token Ring. For I BM's earlier prot ocols, t he m ost significant bit in each byt e was t ransm it t ed first This som et im es is call " big endian" order. I BM did not want t o change t his pract ice for t he Token Ring prot ocol. However, I BM also want ed t o subm it it s Token Ring specificat ion t o t he I EEE. I n it s 802.3 st andard, t he I EEE had specified t hat t he byt es in an Et hernet fram e were t o be t ransm it t ed wit h t he least significant bit first . This was especially im port ant for addresses: •

•

The first ( left m ost ) dest inat ion address bit t hat is t ransm it t ed m ust indicat e whet her it is an individual address ( 0) or a group address ( 1) . The next bit m ust indicat e whet her it is a universal ( 0) or a local ( 1) address.

I BM's com prom ise was t o: •

•

Transm it t he addresses in t he MAC header in t he sam e order as Et hernet addresses wit h t he individual/ group bit first Transfer t he byt es in t he inform at ion field wit h t he m ost significant bit first

However, I BM went one st ep furt her. The com pany reint erpret ed addresses so t hat t he order of t he bit s in each address byt e was reversed. Reversing convert s t he least significant bit int o t he m ost significant bit . For exam ple, I BM defined several special m ult icast addresses ( called funct ional addresses) t hat are used t o ident ify nodes t hat perform various t ypes of Token Ring services. Funct ional addresses st art wit h X'CC ( binary 1100 1100) . The individual/ group bit is t he first ( m ost significant ) bit of t he address. The upper part of Figure 2.3 illust rat es t he order of Token Ring addresses.

Figu r e 2 .3 . I n it ia l bit s in a Tok e n Rin g a ddr e ss.

Because a source address always m ust be an individual address, I BM decided t o use t he individual/ group bit in a source address for a different purpose. I BM used t he bit as a flag t hat indicat es whet her an addit ional field cont aining fram e rout ing inform at ion follows. ( This field is called t he Rout ing I nform at ion Field, or RI F, and is explained in Chapt er 19, " Token Ring and FDDI Overview." ) The lower part of Figure 2.3 illust rat es t his usage. Like Et hernet st at ions, Token Ring st at ions recognize X'FF- FF- FF- FF- FF- FF as a broadcast address. The address X'C0- 00- FF- FF- FF- FF also is designat ed as a Token Ring broadcast address. Not e t hat t he init ial bit of X'CO is a 1, indicat ing t hat t his is a group address.

Addresses Carried in a Token Ring Information Field Som e prot ocol m essages carry MAC addresses wit hin a fram e's inform at ion field. For exam ple, TCP/ I P st at ions broadcast Address Resolut ion Prot ocol ( ARP) request s t hat ask t he owner of an enclosed I P address t o respond and provide it s MAC address. On a Token Ring, t he addresses in t hese m essages are represent ed ( and t ransm it t ed) in exact ly t he sam e way as in t he MAC header.

A Trace Showing Token Ring MAC Addresses For exam ple, I BM has been assigned OUI X'10- 00- 5A. List ing 2.4 shows a t race t hat was obt ained wit h a Net work Associat es Sniffer m onit or. The list ing shows an ARP m essage carried in a Token Ring fram e. The source address is report ed as I BM1 21D4E2. Lat er in t he m essage, t he full address is displayed as X'08- 00- 5A- 21- D4- E2 in t he sender's hardware address field. The act ual NI C address is X'10- 00- 5A- 84- 2B- 47. The address has been expressed wit h t he bit s in each byt e reversed, bot h when it s address appears in t he Token Ring MAC header and when it reappears inside t he ARP dat a. The t ransform at ion is shown as follows. Each byt e is t ranslat ed t o bit s and is form at t ed for easy reading wit h slashes bet ween each group of 8 bit s: X'10- 00- 5A- 84- 2B- 47= 0001 0000/ 0000 0000/ 0101 1010/ 1000 0100/ 0010 1011/ 0100 0111 Reversing t he order wit hin each byt e looks like t his: X'08- 00- 5A- 21- D4- E2 = 0000 1000/ 0000 0000/ 0101 1010/ 0010 0001/ 1101 0100/ 1110 0010 This reversed order is called t he noncanonical form at . When you exam ine t he act ual hexadecim al dat a t hat was t ransm it t ed, you can see t hat t he source MAC address field is displayed as 88 00 5a 21 d4 e2.The 08 has been convert ed t o 88 because t he first bit has been set t o 1, indicat ing t hat a RI F follows.

List in g 2 .4 Tok e n Rin g Addr e sse s ARP: DLC:

----- DLC Header ----DLC: DLC: Frame 31 arrived at 13:35:13.863; frame size is 58 (003A hex) bytes. DLC: FS: Addr recognized indicators: 11, Frame copied indicators: 11 DLC: AC: Frame priority 0, Reservation priority 0, Monitor count 1 DLC: FC: LLC frame, PCF attention code: None DLC: Destination = Station IBM2 A8EDE3 DLC: Source = Station IBM1 21D4E2 DLC: RI: ----- Routing Indicators ----(Routing Information Field) (Other Fields) ARP: ----- ARP/RARP frame ----ARP: ARP: Hardware type = 6 (IEEE 802 Network) ARP: Protocol type = 0800 (IP) ARP: Length of hardware address = 6 bytes ARP: Length of protocol address = 4 bytes

ADDR 0000: 0010: 0020: 0030:

ARP: Opcode 2 (ARP reply) ARP: Sender's hardware address = 08005A21D4E2 ARP: Sender's protocol address = [170.217.24.139] ARP: Target hardware address = 10005AA8EDE3 ARP: Target protocol address = [170.217.17.100] ARP: ARP: HEX (destination) * (source) 18 40 10 00 5a a8 ed e3 88 00 5a 21 d4 e2 08 c0 43 e1 43 b0 41 70 aa aa 03 00 00 00 08 06 00 06 08 00 06 04 00 02 08 00 5a 21 d4 e2 aa d9 18 8b 10 00 5a a8 ed e3 aa d9 11 64

Token Ring Functional Addresses The Token Ring designers defined 31 funct ional address flags t hat are used t o send fram es t o a group of syst em s t hat play specific roles in a Token Ring net work. Funct ional addresses are int roduced by t his 17- bit pat t ern: 1 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 This t ranslat es t o X'C0- 00, followed by anot her 0 bit . The first bit in X'C0 shows t hat t hese are group addresses, and t he second bit shows t hat t hese addresses are classified as local addresses. The rem aining 31 bit s are used as flags t hat ident ify t he t ype ( or t ypes) of dest inat ions t hat should absorb t he fram e. Table 2.1 list s som e funct ional addresses. A single fram e can be sent t o m ult iple groups of syst em s by set t ing several flag bit s t o 1. For exam ple, t o send a fram e t o bot h an act ive m onit or and a ring param et er server, set t he dest inat ion address equal t o t he following: X'C0- 00- 00- 00- 00- 03

Ta ble 2 .1 . Tok e n Rin g Fu n ct ion a l M AC Addr e sse s Fu n ct ion N a m e

M AC Addr e ss

Act ive m onit or

X'C0- 00- 00- 00- 00- 01

Ring param et er server ( RPS)

X'C0- 00- 00- 00- 00- 02

Ring error m onit or ( REM)

X'C0- 00- 00- 00- 00- 08

Configurat ion report server ( CRS)

X'C0- 00- 00- 00- 00- 10

Source rout e bridge

X'C0- 00- 00- 00- 01- 00

FDDI Address Conventions

FDDI inform at ion fram es are based on Token Ring inform at ion fram es. As is t he case for a Token Ring fram e, t he inform at ion in an FDDI fram e is sent wit h t he m ost significant bit first . And, like Token Ring, t he MAC addresses in an FDDI MAC header are expressed in reverse order. Unfort unat ely, MAC addresses em bedded in an FDDI inform at ion field som et im es appear in non- canonical form at ( bit - reversed, like Token Ring) and som et im es appear in canonical form at ( not bit - reversed) . •

•

When t he FDDI ring int erconnect s wit h Token Ring LANs, t he non- canonical ( bit - reversed) form at is used. When t he FDDI ring int erconnect s wit h Et hernet LANs, t he canonical form at is used.

I f bot h Et hernet and Token Ring LANs are connect ed t o an FDDI LAN, bot h canonical and noncanonical form at s can appear on t he FDDI LAN.

A Trace Showing FDDI MAC Addresses List ing 2.5 is a Net work Associat es Sniffer t race t hat shows part s of an FDDI fram e cont aining an ARP m essage t hat includes MAC addresses in it s inform at ion field. The act ual addresses are: Dest inat ion: X'08- 00- 20- 06- C5- D9 Source: X'08- 00- 2B- 0F- E4- 0D The upper port ion of t he t race displays t he act ual addresses, independent of t he order of t ransm ission. The reader can see t hat t he MAC header source address is t he sam e as t he sender's source address in t he ARP part of t he t race. However, t he hexadecim al rendit ion of t he t race shows t hat t he address byt es in t he MAC header and in t he ARP m essage act ually were t ransm it t ed in a different m anner. For t he MAC header, byt es appeared in bit - reversed form at : Dest inat ion: 10 00 04 60 a3 9b Source: 10 00 d4 f0 27 b0 I n t he ARP part , t he addresses appeared in canonical form at : Target hardware address: 08 00 20 06 c5 d9 Sender's hardware address: 08 00 2B 0F E4 0D

List in g 2 .5 M AC Addr e sse s in a n FD D I Fr a m e FDDI: ----- DLC Header -----

FDDI:

FDDI: Frame 5 arrived at 02:17:27.03635; frame size is 67 (0043 hex) bytes. . . . FDDI: Note: Addresses presented in LSb format FDDI: FDDI: Destination = Station Sun 06C5D9 (X'08-00-20-06-C5-D9) FDDI: Source = Station DEC 0FE40D (X'08-00-2B-0F-E4-0D) FDDI: . . . (Other Fields) ARP: ----- ARP/RARP frame ----ARP: ARP: Hardware type = 1 (10Mb Ethernet) ARP: Protocol type = 0800 (IP) ARP: Length of hardware address = 6 bytes ARP: Length of protocol address = 4 bytes ARP: Opcode 2 (ARP reply) ARP: Sender's hardware address = 08002B0FE40D ARP: Sender's protocol address = [128.141.200.5] ARP: Target hardware address = 08002006C5D9 ARP: Target protocol address = [128.141.1.203] ARP: 0000: 50 10 00 04 60 a3 9b 10 00 d4 f0 27 b0 aa aa 03 0010: 00 00 00 08 06 00 01 08 00 06 04 00 02 08 00 2b 0020: 0f e4 0d 80 8d c8 05 08 00 20 06 c5 d9 80 8d 01 0030: cb 01 cd 08 01 03 fd 07 f4 There are swit ches t hat int erconnect Et hernet s, Token Rings, and FDDI LANs. These swit ches have t o deal wit h all of t he form at s and m ake sense of t hem . This is m essy!

N ot e Chapt er 16 describes an I EEE t ag header t hat cont ains priorit y and virt ual LAN inform at ion. This header includes a flag t hat indicat es whet her MAC addresses em bedded in a fram e's inform at ion field appear in canonical or noncanonical form . These t ags provide a rat ional solut ion t o a silly problem and are support ed in up- t odat e equipm ent .

Summary Points • • •

•

•

Every net work int erface card ( NI C) is assigned a unique 6- byt e MAC address. Universally unique m edia access cont rol ( MAC) addresses are cont rolled by t he I EEE, which assigns 3- byt e organizat ionally unique ident ifiers ( OUI s) t o request ing organizat ions. An OUI value is used as t he first 3 byt es of a unique address. Som e LAN adm inist rat ors prefer t o assign t heir own local MAC addresses t o NI Cs. A fram e whose dest inat ion is a broadcast address is sent t o every syst em on a LAN. A fram e whose dest inat ion address is a m ult icast address is t arget ed at a group of syst em s on a LAN.

•

•

• •

Et hernet addresses are t ransm it t ed wit h t he least significant bit first . The first bit s t o be sent are t he unicast / group flag and t he universal/ local flag. The m ost significant bit in a Token Ring inform at ion byt e is t ransm it t ed first . However, addresses are sent wit h t he least significant bit first . To do t his, addresses are redefined wit h a reversed bit order. Addresses appearing in t he inform at ion field have t he sam e bit ordering as addresses in t he MAC header. A Token Ring Rout ing I nform at ion Field ( RI F) lays out a pat h from a source t o t he dest inat ion. The first bit in a source address indicat es whet her a RI F follows. FDDI inform at ion fram es were m odeled on Token Ring inform at ion fram es. The bit ordering for addresses appearing in t he inform at ion field depends on whet her t he FDDI LAN int erconnect s t o Token Ring LANs or Et hernet LANs.

References The I ETF I nt ernet Engineering Not e ( I EN) referenced below cont ains a playful discussion of t he bat t le bet ween t he big- endians and t he lit t le- endians. •

I ETF I EN- 37. " On Holy Wars and a Prayer for Peace." D. Cohen. April 1, 1980.

The following I EEE docum ent t ries t o clear up t he confusion caused by m ult iple " st andard" fram e and address form at s. •

I EEE Draft St andard 802: Overview and Archit ect ure, 1999.

Chapter 3. Ethernet LAN Structure I n 1980, Digit al Equipm ent Corporat ion ( DEC) , I nt el, and Xerox published t he Et hernet Version 1 specificat ion ( also called t he Et hernet Blue Book) . This specificat ion described a 10Mbps LAN t hat operat ed across coaxial cable using baseband t ransm ission ( which m eans t hat bit s are represent ed as a series of volt age pulses) . An im proved version, now called DI X Et hernet or Et hernet Version 2, was published in 1982.

N ot e DI X st ands for Digit al, I nt el, and Xerox. Et hernet Version 2 som et im es is writ t en as Et hernet Version I I , or sim ply Et hernet I I .

I n t he m eant im e, t he I nst it ut e for Elect rical and Elect ronic Engineers ( I EEE) est ablished it s 802 com m it t ee, t asked wit h developing and prom ot ing local and m et ropolit an area net work ( MAN) st andards.

N ot e

The engineers were not very im aginat ive when t hey chose t he nam e of t heir com m it t ee. The 802 com m it t ee was form ed in t he second m ont h ( 2) of 1980 ( 80) .

I n 1981, an 802 subcom m it t ee ( called t he 802.3 Working Group) got t oget her t o com pose a st andard based on DI X Et hernet . By 1983, when t he I EEE st andard was officially published, m any vendors already had endorsed and accept ed bot h DI X and I EEE Et hernet , and som e were shipping product s. Sealing it s success, t he 802.3 st andard was adopt ed by t he Am erican Nat ional St andards I nst it ut e ( ANSI ) and t he I nt ernat ional St andards Organizat ion ( I SO) .

N ot e As Chapt er 4, " The Classical Et hernet CSMA/ CD MAC Prot ocol," will show, t he 802.3 fram e form at differed slight ly from t he DI X fram e form at and im posed a few ext ra byt es of overhead. St andards advocat es have repeat edly declared t hat t he DI X form at was dead, but m ost cust om ers preferred DI X because t hey got a sm idgen of ext ra t hroughput by using it . NI C vendors st ayed out of t he fight and support ed bot h versions on t heir Et hernet cards. Finally, in it s 1998 updat e of Et hernet , t he I EEE 802.3 com m it t ee gave up and incorporat ed t he DI X fram e form at as one of t he accept able ways of doing business. I n fact , t he 802 com m it t ee used t he DI X Et herType t o add som e useful ext ensions t o Et hernet —fram e priorit y levels and virt ual LANs ( VLANs) . These are described in Chapt er 16, " VLANs and Fram e Priorit y." The DI X fram e lives!

Ethernet LAN Architecture Many of t he com ponent s t hat m ake up t oday's Et hernet LANs were defined in t he early years of t he original coaxial cable Et hernet LANs. A brief t our of t he design of t hese early LANs is a quick and painless way t o int roduce Et hernet LAN archit ect ure and t erm inology.

N ot e The purpose of t he discussion t hat follows is t o est ablish som e basic t erm inology and concept s. Many det ails have been om it t ed so t hat som e m aj or pieces of t he fram ework can be nailed in place. Lat er chapt ers will t ell t he whole st ory.

Single Segment Ethernet LAN and CSMA/CD Figure 3.1 shows a very sim ple Et hernet LAN m ade up of several deskt op syst em s and a server at t ached t o a coaxial cable.

Figu r e 3 .1 . Se n ding a fr a m e on a n Et h e r n e t se gm e n t .

A syst em t hat want s t o send a fram e m ust wait unt il t he m edium is quiet —t hat is, unt il no ot her syst em is sending a fram e. Only one fram e is allowed t o t raverse t he cable at any given inst ant . I f t wo st at ions st art t o send at t he sam e t im e, t heir t ransm issions will collide, and each will have t o wait for a random t im eout period before t rying again. This set of rules is called Carrier Sense Mult iple Access wit h Collision Det ect ion ( CSMA/ CD) . All t he syst em s list en t o t he m edium and see every fram e t hat is t ransm it t ed. Each fram e has a header t hat carries t he LAN addresses of it s dest inat ion and it s source. I n Figure 3.1, deskt op syst em A is sending a fram e t o server C. Server C reads t he dest inat ion address in t he fram e's header, recognizes t hat it m at ches it s own address, and accept s t he fram e. Syst em s B and D see t he fram e but , aft er exam ining t he dest inat ion address, ignore t he fram e. The configurat ion shown in Figure 3.1 is called a bus t opology. A single cable serves t o connect t he devices. The LAN in Figure 3.1 illust rat es several concept s: •

•

•

The syst em s are connect ed t o a single coaxial cable bus t hat is called a LAN segm ent . This is a m ult iaccess environm ent t hat is, m ult iple syst em s are connect ed, and a syst em accesses t he m edium t o send a fram e t o anot her syst em . A fram e's dest inat ion address is exam ined by every syst em . This m akes it possible t o use a broadcast address t o send a fram e t o every st at ion, and t o use dist inct ive m ult icast addresses t o send a fram e t o a select ed group of st at ions on t he LAN.

This oft en is called a broadcast m ult iaccess environm ent because m any syst em s are connect ed t o t he m edium , and each syst em 's net work int erface " sees" every fram e.

Using Repeaters to Build Multisegment LANs Early LANs t ended t o be sm all and com pact , but it did not t ake long before users want ed t heir LANs t o grow. However, t he signals t hat represent zeros and ones grow weak ( at t enuat e) as t hey propagat e t hrough a cable segm ent . A segm ent 's lengt h m ust be lim it ed t o assure t hat t he signals t hat it carries are int elligible. The size of a LAN can be increased by inst alling one or m ore repeat ers. A repeat er accept s zeros and ones from one cable segm ent and t ransm it s t hem ont o one or m ore ot her cable segm ent s at full st rengt h. A repeat er is a physical- layer device.

N ot e Hub and concent rat or are ot her nam es for a repeat er. These t erm s are used for repeat ers t hat connect t o m ore t han t wo segm ent s.

Figure 3.2 shows a coax LAN t hat has been ext ended across t hree segm ent s by adding a repeat er. A fram e sent by syst em A will be seen by every device on t he LAN. The fram e in Figure 3.2 is addressed t o syst em X.

Figu r e 3 .2 . Th r e e se gm e n t s con n e ct e d by a r e pe a t e r .

Each LAN segm ent is connect ed t o a port on t he repeat er. A port is a net work int erface. I n t he figure, each repeat er port connect s t o a segm ent via a short at t achm ent cable.

Twisted-Pair Cabling and Hub Repeaters Coaxial cable is heavy and st iff, and coax LANs are hard t o m aint ain. For years, t elephone com panies have used orderly cabling plans based on running unshielded t wist ed- pair cables from wiring closet s t o offices. Buildings are full of unshielded t wist ed- pair cabling. I n t he early 1980s, several vendors ( led by ATT) t hought t hat it would m ake good sense t o creat e a LAN t echnology t hat could follow t his t ype of cabling plan and operat e over unshielded t wist ed- pair t elephone cable. The result was St arLAN, a 1Mbps t wist ed- pair LAN. St arLAN was greet ed wit h som e init ial ent husiasm because a st ar t opology wit h cables radiat ing out from a wiring closet m ade good sense. Using t elephone cable was a great advant age because

buildings already were wired wit h t elephone cable. However, 1Mbps was j ust t oo slow. Event ually, engineers figured out how t o t ransm it 10Mbps ( and lat er, 100Mbps and 1000Mbps) across unshielded t wist ed- pair cabling. LAN cabling got a new look—and a new net work repeat er device—t he 10Mbps t wist ed- pair hub. The change t o t wist ed- pair cabling and a st ar t opology gave hubs a cent ral role. The hub in Figure 3.3 repeat s t he bit s in a fram e t ransm it t ed by st at ion A t o all t he ot her st at ions connect ed t o t he hub.

Figu r e 3 .3 . Pr opa ga t in g a fr a m e t o a ll st a t ion s.

An im port ant feat ure of t his t opology is t hat t here are only t wo nodes on each cable segm ent : a com put er and t he repeat er. There are four segm ent s in Figure 3.3, and each connect s a com put er t o t he hub.

N ot e All t hat t he t wist ed- pair hub in Figure 3.3does is t o repeat bit s from one segm ent ont o anot her. So, why isn't t he device called a t wist ed- pair repeat er? The answer is " m arket ing." Early repeat ers connect ed t wo or t hree coax segm ent s. Vendors want ed t o describe t heir product s using a word t hat indicat ed t hat t heir product s were new. The " hub" product s looked different and connect ed m any t wist ed- pair segm ent s.

Today, t he t erm " hub" som et im es is used for a chassis t hat cont ains repeat ers along wit h ot her net work devices. This is unfort unat e because it causes confusion and dam ages t he m eaning of a convenient piece of net working t erm inology. I n t his book, " hub" will be used t o denot e a repeat er device.

Improving Performance with Bridges The Et hernet specificat ions allow 1,024 st at ions t o be at t ached t o a LAN. But when Et hernet bandwidt h is a shared com m odit y, poor perform ance will m ake users groan and gnash t heir t eet h long before anywhere near t hat num ber of syst em s has been at t ached t o t heir LAN. A bridge is a Layer 2 device t hat t akes a lot of t he pain out of LAN growt h. The t hree LAN segm ent s in Figure 3.4 are connect ed by a bridge. Whenever possible, a bridge blocks fram es from reaching segm ent s t hat have no real need t o carry t hem .

Figu r e 3 .4 . Th r e e se gm e n t s con n e ct e d by a br idge .

For exam ple, if syst em A in Figure 3.4 want s t o send a fram e t o server C, t here is no reason t o t ransm it t hat fram e ont o segm ent 1 or segm ent 2. I f st at ion E want s t o send a fram e t o server H, t here is no reason t o t ransm it t hat fram e ont o segm ent 3. I n fact , during a period when users are sending fram es t o dest inat ions on t heir own segm ent s, t hree fram es could be in t ransit at t he sam e t im e. I n ot her words, for t hree 10Mbps segm ent s, t he available bandwidt h would be 30Mbps during t hat period. Bridges becam e popular because t hey cut down on t he t raffic t hat crosses each segm ent , result ing in m ore bandwidt h for each st at ion on a segm ent .

Collision Domains The LAN segm ent s in Figure 3.2 and Figure 3.3 are connect ed t o one anot her by repeat ers. For eit her of t hese LANs, a fram e t ransm it t ed by any st at ion on t he LAN will be seen by all st at ions on t he LAN. I f t wo st at ions send at t he sam e t im e, t heir fram es will collide. For t his reason, a set of segm ent s connect ed by repeat ers is called a collision dom ain.

N ot e At any given t im e, one st at ion ( at m ost ) in a collision dom ain can t ransm it a fram e successfully. I f one st at ion t alks, t he ot hers should be list ening. For t his reason, CSMA/ CD also is called half- duplex Et hernet t ransm ission.

The bridge in Figure 3.4 split s it s LAN int o t hree separat e collision dom ains. Fram es wit h local dest inat ions do not need t o be forwarded ont o ot her segm ent s. When a fram e m ust be forwarded, t he bridge behaves like a good cit izen of t he dest inat ion collision dom ain: I t list ens t o check whet her t he dest inat ion m edium is available before t ransm it t ing t he fram e. I f t he m edium is busy, t he bridge can hold t he fram e in buffer m em ory unt il t he m edium becom es quiet again. Adm inist rat ors appreciat ed t he fact t hat a bridge could be inst alled by hooking up t he cables and plugging in t he power. A bridge eavesdrops on t raffic originat ing on each segm ent t o discover t he MAC addresses of t he st at ions on t he segm ent . The bridge creat es a t able t hat m aps each MAC address t o t he port t hrough which it is reached. When a MAC address is list ed in t he bridge t able, t he bridge will be capable of forwarding fram es addressed t o t hat MAC address ont o t he correct segm ent . Chapt ers 12- 17 cont ain a full descript ion of what bridges do and how t hey do it .

Leaping to Higher Performance with Switches I n 1993, a com pany nam ed Kalpana int roduced LAN swit ches. This was an ast onishing event , st art ling m any LAN expert s who wondered why t hey had not t hought of it first . A swit ch is a m ult iport bridge t hat can forward several fram es at t he sam e t im e.

Event ually, LAN swit ches evolved int o t he popular Layer 2 swit ches in use t oday. Figure 3.5shows a set of st at ions connect ed t o a Layer 2 swit ch. Twist ed- pair or fiber opt ic cable are t he m edia norm ally used wit h a Layer 2 swit ch.

Figur e 3 .5 . A La ye r 2 sw it ch.

I n Figure 3.5, each link connect ing a st at ion t o a swit ch is a separat e segm ent t hat is bridged t o all ot her segm ent s. The int ernal archit ect ure of t he swit ch allows m any fram es t o be in t ransit at t he sam e t im e, which great ly increases t he LAN bandwidt h.

N ot e Just as " t wist ed- pair hub" is an up- m arket nam e for a m odern repeat er, " Layer 2 swit ch" is an up- m arket nam e for a m odern bridge. A Layer 2 swit ch can have num erous port s and uses bet t er hardware t echnology t han t he bridges of long ago, but funct ionally, it is a bridge. Som e m odern swit ches have been loaded wit h so m any ext ra feat ures and opt ions t hat t he sim ple plug- and- play inst allat ion of earlier t im es has been lost . Fort unat ely, t here also are sim ple workgroup swit ches t hat st ill are a breeze t o inst all.

Switches and Full-Duplex Operation The appeal of m ult iport swit ches was enhanced by an addit ion t o t he 802.3 st andard t hat gave perform ance a big boost . As shown in Figure 3.6, when a single st at ion is

connect ed t o a swit ch port , t he link bet ween t he st at ion and t he swit ch can be used for full- duplex com m unicat ion. Bot h syst em s can t ransm it and receive at t he sam e t im e. For exam ple, across a 100Mbps link, t he st at ion can t ransm it fram es t o t he swit ch at 100Mbps and receive fram es from t he swit ch at 100Mbps.

Figu r e 3 .6 . Fu ll- duple x com m u n ica t ion be t w e e n a st a t ion a n d a sw it ch .

The full- duplex Et hernet MAC prot ocol is very sim ple: Eit her part y can send a fram e whenever it pleases. CSMA/ CD is not needed when full- duplex com m unicat ion is used.

N ot e Full- duplex com m unicat ion can be used bet ween any syst em s t hat are not repeat ers. For exam ple, a full- duplex link can be set up bet ween t wo host s, t wo swit ches ( bridges) , a swit ch and a rout er, or a pair of rout ers.

Sharing the LAN LANs are dat a com m unicat ions workhorses. A LAN fram e can carry t raffic t hat belongs t o any t ype of higher- layer prot ocol. I t is not at all unusual t o see a m ixt ure of prot ocols such as TCP/ I P, Net Ware I PX/ SPX, DECnet , and AppleTalk happily sharing a LAN. Figure 3.7shows how higher- layer prot ocols ride on t op of Layer 1 and Layer 2 LAN prot ocols. Many host s send and receive t raffic for several different prot ocols t hrough a single LAN adapt er.

Figu r e 3 .7 . H igh e r - la ye r pr ot ocols sh a r in g a LAN .

The Role of Routers I t did not t ake long before users want ed t o exchange inform at ion wit h servers locat ed on ext ernal LANs dot t ed across an ent erprise—and, wit h t he advent of t he I nt ernet , wit h servers locat ed around t he world. Rout ers m ake t his com m unicat ion possible. Figure 3.8 shows t wo LANs connect ed by a rout er. The rout er also connect s t hese LANs t o a long- dist ance line. This could lead t o anot her sit e wit hin a com pany or t o t he I nt ernet .

Figu r e 3 .8 . I n t e r con n e ct in g t w o LAN s a n d a W AN lin k .

A rout er is a Layer 3 device. Rout ers have lot s of good feat ures. To m ent ion j ust a few: • • • • •

They can connect different t ypes of LANs gracefully. Unlike a bridge, a rout er does not forward LAN broadcast t raffic or local LAN m ult icast s. A sizeable am ount of bandwidt h can be saved. This is part icularly im port ant when t he peak load on one of t he LANs is close t o t he LANs capacit y. They can connect a set of LANs t o a set of WAN links of different t ypes—for exam ple, dial- up, leased line, fram e relay, or Asynchronous Transfer Mode ( ATM) . They can perform securit y screening and keep risky t raffic off a LAN. An im port ant t hing t o keep in m ind is t hat when dat a from a LAN reaches a rout er, it has passed t hrough a doorway and left t he LAN. The rout er st rips off t he MAC fram e header and t railer. The prot ocol dat a will be encapsulat ed in a new header and t railer before it is forwarded.

Summary Points •

Alt hough LANs originally were t ruly local, t oday a LAN can span a cam pus or can include m ult iple sit es.

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A LAN operat es at t he physical and dat a link layers. Cables, com m unicat ing com put ers, repeat ers/ hubs, and bridges/ swit ches all are LAN com ponent s. An Et hernet collision dom ain is m ade up of end syst em s, cable segm ent s, repeat ers, and hubs. Only one fram e can be in t ransit across a collision dom ain at any given t im e. The Et hernet Carrier Sense Mult iple Access wit h Collision Det ect ion ( CSMA/ CD) m edia access prot ocol is used on a collision dom ain. CSMA/ CD also is called half- duplex Et hernet com m unicat ion. Bridges and swit ches increase a LAN's t ransm ission capacit y by breaking an Et hernet LAN int o m ult iple collision dom ains. The change from coaxial t o t wist ed- pair cable and from a bus t o a st ar t opology was an im port ant st ep in t he evolut ion of Et hernet . When a st at ion is direct ly connect ed t o a swit ch, it can com m unicat e in fullduplex m ode and can t ransm it a fram e whenever it wishes. A single LAN can carry t raffic for m any different higher- layer prot ocols. A rout er can connect several LANs t o one anot her and t o one or m ore wide area circuit s.

References The Universit y of New Ham pshire's I nt erOperabilit y Lab ( I OL) t est s net working product s. I t provides a hom e for m any vendor consort ium s. Each consort ium is devot ed t o a single t echnology. The t echnologies include 100Mbps and Gigabit Et hernet , 100Mbps Token Ring, FDDI , Fibre Channel, 100VG- ANYLAN, virt ual LANs ( VLANs) , bridging, and m ore. Each consort ium m em ber provides plat form s represent ing it s equipm ent and support s t he lab's t est ing act ivit ies. The lab publishes free t ut orials and insight ful whit e papers. These are available at ht t p: / / www.iol.unh.edu/ consort ium s/ fe/ index.ht m l The hist oric publicat ion t hat described DI X Et hernet was " The Et hernet —A Local Area Net work: Dat a Link Layer and Physical Layer Specificat ions." This docum ent was published by Digit al, I nt el, and Xerox in Novem ber 1982. I EEE St andard 802- 1990, " Overview and Archit ect ure," int roduces t he form al I EEE LAN t erm inology and archit ect ure. An updat ed version of t his docum ent is in draft form at t he t im e of writ ing.

Chapter 4. The Classical Ethernet CSMA/CD MAC Protocol The Et hernet m edia access cont rol ( MAC) prot ocol is ext rem ely resilient . I t has been adapt ed t o a wide range of speeds, m edia, and changing LAN t opologies. The current version of t he 802.3 st andard ( defined in t he 1998 edit ion) can operat e at 1Mbps, 10Mbps, 100Mbps, and 1000Mbps.

N ot e The 1Mbps St arLAN version of Et hernet int roduced in t he early 1980s no longer is used and cert ainly does not qualify as a high- speed LAN t echnology. I t will not be discussed furt her in t his book.

Et hernet m edia include various grades of coaxial cable, t wist ed- pair wire, and opt ical fiber. On t wist ed- pair or opt ical fiber, an Et hernet st at ion connect ed t o a swit ch can operat e in full- duplex m ode, doubling t he pot ent ial t hroughput . Rules and param et ers laid down for t he original Et hernet net works have been carried t hrough all t he versions. This has support ed an ast onishing degree of backward com pat ibilit y. A LAN adm inist rat or can leave st able LAN segm ent s t hat work well for t heir users undist urbed, and updat e or add segm ent s based on newer t echnologies when higher perform ance is needed. This chapt er describes t he classical CSMA/ CD half- duplex Et hernet prot ocols. Fullduplex Et hernet is described in Chapt er 5, " Full- Duplex Et hernet Com m unicat ion."

Classic Ethernet Shared Bandwidth LANs Com m unicat ion on a classic Et hernet LAN is half–duplex. A classic LAN consist s of a set of st at ions t hat share a fixed am ount of bandwidt h. Only one fram e can be in t ransit at any given t im e. The first physical m edium t hat was used for Et hernet was t hick 50- ohm coaxial cable, as is illust rat ed in t he t op half of Figure 4.1. Lat er, Et hernet LANs were const ruct ed by connect ing set s of st at ions t o hubs via t wist ed- pair or fiber opt ic cables, as is shown in t he bot t om half of Figure 4.1.

Figu r e 4 .1 . Et h e r n e t st a t ion s sh a r in g a m e diu m .

A st at ion sends inform at ion t o anot her st at ion by wrapping t he inform at ion in a MAC fram e. A MAC fram e is t ransm it t ed as a serial st ream of bit s.

Preamble and Interframe Gap Every fram e is int roduced by a special pat t ern of 1s and 0s called a pream ble. Figure 4.2 illust rat es a series of fram e t ransm issions. The gray areas in t he figure correspond t o fram e pream bles. Fram e t ransm issions m ust be separat ed by a t im e int erval called t he int erfram e gap or int erpacket gap. The t im e period bet ween fram es can vary, but each period m ust be at least as long as t he int erfram e gap.

Figu r e 4 .2 . Fr a m e pr e a m ble s a n d in t e r fr a m e ga p spa cin g.

The int erfram e gap period corresponds t o 96 bit t im es ( t hat is, 12 byt e t im es) . Thus, t he act ual t im e value is different for each speed level. I t is: 96.000 m icroseconds ( = s) for 1Mbps 9.600= s for 10Mbps 0.960= s for 100Mbps 0.096= s for 1000Mbps

I n t e r fr a m e Ga p Sh r in k a ge The size of t he int erfram e gap bet ween fram es can shrink as t he fram es pass t hrough a repeat er. Two fact ors cause t his t o happen: •

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Variat ions in net work delay can cause irregularit ies in t he t im es at which fram es arrive at a repeat er. A repeat er m ust lock on t o an incom ing pream ble signal before it can st art t o ret ransm it t he out going pream ble and fram e bit s. The t im e t hat it t akes t o lock on t o t he signal can vary from fram e t o fram e.

An analogy m ight help t o explain why t he gap som et im es shrinks. Suppose t hat each m inut e t he doorm an at a t heat er allows one cust om er t o ent er t he lobby t o buy a t icket , and norm ally a cust om er com plet es t he t ransact ion wit hin 50 seconds. However, cust om er X walks so slowly t hat he uses up som e of his t im e on t he way t o t he t icket window. Also, t he t icket seller has t rouble underst anding what cust om er X want s ( t hat is, t he seller has t rouble " locking on t o t he signal" ) . The result is t hat t he t ransact ion t akes 54 seconds. The next cust om er arrives at t he t icket window punct ually 6 seconds lat er—t he norm al 10- second int ercust om er gap has shrunk t o 6 seconds.

N ot e 802.3 lim it s t he am ount of int erfram e gap shrinkage t hat is allowed. For exam ple, t he st andard decrees t hat t he int erfram e gap m ust not decrease below 47 bit t im es for 10Mbps Et hernet , or below 64 bit t im es for Gigabit Et hernet . I nt erfram e gap shrinkage is one of t he fact ors t hat lim it s t he num ber of repeat ers in a collision dom ain—and, hence, t he diam et er of a collision dom ain.

The CSMA/CD Protocol St at ions share a m edium by adhering t o a very sim ple m edia access cont rol ( MAC) prot ocol called Carrier Sense Mult iple Access wit h Collision Det ect ion ( CSMA/ CD) . CSMA/ CD is based on a few com m on sense rules: • • • •

Ca r r ie r Se n se —A st at ion list ens t o t he m edium all t he t im e. The st at ion can t ransm it a fram e aft er t he m edium has been quiet for a period at least as long as t he int erfram e gap. Collision D e t e ct ion—Two st at ions m ight st art t o send at roughly t he sam e t im e. While sending, a st at ion cont inues t o list en, checking whet her t here is a concurrent t ransm ission t hat causes it s own signal t o be garbled. Ja m m in g—Aft er det ect ing t hat a collision has occurred, a sender m ust cont inue t o t ransm it bit s t o assure t hat all st at ions will be able t o det ect t he collision. The addit ional num ber of bit s is called t he j am size, and it is equal t o 32 bit s ( 4 byt es) . W a it ing—A st at ion t hat has part icipat ed in a collision m ust rem ain silent for a random am ount of t im e before at t em pt ing t o t ransm it again.

N ot e Alm ost 10 Mbps t hroughput can be at t ained on a CSMA/ CD LAN when only one st at ion t ries t o t ransm it . However, collisions can reduce t he t hroughput across a busy LAN t o 3 - 4 m egabit s per second.

Random Backoff A st at ion t hat has j ust experienced a collision knows t hat t here is at least one ot her syst em on t he LAN t hat also has a fram e t o t ransm it . The st at ion needs t o behave in a way t hat reduces t he likelihood t hat it s next at t em pt t o send t he fram e will cause anot her collision. The basic idea is t o wait a random t im e before resending. I f t he ret ransm it t ed fram e also experiences a collision, t he delay period needs t o be st ret ched out . Specifically, t he delay period is a m ult iple of a param et er called t he slot t im e.

N ot e For 10Mbps and 100Mbps, t he slot t im e is equal t o 512 bit t im es ( t hat is, 64 byt e t im es, t he t im e required t o send a fram e of t he sm allest legal size) . For Gigabit Et hernet , t he slot t im e is 4096 bit t im es ( t hat is, 512 byt e t im es) .

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Aft er one collision, t he st at ion random ly " flips a coin" t o choose whet her t o send im m ediat ely or wait one slot t im e.

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I f a second collision result s, t he st at ion random ly picks one of t he four int egers from 0 t o 3 and ret ransm its aft er t hat num ber of slot t im es. I f a t hird collision occurs, t he num ber of values doubles t o t he range 0 t o 7 slot t im es.

The doubling cont inues for up t o 10 successive collisions and t hen levels off at 1,023 slot t im es. A st at ion ret ries 16 t im es before giving up. St at ed in m at hem at ical t erm s: • • • •

For t he nt h t ry, where n 10, choose a random num ber r wit h: 0 r < 2n Ret ransm it t he fram e aft er wait ing for t he period: r x ( slot t im e)

The form al nam e for t his procedure is t runcat ed binary exponent ial backoff. random backoff I t oft en is referred t o less form ally as random backoff. The num ber of doublings ( 10) is called t he backoff lim it . The t ot al num ber of t ries ( 16) is called t he at t em pt lim it . The use of 1,023 slot t im es m ight sound like a big int erval, but consider t he following: • • •

At 10Mbps, it is equal t o .052 seconds. At 100Mbps, it is equal t o .0052 seconds. At 1000Mbps, which has a bigger slot t im e, t he int erval is .0042 seconds.

N ot e The syst em ( say St at ion A) t hat m anages t o send t he first fram e aft er a collision oft en get s a big advant age. I f m ult iple st at ions have fram es queued up, t his fram e will be followed by anot her collision. But t his is collision 1 for lucky St at ion A and collision 2 ( wit h a longer backoff) for t he ot hers. The odds are t hat St at ion A will get t he opport unit y t o send t he next fram e t oo. This will be followed by collision 3 for t he ot hers, and t hey will back off even longer. Lucky St at ion A oft en get s t o send a long st ring of fram es while ot her st at ions wait . This is called t he capt ure effect .

Ethernet MAC Frames As shown in Figure 4.3, an Et hernet MAC fram e st art s wit h a header t hat includes it s dest inat ion and source MAC addresses. I t ends in a fram e check sequence ( FCS) field t hat cont ains a CRC code t hat is used t o det ect t ransm ission errors.

Figu r e 4 .3 . Ge n e r a l for m a t of a n Et h e r n e t fr a m e .

Preamble and Start Frame Delimiter Patterns A st at ion t ransm it s t wo special bit pat t erns—a pream ble and a st art fram e delim it er—prior t o sending a MAC fram e. Figure 4.4 shows a MAC fram e preceded by it s pream ble and st art fram e delim it er.

Figu r e 4 .4 . Pr e a m ble , st a r t fr a m e de lim it e r , a n d M AC fr a m e .

The pream ble pat t ern t ells ot her st at ions t hat a fram e is on t he way and enables t he physical layers in t he ot her st at ions t o synchronize t heir bit t im ing wit h t he sender's clock by locking on t o t he signal. The pream ble consist s of 7 byt es of alt ernat ing 1s and 0s: 10101010 10101010 10101010 10101010 10101010 10101010 10101010 The special st art fram e delim it er ( SFD) byt e t hat follows t he 802.3 pream ble announces t hat t he MAC fram e follows im m ediat ely. The SFD byt e consist s of t he pat t ern: 10101011

N ot e For som e speeds and physical m edia, t he fram e int roducer is som ewhat different . The det ails are in Chapt er 6, " The Et hernet 10Mbps Physical Layer," Chapt er 7, "The Et hernet 100Mbps Physical Layer," Chapt er 8, " Gigabit Et hernet Archit ect ure," and Chapt er 9, " Gigabit Et hernet Physical Layer."

Ethernet MAC Frame Size Lim it s on t he m inim um and m axim um fram e size were est ablished back when Et hernet was developed. • •

The m inim um MAC fram e size was set t o 64 byt es. Som et im es t he inform at ion field in a sm all fram e m ust be padded t o increase t he t ot al lengt h of t he fram e t o 64 byt es. The m axim um fram e lengt h was set t o 1518 byt es.

N ot e A fram e t hat is t oo short is called a runt . Most runt s are fram es t hat have been chopped off by a collision. Som e result from a burst of t ransm ission errors t hat disrupt s t he fram e.

Relationship Between Frame Size and Collision Domain Size The original 50- ohm coax Et hernet LANs were allowed t o have a m axim um diam et er of 2500 m et ers. The m inim um MAC fram e size was set t o 64 byt es ( 512 bit s) t o assure t hat every st at ion would be capable of sensing a collision across a 2500m et er, 10Mbps Et hernet . Figure 4.5 illust rat es why t he m axim um LAN diam et er depends on t he m inim um fram e size.

Figu r e 4 .5 . M issin g collision s w h e n t h e r ou n d- t r ip t im e is t oo long.

I n Figure 4.5, t he round- t rip t im e bet ween DTE A and DTE B is assum ed t o be 514 bit t im es. This m eans t hat it will t ake 257 bit t im es for a bit t ransm it t ed by DTE A t o reach DTE B. All t im es in t he figure are m easured in bit t im es. The series of event s is as follows: 1. At t im e T= 0, DTE A begins t o t ransm it a 512- bit fram e. 2. At t im e T= 256, DTE B believes t hat t he m edium is free and st art s t o t ransm it a fram e. 3. At t im e T= 257, bit s from DTE A and DTE B have collided. DTE B sends a j am signal. 4. At t im e T= 512, DTE A com plet es it s t ransm ission. I t has not yet heard t he collision or j am signal. DTE A and ot her st at ions t hat are near it believe t hat t he fram e has been sent successfully. DTE A will not back off and t ry again. Com m unicat ion get s com plet ely m uddled when collisions cannot be sensed by every st at ion in a collision dom ain.

MAC Frame Formats The left side of Figure 4.6 shows t he form at of t he original DI X Et hernet fram e. The right side shows t he form at of t he current 802.3 Et hernet fram e. The byt es in an

Et hernet fram e are t ransm it t ed t op t o bot t om and left t o right . The least significant bit of each byt e is t ransm it t ed first .

Figu r e 4 .6 . Et h e r n e t D I X a n d 8 0 2 .3 fr a m e s.

The fram e header ( dest inat ion address, source address, and t ype or lengt h) occupies 14 byt es, and t he FCS t railer occupies 4 byt es. Because t he t ot al fram e lengt h is lim it ed t o 1518 byt es, t he m axim um size of t he inform at ion field is 1500 byt es.

D e st in a t ion a n d Sou r ce Addr e sse s The first t wo fields of a fram e hold 6- byt e dest inat ion and source MAC addresses. MAC addresses were described in Chapt er 2, " LAN MAC Addresses." The source address is t he unicast address of t he st at ion t hat sent t he fram e. The dest inat ion address m ay be a unicast , m ult icast , or broadcast address.

Type or Le n gt h The DI X MAC fram e header and t he original 802.3 Et hernet MAC fram e header were close t o ident ical. They differed only in t he use of t he 2- byt e field t hat follows t he source MAC address. •

•

N ot e

For DI X, t his field cont ains a num ber t hat ident ifies t he t ype of prot ocol unit t hat is carried in t he inform at ion field. For t he original version of 802.3, t his field had t o carry t he lengt h of t he inform at ion field.

I t is im port ant for a fram e t o ident ify t he t ype of prot ocol inform at ion t hat it carries. When t he t hird field in an Et hernet fram e cont ains t he lengt h of t he inform at ion field, a prot ocol t ype ident ifier m ust be placed int o anot her header locat ed at t he st art of t he inform at ion field.

I n spit e of t he fact t hat t he 802 com m it t ee had not blessed t he use of t he DI X " t ype" field, users refused t o give it up. I t worked j ust fine, and DI X fram es did not have t o sacrifice part of t he inform at ion field t o ident ify t he prot ocol being carried. DI X had fewer overhead byt es. Finally, t he use of eit her a t ype or a lengt h value in t his 2- byt e posit ion was absorbed int o t he 802.3 st andard, and t he DI X form at becam e part of t he official st andard.

Et he r ne t Type s An Et hernet fram e is allowed t o carry any t ype of prot ocol dat a, and m ult iple prot ocols can share a LAN cable. I n fact , it is not unusual for a single host t o com m unicat e via several different prot ocols. When a syst em receives a fram e, it needs t o det erm ine what prot ocol is enclosed so t hat it can pass t he dat a field t o t he appropriat e processing m odule. The Et hernet t ype ( also called t he Et herType) ident ifies t he prot ocol. A few not able Et hernet t ypes are list ed in See Table 4.1

N ot e All t he Et hernet t ype values current ly in use have decim al values well above 1500, which is t he size of t he largest Et hernet inform at ion field. Thus, a NI C easily can det erm ine whet her t he t ype/ lengt h field in an incom ing fram e cont ains a t ype or a lengt h value. The hexadecim al represent at ion of 1500 is X'05- DC.

Ta ble 4 .1 . Et h e r ne t Type s H e x Va lu e

D e cim a l Va lu e

Con t e n t of t h e D a t a Fie ld

08- 00

2048

I P dat agram

08- 06

2054

Address Resolut ion Prot ocol ( ARP) m essage

0B- AD

2989

Banyan VI NES

80- 9B

32923

AppleTalk dat a unit s

80- D5

32981

I BM SNA services over Et hernet

81- 37, 81- 38

33079, 33080

Net Ware dat a unit s

86- DD

34525

I P Version 6 dat agram

Ta ble 4 .1 . Et h e r ne t Type s H e x Va lu e

D e cim a l Va lu e

Con t e n t of t h e D a t a Fie ld

60- 03

24579

DECnet Phase I V rout ing inform at ion

60- 04

24580

DEC LAT

N ot e When DI X Et hernet got st art ed, Xerox Corporat ion act ed as an Et hernet t ype regist rat ion aut horit y. Today, t he I EEE act s as t he regist rat ion aut horit y for com panies t hat want t o obt ain new Et hernet t ype num bers. A list of assigned t ype num bers current ly is online at ht t p: / / st andards.ieee.org/ regaut h/ et hert ype/ t ypepub.ht m l. However, t he I EEE list s only nam es of com panies and t he t ype num bers t hat t he com panies have been grant ed. I t does not ident ify t he prot ocols t hat are associat ed wit h t hese t ype num bers. A couple of organizat ions t ry t o keep t rack of t he current st at us of prot ocols and t heir Et hernet t ypes unofficially, using unverified inform at ion cont ribut ed by volunt eers. At t he t im e of writ ing, list s can be found at ht t p: / / www.iana.org/ num bers.ht m l ( Choose Et hernet Num bers) and ht t p: / / www.cavebear.com / CaveBear/ Et hernet / ( Choose Type Codes) .

Size Modification for Gigabit Ethernet The original Et hernet fram e size const raint s st ill hold for t he bulk of t he t raffic t ransm it t ed t oday. Som e ext ra byt es are added when VLANs are used. VLAN fram e form at s are described in Chapt er 16, " VLANs and Fram e Priorit y." The m inim um size of a t ransm ission had t o be changed for half- duplex Gigabit Et hernet .

W a r n in g At t he t im e of writ ing, all Gigabit Et hernet im plem ent at ions are full- duplex, and t his probably will cont inue t o be t rue in t he fut ure. This m eans t hat it is very likely t hat t he discussion t hat follows is st rict ly academ ic. However, t he discussion does shed light on t he reasons t hat half- duplex CSMA/ CD Gigabit Et hernet has not been im plem ent ed.

A fram e lengt h of 64 byt es is far t oo sm all t o assure t hat collisions can be det ect ed on a Gigabit Et hernet CSMA/ CD LAN. At gigabit speed, 512 bit t im es is .000000512 seconds. The est im at ed t im e required j ust t o get t hrough t he source and dest inat ion Gigabit Et hernet DTEs is 864 bit t im es!

Gigabit Et hernet runs across t wist ed- pair and fiber opt ic m edia. Buildings rout inely are wired wit h 100- m et er cable runs. The diam et er of a Gigabit Et hernet collision dom ain cont aining a single hub can be expect ed t o be 200 m et ers. I t t akes m ore t han 2000 bit t im es t o cross 200 m et ers of t wist ed- pair or opt ical cable at Gbps speed. A sender m ust cont inue t o t ransm it for at least t he am ount of t im e t hat it t akes t he first byt e t o t ravel from it s source t o it s dest inat ion. To m ake t his happen at gigabit speed, t he m inim um size of a t ransm ission m ust be increased. I n fact , t he m inim um lengt h of a half- duplex Gbps t ransm ission has been set t o 512 byt es ( 4096 bit s) . This allows m ore t han enough t im e for a byt e t o leave t he source DTE, cross 100 m et ers of cable, pass t hrough a hub, cross anot her 100 m et ers of cable, and ent er t he dest inat ion DTE. Backward com pat ibilit y is very im port ant in t he Et hernet world, and t he half- duplex gigabit problem was solved in a way t hat preserves backward com pat ibilit y. The m inim um fram e size st ill is 64 byt es, but ext ra carrier ext ension byt es are added at t he end of a sm all fram e t o assure t hat t he t ransm ission reaches t he 512- byt e level. Figure 4.7 shows t he form at of a Gigabit Et hernet fram e wit h appended ext ension byt es. Not e t hat t he fram e st ill m ust be at least 64 byt es in lengt h, so if a very sm all am ount of dat a is sent , t he t ransm ission m ight include bot h pad byt es and ext ension byt es.

Figu r e 4 .7 . A giga bit fr a m e w it h a ca r r ie r e x t e n sion .

Having t o add up t o 448 garbage ext ension byt es t o short fram es is very wast eful. To m it igat e t his, Gigabit Et hernet fram es can be t ied t oget her like freight cars int o a long CSMA/ CD t ransm ission, raising t he efficiency. This is called fram e burst ing.

N ot e As was not ed earlier, current ly all Gigabit Et hernet t raffic is sent in full- duplex m ode. The CSMA/ CD rules do not apply t o full–duplex operat ion, and an ext ension does not have t o be added. St andard full- duplex Gigabit Et hernet is com plet ely backward com pat ible wit h classical Et hernet .

However, som e vendors support a nonst andard change t o t he m axim um size of a Gigabit Et hernet fram e. A Jum bo MAC fram e has a size of up t o 9018 byt es. The det ails are explained in Chapt er 8.

DIX Ethernet Frame Trace List ing 4.1 shows a Net work Associat es Sniffer t race of an Address Resolut ion Prot ocol ( ARP) m essage t hat was t ransm it t ed at 10Mbps. Not e t he following: • • • •

The dest inat ion is t he broadcast MAC address. The source has MAC address X'08- 00- 14- 20- 19- 82. The OUI X'08- 00- 14 belongs t o Excelan, and t he Sniffer has replaced t he first 3 byt es of t he source MAC address wit h " Exceln." The Et herType for ARP m essages is X'08- 06. An ARP m essage is short . I t is m ade up of only 28 byt es. The MAC header and fram e check sequence cont ribut e anot her 18 byt es, so t he fram e size is 46 byt es, which is 18 byt es short of t he 64- byt e m inim um . Hence, 18 pad byt es have been insert ed aft er t he ARP dat a. The fram e check sequence ( which is not shown in t he t race) follows t he pad dat a and is com put ed against all 60 of t he previous 60 byt es.

List in g 4 .1 An Et h e r n e t Fr a m e Con t a in in g a n ARP M e ssa ge DLC: Frame 1 arrived at 12:09:34.0000; frame size is 60 (003C hex) bytes. DLC: Destination = BROADCAST FFFFFFFFFFFF, Broadcast DLC: Source = Station Exceln201982 DLC: Ethertype = 0806 (ARP) DLC: ARP: ----- ARP/RARP frame ----ARP: (28 byte ARP message) ARP: ARP: 18 bytes frame padding ARP: HEX ff ff ff ff ff ff 08 00 14 20 19 82 08 06 00 01 08 00 06 04 00 01 08 00 14 20 19 82 81 54 19 02 00 00 00 00 00 00 81 54 19 fe 01 01 00 00 26 3d ea d9 00 00 00 00 6b 69 6c 6c 6a 6f

802.3 LLC/SNAP Frames Ext ra headers are needed t o ident ify t he t ype of prot ocol dat a t hat is enclosed in an Et hernet fram e t hat conform s t o t he original 802.3 specificat ion and has a lengt h field. Figure 4.8 shows t he com m on form at t hat is used t o ident ify a prot ocol t hat has an assigned Et herType code.

Figu r e 4 .8 . For m a t of a n 8 0 2 .3 fr a m e ca r r yin g pr ot ocol da t a t h a t h a s a n a ssigne d Et h e r n e t t ype .

The lengt h field is followed by a 3- byt e Logical Link Cont rol ( LLC) header and a 5byt e Subnet work Access Prot ocol ( SNAP) header. The value in t he LLC field is X'AAAA- 03. The SNAP header consist s of X'00- 00- 00 followed by t he Et hernet t ype code of t he enclosed prot ocol inform at ion. The inform at ion field has been cut back t o 1492 payload byt es because of t he 8 ext ra header byt es. The byt es t hat int roduce t he Et hernet t ype ( X'AA- AA- 03- 00- 00- 00) can be t reat ed like a fixed boilerplat e, but t he sect ion " Source of t he LLC and SNAP Headers," lat er in t his chapt er, has m ore inform at ion if you are curious about where t hese headers cam e from and what t hey m ean.

802.3 LLC/SNAP Frame Trace List ing 4.2 shows part of a Sniffer t race of a Net Ware 802.3 fram e t hat includes LLC and SNAP headers ident ifying t he enclosed prot ocol inform at ion as Novell Net Ware dat a whose Et hernet t ype is X'81- 37.

List in g 4 .2 For m a t of a n 8 0 2 .3 Fr a m e Ca r r yin g a N e t W a r e Pr ot ocol D a t a Un it DLC: Frame 12 arrived at 15:36:18.6314; frame size is 118 (0076 hex) bytes. DLC: Destination = Station WstDigD99D41 DLC: Source = Station Intrln02D520 DLC: 802.3 length = 104 DLC: LLC: ----- LLC Header ----LLC: LLC: DSAP Address = AA, DSAP IG Bit = 00 (Individual Address) LLC: SSAP Address = AA, SSAP CR Bit = 00 (Command) LLC: Unnumbered frame: UI LLC: SNAP: ----- SNAP Header ----SNAP: (00 00 00) SNAP: Type = 8137 (Novell) SNAP: IPX: ----- IPX Header ----IPX: IPX: Checksum = 0xFFFF IPX: Length = 96 . . . HEX 00 00 c0 d9 9d 41 02 07 01 02 d5 20 00 68 03 00 00 00 81 37 ff ff 00 60 . . .

Source of the LLC and SNAP Headers The ext ra LLC and SNAP headers in Figure 4.8and List ing 4.2 result from t he work of t he I EEE 802.2 com m it t ee. This com m it t ee split t he dat a link layer int o t wo sublayers: a Logical Link Cont rol ( LLC) LLC sublayer and a MAC sublayer, as shown in Figure 4.9

Figu r e 4 .9 . Su bla ye r s of t he da t a lin k la ye r .

The Logical Link Cont rol ( LLC) sublayer was designed t o serve t wo purposes: • •

Provide a consist ent int erface bet ween t he net work layer and t he dat a link layer t hat is independent of t he underlying m ode of com m unicat ion, whet her it is Et hernet , Token Ring, FDDI , or a wide area circuit . Provide t hree different t ypes of dat a link service.

The t hree service t ypes are •

•

•

Type 1 —Handles t ransm ission of individual PDUs. No ext ra funct ions are added t o t he dat a link layer. Type 2 —Provides t he capabilit y t o set up a reliable dat a link connect ion wit h a part ner. Dat a sent across t he connect ion is num bered, and incom ing inform at ion PDUs m ust be acknowledged. Aft er a t im eout , unacknowledged inform at ion PDUs are ret ransm it t ed. Type 2 also support s flow cont rol. The Type 2 prot ocol closely resem bles X.25 LAPB. Type 3 —Support s sim ple com m and/ acknowledge int eract ions. The arrival of a com m and PDU requires t he recipient t o send an acknowledgm ent PDU. Aft er a t im eout , a com m and PDU t hat has not been acknowledged is ret ransm it t ed.

Type 1 is t he predom inant dat a link com m unicat ion in use t oday. I BMSNA m akes use of Type 2 connect ions. The aut hor is unaware of any use of Type 3 com m unicat ion.

LSAPs A single com put er can engage in m ult iple concurrent dat a link com m unicat ions. The 802.2 com m it t ee int roduced dat a link layer addresses—called link LSAPs service

access point ( LSAP) addresses—int ended t o help a com put er t o keep t rack of different com m unicat ions flows. LSAPs appear in LLC headers. Figure 4.10 shows t he LLC header form at .

Figu r e 4 .1 0 . For m a t of t he LLC h e a de r .

•

• •

The first field of t he header cont ains t he dest inat ion service access point ( DSAP) num ber. The second field cont ains t he source service access point ( SSAP) num ber. The t hird field cont ains cont rol inform at ion. For Type 1 com m unicat ion, t his is a 1- byt e value equal t o X'03, which m eans unnum bered inform at ion. For Type 2 com m unicat ion, t he cont rol field ident ifies t he t ype of m essage ( for exam ple, inform at ion or flow cont rol) . I t also carries t he num bers used t o sequence and acknowledge dat a.

Set t ing t he DSAP and SSAP equal t o X'AA m eans t hat a SNAP header follows.

SN AP H e a de r s The general form at of a SNAP header is displayed on t he left side of Figure 4.11 The first 3 byt es cont ain an organizat ionally unique ident ifier ( OUI ) assigned by t he I EEE. The rem ainder of t he SNAP header is designed by t hat owning organizat ion.

Figu r e 4 .1 1 . SN AP h e a de r for m a t .

The I EEE assigned t he OUI X'00- 00- 00 t o Xerox back in t he early days of Et hernet . The right side of Figure 4.11 shows t he corresponding SNAP header, which carries a 2- byt e Et herType code.

NetWare over Ethernet Packaging dat a int o a fram e t hat has eit her an Et herType code or a lengt h value, LLC, and SNAP header is a pret t y sim ple j ob. Unfort unat ely, Novell generat ed a fair am ount of confusion when it m ade up t wo m ore ways of packaging Net Ware I PX prot ocol dat a unit s: it s 802.3 " raw" and 802.2 encapsulat ions. Novell ended up wrapping it s dat a int o fram es four different ways: t he t wo st andard form at s, plus t wo t hat it invent ed. Having four different fram e form at s m akes Net Ware t he encapsulat ion king. List ing 4.3 shows a part of a t race of an 802.3 raw fram e cont aining a Net Ware I PX packet . Raw fram es were used in t he earliest days of t he Net Ware product . Not e t hat som et hing is m issing—t here are no 802.2 LLC and SNAP headers t hat ident ify t he prot ocol being carried. Because an 802.2 header really is needed in any 802.3 fram e t hat has a lengt h field, t he packaging in List ing 4.3 is propriet ary. Breaking t he rules seem ed a good idea at t he t im e. LANs were sm all and isolat ed. I f you had Net Ware, why would you ever want t o run any ot her prot ocol on a LAN? And if t here is only one prot ocol, why include a prot ocol ident ifier t hat j ust adds overhead t o each packet ?

List in g 4 .3 A N e t W a r e 8 0 2 .3 " Ra w " Fr a m e Network Associates Sniffer Trace DLC:

IPX:

----- DLC Header ----DLC: DLC: Frame 55 arrived at 17:30:18.8315; frame size is 440 (01B8 hex) bytes. DLC: Destination = Station Accton004720 DLC: Source = Station WstDigD6682B DLC: 802.3 length = 426 DLC: ----- IPX Header ----IPX: IPX: Checksum = 0xFFFF

I t t urned out t hat no prot ocol is an island and t hat t his act ually was not a very good idea. However, placing X'FF- FF int o t he first field ( int ended t o be used as a checksum field) helps NI Cs 0 out t hat t he fram e cont ains I PX. The Net Ware raw encapsulat ion shares LAN m edia wit h ot her prot ocols t oday. List ing 4.4 shows part of a t race of a Net Ware 802.2 encapsulat ion. Not e t hat t he DSAP and SSAP addresses are set t o X'E0.

List in g 4 .4 A N e t W a r e 8 0 2 .2 Fr a m e Network Associates Sniffer Trace DLC: ----- DLC Header ----DLC: DLC: Frame 7 arrived at 15:31:12.6315; frame size is 113 (0071 hex) bytes. DLC: Destination = Station WstDigD99D41 DLC: Source = Station Intrln02D520 DLC: 802.3 length = 99 DLC: LLC: ----- LLC Header ----LLC: LLC: DSAP Address = E0, DSAP IG Bit = 00 (Individual Address) LLC: SSAP Address = E0, SSAP CR Bit = 00 (Command) LLC: Unnumbered frame: UI LLC: IPX: ----- IPX Header ----IPX: IPX: Checksum = 0xFFFF IPX: Length = 96 IPX . . .

Tabulation of Important Ethernet Parameters See Table 4.2 sum m arizes t he official Et hernet MAC param et ers defined for 1Mbps, 10Mbps, 100Mbps, and 1000Mbps Et hernet . Not e t hat m ost of t he param et ers are t he sam e across all speeds. The int erfram e gap is t he sam e when it is expressed in bit t im es. The only except ional param et er is t he Gigabit Et hernet slot t im e. Recall t hat t his would be relevant only if CSMA/ CD Gigabit t ransm ission were used.

Ta ble 4 .2 . Et h e r ne t M AC Pa r a m e t e r s Pa r a m e t e r

1 M bps

1 0 M bps

1 0 0 M bps

1 0 0 0 M bps

slot Tim e

512 bit t im es

512 bit t im es

512 bit t im es

4096 bit t im es

int erFram eGap

96s

9.6s

96s

096s

at t em pt Lim it

16

16

16

16

backoffLim it

10

10

10

10

Ta ble 4 .2 . Et h e r ne t M AC Pa r a m e t e r s Pa r a m e t e r

1 M bps

j am Size

4 byt es

m axFram eSize ( wit hout VLAN header) m inFram eSize [* ]

1 0 M bps 4 byt es

1 0 0 M bps

1 0 0 0 M bps

4 byt es

4 byt es

1518 byt es 1518 byt es

1518 byt es

1518 byt es

64 byt es

64 byt es

64 byt es[ * ]

64 byt es

* A 448- byt e ext ension m ust be added if half- duplex CSMA/ CD is used rat her t han full- duplex CSMA/ CD.

Summary Points • •

•

•

• • •

•

•

•

•

•

Classic Carrier Sense Mult iple Access wit h Collision Det ect ( CSMA/ CD) Et hernet also is called half- duplex Et hernet . At any t im e, only one fram e should be in t ransit . Fram e t ransm issions m ust be separat ed by a t im e int erval t hat corresponds t o 96 bit t im es; t his is called t he int erfram e gap or int erpacket gap. The size of t he int erfram e gap bet ween fram es can shrink as t he fram es pass t hrough a repeat er. Aft er det ect ing t hat a collision has occurred, a sender m ust cont inue t o t ransm it 32 addit ional j am bit s. A st at ion t hat has part icipat ed in a collision m ust rem ain silent for a random am ount of t im e before at t em pt ing t o t ransm it again. The delay period is a m ult iple of a param et er called t he slot t im e. An Et hernet MAC fram e st art s wit h t he fram e's dest inat ion and source MAC addresses and ends wit h an FCS field t hat is used t o det ect t ransm ission errors. A st at ion t ransm it s special bit pat t erns before sending a m edia access cont rol ( MAC) fram e. The m inim um MAC fram e size is 64 byt es. The m inim um t ransm ission size affect s t he diam et er of a collision dom ain. The t hird field in an Et hernet fram e header ident ifies eit her t he t ype or t he lengt h of t he inform at ion field. All of t he Et hernet t ype values current ly in use have decim al values well above 1500, which is t he m axim um inform at ion field size. I f CSMA/ CD were used for Gigabit Et hernet , ext ra ext ension bit s would have t o be t ransm it t ed at t he end of a short fram e. Ext ra LLC and SNAP headers t hat ident ify t he t ype of prot ocol dat a being carried are needed in an Et hernet fram e t hat conform s t o t he original 802.3 specificat ion and has a lengt h field. Net Ware t raffic has been sent using four different form at s: Wit h an Et herType code; in t he convent ional 802.3 form at wit h LLC and SNAP; 802.3 " raw" which has a lengt h field, but no LLC or SNAP; and 802.2 encapsulat ion, which has no SNAP, but uses DSAP and SSAP codes as prot ocol ident ifiers.

References The CSMA/ CD prot ocol is defined in Chapt ers 2, 3, and 4 of

•

I EEE St andard 802.3, 1998 Edit ion. " Carrier Sense Mult iple Access wit h Collision Det ect ion ( CSMA/ CD) Access Met hod and Physical Layer Specificat ions."

Chapter 5. Full-Duplex Ethernet Communication The original Et hernet LANs ran on coaxial cable and used baseband signaling. Only one fram e can be in flight across a baseband coax m edium at any t im e. Just as a car pulling ont o a road m ust wait for a break in t he t raffic before ent ering, t he CSMA/ CD rules require a st at ion t o wait unt il t he m edium is free before it can send anot her fram e. St at ions share t he m edium in a ( m ore or less) fair m anner. However, when LANs are const ruct ed using t wist ed- pair or opt ical fiber m edia connect ed t o swit ches, t he CSMA/ CD rulebook can be t hrown away. Dat a can flow from st at ion t o st at ion, st at ion t o swit ch, or swit ch t o swit ch in full- duplex m ode.

Full-Duplex Architecture A t wist ed- pair or opt ical fiber segm ent connect s only t wo nodes. Furt herm ore, t he following are t rue: •

•

A syst em connect ed t o t wo t wist ed- pair fibers can send dat a on one pair and receive dat a on t he ot her pair. A fiber opt ic adapt er connect s t o t wo opt ical fibers. Dat a is sent on one fiber and received on t he ot her.

The m ost popular t ypes of cable used for Et hernet clearly have t he pot ent ial for fullduplex com m unicat ion. St at ions t hat are connect ed t o a hub form a CSMA/ CD collision dom ain. The hub repeat s bit s ont o every segm ent , and only one signal can be in t ransit at any given t im e. However, it is perfect ly feasible t o send and receive at t he sam e t im e when st at ions are connect ed t o a swit ch. Som e vendors realized t his quit e early and st art ed t o m arket st at ion NI Cs and swit ch port s t hat support ed full- duplex operat ion. Event ually, t he 802.3 st andards were revised t o support t his feat ure. The CSMA/ CD discipline no longer is needed or used when full- duplex t ransm ission is in effect . Figure 5.1 depict s an Et hernet LAN m ade up of syst em s connect ed t o a swit ch and configured for full- duplex com m unicat ion: • • •

Each syst em can send and receive at t he sam e t im e. The CSMA/ CD prot ocol is not applied; t here are no collisions. A syst em can send a fram e whenever it want s, except for t he fact t hat it s fram es st ill m ust be separat ed by t he int erfram e gap.

Figu r e 5 .1 . Fu ll- duple x com m u n ica t ion t hr ou gh a sw it ch .

N ot e The advent of full- duplex com m unicat ion caused CSMA/ CD t o be called half- duplex operat ion.

This really cranks up t he capacit y of an Et hernet LAN. For exam ple, a swit ch wit h t went y 10Mbps port s has a t op capacit y of 200Mbps. A swit ch wit h t went y 100Mbps port s has a t op capacit y of 2000Mbps.

N ot e The t rue m axim um t hroughput depends on t he capabilit y of t he swit ch t o handle a heavy load of t raffic. The swit ch vendor needs t o guarant ee a generous backplane speed and an adequat e supply of buffer m em ory. The swit ch m ust be guarant eed t o be " fully non- blocking" t o m ake t he grade.

Full-Duplex Parameters Table 5.2 list ed Et hernet MAC param et ers. However, only t hree of t hese param et ers are relevant for full- duplex operat ion: • • •

The int erfram e gap, which is 96 bit t im es The m inim um MAC fram e size, which is 64 byt es The m axim um MAC fram e size, which is 1518 byt es

These param et ers are t he sam e for all speeds: 1Mbps, 10Mbps, 100Mbps, and 1000Mbps.

Assuring Backward Compatibility Full- duplex operat ion int roduces a com pat ibilit y problem because older NI Cs do not support t he feat ure. When an up- t o- dat e swit ch port is connect ed t o a hub or t o a st at ion wit h an old NI C card, t he swit ch port m ust figure out t hat it m ust t alk t o t hat node in half- duplex m ode. Backward com pat ibilit y was est ablished by int roducing an Aut o- Negot iat ion funct ion t hat enables nodes t o announce t heir capabilit ies t o one anot her. I f a part ner has an old int erface and cannot respond t o Aut o- Negot iat ion m essages, t he newer device knows t hat it m ust rat chet down t o half- duplex ( CSMA/ CD) com m unicat ion on t hat segm ent . Aut o- Negot iat ion is described in Chapt er 11, " Aut o- Negot iat ion."

N ot e Aut o- Negot iat ion support s backward com pat ibilit y in ot her ways, t oo. One of t he Aut o- Negot iat ed feat ures is t he int erface speed. For exam ple, m any hub and swit ch devices have port s t hat can operat e at eit her 10Mbps or 100Mbps. When a cable connect ing t o a st at ion wit h a 10Mbps NI C is plugged int o one of t hese hub or swit ch port s, t he port operat es at 10Mbps. I f a new 100Mbps NI C is inst alled in t he st at ion, t he speed aut om at ically is negot iat ed up t o 100Mbps.

Handling Congestion Full- duplex com m unicat ion gives a big boost t o capacit y, but it also creat es a problem t hat can degrade net work perform ance. For exam ple, Figure 5.2, several client s are sending dat a t o a file server at t he sam e t im e. The swit ch is incapable of forwarding t he fram es as quickly as t hey are arriving.

Figu r e 5 .2 . Con ge st ion .

A swit ch rout inely handles t em porary pileups of fram es by st oring t he ext ra input in buffer m em ory. I f swit ch m em ory fills up, however, som e fram es m ust be discarded. Usually, discarded fram es are ret ransm it t ed lat er by a higher prot ocol layer. But processing t he sam e fram es t wo or m ore t im es reduces t he swit ch's effect ive t hroughput and slows end- user response t im e. A swit ch t ries t o avoid fram e loss by t elling at t ached syst em s: " St op while I clean house! " A flow cont rol operat ion ( originally defined in st andard I EEE 802.3x and now part of 802.3) does t his j ob. The part y at eit her end of a full- duplex Et hernet link can ask it s part ner t o st op t ransm it t ing fram es for a specified lengt h of t im e. The request is carried in a special PAUSE fram e. This is im plem ent ed only in a full- duplex environm ent because it m ust be possible t o ask a part ner t o pause right away, even while t he part ner is in t he m iddle of a t ransm ission. Alt hough support for t he PAUSE funct ion current ly is an opt ional feat ure of t he 802.3 st andard, Et hernet net work int erface card and swit ch vendors have im plem ent ed t he feat ure in m any of t heir product s.

The MAC Control Sublayer The 802 com m it t ee creat ed an archit ect ural hom e for PAUSE m essages by defining a new MAC cont rol sublayer, which is shown in Figure 5.3. Current ly, sending and receiving PAUSE m essages is t he only funct ion perform ed by t his sublayer, but ot hers could be added in t he fut ure.

Figu r e 5 .3 . The M AC con t r ol su bla ye r .

An end syst em 's MAC cont rol sublayer is sandwiched bet ween t he MAC sublayer and t he Logical Link Cont rol ( LLC) sublayer. A swit ch's MAC cont rol sublayer is sandwiched bet ween t he MAC sublayer and t he swit ch fram e relaying funct ion.

MAC Control Frames and PAUSE Frames Figure 5.4 shows t he form at of a MAC cont rol fram e. MAC cont rol fram es have a fixed size of 64 byt es, equal t o t he m inim um fram e size. They are ident ified by an Et herType value of X'88- 08. The first field in t he MAC cont rol fram e dat a area is a 2byt e operat ion code t hat ident ifies t he kind of cont rol dat a t hat follows.

Figu r e 5 .4 . The ge n e r a l M AC con t r ol fr a m e for m a t a n d a PAUSE fr a m e .

The form at of t he only current ly defined MAC cont rol fram e, t he PAUSE fram e, also is displayed in Figure 5.4 •

PAUSE fram es are sent t o t he m ult icast dest inat ion address: X'01- 80- C2- 00- 00- 01 The source and dest inat ion int erfaces bot h m ust be preconfigured t o accept fram es sent t o t his address.

• •

•

The PAUSE operat ion code is equal t o X'00- 01. A PAUSE fram e cont ains a 2- byt e pause num ber t hat st at es t he lengt h of t he pause in unit s of 512 bit t im es. For exam ple, at 10Mbps, a value of 1000 t ranslat es t o 512,000 bit t im es, or .0512 seconds. The biggest pause num ber is 65,535, which t ranslat es t o 3.355 seconds at 10Mbps, .3355 seconds at 100Mbps, and .03355 seconds at 1000Mbps. The rem ainder of t he dat a field is filled wit h X'00 byt es.

The t im e needed t o clear out buffers m ight be hard t o est im at e accurat ely. The prot ocol avoids unnecessarily long wait s by perm it t ing t he pauser t o send anot her PAUSE fram e wit h a 0 wait t im e t o signal t hat t ransm ission can resum e im m ediat ely. The use of PAUSE fram es is a feat ure t hat needs t o be agreed upon by m eans of t he Aut o- Negot iat ion Prot ocol, described in Chapt er 11.

Just t o com plicat e t hings a lit t le, t he part ners at t he ends of a link can decide t o use PAUSE fram es t o im plem ent flow cont rol in only one direct ion or in bot h direct ions. This act ually leads t o t hree choices. For exam ple, in Figure 5.5, st at ion A and swit ch B could agree t hat : •

•

•

St at ion A will send PAUSE m essages, and swit ch B will receive t hem . St at ion A will flow cont rol swit ch B, but swit ch B will not flow cont rol st at ion A. Swit ch B will send PAUSE m essages, and st at ion A will receive t hem . Swit ch B will flow cont rol st at ion A, but st at ion A will not flow cont rol swit ch B. Bot h st at ion A and swit ch B will send and receive PAUSE m essages. They will flow cont rol one anot her.

Figu r e 5 .5 . Flow con t r ol a lt e r n a t ive s.

The first t wo alt ernat ives are called asym m et ric flow cont rol. The t hird alt ernat ive is called sym m et ric flow cont rol.

Implementation The 802.3 flow cont rol specificat ion describes t he rules for using PAUSE fram es but does not provide any clues as t o how a syst em decides when a PAUSE fram e should be sent . However, a t ypical im plem ent at ion is not com plicat ed. For exam ple, t he swit ch in Figure 5.2 usually would be configured wit h a lim it on t he am ount of buffer m em ory t hat is available t o hold incom ing fram es arriving at each of it s port s. I f t he swit ch has port s capable of operat ing at m ore t han one speed ( for exam ple, 10Mbps and 100Mbps, or 100Mbps and 1000Mbps) , m ore m em ory can be assigned t o t he port s current ly operat ing at t he higher speed. I n Figure 5.2, t here are t oo m any fram es heading for t he swit ch. This causes som e of t he arriving fram es t o be buffered. When t he inbound m em ory allocat ion for a port is alm ost exhaust ed, t he swit ch sends a PAUSE t hrough t hat port . The im plem ent at ion at an end- user st at ion is st raight forward. I f t he CPU and disk I / O com ponent s cannot keep up wit h t he incom ing flow of fram es, m em ory allocat ed for net work I / O st art s t o fill quickly. When a t hreshold level is exceeded, t he syst em can send a PAUSE fram e t o it s neighboring swit ch.

Summary Points •

•

•

•

• • •

When t wist ed- pair or fiber opt ical m edia is used, dat a can flow from st at ion t o st at ion, st at ion t o swit ch, or swit ch t o swit ch in full- duplex m ode. For full- duplex operat ion, t he CSMA/ CD prot ocol is not applied; t here are no collisions. The param et ers relevant for full- duplex operat ion are t he int erfram e gap, t he m inim um MAC fram e size, and t he m axim um MAC fram e size. Backward com pat ibilit y wit h half- duplex int erfaces was est ablished by int roducing an Aut o- Negot iat ion funct ion t hat enables nodes t o agree on using capabilit ies t hat bot h support . Special PAUSE fram es t ell a part ner t o t em porarily st op sending fram es. The MAC cont rol sublayer was creat ed t o perform PAUSE funct ions ( and ot her funct ions t hat m ight be defined in t he fut ure) . Traffic cont rol via PAUSE fram es can be sym m et ric or asym m et ric ( only one part y is cont rolled) .

References The prot ocols present ed in t his chapt er are defined in •

I EEE St andard 802.3, 1998 Edit ion. " Carrier Sense Mult iple Access wit h Collision Det ect ion ( CSMA/ CD) Access Met hod and Physical Layer Specificat ions."

Full- duplex operat ion is discussed in Chapt er 4 of t he 802.3 st andard. The MAC Cont rol sublayer is described in Chapt er 31. The PAUSE operat ion and t he form at of a PAUSE fram e are described in Annex 31B.

Chapter 6. The Ethernet 10Mbps Physical Layer This chapt er describes t he physical charact erist ics of Et hernet LANs built using 10Mbps coax, t wist ed- pair, and fiber opt ic Ethernet m edia. Som e Et hernet LANs are const ruct ed using a single m edium , such as t wist ed- pair cabling. However, m any have been built using a m ixt ure of m edia t ypes. Alt hough m any people t hink of coax Et hernet as a t echnology of t he past , a quick check of cabling and com ponent s cat alogs shows t hat a m arket for t he t echnology st ill exist s. There is an inst alled base of coax LANs t hat are working well and whose users are sat isfied. I t is very unlikely t hat you will inst all a new coax Et hernet LAN. However, t he archit ect ure and key param et ers for Et hernet LANs were defined for t he coax LAN environm ent , so it m akes sense t o exam ine t he coax environm ent first . LANs based on t hick coax m edia ( called 10BASE5) and t hin coax m edia ( called 10BASE2) are described in t he opening sect ions of t his chapt er. The m ost com m on Et hernet LAN m edium in current use is t wist ed- pair cabling, and 10BASE- T LANs are discussed aft er t he coax LANs. The final part of t he chapt er describes t he physical charact erist ics of fiber opt ic m edia and shows how fiber opt ic cables are int egrat ed int o Et hernet LANs.

Baseband Ethernet on Thick Coaxial Cable (10BASE5) The original Et hernet LAN defined in t he 1980s was built from sect ions of t hick, heavy 50- ohm coaxial cable. A LAN segm ent is eit her a single cable sect ion or is m ade up of t wo or m ore sect ions j oined by connect ors ( called barrel connect ors because t hey are shaped like barrels) . The segm ent at t he t op of Figure 6.1 is m ade up of t wo sect ions j oined by a barrel connect or.

Figu r e 6 .1 . A br a n ch in g bu s Et h e r n e t .

A com ponent called a t erm inat or is at t ached t o each end of a segm ent . Each 50- ohm t erm inat or absorbs signals and prevent s t hem from being reflect ed back int o t he cable. Coaxial cable should be grounded at exact ly one point , and one of t he t erm inat ors oft en is select ed as t he locat ion for t he ground connect ion.

W a r n in g A defect ive, loose, or m issing t erm inat or kills a LAN. Because of t he reflect ed signals, st at ions becom e incapable of locking on t o t he t rue signals and receiving incom ing dat a bit s.

Mult iple coax segm ent s can be connect ed by repeat ers t o form a branching bus t ree t opology like t he one shown in Figure 6.1. For a t hick Et hernet 10BASE5 LAN • • •

10 st ands for 10 m egabit s per second ( Mbps) . BASE denot es t hat dat a is represent ed by baseband signals. 5 corresponds t o t he m axim um segm ent lengt h, which is 500 m et ers.

For baseband t ransm ission, only one signal can be on a cable at any given t im e. The 0 and 1 bit s are t ransm it t ed across an Et hernet segm ent using Manchest er encoding,

which is described in Appendix A, " Physical Layer Encodings for Et hernet , Fibre Channel, FDDI , and Token Ring."

Broadband Ethernet An alt ernat ive broadband version of Et hernet was int roduced in t he early days of Et hernet . I n a broadband net work, several channels operat ing on different frequency bands share t he cable and m ult iple t ransm issions occur in parallel. A broadband Et hernet ( which was called 10BROAD36) is im plem ent ed as a single long coaxial cable segm ent . For a while, t he m erit s of baseband versus broadband were hot ly debat ed. However, st at ions had t o be connect ed t o a broadband LAN by cost ly frequency m odulat ion t ransm it t ers/ receivers ( t ransceivers) , and t he connect ion process required t uning and t est ing. Baseband becam e t he norm because it s t ransceivers were m uch cheaper and t he inst allat ion process was far sim pler. Broadband t ransm ission has re- surfaced t oday in t he I nt ernet cable m odem service. However, t he cable m odem net works use propriet ary t echnology t hat does not adhere closely t o t he original Et hernet broadband st andard.

Connecting to 10BASE5 Coax Et hernet st at ions do not connect direct ly t o a 10BASE5 cable. Figure 6.2 illust rat es how a net work int erface card ( NI C) connect s t o t he bus. One end of an at t achm ent unit int erface ( AUI ) drop cable plugs int o t he NI C, and t he ot her end plugs int o a t ransceiver. The t ransceiver also is called a m edium at t achm ent unit ( MAU) .

Figu r e 6 .2 . At t a ch in g t o a coa x Et h e r n e t .

MAU is t he t erm used in t he I EEE st andards and in som e t ext books. However, vendors call t heir at t achm ent product s t ransceivers.

The use of an AUI drop cable is convenient . The t hick Et hernet backbone cable can be inst alled in ceiling or floor duct s. The AUI cable runs from t he ceiling or floor t o t he st at ion.

N ot e An AUI cable consist s of four individually shielded pairs of wire surrounded by an overall cable shield. The double shielding m akes AUI cable resist ant t o elect rical signal int erference. An AUI cable connect s t o t he t ransceiver and t he NI C via 15- pin DB15 connect ors.

Two st yles of t hicknet t ransceivers exist , as is shown on t he left side of Figure 6.3. To inst all an int rusive t ransceiver, t he cable segm ent m ust be cut and at t ached t o connect ors on t he t ransceiver. The well- nam ed vam pire t ap t ransceiver is nonint rusive; it is clam ped ont o t he cable and connect s by piercing t he cable.

Figu r e 6 .3 . Thick ne t t r a n sce ive r s.

The right side of Figure 6.3 shows a com plet e connect ion bet ween a DTE and a t hick Et hernet cable. A st at ion's NI C includes a fem ale D connect or. The 15- pin m ale end of an AUI cable plugs int o t his, and t he opposit e fem ale end of t he cable plugs int o a m ale D connect or on t he t ransceiver.

N ot e Specialized connect ors are defined for each of t he ot her m edia ( t hin coax, t wist edpair, and opt ical fiber cabling) discussed in t his chapt er. However, t his init ial configurat ion, consist ing of a net work int erface wit h a fem ale D connect or and an AUI cable connect ing t o a t ransceiver, also could be used for each of t he m edia. I t is not used because t he m edia- specific NI Cs are very cheap. Using a separat e t ransceiver would add great ly t o t he cost .

Figure 6.44 shows a repeat er t hat connect s t o t wo coaxial cable segm ent s via t wo AUI cables.

Figu r e 6 .4 . Con n e ct in g coa x se gm e n t s u sin g a r e pe a t e r .

Transceiver (MAU) Functions Vendors m ight have decided t o use t he nam e t ransceiver inst ead of m edium at t achm ent unit because an MAU does a lot m ore t han at t ach t o a m edium . I t also sends and receives bit s and perform s a lot of ot her funct ions—for exam ple, when it det ect s t hat a fram e t hat it is sending is experiencing a collision, it t ransm it s a j am signal.

N ot e St at ion MAUs are not t he only ones t hat send j am signals. When an MAU at t ached t o a repeat er det ect s t hat t he fram e t hat it current ly is repeat ing is experiencing a collision, it has t o send a j am signal ont o all t he ot her segm ent s t o m ake sure t hat all st at ions are alert ed t o t he collision. A t ransceiver perform s t wo ot her wort hwhile funct ions:

• •

Jabber cont rol Signal qualit y error m essages

Ja bbe r Con t r ol Jabber occurs when a st at ion " t alks" t oo long. That is, it t ransm it s cont inuously for m ore t im e t han is needed t o send a fram e of m axim um size. This is caused by a m alfunct ion. An 802.3 t ransceiver cont rols j abber, cut t ing off t ransm ission aft er a t im e t hreshold has been exceeded.

Sign a l Qu a lit y Er r or M e ssa ge s Aft er a t ransceiver has finished t ransm it t ing t he last bit of a fram e, it passes a special signal up t o it s own st at ion. The signal verifies t hat t he t ransceiver has done it s j ob and is operat ing correct ly. This m echanism is called t he signal qualit y error t est ( SQE t est ) . SQE signals are used for t wo ot her funct ions: • •

They report collisions. They report t hat im proper pulses were sensed on t he m edium . ( This act ually m ight be caused by a fault in t he t ransceiver rat her t han a problem on t he LAN cable.)

The t ransceiver funct ions were carried over t o t he ot her Et hernet m edia. Jam t ransm ission, j abber cont rol, and t he SQE t est funct ion are im plem ent ed for baseband coax, broadband coax, t wist ed- pair, and opt ical fiber m edia.

10BASE5 Collision Domain Parameters A 10BASE5 collision dom ain is m ade up of segm ent s connect ed by repeat ers. The im port ant param et ers t hat govern t he size and shape of a t hick Et hernet collision dom ain are present ed in Table 6.1.

Ta ble 6 .1 . 1 0 BASE5 Collision D om a in Pa r a m e t e r s Pa r a m e t e r or Cha r a ct e r ist ic

Va lu e

Ta ble 6 .1 . 1 0 BASE5 Collision D om a in Pa r a m e t e r s Pa r a m e t e r or Cha r a ct e r ist ic

Va lu e

Topology

Branching bus

Type of segm ent cable

Thick, 50- ohm coax

Connect or

" D" wit h AUI cable and t ransceiver

Lengt h of an AUI at t achm ent cable

5- 50 m et ers

Maxim um lengt h of a segm ent

500 m et ers

Maxim um propagat ion delay for a segm ent

2165 nanoseconds

Maxim um num ber of nodes ( st at ions and repeat ers) t hat can be at t ached t o a segm ent

100

Minim um dist ance bet ween t wo t ransceivers

2.5 m et ers

Maxim um collision dom ain diam et er

2500 m et ers

Maxim um num ber of cable segm ent s t raversed bet ween a 5 source t o a dest inat ion Maxim um num ber of repeat ers bet ween a source and dest inat ion

4

Maxim um num ber of populat ed segm ent s bet ween a source and dest inat ion

3

The placem ent of st at ions on a t hick coax cable is not arbit rary. The 10BASE5 bus cable ( which usually has a yellow or orange j acket ) is m arked at 2.5- m et er int ervals wit h t he opt im al posit ions at which t ransceivers can be at t ached.

5 - 4 - 3 Ru le The last t hree it em s in Table 6.1 represent t he fam ous Et hernet 5- 4- 3 rule, which st at es t hat any pat h t hrough a collision dom ain m ay: • • •

Traverse at m ost five segm ent s Go t hrough at m ost four repeat ers Cross at m ost t hree segm ent s t hat cont ain st at ions

The 5- 4- 3 rule is illust rat ed in Figure 6.5 The figure includes seven segm ent s, but t his is not a problem —t he 5- 4- 3 rule applies t o pat hs. Because of t he t ree st ruct ure, a pat h bet ween any pair of st at ions t raverses no m ore t han five segm ent s. For exam ple, t he pat h from st at ion A t o st at ion B crosses five segm ent s and passes t hrough four repeat ers. Only t hree of t hese segm ent s are populat ed wit h end st at ions. Sim ilarly, t he pat h from st at ion B t o st at ion C crosses five segm ent s, only t hree of which are populat ed.

Figu r e 6 .5 . A coa x Et h e r n e t LAN e x t e n de d u sin g r e pe a t e r s.

Pr opa ga t ion D e la y The propagat ion delay is t he am ount of t im e t hat elapses when a signal t ravels from one end of a cable t o t he ot her end. This is t he param et er t hat act ually is responsible for t he const raint s t hat lim it t he size of a collision dom ain. I f t wo or m ore st at ions st art t o t ransm it at close t o t he sam e t im e, t heir signals will collide. The net work m ust be sm all enough so t hat if m ult iple st at ions t ransm it m inim al- sized fram es at t he sam e t im e, all of t hem will det ect t he collision. All t he receiving st at ions also m ust be capable of det ect ing t he collision. The net work diam et er is t he longest pat h bet ween t wo point s in a collision dom ain. The five- segm ent m axim um lim it s t he biggest diam et er of a 10BASE5 collision dom ain t o 2,500 m et ers. However, all count s end at a bridge, and bridges can be used t o great ly ext end t he size of a LAN. Chapt er 12, " Et hernet Bridges and Layer 2 Swit ches," has t he det ails.

N ot e

An Et hernet LAN can include up t o 1,024 st at ions. ( Repeat ers are not included in t his count .)

Unpopulat ed LAN segm ent s are used as t ransit links t hat carry signals from one part of t he LAN t o anot her part of t he LAN. The diam et er of a collision dom ain can be increased by using fiber opt ic cable for t he unpopulat ed t ransit sect ions. This is discussed lat er in t his chapt er.

Practical Problems with 10BASE5 The coaxial cable used for 10BASE5 was hard t o work wit h because it was cost ly, heavy, and inflexible. Many organizat ions st art ed out wit h sm all LANs. As PCs and workst at ions proliferat ed, a LAN would be ext ended in a piecem eal fashion, and oft en an organizat ion would lose t rack of how m uch cable was inst alled and where it was. This m ade com put er m oves and changes a big hassle. LAN growt h was difficult as well. New t ransceivers had t o be added carefully at m arked locat ions. Com m unicat ion across a segm ent was int errupt ed whenever an int rusive t ransceiver was inst alled or when a new segm ent bounded by barrel connect ors was insert ed. A fault y sect ion of cable, a bad t erm inat or, a loose barrel connect or, or a bad t ransceiver could disrupt com m unicat ion for everyone on a segm ent , and t he source of t he problem was hard t o t rack down.

Baseband Ethernet on Thin Coax Cable (10BASE2) A new version of coax Et hernet was int roduced t o overcom e som e of t he problem s posed by 10BASE5. This version is built using a light er, m ore flexible cable called t hinnet ( or " cheapernet " ) and is called 10BASE2. As before, 10 m eans 10Mbps, and BASE st ands for baseband; t he 2 represent s 200 m et ers. However, t his is t he rounded- up value for t he act ual m axim um segm ent lengt h, which is 185 m et ers. Alt hough AUI cables and ext ernal t ransceivers could be used wit h 10BASE2, a far less cost ly configurat ion becam e t he norm . The t ransceiver funct ion was int egrat ed ont o a NI C t hat connect s direct ly t o t he coax bus via a Bayonet Neil- Concelm an ( BNC) T connect or, as is shown in Figure 6.6.

Figu r e 6 .6 . Et h e r n e t N I C w it h in t e gr a t e d t r a n sce ive r a n d BN C con n e ct or .

N ot e A form al int erface called t he AUI st ill exist s bet ween t he t ransceiver chip and t he ot her com ponent s on a NI C, but t his int erface is on t he card inst ead of being represent ed by a cable.

Figure 6.7illust rat es t he st ruct ure of a 10BASE2 net work. A 10BASE2 segm ent is m ade up of short lengt hs of cable connect ed t o one anot her by BNC T connect ors. I n ot her words, a segm ent is a daisy chain of short cables and BNC Ts.

Figu r e 6 .7 . D a isy- ch a in e d 1 0 BASE2 se gm e n t s.

•

•

For m ost of t he st at ions, t he BNC T is pushed ont o a round post ext ending from t he st at ion's 10BASE2 NI C. For a few, t he NI C m ay have a fem ale D socket t hat m ust be connect ed t o an AUI cable.

The repeat er in Figure 6.7 is connect ed t o a pair of t ransceivers via AUI cables and D connect ors. Each t ransceiver also has a BNC int erface. The t radeoff for t he cheaper backbone cable and com ponent s, and ease of use is t hat t he coax segm ent s m ust be short er. At m ost , 30 nodes ( including bot h st at ions and repeat ers) can be at t ached t o a t hinnet coax segm ent .

10BASE2 Collision Domain Parameters Table 6.2 has t he vit al st at ist ics for a 10BASE2 collision dom ain. The 5- 4- 3 rule cont inues t o hold. Therefore, a 10BASE2 collision dom ain has a m axim um diam et er of 925 m et ers. The collision dom ain size can be increased by using t hicknet Et hernet ( or opt ical fiber) for som e of t he segm ent s, or by int roducing bridges.

Ta ble 6 .2 . 1 0 BASE2 Collision D om a in Pa r a m e t e r s Pa r a m e t e r or Cha r a ct e r ist ic

Va lu e

Topology

Branching bus

Type of segm ent cable

Thin 50- ohm coax

Connect or

BNC

Maxim um lengt h of a segm ent

185 m et ers

Maxim um propagat ion delay for a segm ent

950 nanoseconds

Maxim um num ber of nodes ( st at ions and repeat ers) t hat can be at t ached t o a segm ent

30

Minim um dist ance bet ween t wo nodes

.5 m et er

Maxim um collision dom ain diam et er

925 m et ers

Maxim um num ber of cable segm ent s t raversed bet ween a source and dest inat ion

5

Maxim um num ber of repeat ers bet ween a source and dest inat ion

4

Maxim um num ber of populat ed segm ent s bet ween a source and dest inat ion

3

Practical Problems with 10BASE2 Making room for new st at ions on a LAN segm ent is easy: You j ust insert new sect ions and addit ional BNC T connect ors. However, com m unicat ion across t he LAN halt s while you are doing t his. Furt herm ore, t he sim plicit y can be a pit fall. Users have been known t o disconnect LAN cables while rearranging t he furnit ure in t heir offices, unaware t hat t hey have st opped all LAN t raffic dead in it s t racks. As was t he case for 10BASE5, fault y sect ions and bad connect ors or t erm inat ors can disrupt com m unicat ions for everyone on a 10BASE2 segm ent .

10Mbps Twisted-Pair Ethernet (10BASE-T) I t was a banner day for Et hernet when researchers discovered how t o cram 10Mbps across unshielded t wist ed- pair ( UTP) cable. I n 1990, 10BASE- T becam e an I EEE st andard. The days of snaking heavy coaxial cable t hrough a building—and, because of m oves and changes, oft en losing t rack of what act ually was hidden in t he ceilings and walls—were over. Now a building could be st ar- wired for dat a in t he sam e way t hat it was st ar- wired for t elephony. I n m any cases, ext ra wire pairs t hat already were inst alled could be used. The label chosen for 10Mbps t wist ed- pair Et hernet is 10BASE- T. Two wire pairs are used.

10BASE-T Segments A t wist ed- pair segm ent can connect t o only t wo nodes: one at each end of t he segm ent . The m axim um 10BASE- T segm ent size is 100 m et ers. Figure 6.8shows several t ypes of 10BASE- T segm ent connect ions. At t he t op, 10BASE- T segm ent s link st at ions t o a hub. The hub is j oined t o a swit ch by a 10BASE- T segm ent , and furt her segm ent s connect t he swit ch t o t wo servers and t o a rout er. Below t hat , t he figure shows t wo st at ions t hat are direct ly connect ed t o one anot her by a 10BASE- T segm ent . This is perfect ly feasible but is not oft en done.

Figu r e 6 .8 . 1 0 BASE- T se gm e n t s.

Half-Duplex Operation Twist ed- pair hubs were int roduced t o t ie several st at ions int o a com m on collision dom ain. For exam ple, t he hub in Figure 6.99 repeat s t he bit s in a fram e t ransm it t ed by st at ion A t o t he ot her st at ions connect ed t o t he hub. Only one st at ion connect ed t o a hub can t ransm it at any t im e. Com m unicat ion is cont rolled by t he CSMA/ CD prot ocol and is said t o be half- duplex.

Figu r e 6 .9 . H a lf- du ple x com m u n ica t ion t hr ou gh a h u b.

1 0 BASE- T Collision D om a in Pa r a m e t e r s Table 6.3 displays t he param et ers for a 10BASE- T Et hernet collision dom ain. A collision dom ain is const ruct ed by connect ing every st at ion t o a hub and int erconnect ing several hubs. I t is im port ant t o use t wist ed- pair cable t hat m eet s st andard qualit y requirem ent s. Chapt er 10, " Twist ed- Pair Cabling St andards and Perform ance Requirem ent s," discusses various cable cat egories and describes t est s t hat are applied t o t wist ed- pair cables. The 5- 4- 3 rule st ill holds. Because t he m axim um t wist ed- pair segm ent lengt h is 100 m et ers, t he diam et er of a collision dom ain t hat is const ruct ed using only t wist ed pair segm ent s is lim it ed t o 500 m et ers, at m ost .

N ot e The diam et er of a collision dom ain t hat is based predom inant ly on t wist ed- pair cabling can be increased by insert ing som e opt ical fiber links bet ween hubs. Of course, a very large LAN can be built using swit ches t o int erconnect m ult iple collision dom ains.

Ta ble 6 .3 . 1 0 BASE- T Collision D om a in Pa r a m e t e r s Pa r a m e t e r or Cha r a ct e r ist ic

Va lu e

Topology

A t ree of st ars

Pat ch cords

Up t o 10 m et ers t ot al

Type of segm ent cable

2 t wist ed pairs, Cat egory 3 or bet t er

Connect or

RJ- 45 j ack and plug

Maxim um lengt h of a segm ent

100 m et ers

Ta ble 6 .3 . 1 0 BASE- T Collision D om a in Pa r a m e t e r s Pa r a m e t e r or Cha r a ct e r ist ic

Va lu e

Maxim um num ber of nodes t hat can be at t ached t o a segm ent

2

Maxim um collision dom ain diam et er

500 m et ers

Maxim um num ber of cable segm ent s t raversed bet ween a source and dest inat ion

5

Maxim um num ber of repeat ers ( hubs) bet ween a source and dest inat ion

4

Maxim um num ber of populat ed segm ent s bet ween a source and dest inat ion

3

1 0 BASE- T Collision D om a in Ca blin g Topology The overall t opology of a 10BASE- T LAN is a t ree of st ars, as is illust rat ed in Figure 6.10. The LAN in Figure 6.10 10 is const ruct ed around four hubs and is a single collision dom ain. Each segm ent is at m ost 100 m et ers in lengt h. The longest pat h t hrough t he net work passes t hrough four hubs and crosses five segm ent s.

Figu r e 6 .1 0 . A sim ple 1 0 BASE- T LAN .

N ot e The rule t hat st at es t hat at m ost t hree segm ent s along a pat h can cont ain st at ions always holds for a 10BASE- T collision dom ain. I n fact , t here are only t wo st at ions on any pat h. All t he ot her nodes along a pat h are repeat ers ( hubs) .

An unbroken 100- m et er run oft en is used t o connect t wo hubs or t o j oin a hub t o a bridge. However, a horizont al cable run bet ween a workst at ion and a hub norm ally is broken up int o pieces, as Figure 6.11 shows. At t he st at ion end, a NI C is connect ed t o a t elecom m unicat ions out let by a short cable called a pat ch cord. Wit hin t he wiring closet at t he ot her end, pat ch cords are used t o com plet e t he connect ion t o a specific hub or swit ch.

Figu r e 6 .1 1 . Com pon e n t s of a 1 0 BASE- T LAN .

N ot e A pat ch cord also is called a pat ch cable or j um per cord. A pat ch cord t hat connect s a user's com put er t o an out let oft en is called a work area cable. A pat ch cord t hat connect s a device t o a hub or swit ch in a wiring closet oft en is called an equipm ent cable.

A t ypical breakdown is 90 m et ers for t he long run bet ween t he t elecom m unicat ions out let and pat ch panels in t he wiring closet and up t o 10 m et ers for t he pat ch cords used at each end. The pat ch cord connect ion bet ween t he st at ion and t he t elecom m unicat ions out let norm ally is lim it ed t o a lengt h of 3 m et ers, at m ost . Pat ch panels in a wiring closet enable t he connect ions bet ween end- user st at ions and hubs or swit ches t o be rearranged by unplugging and replugging pat ch cables. The pat ches shown at t he t op of Figure 6.12 are called int erconnect s. A st at ion is at t ached t o a new hub by unplugging t he pat ch cable from t he old hub and plugging it int o t he new hub. The pat ches shown in t he lower part of Figure 6.12 are called cross- connect s. Cables connect ed t o t he hubs do not need t o be t ouched. An at t achm ent is changed by plugging one end of a pat ch cord int o a different socket . Cross- connect s are very convenient . However, t hey can degrade signals since t hey int roduce m ore connect or hardware int o t he cable pat h.

Figu r e 6 .1 2 . I n t e r con n e ct a n d cr oss- con n e ct .

N ot e Most NI Cs bought t oday operat e at eit her 10 or 100Mbps. A st at ion wit h a 10/ 100Mbps adapt er can be upgraded t o 100Mbps by changing it s hub or swit ch connect ion so t hat it at t aches t o a hub or swit ch port capable of operat ing at 100Mbps. As soon as t he new connect ion is set up, t he st at ion's NI C and t he hub or swit ch port aut om at ically perform a negot iat ion t hat upgrades t he link t o t he higher speed.

Figure 6.13 shows a sam ple layout for a LAN t hat spans a building. Twist ed- pair cabling bundles t ypically cont ain 2, 4, or 25 pairs.

Figu r e 6 .1 3 . Ca blin g a bu ildin g.

Full-Duplex Operation Many LANs in operat ion t oday are built around hubs and half- duplex ( CSMA/ CD) com m unicat ion, and t hey are subj ect t o t he CSMA/ CD 5- 4- 3 rule. However, t he price of swit ches has dropped at an ast onishing rat e, and hubs are dest ined t o gradually fade away from t he scene. Because dat a is sent and received on different pairs, fullduplex t ransm ission is a regular feat ure of t wist ed- pair swit ches. Full- duplex com m unicat ion was described in Chapt er 5, " Full- Duplex Et hernet Com m unicat ion." Today's swit ches operat e at 10Mbps, 100Mbps, and 1000Mbps speeds. Up- t o- dat e NI Cs and swit ch ( and hub) port s support an Aut o- Negot iat ion funct ion t hat enables peer int erfaces t o select t he speed at which t hey will operat e and t o decide whet her t hey will use full- duplex operat ion. Aut o- Negot iat ion is discussed in Chapt er 11. The advant ages of using swit ches include • • •

Full- duplex operat ion for each port Bandwidt h proport ional t o t he num ber of port s No collisions

•

No 5- 4- 3 rule

The archit ect ure and feat ures of swit ches are described in Part I I , " Bridging, Swit ching, and Rout ing."

10Mbps Twisted-Pair Transmission 10Base- T carries dat a across t wo t wist ed pairs of wire. Dat a is t ransm it t ed from a syst em on one pair and is received on t he ot her pair. The t erm t wist ed- pair refers t o t wo wires wrapped around each ot her. For 10BASE- T, t he sam e signal is sent across bot h of t he wires in a pair but wit h reverse polarit y. This m eans t hat when a posit ive volt age is placed on one wire, an equal negat ive volt age is placed on t he ot her. There is a good reason for t his. A current sent down a copper wire creat es an elect rom agnet ic field t hat induces current s in nearby wires. Som e people experience t his effect ( called crosst alk) during a t elephone call when t hey suddenly begin t o hear a conversat ion t aking place on anot her set of wires. When t wo wires carrying t hese reversed signals are t wist ed around one anot her, t he elect rom agnet ic fields around each wire com e very close t o canceling each ot her out . The arrangem ent is called a balanced cable. The m ore t wist s per foot , t he bet t er t his works. The num ber of t wist s ranges from 2 t o 12 per foot . Figure 6.14 illust rat es t he relat ionship bet ween t he pairs. The wire labeled Tx+ carries t he t ransm it t ed signal, and Tx- carries t he invert ed t ransm it signal. The wire labeled Rx+ carries t he signal t hat was t ransm it t ed from t he rem ot e end, and Rx- carries t he invert ed signal.

Figu r e 6 .1 4 . Tr a n sm it a n d r e ce ive t w ist e d pa ir s.

There is anot her good reason for t his paired t ransm ission. I f a noise spike occurs along t he way, bot h signals in a pair will be subj ect ed t o t he sam e spike. On recept ion, t he invert ed signal on t he incom ing Rx- cable is invert ed again. This causes t he noise spike t o be invert ed. The invert ed Rx- signal t hen is added t o t he Rx+ signal, and t he result is t hat t he noise spike cancels it self out .

The t wo t wist ed pairs are t erm inat ed by a RJ- 45 plug at each end. This plug has eight cont act s, num bered 1 t o 8. Only four are needed, and t he ones t hat are used are:

1

Tx+

2

Tx-

3

Rx+

6

Rx-

Each wire connect s a t ransm it pin at one end t o a receive pin at t he ot her end. The t op of Table 6.15 shows t he proper m at chup. For exam ple, t he t ransm it wire t hat is at t ached t o pin 1 on t he sender side m ust be at t ached t o pin 3 on t he receiver side. The bot t om of t he figure shows a t wist ed- pair j ack.

Figu r e 6 .1 5 . Pin , w ir e , a n d j a ck con n e ct ion s.

Figure 6.16 shows how a crossover cable connect s t ransm it and receive ends.

Figu r e 6 .1 6 . Cr ossove r ca ble w ir e s.

Hub and Switch Connections An ordinary garden- variet y port t hat t ransm it s on pins 1 and 2 and receives on pins 3 and 6 is called a m edium dependent int erface ( MDI ) port . St at ions have MDI port s. You m ight expect t hat a crossover cable like t he one in Figure 6.16 would be needed bet ween a st at ion and a hub, but hub port s are st ruct ured so t hat t he crossover is perform ed inside t he port . These port s are said t o be of t ype m edium dependent int erface crossover ( MDI - X) . A st raight - t hrough cable is used bet ween an MDI port in an end syst em and an MDI - X port in a hub. What do you do when a hub has t o be connect ed t o anot her hub? A st raight - t hrough cable won't work bet ween t wo MDI - X port s. Transm it would be connect ed t o t ransm it and receive t o receive. Fort unat ely, m ost hubs have a special MDI port t hat is provided for t his purpose. A st raight - t hrough cable is used t o connect an MDI port on one hub t o an MDI - X port on t he ot her hub. Users prefer not t o wast e a port on a hub t hat does not need t o be connect ed t o anot her hub. Many hubs have a special port t hat is m arked MDI / MDI - X. I t can be set t o MDI or MDI - X by pushing or releasing a but t on or by m oving a slider. Choosing MDI - X let s you connect an end- user st at ion t o t he port via a st raight - t hrough cable. Choosing MDI enables you t o connect t o an MDI - X port on anot her hub using a s t raight - t hrough cable.

N ot e The previous discussion also applies t o swit ch port s. End syst em s connect t o MDI - X swit ch port s via st raight - t hrough cables. A swit ch can be connect ed t o a hub or t o anot her swit ch via a st raight - t hrough cable by using an MDI - X port on one end and an MDI port at t he ot her end.

I t is very convenient t o be able t o use st raight - t hrough cables for all your connect ions. I t is all t oo easy t o plug in t he wrong t ype of cable if you have bot h t ypes lying around. However, if a hub needs t o be connect ed t o m ore t han t wo hub or swit ch devices, you could run out of available MDI port s. I n t his case, you would need t o use MDI - X port s at bot h ends and connect t hem wit h a crossover cable.

N ot e I t is a good idea t o buy color- coded cables. All crossover cables should have a charact erist ic color t hat st ands out , such as hot pink.

Figure 6.17 illust rat es valid hub connect ions.

Figu r e 6 .1 7 . Por t a n d ca ble t ype s.

Twisted-Pair Link Integrity Test The fact t hat dat a is sent and received across a 10BASE- T link on different wires allowed a useful self- t est capabilit y t o be int roduced. Twist ed- pair int erfaces det ect cable failures aut om at ically by perform ing an ongoing link int egrit y t est . Each int erface im plem ent s t his by sending a special link int egrit y signal across each pair at ( approxim at ely) 16- m illisecond int ervals when t he dat a t ransm it t er is idle. The signal consist s of a special nondat a pulse called a norm al link pulse ( NLP) . As long as t he int erface at t he ot her end of t he link receives t his signal, it knows t hat t he link is working. I f no t est signal is received during a specified period of t im e, dat a t ransfer t hrough t he int erface is disabled. Eit her part y can send t hese t est pulses t o it s part ner on it s t ransm it wires as long as it s dat a t ransm it t er is idle.

Building a Mixed Coax/Twisted-Pair Collision Domain The m axim um lengt h of a t hick Et hernet segm ent is 500 m et ers, and a t hin Et hernet segm ent is, at m ost , 185 m et ers long. The lengt h of a t wist ed- pair segm ent , however, is lim it ed t o 100 m et ers. Figure 6.18 shows what happens when you build a LAN t hat m ixes 10BASE5 coax and t wist ed- pair segm ent s. The m axim um lengt h of each segm ent is dict at ed by it s m edium t ype.

Figu r e 6 .1 8 . M ix e d coa x a n d t w ist e d- pa ir LAN .

The pat h from st at ion A t o server X passes t hrough four repeat ers ( t hree hubs and a coax repeat er) . I t crosses t hree t wist ed- pair segm ent s and t wo coax segm ent s. The t hin lines in t he figure represent AUI cables. These do not count as segm ent s.

N ot e The recom m ended segm ent - lengt h rest rict ions have been est ablished t o assure t hat collisions will be heard by every syst em . However, it is possible t o build LANs t hat cont ain som e segm ent s t hat are longer if t hey are offset by short er segm ent s elsewhere. Chapt er 13 of t he 1998 I EEE 802.3 st andard present s a m et hod of calculat ing whet her t he delay across t he worst pat h in a net work exceeds t he perm issible bound.

When nonst andard cable lengt hs are used, it is very im port ant t o m aint ain very det ailed docum ent at ion of t he LAN st ruct ure and t o flag t he except ions clearly. All LAN adm inist rat ors need t o be kept aware of t he except ions. Ot herwise, som eone m ight innocent ly add a cable t hat creat es a pat h t hat is t oo long. This leads t o errat ic behavior t hat is hard t o pinpoint and fix. I t is a good idea t o st ick wit h t he norm al lengt h rest rict ions whenever possible!

10Mbps on Fiber Optic Cable Fiber opt ic cable easily out perform s copper m edia. I t is capable of carrying a huge capacit y for ext ended dist ances, and it is not subj ect t o t he elect rom agnet ic dist urbances t hat cause dist ort ions and errors in copper wires. No crosst alk problem occurs, eit her. Opt ical fiber also offers superior securit y. Unlike elect rical t ransm ission, it does not em it signals t hat can be capt ured by elect ronic eavesdropping equipm ent . The use of opt ical fiber for t elecom m unicat ions dat es back t o 1977, when t he m edium was first used t o carry long- dist ance t elephone calls. Fiber opt ic cable was int roduced int o t he Et hernet local area net working arena in t he 1980s. Et hernet fiber opt ic links init ially were used for long runs of cable wit hin a building or bet ween t wo buildings. Originally, vendors built propriet ary product s t hat sat isfied t he dem and for t hese long links. I n 1987, t he I EEE 802.3 com m it t ee int roduced t he 10Mbps fiber opt ic int er- repeat er link ( FOI RL) st andard. FOI RL enabled vendors t o 10 Mbps FOI RL build int eroperable fiber opt ic int erfaces. A FOI RL link could be used t o connect a repeat er t o anot her repeat er or t o a st at ion. At t his t im e, opt ical fiber was cost ly and difficult t o work wit h. Event ually, t he price of t he cable and com ponent s cam e down, and new connect ors m ade fiber far easier t o inst all and m aint ain. The m edium becam e increasingly popular, and t here was an im pet us t o updat e t he t echnology and support even longer cable runs. The updat ed set of I EEE 802.3 10Mbps opt ical fiber st andards was dubbed 10BASE- F. 10BASE- F includes t hree different configurat ions: • • •

1 0 BASE- FL An upgrade of FOI RL t hat int eroperat es wit h FOI RL. 10BASE- FL support s st at ion- t o- st at ion, st at ion- t o- repeat er, and repeat er- t o- repeat er links. 1 0 BASE- FB A specificat ion for repeat er- t o- repeat er backbone links. 1 0 BASE- FP A specificat ion for a passive opt ical device designed t o int erconnect m ult iple st at ions. I t was not im plem ent ed by vendors and will not be described furt her.

Features of Fiber Optic Links A fiber opt ic link is m ade up of t wo fibers. A separat e fiber is used for each direct ion of t ransm ission. Figure 6.19 shows how t wo devices are connect ed by a link t hat consist s of t wo fibers.

Figu r e 6 .1 9 . Se n d a n d r e ce ive fibe r s.

The carefully layered design of 802.3 int erfaces paid off when t he I EEE int roduced opt ical fiber. Syst em s can be connect ed t o fiber by swapping out a copper m edium t ransceiver and swapping in a fiber opt ic t ransceiver. The upper part of t he Figure 6.19 shows a schem at ic t hat illust rat es flows of dat a across t wo fibers. The lower part shows a st at ion t hat has a NI C wit h a st andard 15pin socket . The st at ion is prepared for fiber opt ic t ransm ission by plugging in an AUI cable t hat connect s t o a fiber opt ic t ransceiver. ( Alt ernat ively, a NI C wit h an int egrat ed fiber opt ic t ransceiver could be inst alled in t he st at ion.) I n t he figure, a pair of fiber cables connect s t he st at ion t o a hub. FOI RL, 10BASE- FL, and 10BASE- FB int erfaces have several feat ures in com m on wit h 10Mbps coax and t wist ed- pair im plem ent at ions. These feat ures include •

•

•

•

The use of Manchest er encoding t o t ransm it 0s and 1s. Manchest er encoding is described in Appendix A. Jabber cont rol—t hat is, t he capabilit y t o cut off an inappropriat e t ransm ission aft er a t im e t hreshold has been exceeded. The use of SQE signals t o announce collisions and j abbers or ot her errors t o t he local syst em . For end st at ions, t he im plem ent at ion of t he SQE t est funct ion—t hat is, an SQE signal t hat announces t he end of each dat a t ransm ission t o t he st at ion.

Som e feat ures are unique t o t he opt ical fiber m edium : •

• •

The use of special idle signals. When it is not t ransm it t ing dat a, an opt ical int erface sends a st eady st ream of special idle signals ont o it s t ransm it fiber. This enables t he int egrit y of each fiber t o be checked very easily because a signal of som e t ype always should be arriving across a fiber t hat is funct ioning properly. The fiber opt ic link int egrit y t est consist s of m onit oring t he cont inuous incom ing signals. A low light - det ect ion funct ion. Any t im e t hat t he light signal arriving on a fiber is t oo weak, t he int erface shut s down it s receive dat a funct ion and sends only idle signals ont o it s t ransm it fiber unt il t he incom ing signal ret urns t o norm al. For 10BASE- FB, t he capabilit y t o send a signal across t he t ransm it fiber t hat indicat es t hat t here is a fault condit ion on t he receive fiber.

N ot e For 10BASE- FB, idles are sent in t he form of special synchronous signals, which m eans t hat t he receiver can m aint ain bit t im ing using t hese signals. A consequence is t hat a 10BASE- FB int erface does not have t o resynchronize it s t im ing every t im e a fram e arrives. This offers t he advant age of reducing t he int erfram e gap shrinkage t hat occurs when fram es pass t hrough a repeat er.

Fiber Optic Inter-Repeater Link The fiber opt ic int er- repeat er link was int roduced t o connect a repeat er t o anot her repeat er or t o a st at ion across a long and reliable link. Figure 6.20 shows t wo FOI RL links t hat span a dist ance of up t o 1000 m et ers bet ween a pair of copper segm ent s.

Figu r e 6 .2 0 . A 1 0 M bps fibe r opt ic int e r - r e pe a t e r lin k .

A repeat er can be convert ed t o an FOI RL repeat er by at t aching an FOI RL t ransceiver t o t he end of an AUI cable. On t he left side of Figure 6.20, an AUI cable and t ransceiver connect each repeat er t o t he opt ical fiber link. Vendors also built FOI RL repeat ers t hat cont ained int egrat ed opt ical fiber t ransceivers. I n t his case, t he fiber opt ic cable is plugged direct ly int o a port on t he repeat er, as is shown on t he right side of Figure 6.20. I n eit her case, t he fiber segm ent can be up t o 1000 m et ers in lengt h.

FOIRL Links in a Collision Domain The m axim um lengt h of a part icular FOI RL link wit hin a collision dom ain depends on t he t opology of t he collision dom ain: •

•

A FOI RL link on a pat h wit h five segm ent s and four repeat ers can be 500 m et ers in lengt h, at m ost . For pat hs wit h four segm ent s and t hree repeat ers ( or less) , a FOI RL link can be any lengt h up t o t he m axim um of 1000 m et ers.

Figure 6.21 shows a valid configurat ion for a pat h ( from st at ion A t o server X) t hat t raverses five segm ent s and four repeat ers ( hubs) . I t includes t hree 500- m et er FOI RL links.

Figu r e 6 .2 1 . FOI RL le n gt h r e st r ict ion for a pa t h w it h five se gm e n t s.

Figure 6.22 shows a valid configurat ion for a pat h ( from st at ion B t o server X) t hat t raverses four segm ent s and t hree repeat ers. I t includes t wo 1000- m et er FOI RL links.

Figu r e 6 .2 2 . 1 0 0 0 - m e t e r FOI RL lin k s for a pa t h w it h fou r se gm e n t s.

10BASE-FL 10BASE- FL is an im proved version of FOI RL t hat • • •

•

I s based on m ore up- t o- dat e connect ors and t ransm ission t echnology. Can be used for st at ion- t o- st at ion links as well as for repeat er- t o- repeat er and repeat er- t o- st at ion links. Support s links t hat are up t o 2000 m et ers in lengt h—double t he FOI RL dist ance. ( An addit ional 25- m et er AUI cable can be used at each end.) I s backward com pat ible. A 10BASE- FL int erface can com m unicat e wit h a FOI RL int erface across a 1000- m et er link bet ween repeat ers.

A 10BASE- FL link bet ween t wo DTEs can operat e in full- duplex m ode. However, Aut o- Negot iat ion is not support ed, and bot h st at ions m ust be m anually configured for full- duplex operat ion. A 10BASE- FL link can be up t o 2000 m et ers long, but t he dist ance m ust be sm aller in a collision dom ain whose longest pat h includes t hree or four repeat ers: •

•

A 10BASE- FL repeat er- t o- repeat er link on a pat h wit h five segm ent s and four repeat ers can be, at m ost , 500 m et ers in lengt h. A 10BASE- FL repeat er- t o- repeat er link on a pat h wit h four segm ent s and t hree repeat ers can be, at m ost , 1000 m et ers in lengt h.

•

A 10BASE- FL repeat er- t o- st at ion link on a pat h wit h four segm ent s and t hree repeat ers can be, at m ost , 400 m et ers in lengt h.

Figure 6.23 shows an exam ple of a LAN whose longest pat h crosses t hree hubs and four segm ent s. Three of t he segm ent s are 10BASE- FL links, and t he fourt h is an FOI RL link. Because t here are t hree repeat ers, t he repeat er- t o- st at ion 10BASE- FL links m ust be lim it ed t o 400 m et ers.

Figu r e 6 .2 3 . 1 0 BASE- FL a n d FOI RL lin k s.

Any of t he FOI RL links in Figure 6.21 and Figure 6.22 can be replaced by a 10BASEFL link of t he sam e lengt h. Figure 6.24 shows a 2000- m et er 10BASE- FL link t hat connect s a pair of t wist ed- pair hubs.

Figu r e 6 .2 4 . A 2 0 0 0 - m e t e r 1 0 BASE- FL lin k .

10BASE-FB As t he " B" in t he nam e suggest s, 10BASE- FB defines an int erface t o a backbone link—t hat is, a 10BASE- FB link is used t o connect a pair of repeat ers ( hubs) . A 10BASE- FB link can be up t o 2000 m et ers long, but t he dist ance m ust be sm aller in a collision dom ain whose longest pat h includes t hree or four repeat ers: •

•

A 10BASE- FB link on a pat h wit h five segm ent s and four repeat ers can be, at m ost , 500 m et ers in lengt h. A 10BASE- FB link on a pat h wit h four segm ent s and t hree repeat ers can be, at m ost , 1000 m et ers in lengt h.

A 10BASE- FB link is not longer t han a 10BASE- FL link, and 10BASE- FB is less versat ile because it cannot be used t o connect a st at ion t o a repeat er. However, 10BASE- FB does have ext ra feat ures t hat are useful in a repeat er- t o- repeat er backbone link. The synchronous idles reduce int erfram e gap shrinkage, and a 10BASE- FB repeat er int erface can send a special signal t o t he repeat er at t he ot her end of t he link t o report fault condit ions.

Summary of Fiber Distance Specifications Som e dist ance lim it at ions for FOI RL, 10BASE- FL, and 10BASE- FB segm ent s are sum m arized in Table 6.4. The t hree m iddle colum ns sum m arize t he lengt h rest rict ions for a collision dom ain: • • • •

The usual 5- 4- 3 rule ( at m ost five segm ent s, four repeat ers, and t hree populat ed segm ent s) holds when fiber opt ic segm ent s are added t o a collision dom ain. The second colum n in Table 6.4 shows t hat a fiber opt ic segm ent can be, at m ost , 500 m et ers in lengt h if t he longest pat h cont aining t he segm ent crosses five segm ent s and four repeat ers. The t hird colum n indicat es t hat t he lengt h can be increased t o 1000 m et ers if t he longest pat h cont aining t he segm ent crosses four segm ent s and t hree repeat ers. The lengt h of a 10BASE- FL segm ent t hat is connect ed t o a DTE is lim it ed t o 400 m et ers if t he longest pat h cont aining t he segm ent crosses four fiber opt ic

segm ent s and t hree repeat ers, and t he lengt h of each repeat er- t o- repeat er link is 1000m . Keep in m ind t hat t hese lim it at ions can be lift ed by using bridges or swit ches t o reduce t he num ber of segm ent s in pat hs across a collision dom ain. Anot her alt ernat ive is t o use an inexpensive m edia convert er, which m akes t wo segm ent s behave like one ext ended segm ent . The advant age of doing t his is t hat a fiber opt ic cable can be added t o a collision dom ain wit hout increasing t he current segm ent and repeat er count .

Ta ble 6 .4 . 1 0 M bps Fibe r Opt ic D ist a n ce s Ca ble

Collision D om a in Re st r ict ion s

M a x im um Le n gt h

Five Segm ent s, Four Repeat ers, Repeat er- t oRepeat er Link

Four Segm ent s, Three Repeat ers, Repeat er- t oRepeat er Link

Four Segm ent s, Three Repeat ers, Repeat er- t o- DTE Link

FOI RL

500m

1000m

–

1000m

10BASEFL

500m

1000m

400m

2000m

10BASEFB

500m

1000m

–

2000m

The Structure of a Fiber As Figure 6.25 shows, an opt ical fiber consist s of a glass core surrounded by anot her layer of glass t hat is called cladding. One or m ore layers of prot ect ive buffer coat ing are wrapped around t he cladding. A j acket is wrapped around t he buffer.

Figu r e 6 .2 5 . St r u ct u r e of a n opt ica l fibe r ca ble .

N ot e

Opt ical fiber cables also can be m ade out of plast ic, but glass is m uch preferred. Alt hough plast ic is cheaper, it current ly support s t ransm ission across only very short cable dist ances. For exam ple, it can be used for 50- t o 100- m et er office connect ions.

Dat a is t ransm it t ed by sending light signals t hrough t he core. The cladding m at erial is st ruct ured t o keep t he light wit hin t he core. More specifically, t he cladding and t he core have different indices of refract ion. The index of refract ion of a m edium affect s t he way t hat light is bent when it st rikes t he m edium . When a ray of light t raversing t he core st rikes t he cladding, t he difference bet ween t he indices causes t he light t o be bent back int o t he core. The t wo com m on t ypes of opt ical fiber t ransm ission syst em s are •

•

Sin gle m ode A laser t ransm it s a focused ray of light int o a cable whose core has an ext rem ely sm all diam et er: 8- 10 m icrom et ers ( m ) . A single- m ode syst em is capable of carrying enorm ous am ount s of dat a across long dist ances, such as t ens or hundreds of kilom et ers. Single- m ode fiber is t he m edium of choice for wide area t elecom m unicat ions net works. I t also is used for gigabit LAN connect ions. M ult im ode A light em it t ing diode ( LED) em it s m ult iple rays of light int o a fiber wit h a larger core, norm ally 62.5m . The rays are called m odes. A m ult im ode syst em can carry dat a across dist ances of up t o 2km . Lasers are expensive, and LEDs are cheap. Mult im ode fiber and LEDs are used for 10Mbps fiber opt ic im plem ent at ions.

N ot e There is a t hird fiber opt ic t ransm ission syst em : A Vert ical Cavit y Surface Em it t ing Laser ( VCSEL) can be used t o t ransm it light signals ont o m ult im ode fiber. VCSELs are less cost ly t han ordinary lasers and are used for gigabit t ransm ission. VCSELs are discussed in Chapt er 9, " Gigabit Et hernet Physical Layer."

Opt ical fiber cables are described by t wo num bers separat ed by a slash. The first num ber ident ifies t he diam et er of t he core, and t he second is t he diam et er of t he cladding. The st andard diam et er for cladding is 125m . The t ypes of fiber used for 10Mbps Et hernet LAN com m unicat ion are shown here: Mult im ode fiber of size 62.5/ 125m is t he t ype recom m ended for 10Mbps Et hernet LANs and is t he m ost popular. The 50/ 125m size som et im es is used. 100/ 140m and 85/ 125= m also are accept able, but t heir use is rare.

N ot e Single- m ode fiber is used for higher- speed LANs. Sizes such as 8.3/ 125m , 9/ 125m , or 10/ 125m are com m only used for single- m ode fiber. The core is t iny; a hum an hair has a diam et er t hat is roughly 80m .

Multimode Transmission A LED em it s m ult iple rays of light int o t he core of a m ult im ode fiber. These rays spread out and bounce off t he cladding of t he m ult im ode cable. This causes som e rays t o follow a longer pat h t hrough t he cable and arrive lat er. When t he dispersion of t he signal becom es t oo great , t he signal is corrupt ed and cannot be int erpret ed correct ly. Figure 6.26 illust rat es t he dispersion of rays t hrough m ult im ode. The upper part of t he figure shows t he older st ep index fiber, which has a core whose index of refract ion is const ant . The lower part of Figure 6.26 shows graded index fiber, whose core index of refract ion varies. I t is highest at t he cent er and sm allest at t he edges. The result is t hat light t ravels fast er near t he edges, allowing t he rays t hat t raverse longer pat hs t o " cat ch up" wit h rays t hrough t he cent er.

Figu r e 6 .2 6 . M u lt im ode st e p in de x a n d gr a de d in de x fibe r .

The wavelengt h of t he light t ends t o vary slight ly depending on t he dist ance from t he cent er. The recom m ended ranges of cent er wavelengt hs are • •

From 800 t o 910 nanom et ers ( nm ) for 10BASE- FB and 10BASE- FL From 790 t o 860nm for FOI RL

Optical Fiber Connectors Current ly, t he connect or of choice is t he square connect or ( SC) . A version of t his connect or is shown in figure 6.27

Figu r e 6 .2 7 . Fibe r opt ic con n e ct or s.

The " bayonet " st raight t ip ( ST) connect ors are recom m ended in t he 10Mbps Et hernet st andards docum ent s. They som et im es st ill are used, and t here is a large inst alled base of t hese older connect ors, which also are shown in figure 6.27

N ot e The ST connect or is form ally called BFOC/ 2.5 in st andards docum ent s. The ST plug connect s in a sim ilar m anner t o a 10BASE2 BNC connect or.

A num ber of vendors have designed connect ors t hat offer som e special advant ages, such as easy cable t erm inat ion and a sm aller connect ion foot print . These are referred t o as sm all form fact or ( SFF) connect ors. At t he t im e of writ ing, a TI A com m it t ee is reviewing a SG connect or, whose clever design m akes it inexpensive t o m anufact ure, easy t o inst all, and less likely t o pull loose accident ally. I n addit ion, t he SG requires half as m uch space as a duplex SC connect or. figure 6.27 also displays a sub m iniat ure t ype A ( SMA) connect or t hat som et im es is used in Et hernet LANs, and t he old m edium int erface connect or ( MI C) t hat was defined for use in FDDI LANs and t hat st ill is occasionally used in t hese LANs.

N ot e The SC connect or was designed by Nippon Telephone and Telegraph ( NTT) . AT&T designed t he ST connect or. 3M and ot her vendors have cont ribut ed t o t he SG connect or design effort .

Summary Points •

A collision dom ain obeys t he 5- 4- 3 rule: Every pat h cont ains at m ost five segm ent s, four repeat ers, and t hree populat ed segm ent s.

For coaxial cable LANs •

• • •

•

• • •

The original baseband Et hernet LAN defined in t he 1980s was built from sect ions of t hick, heavy 50- ohm coaxial cable. Only one signal can be on a cable at any given t im e. St at ions at t ach t o t he m edium via at t achm ent unit int erface ( AUI ) cables and t ransceivers. Transceivers send and receive, send j am s, cont rol j abber, and det ect collisions. SQE signals are used t o report t he com plet ion of a fram e t ransm ission, a collision, or im proper pulses on t he cable. Thin ( 10BASE2) coax is cheaper and easier t o work wit h t han t hick coax. A t hin coax segm ent has a daisy chain st ruct ure. Syst em s connect t o a 10BASE2 segm ent via BNC connect ors.

A 10BASE- T int erface can • • • • • •

Transm it and receive dat a Part icipat e in a hub- based CSMA/ CD collision dom ain Det ect collisions Perform signal qualit y error ( SQE) t est s Cut off excessively long t ransm issions ( j abbers) Test t he link in each direct ion of t ransm ission via link int egrit y t est pulses

An up- t o- dat e 10BASE- T int erface can •

Operat e in full- duplex m ode across st at ion- t o- swit ch and swit ch- t o- swit ch links

•

Support Aut o- Negot iat ion, which enables it t o det ect whet her it s peer support s full- duplex com m unicat ion and enables t he peer t o operat e at a m at ching speed

For fiber opt ic m edia •

•

• •

• •

• •

•

Fiber opt ic int er- repeat er links ( FOI RL) were t he first st andard fiber opt ic links used for 10Mbps Et hernet . 10BASE- FL is an upgrade of FOI RL t hat int eroperat es wit h FOI RL. I t support s st at ion- t o- st at ion, st at ion- t o- repeat er, and repeat er- t o- repeat er links. 10BASE- FB is a specificat ion for repeat er- t o- repeat er backbone links. Fiber int erfaces send idle signals bet ween dat a t ransm issions. For 10BASE- FB, t hese are synchronous, and t he receiver can use t hem t o m aint ain bit clocking. An FOI RL link can be up t o 1000 m et ers in lengt h. 10BASE- FL and 10BASE- FB links can be up t o 2000 m et ers in lengt h. However, t he lengt hs m ust be short er when pat hs in a collision dom ain pass t hrough t hree or m ore repeat ers. An opt ical fiber consist s of a core surrounded by anot her layer called cladding and is wrapped in buffer m at erial and a j acket . A link consist s of t wo fibers: one for each direct ion of t ransm ission. The recom m ended cable for 10Mbps Et hernet fiber links is 62.5/ 125m m ult im ode fiber. The SC connect or current ly is t he m ost popular choice, alt hough t here is a big inst alled base of older ST connect ors.

References A det ailed descript ion of t he Et hernet 10Mbps physical layer can be found in •

I EEE St andard 802.3, 1998 Edit ion. " Carrier Sense Mult iple Access wit h Collision Det ect ion ( CSMA/ CD) Access Met hod and Physical Layer Specificat ions." Chapt ers 1- 20

Lant ronics has published several Et hernet t ut orials online at ht t p: / / www.lant ronics.com / . ST connect ors are described in t he st andards docum ent s: •

•

I EC 60874- 10. " Connect ors for Opt ical Fibres and Cables: Part 10: Sect ional Specificat ion, Fibre Opt ic Connect or Type BFOC/ 2.5." 1992. " TI A Fiber Opt ic Connect or I nt erm at eabilit y St andard 2 ( FOCI S- 2) ." SC com ponent s are described in

•

" TI A Fiber Opt ic Connect or I nt erm at eabilit y St andard 3 ( FOCI S- 3) ."

Opt ical Cable Corporat ion ( ht t p: / / www.occfiber.com / ) has published an excellent set of t echnology whit e papers.

Chapter 7. The Ethernet 100Mbps Physical Layer The j ob of rat chet ing up Et hernet speed t o 100Mbps was t ackled in t he early 1990s. I n 1993, a group of vendors form ed t he Fast Et hernet Alliance and produced a 100Mbps Et hernet specificat ion t hat was backward com pat ible wit h 10BASE- T and 10BASE- F. They subm it t ed t his t o t he I EEE, and t he result ( originally published as 802.3u) was absorbed int o t he 802.3 st andard. Today, Et hernet vendor cooperat ion cont inues under t he um brella of t he Fast Et hernet Consort ium . The Consort ium sponsors int eroperabilit y t est ing t hat is perform ed at t he Universit y of New Ham pshire I nt erOperabilit y Lab ( I OL) ( ht t p: / / www.iol.unh.edu/ consort ium s/ fe/ index.ht m l) . The surprising t hing about Fast Et hernet is how m uch of t he original st ruct ure of Et hernet is ret ained in it s design: • •

The m inim um and m axim um Et hernet fram e sizes are unchanged. The rules for handling collisions are t he sam e.

The differences are confined t o t he physical layer. The m ost significant change caused by increasing t he t ransm ission speed is t he decrease in t he m axim um diam et er of a collision dom ain. The reason for t his is t hat at 100Mbps, an Et hernet fram e is t ransm it t ed in roughly 1/ 10 of t he t im e required t o t ransm it at 10Mbps. To be sure t hat a t ransm it t ing st at ion det ect s a colliding fram e before t he st at ion has sent 64 byt es, t he m axim um dist ance bet ween st at ions also m ust shrink.

N ot e When 100Mbps Et hernet was first considered, t here act ually were t wo com pet ing proposals. The Fast Et hernet Alliance cham pioned a backward- com pat ible version t hat quickly won accept ance from users. Hewlet t - Packard and AT advocat ed a new prot ocol, 100VG- AnyLAN, t hat was int ended t o replace bot h Et hernet and Token Ring. The cont est bet ween t hese proposals held up t he est ablishm ent of a st andard for quit e a while. Finally, t he I EEE accept ed Fast Et hernet as part of 802.3 and published 100VG- AnyLAN as a separat e st andard, 802.12. Despit e t his, 100VG- AnyLAN did not m ake m uch of a dent in t he m arket and is a dying t echnology.

This chapt er deals wit h 802.3 100Mbps Et hernet . For com plet eness, t he 100VGAnyLAN prot ocol is described briefly at t he end of t he chapt er. Fast Et hernet runs on bot h t wist ed- pair copper m edia and fiber opt ic cable. As is shown in Table 7.1, t hree different t wist ed- pair t echnologies and one opt ical fiber t echnology were defined.

Ta ble 7 .1 . 1 0 0 M bps Et h e r n e t Te ch n ologie s Te chnology

Type of Ca ble

St a nda r d M a x im um Ca ble Le n gt h in M e t e r s

M ode of Ope r a t ion

100BASE- TX Two pairs of Cat egory 5 unshielded 100 t wist ed- pair ( UTP) cabling or t wo pairs of I BM Type 1 shielded t wist ed–pair cabling

Half- or fullduplex

100BASE- T4 Four pairs of Cat egory 3 UTP or bet t er

100

Half- duplex only

100BASE- T2 Two pairs of Cat egory 3 UTP or bet t er

100

Half- or fullduplex

100BASE- FX Two m ult im ode opt ical fibers

412 half- duplex, 2000 full- duplex

Half- or fullduplex

The first t wo copper int erfaces, 100BASE- TX and 100BASE- T4, and t he opt ical fiber int erface, 100BASE- FX, have been im plem ent ed as product s. 100BASE- T2, however, never m ade it t o t he m arket place. The 100BASE- FX fiber opt ic int erface allows for longer cable runs t han t he t wist ed- pair im plem ent at ions. The physical layer for 100BASE- FX is based on t he physical layer of a 100Mbps LAN t hat has been around for a long t im e: Fibre Dist ribut ed Dat a I nt erface ( FDDI ) . 100BASE- TX is based on t he physical layer of t he copper version of t his LAN ( CDDI ) . ANSI st andards com m it t ee X3T9.5 defined CDDI t o int roduce t wist ed- pair t ransm ission int o FDDI LANs. I t 's confusing enough t o sort out t he m eaning of four different " 100BASE" t it les, but yet anot her t erm som et im es is used. Because 100BASE- FX and 100BASE- TX have a lot in com m on, t his pair of t echnologies has been dubbed 100BASE- X.

Coexistence and Migration with Auto-Negotiation Vendors underst and t he best way t o m arket Fast Et hernet t wist ed- pair int erfaces. They sell 10/ 100 t wist ed- pair adapt ers t hat perm it an easy m igrat ion from 10Mbps t o 100Mbps Et hernet . An Aut o- Negot iat ion capabilit y was int roduced t o aut om at e t hat m igrat ion. The Aut o- Negot iat ion Prot ocol enables an adapt er t o discover t he highest level of funct ionalit y t hat it shares wit h it s peer at t he ot her end of a t wist edpair link. This is done in t wo ways. I f t he int erfaces at bot h ends of a segm ent are " sm art " — t hat is, capable of perform ing t he Aut o- Negot iat ion Prot ocol—t hen t he peers exchange m essages t o announce t heir capabilit ies. I f only one of t he int erfaces is sm art , it analyzes incom ing signals t o recognize a 10BASE- T, 100BASE- TX, or 100BASE- T4 part ner. Aut o- Negot iat ion is a requirem ent

for 1000BASE- T int erfaces, so link param et ers always can be negot iat ed wit h a part ner whose adapt er support s 1000BASE- T. The 802.3 com m it t ee m ade Aut o- Negot iat ion an opt ional feat ure for 10Mbps and 100Mbps devices. So, alt hough all t he leading vendors provide it , som e product s do not . Aut o- Negot iat ion pays for it self by prevent ing problem s t hat could be very t im econsum ing t o solve, and it cert ainly should be on t he list of essent ial product requirem ent s. 10/ 100Mbps t wist ed- pair NI Cs are not expensive, and m any organizat ions buy t hem in bulk. Then an adm inist rat or inst alls a 10/ 100 card whenever a 10Mbps NI C m ust be added t o a new device or replaced in an old device. This is a good st rat egy even if som e of t he hubs and swit ches t hat are st ill in use are old 10Mbps devices. To upgrade a link originat ing at a st at ion wit h a 10/ 100 NI C t o 100Mbps, you j ust have t o connect t he ot her end of t he cable t o a hub or swit ch port capable of operat ing at 100Mbps.Aut o- Negot iat ion part ners will select t he highest speed support ed at bot h ends.

100Mbps Ethernet on Twisted-Pair Cabling During t he 100Mbps Et hernet developm ent effort , t hree separat e t eam s of engineers pursued different t echnical approaches t o pushing 100Mbps across t wist ed–pair cabling. As a result , t hree dist inct t wist ed- pair physical specificat ions were writ t en: • •

•

100BASE- TX—This version was m arket ed first and is by far t he m ost popular. I t runs across t wo pairs of wires, eit her Cat egory 5 UTP cable or I BM Type 1 shielded t wist ed–pair cable. 100BASE- T4—This version runs across four Cat egory 3 ( or bet t er) t wist ed pairs. Alt hough far less popular t han t he TX version, it is useful for environm ent s t hat do not have eit her Cat egory 5 or shielded t wist ed- pair cabling. I t has som e short com ings, however: The worst is t hat it does not support full- duplex operat ion. 100BASE- T2—The goal of t his version was com plet e backward com pat ibilit y wit h 10Base- T—nam ely, operat ion at 100Mbps across t wo pairs of Cat egory 3 ( or bet t er) cable. 100BASE- T2 t ook a long t im e t o develop. I n t he m eant im e, 100BASE- TX becam e well ent renched. A st rong and com pet it ive m arket for 100BASE- TX brought prices down rapidly. 100BASE- T2 required com plex engineering t hat would have m ade product s quit e cost ly. As a result , when t he 100BASE- T2 was com plet ed, vendors did not im plem ent t he t echnology. However, it s m echanism s used for it s physical layer becam e t he basis for gigabit t wist ed- pair t ransm ission.

All t hese versions were designed t o operat e across up t o 100 m et ers of cable.

100BASE-TX The 100BASE- TX physical layer has a lot in com m on wit h 10BASE- T: •

I t runs across t wo t wist ed pairs.

• •

•

• •

A segm ent lengt h of up t o 100 m et ers is support ed. I t s t wist ed- pair cables are t erm inat ed by RJ- 45 connect ors, and pin usage is t he sam e for 10BASE- T and 100BASE- TX. Transm it wires at one end of t he link becom e receive wires at t he ot her end in t he usual crossover fashion. Dat a is t ransm it t ed on one cable pair and received on t he ot her pair. As was t he case for 10BASE- T, pins 1 and 2 are used t o t ransm it , and pins 3 and 6 are used t o receive. Full duplex com m unicat ion can be est ablished bet ween a st at ion and a swit ch ( or on a direct link bet ween t wo st at ions) .

Som e im port ant differences also exist bet ween 100BASE- TX and 10BASE- T: •

• •

100BASE- TX requires Cat egory 5 unshielded t wist ed- pair ( UTP) cable or I BM Type 1 150- ohm shielded t wist ed- pair cable. A different m et hod is used t o encode dat a ont o t he wire. Link pulses are sent across t he link only during init ializat ion. ( These pulses are used t o t ransm it aut oconfigurat ion inform at ion.) During norm al operat ion, a st eady st ream of special idle sym bols is t ransm it t ed bet ween fram es.

N ot e I dle sym bols are used as int erfram e filler for all 100Mbps and 1000Mbps t echnologies t hat are physically capable of full- duplex t ransm ission. Transm it t ing idles bet ween fram es keeps receive clocks in synch wit h send clocks at all t im es. A link problem can be det ect ed im m ediat ely because it will result in garbled or weak idle signals.

Use of CD D I M e ch a n ism s Vendors were able t o rush 100BASE- TX t o m arket very quickly because it s physical layer was based on t he exist ing CDDI st andard. This enabled Fast Et hernet vendors t o reuse readily available com ponent s when building t heir product s.

N ot e Support for shielded t wist ed- pair cabling was t hrown int o t he 100BASE- TX st andard because it already had been im plem ent ed for CDDI LANs.

For CDDI ( and FDDI ) , groups of 4 bit s are encoded ont o a wire using an encoding schem e called 4B/ 5B. This encoding schem e also was used for 100BASE- FX. The 4B/ 5B schem e m aps 4- bit nibbles t o 5- bit pat t erns of 0s and 1s t hat are t ransm it t ed across t he wire. Appendix A, " Physical Layer Encodings for Et hernet , Fibre Channel, FDDI , and Token Ring," has t he full det ails. One of t he advant ages of t his schem e is t hat t here are ext ra 5- bit pat t erns t hat can be useda—lone or com bined in pairs—as special cont rol codes. Three im port ant pat t erns are

• • •

I dle ( 1 1 1 1 1 ) —The idle pat t ern is sent cont inuously bet ween fram es. St a r t - of- st r e a m de lim it e r ( 1 1 0 0 0 1 0 0 0 1 ) —A new fram e is int roduced by t he st art - of- st ream delim it er. En d- of- st r e a m de lim it e r ( 0 1 1 0 1 0 0 1 1 1 ) —An end- of- st ream delim it er is sent at t he end of a fram e.

This use of t hese cont rol codes int roduces som e significant differences bet ween t he 100BASE- TX and 10BASE- T physical- layer prot ocols. Unlike 10BASE- T, t he following is t rue of 10BASE- TX: •

• •

There is no need t o t est t he int egrit y of a link using norm al link pulses. The cont inuous idle pat t erns serve t his purpose. The first byt e of a fram e's pream ble is replaced by a st art - of- fram e delim it er. An end- of- st ream delim it er is t ransm it t ed at t he end of a fram e.

100BASE-T4 The only m erit of 100BASE- T4 is t hat it runs over Cat egory 3 t wist ed–pair cabling. There st ill are pocket s of Cat egory 3, alt hough m ost sit es have been rewired wit h Cat egory 5. As was t he case for 100BASE- TX, t he following is t rue of 100BASE- T4: • •

A segm ent lengt h of 100 m et ers is support ed. The t wist ed- pair cables are t erm inat ed by RJ- 45 connect ors.

However, t here also are several differences bet ween 100BASE- T4 and 100BASE- TX: • • •

•

Four t wist ed pairs are used. Full- duplex t ransm ission is not support ed. Groups of bit s are encoded ont o t he wires using an encoding schem e called 8B/ 6T. Appendix A describes 8B/ 6T encoding. An int erface t ransm it s link int egrit y pulses across one of t he wire pairs when it is not sending dat a.

Four t wist ed pairs m ight sound like a handful, but t he cable cont aining t hem st ill can be t erm inat ed by RJ- 45 plugs. Now, however, all eight pins are used inst ead of t he four pins t hat are used for 10BASE- T and 100BASE- TX. The t ot al 100Mbps capacit y is split int o t hree 33 1/ 3Mbps part s. Just as was t he case for 10BASE- T, one wire in a pair carries t he norm al t ransm ission, while t he ot her wire carries a copy t hat has been invert ed. The way t hat t hree of t he four pairs are select ed t o t ransm it dat a or receive dat a looks a lit t le st range at first , but it act ually m akes very good sense. Figure 7.1 provides an idea of how it works. Pair 2 always act s as a receive pair, and it also cont rols what is going on. A syst em list ens on pair 2 t o det erm ine whet her it s part ner is t ransm it t ing.

Figu r e 7 .1 . 1 0 0 BASE- T4 t r a n sm ission .

I f t he local syst em is not sending and pream ble bit s st art t o arrive on pair 2, pairs 3 and 4 are recruit ed t o act as addit ional receive pairs for t he incom ing dat a. I f t he local syst em has dat a t o t ransm it , it list ens t o pair 2 t o check t hat t he m edium is free. The syst em t hen t ransm it s dat a across pairs 1, 3, and 4. The syst em cont inues t o list en on pair 2 t o det ect a collision. I f pream ble bit s arrive on pair 2 while t he syst em is t ransm it t ing, a collision is occurring and j am m ing bit s are sent out of t he ot her pairs. Not e t he special role of pairs 1 and 2. Pair 1 can only t ransm it , and pair 2 can only receive. By list ening t o pair 2, t he syst em finds out whet her t he m edium is free so t hat it can t ransm it . I f so, pairs 3 and 4 j oin pair 1 as send pairs. When dat a is arriving on pair 2, however, pairs 3 and 4 act as receive pairs. Now you can see why full- duplex operat ion does not work for 100BASE- T4. Pairs 3 and 4 can be set t o work as send pairs, or t hey can be set t o work as receive pairs. There is no way t hat t hey can send and receive at t he sam e t im e. As was t he case for 10BASE- T, wires need t o be crossed so t hat a send wire becom es a receive wire at t he ot her end. The crossover is perform ed wit hin a hub or swit ch port , or occasionally by a crossover cable. Figure 7.2 shows t he cross- connect ions bet ween t he pins at each end. Not e t hat t he pin num bers are not in sequent ial order. The pin order in Figure 7.2 is t he one t hat norm ally is displayed because it produces a t idy diagram .

Figu r e 7 .2 . 1 0 0 BASE- T4 cr oss- con n e ct ion s.

Special st art - of- st ream and end- of- st ream sym bol pat t erns are sent across pairs 1, 3, and 4 t o m ark t he beginning and end of a fram e. However, idle sym bols are not used. During an idle period, an int erface sends link int egrit y pulses out of TX_D1 and receives t hem on RX_D2. During init ializat ion, special Aut o- Negot iat ion link pulses are sent and received on t he sam e pairs.

100BASE-T2 100BASE- T2 will be discussed only briefly because it has not been im plem ent ed in product s. 100BASE- T2 differed from 100BASE- TX in t he fact t hat it was designed t o run over Cat egory 3 UTP and in it s com plex encoding schem e ( which is sket ched in Appendix A) . I t s im plem ent at ion required cost ly digit al signal processors and ot her special elect ronic com ponent s, in cont rast wit h t he cheap off- t he- shelf com ponent s t hat were used for 100BASE- TX. Funct ionally, 100BASE- T2 has som e feat ures in com m on wit h 100- BASE- TX: • • • •

A segm ent lengt h of up t o 100 m et ers is support ed. The Cat egory 3 or bet t er UTP cables are t erm inat ed by RJ- 45 connect ors. Dat a is t ransm it t ed on one cable pair and received on t he ot her pair. As was t he case for 10BASE- T, pins 1 and 2 are used t o t ransm it , and pins 3 and 6 are used t o receive. Full- duplex com m unicat ion can be est ablished bet ween a st at ion and a swit ch.

100BASE-FX and FDDI A 100Mbps opt ical fiber im plem ent at ion was produced very quickly by borrowing t he physical layer from t he FDDI physical layer defined by ANSI st andards com m it t ee X3T9.5. The im port ant charact erist ics of 100BASE- FX are list ed here: •

•

I t runs across a pair of opt ical fibers. 62.5/ 125 m icron m ult im ode fiber is specified in t he 802.3 st andard. A segm ent lengt h of up t o 412 m et ers is support ed for half- duplex com m unicat ion. Up t o 2000 m et ers are support ed across a full- duplex link.

• • •

The fiber opt ic cables are t erm inat ed by t he sam e SC or ST connect ors t hat were m ent ioned in Chapt er 6, " The Et hernet 10Mpbs Physical Layer" ( see Figure 6.27) . Dat a is t ransm it t ed ont o one fiber and received on t he ot her. Full- duplex com m unicat ion can be est ablished bet ween DTEs ( t hat is, a st at ion and a swit ch, a pair of st at ions, a pair of swit ches, a swit ch and a rout er, and so on) .

As was not ed earlier, bot h 100BASE- TX and 100BASE- FX use t he 4B/ 5B encoding schem e defined by ANSI for t he signals t hat are act ually sent across t he wire. Just as was t he case for 100BASE- TX, t hree im port ant special pat t erns are used wit h 100BASE- FX: • • •

I dle ( 11111) —The idle pat t ern is sent cont inuously bet ween fram es. St art - of- st ream delim it er ( 11000 10001) —A new fram e is int roduced by t he st art - of- st ream delim it er. End- of- st ream delim it er ( 01101 00111) —An end- of- st ream delim it er is sent at t he end of a fram e.

Appendix A cont ains a det ailed descript ion of t he physical encoding t hat is used for 100BASE- FX.

Media Independent Interface An adm inist rat or who is preparing a st at ion t o part icipat e in a Fast Et hernet LAN m ust inst all an appropriat e net work adapt er: •

•

•

•

100BASE- T4 cabling 100BASE- TX cabling 100BASE- FX inst alled 100BASE- FX inst alled

wit h an RJ- 45 socket for four pairs of Cat egory 3 t wist ed- pair wit h an RJ- 45 socket for t wo pairs of Cat egory 5 t wist ed- pair wit h an ST socket if opt ical cables wit h ST plugs have been wit h an SC socket if opt ical cables wit h SC plugs have been

Given t he num ber of m oves and changes t hat occur in a t ypical organizat ion, an adm inist rat or could be kept pret t y busy pulling adapt ers out of syst em s and inst alling new ones t hat m at ch t he cabling at a new locat ion. For exam ple, a user wit h a high- powered st at ion t hat has been com m unicat ing via a 100BASE- TX connect ion m ight m ove t o a new office and discover t hat t his office is m ore t han 100 m et ers away from t he nearest 100Mbps hub or swit ch and t hat t he office has been wired wit h opt ical fiber. One way t o be ready for sit uat ions like t his is t o inst all NI Cs wit h a m edia independent int erface ( MI I ) in syst em s t hat are likely t o be m oved and t hen t o buy hubs and swit ches t hat have som e MI I port s. When a syst em has a MI I int erface, get t ing ready for 100BASE- T4, 100BASE- TX, or 100BASE- FX t ransm ission is easy: You j ust get t he appropriat e t ype of t ransceiver and plug it int o t he MI I int erface.

Physically, t he MI I int erface is a fem ale 40- pin D connect or, as shown in Figure 7.3. Som e t ransceivers have a corresponding 40- pin m ale connect or at one end t hat plugs direct ly int o t he MI I . For ot hers, t he 40- pin m ale connect or is locat ed at t he end of a short cable ( up t o half a m et er in lengt h) , as shown in Figure 7.3. The desired physical m edium at t achm ent is at t he ot her end of t he t ransceiver.

Figu r e 7 .3 . A m e dia in de pe n de n t in t e r fa ce ( M I I ) .

A MI I connect or provides flexibilit y and is useful t o m eet special needs. However, ext ernal t ransceivers are expensive, and t his is not a solut ion t hat norm ally is used for a large num ber of NI Cs.

N ot e Anot her good solut ion exist s for sit es t hat have a m ixt ure of Cat egory 5 t wist ed- pair and fiber opt ic cabling: 100BASE- TX t o 100BASE- FX m edia convert ers can be deployed t o exact ly t he syst em s t hat need t hem .

100Mbps Collision Domain Diameter The sm allest Et hernet fram e is 64 byt es ( 512 bit s) in lengt h. To m ake sure t hat every st at ion in a collision dom ain becom es aware of a collision, t he round- t rip t im e bet ween t he t wo st at ions t hat are fart hest apart m ust be less t han 512 bit t im es. At 100Mbps, a round t rip of 512 bit t im es t ranslat es t o .00000512 seconds. This is not m uch t im e! The speed of light ( c) is 299,792,458 m et ers per second. The speed of elect rons t hrough a cable is bet ween .6c and .9c for m ost cable t ypes. I n addit ion t o t im e spent t raversing cables, som e t im e is spent processing bit s as t hey ent er and leave

each DTE and int erm ediat e repeat er. As a result , you can't go very far in 512 bit t im es. The result is t hat a 100BASE- T collision dom ain has a sm all diam et er. Figure 7.4 illust rat es t he com ponent s of t he end- t o- end propagat ion delay for a bit sent from one st at ion t o anot her. The delay is t he sum of t he t im es required t o

Figu r e 7 .4 . Com pon e n t s of 1 0 0 M bps t r a n sit de la ys.

• • • • • • •

Transm it a signal t hrough t he DTE int erface at st at ion A Traverse cable 1 Receive a signal t hrough t he incom ing hub int erface Process t he signal Transm it a signal t hrough t he out going hub int erface Traverse cable 2 Receive a signal t hrough t he DTE int erface at server X

Repeater (Hub) Classes The 100Mbps t im e const raint s are so t ight t hat t he t im e t hat it t akes t o process signals wit hin a hub m at t ers quit e a lot . I n fact , t wo different classes of hubs have been defined based on different processing t im e requirem ent s:

•

•

Class I —This t ype of hub is designed t o support a m ixt ure of 100BASE- T4 and 100BASE- X ( t hat is, 100BASE- TX or FX) int erface t ypes. Because t he signal encodings for 100BASE- T4 and 100BASE- X port s are very different , each incom ing signal first m ust be convert ed t o a bit pat t ern and t hen m ust be recoded int o an out going signal. This int roduces a delay int o t he forwarding process. The result is t hat norm ally only one Class I repeat er can be used in a 100Mbps collision dom ain. Class I I —All hub int erfaces are of t he sam e t ype ( eit her all 100BASE- T4 or all 100BASE- X) . Thus, an incom ing signal can be repeat ed direct ly out of t he ot her port s. The hub forwarding t im e is fast enough t o allow t wo Class I I hubs t o be used in a 100Mbps collision dom ain ( alt hough t he benefit t urns out t o be m arginal) .

Thus, Class I I hubs are used when t he incom ing and out going signals are encoded in t he sam e way. That is, a Class I I hub can be used in eit her of t hese cases: • •

Every hub int erface is of t ype 100BASE- TX or of t ype 100BASE- FX. Every hub int erface is of t ype 100BASE- T4.

I f t wo st at ions are at t ached t o a Class I hub by 100- m et er copper cable segm ent s, t he t im e budget is exhaust ed. There isn't enough t im e left t o t raverse anot her hub. I f all your st at ions are connect ed t o a hub by short er cables, you could com put e t he com ponent delays on your longest pat h t o see whet her a second Class I hub could be added. However, t his would be risky. I f som eone lat er at t aches a new st at ion using a longer cable, your ent ire collision dom ain would be dest abilized. Most adm inist rat ors follow t he conservat ive course and st ick t o a single Class I hub.

N ot e Before plugging a hub int o a LAN, it is im port ant t o know which class it is. The I EEE requires vendors t o m ark a hub wit h a Rom an num eral I or I I surrounded by a circle.

Collision Domain Configurations The figures t hat follow show dist ance rest rict ions for 100Mbps collision dom ains. The num bers shown for t wist ed- pair copper cable are based on t he use of Cat egory 5 cable. Figure 7.5 illust rat es t he m axim um dist ances for a DTE- t o- DTE connect ion, such as a link bet ween a pair of swit ches, a pair of rout ers, or a st at ion and a swit ch. Not e t hat a crossover cable needs t o be used for a DTE- t o- DTE link.

Figu r e 7 .5 . D TE- t o- D TE h a lf- du ple x con n e ct ion s.

Figure 7.6 shows t he m axim um dist ances bet ween DTEs for pat hs t hat pass t hrough a Class I hub. Not e t hat t he dist ances depend on t he com binat ions of m edia t hat are used.

Figu r e 7 .6 . D ist a n ce s for a Cla ss I h u b.

Figure 7.7 shows t he m axim um dist ances bet ween DTEs for pat hs t hat pass t hrough one Class I I hub. The fiber cable lengt hs are bigger because it t akes less t im e t o t raverse t he hub.

Figu r e 7 .7 . D ist a n ce s for on e Cla ss I I h u b.

Figure 7.8 shows t he m axim um dist ances bet ween DTEs for pat hs t hat pass t hrough t wo Class I I hubs. The diam et er of t he collision dom ain diam et er act ually is decreased by t he presence of t he second hub. I t m akes m ore sense t o use a st ack of hubs t hat behave like a single hub.

Figu r e 7 .8 . D ist a n ce s for t w o Cla ss I I h u bs.

Computing a Special Collision Domain Configuration Table 7.2 cont ains t he I EEE 802.3 t abulat ion of round- t rip delay t im es t hat can be used t o direct ly calculat e m axim um collision dom ain sizes. For m ore precise values, t he I EEE num bers could be replaced by num bers obt ained from your specific cable, adapt er, and hub vendors.

Ta ble 7 .2 . Com pone nt s of Round Tr ip D e la y Com pon e nt

Roun d- Tr ip Ca ble D e la y in Bit Tim e s Pe r M e t e r

M a x im um N u m be r of Bit Tim e s

Tim e t o exit / ent er t wo TX/ FX N/ A DTEs

100

Tim e t o exit / ent er t wo T4 DTEs

N/ A

138

Tim e t o exit / ent er one T4

N/ A

127

Ta ble 7 .2 . Com pone nt s of Round Tr ip D e la y Com pon e nt

Roun d- Tr ip Ca ble D e la y in Bit Tim e s Pe r M e t e r

M a x im um N u m be r of Bit Tim e s

and one TX/ FX DTE Cat egory 3 or Cat egory 4 segm ent

1.14

114 ( for 100 m et ers)

Cat egory 5 or STP segm ent

1.112

111.2 ( for 100 m et ers)

Fiber opt ic segm ent

1.0

412 ( for 412 m et ers)

Class I hub

N/ A

140

Class I I TX/ FX hub

N/ A

92

Class I I T4 hub

N/ A

67

To illust rat e how Table 7.2 is used, you can verify t he value used for t he fiber opt ic cable in t he configurat ion at t he bot t om of Figure 7.7, which shows t he m axim um diam et er of a collision dom ain t hat cont ains bot h 100BASE- TX Cat egory 5 copper links and fiber opt ic links connect ed t o a Class 2 hub. Assum ing t hat 100- m et er copper links will be used, t he DTE, copper, and hub delays are: Ent er and leave t wo TX/ FX DTEs 100 bit t im es Round t rip on 100m Cat egory 5 cable 111.2 bit t im es Ent er and leave Class I I hub t wice 92 bit t im es Tot al 303.2 bit t im es Tim e rem aining out of 512 bit t im es for t he fiber opt ic link 208.8 bit t im es The round- t rip t im e for fiber opt ic cable is 1 bit t im e per m et er, so you could use a 208.8- m et er fiber opt ic cable. I t act ually is desirable t o use a fiber opt ic cable t hat is 1–5 m et ers less t han 208.8 t o keep t he round- t rip t im e com fort ably under 512 bit t im es.

Curing the Diameter Problem with Switches The advent of cheap swit ches t akes t he st ing out of t he collision dom ain rest rict ions. You can m ake a LAN span a large area by connect ing m any sm all collision dom ains wit h swit ches.

I n t he LAN in Figure 7.9, all t he deskt op syst em s are connect ed t o a hub by runs of 60 m et ers or less. A long 250- m et er fiber opt ic run has been connect ed t o a server t hat is far from t he wiring closet .

Figu r e 7 .9 . A spe cia l con figu r a t ion .

The round- t rip bit t im es for each com ponent are shown in t he figure. The t wo DTEs account for 100 bit t im es, t he Class I I hub uses up 92 during t he round t rip, t he t wist ed- pair cable account s for 66.72, and t he fiber opt ic cable account s for 250. The t ot al is 508.72, which is wit hin t he 512 bit - t im e budget . However, anot her LAN adm inist rat or m ight innocent ly plug in a new deskt op st at ion using a 100- m et er t wist ed- pair cable run. Missed collisions would lead t o som e very st range error sym pt om s, and t he problem could be difficult t o t rack down. This sit uat ion illust rat es t he following: • • •

Swit ches can be a good invest m ent . I t is a good idea t o st ick t o t he safe, conservat ive cable- lengt h lim it s. I t is very im port ant t o docum ent t he st ruct ure of a LAN carefully, especially if you violat e t he usual lim it s.

100VG-AnyLAN 100VG- AnyLAN is a t echnology t hat was proposed as an alt ernat ive t o 100Mbps Et hernet . 100VG- AnyLAN did not succeed in gat hering sufficient vendor or cust om er support t o m ake it int o a serious cont ender. Today, even Hewlet t - Packard, it s st aunchest support er, offers a m eager scat t ering of VG product s t hat are alm ost lost

wit hin HP's list of Et hernet offerings. The m arket has cast a deciding ballot for Et hernet . However, it is easy t o see why t he I EEE grant ed 100VG- AnyLAN st andards st at us, publishing it as I EEE 802.12. This is an int erest ing t echnology, and it is out lined briefly in t he sect ions t hat follow.

100VG-AnyLAN Topology The 100VG- AnyLAN t opology is a t ree of repeat ers t hat can be nest ed five levels deep. The repeat er at t he t op of t he t ree is called t he root repeat er. A t hree- level LAN is shown in Figure 7.10. I n a given 100VG- AnyLAN, all fram es conform t o eit her an Et hernet form at or a Token Ring form at .

Figu r e 7 .1 0 . Ca sca din g 1 0 0 VG An yLAN r e pe a t e r s.

The reasoning behind offering a choice of fram e form at s was t o m ake it easy t o creat e 100VG- AnyLAN islands wit hin an Et hernet shop or wit hin a Token Ring shop, evolving gradually t o a pure 100VG- AnyLAN t echnology. A 100VG- AnyLAN can be connect ed t o adj acent Et hernet s or Token Rings via bridges. Figure 7.10 includes a bridge t o Et hernet .

Cables and Connectors for 100VG AnyLAN Several m edia are support ed for repeat er- t o- DTE links and repeat er- t o- repeat er links. Table 7.3 list s t he m edia t hat can be used and t he lengt h lim it at ions for each t ype of segm ent . RG- 45 plugs and socket s are used wit h t he t wist ed- pair m edia. SC connect ors are used wit h fiber opt ic m edia.

Ta ble 7 .3 . M e dia Type s a n d Se gm e n t Le n gt h s for 1 0 0 VGAn yLAN M e diu m Type

Se gm e n t Le n gt h

Four pairs of 100 m et ers Cat egory 3 or 4 UTP Four pairs of Cat egory 5 UTP

200 m et ers

Two pairs of STP

100 m et ers

One pair of m ult im ode fibers

500 m et ers wit h an 800- nanom et er t ransceiver 2000 m et ers wit h a 1300- nanom et er t ransceiver

The diam et er of a 100VG- AnyLAN can be subst ant ial. However, delays caused by t raversing m ult iple repeat ers shrink t he diam et er. Figure 7.11 shows t he largest LANs t hat can be built using t wo, t hree, four, or five levels of repeat ers. The diam et ers range from a m axim um of 4 kilom et ers wit h t wo repeat ers, t o 1 kilom et er wit h five repeat ers. The reason t hat t hese large diam et ers can be at t ained is t hat collisions do not occur for t his t echnology.

Figu r e 7 .1 1 . M a x im u m VG- An yLAN dia m e t e r s for e a ch le ve l of r e pe a t e r s.

Initialization and Port Types I t is easiest t o underst and how a 100VG- AnyLAN operat es by st art ing off wit h a LAN t hat consist s of a set of DTEs connect ed t o a single repeat er, as shown in Figure 7.12.

Figu r e 7 .1 2 . A sin gle r e pe a t e r 1 0 0 VG- An yLAN .

Two end- user workst at ions, a server, a net work m anagem ent st at ion, and a net work m onit or are at t ached t o t he repeat er. St at ion A in t he figure has j ust been plugged in and has st art ed an init ializat ion process. I nit ializat ion consist s of t he exchange of a sequence of special t raining fram es. St at ion A st art s t he process by announcing t he following: • • • •

What it s MAC address is Whet her it is a st at ion or a next - level repeat er Whet her it wishes t o receive unicast fram es addressed t o it s own MAC address ( privacy m ode) , or wishes t o receive copies of all unicast fram es ( prom iscuous m ode) Whet her it uses an Et hernet or Token Ring fram e form at

The upst ream repeat er responds wit h indicat ions of t he following: • •

•

• •

Whet her t he request ed configurat ion is com pat ible wit h t he net work Whet her t he node has been accept ed as a st at ion or a repeat er, or has been refused access Whet her t he node will receive only broadcast s, m ult icast s, and unicast fram es addressed t o it s own MAC address, or will receive copies of all fram es Whet her t he node's MAC address is a duplicat e address Whet her an Et hernet or Token Ring fram e form at m ust be used

Not e t hat t he repeat er learns t he MAC address of each connect ed syst em during t his init ializat ion ( " t raining" ) process. The norm al expect at ion is t hat end- user syst em s will operat e in privacy m ode. I n fact , it is prudent for an adm inist rat or t o configure each repeat er port so t hat t his will be t he case. However, in Figure 7.12, exact ly one

special port has been set up in prom iscuous m ode so t hat operat ions st aff can m onit or t he net work. A VG- AnyLAN repeat er perform s t he following: •

•

Forwards a broadcast or m ult icast fram e t hrough all port s ( except for t he one on which it arrived) Forwards a unicast fram e t hrough t he unique port t hat leads t o it s dest inat ion MAC address, as well as t hrough any prom iscuous port s

The Medium Access Protocol for One Repeater The m edium access prot ocol is called dem and priorit y. This m eans t hat a syst em needs t o ask it s repeat er for perm ission t o send a fram e and cannot t ransm it unt il t hat perm ission is grant ed. Dem and priorit y support s t wo priorit y levels: norm al and high. High- priorit y fram es are sent before norm al- priorit y fram es. Referring t o Figure 7.12, t he repeat er perform s t hese t asks: • •

Polls each downlink node in round- robin order t o find out whet her it has a fram e t hat needs t o be t ransm it t ed, and t o det erm ine whet her t he priorit y is norm al or high Grant s perm ission t o each high- priorit y request before grant ing perm ission t o send norm al- priorit y fram es

N ot e To prevent lockout , norm al fram es are prom ot ed t o high- priorit y st at us aft er a t im eout period so t hat every fram e will get it s chance t o be sent .

All syst em s are polled at least once per fram e t ransm ission, so t he st ack of request s is kept up- t o- dat e.

The Medium Access Protocol for Two Levels of Repeaters The way t hat polling is carried out in a t ree of repeat ers is designed so t hat t he LAN will behave like one big repeat er. Take t he t wo- level t ree in Figure 7.13 for exam ple. I f a request is m ade by DTE 7, which is connect ed t o a lower- level repeat er 2, t he repeat er signals a request t o t he root . The root will pass cont rol t o t he lower- level repeat er when it is t im e t o grant t he port connect ing t o repeat er 2 port perm ission t o send.

Figu r e 7 .1 3 . Pollin g a n d pe r m ission s in a t r e e of r e pe a t e r s.

For exam ple, if norm al request s are m ade by DTEs 1, 7, 8, and 9, t he root repeat er will receive t he request s via port s 1, 2, and 5. 1. The root grant s perm ission t o DTE 1 t o send a fram e. 2. Moving t o port 2, t he root delegat es aut horit y t o repeat er 2. Repeat er 2 can now schedule a full round- robin cycle of t ransm issions. I t grant s perm ission t o send t o DTE 7 and DTE 8 in t urn. 3. Next , t he root delegat es aut horit y t o repeat er 5, which allows DTE 9 t o send a fram e. 4. The end result is t hat fram es have been sent by DTEs 1, 7, 8, and 9, in t hat order. The process works t he sam e way for a deeper t ree. When a node t ransm it s a fram e, t he fram e is forwarded t o every repeat er in t he LAN. For exam ple, when DTE 7 t ransm it s a fram e, repeat er 2 forwards it t o t he root , which forwards it t o t he ot her repeat ers. A broadcast or m ult icast fram e will be delivered t hrough every DTE port . A repeat er t hat recognizes t hat a unicast dest inat ion MAC address belongs t o one of it s DTE port s forwards t he fram e t hrough t hat port . The fram e also is sent t o any prom iscuous DTE connect ed t o t he repeat er.

Summary Points • • •

•

•

Fast Et hernet runs on bot h t wist ed- pair and fiber opt ic cable. 100- m et er t wist ed- pair links are support ed for Fast Et hernet . 100BASE- FX and 100BASE- TX have a lot in com m on, and t he pair of t echnologies has been called 100BASE- X. 100BASE- TX runs on t wo pairs of Cat egory 5 t wist ed- pair cabling and support s full- duplex operat ion. 100BASE- T4 runs on four pairs of Cat egory 3 t wist ed- pair cabling and does not support full- duplex operat ion. Dat a is t ransm it t ed on t hree pairs, and t he fourt h is used t o sense collisions.

• • •

• • •

•

•

•

•

100BASE- FX runs on t wo opt ical fibers and support s full- duplex operat ion. 100BASE- TX and 100BASE- FX t ransm ission are based on FDDI and CDDI t ransm ission. All t hese t echnologies use 4B/ 5B encoding. The goal of 100BASE- T2 was backward com pat ibilit y wit h 10Base- T—nam ely, operat ion at 100Mbps across t wo pairs of Cat egory 3 cable. The t echnology was not im plem ent ed in product s. I dle sym bols are used as int erfram e filler for 100BASE- TX and 100BASE- FX. I t is possible t o plug a 100BASE- T4, 100BASE- TX, or 100BASE- FX t ransceiver int o a m edia independent int erface ( MI I ) . The diam et er of a Fast Et hernet collision dom ain ranges from 200 m et ers t o 320 m et ers, depending on t he m edia and t he class of hub t hat is used. A Class I hub support s a m ixt ure of 100BASE- T4 and 100BASE- X links. Only a single Class I hub m ay be used in a collision dom ain. All t he port s on a Class I I hub m ust be of t he sam e t ype. Two Class I I hubs m ay be used in a collision dom ain. When Fast Et hernet init ially was proposed, t here was a com pet ing t echnology called 100VG- AnyLAN. 100VG- AnyLAN is lit t le used t oday. The 100VG- AnyLAN t opology is a t ree of repeat ers t hat can be nest ed five levels deep.

References 100Mbps Et hernet is described in •

I EEE St andard 802.3, 1998 Edit ion. " Carrier Sense Mult iple Access wit h Collision Det ect ion ( CSMA/ CD) Access Met hod and Physical Layer Specificat ions." See Chapt ers 21- 30.

100VG- AnyLAN is described in •

I EEE St andard 802.12, 1998 edit ion. " Dem and- Priorit y Access Met hod, Physical Layer and Repeat er Specificat ions."

Chapter 8. Gigabit Ethernet Architecture 100Mbps Et hernet becam e an I EEE st andard in m id- 1995. By t he end of 1995, a group of vendors had form ed t he Gigabit Et hernet Consort ium and had st art ed work on 1000Mbps Et hernet . They worked at a rem arkable pacewit hin 13 m ont hs, a specificat ion t hat described gigabit t ransm ission across m ult im ode and single- m ode opt ical fibers and special shielded copper cables had been writ t en and approved by t he I EEE 802.3 com m it t ee. One piece was m issing, t hough: a specificat ion for Cat egory 5 t wist ed- pair cabling. This was published in 1999.

N ot e At t he t im e of writ ing, work is proceeding on t he next st ep: 10Gbps Et hernet .

The bat t le bet ween t aking a backward- com pat ible approach versus using a new t echnology ended when users em braced Fast Et hernet and rej ect ed 100VG- AnyLAN. Engineers working on t he Gigabit Et hernet proj ect were det erm ined t o m ake 1000Mbps Et hernet as com pat ible wit h t he earlier versions as was possible. The full- duplex version of Gigabit Et hernet is t ot ally com pat ible wit h t he 10Mbps and 100Mbps versions. However, it was im possible t o build CSMA/ CD ( half- duplex) Gigabit Et hernet t hat was com plet ely backward com pat ible. Som e changes were needed, and a lot of effort was invest ed in ext ending t he CSMA/ CD prot ocol so t hat it could work at gigabit speed. This effort m ight have been wast ed: At t he t im e of writ ing no vendors offer half- duplex Gigabit Et hernet ; all act ual product s are designed for full- duplex operat ion. There are good reasons for t his: • •

•

Ext ension byt es need t o be added t o short fram es t o m ake CSMA/ CD gigabit feasible. Because short fram es norm ally m ake up a significant port ion of LAN t raffic, t hese em pt y byt es wast e bandwidt h and reduce t hroughput . Even wit h fram e ext ension, t he diam et er of a gigabit collision dom ain m ust be quit e sm all t o enable all st at ions t o det ect collisions. Gigabit int erfaces are cost ly. I t m akes sense t o use t hem in a way t hat support s t he full bandwidt h and allows for long cable runs.

Norm ally, a full- duplex LAN is const ruct ed by at t aching st at ions t o swit ches. However, gigabit swit ches are expensive. Vendors want ed t o offer an addit ional, less cost ly alt ernat ive. This alt ernat ive is a new net work device called a full- duplex repeat er or buffered dist ribut or. Full- duplex repeat ers are described lat er in t his chapt er. Cust om ers who buy gigabit product s are looking for high perform ance, and som e vendors have im plem ent ed a Jum bo fram e feat ure t hat im proves t hroughput . The size of a Jum bo MAC fram e ranges up t o 9018 byt es. Jum bo fram es are nonst andard, and t here is no sign t hat t he I EEE 802.3 com m it t ee is ready t o em brace t hem as part of t he Et hernet st andard anyt im e soon. However, t he m arket place will m ake t he final decision on whet her Jum bos are a good idea. This chapt er concent rat es on Gigabit Et hernet archit ect ure and prot ocols. Full- duplex Gigabit Et hernet is discussed first . Then, for com plet eness, t he adapt at ions t hat were needed t o im plem ent half- duplex ( CSMA/ CD) are described. Chapt er 9, " Gigabit Et hernet Physical Layer," deals wit h gigabit physical im plem ent at ion issues; Chapt er 10, " Twist ed- Pair Cabling St andards and Perform ance Requirem ent s," describes t he perform ance t est s required t o check cabling syst em s t hat will be used for gigabit t ransm ission; and Appendix A, " Physical Layer Encodings for Et hernet , Fibre Channel, FDDI , and Token Ring," out lines t he way t hat dat a is encoded before it is t ransm it t ed.

Gigabit Configurations

Figure 8.1 depict s an environm ent t hat benefit s from a gigabit link. The servers in t he figure are linked t o 100Mbps swit ches. The swit ches also int erconnect com m unit ies of workst at ions. Each 100Mbps swit ch has one gigabit port , and a gigabit link bet ween t he 100Mbps swit ches provides am ple bandwidt h for client / server and server/ server com m unicat ions.

Figu r e 8 .1 . Con n e ct in g 1 0 0 M bps sw it ch e s w it h a giga bit lin k .

I n Figure 8.2, gigabit NI Cs have been inst alled in a set of high- perform ance servers t hat are connect ed t o a 100/ 1000Mbps swit ch. Gigabit NI Cs also have been inst alled in client workst at ions t hat belong t o a workgroup perform ing net work- int ensive graphics work. These workst at ions share t he bandwidt h provided by a full- duplex ( buffered) repeat er. A set of 10Mbps swit ches also connect s t o t he cent ral 100/ 1000Mbps swit ch via 100Mbps uplinks.

Figu r e 8 .2 . Sw it ch a n d fu ll- du ple x r e pe a t e r con n e ct ion s.

Full-Duplex Gigabit Ethernet Full- duplex Gigabit Et hernet is used t oday. I t is ent irely nat ural t o use t he full- duplex Et hernet prot ocol because, down at t he physical level, bot h t he fiber opt ic and copper gigabit im plem ent at ions t ransm it signals in bot h direct ions concurrent ly. Chapt er 9 and Appendix A discuss how t his is done. Full- duplex Et hernet was described in Chapt er 5, " Full- Duplex Et hernet Com m unicat ion." There is not hing com plicat ed about it : •

•

CSMA/ CD is not used. Full- duplex m ode is used on links for which t here are no collisions. A syst em can send a fram e whenever it want s, except for t he fact t hat fram es m ust be separat ed by an int erfram e gap and PAUSE request s m ust be honored.

Full- duplex com m unicat ion at 1000Mbps is no different from full- duplex com m unicat ion at 10Mbps or 100Mbps, except for t he possible use of nonst andard Jum bo fram es. Table 8.1. present s t he param et ers t hat are relevant t o full- duplex operat ion.

Ta ble 8 .1 . Fu ll- D u ple x Et h e r n e t Pa r a m e t e r s Pa r a m e t e r

1 0 M bps

1 0 0 M bps

1 0 0 0 M bps

I nt erFram eGap

9.6µs ( 96 bit t im es)

.96µs ( 96 bit t im es)

.096µs ( 96 bit t im es)

m axFram eSize ( wit hout VLAN header)

1518 byt es

1518 byt es

1518 byt es( 9018, if Jum bo fram es are support ed)

m inFram eSize

64 byt es

64 byt es

64 byt es

Full-Duplex Repeaters The device called a full- duplex repeat er or buffered dist ribut or was designed t o overcom e t he defect s of an ordinary hub. A full- duplex repeat er connect s t o full- duplex links. Like a swit ch, it can t em porarily st ore incom ing fram es in buffer m em ory. Unlike a swit ch, however, it does not have a filt ering t able or t he high t hroughput t hat result s from t ransm it t ing independent st ream s of fram es in parallel. A full- duplex repeat er perform s t he following t asks: •

• •

•

•

•

Forwards each incom ing fram e t hrough all port s ot her t han t he one on which it arrived, j ust as a hub would. Connect s t o each syst em via a full- duplex link. Queues incom ing fram es and schedules t hem for t ransm ission in a first - in first - out ( FI FO) m anner. Shares 1000Mbps full- duplex bandwidt h ( a t ot al of 2000Mbps) am ong all t he connect ed devices. Connect s t o syst em s by links whose lengt h is lim it ed by m axim um cable dist ances rat her t han by collision dom ain dist ances. Transm it s PAUSE fram es t o im pose flow cont rol on endpoint st at ions whose input queue allocat ion is alm ost exhaust ed. This prevent s an overflow sit uat ion t hat would cause fram es t o be discarded.

The repeat er can use flow cont rol for it s at t ached devices, but t he devices are not allowed t o send PAUSE fram es t o t he repeat er.

N ot e At t he t im e of writ ing, gigabit NI Cs which require lot s of sophist icat ed elect ronics, are quit e expensive. Adding t he m em ory and processing capabilit y required t o build a full- duplex repeat er ( inst ead of a hub) is cheap when com pared t o t he cost of t he int erfaces. Because m any endpoint syst em s cannot operat e at speeds anywhere near t he gigabit level, t he need t o share 2Gbps bandwidt h oft en is not a serious det rim ent .

Figure 8.3 provides a rough idea of how a full- duplex repeat er works. The det ails of how input and out put queues are m anaged and fram e t ransm issions are scheduled are up t o t he vendor.

Figu r e 8 .3 . A giga bit - pe r - se con d fu ll- du ple x r e pe a t e r .

I n t he figure, each capit al let t er represent s a fram e. The alphabet ic t ags correspond t o t he order in which t he fram es arrive at t he repeat er. I n t he upper port ion of t he figure, t he input queue for st at ion 1 is alm ost full. The repeat er is sending st at ion 1 a flow cont rol m essage t o pause input for a while. The lower part of t he figure represent s t he sam e full- duplex repeat er a short t im e lat er. Every incom ing fram e m ust be queued t o all port s except for t he one on which it arrived. Fram e A current ly is in t ransit t o st at ions 2, 3, and 4, while fram e B is queued up t o be t ransm it t ed t o st at ions 1, 2, and 3. Not e t hat st at ion 4 is t ransm it t ing fram e G t o t he repeat er at t he sam e t im e t hat it is receiving fram e A.

Jumbo Frames Lot s of full- sized fram es are t ransm it t ed during a file t ransfer, Web page download, or ot her bulk dat a t ransfer operat ion. The m axim um MAC fram e size is 1518 byt es.

The pream ble, st art fram e delim it er, and int erfram e gap add 20 byt es of prot ocol overhead. Hence, a t ot al of 12,304 bit t im es is consum ed by each m axim um size fram e. Up t o 81,274 m axim um size fram es can be t ransm it t ed across a gigabit link in a second. Even m ore can be sent when t here is a m ix of fram e sizes. A processing overhead is associat ed wit h handling each fram e, and m any com put ers sim ply cannot keep up wit h t he num ber of fram es t hat can arrive across a gigabit link. Several vendors have decided t o solve t his problem by int roducing non- st andard 9018- byt e Jum bo fram es. The inform at ion field in a Jum bo fram e cont ains 9000 byt es, t he sam e num ber of payload byt es as six m axim um - sized ordinary Et hernet fram es. The use of Jum bo fram es cut s down on t he num ber of sm all fram es as well as t he num ber of big fram es. This is because m any of t he sm all fram es are sent t o acknowledge incom ing dat a. An acknowledgem ent of a Jum bo t akes t he place of six acknowledgem ent s for ordinary fram es. Jum bo fram es are not really unusually large. Bigger fram es are used on 16Mbps Token Rings and across ATM net works.

Effe ct of Ju m bos on Th r ou gh pu t Jum bos significant ly reduce t he processing load on com put ers t hat t ransm it and receive fram es across a gigabit link. This reduct ion in processing overhead can result in a large increase in t hroughput . Syst em s sim ply are capable of sending and receiving m ore dat a when t hey have fewer fram es t o process. Also, a sm all—but st ill significant —im provem ent in t hroughput result s from t he reduct ion in overhead bandwidt h when Jum bo fram es are used. A rough calculat ion provides an idea of it s order of m agnit ude. •

•

Adding t he 18 fram e header and t railer byt es t o t he 20 prot ocol overhead byt es yields a t ot al of 38 byt es ( 304 bit s) of overhead per fram e. For a st eady st ream of 1518- byt e fram es, t his overhead adds up t o alm ost 3,100,000 byt es per second. The overhead is reduced t o less t han 526,000 byt es per second for a st ream of Jum bo fram es. This difference can be used t o carry ext ra payload.

D r a w ba ck s a n d Be n e fit s of Ju m bo Fr a m e s The I EEE 802.3 com m it t ee has held back on endorsing Jum bos because of fears of int eroperabilit y problem s. For exam ple, Jum bo fram es cannot be used wit h a buffered repeat er unless every repeat er port and every NI C connect ed t o t he repeat er support t he feat ure. One applicat ion t hat benefit s great ly from Jum bo fram es is t he file access service provided by t he Net work File Server ( NFS) prot ocol. NFS client and server soft ware is bundled wit h UNI X com put ers and is available for PCs. A client com put er on a LAN

can " m ount " a direct ory locat ed at a NFS file server int o it s own direct ory syst em and t hen can access files in t he direct ory as if t hey are local. Dat a read from a NFS server is t ransm it t ed in 8,192- byt e blocks. The TCP/ I P User Dat a Prot ocol ( UDP) oft en is used t o carry t hese blocks, which m ust be fragm ent ed int o six pieces before t hey can be sent in convent ional Et hernet fram es. Fragm ent at ion and reassem bly of t he blocks im poses a significant burden on bot h client syst em s and file servers. No fragm ent at ion is needed when Jum bo fram es are used, and t his cont ribut es t o fast er and m ore efficient file server perform ance.

Half-Duplex Gigabit Ethernet Backward com pat ibilit y broke down when t he 802.3 com m it t ee wrot e t he rules for gigabit hubs operat ing in a CSMA/ CD environm ent . Som e Et hernet prot ocol elem ent s had t o be changed t o accom m odat e half- duplex Gigabit Et hernet . Half- duplex Gigabit Et hernet is not used at t he t im e of writ ing, but a descript ion of t he prot ocol elem ent s is included for com plet eness. The discussion will uncover anot her reason t hat m ot ivat ed vendors t o im plem ent only full- duplex Gigabit Et hernet —nam ely, t he sm all diam et er of a gigabit collision dom ain.

Modification to Frame Format for Half-Duplex Mode Chapt er 4, " The Classical Et hernet CSMA/ CD MAC Prot ocol," present ed one of t he changes t hat was needed t o im plem ent CSMA/ CD ( half- duplex m ode) at gigabit speed: t he addit ion of ext ension bit s t o short fram es. A 64- byt e ( 512- bit ) fram e is on a gigabit m edium for far t oo short a t im e. Collisions of unext ended 64- byt e fram es cannot be det ect ed in a gigabit collision dom ain. Ext ension byt es need t o be added t o sm all fram es t o keep a MAC fram e t ransm ission on t he m edium for 4,096 bit t im es ( t he t im e t o t ransm it 512 byt es) . Figure 8.4 shows a MAC fram e t hat has been ext ended.

Figu r e 8 .4 . A fr a m e w it h a ca r r ie r e x t e n sion t h a t w ou ld be u se d for h a lf- du ple x Giga bit Et h e r n e t .

N ot e The ext ension byt es are represent ed on t he physical m edium by special nondat a sym bols. These special sym bols enable a receiver t o det ect exact ly where a fram e ends and an ext ension field begins. Appendix A explains how ext ension sym bols are encoded.

Frame Bursting for Half-Duplex Operation Anot her adapt at ion called fram e burst ing or burst m ode was int roduced t o reduce t he wast e caused by t ransm it t ing a lot of fram e ext ensions and, hence, t o raise t he t hroughput across a gigabit collision dom ain. As t he nam e suggest s, burst m ode enables a syst em t o t ransm it m ult iple fram es in sequence aft er it has capt ured t he m edium . Figure 8.5 illust rat es why t his capabilit y was viewed as desirable. The figure shows a 64- byt e fram e t hat has t he required 448 byt es of carrier ext ension. Toget her, t he fram e and it s ext ension occupy 512 byt es. Adding an init ial 8- byt e pream ble and st art - of- fram e delim it er and a 12- byt e- t im e int erfram e gap brings t he t ot al up t o 532 byt es. Only 12 percent of t he ut ilized bandwidt h is occupied by t he 64- byt e fram e.

Figu r e 8 .5 . Ove r h e a d for a 6 4 - byt e fr a m e .

I f you add t o t his t he fact s t hat sm all fram es are com m onplace and t hat collisions bring down t he overall bandwidt h t hat is available on a half- duplex gigabit LAN, it s price/ perform ance loses all at t ract ion. Clearly som et hing had t o be done. Burst m ode was designed t o im prove half- duplex perform ance. Figure 8.6 shows how burst m ode works:

Figu r e 8 .6 . Fr a m e bu r st in g in a h a lf- du ple x Giga bit Et h e r n e t LAN .

1. A st at ion wit h several fram es t o send t ransm it s t he first fram e. Ext ension bit s are appended if t he fram e is less t han 512 byt es long. 2. The st at ion cont inues t o send ext ension bit s during t he int erfram e gap t im e. Ot her st at ions recognize t hat t he m edium has not been released and do not at t em pt t o t ransm it . 3. The st at ion sends t he next fram e. Even if t he fram e is short , it does not need t o be ext ended. The m edium already has been held successfully for enough t im e t o assure t hat a collision will not occur. 4. I f t he st at ion st ill has m ore fram es t o send, it again fills t he int erfram e gap wit h ext ension bit s.

5. The st at ion is allowed t o init iat e fresh fram es unt il t he burst lim it expires. The burst lim it is 8,192 byt es ( 65,536 bit s) . Not e t hat an act ual t ransm ission can be longer t han t he burst lim it because t he sender is allowed t o com plet e a fram e t hat it st art ed t o t ransm it when t he burst lim it had not yet been exhaust ed. Even wit h t he added boost provided by fram e burst ing, half- duplex hubs cannot provide t hroughput t hat can com pet e wit h t he full- duplex repeat ers described earlier.

1000Mbps Half-Duplex Hubs An ordinary half- duplex hub could be used t o int erconnect a set of gigabit syst em s int o a collision dom ain in which t he overall 1000Mbps bandwidt h is shared am ong t he syst em s. Alt hough t hese hubs are m yt hical devices at t he t im e of writ ing, for com plet eness, t his sect ion discusses t he feat ures and lim it at ions of a half- duplex hub im plem ent at ion. Tight t im ing on collision det ect ion squeezed down t he diam et er of 100Mbps t wist edpair collision dom ains. The rest rict ions on t he diam et er are even m ore severe at a gigabit per second. At m ost , a single hub can be used t o connect gigabit devices. Table 8.2. displays t he I EEE recom m ended size for t he longest pat h in a gigabit collision dom ain for various t ypes of m edia. Keeping cable lengt hs wit hin t he conservat ive lim it s t hat are shown will assure t hat collisions can be det ect ed for a m inim um 512- byt e t ransm ission. Not e t hat t he diam et er of a collision dom ain is quit e sm all even when opt ical fiber is used.

N ot e The 25- m et er shielded copper cables m ent ioned in Table 8.2 t ypically are used t o int erconnect syst em s locat ed in a com put er room —for exam ple, t o link a server t o a swit ch or connect a pair of swit ches t o one anot her.

Ta ble 8 .2 . Giga bit Collision D om a in D ia m e t e r s in M e t e r s Configu r a t ion

One Hub

Bot h UTP

Bot h Sh ie lde d Coppe r

200( 100+ 100) 50( 25+ 25)

Bot h Opt ica l Fibe r 220

M ix e d UTP a n d Fibe r

210( 100+ 110)

M ix e d Sh ie lde d Coppe r a n d Fibe r 220 ( 25+ 195)

The est im at ed delay for each net work com ponent is shown in Table 8.3. Not e t hat it requires 1,840 bit - t im es sim ply t o process incom ing and out going bit s at DTE and repeat er int erfaces. Also, a round t rip across a 100- m et er t wist ed- pair segm ent fiber

t akes roughly t he sam e am ount of t im e as a round t rip across a 110- m et er fiber opt ic segm ent .

Ta ble 8 .3 . Giga bit N e t w or k Com pon e n t D e la ys N e t w or k Com pon e nt

Roun d- Tr ip D e la y in Bit Tim e s Pe r M e t e r of Ca ble

M a x im um Roun d- Tr ip D e la y in Bit Tim e s

Two DTEs

N/ A

864

Cat egory 5 UTP Cable Segm ent

11.12

1112 ( 100m )

Special Shielded Copper Cable

10.10

253 ( 25m )

Fiber Opt ic Cable Segm ent

10.10

1111 ( 110m )

Repeat er

N/ A

976

Now t hat t he special feat ures of t he Gigabit Et hernet prot ocol have been described, it is t im e t o exam ine t he way t hat t he physical layer was adapt ed t o 1000Mbps speed. Chapt er 9 and Appendix A invest igat e t he physical layer in dept h.

Summary Points •

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•

• •

•

The full- duplex version of Gigabit Et hernet is t ot ally com pat ible wit h t he 10Mbps and 100Mbps versions. For full- duplex operat ion, CSMA/ CD is not used, and a syst em can send a fram e whenever it want s, except for t he fact t hat fram es m ust be separat ed by an int erfram e gap and PAUSE request s m ust be honored. St at ions connect t o a device called a full- duplex repeat er or buffered dist ribut or via full- duplex links. The st at ions share t he gigabit bandwidt h. Nonst andard 9018- byt e Jum bo fram es im prove perform ance by reducing t he num ber of fram es t hat need t o be sent, which reduces a st at ion's processing load. The half- duplex version of Gigabit Et hernet requires m odificat ions t o t he prot ocol. At t he t im e of writ ing, t here are no half- duplex product s. For half- duplex operat ion, ext ension byt es m ust be added t o short fram es, bringing t heir lengt h t o 512 byt es. Ext ension byt es are t ransm it t ed as special nondat a sym bols. For half- duplex operat ion, burst m ode enables a syst em t o t ransm it m ult iple fram es in sequence when it has capt ured t he m edium . Only one hub could be used in a half- duplex Gigabit Et hernet collision dom ain.

References A lot of useful inform at ion is available at t he Gigabit Et hernet Consort ium Web sit e , at ht t p: / / www.gigabit - et hernet .org/ .

Gigabit Et hernet st andards include • •

I EEE St andard 802.3, 1998 Edit ion. " Carrier Sense Mult iple Access wit h Collision Det ect ion ( CSMA/ CD) Access Met hod and Physical Layer Specificat ions." See Chapt ers 34, 36, 41, and 42. I EEE St andard 802.3ab, 1999. " Supplem ent t o Carrier Sense Mult iple Access wit h Collision Det ect ion ( CSMA/ CD) Access Met hod and Physical Layer Specificat ions—Physical Layer Param et ers and Specificat ions for 1000Mbps Operat ion Over 4- Pair of Cat egory 5 Balanced Copper Cabling, Type 1000BASE- T."

Chapter 9. Gigabit Ethernet Physical Layer Four st andardized physical im plem ent at ions of Gigabit Et hernet exist : • • •

•

1 0 0 0 BASE- SX— A fiber opt ic im plem ent at ion for a pair of m ult im ode fibers. SX corresponds t o short wavelengt h. Lasers t ransm it t ing light at a wavelengt h of 850 nanom et ers ( nm ) are used. 1 0 0 0 BASE- LX— A fiber opt ic im plem ent at ion for a pair of single- m ode or m ult im ode fibers. LX corresponds t o long wavelengt h. Lasers t ransm it t ing light at a wavelengt h of 1300nm are used. 1 0 0 0 BASE- CX— A short shielded copper cable used in wiring closet s and equipm ent room s. CX corresponds t o copper 1 0 0 0 BASE- T— An unshielded t wist ed- pair im plem ent at ion t hat operat es across four pairs of Cat egory 5 t wist ed- pair cabling.

The first t hree were developed concurrent ly and, as a group, are called 1000BASE- X. They were based on exist ing fibre channel t ransm ission t echnology and share a com m on m et hod of encoding sym bols ont o a m edium . 1000BASE- T, t he t wist ed- pair version of Gigabit Et hernet , ut ilizes a different and highly com plex encoding m et hod, and it required a new t ransm ission t echnology. The 1000BASE- T im plem ent at ion includes sophist icat ed elect ronics. Som e vendors have ext ended t he 1000BASE- LX single- m ode fiber cable lengt h by int roducing ( current ly) nonst andard t ransceivers. The im plem ent at ions are dubbed 1000BASE- LH. LH corresponds t o long haul.

Features of Gigabit Ethernet There are som e charact erist ics t hat all of t he gigabit im plem ent at ions share in com m on: •

On init ializat ion or reset , a gigabit int erface perform s an Aut o- Negot iat ion procedure wit h it s link part ner t o est ablish t he ground rules for com m unicat ion.

•

Aft er init ializat ion, at t he physical level, signals are t ransm it t ed concurrent ly in bot h direct ions on a cont inuous basis.

These feat ures are described in t he sect ions t hat follow.

Auto-Negotiation All t he gigabit t echnologies support t he exchange of link param et ers using an Aut oNegot iat ion Prot ocol. The following is t rue of Aut o- Negot iat ion: • • •

I t confirm s t hat bot h ends of t he link can operat e at 1000Mbps. I t enables each int erface t o announce whet her it wishes t o operat e in fullduplex or half- duplex ( CSMA/ CD) m ode. Full- duplex m ode is used when it is select ed by bot h part ies. For full- duplex links, it est ablishes whet her PAUSE flow- cont rol fram es will be used and whet her flow cont rol will be sym m et ric or asym m et ric.

N ot e For t wist ed- pair cabling, Aut o- Negot iat ion also could be used t o est ablish a different speed of operat ion. At t he t im e of writ ing, gigabit NI Cs are significant ly m ore expensive t han 100Mbps int erface cards. The result is t hat t here appears t o be no dem and for gigabit - capable adapt ers t hat support m ult iple speeds. However, t his could change as prices com e down or if 10Gbps adapt ers are int roduced.

Chapt er 11, " Aut o- Negot iat ion," has all t he det ails on how Aut o- Negot iat ion works.

Bidirectional Gigabit Transmission All of t he current Gigabit Et hernet t echnologies are inherent ly full- duplex. At t he physical level, signals are t ransm it t ed in bot h direct ions on a cont inuous basis. An int erface sends idle codes when it is not t ransm it t ing a fram e. I f a gigabit link were configured as a half–duplex link, an int erface would have t o be inhibit ed from sending a fram e at t he sam e t im e t hat it was receiving one. The fact t hat t he link act ually is physically full- duplex undoubt edly cont ribut ed t o t he fact t hat vendors decided t o im plem ent only full- duplex Gigabit Et hernet . The t op port ion of See figure 9.1 indicat es how 1000BASE- SX or LX t ransm ission operat es. A separat e fiber is used in each direct ion. The m iddle part of figure 9.1 illust rat es a 1000BASE- CX im plem ent at ion. A separat e pair is used in each direct ion. Fram es or special idle sym bols are sent across each fiber or wire cont inuously.

Figu r e 9 .1 . Giga bit t r a n sm ission a r ch it e ct u r e .

The bot t om part of t he figure represent s 1000BASE- T physical t ransm ission. Four t wist ed pairs are used. Transm ission is bidirect ional across each pair. Here again, t ransm ission is cont inuous wit h idle sym bols being sent bet ween fram es.

Physical Characteristics of Gigabit Ethernet Gigabit Et hernet has been im plem ent ed on m ult im ode fiber opt ic cable, single- m ode fiber opt ic cable, short shielded cables, and unshielded t wist ed- pair ( UTP) cable. See Tabel 9.1 list s t he cable lengt h ranges for a variet y of gigabit t ransm ission opt ions. Not e t hat all t he short wavelengt h ( SX) im plem ent at ions and all but one of t he long wavelengt h ( LX) im plem ent at ions, run on m ult im ode fiber. One LX im plem ent at ion and all LH im plem ent at ions require single- m ode fiber. The reason t hat t here are several m ult im ode SX and LX ent ries in t he t able is t hat t he lengt h of t he m ult im ode cable t hat can be used depends on a cable param et er called t he m odal bandwidt h. The m odal bandwidt h affect s t he capabilit y of a part icular t ype of m ult im ode cable t o t ransm it inform at ion across a given dist ance. Form ally, t he m odal bandwidt h is defined as t he worst - case 3- decibel bandwidt h t hat will be achieved by a part icular t ype of cable.

The m odal bandwidt h is report ed in unit s of MHz ÷ km . To com put e t he bandwidt h in MHz for a given lengt h of cable, t he m odal bandwidt h m ust divided by t he cable lengt h ( in km unit s) . For exam ple, t he bandwidt h in MHz of a 500- m et er ( .5km ) cable wit h m odal bandwidt h equal t o 500 is: ( 500/ .5) = 1000MHz.

N ot e Modal dispersion is t he m ain fact or t hat det erm ines t he m odal bandwidt h. Modal dispersion result s from t he fact t hat m any rays of light are em it t ed int o t he relat ively big core of a m ult im ode fiber. These rays bounce off t he cladding of t he cable. Som e of t he rays follow a longer pat h t hrough t he cable and arrive lat er t han ot hers—t hat is, t heir arrivals are dispersed. Figure 6.26 A significant am ount of m odal dispersion int erferes wit h t he receiver's capabilit y t o int erpret incom ing signals correct ly. The am ount of dispersion t hat will occur depends on t he st ruct ure of t he cable, t he way light is inj ect ed, and t he lengt h of t he cable.

The ent ry for 1000BASE- LH in See Table 9.1 indicat es t hat t here is a very broad range of lengt hs for t his nonst andard opt ion. Som e 1000BASE- LH t ransceivers are sim ply high- qualit y 1000BASE- LX t ransceivers whose vendor has guarant eed t hat t ransm ission will rem ain reliable across a dist ance of 10km . Ot hers are propriet ary variat ions of 1000BASE- LX t hat support very long cables. At t he t im e of writ ing, t here are 1000BASE- LH t ransceivers t hat operat e across up t o 100km of single- m ode cable. Som e of t he lasers t hat are used for 1000BASE- LH t ransm it light at wavelengt hs t hat are longer t han t he usual 1300nm LX wavelengt h.

Ta ble 9 .1 . Giga bit Et h e r n e t Opt ion s a nd Ca ble Le n gt h s Tr a n sce ive r Type

Ca ble Type

D ia m e t e r of Fibe r Cor e in M icr om e t e r s (mm)

Fibe r M oda l Ba n dw idt h ( MHz x km )

Ca ble Ra n ge in Meters

1000BASE- CX

Special shielded balanced copper cable

N/ A

N/ A

0.1–25

1000BASE- SX

MMF

62.5

160

2–220

1000BASE- SX

MMF

62.5

200

2–275

1000BASE- SX

MMF

50

400

2–500

1000BASE- SX

MMF

50

500

2–550

1000BASE- LX

MMF

62.5

500

2–550

1000BASE- LX

MMF

50

400

2–550

Ta ble 9 .1 . Giga bit Et h e r n e t Opt ion s a nd Ca ble Le n gt h s Tr a n sce ive r Type

D ia m e t e r of Fibe r Cor e in M icr om e t e r s (mm)

Ca ble Type

Fibe r M oda l Ba n dw idt h ( MHz x km )

Ca ble Ra n ge in Meters

1000BASE- LX

MMF

50

500

2–550

1000BASE- LX

SMF

10

N/ A

2–5000

1000BASE- LH

SMF

9

N/ A

1000– 100,000

1000BASE- T

4 UTP

N/ A

N/ A

100

MMF= m ult im ode fiber, SMF= single- m ode fiber, 1000BASE- LH is nonst andard

1000BASE-X Technology As was t he case for 100Mbps Et hernet , an exist ing t ransm ission t echnology was reused t o speed t he developm ent of Gigabit Et hernet . The encoding and t he t ransm ission m et hods already in use for fibre channel t echnology were adopt ed for t he t hree 1000BASE- X im plem ent at ions ( 100BASE- LX, 100BASE- SX, and 100BASECX) . This m ade it possible t o reuse exist ing physical com ponent s and helped vendors roll out product s quickly. The sect ions t hat follow discuss t he encoding m et hod, t ransm ission m echanism s, and connect ors for t he t hree 1000BASE- X im plem ent at ions. 1000BASE- T has very different feat ures and will be described at t he end of t he chapt er.

8B/10B Encoding For 1000BASE- SX, 1000BASE- LX, and 1000BASE- CX, each byt e in a fram e is m apped t o a 10- bit code prior t o t ransm ission. The t ranslat ion is called 8B/ 10B encoding.

N ot e Not e t hat because of 8B/ 10B encoding, 1,250,000,000 bit s per second m ust be t ransm it t ed t o send 1000Mbps of dat a.

The 10- bit codes are called code- groups. Not e t hat t here are only 256 8- bit pat t erns, while t here are 1024 10- bit pat t erns. Only a subset of t he full num ber of 10- bit codes needs t o be used t o represent dat a byt es. This subset has been select ed carefully.

Several reasons exist for perform ing t he t ranslat ion and using code- groups: • •

• •

The code- group pat t erns t hat have been select ed t o represent byt es cont ain a well- dist ribut ed m ixt ure of 0s and 1s. This helps t he receiver keep it s bit clocking in synch. A balanced m ixt ure of 0s and 1s im proves t he elect rical behavior of copper cables and prevent s opt ical cable lasers from overheat ing. I f one or m ore bit errors occur during dat a t ransm ission, it is likely t hat a valid 10- bit pat t ern will be changed int o a 10- bit pat t ern t hat does not represent a dat a byt e. Thus, error det ect ion is enhanced. There are m any ext ra 10- bit pat t erns. This m akes it possible t o define codegroup com binat ions t hat represent idle signals and fram e ext ension byt es, and m ark t he beginning and end of a fram e.

Det ails on t he exact m apping of byt es t o 10- bit code- groups are present ed in Appendix A, " Physical Layer Encodings for Et hernet , Fibre Channel, FDDI , and Token Ring."

1000BASE-SX and LX Transmission: Lasers and VCSELs Gigabit Et hernet runs on bot h m ult im ode and single- m ode opt ical fiber. Som e differences exist bet ween t he fiber opt ic t ransceivers used for Gigabit Et hernet and t he ones t hat were used for 10Mbps and 100Mbps Et hernet . Alt hough lasers were used for t ransm ission across long- dist ance 10Mbps and 100Mbps Et hernet single- m ode links, less expensive light em it t ing diodes ( LEDs) were used for 10Mbps and 100Mbps t ransm ission across short er m ult im ode fiber lines. However, LEDs cannot operat e at 1000Mbps. At gigabit speed, lasers are required for m ult im ode fiber as well as single- m ode fiber. Fort unat ely, t here is a t ype of laser at hand t hat is quit e inexpensive and can be used wit h m ult im ode fiber: t he Vert ical Cavit y Surface Em it t ing Laser ( VCSEL) . VCSEL em it t ers t ransm it t ing light whose wavelengt h is 850nm can support gigabit t ransm ission across cables t hat are hundreds of m et ers in lengt h. 850nm laser t ransceivers are used in t he short wavelengt h ( SX) im plem ent at ions.

N ot e The SX wavelengt h act ually varies bet w een 770nm and 860nm , but t he product s nonet heless are called 850nm t ransceivers.

The m ore cost ly lasers used for dat a t ransm ission across single- m ode fiber are operat ed at a wavelengt h of 1300nm for t he long wavelengt h ( LX) im plem ent at ion. LX lasers are used wit h bot h m ult im ode and single- m ode cables. However, even wit h long wavelengt h lasers, m ult im ode cable lengt hs are rest rict ed t o at m ost 550 m et ers. I n cont rast , 1000BASE- LX single- m ode cables can be t housands of m et ers in lengt h.

N ot e The LX wavelengt h act ually varies bet ween 1270nm and 1355nm . Even longer wavelengt hs are being used for som e 1000BASE- LH im plem ent at ions.

LX t ransm ission across m ult im ode fiber requires t he help of a special m odecondit ioning pat ch cord. This t ype of pat ch cord is described in t he sect ion " ModeCondit ioning Pat ch Cords," aft er t he discussion of fiber opt ic connect ors t hat follows.

Connectors for 1000BASE-SX, 1000BASE-LX, and 1000BASE-LH The recom m ended plug for 1000BASE- SX and 1000BASE- LX is a duplex SC connect or, depict ed in See figure9.2 I t is used for bot h m ult im ode and single- m ode fiber opt ic gigabit inst allat ions. This connect or also is used for 1000BASE- LH.

Figu r e 9 .2 . A du ple x SC fibe r opt ic con n e ct or .

As shown in See figure 9.2, one side of t he connect or has t wo flanges t hat m at ch not ches in t he out let boxes.

N ot e As was not ed in Chapt er 6, " The Et hernet 10Mbps Physical Layer," vendors have designed ot her connect ors t hat are easy t o at t ach t o cables and have a sm all connect ion foot print . Work is in progress on a st andard SG connect or.

M ode - Con dit ion in g Pa t ch Cor ds A problem arises when an LX laser source is aim ed at t he cent er of t he core of a m ult im ode fiber. Mult iple overlapping signals are creat ed, and t he receiver m ay not be able of int erpret ing incom ing signals correct ly. I t t urns out t hat offset t ing t he beam from t he cent er of t he m ult im ode core solves t he problem . See figure 9.3 shows how t his works. An LX laser t ransm it t er is

connect ed t o a special m ode- condit ioning pat ch cord t hat consist s of a single- m ode fiber spliced t o an off- cent er posit ion on a m ult im ode fiber. The light beam is aim ed int o t he single- m ode end of t he spliced fiber.

Figu r e 9 .3 . A m ode - con dit ion in g pa t ch cor d.

The t ype of m ult im ode fiber t hat is used in t he pat ch cord m ust be t he sam e t ype t hat is used in t he at t ached cable run.

N ot e For a m ode- condit ioning pat ch cord, t he st andards recom m end t hat a blue color ident ifier should be used on t he single- m ode fiber connect or, and a beige color ident ifier should be used on t he ot her connect ors.

1000BASE-CX Copper Cables and Connectors A special short copper cable was designed for use in wiring closet s and com put er room s. I t s purpose is t o connect t wo pieces of equipm ent t oget her via a gigabit —link for exam ple, t o connect a server t o a swit ch or t o int erconnect a pair of rout ers. 1000BASE- CX cables are sold in pre- assem bled form and can be ordered in various cable lengt hs.

N ot e The fibre channel st andard support s a t winax cable as well as t his special t ype of cable.

The 1000BASE- CX cable is a 150- ohm shielded balanced cable t hat cont ains four wires. One pair is used t o t ransm it , and t he ot her is used t o receive. As was t he case for t wist ed- pair cabling, a posit ive signal on one wire is balanced by a negat ive signal on t he ot her wire in t he pair. Each pair m ust be grounded. The cable is wired in crossover fashion, wit h a t ransm it line at one end connect ed t o a receive line at t he ot her end. There are t wo t ypes of connect ors, called st yle 1 and st yle 2. St yle 1 is t he popular 9- pin shielded D- subm iniat ure connect or t hat is shown in See figure 9.4 The connect or is m ale on t he cable and fem ale on t he recept acle. The figure also shows t he pin assignm ent s at each end of a crossover cable.

Figu r e 9 .4 . St yle 1 1 0 0 0 BASE- CX con n e ct or a n d pin a ssign m e n t s.

The t rouble wit h st yle 1 is t hat t hese 9- pin D connect ors are very com m on, and it would be all t oo easy t o plug one of t he ends of a CX cable int o an int erface t hat has not hing t o do wit h 1000BASE- CX. For t his reason, t he 8- pin st yle 2 connect or, which was designed especially for 1000BASE- CX, is preferred. This is called t he High Speed Serial Dat a Connect or ( HSSDC) .See figure 9.5 depict s a st yle 2 connect or and t he pin assignm ent s at each end of t he crossover cable.

Figu r e 9 .5 . St yle 2 1 0 0 0 BASE- CX con n e ct or a n d pin a ssign m e n t s.

Gigabit Interface Converters Rem ovable out let boxes called gigabit int erface convert ers ( GBI Cs) , originally designed for fibre channel int erfaces, also are used in Gigabit Et hernet product s. The use of GBI Cs gives bot h vendors and cust om ers a lot of flexibilit y. A vendor can build a swit ch t hat has slot s int o which GBI Cs can be insert ed. A cust om er t hen can use a slot for any 1000BASE- X t echnology by plugging in t he appropriat e GBI C. Changes are easy. For exam ple, a port can be changed from 1000BASE- SX t o 1000BASE- LX by swapping in an appropriat e GBI C. Furt herm ore, GBI Cs can be replaced wit hout powering off a device. The GBI C specificat ion was writ t en by a vendor group called t he SFF Com m it t ee. The group originally was form ed t o design sm all form fact or ( SFF) disk drives. Aft er com plet ing it s original t ask, t he group t ook on m any ot her proj ect s.

1000BASE-T Technology There is a very large inst alled base of Cat egory 5 unshielded balanced t wist ed- pair wiring, and t he engineers designing Gigabit Et hernet believed t hat it was very im port ant t o creat e an im plem ent at ion t hat could run across 100- m et er lengt hs of high- qualit y Cat egory 5 cabling t erm inat ed by RJ- 45 connect ors. 1000BASE- T sat isfies t his need. Four unshielded t wist ed pairs ( UTPs) are required t o at t ain gigabit bandwidt h.

1000BASE-T Encoding Operat ing bidirect ionally on t wist ed pairs is a challenge. The encoding used for 1000BASE- T is highly com plex. I t not only m ust achieve a balance of 0s and 1s on each cable, but it also m ust creat e a balanced elect rom agnet ic condit ion across all four pairs. The encoding m et hod consist s of several st eps: 1. Each byt e is scram bled. 2. The scram bled byt e is t ranslat ed t o a 4- t uple of special sym bols via a m apping called 8B1Q4. 3. Each sym bol is represent ed on t he physical m edium by a volt age. Five different volt age levels are used, corresponding t o an alphabet of five different sym bols. The physical t ransm ission m et hod is called 4- dim ensional 5- level pulse am plit ude m odulat ion ( 4D- PAM5) . More det ails are present ed in Appendix A.

Getting the Cable Ready A t wist ed- pair cabling syst em t hat will be used for Gigabit Et hernet m ust sat isfy st ringent qualit y requirem ent s. A bat t ery of t est s m ust be applied t o ensure t hat t he cabling syst em can deliver a gigabit load. Chapt er 10, " Twist ed- Pair Cabling St andards and Perform ance Requirem ent s," defines t he param et ers t hat need t o be t est ed and discusses t est t ools. Enhanced Cat egory 5 ( Cat egory 5E) cabling syst em s, which are discussed in Chapt er 10, consist of high- qualit y Cat egory 5 cables and connect ors t hat have passed t hese t est s. New cabling needs t o be inst alled wit h care. I f exist ing Cat egory 5 cable will be used, it m ust be t est ed and recert ified, and som e changes m ight be needed t o get it t o work reliably. For exam ple, crosst alk from ot her cables wit hin a bundle ( " alien crosst alk" ) disrupt s gigabit t ransm ission, so m ult iple four- pair set s should not be bundled t oget her. I n part icular, t he 25- pair bundles t hat are com m on in m any inst allat ions cannot be used for Gigabit Et hernet . I n addit ion, problem s are likely t o be caused by t he following: •

• •

•

Using t oo m any connect ors along t he cable pat h from office equipm ent t o equipm ent in a wiring closet The presence of subst andard connect ors Sloppy inst allat ion t echniques t hat result in cable fault s or cause loose wire t wist s near connect ors The use of poor- qualit y equipm ent cables or pat ch cables

Much of t he exist ing inst alled Cat egory 5 horizont al cabling will not cause t rouble as long as m ult iple four- pair set s are not bundled t oget her. This is good news because it

is a lot easier ( and less cost ly and disrupt ive) t o fix connect or and pat ch cable problem s t han t o pull new horizont al cable. Five correct ive act ions are recom m ended by TI A/ EI A TSB- 95: •

•

•

•

•

Replace equipm ent cords and pat ch cords wit h cords const ruct ed from cable t hat m eet s t he Cat egory 5E specificat ion. I f t he link has a cross- connect , reconfigure t he cross- connect as an int erconnect . Rem ove any t ransit ion point connect or. A t ransit ion point is a locat ion where flat undercarpet cabling connect s t o round cabling. Replace t he work area out let wit h an out let t hat m eet s t he Cat egory 5E specificat ion. Replace a subst andard int erconnect wit h an int erconnect t hat m eet s t he Cat egory 5E specificat ion.

St at ed succinct ly, it helps t o use cables and connect ors t hat m eet t he st rict specificat ions and t o reduce t he num ber of connect ors along a link t o t hree. See figure 9.6 shows before and aft er configurat ions t hat reduce t he num ber of connect ors on a link. The horizont al run in t he " before" figure includes a t ransit ion point .

Figu r e 9 .6 . I m pr ovin g sign a l qu a lit y by r e ducin g t h e n u m be r of con n e ct or s.

Encoders, Decoders, and Hybrids See figure9.7 illust rat es how signals are sent across a 1000BASE- T int erface. A t riangle cont aining a T represent s a t ransm it encoder. A t riangle cont aining a R represent s a receive decoder. Recall t hat t he encoding process convert s a byt e int o four code sym bols. Each of t he four pairs carries one of t hese sym bols. The four sym bols are t ransm it t ed across t he four pairs at t he sam e t im e. Then t he 4- t uple is decoded int o a byt e.

Figu r e 9 .7 . 1 0 0 0 BASE- T t r a n sm ission a cr oss fou r t w ist e d pa ir s.

The hybrid com ponent shown in t he figure prevent s t he locally t ransm it t ed signals ( and any echoed signals t hat result from reflect ions) from int erfering wit h t he incom ing signals t hat are sent from t he far end.

Master/Slave Timing Synchronizat ion of bit t im ing bet ween part ners is especially im port ant because dat a is being sent and received on t he sam e wire. The bit s are synchronized by est ablishing one end of t he link as t he t im ing m ast er. The m ast er uses it s int ernal clock for bit t im ing. The ot her end is t he t im ing slave. I t recovers t he clock from incom ing bit s and uses t his clocking for it s t ransm ission. The m ast er and slave roles eit her are pre- est ablished by m anual configurat ion or are det erm ined during an Aut o- Negot iat ion int eract ion.

N ot e I f neit her side has a preconfigured role, a m ult iport device ( hub or swit ch) act s as m ast er when com m unicat ing wit h a single- port device. When peers com m unicat e ( single- port t o single- port , or m ult iport t o m ult iport ) each endpoint generat es a random num ber called a seed and sends it t o it s part ner. The part y t hat generat es t he bigger seed value becom es t he m ast er. I n t he unlikely event of a t ie, t hey st art all over. Chapt er 11 describes t he Aut o- Negot iat ion Prot ocol in det ail.

Aft er Aut o- Negot iat ion is com plet e, t he m ast er init iat es a period of t raining. During t raining, t he m ast er sends a sequence of idles t hat enable t he slave endpoint t o synchronize it s t im ing wit h t he m ast er. When t raining is com plet e, t he part ies are ready t o send dat a t o one anot her.

Auto-Negotiation and Crossover Cable See figure9.7 is slight ly m isleading because it gives t he im pression t hat st raight t hrough cabling is used for 1000BASE- T. I n fact , crossovers have t o be used for 1000BASE- T, j ust as for 10BASE- T, 100BASE- TX, and 100BASE- T4. The reason for t his is t hat , at init ializat ion t im e, a Gigabit Et hernet int erface needs t o be capable of exchanging Aut o- Negot iat ion inform at ion wit h what ever t ype of t wist ed- pair Et hernet int erface is at t he ot her end of t he link. Using crossovers m akes 1000BASE- T physically com pat ible wit h t he ot her t wist ed- pair Et hernet im plem ent at ions. See figure9.8 illust rat es t he pin and cable relat ionships for 1000BASE- T when a MDI int erface in a st at ion is connect ed t o an int ernally crossed- over MDI - X int erface in a buffered dist ribut or or swit ch.

Figu r e 9 .8 . Cr ossove r M D I / M D I - X w ir in g for 1 0 0 0 BASE- T.

The BI _D prefixes labeling each wire st and for bidirect ional dat a. The st ory of Gigabit Et hernet is not yet com plet e. Chapt er 10 invest igat es t he t est ing t hat m ust be done t o qualify t wist ed- pair cable. Appendix A present s t he encodings. Finally, Chapt er 11 describes t he negot iat ion t hat occurs when a link init ializes.

Summary Points

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Four different st andardized physical im plem ent at ions of Gigabit Et hernet exist . 1000BASE- SX and 1000BASE- LX are based on fiber opt ic m edia. 1000BASECX is a shielded copper cable. 1000BASE- T is an unshielded t wist ed- pair im plem ent at ion t hat operat es across four pairs of Cat egory 5E wire. 1000BASE- SX, 1000BASE- LX, and 1000BASE- CX are based on fibre channel t echnology. The group is called 1000BASE- X. For bot h t he 1000BASE- X and 1000BASE- T physical im plem ent at ions, fram es or idles are t ransm it t ed cont inually in bot h direct ions. Lasers are used for bot h m ult im ode and single- m ode 1000BASE- X fiber im plem ent at ions. I nexpensive Vert ical Cavit y Surface Em it t ing Lasers ( VCSELs) support gigabit t ransm ission across m ult im ode cables t hat are hundreds of m et ers in lengt h. The st andard single- m ode 1000BASE- LX im plem ent at ion support s cable lengt hs ranging up t o 5000 m et ers. Nonst andard 1000BASE- LH im plem ent at ions support cable lengt hs ranging up t o 100,000 m et ers. 8B/ 10B encoding, which was int roduced for fibre channel, is used for t he 1000BASE- X fam ily. Each byt e is t ranslat ed t o a 10- bit code- group. There are special 8B/ 10B code- group pat t erns t hat represent idles and fram eext ension byt es, and t hey m ark t he beginning and end of a fram e. The st andard plug recom m ended for 1000BASE- SX and 1000BASE- LX in t he 802.3 st andard is a duplex SC connect or. Overlapping signals are creat ed when a 1000BASE- LX laser beam is aim ed at t he cent er of t he core of a m ult im ode fiber. The problem is solved by t ransm it t ing t he light beam int o a single- m ode fiber m ode- condit ioning pat ch cord. Two t ypes of connect ors exist for 1000BASE- CX cables: a st yle 1 9- pin Dsubm iniat ure and a special st yle 2 8- pin connect or. The use of rem ovable out let boxes called gigabit int erface convert ers ( GBI Cs) provides vendors and users wit h a lot of flexibilit y in t heir 1000BASE- X im plem ent at ions. 1000BASE- T support s 100- m et er cables. High- qualit y Cat egory 5E ( or bet t er) cabling and connect ors m ust be used for 1000BASE- T. A m inim al num ber of connect ors should be used across t he link bet ween a st at ion and a repeat er or swit ch. The m ast er end of a 1000BASE- T link uses int ernal t im ing. The slave end synchronizes wit h t he m ast er.

References Gigabit Et hernet is defined in • •

I EEE St andard 802.3, 1998 Edit ion. " Carrier Sense Mult iple Access wit h Collision Det ect ion ( CSMA/ CD) Access Met hod and Physical Layer Specificat ions." See Chapt ers 3, 4, and 34–39. I EEE 802.3- ab- 1999. " Physical Layer Param et ers and Specificat ions for 1000 Mb/ s Operat ion Over 4- Pair of Cat egory 5 Balanced Copper Cabling, Type 1000BASE- T."

The fibre channel physical layer is described in

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ANSI St andard X3.230- 1994 ( FC- PH) . " I nform at ion Technology—Fibre Channel—Physical and Signaling I nt erface."

SFF inform at ion ( including t he GBI C specificat ion) is available at ft p: / / fission.dt .wdc.com / pub/ st andards/ sff/ spec For com plet e det ails of t he physical engineering of 1000BASE- X Gigabit Et hernet , see •

Cunningham , David, and William Lane. Gigabit Et hernet Net working. I ndianapolis, I N: Macm illan Technical Publishing, 1999.

Chapter 10. Twisted-Pair Cabling Standards and Performance Requirements As is t he case for t he ot her aspect s of dat a com m unicat ions, cabling st andards are im port ant in order t o fost er a com pet it ive m arket for product s t hat do what t hey are supposed t o do. Cabling st andards also give t est equipm ent vendors t he basis on which t o build t est product s. Twist ed- pair cabling is t he m ost frequent ly used Et hernet LAN m edium . This chapt er focuses on t wist ed- pair cabling requirem ent s and t he t est s t hat you m ust perform t o check whet her your cable conform s t o t hese requirem ent s. Test s for fiber opt ic cable are discussed briefly at t he end of t he chapt er. Cable t est ing equipm ent , which is an essent ial part of a LAN t echnician's arsenal, can check out t he perform ance param et ers t hat are described in t his chapt er.

Cabling Standards Bodies The body t hat rules Unit ed St at es cabling st andards is t he Telecom m unicat ions I ndust ry Associat ion ( TI A) whose parent organizat ion is t he Elect ronic I ndust ries Alliance ( EI A) . Cabling requirem ent s are described in t he TI A/ EI A- 568A Com m ercial Building Telecom m unicat ions Cabling St andard, first published in 1991. This st andard has been updat ed on a regular basis. A Canadian equivalent of t his U.S. st andard is called CSA T529. The I SO/ I EC 11801 st andards are followed in Europe and in m any ot her part s of t he world. These st andards are based part ly on TI A/ EI A specificat ions and part ly on st andards t hat reflect special European condit ions. A num ber of organizat ions influence cabling st andards, including t he I EEE 803 com m it t ee, t he ATM Forum , and t he Com it e Europeen de Norm alisat ion Elect rot echnique ( CENELEC) , am ong ot hers.

TIA/EIA Categories The focal point of t he st andards act ivit y is t o define various cat egories of cabling t hat are m at ched t o t he needs of different applicat ions. A cat egory num ber is a st andard rat ing t hat is applied t o an ent ire cabling syst em , which in addit ion t o cable runs, includes out let s, pat ch cords, panels, connect ors, and cross- connect blocks. Rat ing and t est ing all of t he com ponent s of a syst em m akes a lot m ore sense t han focusing only on t he cable.

N ot e I SO/ I EC and CENELEC use som e t erm inology t hat differs from t he TI A/ EI A language. As wit h TI A/ EI A, connect ors and cables are rat ed by cat egory num ber. However, I SO/ I EC and CENELEC use alphabet ical class values t o grade link perform ance. Class C corresponds t o TI A/ EI A Cat egory 3 perform ance, Class D t o Cat egory 5, Class E t o Cat egory 6, and Class F t o Cat egory 7.

Engineers keep reaching for higher capacit ies for t wist ed- pair syst em s, and recent ly t here has been a flurry of work on new cable cat egories. At t he t im e of writ ing, seven cat egories of t wist ed- pair cable exist , num bered 1, 2, 3, 4, 5, 5E, and 6; discussion of a Cat egory 7 st andard already has st art ed as well. A higher num ber corresponds t o bet t er qualit y cable. The current inst alled base consist s alm ost ent irely of Cat egory 5, 5E, and 3 cable. Each cat egory above 1 specifies a guarant eed bandwidt h m easured in m egahert z ( MHz) t hat a cable m ust be capable of t ransm it t ing in a dependable m anner. ( The cable m ay, in fact , support a higher bandwidt h.) No sim ple form ula m aps bandwidt h, which is m easured as a range of frequencies, int o bit s per second. Again and again, a clever new t ransm ission t echnology has raised t he num ber of bit s per second t hat could be sent across a cable wit h a specified bandwidt h.

N ot e I t is im port ant t o t hink of a cat egory as a st andard t hat you t est for, not j ust as a set of product s t hat you buy. A bot ched inst allat ion can t urn t he best cable and com ponent s int o wort hless j unk. I t doesn't t ake m uch t o bot ch it —loosening t he t wist s on a cable when adding a connect or, pulling t he cable wit h t oo m uch force ( m ore t han 25 pounds) , posit ioning t he cable close t o a source of int erference such as elect ric power cables or fluorescent light s, creat ing t oo sharp a bend, t wist ing t he cable, or causing a kink. An ent ire cable plant m ust be cert ified by t est ing aft er it is inst alled.

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Cat egory 1 som et im es is referred t o as " barbed wire," so you should not expect m uch from it . Cat egory 2 is an im proved version t hat prim arily carries digit al voice and connect s t o digit al PBX swit ches. Cat egory 3 cable is used in m any 10BASE- T LANs. I t is m ade up of t wo or m ore pairs of copper wires wit h each wire wrapped in insulat ion. Cat egory 4 is sim ilar t o Cat egory 3, but t he qualit y of t he cable and com ponent s is som ewhat bet t er. Cat egory 5 cable is a popular dat a grade. The qualit y of t he cable and com ponent s is bet t er t han t hat of Cat egory 4, and t he wires are m ore t ight ly wound. Alt hough 100Mbps and Gigabit Et hernet can run on Cat egory 5 UTP cable, problem s arise if t he ent ire cable syst em is not of sufficient ly high qualit y. Cat egory 5E was int roduced t o define t he t est s t hat assure t hat a cable syst em is capable of support ing Gigabit Et hernet . Som e exist ing Cat egory 5 cable plant s have been reclassified as Cat egory 5E aft er t est ing. At t he t im e of writ ing, t he Cat egory 6 st andard st ill had draft st at us. However, vendors already sell cables, com ponent s, and t est ers t hat m eet t he current draft specificat ions. Cat egory 6 cable m ust support a bandwidt h of up t o 250MHz across 100 m et ers. I t was designed wit h backward com pat ibilit y in m ind, so a RJ- 45 st yle connect or st ill is used. Alt hough t he com ponent s are com pat ible wit h Cat egory 5, t hey m ust pass t est s t hat require a higher level of qualit y. Cat egory 7, which is aim ed at very high t ransm ission capacit ies, is expect ed t o be a big depart ure from t he ot her cabling st andards. Each t wist ed–pair segm ent is shielded, and t he cable as a whole also is wrapped in shielding. This cable would be st iff and heavy, and it m ight t urn out t o be m ore cost ly t han opt ical fiber. Opt ical fiber can carry far m ore inform at ion t han even t he best t wist ed–pair cable.

N ot e Most of t he copper cable t hat is sold t oday is Cat egory 5 or bet t er. Vendors already offer cable t hat exceeds t he perform ance levels t hat are t arget s for Cat egory 6. While som e organizat ions are m oving quickly t o inst all high- perform ance cable, quit e a few sit es cont inue t o use Cat egory 3 or t hin coaxial cable for t heir LANs. There st ill is a sm all m arket for Cat egory 3 cable.

The charact erist ics of each cat egory are sum m arized briefly in Table 10.1.

Ta ble 1 0 .1 . UTP Ca t e gor ie s Ca t e gor y

Ba n dw idt h

D e scr ipt ion

1

—

Meet s t he m inim um requirem ent s for plain old t elephone service ( POTS) .

2

—

Som et im es is used for digit al PBX or I SDN connect ions.

Ta ble 1 0 .1 . UTP Ca t e gor ie s Ca t e gor y

Ba n dw idt h

D e scr ipt ion

3

16MHz

Meet s t he requirem ent s of 10BASE- T Et hernet , 4Mbps Token Ring, and 100BASE- T4 ( as well as voice) .

4

20MHz

Meet s t he requirem ent s of 10BASE- T Et hernet and 16Mbps Token Ring.

5

100MHz

Meet s t he requirem ent s of 100Mbps Et hernet and, if it passes cert ificat ion t est s, 1000Mbps Et hernet . Also used for CDDI ( FDDI over copper) .

5E

100MHz

Meet s t he requirem ent s of 100Mbps and 1000Mbps Et hernet . Cat egory 5E st ands for Enhanced Cat egory 5. The cable has passed specified perform ance t est s, and t he RJ- 45 connect or com ponent s have been t est ed and m at ched t o t he requirem ent s of high- speed t ransm ission.

6

250MHz

Current ly a draft st andard, aim ed at 1000Mbps and above. The cable m ust pass m ore t est s t han Cat egory 5E. The RJ45 connect or form is used, but t he com ponent s sat isfy ext ra perform ance requirem ent s. Com ponent s m ust be carefully m at ched.

7

600MHz t o 1200MHz

Current ly a draft st andard, aim ed at 1000Mbps and above. Shielded screened t wist ed- pair cable is used. Each pair is individually shielded. A new connect or form is used.

Wire Features The wire used for a long run of t wist ed- pair cable is a solid copper cylinder. For Cat egory 1- 5E, 100- ohm wire is used. Anot her feat ure of t he wire is t hat it s diam et er is st andardized. However, t he diam et er is report ed indirect ly, using an invert ed m easurem ent called t he Am erican Wire Gauge ( AWG) . The AWG value is t he inverse of t he t hickness ( in inches) of a wire. For exam ple, t he diam et er of 24- AWG wire is 1/ 24 inch. Not e t hat t hicker wire has a sm aller AWG value. Cat egory 1- 5E LAN cable is predom inant ly 24 AWG ( alt hough 22 AWG was specified as accept able in t he st andards) . A bigger diam et er ( such as 23 AWG) m ight be used for Cat egory 6. Pat ch cords, which are used in work areas and wiring closet s, need t o be flexible. The wire in a pat ch cord is m ade up of m any t hin st rands, which m akes it far easier t o bend. A solid wire cable does not bend easily, fat igues when it is bent , and can be dam aged by a bend wit h t oo t ight a radius.

Cabling Layouts A t ypical cabling layout was described in Chapt er 6, " The Et hernet 10Mbps Physical Layer." This layout , which conform s t o ANSI / TI A/ EI A 568- A, specifies a t ot al of 10

m et ers for equipm ent cords and pat ch cords, and a 90- m et er cable run bet ween t he office t elecom m unicat ions out let and cross- connect panels in t he wiring closet . An addit ional det ail was added in Chapt er 9, " Gigabit Et hernet Physical Layer." A horizont al run can include a t ransit ion point , which is a locat ion where flat undercarpet cabling connect s t o round cabling. ( See Figure 9.6.) There are a couple of ot her cabling layout opt ions ( which are specified in ANSI / TI A/ EI A TSB 75) . One incorporat es a m ult iuser t elecom m unicat ions out let ( MUTO) . Up t o 24 work area cables ( each as long as 20 m et ers) can run direct ly from office equipm ent t o a MUTO. The MUTO m ust be wit hin 70 m et ers of t he wiring closet . Because t he MUTO replaces a work area j ack, it does not add a connect ion point t o t he cable pat h. However, it oft en is used t o gat her pairs t oget her int o a 25pair cable ( which should not be done wit h Gigabit Et hernet ) . The ot her opt ion, called a consolidat ion point , int roduces an ext ra connect or int o t he horizont al run. The horizont al cabling t hat leads from t he t elecom out let t o t he wiring closet consist s of t wo sect ions t hat m eet at t he consolidat ion point . The m axim um com bined lengt h of t he t wo horizont al cable sect ions is 90 m et ers. Up t o 24 horizont al cables can m eet at a consolidat ion point . ( To com pensat e for t he ext ra connect or, t he work area cable m ust be rest rict ed t o a m axim um lengt h of 3 m et ers.) Because t his arrangem ent adds an ext ra connect ion point , it can cause problem s if used wit h Gigabit Et hernet connect ions.

Unshielded Twisted-Pair Cabling Performance Parameters Cabling perform ance param et ers are not m yst erious. They are sym pt om s t hat enable you t o t rack down flaws t hat can dist ort and ruin t he signal on t wist ed- pair cable. For exam ple: • • •

The signal m ight be get t ing t oo faint as it t raverses t he cable. The copper wires in a pair m ight not be t wist ed t ight ly all along t he cable. This prevent s t he posit ive and negat ive signals in a pair from balancing out and creat es an elect rom agnet ic field. This elect rom agnet ic field induces false signals on neighboring cables and m ay corrupt dat a t ransm ission. Flaws in t he cable st ruct ure m ight be causing part of t he signal t o be reflect ed back t o t he source, result ing in problem s t here.

Expressing perform ance charact erist ics as m easurable quant it ies m akes it possible t o set cable qualit y st andards and use cable- t est ing equipm ent t o det ect fault s.

Parameters for All Ethernet LANs Som e basic TI A/ EI A t est requirem ent s m ust be sat isfied for all t wist ed- pair cables used for Et hernet LANs. These basic requirem ent s set accept able levels for t he following: •

At t e nu a t ion — The proport ion of power t hat is lost as a signal t raverses a cable. At t enuat ion is m easured in decibels ( dB) .

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N e a r e n d cr osst a lk ( N EXT) — The dist ort ion of a weak incom ing signal by a st rong out going signal on a neighboring wire pair. NEXT is m easured in decibels. I m pe da n ce — A m easure of t he opposit ion t o t he flow of elect ricit y down t he wire. I rregularit ies in t he im pedance of a cable, or int erconnect ing t wo cables t hat have different im pedance rat ings, causes part of an out - going signal t o be reflect ed back. I m pedance is m easured in ohm s.

A checkup on t he healt h of a cable plant also includes wirem ap t est ing. This procedure checks t he following for every conduct or in a cable: • • • • •

Proper pin t erm inat ion at each end Cont inuit y t o t he rem ot e end Crossed pairs or reversed pairs Short s bet ween t wo or m ore conduct ors Split pairs ( t hat is, t he wires of t wo different pairs are cross- connect ed)

The 802.3 specificat ion defines som e furt her wiring const raint s needed t o assure t hat Et hernet will funct ion correct ly. I t specifies m axim um values for t he following: • • •

Cable lengt h Jit t er ( t hat is, variat ion in t he int erval bet ween signal t ransit ions) Propagat ion delay

Parameters for High-Speed LANs Not surprisingly, m ore st ringent requirem ent s m ust be applied t o cable t hat is used t o carry dat a at gigabit speeds. Also, a new fact or arises for 1000BASE- T Et hernet . 1000BASE- T t ransm ission occurs sim ult aneously across four t wist ed pairs. The fact t hat dat a is being t ransm it t ed at high frequencies across four neighboring wire pairs inst ead of t wo creat es t he pot ent ial for severe crosst alk problem s. This led t o t he est ablishm ent of a slew of new param et er definit ions and t est s. The param et ers include • • • • • • • •

Far end crosst alk ( FEXT) Equal level far end crosst alk ( ELFEXT) Power sum near end crosst alk ( PSNEXT) Worst pair- t o- pair ELFEXT Power sum ELFEXT At t enuat ion t o crosst alk rat io ( ACR) Delay skew Ret urn loss

N ot e Cat egory 5 cable is qualified for use at high speeds by t est ing t hat t he values of t hese param et ers lie wit hin specified bounds. Current ly, t he sam e set of param et ers is used t o qualify Cat egory 6 cable, but t he qualit y requirem ent s are m ore st ringent .

Parameter Descriptions Bot h basic and high- speed perform ance param et ers are described in deeper det ail in t he sect ions t hat follow.

Jit t e r At each t ransm ission speed, t here is a fixed t im e int erval during which a bit is t ransm it t ed. A bit is represent ed as a variat ion in t he signal sent across a cable—for exam ple, by changing a volt age from high t o low or from low t o high. Jit t er is a deviat ion in t he periodic int erval at which signal t ransit ions occur. Transm ission st andards set lim it s on j it t er. For exam ple, when t est ed, a 10BASE- T cable should not exhibit m ore t han 5 nanoseconds of j it t er.

At t e n u a t ion a n d D e cibe ls At t enuat ion m easures t he am ount of power loss of t he elect rical signal when it t ravels t hrough a cable. I t is calculat ed in logarit hm ic unit s called decibels ( dB) . The form ula is shown here:

Not e t hat if t he far end power ( t he power- out ) is 1/ 10 of t he original power ( t he power- in) , t he at t enuat ion is

I f t he far end power is 1/ 100 of t he power- in, t he at t enuat ion is

The fart her a signal t ravels, t he m ore it at t enuat es. Thus, at t enuat ion depends on t he cable lengt h. St andards docum ent s handle t his by st at ing t he value for a specific lengt h ( such as per kilom et er) .

Because t he power- out at t he far end always is less t han t he power- in, t he at t enuat ion always is negat ive. The m inus sign oft en is om it t ed in docum ent s t hat set at t enuat ion levels. For exam ple, in t he st at em ent " St andard 568- A lim it s at t enuat ion in a Cat egory 5 syst em t o 24dB for a 100MHz signal," it is underst ood t hat t he m easurem ent act ually is –24dB. Not e t hat t his m eans t hat sm aller m easurem ent s are good news—t hey indicat e less at t enuat ion. The at t enuat ion across a cable act ually is different at each frequency level. Higher frequencies experience m ore at t enuat ion.

N ot e Tem perat ure and cable diam et er also have an effect on at t enuat ion. At t enuat ion increases as t he t em perat ure goes up. Enlarging t he diam et er of a cable decreases at t enuat ion. This is t he reason t hat a bigger diam et er t han 24 AWG m ight be used for Cat egory 6 cable.

N e a r End Cr osst a lk A st rong signal t ransm it t ed from a st at ion can dist ort a weak dat a signal arriving at t he st at ion on an adj acent pair of wires. This happens because current flowing t hrough a wire creat es an elect rom agnet ic field t hat induces signals on adj acent wires. The am ount of dist ort ion is m easured in decibels and is called t he near end crosst alk ( NEXT) . More precisely, t he near end crosst alk is t he am ount of energy t ransferred t o a weak incom ing signal from a st rong out going signal on an adj acent wire. This am ount increases as t he frequency level of t he t ransm ission increases. Recall t hat t he sam e signal is sent across bot h of t he wires in a pair, but wit h reverse polarit y. Twist ing t he wires t ight ly causes t he t wo elect rom agnet ic fields t o cancel one anot her. A st rong NEXT effect is a sym pt om t hat t he wires are not t wist ed properly.

M e a su r in g N EXT The effect of crosst alk is m easured by t ransm it t ing a specified signal level on a t ransm it pair. The near end crosst alk is t he proport ion of t hat signal t hat is t ransferred t o t he local receive pair. Figure 10.1 illust rat es NEXT. I n t he figure, a st rong out going signal induces a NEXT signal on a neighboring pair t hat could badly dist ort t he weak incom ing signal.

Figu r e 1 0 .1 . A st r on g out goin g signa l in du cin g N EXT on a n e igh bor in g pa ir .

NEXT is m easured in decibels. You want t he NEXT m easurem ent t o be sm all—a sm all proport ion t ranslat es t o a large negat ive dB value. As was t he case for at t enuat ion, t he negat ive sign com m only is dropped in docum ent at ion. For exam ple, t he st at em ent " The am ount of NEXT bet ween pairs in a four- pair cable should be at least 60dB for a 10MHz link" m eans t hat it should be - 60dB or less. NEXT levels are im proved by using wire pairs t hat have been t ight ly t wist ed. A cert ain am ount of unt wist ing is bound t o happen near a t erm inat ing plug. I t is im port ant t o m ake sure t hat t he wires are not unt wist ed for m ore t han 1/ 2 inch from t he t erm inat ion point . The use of high- grade plugs and j acks also helps. NEXT is affect ed by t he frequency of t he signal and increases as t he frequency increases. Som e of t he vendors t hat m anufact ure high- perform ance cables conform ing t o Cat egory 6 perform ance levels isolat e t he four pairs from one anot her using separat ors called splines. Figure 10.2 illust rat es one im plem ent at ion of splines in a sheat h cont aining four t wist ed pairs. These splines m aint ain int erpair spacing, even when t he cable is bent m ore sharply t han it should be. The use of splines reduces near end crosst alk, at t enuat ion, and far end crosst alk.

Figu r e 1 0 .2 . Se pa r a t in g t w ist e d pa ir s w it h in a fou r - pa ir ca ble .

Fa r En d Cr osst a lk Far end crosst alk ( FEXT) is t he am ount of energy t ransferred t o an out going signal by an incom ing signal on an adj acent wire. Maint aining t ight t wist s and using good connect ors at t he receive end prevent s t he FEXT level from get t ing out of bounds. Figure 10.3 illust rat es FEXT. I n t he figure, t he incom ing signal has been at t enuat ed by it s t rip along t he cable, but it st ill m ight be capable of inducing noise on an adj acent cable.

Figu r e 1 0 .3 . A w e a k in com in g sign a l in du cin g FEXT on a n e igh bor in g pa ir .

M e a su r in g FEXT a n d Com pu t in g ELFEXT

Like NEXT, FEXT is m easured by t ransm it t ing a specified signal level on a t ransm it pair. The proport ion of t he signal t hat is t ransferred t o t he t ransm it pair at t he far end t hen is m easured in decibels. However, t his m easurem ent m ust be adj ust ed t o m ake it m eaningful. The st rengt h of a signal decreases due t o at t enuat ion as t he signal t ravels down a lengt h of cable. Thus, t he FEXT value depends on t he lengt h of t he cable and t he am ount of at t enuat ion. FEXT is t urned int o a m eaningful m easurem ent by subt ract ing t he at t enuat ion. The result is called t he equal level far end crosst alk ( ELFEXT) : ELFEXT is independent of lengt h and m easures a charact erist ic of a cable product .

PSN EXT, PSELFEXT, a n d W or st Pa ir - t o- Pa ir ELFEXT Four- pair cables are used for 100- BASE- T4 and 1000BASE- T t ransm ission. Every pair causes crosst alk on t he t hree ot her neighboring pairs. Thus, for pairs A, B, C, and D, t he following is t rue: • • • •

B, A, A, A,

C, C, B, B,

and and and and

D D D C

cause cause cause cause

near near near near

and and and and

far far far far

end end end end

crosst alk crosst alk crosst alk crosst alk

on on on on

A. B. C. D.

This adds up t o a long list of 24 crosst alk m easurem ent s. Som e com bined m easurem ent s have been defined t o pare t he list int o a sm aller set of num bers t hat can be used t o evaluat e a cable: • • •

Power sum near end crosst alk ( PSNEXT) Power sum equal level far end crosst alk ( PSELFEXT) Worst pair- t o- pair equal level far end crosst alk

The power sum near end crosst alk ( PSNEXT) is t he sum of t he NEXT effect s on a pair by t he ot her t hree pairs. For exam ple, t he PSNEXT for pair A is t he sum of t he near end crosst alk values caused by B, C, and D. There is a separat e PSNEXT value for each of t he four pairs. Sim ilarly, t here is a separat e power sum equal level far end crosst alk ( PSELFEXT) value for each of t he four pairs. The PSELFEXT is t he sum of t he equal level far end crosst alk ( ELFEXT) effect s on a pair by t he ot her t hree pairs. For exam ple, t he PSELFEXT for pair A is t he sum of t he equal level far end crosst alk values caused by B, C, and D. Finally, t he worst pair- t o- pair equal level far end crosst alk is t he biggest ELFEXT effect of one pair on anot her.

Attenuation to Crosstalk Ratio The at t enuat ion t o crosst alk rat io ( ACR) is a m easurem ent of t he signal t o noise rat io at t he receive end of a pair. This m easurem ent is expressed in decibels. I f t he signal is st ronger t han t he noise, t he ACR is posit ive. Bigger ACR values correspond t o bet t er signals. The ACR m easurem ent act ually is equivalent t o t he ELFEXT and can be convert ed int o an ELFEXT value.

Structural Return Loss and Return Loss I f a wire's st ruct ure is not uniform , it s im pedance varies along it s lengt h. Changes in im pedance cause a signal sent down t he wire t o lose st rengt h because part of t he signal is reflect ed back t o it s origin. St ruct ural ret urn loss ( SRL) is a m easure of t his loss, expressed in decibels. Ret urn loss is a m easure of t he relat ive am ount of a signal t hat is reflect ed back t o it s source. I t is relat ed t o t he uniform it y of a cable relat ive t o a given t arget value for it s im pedance—100 ohm s, in t he case of t wist ed–pair cabling.

Propagation Delay and Time Domain Reflectometry Recall t hat t he propagat ion delay is t he am ount of t im e t hat it t akes a signal t o t ravel from one end of a wire t o t he ot her end. Obviously, t his depends on t he lengt h of t he wire and t he speed at which elect rons t ravel t hrough t he wire. The propagat ion delay for a part icular t ype of cable can be com put ed from a rat ing called t he nom inal velocit y of propagat ion ( NVP) . The NVP is t he t ransm ission speed along a wire relat ive t o t he speed of light in a vacuum . I t is expressed as a percent age of t he speed of light . Having a st andardized and pre- t est ed NVP for a cable enables a net work t echnician t o locat e a cable fault using a Tim e Dom ain Reflect om et ry ( TDR) t est device. Before t est ing a cable, t he user ent ers t he cable's rat ed NVP int o t he device. On com m and, t he t est device em it s a signal. I f t here is a fault such as an open cable, short circuit , or bad connect ion, all or part of t he signal pulse will be reflect ed back. The t est device can est im at e t he dist ance t o t he fault based on t he velocit y of t he signal and t he am ount of t im e t hat elapses bet ween sending t he pulse and receiving t he reflect ed signal. TDR t ools exist for bot h coax and t wist ed- pair cable.

Delay Skew 1000BASE- TX t ransm it s signals across four pairs concurrent ly. The m ult iple incom ing signals m ust be synchronized so t hat t hey can be recom bined int o t he original signal. A receiver can cope wit h slight variat ions in delay, but if t oo large a difference exist s bet ween t he propagat ion t im es of t he pairs, com m unicat ion will fail.

The delay skew is t he difference bet ween t he propagat ion delays of t he slowest and fast est pairs.

Managing Twisted-Pair Cabling Changing from coaxial cable t o t wist ed- pair cable did m ore t han rat ionalize t he cabling layout of Et hernet LANs. I t im proved LAN reliabilit y, availabilit y, and m anageabilit y. A coax bus is vulnerable t o cable or connect or flaws t hat can shut down or degrade t he perform ance of t he ent ire LAN. These flaws have always been hard t o t rack down. Fault s st ill happen in t wist ed- pair LANs, but t he effect of a fault is lim it ed. A cable failure bet ween a hub or swit ch port and a st at ion affect s only t hat st at ion. Link int egrit y t est s det ect a cable failure very quickly. Using an SNMP m anagem ent st at ion or checking LED light s can pinpoint a t rouble spot . A fault on a link bet ween hubs or bridges t em porarily can segm ent t he LAN int o t wo pieces, but each part st ill can cont inue t o funct ion on it s own.

Test Tools A cable t est t ool is t he LAN t echnician's best friend. There are basically t wo t ypes of t est t ools: • •

Cont inuit y/ cable t est ers Cert ificat ion t ools

Bulky t est devices once were t he norm , but now handheld equipm ent t hat m eet s high- qualit y st andards is available.

Con t in u it y/ Ca ble Te st e r Called a cont inuit y t est er by som e vendors and a cable t est er by ot hers, t hese devices offer a helping hand t o a som eone who is inst alling a cable in an office or t rying t o t rack down a cable fault . The device •

•

•

Calculat es t he t ot al cable lengt h from t he office t o t he hub or swit ch in t he wiring closet Perform s wirem ap t est s t o verify cont inuit y t o t he rem ot e end; t o check for crossed, reversed, or split pairs; and t o m ake sure t hat t here are no short s Perform s TDR t est s

Many t est product s include t he com ponent s needed t o perform a t one t est . To do t his, a t one t est set is at t ached t o one end of a cable and generat es a t one t hat is t ransm it t ed ont o a wire. When a separat e piece of equipm ent called a probe is placed near a bundle of wires, it can det ect which wire in t he bundle carries t he t one.

Tone t est equipm ent m akes it possible t o m at ch one end of a cable locat ed in an office t o t he ot her end in a wiring closet . I t is a valuable t ool for inst allat ions, for m oves and changes, and for t racing fault y wires.

Ce r t ifica t ion Tool A cert ificat ion t ool is a high- end t est er t hat can det erm ine whet her inst alled cables m eet TI A Cat egory 5, 5E, or proposed Cat egory 6 or 7 requirem ent s. The t ool checks t he perform ance param et ers aut om at ically. A product usually also includes t he capabilit ies of a cont inuit y/ cable t est er as well as opt ional kit s, such as fiber opt ic t est com ponent s.

N ot e Cert ificat ion t ools differ in t heir level of accuracy. I n fact , st andards have been set for t he accuracy of t heir m easurem ent s. The least accurat e devices are rat ed as Level I t est ers. Level I I t est ers at t ain a higher degree of accuracy. Level I I - E t est ers also can calculat e quant it ies such as PSNEXT and ELFEXT needed t o qualify Cat egory 5E cables, and Level I I I t est ers can check out Cat egory 6 cables.

The Scope of a Test There are t wo scopes for t est s: channel or perm anent link ( also called a basic link) . These are illust rat ed in t he upper part of Figure 10.4:

Figu r e 1 0 .4 . Te st in g a ch a n n e l or a pe r m a n e n t lin k .

• •

A channel includes all of t he cabling syst em com ponent s bet ween a st at ion and a hub or a swit ch in a wiring closet . I t includes up t o 90 m et ers of horizont al cabling, a work area pat ch cord plugged int o an out let , and crossconnect s or int erconnect s in t he wiring closet . A perm anent link excludes t he pat ch cords. I t includes t he horizont al cabling and t he connect ors at each end of t he horizont al cabling.

N ot e The horizont al cabling port ion m ight include a t ransit ion point or a consolidat ion point .

The lower part of Figure 10.4 shows t he com ponent s t hat are included in a channel t est . The syst em s at t he ends of t he channel have been replaced wit h t est equipm ent .

Testing Fiber Optic Cable

An underst anding fiber opt ic cabling of fiber opt ic cable t est ing requires m ore background inform at ion t han can be included in t his book. This sect ion provides a brief discussion of a few of t he fact ors t hat are involved in t est ing fiber opt ic cable. First of all, t here is one boon. Crosst alk, a worrisom e problem for t wist ed- pair m edia, does not occur for fiber opt ic m edia. Som e of t he it em s t hat m ust be t est ed for fiber opt ic m edia include: •

•

• •

Sour ce opt ica l pow e r — I f t he source power is inadequat e, inform at ion t ransfer could be error prone or could fail. Signa l a t t e n u a t ion— Most at t enuat ion is caused by scat t ering of light due t o bouncing off t he cladding or bouncing off at om s in t he glass. Som e at t enuat ion is caused by absorpt ion of light by dopant s added t o t he glass. ( A dopant is an im purit y int ent ionally added t o an opt ical m edium t o change it s opt ical propert ies.) The light t hen is convert ed t o heat . W a ve le ngt h m e a sur e m e n t — Dat a m ust be t ransm it t ed using specified wavelengt hs. Less scat t ering occurs for long wavelengt hs, so great er cable lengt hs can be support ed wit h long wavelengt hs. Ca ble con t in u it y— The light used t o t ransm it dat a is invisible. A cable cont inuit y check is carried out by shining a harm less visible light ont o t he fiber. This m akes bends and breaks visible if t he sheat h has t orn.

N ot e A user should not look int o a fiber opt ic cable while a laser is act ively t ransm it t ing. Laser energy is high, and exposure t o a laser beam can cause blindness.

Cable fault s can be locat ed using an opt ical t im e- dom ain reflect om et er ( OTDR) . An OTDR also can m easure t he am ount of light t hat is being scat t ered back t o t he source. However, OTDRs are cost ly and usually are used only for long- dist ance single- m ode fiber links. A sim plified product called a fault finder can be used for LANs.

Summary Points • • • •

U.S. cabling st andards are published by t he Telecom m unicat ions I ndust ry Associat ion ( TI A) and t he Elect ronic I ndust ries Associat ion ( EI A) . I SO/ I EC st andards are used in ot her part s of t he world. A cat egory num ber is a st andard rat ing t hat is applied t o an ent ire cabling syst em , which in addit ion t o cable runs, includes out let s, pat ch cords, panels, connect ors, and cross- connect blocks. There are seven current ly defined cat egories of unshielded t wist ed- pair cable, num bered 1, 2, 3, 4, 5, 5E, and 6. A higher num ber corresponds t o bet t erqualit y cable. Most of t he inst alled base of cable is Cat egory 5 or Cat egory 3. Cat egory 5E and 6 are appropriat e for gigabit speeds.

•

•

•

• • •

•

An Am erican Wire Gauge ( AWG) value is t he inverse of t he t hickness ( in inches) of a wire. Cat egory 1- 5E LAN cable is predom inant ly 24 AWG. All dat a cable m ust be t est ed for im pedance, at t enuat ion, and near end crosst alk ( NEXT) . High- speed LAN cable is checked for equal level far end crosst alk ( ELFEXT) , power sum near end crosst alk ( PSNEXT) , worst pair- t opair ELFEXT, delay skew, and ret urn loss. Wirem ap t est ing checks pin t erm inat ion, cont inuit y, crossed or reverse pairs, short s, and split pairs. A Tim e Dom ain Reflect om et ry t est device is used t o locat e cable fault s. A cont inuit y and cable t est er can calculat e cable lengt h, perform wirem ap t est s, and perform TDR t est s. The product usually includes t one t est equipm ent . A cert ificat ion t ool is a high- end t est er t hat can det erm ine whet her inst alled cables m eet TI A Cat egory 5, 5E, or proposed Cat egory 6 or 7 requirem ent s. Fiber opt ic cable m easurem ent s include source opt ical power, signal at t enuat ion, wavelengt h m easurem ent , and cable cont inuit y.

References Copper and fiber cabling perform ance guidelines are specified in ANSI / TI A/ EI A- 568, " Com m ercial Building Telecom m unicat ions Cabling St andard," 2000. Guidelines for field t est ing Cat egory 3, 4, and 5 UTP cabling can be found in TI A/ EI A Technical Service Bullet in 67 ( TSB- 67) , " Transm ission Perform ance Specificat ions for Field Test ing of Unshielded Twist ed- Pair Cabling Syst em s," 1995. Ret urn loss and ELFEXT link perform ance param et ers are specified in TI A/ EI A- TSB95, " Addit ional Transm ission Perform ance Guidelines for 4- Pair 100 Ohm Cat egory 5 Cabling," 1999. Ot her docum ent s t hat m ay be of int erest include •

•

•

• • • •

TI A/ EI A- 569. " Com m ercial Building St andard for Telecom m unicat ions Pat hways and Spaces." 1998. TI A/ EI A- 570. " Resident ial and Light Com m ercial Telecom m unicat ions Wiring St andard." 1997. TI A/ EI A- 606. " Adm inist rat ion St andard for t he Telecom m unicat ions I nfrast ruct ure of Com m ercial Buildings." 1993. TI A/ EI A- 607. " Com m ercial Building Grounding/ Bonding Requirem ent s." 1994. Microt est , I nc. " Field Test ing Of High Perform ance Prem ise Cabling." 1998. " Cablet ron Syst em s Cabling Guide." 1996. ht t p: / / www.cablet ron.com / " How t o Effect ively Manage Your St ruct ured Cabling I nfrast ruct ure, An Anixt er Technology Whit e Paper." ht t p: / / www.anixt er.com /

Chapter 11. Auto-Negotiation The versat ilit y of Et hernet m eans t hat users have m any choices of m edia, speed, and feat ures. This could have t urned Et hernet int o a configurat ion night m are.

Fort unat ely, aut om at ic negot iat ion m echanism s were int roduced in t he m id- t o lat e 1990s. Aut o- Negot iat ion enables a pair of link part ners t o com m unicat e whenever eit her end is init ialized, reset , or reconfigured. The negot iat ion enables t hem t o discover an opt im al set of m ut ually support ed capabilit ies. Current ly, t wo separat e Aut o- Negot iat ion t echnology fam ilies exist : • •

1 0 M bps, 1 0 0 M bps, a n d 1 0 0 0 M bps t w ist e d- pa ir in t e r fa ce s Vendors have built single- speed and m ult ispeed t wist ed- pair int erfaces t hat support Aut oNegot iat ion. Twist ed- pair link part ners can negot iat e t heir best set of shared capabilit ies. 1 0 0 0 BASE- SX, 1 0 0 0 BASE- LX, a n d 1 0 0 0 BASE- CX in t e r fa ce s A uniform Aut o- Negot iat ion Prot ocol is used by all t he devices in t his fam ily, but t he prot ocol can be used only when like connect s t o like ( for exam ple, 1000BASESX t o 1000BASE- SX) . Param et ers such as t he use of full- duplex m ode and flow cont rol are negot iat ed.

This chapt er explains t wist ed- pair Aut o- Negot iat ion first . 1000BASE- X Aut oNegot iat ion has a subset of t he funct ionalit y of t he t wist ed- pair version and is discussed at t he end of t he chapt er.

Auto-Negotiation for Twisted-Pair Interfaces Som e of t he vendors t hat built t he first 10/ 100 t wist ed- pair Et hernet adapt ers and hubs creat ed propriet ary prot ocols t hat enabled link part ners t o negot iat e and check whet her bot h ends could operat e at 100Mbps and whet her bot h could operat e in fullduplex m ode. Users want ed a st andard negot iat ion prot ocol. The I EEE 802.3 com m it t ee act ed quickly and published 802.3u, which was based on Nat ional Sem iconduct or's NWay negot iat ion m et hod. I EEE Aut o- Negot iat ion was defined as an opt ional feat ure of 10BASE- T, 100BASE- T4, and 100BASE- TX int erfaces. I t becam e a m andat ory part of t he 1000BASE- T and 1000BASE- X int erfaces t hat were int roduced lat er. Twist ed- pair Et hernet int erfaces t hat support Aut o- Negot iat ion can aut om at ically det erm ine t he following: • • •

The speed at which t hey should operat e Whet her bot h part ners are capable of full- duplex m ode Whet her flow cont rol should be used in one direct ion, bot h direct ions, or not at all

Aut o- Negot iat ion m essages can carry som e addit ional inform at ion: • •

They can report rem ot e fault condit ions. They can report vendor- specific or product - specific dat a.

Auto-Negotiation and Ethernet Upgrades The capabilit y t o aut om at ically negot iat e link param et ers m akes a net work upgrade an easier j ob. For exam ple, t he devices in Figure 11.1 are connect ed t o a 10Mbps t wist ed- pair hub st ack. All t he syst em s in t his collision dom ain share t he 10Mbps bandwidt h. As m ore st at ions are added t o t he LAN in t he figure, each st at ion's share of t he bandwidt h shrinks. Event ually, t he LAN becom es congest ed and response t im e suffers.

Figu r e 1 1 .1 . Syst e m s con n e ct e d t o a st a ck of 1 0 M bps h u bs.

Using up- t o- dat e m ult ispeed Et hernet adapt ers sm oot hes t he pat h t o an upgrade. Current ly, 10/ 100 adapt ers are cheap. I f t he hub in Figure 11.1 is replaced wit h a 10/ 100 hub, perform ance im proves im m ediat ely for all t he syst em s t hat have 10/ 100 NI Cs. Each syst em will reinit ialize and st art t o operat e at 100Mbps.

N ot e 10Mbps segm ent s and 100Mbps segm ent s cannot belong t o t he sam e collision dom ain. A 10/ 100 " hub" act ually cont ains int ernal bridging t hat separat es t he 10Mbps segm ent s and 100Mbps segm ent s int o separat e collision dom ains. St at ions wit h old 10Mbps adapt ers will share a bandwidt h of 10Mbps. St at ions wit h 10/ 100 adapt ers will share a bandwidt h of 100Mbps.

N ot e The t op t hroughput on a busy collision dom ain is less t han 40 percent of t he physical capacit y—for exam ple, less t han 4Mbps on a 10Mbps LAN. Replacing a hub wit h a swit ch gives each st at ion a full- duplex 10Mbps or 100Mbps and can easily increase t hroughput by a fact or of 25 or bet t er.

Moving t o higher speeds, Figure 11.2 shows a set of 10/ 100 swit ches and a gigabit full- duplex repeat er t hat are connect ed t o a gigabit swit ch. The gigabit swit ch has 10 full- duplex port s. This can add up t o a 20Gbps capacit y.

Figu r e 1 1 .2 . Upgr a de opt ion s in a n Et h e r n e t LAN .

N ot e

A swit ch needs lot s of CPU power and big RAM m em ory resources t o handle fullduplex gigabit t raffic on all it s port s concurrent ly. Check vendor specificat ions and t est result s t o verify t he act ual num ber of short and long fram es per second t hat a gigabit swit ch can handle. The buzzword t o wat ch for is nonblocking. This m eans t hat t he swit ch can deliver all it s incom ing fram es wit hout dropping any, even when fram es are arriving at all port s at t op speed.

Aut o- Negot iat ion eases m oves and changes at t he gigabit level. For exam ple, server A in figure 11.2 shares bandwidt h wit h high- perform ance st at ions at t ached t o t he full- duplex repeat er on t he right . I f an increasing num ber of st at ions on t he left need t o access server A and t raffic is st eadily growing, server A can be m oved t o t he cent ral gigabit swit ch. I t will im m ediat ely have access t o a dedicat ed full- duplex gigabit bandwidt h of it s own.

N ot e Recall t hat a gigabit full- duplex repeat er also is called a buffered dist ribut or.

Aut o- Negot iat ion has anot her benefit : I t prevent s an inappropriat e device from connect ing t o a LAN. For exam ple, if a user connect s a 100BASE- T4 device t o a 10BASE- T/ 100BASE- TX hub and neit her t he device nor t he hub support s Aut oNegot iat ion, t he ent ire collision dom ain will be disrupt ed. However, a hub t hat support s Aut o- Negot iat ion will refuse t o set up a link t o an incom pat ible device and will prot ect t he ot her st at ions in t he collision dom ain.

Checking and Controlling Interfaces A LAN adapt er usually has a visible set of LEDs t hat report t he adapt er st at e and st at us. For exam ple, a green- lit LED m arked 100 indicat es t hat 100Mbps has been est ablished on t he link and t hat t he link is operat ional. Som et im es an ext ra LED blinks when dat a is being t ransm it t ed t hrough t he int erface. ( A st eady light indicat es t hat t he port is very busy.) SNMP can be very helpful when you want t o check t he speed of an int erface wit hout walking up t o it and looking at LEDs. SNMP also can be used t o check t he ot her param et ers t hat have been negot iat ed for t he int erface, such as full/ half- duplex m ode and flow cont rol. A net work adm inist rat or can configure a NI C m anually ( or via SNMP) , if t here is a reason t o do so. This can be applied eit her of t wo ways: •

The adm inist rat or leaves Aut o- Negot iat ion enabled, but specifies which capabilit ies will be announced. For exam ple, t he adm inist rat or m ight want t o force 100BASE- TX and prevent 10BASE- T.

•

The adm inist rat or specifies t he exact behavior t hat is desired and disables Aut o- Negot iat ion.

N ot e Som e vendor m anuals recom m end t urning off Aut o- Negot iat ion for rout er and server port s. Their logic is t hat it is bet t er t o cause t he link t o die if it cannot operat e exact ly as you wish ( for exam ple, in full- duplex rat her t han half–duplex m ode) . Unfort unat ely, people m ake m ist akes, and t he m anually configured int erfaces at each end of a link m ight be incom pat ible. A sensible alt ernat ive is t o configure each int erface t he way you t hink it should behave and t hen let it perform Aut o- Negot iat ion so t hat each end verifies t hat it s part ner is com pat ible. You can check link param et ers via a periodic SNMP poll t o m ake sure t hat you are get t ing what you expect .

Auto-Negotiation Functionality for Twisted-Pair Interfaces I f a pair of devices connect ed by a t wist ed- pair link support Aut o- Negot iat ion: • • •

Each int erface announces t he t echnology choices t hat it support s. Each indicat es whet her it support s full- duplex com m unicat ion. Each announces it s flow cont rol preferences.

A pair of 1000BASE- T int erfaces also negot iat es t o decide which will act as m ast er t im er or slave t im er for t he link. The m ast er uses it s own clock, and t he slave bases it s t im ing on t he incom ing dat a st ream .

N ot e Current ly, 100BASE- T2 is not im plem ent ed. I n t he fut ure, if som e vendor builds 100BASE- T2 adapt ers, t hese adapt ers also will perform a m ast er/ slave negot iat ion.

Twisted-Pair Technology Capabilities The negot iat ion procedure t hat is used is quit e st raight forward: 1. Each part y sends it s part ner a checklist indicat ing all of t he half- and fullduplex t echnologies t hat it can support . 2. A set of rules is applied t o t he values provided by each part ner. 3. The out com e of t hese rules det erm ines t he way t he link will operat e. Table 11.1displays t he current list of t ransm ission t echnologies. The choices are list ed in order of preference, from highest t o lowest . The highest - ranking capabilit y t hat bot h link part ners have in com m on is select ed. Not e t hat t he full- duplex version of a t echnology always ranks above t he half- duplex version.

I t m ight seem odd t hat t he 100BASE- T4 half–duplex version is ranked above t he 100BASE- TX half- duplex version. I f bot h part ners could support bot h of t hese t echnologies, 100BASE- T4 would be select ed. The reason is t hat 100BASE- T4 can run on Cat egory 3 cable, but 100BASE- TX requires Cat egory 5. The less dem anding t echnology is given preference j ust in case t he cable plant is not up t o par.

Ta ble 1 1 .1 . N e got ia ble Tr a n sm ission Te chn ologie s Te ch nology

Acce pt a ble Ca bling

1000BASE- T full- duplex

Four pairs of Cat egory 5 UTP

1000BASE- T half- duplex

Four pairs of Cat egory 5 UTP

* 100BASE- T2 full- duplex

Cat egory 3 UTP

100BASE- TX full- duplex

Two pairs of Cat egory 5 UTP

* 100BASE- T2 halfduplex

Cat egory 3 UTP

100BASE- T4 half- duplex

Four pairs of Cat egory 3 UTP

100BASE- TX half- duplex

Two pairs of Cat egory 5 UTP

10BASE- T full- duplex

Two pairs of Cat egory 3 UTP

10BASE- T half- duplex

Two pairs of Cat egory 3 UTP * Not im plem ent ed in product s

Negotiating Support for Flow Control Part ners t hat support full- duplex com m unicat ions will reveal whet her t hey can support flow cont rol m essages. Flow cont rol m ay be sym m et ric ( bot h part ners send PAUSE m essages) or asym m et ric ( only one part y sends PAUSE m essages) .

N ot e Flow cont rol and PAUSE m essages were described in Chapt er 5, " Full- Duplex Et hernet Com m unicat ion."

Two bit s are used t o describe a syst em 's flow cont rol capabilit y, which m ight be: • • • •

Does not support any flow cont rol capabilit y ( 00) Wishes t o send PAUSE m essages t o t he part ner, but does not wish t o receive t hem ( 01) Wishes t o send and receive PAUSE m essages ( 10) I s willing eit her t o send and receive PAUSE m essages, or j ust t o receive PAUSE m essages ( 11)

I f part ner A is capable of sending PAUSE m essages and part ner B is capable of receiving t hem , t he flow cont rol capabilit y will be enabled for t hat direct ion of t ransm ission. I f part ner B also can send and part ner A also can receive, flow cont rol is used in bot h direct ions ( sym m et rically) .

Determining Master/Slave Timer Roles 1000BASE- T com m unicat ions require one part ner t o act as link m ast er and t he ot her t o act as slave. The m ast er uses it s local clock t o t im e t ransm issions. The slave recovers clock t im ing from t he signals received from t he m ast er. Aut o- Negot iat ion chooses t he roles based on t hree rules: •

•

•

I f an adm inist rat or has m anually configured one end as m ast er and t he ot her end as slave, t he part ies will be given t hese roles. I f neit her side has a preconfigured role, a m ult iport device ( such as a hub or swit ch) will act as m ast er when com m unicat ing wit h a single- port device. I f neit her is preconfigured and bot h are single- port or bot h are m ult iport , each endpoint generat es a random num ber called a seed and sends it t o it s part ner. The part y t hat generat es t he bigger seed value becom es t he m ast er. I n t he unlikely event of a t ie, t hey st art over.

Parallel Detection for Twisted-Pair Cabling What happens if t he int erface at one end of t he link support s Aut o- Negot iat ion but t he ot her int erface does not ? A funct ion called parallel det ect ion is included in t he Aut o- Negot iat ion specificat ion. Parallel det ect ion enables a part y t o discover whet her a non- negot iat ing part ner has a 10BASE- T, 100BASE- T4, or 100BASE- TX int erface by exam ining t he signals arriving from t he part ner. I f t he local int erface support s t he part ner's t echnology, it t hen can swit ch t o t hat t echnology. However, only half- duplex m ode can be used, because t here is no way t o find out whet her t he part ner support s full–duplex com m unicat ion. The only way t o swit ch t he link t o full- duplex operat ion is t o configure it m anually at bot h ends and disable Aut o- Negot iat ion at t he capable end. Som e vendors have im plem ent ed a propriet ary feat ure t hat enables an int erface t o det ect whet her it is at t ached t o properly inst alled Cat egory 3, 4, or 5 cable. This could affect t he choice of speed or prevent a link from being init ialized if all perm it t ed opt ions required Cat egory 5 cable.

Exchanging Auto-Negotiation Data across Twisted-Pair Media How do you exchange configurat ion dat a wit h a t wist ed- pair link part ner if you don't know t he speed of t he part ner's int erface or how t he part ner encodes bit s ont o t he link? And how do you im plem ent it in a way t hat is backward com pat ible wit h old 10BASE- T adapt ers t hat were built before t here was any need for an aut oconfigurat ion prot ocol?

The solut ion t hat was chosen builds on a low- level m echanism t hat was int roduced wit h 10BASE- T. Recall t hat a 10BASE- T int erface execut es an ongoing cable check by sending a special signal called a link int egrit y pulse. This pulse is sent periodically ( roughly every 16 m illiseconds) when t he dat a t ransm it t er is idle. The link int egrit y pulse signal also is called a norm al link pulse ( NLP) . The t op of Figure 11.3 illust rat es t wo NLPs separat ed by a 16- m illisecond idle period.

Figu r e 1 1 .3 . N LPs a n d FLPs.

Old 10BASE- T int erfaces cont inue t o operat e correct ly if a burst of pulses is sent t o t hem inst ead of a single pulse. A burst is called a fast link pulse ( FLP) . All t wist edpair int erfaces capable of perform ing Aut o- Negot iat ion send FLPs during init ializat ion. Configurat ion param et ers are encoded wit hin each FLP. A pair of FLPs is illust rat ed in t he m iddle of Figure 11.3 During Aut o- Negot iat ion, a bundle of FLPs is sent roughly every 16 m illiseconds. Each FLP consist s of 33 pulse posit ions and carries a short 16- bit m essage. Sevent een odd- num bered pulses are used for clocking. Each of t he rem aining 16 even- num bered pulse posit ions represent s a dat a bit :

• •

The presence of a pulse in an even- num bered posit ion represent s 1. The absence of a pulse in an even- num bered posit ion represent s 0.

The bot t om of Figure 11.3 shows a dat a pat t ern t hat st art s wit h 1100111. The lines t hat represent clocking pulses are dot t ed t o m ake t he dat a signals st and out . FLPs are roughly 62.5µs apart .

Ba se Pa ge , M e ssa ge Pa ge s, a nd Un for m a t t e d Pa ge s The first 16- bit m essage is called t he base page. ( Som et im es it is called t he base pages base link code word.) Addit ional m essages consist of t hese com ponent s: •

•

An init ial m essage page t hat includes a code t hat ident ifies t he t ype of m essage. Depending on t he m essage t ype, zero, one, or m ore supplem ent ary unform at t ed pages. The t erm unform at t ed page is m isleading. Each of t hese pages has a specific form at t hat depends on t he t ype of m essage.

For m a t of t h e Ba se Pa ge Figure 11.4 shows t he layout of a base page. I t includes t hese com ponent s: • •

•

•

•

A 5 - bit se le ct or fie ld This field ident ifies t he base page t ype. An Et hernet ( 802.3) base page is ident ified by code 00001. ( The only ot her defined t ype is 802.9. The 802.9 st andard defines a prot ocol for carrying I SDN calls across a LAN.) Te ch nology a bilit y bit s This field consist s of 8 bit s t hat are used t o ident ify support ed t echnologies. ACK bit This bit is set t o 1 t o indicat e t hat at least t hree copies of t he part ner's base page have been received. Re m ot e fa u lt ( RF) bit This bit is set t o 1 t o signal t hat t here is som e kind of link fault . N e x t pa ge ( N P) bit I f set t o 1, t his indicat es t hat t he originat or is willing t o exchange one or m ore addit ional m essages. ( For exam ple, an addit ional m essage could be used t o describe a fault t hat has been det ect ed.)

Figu r e 1 1 .4 . For m a t of a t w ist e d- pa ir ba se pa ge .

N ot e An exchange of addit ional m essages occurs only if bot h t he local device and it s link part ner have set t heir next page bit s t o 1 during t he base page exchange.

The m eaning of each bit of t he base page t echnology abilit y field is displayed in Table 11.2. Set t ing a bit t o 1 m eans t hat t he adapt er is willing and able t o perform ing t he corresponding t echnology. An adapt er t ypically t ells it s part ner t hat several alt ernat ives are accept able. For exam ple, a 10/ 100Mbps adapt er can set bit s A0, A1, A2, and A3 t o 1 t o indicat e t hat any of t he following are accept able: • • • •

100BASE- TX 100BASE- TX 10BASE- T in 10BASE- T in

in full- duplex m ode in half- duplex m ode full- duplex m ode half- duplex m ode

The first it em on t his list t hat is accept able t o t he sender's part ner will be t he one t hat is chosen. I f t he part ner is picky and can operat e only in 100BASE- T4 or 1000BASE- T, t he negot iat ion fails and t he link is not set up. The t wo PAUSE bit s ( A5 and A6) were described earlier in t he sect ion " Negot iat ing Support for Flow Cont rol."

Ta ble 1 1 .2 . Te ch n ology Abilit y Bit Assignm e n t s Bit

Te ch nology

A0

10BASE- T

A1

10BASE- T full- duplex

A2

100BASE- TX

A3

100BASE- TX full- duplex

A4

100BASE- T4

A5

First PAUSE bit

A6

Second PAUSE bit

A7

Reserved for fut ure t echnology

The next four sect ions cont ain t echnical reference m at erial. You m ay wish t o skip ahead t o t he sect ion ent it led " Aut o- Negot iat ion for 1000BASE- X I nt erfaces."

For m a t of M e ssa ge Pa ge s a n d Un for m a t t e d Pa ge s Not e t hat 1000BASE- T is not m ent ioned in Table 11.2.1000BASE- T t echnology capabilit ies are announced in an addit ional m essage t hat consist s of a m essage page followed by t wo unform at t ed pages. The upper part of Figure 11.5 shows t he general layout of a m essage page. The first 11 bit s of a m essage page cont ain a m essage code t hat ident ifies t he m essage t ype. The rem aining 5 bit s ( from right t o left ) are described here:

Figu r e 1 1 .5 . For m a t of m e ssa ge pa ge s a n d u n for m a t t e d pa ge s.

• • •

•

•

N e x t pa ge bit — The value is 1 if anot her page will follow . M e ssa ge pa ge fla g— This bit is set t o 1 if t his is a m essage page. ACK bit — This bit is set t o 1 aft er receiving at least t hree copies of a page from t he part ner. ACK2 bit — This bit is set t o 1 t o indicat e t hat t he receiver support s t he t ype of m essage in t he received page and can act in accordance wit h t he inform at ion t hat it received. Toggle ( T) bit — This bit alt ernat es bet ween 0 and 1 in t he pages t hat are t ransm it t ed and is a very sim ple m essage- num bering m echanism . The receiver uses t he t oggle bit t o check t hat pages are being received in order and wit hout loss. ( The init ial value of t he t oggle bit is t he opposit e of t he value of bit A6 in t he sender's base page.)

A m essage page can be followed by zero, one, or m ore unform at t ed pages. The lower part of Figure 11.5shows t he layout of an unform at t ed page. The cont ent s of t he first 11 bit s depend on t he preceding m essage code. The last 5 bit s ( from right t o left ) have t he sam e m eaning as t he last 5 bit s of a m essage page. The m essage page flag has value 0 in an unform at t ed page. A device indicat es t hat it has no m ore pages t o send by set t ing t he next page bit t o 0.

I f part ner A has sent all of it s m essages, while part ner B has m ore t o send, part ner A t ransm it s null m essages t o acknowledge receipt of part ner B's m essages. A null m essage is a m essage page whose m essage code is 1.

Pa ge Ex ch a nge Pr ot ocol The prot ocol requires pages t o be sent m ult iple t im es t o be sure t hat t hey have been received successfully. The st eps are shown here: 1. Bot h syst em s send t heir base pages repeat edly wit h t he ACK bit set t o 0. 2. Aft er receiving at least t hree consecut ive consist ent copies of t he part ner's base page, a st at ion will t ransm it several copies of it s base page wit h t he ACK bit set t o 1. 3. I f bot h part ners have set t he next page bit t o 1 in t heir base pages, t he st at ions will t ransm it addit ional m essages. 4. On com plet ion, if bot h part ners have indicat ed support for a com pat ible configurat ion, t he link will be est ablished. Ot herwise, link set up fails and dat a cannot be t ransferred.

N ot e The reason for all t he repet it ion is t hat t his is very low- level com m unicat ion, and no cyclic redundancy check is included in a m essage. Receiving lot s of consist ent copies of a m essage is one way t o convince yourself t hat it is free of errors. Not e t hat m ult iple copies of a page are sent out very quickly because a fresh FLP is sent every 16m s. For exam ple, it t akes only 96m s t o send out six copies of a page.

Message Pages for 1000BASE-T Capabilities Table 11.3.describes t he layout of t he t hree pages t hat are used t o carry 1000BASET capabilit ies. A m essage code of 8 announces t hat 1000BASE- T inform at ion follows. Bit s U3 and U4 on t he first unform at t ed page announce whet her t he 1000BASE- T int erface is willing and able of operat ing in full- duplex or half–duplex m ode. Bit s U0, U1, and U2 det erm ine t he m ast er and slave roles for t he link part ners. Som et im es t he " seed" values in t he first 11 bit s of t he second unform at t ed page m ust be com pared t o break a t ie.

Ta ble 1 1 .3 . 1 0 0 0 BASE- T M e ssa ge Pa ge s Bit or Fie ld

D e scr ipt ion

Message Page First .11.1bit s Unform at t ed

Message code= 8; indicat es t hat 1000BASE capabilit ies follow

Ta ble 1 1 .3 . 1 0 0 0 BASE- T M e ssa ge Pa ge s Bit or Fie ld

D e scr ipt ion

Page 1 U0, U1, U2

Mast er/ slave bit s.

U0

1= Mast er/ slave role has been configured m anually. 0= Mast er/ slave role has not been configured m anually.

U1

I f m ast er/ slave role has been configured m anually ( U0= 1) , t hen 1= m ast er role and 0= slave role.

U2

I f m ast er/ slave role has not been configured m anually ( U0= 0) , t hen 1= m ult iport and 0= single- port .

U3

1000BASE- T full- duplex.

U4

1000BASE- T half- duplex.

U5- U10

Reserved, set t o 0.

Unform at t ed Page 2 U0- U10

Seed value for m ast er/ slave negot iat ion.

Table 11.4 cont ains t he det ails of how t he m ast er and slave roles are assigned. Basically, it com es down t o eit her assigning t he roles ( carefully) via m anual configurat ion or leaving t he choice open. I f t he choice is open, a m ult iport syst em becom es m ast er and it s at t ached single- port devices assum e t he slave role. The seed is used t o break t ies bet ween m ult iport / m ult iport or single- port / single- port link part ners.

Ta ble 1 1 .4 . D e t e r m inin g M a st e r a n d Sla ve Role s U0 U1 U2 for Pa r t n e r A

U0 U1 U2 for Pa r t n e r B

Ou t com e

1 1 X

1 0 X

Part ner A will be m ast er, and part ner B will be slave. ( They were m anually configured wit h t hese roles.)

1 1 X

1 1 X

Not allowed; configurat ion fails. ( Bot h are m ast ers.)

1 0 X

1 0 X

Not allowed; configurat ion fails. ( Bot h are slaves.)

0 X1

0 X 0 or

Part ner A is m ult iport , and part ner B eit her is single- 1 0 X port or has been configured as a slave. Part ner A is t he m ast er, and part ner B is t he slave.

Ta ble 1 1 .4 . D e t e r m inin g M a st e r a n d Sla ve Role s U0 U1 U2 for Pa r t n e r A

U0 U1 U2 for Pa r t n e r B

Ou t com e

0 X1

1 1 X

Alt hough part ner A is m ult iport , part ner B has been m anually configured as m ast er and part ner A will be slave.

0 X1

0 X1

Bot h are m ult iport . The part ner wit h t he higher seed becom es m ast er.

0 X0

0 X0

Bot h are single- port . The part ner wit h t he higher seed becom es m ast er.

Ot her com binat ion

Aut oconfigurat ion fails.

Message Codes The previous sect ion described t he 1000BASE- T pages t hat are sent wit h m essage code 8. Ot her m essage codes have been defined, and several are shown in Table 11.5. Message code 5 int roduces an organizat ionally unique ident ifier ( OUI ) and a dat a field whose cont ent has been defined by t he organizat ion t hat owns t he OUI . Recall t hat OUI s are 3- byt e prefixes adm inist ered by t he I EEE. ( See Chapt er 2, " LAN MAC Addresses." ) Message code 6 int roduces a PHY ident ifier and a dat a field whose cont ent is specific t o t he product wit h t hat PHY ident ifier. A PHY I dent ifier consist s of bit s 3- 24 of t he OUI assigned t o an int erface m anufact urer by t he I EEE, followed by a 6- bit m anufact urer's m odel num ber and a 4- bit m anufact urer's revision num ber. This adds up t o a t ot al of 32 bit s.

N ot e Not e t hat m essage codes 5 and 6 open up t he Aut o- Negot iat ion Prot ocol t o vendorspecific and product - specific enhancem ent s.

Ta ble 1 1 .5 . M e ssa ge Code s M e ssa ge Code #

M e ssa ge Code D e scr ipt ion

0

Current ly reserved for fut ure enhancem ent s.

1

The code for null m essages. The local part y sends null m essages when it has com plet ed it s t ransm ission and t he part ner is st ill sending.

Ta ble 1 1 .5 . M e ssa ge Code s M e ssa ge Code #

M e ssa ge Code D e scr ipt ion

2

Reserved for fut ure expansion of t he t echnology abilit y field. One unform at t ed page cont aining a t echnology abilit y field follows.

3

Reserved for fut ure expansion of t he t echnology abilit y field. Two unform at t ed pages cont aining a t echnology abilit y field follow.

4

Used for describing fault s. One unform at t ed page cont aining a rem ot e fault code follows. Fault codes include t hese: 0. Test ing t he rem ot e fault report ing operat ion1. Link loss2. Jabber3. Parallel det ect ion fault

5

Organizat ionally unique ident ifier t agged m essage. Four unform at t ed pages follow. They cont ain an OUI ( spread across t hree pages) and a dat a field specific t o t hat OUI .

6

PHY ident ifier t ag code m essage. Four unform at t ed pages follow. They cont ain a PHY ident ifier and a dat a field specific t o a device wit h t hat PHY I D.

7

100BASE- T2 t echnology m essage. 100BASE- T2 capabilit ies follow in t wo unform at t ed pages.

8

1000BASE- T t echnology m essage. 1000BASE- T capabilit ies follow in t wo unform at t ed pages.

Auto-Negotiation for 1000BASE-X Interfaces Aut o- Negot iat ion is a m andat ory funct ion for 1000BASE- X int erfaces. These include t he 1000BASE- SX and 1000BASE- LX fiber opt ic int erfaces and t he 1000BASE- CX copper int erface. I t is physically im possible for incom pat ible t ypes ( for exam ple, 1000BASE- SX and 1000BASE- LX, or 1000BASE- LX and 1000BASE- CX) t o com m unicat e across a link, so t he t echnology t ype is not negot iable. The capabilit ies t hat are negot iat ed include • •

Full–duplex or half- duplex capabilit ies. Flow cont rol capabilit y ( for a full–duplex link) . As was t he case for t wist edpair cabling, 2 bit s are used t o describe whet her t he syst em can send and/ or receive PAUSE m essages.

These Aut o- Negot iat ion m essages opt ionally also can describe rem ot e fault condit ions or include vendor- or product - specific dat a.

1000BASE-X Auto-Negotiation Implementation 1000BASE- X Aut o- Negot iat ion inform at ion is not sent in special pulses. Alt hough t he inform at ion cannot be sent in fram es, t he byt es t hat m ake up an Aut o- Negot iat ion page are t ransm it t ed t he sam e way t hat ot her dat a byt es are sent .

Recall t hat for 1000BASE- X, each dat a byt e is t ranslat ed int o a 10- bit dat a codegroup before it is sent across t he m edium . I n addit ion t o dat a code- groups, t here are several special nondat a code- groups t hat represent idle sym bols and m ark t he beginnings and ends of fram es. There also is a special com binat ion of t wo codegroups t hat m arks t he beginning of an Aut o- Negot iat ion page. Figure 11.6shows how an Aut o- Negot iat ion page is sent across a link.

Figu r e 1 1 .6 . Se n din g a n Au t o- N e got ia t ion pa ge a cr oss a 1 0 0 0 BASE- X lin k .

On arrival, t he t wo int roduct ory code groups are recognized and st ripped off. The t wo code- groups t hat follow are t ranslat ed t o 2 dat a byt es. These 2 byt es ( 16 bit s) m ake up an Aut o- Negot iat ion page.

N ot e The t ranslat ion of byt es int o code- groups is explained in Appendix A, " Physical Layer Encodings for Et hernet , Fibre Channel, FDDI , and Token Ring." Det ails are shown in Table 11.9.

Figure 11.6 shows t he form at of a 1000BASE- X base page: • •

•

• • •

The speed m ust be 1000Mbps, and t he part ners cannot t alk t o each ot her unless t heir t echnologies m at ch. Hence, none of t he bit s are used t o set a speed or define a t echnology. A pair of 1- bit flags is used t o st at e whet her t he int erface can operat e in halfduplex m ode or in full- duplex m ode. Two pause bit s announce whet her PAUSE fram es are support ed and indicat e t he desired asym m et ric or sym m et ric usage. These are coded exact ly as was described earlier, nam ely: 00= Do not support any flow cont rol capabilit y.01= Wish t o send PAUSE m essages but not receive t hem .10= Wish t o send and receive PAUSE m essages.11= Willing t o send and receive, or j ust receive. Two rem ot e fault bit s exist inst ead of one: 00= Link OK or device is incapable of fault det ect ion.01= Device is going offline.10= Link failure.11= Aut oNegot iat ion error; com m unicat ion not possible. The ACK bit is set t o 1 if aut oconfigurat ion m essages have been received from t he part ner. ( The bit is set aft er at least t hree consecut ive and m at ching copies of a page have been received.) The next page bit is set t o 1 t o indicat e a willingness t o send and receive addit ional m essages.

Succeeding m essage pages and unform at t ed pages follow t he sam e form at t hat was shown earlier in Figure 11.5.

Figu r e 1 1 .7 . Au t o- N e got ia t ion ba se pa ge for 1 0 0 0 BASE- X.

This im plem ent at ion has ret ained as m uch com pat ibilit y wit h t he t wist ed- pair im plem ent at ion as was possible.

Summary Points • •

•

•

•

• •

•

•

•

Aut o- Negot iat ion enables a pair of link part ners t o com m unicat e whenever eit her int erface is init ialized, reset , or reconfigured and t o discover an opt im al set of m ut ually support ed capabilit ies. There are t wo separat e Aut o- Negot iat ion t echnology fam ilies: t wist ed- pair int erfaces and 1000BASE- X int erfaces. Twist ed- pair Et hernet int erfaces can aut om at ically negot iat e t ransm ission speed, half- or full–duplex operat ion, and use of flow cont rol. I n addit ion, 1000BASE- T and 100BASE- T2 int erfaces can negot iat e m ast er/ slave roles. 1000BASE- X int erfaces can aut om at ically negot iat e half- or full- duplex operat ion and t he use of flow cont rol. Bot h fam ilies can use Aut o- Negot iat ion m essages t o describe a fault or t o report vendor- or product - specific inform at ion. Aut o- Negot iat ion can sim plify an adm inist rat or's j ob and ease upgrades. Parallel det ect ion enables a part y t o discover whet her a nonnegot iat ing t wist ed- pair part ner has a 10BASE- T, 100BASE- T4, or 100BASE- TXint erface by exam ining t he signals arriving from t he part ner. Aut o- Negot iat ion inform at ion is t ransm it t ed across a t wist ed- pair link encoded in fast link pulses ( FLPs) . Each FLP cont ains a 16- bit page. There are t hree t ypes of pages: base pages, m essage pages, and unform at t ed pages. For 1000BASE- X, pages are int roduced by a special pair of code- groups.

References Aut o- Negot iat ion is described in •

I EEE St andard 802.3, 1998 Edit ion. " Carrier Sense Mult iple Access wit h Collision Det ect ion ( CSMA/ CD) Access Met hod and Physical Layer Specificat ions." Chapt er 21, 28, 34, and 37.

For a good int roduct ion t o Aut o- Negot iat ion, see •

Verschueren, Ben. " Aut o- Negot iat ion I s Our Friend." Universit y of New Ham pshire I nt eroperabilit y Lab. Available from I nt ernet : ht t p: / / www.iol.unh.edu/

Part II: Bridging, Switching, and Routing 12 Et hernet Bridges and Layer 2 Swit ches 13 The Spanning Tree Prot ocol 14 Swit ches and Mult icast Traffic 15 Link Aggregat ion 16 VLANs and Fram e Priorit y 17 Source- Rout ing, Translat ional, and Wide Area Bridges 18 Rout ing and Layer 2/ 3 Swit ches

Chapter 12. Ethernet Bridges and Layer 2 Switches Chapt er 3, "Et hernet LAN St ruct ure," cont ained a brief sket ch of Et hernet bridges. This chapt er st art s t o fill in t he pict ure. Bridges have been around for a long t im e. Bridge product s evolved int o t oday's Layer 2 swit ches. Funct ionally, a Layer 2 swit ch is a bridge, but calling it a Layer 2 swit ch is good m arket ing. The new t it le t ells a cust om er t hat t he product has an up- t o- dat e im plem ent at ion. Com pared t o t he bridge product s of t he past , t oday's Layer 2 swit ches are fast er, have m ore port s, hold m ore inform at ion, have m ore securit y opt ions, and som et im es support virt ual LANs ( VLANs) . I f t he vendors who have m ade t hese im provem ent s want t o call t heir product s Layer 2 swit ches, t hat 's okay— but t hey st ill are bridges! The t erm s bridge and Layer 2 swit ch ( som et im es short ened t o swit ch) are int erchangeable in t his chapt er. The classic t erm , bridge, is used in t he init ial sect ions. The funct ions perform ed by bridges/ Layer- 2- swit ches are described in t he I EEE st andards docum ent s: • •

" 802.1D Media Access Cont rol ( MAC) Bridges" " 802.1Q Virt ual Bridged Local Area Net works"

Main Bridge/Switch Functions A bridge carries out t hree m ain funct ions: •

I t im proves perform ance by keeping local t raffic wit hin a lim it ed part of t he LAN.

• •

I t m akes it possible t o build a m ixed- speed LAN. For exam ple, t raffic m ust be bridged bet ween a 10Mbps Et hernet segm ent and a 100Mbps segm ent , or a 100Mbps segm ent and a 1000Mbps segm ent . Not e t hat a 10/ 100Mbps or 100/ 1000Mbps " hub" cont ains a hidden bridge. I t ext ends t he area covered by a LAN. The size of a collision dom ain is lim it ed by rules t hat rest rict cable lengt hs and t he num ber of repeat ers in a pat h. The scope of t hese rules ends when a fram e reaches a bridge port . The fram e m akes a fresh st art when it is t ransm it t ed out anot her bridge port .

For exam ple, in Figure 12.1 m ost of t he t raffic for t he workgroup on t he left is exchanged bet ween t he deskt op syst em s and server B. All syst em s in t he workgroup are at t ached t o 10Mbps hubs.

Figur e 1 2 .1 . Con t r ollin g t r a ffic a n d e x t e n ding t h e LAN dia m e t e r w it h a br idge .

During t he day, a sm all am ount of t raffic is direct ed at dat abase server C, which is connect ed t o a 100Mbps segm ent . Overnight , workgroup server B is backed up t o headquart ers file server D, locat ed 500 m iles away. Figure 12.1 •

Bridge 1 prevent s local workgroup t raffic bet ween deskt ops and server B from being t ransm it t ed t o t he 100Mbps hub on t he right or across t he wide area link t o headquart ers.

• •

Traffic bet ween st at ion A and dat abase server C passes across six segm ent s. However, because t he segm ent count ends and rest art s at bridge 1, t his pat h is perfect ly accept able. Traffic bet ween workgroup server B and file server D t ravels far beyond t he Et hernet LAN dist ance lim it at ions t hat were described in earlier chapt ers. Bridging m akes " local" area net works t hat span wide- area links com plet ely legit im at e.

I n earlier t im es, a LAN bridge was a sim ple device: You plugged it in, and it did it s j ob. Net work adm inist rat ors loved t he sim plicit y of t hese product s but alm ost im m ediat ely st art ed t o ask for ext ra feat ures t hat m et special needs. Product m anuals t hat originally consist ed of inst ruct ions on how t o unpack t he device and plug it in grew t hick and full of com plicat ed inst ruct ions. I t is t he goal of t his chapt er t o enable an adm inist rat or t o underst and t oday's fully feat ured Layer 2 swit ching product s and t o be able t o configure and m anage t hem .

Other Bridge Functions Bridges have been enhanced wit h funct ions t hat im prove perform ance, securit y, availabilit y, and m anageabilit y. Specific capabilit ies include • • •

•

•

•

Prevent ing som e t ypes of fram es from being forwarded t o part s of a LAN. This is done t o elim inat e superfluous t raffic or t o im pose som e securit y const raint s. An adm inist rat or m anually configures filt ering rules t o be applied t o fram es. Filt ering is described in t his chapt er. Elim inat ing single point s of failure by inst alling backup bridges and creat ing backup links bet ween bridges. The Spanning Tree Prot ocol ( STP) is t he m ost popular backup facilit y. The Spanning Tree Prot ocol is int roduced in t his chapt er and is st udied in det ail in Chapt er 13, "The Spanning Tree Prot ocol." Com bining a group of links bet ween bridges so t hey appear t o be a single link. This is called link aggregat ion. Aggregat ing links is preferable t o using backup links because t he full bandwidt h provided by ext ra links is available during norm al operat ion. Link aggregat ion is discussed in Chapt er 15, " Link Aggregat ion." Set t ing priorit ies so t hat t im e- sensit ive fram es can be t ransm it t ed quickly. Priorit y t ags are described in Chapt er 16, " VLANs and Fram e Priorit y." Support ing VLANs, which can group users by t heir need t o com m unicat e wit h one anot her and share com m on dat a rat her t han t heir physical locat ion. VLANs are covered in Chapt er 16. Support ing net work m anagem ent via t he Sim ple Net work Managem ent Prot ocol ( SNMP) . High- end product s also have a rem ot e m onit or ( RMON) capabilit y.

Vendors always are t rying t o get ahead of t he pack. To get a com pet it ive edge, som e vendors offer new feat ures t hat are based on propriet ary prot ocols. For exam ple, several vendors have creat ed propriet ary solut ions t hat m ake a set of parallel links behave like a single link. The I EEE 802.1 com m it t ee has writ t en a specificat ion ( 802.3ad) t hat describes a st andards- based way t o aggregat e links. Som e vendors current ly offer a m odified Spanning Tree Prot ocol t hat swit ches over from a failed

com ponent t o live backup equipm ent m ore quickly. Event ually, t his also m ight becom e st andardized. When a vendor com es up wit h a new winning st rat egy, t he advant age usually does not last long. Ot her vendors are quick t o copy t he capabilit y. Cust om ers t hen clam or for a version t hat works across vendor product s. Event ually, it m akes sense t o cooperat e on producing a st andard version, and so yet anot her I EEE subcom m it t ee is convened. Bridge st andards are published in t he 802.1 series of publicat ions. Each new subcom m it t ee is given a nam e t hat t acks one or m ore let t ers ont o 802.1. The fact t hat bridges ( t hat is, Layer 2 swit ches) are a hot area is reflect ed in t he fact t hat all of t he let t ers from a t o z have been used up. Current com m it t ees are nam ed in a new t wo- let t er sequence ( aa t o az) .

Collision Domains At t he out set , it probably is easiest t o underst and bridge funct ions in t he cont ext of t he classic bridges used in t radit ional coax Et hernet s. A st at ion on a coax LAN segm ent can see all t he fram es t hat are t ransm it t ed by any st at ion on it s segm ent . I f t hat segm ent is connect ed t o ot her segm ent s by repeat ers, it sees every fram e t hat is sent by t he st at ions on any of t hese segm ent s. I f t wo st at ions st art t o send at t he sam e t im e, t heir fram es will collide. This is t he reason t hat a net work m ade up of segm ent s connect ed t o one anot her by repeat ers was called a collision dom ain. Figure 12.2 displays a coax- based collision dom ain m ade up of t hree segm ent s. When st at ion A on segm ent 1 t ransm it s a fram e, it is repeat ed ont o segm ent s 2 and 3.

Figur e 1 2 .2 . A collision dom a in m a de u p of coa x se gm e n t s.

Figure 12.3 shows a 10BASE- T collision dom ain m ade up of 10 t wist ed- pair segm ent s connect ed by hubs. When st at ion A sends a fram e, hub 1 repeat s it ont o all it s ot her segm ent s, including segm ent s t hat connect t o hub 2 and hub 3. Hub 2 and hub 3 each repeat t he fram e ont o all segm ent s except for t he one on which it arrived. I f t wo st at ions t ransm it fram es at roughly t he sam e t im e, t he fram es will collide.

Figu r e 1 2 .3 . Figu r e 1 2 .3 A collision dom a in for t w ist e d- pa ir se gm e n t s.

Bridged Collision Domains Bridges part it ion a LAN int o separat e collision dom ains and prevent purely local t raffic from leaving a collision dom ain. I f t he LAN can be part it ioned using bridges so t hat each workgroup is cont ained wit hin it s own separat e collision dom ain, t he am ount of t raffic in each collision dom ain and t he num ber of collisions—is reduced appreciably. This t ranslat es int o m ore bandwidt h for each user and quicker response t im es. The bridge in Figure 12.4 part it ions a LAN int o t hree collision dom ains: X, Y, and Z. Figure 12.4 shows a classical Et hernet LAN wit h a coaxial cable m edium .

Figur e 1 2 .4 . A coa x Et h e r n e t LAN m a de u p of collision dom a in s con n e ct e d by a br idge .

Segm ent 1 and segm ent 2 in Figure 12.4 are connect ed by a repeat er, so t hese segm ent s m ake up a single collision dom ain. This m eans t hat if st at ion A and st at ion D st art t o t ransm it at roughly t he sam e t im e, t he fram es will collide, and bot h st at ions will have t o back off and t ry again lat er. Segm ent 3 and segm ent 4 each are separat e collision dom ains.

Transparent Bridging The bridges t hat are used in Et hernet LANs are called t ransparent bridges. When a fram e is sent from a source in one collision dom ain t o a dest inat ion in anot her, a t ransparent bridge forwards t he fram e based on inform at ion it has learned by observing t he t raffic at each port . For exam ple, t he bridge in Figure 12.4 learns addresses as follows: •

Through port 1, t he bridge can see all t he fram es t hat t raverse collision dom ain Y. I t records t heir source MAC addresses.

Assum ing t hat all st at ions in t he figure are act ively using t he net work, t he bridge will discover t he port t hrough which each syst em is reached. I f st at ion A sends a fram e t o server E, t he bridge will ignore it . But if st at ion A sends a fram e t o server I , t he bridge will absorb t he fram e and t ransm it it t hrough port 2.

By isolat ing local t raffic, t he bridge in Figure 12.4 m akes it possible for t hree fram es t o be t ransm it t ed at t he sam e t im e. For exam ple, st at ion A could t ransm it a fram e t o server E at t he sam e t im e t hat server G sends a fram e t o st at ion F and server I sends a fram e t o st at ion J.

A Br idge in a Tw ist e d- Pa ir LAN Figure 12.5 shows an Et hernet LAN const ruct ed using hubs and t wist ed- pair cable. The role of t he bridge is t he sam e. The t wo hubs at t he t op of Figure 12.5 are connect ed and form a single collision dom ain. Just as in Figure 12.4, t he bridge part it ions t he LAN int o t hree collision dom ains. The bridge in Figure 12.5 m akes it possible for t hree fram es t o be t ransm it t ed at t he sam e t im e.

Figu r e 1 2 .5 . Figu r e 1 2 .5 A t w ist e d- pa ir Et h e r n e t LAN m a de u p of collision dom a in s con n e ct e d by a br idge .

A Br idge in a M ix e d- M e dia LAN

Wit h a helping hand from bridges, a LAN can be const ruct ed from a m ixt ure of coax segm ent s and t wist ed- pair segm ent s t hat operat e at different speeds. I n Figure 12.6, coax and t wist ed- pair collision dom ains are connect ed by a bridge.

Figur e 1 2 .6 . A LAN m a de u p of coa x a n d t w ist e d- pa ir collision dom a in s.

A St a t e - of- t h e - Ar t Sw it ch e d LAN Figure 12.7 shows a st at e- of- t he- art LAN built using four Layer 2 swit ches. Each swit ch port connect s t o a single segm ent .

Figur e 1 2 .7 . A sw it ch e d t w ist e d- pa ir Et h e r n e t .

I f a workst at ion or server has an old adapt er t hat operat es only in half- duplex m ode, it s segm ent will be a collision dom ain. I f a st at ion has an up- t o- dat e adapt er, it will operat e in full- duplex m ode, and CSMA/ CD will not be used. A segm ent t hat links a pair of swit ches will operat e in full- duplex m ode.

N ot e I f each of t he 10 syst em s in t he Figure 12.7 had an old adapt er t hat was incapable of full- duplex operat ion, t here would be 10 collision dom ains in t he figure.

The swit ches am plify t he bandwidt h available on t he LAN by a large fact or. Fram es can be in t ransit across every segm ent at t he sam e t im e. The t hroughput is even higher for endpoint s t hat support full- duplex t ransfer because t wo fram es can be in t ransit on a segm ent at t he sam e t im e: one in each direct ion. Not e t hat from t he point of view of a swit ch port , a LAN is divided int o t wo com ponent s. One com ponent consist s of syst em s t hat are reached t hrough t hat port , and t he ot her com ponent cont ains t he rest of t he LAN. For exam ple, in Figure 12.7, for port 1 on swit ch 1, t he first com ponent consist s of st at ion A, and t he second is everyt hing else. But for port 1 on swit ch 3, t he first com ponent includes all t he syst em s connect ed t o swit ch 1 and swit ch 2, and t he second includes all t he syst em s at t ached t o swit ch 3 and swit ch 4. Figure 12.8 shows what happens when a hub linked t o several st at ions is connect ed t o port 7 on swit ch 4. That port m ust now operat e in half- duplex CSMA/ CD m ode.

Figur e 1 2 .8 . Con n e ct in g a sw it ch t o a h u b.

Transparent Bridge Internals Now it is t im e t o look inside bridges and find out what m akes t hem t ick. The m ain funct ion of a bridge is t o forward any fram e whose source and dest inat ion are on different sides of t he bridge. •

•

I f t he fram e's dest inat ion is reached t hrough t he fram e's arrival port , t he bridge discards t he fram e. I f t he fram e's dest inat ion is not reached t hrough t he arrival port , t he bridge forwards t he fram e.

Learning How does a bridge know where a st at ion wit h a part icular MAC address is locat ed? The m et hod, which is called learning, is st raight forward: • •

•

The bridge wat ches all t he t raffic at each of it s port s. The bridge not es t he source MAC address of each fram e and t he port at which t he fram e was observed. The bridge adds what it learns t o a t able called t he filt ering t able. Learned ent ries also are called dynam ic ent ries.

N ot e The I EEE nam e for t his t able is t he filt ering dat abase. Vendors have added lot s of ot her nam es, however. Som e use t he t erm forwarding dat abase or forwarding t able for t he set of dynam ically learned ent ries and use st at ic dat abase or filt ering dat abase for t he set of m anually configured ent ries. I n a real im plem ent at ion, a bridge perform s a single t able lookup t o decide how a fram e should be handled. The t erm filt ering t able has been chosen t o represent t hat t able in t his book.

Som e bridge ( Layer 2 swit ch) product s support t ables t hat can hold t housands or t ens of t housands of learned ent ries. Now t he processing st eps can be rephrased m ore accurat ely: 1. I f t he fram e's dest inat ion is in t he filt ering t able and is reached t hrough t he arrival port , t he bridge discards t he fram e. 2. I f t he fram e's dest inat ion is in t he filt ering t able and it s exit port is different from t he arrival port , t he bridge forwards t he fram e t hrough t he exit port . 3. I f t he fram e's dest inat ion is not in t he filt ering t able, t he bridge forwards t he fram e t hrough all port s ot her t han t he arrival port . This reason t hat t his t ype of bridge is called " t ransparent " is t hat you can plug it in and forget it . I t is effect ive, and it s act ions are invisible t o users.

Filtering Table Entries Table 12.1 shows a few sam ple learned filt ering t able ent ries. Each ident ifies a MAC address and t he num ber of t he port at which t he address was observed. Use of t he t able is st raight forward. For exam ple, if a fram e wit h t he dest inat ion MAC address shown in t he first row ( nam ely, 00- 60- 08- 1E- AE- 42) arrives from bridge port 2, t he bridge will ignore it . But if a fram e wit h t his dest inat ion arrives from any ot her port , t he bridge will t ransm it t he fram e out port 2.

Ta ble 1 2 .1 . Sa m ple En t r ie s fr om a Br idge Filt e r in g Ta ble D e st in a t ion M AC Addr e ss

Tr a n sm it Por t

St a t u s

00- 60- 08- 1E- AE- 42

2

Learned

00- 60- 08- BD- 7D- 1A

3

Learned

00- 90- 27- AE- B9- 1D

1

Aged out

Not e t hat t he t hird ent ry in t he t able is m arked " aged out ." Every t im e a bridge observes a MAC address, t he bridge rest art s a t im er associat ed wit h it s ent ry. I f t he st at ion wit h t hat address st ops sending fram es, it s ent ry will t im e out and be rem oved from t he t able. This m akes good sense: There is no point in clut t ering t he t able wit h MAC addresses for st at ions t hat have been shut down for t he day, have been given a new NI C, or have been rem oved from t he LAN. A bridge aut om at ically t hrows out old ent ries t o keep up wit h t he changing LAN environm ent . The usual default age- out t im e is 5 m inut es, but t his can be changed by t he LAN adm inist rat or. A short t im e such as 5 m inut es is a good choice if you want t o keep t he t able size as lean as possible. I t also assures t hat t raffic will be capable of reaching a syst em short ly aft er it has been m oved or it s NI C has been replaced.

N ot e To m ake room for new ent ries, som e bridges drop a configured percent age of t he oldest filt ering t able ent ries when t he num ber of ent ries nears t he t able's m axim um capacit y.

Static Filtering Information LAN adm inist rat ors loved t he plug- and- play sim plicit y of t ransparent bridges, but t hey prom pt ly dem anded an abilit y t o configure and cont rol t heir bridges. Vendors react ed quickly, giving LAN adm inist rat ors t he abilit y t o select ively cont rol how t raffic is forwarded by m anually configuring st at ic filt ering t able ent ries. St at ic ent ries do not age out . St at ic ent ries are used for t he following purposes: • • •

To expedit e t he forwarding of t raffic t o a server To im pose securit y const raint s To im prove perform ance by blocking out ext raneous fram es

Usin g a St a t ic En t r y for Efficie n t For w a r ding

You m ight want t o prevent t he filt ering t able ent ry of an im port ant server from ever aging out of t he t able. I t is easy t o do t his: You j ust creat e a st at ic filt ering t able ent ry t hat m aps t he server's MAC address t o it s t ransm it port . For exam ple, for t he net work in Figure 12.9, an ent ry can st at e t hat Web server A ( wit h MAC address 08- 00- 20- 5A- 01- 6C) is reached via bridge port 2. This prevent s t he flooding of fram es ont o ot her segm ent s t hat norm ally would occur if t here current ly was no dynam ic ent ry for t he dest inat ion. The result is a reduct ion in ext raneous t raffic.

Figu r e 1 2 .9 . For w a r din g fr a m e s u sin g a st a t ic e n t r y.

Sa m ple Filt e r in g Ta ble For m a t Table 12.2 shows an exam ple of how filt ering inform at ion can be organized int o a t able. Ent ry 1 was learned dynam ically; t hrough observat ion, it was learned t hat MAC address 00- 60- 08- 1E- AE- 42 is reached t hrough port 2. The second ent ry cont ains t he st at ic forwarding inform at ion for Web server A. Any fram es arriving from port s ot her t han 2 and addressed t o Web server A will be t ransm it t ed t hrough bridge port 2. Ent ries 3 and 4 are used t o cont rol access t o t he Hum an Resources Server, and ent ry 5 is used t o cut out excess I PX t raffic. These are explained in t he sect ions t hat follow. Each ent ry in Table 12.2 ident ifies only one out going t ransm it port . However, in general, an ent ry can list several out going port s t hrough which fram es should be

t ransm it t ed. For exam ple, fram es sent t o a part icular m ult icast address m ight need t o be forwarded t hrough m ult iple port s.

Ta ble 1 2 .2 . St a t ic M AC Addr e ss a n d Filt e r in g En t r ie s En t r y N u m be r

D e st ina t ion

Pr ot ocol Sour ce Por t

Tr a n sm it Por t s

St a t u s

1

00- 60- 08- 1E- Any AE- 42

2

Any

2

Learned

08- 00- 20- 5A- Any 01- 6C

Any

2

St at ic

To Web server A.

3

00- 10- 83- 34- Any BA- 12

1

None

St at ic

Prot ect s HR Server from users on segm ent 1.

4

00- 10- 83- 34- Any BA- 12

2

3

St at ic

Allows access t o HR Server by users on segm ent 2.

5

Any

Any

None

St at ic

All I PX t raffic is local t o segm ent 1.

I PX

Com m e n t

The disadvant age of using a st at ic ent ry is t hat t he adm inist rat or m ust be very careful t o updat e a syst em 's ent ry when t he syst em is m oved or when it s NI C is replaced. I f t here is a st at ic ent ry for a MAC address, no dynam ic dat a is recorded for t hat MAC address. I f t he syst em is m oved, t he correct dynam ic inform at ion describing it s new locat ion will not be put int o t he filt ering t able because of t he presence of a st at ic ent ry. The filt ering t able also m ust be updat ed if t here is a st at ic ent ry for a syst em t hat is rem oved from t he LAN. Ot herwise, t he t able will becom e clogged wit h st ale, useless dat a.

I m posin g a Se cu r it y Con st r a in t Adm inist rat ors oft en wish t o cont rol t he flow of fram es across a bridge in order t o im pose som e securit y rules. An adm inist rat or m ight want t o rest rict access t o a specific dest inat ion MAC address based on t he bridge port at which a fram e arrives. For exam ple, in Figure 12.10, t he LAN adm inist rat or want s t o prevent all users at t ached t o segm ent 1 from accessing t he Hum an Resources Server on segm ent 3, while users on segm ent 2 are allowed t o access t he server.

Figu r e 1 2 .1 0 . Con t r ollin g a cce ss via a st a t ic filt e r in g t a ble e n t r y.

The st at ic inform at ion used t o prot ect t he Hum an Resources Server is displayed in ent ries 3 and 4 in Table 12.2.

I m pr ovin g Pe r for m a n ce by Filt e r in g on a Pr ot ocol Many bridge product s enable an adm inist rat or t o cont rol t raffic based on t he Layer 3 prot ocol carried in t he fram e. For exam ple, Figure 12.11 shows t hat t he workgroup syst em s on segm ent 1 access a local Net Ware file server via I PX. The syst em s use TCP/ I P t o connect t o t he Web servers t hat are on ot her segm ent s. The adm inist rat or wishes t o block I PX t raffic from ent ering or leaving segm ent 1. There is no valid reason for I PX t raffic t o flow bet ween t his segm ent and t he ot her LAN segm ent s.

Figu r e 1 2 .1 1 . Block in g pr ot ocol t r a ffic.

I f a fram e wit h an unrecognized dest inat ion MAC address t hat carries I PX inform at ion is received at port 1, t he bridge will not t ransm it t he fram e t hrough port s 2 and 3. However, a fram e t hat carries I P inform at ion will be t ransm it t ed. I n addit ion, I PX fram es originat ing on segm ent 2 or 3 will be blocked from being sent t hrough port 1. Ent ry 5 in Table 12.2 m akes sure t hat no I PX t raffic is carried across t he bridge.

Con t r ollin g Tr a ffic Using Ot h e r At t r ibu t e s Som e bridges can im pose const raint s t hat are based on ot her header inform at ion. For exam ple, fram es t hat carry TCP/ I P applicat ion dat a have Layer 4 headers t hat ident ify t he applicat ion. Som e bridges even can m ake forwarding decisions based on whet her a part icular byt e pat t ern appears at som e specific locat ion in a fram e. Anot her opt ion t hat has becom e popular is t he abilit y t o lim it t he t ot al num ber of broadcast or m ult icast fram es t hat m ay be forwarded per second.

N ot e The I EEE 802.1D bridge st andard st at es t hat const raint s can be based on t he source port and dest inat ion MAC address. Som e vendors added t he abilit y t o cont rol t raffic using elaborat e crit eria because som e cust om ers want ed t his feat ure. Ot her vendors offer low- cost product s t hat can filt er only on t he dest inat ion MAC address.

Layer 2 Switch Architecture What happened t hat caused vendors t o build a new generat ion of bridge product s and invent a new nam e for t hem ? • • • •

LANs got bigger. Bandwidt h needs increased. Vendors got sm art er. Hardware got bet t er and cheaper.

The m ost im port ant hardware innovat ion was t he int roduct ion of low- cost applicat ion specific int egrat ed circuit ( ASI C) chips. The st eps in a com put er program ( or in a group of com put er program s) can be convert ed t o hardware logic t hat is et ched ont o an ASI C chip. The result is t hat t he program s run a lot fast er. This act ually is not a new idea. For years, t here have been chips t hat perform ed specialized com put e- int ensive funct ions such as encrypt ion or dat a com pression. What is new is t he abilit y t o design special- purpose chips quickly and produce t hem relat ively inexpensively. The ASI C chips used in swit ches cont ain on- chip program logic, a sm all general m icroprocessor, and m em ory. The am ount of logic or m em ory t hat fit s on a chip increases every year.

Store-and-Forward versus Cut-Through According t o t he I EEE 802.1D bridge st andard, a bridge should process each incom ing user fram e in a st ore- and- forward m anner. That m anner is det ailed here: 1. Wait unt il an ent ire fram e has been received. 2. Check t he fram e- check sequence, and discard t he fram e if it has been corrupt ed. 3. Discard runt s t hat result from collisions, overly long fram es, and m alform ed fram es. 4. For a good fram e, check t he filt ering t able and, if appropriat e, relay t he fram e t hrough one or m ore port s. Operat ing in st ore- and- forward m ode enables a bridge t o weed out a lot of net work debris inst ead of forwarding it . However, st ore- and- forward also slows down delivery. Several vendors offer one or bot h of t he following fast - forwarding cut - t hrough opt ions: •

•

Fram e t ransm ission st art s as soon as t he dest inat ion address has been received and looked up. Hence, t he bit s at t he head end of a fram e will be going out on t he wire while t he rest of t he fram e st ill is being received. This m eans t hat fram es t hat are m alform ed or t hat have a bad fram e check sequence are forwarded unt il t he flaw is discovered. Alt ernat ively, fram e t ransm ission st art s as soon as an init ial valid 64 bit s have been received. This reduces t he chance of forwarding an errored fram e because collision fragm ent s and ot her m alform ed fram es will be det ect ed and discarded.

Som e product s t ry t o give you t he best of bot h worlds. I f t he error rat e reaches a preset t hreshold, t he swit ch changes from cut - t hrough t o st ore- and- forward m ode.

Parallel Processing with ASICs Whet her st ore- and- forward or cut - t hrough m ode is used, whenever a fram e arrives, a bridge m ust search it s filt ering t able t o see whet her t here is an ent ry t hat indicat es what should be done wit h t he fram e. As t he size of LANs has increased, t he size of t his t able also has increased. This is a j ob for ASI Cs! Typically, each swit ch line card in a Layer 2 swit ch cont ains ASI Cs t hat process incom ing fram es. Alt hough t he bridge filt ering t able is supervised and m aint ained by a cent ral CPU, working copies of t he t able are loaded int o t he m em ory of each ASI C. Table lookups are perform ed by each ASI C, enabling m any lookups t o be carried out in parallel. This gives perform ance a big boost . For furt her com m ent s on how fram es are processed, see Chapt er 18, " Rout ing and Layer 2/ 3 Swit ches," which cont ains a discussion of Layer 2/ 3 swit ch archit ect ure.

Basic Bridge Layer Structure Figure 12.12 shows t he sim ple prot ocol st ruct ure of a basic bridge. Each port has a physical- layer and MAC- layer com ponent . The bridge perform s learning, filt ering t able lookups, and relaying of fram es.

Figu r e 1 2 .1 2 . St r u ct u r e of a ba sic br idge .

Building Redundancy into a LAN

The effect of a link or bridge failure ranges from a m inor inconvenience t o a crisis t hat result s in t he loss of subst ant ial am ount s of product ivit y. Many users want t o build redundancy int o t heir bridged net works t o prevent a single point of failure. The Spanning Tree Prot ocol is t he st andard, universally accept ed m et hod of building redundancy int o a bridged Et hernet LAN. I t enables t he bridges in a LAN t o act ivat e backup links when connect ivit y fails. A second m echanism called link aggregat ion enables a set of links connect ing t wo syst em s t o behave like a single link. I f one link fails, it s t raffic is divert ed ont o t he rem aining live links. The subsect ions t hat follow present brief int roduct ions t o t hese t echnologies. Spanning Tree det ails are present ed in Chapt er 13, and link aggregat ion is discussed in Chapt er 15.

The Spanning Tree Protocol Transparent bridges and redundant pat hs do not m ix. Figure 12.13 shows a sim ple exam ple. The t op part of t he figure shows syst em s connect ed on a coax LAN. The bot t om part shows t he sam e set of syst em s connect ed using t wist ed- pair cabling and hubs. Bot h versions are bridged in t he sam e way.

Figu r e 1 2 .1 3 . A r e du n da n t br idge con n e ct ion .

I f st at ion A t ransm it s a fram e addressed t o server B on collision dom ain Y, bot h bridges will forward t his fram e ont o collision dom ain Y. When bridge 2 t ransm it s it s copy of t he fram e, port 2 on bridge 1 observes t he fram e, reads it s source address, and concludes t hat st at ion A is on collision dom ain Y. Now bridge 1 t hinks t hat st at ion A is on bot h collision dom ain X and collision dom ain Y! The sam e t hing happens t o bridge 2 when port 2 at bridge 2 observes t he copy of t he fram e t hat was forwarded ont o collision dom ain Y by bridge 1. I t does not t ake long for t he whole LAN t o fall down in a heap of confusion. I n fact , t here can be only one pat h bet ween any t wo st at ions t hat are on t he sam e Et hernet LAN. Two pat hs creat e a loop. A LAN built using t ransparent bridges can't have any loops in it ; it m ust have a t ree st ruct ure.

N ot e

Bridges are t he nodes in t he t ree.

Fort unat ely, t he Spanning Tree Prot ocol let s you have your t ree and get redundancy, t oo. The Spanning Tree Prot ocol convert s a LAN t hat cont ains one or m ore loops int o a t ree- shaped LAN by blocking redundant bridge port s. To accom plish t his, bridges exchange Bridge Prot ocol Dat a Unit ( BPDU) m essages t hat enable t hem t o agree on an init ial t ree- shaped t opology and, aft er t he failure of som e com ponent , change t he t opology t o repair broken pat hs. I n Figure 12.14, a redundant t opology has been convert ed t o a t ree by blocking port 2 on bridge 2. A blocked port does not forward t raffic and does not learn new ent ries for it s bridge's filt ering t able.

Figu r e 1 2 .1 4 . Re m ovin g a loop by block in g a por t .

More det ails about t he inner workings of t he Spanning Tree Prot ocol are available in Chapt er 13.

Link Aggregation Several Layer 2 swit ch vendors offer a product feat ure t hat enables several lines t o act like a single line. St at ed different ly, several port s on a swit ch cooperat e and behave like one port . Vendors have several nam es for t his capabilit y: t runking, port aggregat ion, inverse m ult iplexing, and link aggregat ion. The I EEE has chosen t he t erm link aggregat ion, so t hat is t he one t hat will be used here.

Link aggregat ion is illust rat ed in Figure 12.15. I n t he figure, four 100Mbps links bet ween swit ch 1 and swit ch 2 have been com bined int o one 400Mbps aggregat ed link.

Figu r e 1 2 .1 5 . Lin k a ggr e ga t ion .

The Spanning Tree Prot ocol t reat s an aggregat ed link as a single link.

Handling Multicasts The bridge learning process im proves perform ance: A unicast fram e whose dest inat ion has been learned is forwarded only t hrough t he port t hat leads t o t he dest inat ion—it does not get sent ont o ot her part s of t he LAN. An adm inist rat or can ent er st at ic inform at ion t hat reduces unnecessary unicast t raffic even furt her. St em m ing t he flow of m ult icast fram es is anot her challenge. According t o t he original rule of m ult icast ing, if even one st at ion on a LAN j oins a m ult icast group, every st at ion on t he LAN will see all t he fram es sent t o t hat m ult icast group. As m ult icast applicat ions becom e m ore prevalent , following t his rule will undo t he efficiencies t hat have carefully been built int o t he new swit ch devices.

IGMP Snooping and GARP Bridge vendors quickly devised a st opgap m et hod called I GMP snooping t hat channels m ult icast fram es t o group m em bers. The I nt ernet Group Managem ent Prot ocol ( I GMP) is t he prot ocol t hat TCP/ I P syst em s use t o j oin and leave an I Pbased m ult icast group. Bridges eavesdrop on I GMP m essages t o build t able ent ries t hat ident ify t he port s t hat lead t o m em bers of a m ult icast group. I GMP snooping is far from ideal, and a long- t erm solut ion called t he GARP Mult icast Regist rat ion Prot ocol ( GMRP) was defined by an I EEE 802.1 working group. GMRP enables syst em s t hat wish t o j oin a m ult icast group t o regist er wit h a neighboring bridge. Bridges exchange regist rat ion inform at ion t hat assures t hat m ult icast fram es will be forwarded t o group m em bers.

I GMP snooping and GMRP are described in Chapt er 14, " Swit ches and Mult icast Traffic."

Functional Structure of a Layer 2 Switch Bridges originally were designed t o eit her ignore or forward a fram e; t hey were not t he sources or dest inat ions of any fram es. This changed when feat ures such as t he Spanning Tree Prot ocol, link aggregat ion, GMRP, and SNMP m anagem ent were int roduced. Bridges exchange prot ocol t raffic wit h one anot her and exchange SNMP m essages wit h net work m anagem ent st at ions. Figure 12.16 shows t he layered st ruct ure of a m odern Layer 2 swit ch. Each bridge port has it s own MAC address. This enables t he bridge port t o be t he source or dest inat ion of a fram e.

Figu r e 1 2 .1 6 . La ye r e d st r u ct u r e of a br idge .

Link aggregat ion runs direct ly on t op of t he MAC sublayer. However, t he m essages used in t he Spanning Tree Prot ocol and t he GARP Mult icast Regist rat ion Prot ocol cont ain Logical Link Cont rol ( LLC) headers, and t hese prot ocols operat e above t he LLC layer.

I GMP snooping runs on t op of I P, and SNMP usually runs on t op of t he User Dat agram Prot ocol ( UDP) and I P. The st age now is set for a present at ion of all t he prot ocol det ails. This is done in t he chapt ers t hat follow.

Summary Points • •

•

• • • • •

• • • •

• •

Layer 2 swit ches are m odern bridge product s. A bridge im proves perform ance by keeping local t raffic wit hin a lim it ed part of a LAN. A bridge m akes it possible t o build a m ixed- speed LAN. For exam ple, t raffic m ust be bridged bet ween a 10Mbps segm ent and a 100Mbps segm ent . A bridge can ext end t he diam et er of a LAN. An Et hernet t ransparent bridge can dynam ically record t he port t hrough which a MAC address can be reached in a filt ering t able. This process is called learning. St at ic ent ries can be configured int o a filt ering t able and used t o expedit e t he forwarding process, im pose securit y const raint s, and im prove perform ance by blocking select ed fram es from ent ering part s of t he LAN. LAN availabilit y can be im proved by inst alling backup bridges and backup links. The Spanning Tree Prot ocol ( STP) enables bridges t o aut om at ically reconfigure t he LAN t opology t o use backup resources. Som e bridge product s can t reat a set of links bet ween a pair of devices like a single link. This is called link aggregat ion. Som e bridge product s can give priorit y t reat m ent t o preferred fram es. Som e bridge product s support virt ual LANs ( VLANs) . Bridges eavesdrop on I nt ernet Group Managem ent Prot ocol ( I GMP) m essages t o build filt ering t able ent ries t hat ident ify t he port s t hat lead t o m em bers of a m ult icast group. The GARP Mult icast Regist rat ion Prot ocol ( GMRP) enables syst em s t hat wish t o j oin a m ult icast group t o regist er wit h a neighboring bridge. Support for Sim ple Net work Managem ent Prot ocol ( SNMP) is alm ost universal. I n high- powered Layer 2 swit ches, each swit ch line card cont ains applicat ion specific int egrat ed circuit s ( ASI Cs) t hat process incom ing fram es. A working copy of t he filt ering t able is loaded int o t he m em ory of each ASI C, and m any t able lookups occur in parallel.

References Bridge archit ect ure and funct ions are described in •

I EEE 802.1D. " Local and m et ropolit an area net works—Com m on specificat ions—Media Access Cont rol ( MAC) Bridges." 1998.

Chapter 13. The Spanning Tree Protocol

The cost of Spanning Tree Prot ocol ( STP) ext ra equipm ent oft en is negligible com pared t o t he cost of a net work failure. The usual way t o prevent net work failures is t o inst all ext ra equipm ent and ext ra net work links. However, as discussed in Chapt er 12, " Et hernet Bridges and Layer 2 Swit ches," m ult iple pat hs leading t o t he sam e LAN dest inat ion form loops t hat cause bridges t o crash. I f redundant LAN pat hs are creat ed, som e m ust be placed in backup m ode. I f t hese pat hs are needed t o cope wit h a net work failure, t hey m ust be upgraded t o act ive st at us. Manually cont rolling t he t opology of a big, com plicat ed LAN can be a t ough j ob especially since several adm inist rat ors m ay be allowed t o add and rem ove equipm ent and m ake param et er changes independent ly. I t is very desirable t o have a prot ocol t hat can cont rol LAN t opology aut om at ically. The I EEE 802.1D Spanning Tree Prot ocol ( STP) was designed t o do t his. The prot ocol enables t he bridges in a LAN t o perform t he following act ions: • •

Discover and act ivat e an opt im al t ree t opology for t he LAN Det ect failures and subsequent recoveries, and aut om at ically updat e t he t opology so t hat t he "best " t ree st ruct ure possible is select ed at any given t im e

The t opology of t he LAN aut om at ically is calculat ed from a set of bridge configurat ion param et ers t hat are est ablished by adm inist rat ors. The t ree will be t he " best " one t hat can be const ruct ed using t hese param et ers. I f t he configurat ion j ob is bot ched, t he result s m ight not be t he best t hat act ually is possible. The 802.1D STP has won st rong accept ance in t he m arket place. This prot ocol always has been popular for Et hernet LANs, but it is increasingly used in Token Ring LANs as well. The prot ocol was designed t o operat e wit h any t ype of LAN MAC prot ocol, not j ust Et hernet .

N ot e Several vendors have creat ed propriet ary special purpose variant s of t he STP. For exam ple, t he I BM variant prevent s Token Ring explorer fram es from following a looped pat h. ( Explorer fram es are used t o set up a pat h across a series of Token Ring Source Rout e bridges. These bridges are described in Chapt er 19, " Token Ring and FDDI Overview." ) Use of propriet ary variant s is fading, and t hese variant s will not be discussed in t his chapt er.

LAN Structure and SubLANs Bridges ( swit ches) provide t he infrast ruct ure t hat holds t he pieces of a LAN t oget her. Each bridge port connect s t o one of t hese pieces.

N ot e

No st andard t erm inology exist s for t he piece of a LAN t hat is connect ed t o a bridge port . The t erm subLAN is used for t his purpose in t his book.

The LAN pieces ( subLANs) look quit e different depending on t heir int ernal t echnology. For exam ple, subLAN A, in t he upper- left corner of See figure 13.1, is a coax- based collision dom ain t hat includes t wo segm ent s and a repeat er. SubLAN B, in t he upperright corner, is a t wist ed- pair collision dom ain t hat is built around t wo hubs. Not e t hat t raffic m oving bet ween bridge 1 and bridge 2 m ust cross subLAN B.

Figur e 1 3 .1 . Pie ce s ( su bLAN s) of a n Et h e r n e t LAN .

Assum ing t hat all t he links in t he lower half of t he figure are full- duplex links, subLANs C and D in See figure 13.1 are j ust full- duplex links t hat connect bridge 2 t o bridge 3 and bridge 4. SubLANs E t o L are full- duplex links t hat connect workst at ions and servers t o bridges. SubLANs A and E t o L are called leaves, which m eans t hat t hey do not carry t raffic from one bridge t o anot her. These subLANs are t he " end of t he line" and are not very int erest ing from t he point of view of LAN st ruct ure.

See figure 13.2 shows t he kind of pict ure t hat will be used in t his chapt er t o depict LAN st ruct ures. All t he leaves have been st ripped from t he LAN. Each subLAN t hat int erconnect s bridges is represent ed by a bar wit h boxes t hat represent point s of connect ion.

Figu r e 1 3 .2 . Th e st r u ct u r e of a br idge d LAN .

The bar can represent a subLAN t hat is a collision dom ain ( as is t he case wit h subLAN B) , or t he bar m ay be superim posed on a sim ple point - t o- point link bet ween a pair of bridges ( as is t he case wit h subLANs C and D) .

N ot e I f you are t rying t o underst and t he LAN skelet on, it does not m at t er what t ype of subLAN t he bar represent s. However, when you are t rying t o pick param et ers t hat will help t he STP set up a good t opology, it m at t ers a lot whet her a subLAN is a busy 10Mbps collision dom ain or a full- duplex gigabit link.

Protocol Overview This sect ion provides a rough underst anding of t he m aj or spanning t ree concept s. Lat er sect ions present det ailed definit ions and describe t he prot ocol m echanism s.

Electing the Root Bridge The first st ep in t he STP is t o choose a bridge t hat will act as t he root of t he t ree. The t ree t hat is form ed will consist of pat hs t hat radiat e from t he root and reach every bridge and every subLAN. The sam ple bridged LAN in See figure 13.3 illust rat es how a LAN's root bridge is chosen.

Figu r e 1 3 .3 . Ele ct in g t h e r oot br idge .

• •

•

Every bridge is assigned a MAC address t hat is used as it s unique bridge address. The cust om ary convent ion is t o reuse t he sm allest port MAC address as t he bridge address. To avoid clut t er in t he figure, t he bridge addresses in See figure 13.3 are writ t en as MAC1, MAC2, and so on. Each bridge also is assigned a priorit y num ber. The priorit y num ber followed by t he bridge address is called t he bridge ident ifier. The root of t he t ree is t he bridge wit h t he sm allest bridge ident ifier. To find t he root , select t he bridge ( or bridges) wit h t he sm allest priorit y num ber. I f t here is a t ie, com pare t he bridge ( MAC) addresses t o break t he t ie.

Bridges discover which one is t he root by exchanging t heir bridge ident ifiers. This process is called an elect ion. A wise adm inist rat or rigs t he result of t he root elect ion by assigning t he lowest priorit y num ber t o t he bridge best suit ed t o be t he root and

t hen assigning t he next lowest t o bridges t hat can act as backup root s. I f an adm inist rat or does not assign a specific value t o a bridge, it s priorit y will default t o 32,768. I n See figure 13.3, bridge 1 has priorit y num ber 0 and is t he clear winner of t he root cont est .

N ot e I t is convenient t o place t he root at t he t op of a LAN st ruct ure diagram . This conform s t o t he usual com put er science convent ion for drawing t ree st ruct ures.

An adm inist rat or should consider several fact ors when deciding which bridge would be t he best one t o win t he cont est for root : • • •

•

The root should have a cent ral posit ion in a LAN. As you shall soon see, STP m essages flow from t he root t o all ot her bridges on a periodic basis. A cent ral posit ion m inim izes t he dist ance t hat t hese m essages m ust t ravel. The preferred root bridge should be a solid, reliable piece of equipm ent . I f t he root bridge fails, anot her root m ust be elect ed. LAN t raffic is disrupt ed during t he changeover process. The root should be given am ple bandwidth by locat ing it on t he backbone of t he LAN. The root should be locat ed in an area t hat get s plent y of support and at t ent ion.

Generating the Tree I t is desirable t o configure bridges so t hat t he Spanning Tree algorit hm will choose and act ivat e t he pat hs branching out from t he root t hat deliver t he best perform ance. An easy way t o plan for t his is t o do t he following: • •

Assign a cost num ber t o each subLAN. Com put e t he cost of a pat h by adding cost num bers t hat are assigned t o each subLAN t raversed by t he pat h. The pat h t hat has t he least cost is t he best .

The process by which cost num bers are configured int o bridges will be explained lat er, in t he sect ion " Port Cost s and t he Root Pat h Cost ." For now, we'll st ick t o t his sim ple, int uit ive m odel.

Assign in g Cost s See Table 13.1 13.1 shows t he cost values recom m ended by t he I EEE. Appropriat ely, t he lowest cost s are assigned t o subLANs t hat operat e at t he highest bandwidt hs. When you pick a least - cost pat h, you want it t o be t he one t hat provides t he best perform ance.

The t able also shows t he range of values t hat can be used by an adm inist rat or who want s t o fine- t une a cost . For exam ple, a subLAN t hat consist s of a st ack of 10Mbps hubs connect ed t o a large and busy workgroup could be assigned a cost value t hat is great er t han 100 ( for exam ple, 300) ; likewise, a subLAN t hat consist s of a single 10Mbps link could be assigned a cost value below 100. A full- duplex link has m ore capacit y t han a half- duplex link, and t he cost should reflect t his. For exam ple, a half- duplex 10Mbps link connect ing a pair of bridges m ight be rat ed at 75, while a full- duplex 10Mbps link m ight be rat ed at 50.

Ta ble 1 3 .1 . Re com m e n de d Se gm e n t Cost Va lu e s Lin k Spe e d

Re com m e n de d Va lue

Re com m e n de d Ra nge

4Mbps

250

100- 1000

10Mbps

100

50- 600

16Mbps

62

40- 400

100Mbps

19

10- 60

1Gbps

4

3- 10

10Gbps

2

1- 5

I n See figure 13.4, A and B are 1Gbps Et hernet subLANs, C is a 100Mbps Et hernet subLAN, and D and E are 10Mbps subLANs. The default cost s have been assigned t o each subLAN.

Figur e 1 3 .4 . Assign in g su bLAN cost s.

The cost of reaching a bridge is t he sum of t he cost s of t he subLANs t hat are crossed when t raveling from t he root t o t he bridge. The cost of reaching a subLAN is t he sam e as t he cost of reaching t he bridge adj acent t o t he subLAN on t he pat h from t he root .

Opt im a l Pa t h Ex a m ple The discovery of opt im al pat hs st art s at t he root and works it s way t hrough t he net work. I n See Figure 13.4 • •

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•

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The cost of t he pat h from t he root t o bridge 2 is 4. The cost of t he pat h from t he root t o bridge 3 also is 4. Not e t hat bot h bridges connect t o subLAN D, and t hey provide pat hs of equal cost from t he root t o subLAN D. Only one of t hese pat hs can be used. The t ie is broken by com paring t he bridge priorit ies. Bridge 3 has a sm aller priorit y num ber and wins. The best pat h from t he root t o bridge 4 crosses subLANs A and C and has a t ot al cost of 23. The best pat h from t he root t o bridge 5 crosses subLANs B and D and has a t ot al cost of 104. SubLAN E can be reached via bridge 4 ( cost 23) or bridge 5 ( cost 104) . Bridge 4 wins.

Two port s on bridge 4 have been connect ed t o subLAN E t o provide a backup in case of port failure. Only one can be act ive ( ot herwise, a loop would form ) , and a preferred port m ust be chosen. Every port on a bridge is configured wit h a priorit y num ber. ( The default port priorit y is 32.) I n t his exam ple, t he LAN adm inist rat or has ensured t hat port 2 will be preferred t o port 1 by assigning port 2 a sm aller priorit y num ber. I f t he priorit ies had been equal, t he sm aller port num ber would have been preferred. The com binat ion of t he port priorit y and t he port num ber is called t he port ident ifier.

N ot e Why would anyone care which port on a bridge was used t o connect t o a subLAN? Keep in m ind t hat t he subLAN m ight be a large collision dom ain consist ing of a cascade of hubs. One of t he bridge port s m ight be connect ed t o a preferred cent ral hub, and t he ot her m ight be connect ed t o a different hub. The second connect ion m ight have been inst alled t o keep at least part of t he subLAN connect ed t o t he LAN if t he cent ral hub failed.

See Figure 13.5 shows t he t ree t hat result s from choosing opt im al pat hs. The dot t ed lines correspond t o blocked port s. Port 3 on bridge 2, port 1 on bridge 4, and port 1 on bridge 5 have been blocked t o form t he t ree. The set of pat hs t hat have been select ed is called t he act ive t opology.

Figur e 1 3 .5 . Ch oosin g t h e a ct ive t opology.

Elements of the Protocol The st age now is set for a closer view of t he inner workings of t he STP. Bridges get t he inform at ion t hat t hey need t o build an opt im al t ree t opology by exchanging Bridge Prot ocol Dat a Unit ( BPDU) m essages. These m essages are addressed t o t he m ult icast address X'01- 80- C2- 00- 00- 00, which t he I EEE has assigned t o bridges part icipat ing in t he STP.

Port Costs and the Root Path Cost An adm inist rat or m ust ent er configurat ion dat a at each bridge—t he dat a does not float around on subLANs. I nst ead of associat ing a cost wit h a subLAN, a corresponding pat h cost is associat ed wit h each port . I n See figure 13.6, t he cost t hat earlier was associat ed t o each subLAN in Figure See 13.5 has been configured as t he pat h cost for each port t hat connect s t o t he subLAN.

Figur e 1 3 .6 . For m a l pa t h cost s a ssocia t e d w it h por t s.

The cost of get t ing from t he root t o a bridge is called t he bridge's root pat h cost : •

•

A bridge's root pat h cost is com put ed by adding port pat h cost num bers along t he pat h from t he root t o t he bridge. Cost num bers are added only for port s t hat receive fram es forwarded from t he root t o t he bridge.

For exam ple, t o com put e t he root pat h cost for bridge 4 in See figure 13.6, you would add t he following pat h port cost s: • •

Bridge 2, port 2 ( cost 4) Bridge 4, port 3 ( cost 19)

The t ot al is 23. Bridge 1, port 1 and bridge 2, port 1 send fram es on t he pat h, and t heir values are not added in.

The answers com put ed using t he port cost s in See figure 13.6 are exact ly t he sam e as t he answers com put ed using t he subLAN cost s in See figure 13.4. For exam ple, t he root pat h cost for bridge 4 st ill is 23, and t he root pat h cost for bridge 5 st ill is 104. Cost s for t he sending port s along t he way are not count ed because t hat would cause each subLAN cost t o be count ed t wice. The root 's own port num bers never get added in, so should an adm inist rat or bot her t o assign cost num bers t o t he root 's port s? The answer is yes. Anot her adm inist rat or m ight com e along and change a bridge priorit y num ber som ewhere, causing a different bridge t o becom e root . The current root t hen becom es an ordinary cit izen. I f no num bers have been assigned t o it s port s, t heir cost s default t o 128. This could warp t he t opology of t he result ing t ree. Alt hough it usually m akes sense t o assign t he sam e cost t o every port t hat connect s t o t he sam e subLAN, t he rules do not require adm inist rat ors t o do t his. Alt hough it m ight seem illogical, adm inist rat ors can assign different cost values t o different port s t hat connect t o t he sam e subLAN. Act ually, it m akes polit ical sense t o allow t his because, in real life, several adm inist rat ors m ight be responsible for configuring bridges in a LAN. Each m ight cont rol a different set of bridges, and each m ight have a different opinion about t he cost t hat should be assigned t o a port connect ed t o a part icular subLAN. The adm inist rat ors don't have t o fight it out . The STP st ill will work perfect ly well even if t he num bers are different . However, t he adm inist rat ors m ight not all agree t hat t he t ree t hat finally is produced by t he prot ocol is " opt im al."

Root Port, Designated Bridge, and Designated Port Som e addit ional definit ions are needed before describing in det ail how t he prot ocol chooses a root bridge, discovers opt im al pat hs, and updat es t he t ree t opology. Som e key definit ions are list ed here: • • •

Root port —The port on a bridge t hat connect s t o it s best pat h t o t he root . Designat ed bridge for a subLAN—The bridge t hat t ransm it s fram es com ing from t he direct ion of t he root ont o t he subLAN, and t hat t ransm it s fram es from t he subLAN t oward t he root . The designat ed bridge is on a least - cost pat h bet ween t he subLAN and t he root . Designat ed port —The single unblocked port t hat connect s a subLAN t o it s designat ed bridge.

Choosing the Root Every bridge st art s out wit h dream s of glory: I t assum es t hat it is t he root and is t he designat ed bridge for each subLAN t o which it is direct ly connect ed. I f m ult iple port s connect a bridge t o a part icular subLAN, t he port wit h t he sm allest priorit y num ber is used, and t he ot hers are blocked. I f t he priorit y num bers are equal, t he sm allest port num ber wins. A bridge t hat t hinks t hat it is t he root m ult icast s Configurat ion BPDUs ont o each subLAN t o which it is connect ed. I f anot her bridge on t hat subLAN knows bet t er, it

t ransm it s a Configurat ion BPDU ont o t he subLAN t hat nam es it s candidat e for root —a bridge t hat has a bet t er ( priorit y, MAC) bridge I D com binat ion. For exam ple, in See figure 13.3, when bridge 4 init ializes, it assum es t hat it is t he root . I t soon receives a m essage from bridge 2 t hat nom inat es it self because bridge 2 has a bet t er priorit y num ber. However, bridge 2 soon learns t hat bridge 1 has an even bet t er priorit y value ( 0, which is t he best possible) . Bridge 2 passes t hat inform at ion along in t he next BPDU t hat it sends across subLAN C. List ing 13.1 shows part of t he t race of a Configurat ion BPDU. I n t he t race, t he sender ident ifies t he priorit y and MAC num ber of t he bridge t hat it believes t o be t he root — nam ely, it self. This can be seen by com paring t he root ident ifier field wit h t he sending bridge I D field. The cost of get t ing from t he root t o it self is 0, so t he sender report s a root pat h cost of 0. The sender has priorit y value X'80- 00 ( 32,768) , which is t he default . The 2- byt e port ident ifier ( X'8002) shows t hat t he port has been assigned t he default port priorit y ( X'80, which is decim al 128) and t hat t he sender t ransm it t ed t he m essage t hrough port X'02 ( decim al 2) .

List in g 1 3 .1 I nit ia l Fie lds in a Con figu r a t ion BPD U BPDU: Root Identifier = 8000.0200000A402C BPDU: Priority = 8000 BPDU: MAC Address = 0200000A402C BPDU: BPDU: Root Path Cost = 0 BPDU: BPDU: Sending Bridge Id = 8000.0200000A402C.8002 BPDU: Priority = 8000 BPDU: MAC Address = 0200000A402C BPDU: Port = 8002 I f t his bridge receives a BPDU t hat has a sm aller value in it s root ident ifier field, it will accept t hat syst em as t he new root . Through a series of correct ions, all t he bridges discover t he real root very quickly.

Discovering the Root Paths Aft er t he root has been discovered, it does not t ake long t o generat e t he rest of t he t ree. The root m ult icast s a Configurat ion BPDU across each of it s adj acent subLANs. This BPDU is received by each bridge connect ed t o t hese subLANs. Each receiver creat es a new Configurat ion BPDU by • •

Replacing t he 0 value in t he root pat h cost field wit h t he cost associat ed wit h it s arrival port . For exam ple, bridge 2 in Figure 13.6 would put a value of 4 in t he root pat h cost field. Placing it s own ident ifier in t he sending bridge ident ifier field.

The bridge t hen sends t his updat ed BPDU ont o every subLAN for which it believes it is t he designat ed bridge. I t is possible t hat anot her bridge also believes t hat it is t he designat ed bridge for one of t hese subLANs. I f so, t hat bridge will send it s own BPDU ont o t he subLAN. Each bridge com pares t he root pat h cost value in an incom ing BPDU wit h it s own value. The bridge wit h t he sm allest root pat h cost wins. A t ie is broken in t he usual way, by com paring bridge ident ifiers. For exam ple, in Figure 13.6, bridge 2 and bridge 3 send BPDUs ont o subLAN D t o report ident ical root pat h cost s of 4. The t ie is broken in favor of bridge 3 because it s priorit y num ber, 5, is t he low value. Updat ed BPDUs are propagat ed down t he t ree unt il every bridge discovers it s root port , it s root pat h cost , and it s designat ed port s. For exam ple, in See figure 13.6 1. Bridge 5 receives t he BPDUs sent across subLAN D by bridges 2 and 3, and discovers t hat adj acent bridge 3 lies on it s pat h t o t he root . Hence, port 2 is t he root port for bridge 5, and it s root pat h cost is 104. 2. Bridge 5 t hen sends a BPDU t hat report s it s root pat h cost ont o subLAN E. 3. When bridge 5 receives a BPDU from bridge 4 report ing a root pat h cost of 23, bridge 5 discovers t hat it is not t he designat ed bridge for subLAN E and t hat port 1 is not a designat ed port .

N ot e The t opology of t he t ree is com plet ely described by t he ident it y of t he root bridge, t he root port s, and t he designat ed port s. The t ree is form ed by " t urning on" t he root port s and designat ed port s, enabling t hem t o forward fram es. The ot her port s are blocked and cannot send or receive dat a fram es, alt hough t hey will cont inue t o send and receive BPDUs.

Topology Changes and Port State Changes The t ree t opology m ust be recom put ed aft er t he failure or recovery of a bridge or a bridge port . One or m ore blocked port s m ight need t o be placed on act ive dut y. For exam ple, See figure 13.7 shows how t he t opology t hat was est ablished in figure 13.5 changes aft er bridge 4 crashes. Port 1 on bridge 5 needs t o be unblocked and m ust learn t he MAC addresses of t he syst em s on subLAN E.

Figu r e 1 3 .7 . Figu r e 1 3 .7 Topology ch a n ge s a ft e r a br idge cr a sh e s.

Ot her bridges have t o do som e relearning as well. For exam ple, before t he crash, t he following was t rue: •

•

Bridge 2 learned t he MAC addresses of stat ions on subLAN E from fram es t hat were t ravelling up t he t ree from subLAN E t o ot her subLANs. Bridge 2 was forwarding fram es sent from subLAN A t o subLAN E t hrough it s port 1.

Aft er t he crash, bridge 2 m ust unlearn t he subLAN E dest inat ion addresses quickly. Traffic from subLAN A t o subLAN E will have t o pass t hrough bridges 1, 3, and 5. To avoid chaos during a change ( for exam ple, while bridge 4 ages out it s subLAN E addresses and bridge 5 learns t hem ) , a bridge needs t o act ivat e a blocked port in st ages. First , t he port needs t o rem ain quiet unt il t he t opology of t he new t ree has set t led down. I f bot h old port s and new port s are funct ioning at t he sam e t im e, t raffic will circle around looped pat hs. Secondly, a port t hat is unblocking needs t o ent er a learning phase t o discover t he MAC addresses t hat are reached t hrough t he port . I n fact , during a t opology change,

all port s in t he t ree m ust age out t heir dynam ic filt ering t able ent ries quickly and relearn t heir dynam ic address inform at ion.

N ot e Recall t hat t he filt ering t able includes bot h dynam ically learned and m anually configured ent ries.

See Table 13.2 displays t he com plet e set of port st at es and t he funct ions perform ed while in each st at e. A forward delay t im er cont rols t he durat ion of t he list ening and learning st at es in See Table 13.2. This t im er also is used t o age out dynam ic filt ering t able ent ries following a t opology change.

Ta ble 1 3 .2 . Spa n n ing Tr e e Br idge Por t St a t e s St a t e

Fu n ct ion s

Disabled

The port is incapable of funct ioning, eit her because of equipm ent failure or act ion by an adm inist rat or.

Blocking

The port can only send and receive BPDUs.

List ening

A t im er is set and t he port is wait ing quiet ly for a period ( t he forward delay t im e) t o allow ot her bridges t o discover t he new t opology. The port cont inues t o send and receive BPDUs.

Learning

Aft er t he list ening period has expired, t he t im er is reset t o t he forward delay t im e, and t he port st art s t o learn MAC address inform at ion and add it t o t he filt ering dat abase. The port cont inues t o send and receive BPDUs.

Forwarding Aft er t he t im er has expired, t he port is ready t o receive and forward fram es. The port cont inues t o learn MAC address inform at ion t hat is added t o t he filt ering dat abase and is capable of sending and receiving BPDUs.

N ot e I n addit ion t o t he funct ions list ed in See Table 13.2, every nondisabled port is capable of sending and receiving SNMP net work m anagem ent m essages.

Maintaining the Tree Each bridge records som e t opology inform at ion. For exam ple, it keeps t rack of t he following: •

The ident ifier of t he root bridge

• • •

The bridge's root port The bridge's root pat h cost The bridge's designat ed port s

This inform at ion needs t o be refreshed periodically. I t is t he j ob of t he root t o launch a round of inform at ion exchanges at regular int ervals. The root sends out a fresh Configurat ion BPDU every t im e a period called t he hello t im e has elapsed. This t riggers a cascade of new BPDUs away from t he root t oward t he bot t om of t he t ree, and enables each bridge t o validat e or updat e it s inform at ion. I f a problem prevent s Configurat ion BPDUs from arriving at a bridge, it is very likely t hat som e of t he bridge's t opology inform at ion is out of dat e. The t opology inform at ion is discarded if no BPDUs have been received during a t im eout period whose value is called t he m axim um age. The current t ree can be disrupt ed by t he failure of a port or of an ent ire bridge, by adm inist rat ive changes in configurat ion dat a, or by t he addit ion of m ore equipm ent . Any bridge t hat has evidence t hat t he t opology inform at ion needs t o change t ransm it s a Topology Change Not ificat ion BPDU t hrough it s root port . This BPDU is relayed upward t oward t he root . The root react s by sending out a Configurat ion BPDU t o acknowledge t hat it has received t he Topology Change Not ificat ion.

Protocol Messages Recall t hat fram es cont aining Bridge PDUs are addressed t o t he m ult icast address X'01- 80- C2- 00- 00- 00. These fram es cont ain t he 3- byt e Logical Link Cont rol ( LLC) header t hat is displayed in See figure 13.8. The LLC dest inat ion service access point ( DSAP) and source service access point ( SSAP) addresses bot h are X'42. The cont rol field value, X'03, m eans " unnum bered inform at ion."

Figur e 1 3 .8 . LLC h e a de r in BPD U fr a m e s.

N ot e

Logical Link Cont rol headers, DSAPs, SSAPs, and cont rol fields were discussed in Chapt er 4, " The Classical Et hernet CSMA/ CD Mac Prot ocol," in t he sect ion called " LSAPs."

The first 2 byt es of a BPDU cont ain t he prot ocol ident ifier X'00- 00.

The Configuration BPDU See Figure 13.9 shows t he layout of a com plet e Configurat ion BPDU m essage. The last t hree param et ers ( m axim um age, hello t im e, and forward delay) are values t hat have been configured at t he root bridge. The root announces t hem t o t he ot her bridges t o synchronize t he act ions of t he prot ocol across all t he bridges. I n part icular

Figur e 1 3 .9 . A Con figur a t ion BPD U.

•

The m axim um age is used t o t im e out st ale prot ocol inform at ion.

• •

The hello t im e set s t he int erval at which t he root sends BPDUs. The forward delay t im es out port st at e changes.

Aft er t he root bridge sends a BPDU cont aining t hese values, ot her bridges save t hem and copy t hem int o t he BPDUs t hat t hey t ransm it . All bridges quickly learn t hese values, and t he bridges are governed by t hem unt il a different bridge becom es root . Every bridge is configured wit h it s own m axim um age, hello t im e, and forward delay. However, a bridge's values are not used unless t he bridge becom es t he root . One rem aining param et er has not been explained. The value in t he m essage age field st art s out at 0 and is increm ent ed at each bridge downst ream from t he root . An incom ing m essage whose age exceeds t he m axim um age t im er value is discarded. List ing 13.2 displays a DI X Et hernet fram e t hat was capt ured by a Net work Associat es Sniffer and t hat cont ains a Configurat ion PDU. Not e t he following: •

•

•

• • •

•

• • • •

The fram e's dest inat ion MAC address is m ult icast address X'01- 80- C2- 00- 0000, which t he I EEE has assigned t o bridges part icipat ing in t he STP. The m essage has an LLC header wit h DSAP and SSAP equal t o X'42 and a cont rol field cont aining X'03, which m eans " unnum bered inform at ion." The prot ocol ident ifier X'00- 00 corresponds t o t he STP. The prot ocol version is X'00. BPDU t ype X'00 indicat es t hat t his is a Configurat ion m essage. Only t wo of t he flag bit s are used. These bit s are em ployed in t he t opology change procedure. The root ident ifier includes t he root 's bridge ident ifier t hat is, it s priorit y num ber and t he MAC address select ed t o uniquely ident ify t hat bridge. The sender believes t hat it is t he root and, hence, report s it s root pat h cost as 0 and t he m essage age as 0. The sending bridge ident ifier is t he sam e as t he root ident ifier. The sending port 's priorit y and port num ber are included. I n t his exam ple, a m essage older t han t he m axim um age of 20 seconds ( which is called t he inform at ion lifet im e in t he t race) should be ignored and discarded. The root will generat e a fresh BPDU every 2 seconds. The forward delay t im e in t he t race is 15 seconds. This m eans t hat it will t ake a t ot al of 30 seconds for a blocked port t o t ransit ion t hrough list ening and learning st at es t o t he forwarding st at e.

List in g 1 3 .2 A Com ple t e Con figur a t ion BPD U DLC: DLC: DLC: LLC: ----LLC: LLC: LLC: LLC: LLC: BPDU: ----BPDU: BPDU:

Source = Station 0200000A40CC 802.3 length = 38 LLC Header ----DSAP Address = 42, DSAP IG Bit = 00 (Individual Address) SSAP Address = 42, SSAP CR Bit = 00 (Command) Unnumbered frame: UI Bridge Protocol Data Unit Header ----Protocol Identifier = 0000

BPDU: BPDU: BPDU: BPDU: BPDU: BPDU: BPDU: BPDU: BPDU: BPDU: BPDU: BPDU: BPDU: BPDU: BPDU: BPDU: BPDU: BPDU: BPDU: BPDU: BPDU: BPDU:

Protocol Version

= 00

BPDU Type = 00 (Configuration) BPDU Flags 0... .... .... ...0 .000 000.

= = = =

00 Not Topology Change Acknowledgment Not Topology Change Unused

Root Identifier Priority MAC Address

= 8000.0200000A402C = 8000 = 0200000A402C

Root Path Cost

= 0

Sending Bridge Id = 8000.0200000A402C.8002 Priority = 8000 MAC Address = 0200000A402C Port = 8002 Message Age = 0.000 seconds Information Lifetime = 20.000 seconds Root Hello Time = 2.000 seconds

Every field in t he Configurat ion BPDU has a purpose t hat is clear—except for t he sending port priorit y and t he port num ber. I n fact , t his inform at ion is needed when a bridge t alks t o it self! I f a LAN adm inist rat or has hooked up t wo or m ore port s on a bridge t o t he sam e subLAN ( as is t he case for bridge 4 in See Figure 13.7) , t here is no way for t he bridge aut om at ically t o be aware t hat bot h port s at t ach t o t he sam e subLAN. I ncluding t he sending port ident ifier in t he BPDU reveals what is going on. For exam ple, bridge 4 in figure 13.7 is connect ed t o subLAN E via port s 1 and 2. When t he bridge t ransm it s a Configurat ion BPDU out one of t he port s—say, port 1— t he BPDU will be received by t he ot her port . The bridge will not e t hat it sent t his BPDU and t hen will discover t hat it has t wo connect ions t o subLAN E.

The Topology Change BPDU One m ore t ype of BPDU exist s. A bridge t hat has det ect ed a t opology change sends a Topology Change Not ificat ion BPDU up t he t ree t hrough it s root port . Each bridge t hat receives t his t ype of m essage also t ransm it s a Topology Change Not ificat ion BPDU t hrough it s root port . The m essage m akes it s way up t he t ree unt il a not ificat ion reaches t he root . Topology Change BPDUs are short and sweet , as List ing 13.3 illust rat es. The m essage cont ent consist s of 5 byt es: • • •

The prot ocol ident ifier, X'00- 00 for t he STP The prot ocol version, X'00 The BPDU t ype, which is Topology Change Not ificat ion ( X'80)

There are no param et ers in t he m essage. The rest of t he fram e's dat a area cont ains 39 byt es of padding.

List in g 1 3 .3 A Topology Ch a n ge N ot ifica t ion BPD U BPDU: ----BPDU: BPDU: BPDU: BPDU: BPDU: BPDU:

Bridge Protocol Data Unit Header ----Protocol Identifier = 0000 Protocol Version = 00 BPDU Type = 80 (Topology Change)

DLC: Frame padding= 39 bytes Aft er a Topology Change BPDU has m ade it s way up t o t he root , t he root t ransm it s Configurat ion BPDUs whose Topology Change Acknowledgem ent flag is set t o 1. This flag not ifies all bridges t hat t he change has been seen. The root sends Configurat ion BPDUs whose Topology Change flag is set t o 1 for a period of t im e equal t o t he sum of t he bridge m axim um age and bridge forward delay. This warns all t he bridges t hat t hey should age out t he dynam ic ent ries in t heir filt ering dat abases quickly, using t he forwarding delay as t he t im eout .

Summary Points • •

•

•

•

•

• • • •

•

•

•

•

•

The I EEE 802.1D Spanning Tree Prot ocol ( STP) was designed t o aut om at ically generat e a t ree- shaped LAN t opology t hat does not cont ain loops and t o updat e t he t opology t o use backup resources if som e act ive com ponent fails. The t ree will be t he " best " one t hat can be const ruct ed using t he bridge param et ers t hat have been est ablished by LAN adm inist rat ors. The first st ep in t he STP is t o choose a bridge t hat will act as t he root of t he t ree. The t ree t hat is form ed will consist of pat hs t hat radiat e out from t he root and reach every bridge and every subLAN. A bridge ident ifier is m ade up of a priorit y num ber and t he unique bridge MAC address. The root of t he t ree is t he bridge wit h t he sm allest bridge ident ifier. The root should have a cent ral posit ion in a LAN, be a solid, reliable piece of equipm ent , have access t o robust bandwidt h, and be locat ed in an area t hat get s plent y of support and at t ent ion. A port ident ifier is m ade up of a port priorit y and t he port num ber. Every port is assigned a pat h cost . A bridge's root pat h cost is com put ed by adding port pat h cost num bers for port s t hat receive fram es along t he pat h from t he root t o t he bridge. The port on a bridge t hat connect s t o it s best pat h t o t he root is called t he root port . The bridge t hat t ransm it s fram es com ing from t he direct ion of t he root ont o t he subLAN is called it s designat ed bridge. The single unblocked port t hat connect s a subLAN t o it s designat ed bridge is called it s designat ed port . The root bridge, root port s, root pat h cost s, and designat ed port s are select ed by an exchange of Configurat ion BPDUs. A bridge signals a change by sending a Topology Change BPDU up t he t ree t oward t he root . There are five port st at es: disabled, blocking, list ening, learning, and forwarding.

•

Each bridge is configured wit h hello t im e, forward delay, and m axim um age param et ers t hat it will advert ise if and when it is t he root .

References The Spanning Tree Algorit hm and Prot ocol are described in •

I EEE 802.1D. " Local and Met ropolit an Area Net works—Com m on Specificat ions—Media Access Cont rol ( MAC) Bridges." 1998. Chapt ers 8 and 9.

Chapter 14. Switches and Multicast Traffic One of t he core funct ions perform ed by a bridge is t o prot ect t he bandwidt h available wit hin a collision dom ain by blocking off t raffic t hat is not addressed t o a st at ion in t hat dom ain. Chapt er 12, " Et hernet Bridges and Layer 2 Swit ches," showed t hat bridges do a good j ob of channeling unicast fram es t o t heir dest inat ions. Cont rolling m ult icast t raffic is a m ore difficult challenge. Mult icast ing was designed in t he days of broadcast - st yle LANs, and t he original assum pt ion was t hat m ult icast fram es would be visible t o every st at ion on a LAN. For t oday's large and geographically ext ended LANs, it m akes no sense t o glut t he whole LAN wit h t raffic t o deliver a st ream of m ult icast s t o one or t wo st at ions. The first m echanism t hat bridge vendors used t o st em t he t ide was t arget ed at I P m ult icast t raffic. I P syst em s send out I nt ernet Group Managem ent Prot ocol ( I GMP) m essages t o not ify I P rout ers t hat t hey want t o j oin an I P m ult icast group—t hat is, t hat t hey want t o receive I P t raffic sent t o a part icular I P m ult icast address. The m echanism t hat was int roduced is called I GMP snooping. Bridges eavesdrop on I GMP m essages t o discover which subLANs cont ain users t hat have j oined I P m ult icast groups. I P m ult icast t raffic is delivered across a LAN in fram es wit h m ult icast dest inat ion MAC addresses. I GMP snooping has som e short com ings. To m ent ion a couple, it im poses a big overhead burden on bridges, and it works only for I P t raffic. A bet t er solut ion was defined by an I EEE 802.1 working group. This group creat ed a Layer 2 prot ocol t hat enables a syst em t o regist er wit h neighboring bridges. The syst em asks t he bridges t o be sure t o send it fram es t hat have part icular dest inat ion m ult icast MAC addresses. Regist rat ion inform at ion is propagat ed from bridge t o bridge across a LAN, and every bridge becom es aware of t he port s t hrough which it needs t o forward fram es addressed t o a specific m ult icast address. This solut ion is called t he GARP Mult icast Regist rat ion Prot ocol ( GMRP) . GMRP provides pinpoint cont rol, enabling bridges t o carry m ult icast s exact ly where t hey need t o go. Vendors have welcom ed t his solut ion and have been quick t o im plem ent it .

N ot e GARP st ands for t he Generic At t ribut e Regist rat ion Prot ocol. This prot ocol GARP provides a general regist rat ion m echanism for use in a bridged LAN. GMRP was t he first prot ocol t hat was built on t op of GARP.

Bot h I GMP snooping and GMRP are described in t his chapt er.

Multicasting Mult icast MAC addresses enable a single fram e t o be addressed t o a group of st at ions. A m ult icast address also is called a group address, and st at ions t hat have been configured t o receive t raffic addressed t o a part icular group address are said t o be m em bers of a m ult icast group. Mult icast MAC addresses are used in t wo ways: • •

St andard m ult icast MAC addresses have been assigned t o specific t ypes of devices and or net work roles. For exam ple, a m ult icast sent t o dest inat ion MAC address X'01- 00- 5E- 00- 00- 02 is direct ed t o every m ult icast - capable rout er on a LAN. A pool of addresses is reserved for ad hoc m ult icast groups. These addresses are used for applicat ions such as conferences, dist ance learning, or ent ert ainm ent . An address can be checked out of t he pool and used for a lim it ed am ount of t im e.

N ot e I t is perfect ly legit im at e for a st at ion t hat does not belong t o a m ult icast group t o send dat a t o a m ult icast group. I n t his case, t he sender does not accept any fram es t ransm it t ed t o t he group address. Think of a lect urer addressing a large group of at t endees at a convent ion—wit h no quest ions from t he audience allowed! The m em bers of t he audience are receivers, but t he lect urer is not a receiver.

A net work device such as a bridge or rout er is shipped from t he fact ory wit h it s adapt er ready t o receive fram es sent t o st andard, device- specific m ult icast addresses. Joining an ad hoc m ult icast is like picking a cable TV channel. The user picks a m ult icast group, and t he user's applicat ion soft ware not ifies t he device driver t o j oin t he group.

The Need to Control Multicasts A LAN m ult icast adds no special load t o a sm all, broadcast - st yle LAN. For exam ple, t he hub in Figure 14.1 repeat s all fram es—unicast and m ult icast —ont o every

segm ent . Every st at ion aut om at ically sees every fram e. The NI C in a syst em t hat has j oined a m ult icast group absorbs fram es sent t o t he m ult icast group address in addit ion t o t hose sent t o it s unicast address. St at ions t hat are not group m em bers sim ply ignore t he m ult icast fram es.

Figu r e 1 4 .1 . A h u b r e pe a t in g a ll fr a m e s.

As LANs have got t en larger, it has becom e desirable t o prevent group fram es from chewing up t he bandwidt h in subLANs t hat neit her cont ain group m em bers nor are on t he pat h t o a subLAN t hat holds m em bers. For exam ple, in Figure 14.2, a group of users at sit e 1 is planning t o hold a local elect ronic conference. All t he part icipant s are connect ed t o subLAN A.

Figu r e 1 4 .2 . Con st r a in in g t h e scope of m u lt ica st fr a m e s.

There is no reason t o forward t he group's m ult icast fram es ont o subLANs B, C, and D. I n fact , sending t hese fram es across t he wide area bridge t hat connect s sit e 1 t o rem ot e sit e 2 would needlessly use up a lot of valuable bandwidt h. The need t o lim it t he scope of m ult icast s has becom e even m ore urgent wit h t he growing popularit y of connect ing LAN st at ions direct ly t o swit ch port s. The benefit of a swit ched LAN is precisely t hat everybody does not have t o see all t he t raffic, enabling m ult iple independent st ream s of t raffic t o flow t hrough t he swit ch. Convent ional m ult icast ing, however, requires t he fram es t o be delivered t hrough every port in t he LAN, even if only one st at ion needs t o see t he t raffic. Queuing m ult icast fram es t o every port can clog up a LAN swit ch and slow swit ch perform ance. Clearly, it is desirable t o learn which port s lead t o st at ions t hat have j oined a m ult icast group, and t hen forward t he fram es addressed t o t hat group only t hrough t hose port s. You could t ry t o cont rol t he forwarding of fram es sent t o st andard, role- based group addresses by m anual configurat ion. However, t his will not work very well if your net work is growing or changing. I n any case, it is not feasible t o m aint ain m anual ent ries for m ult icast conferences t hat an organizat ion set s up on an ad hoc basis. I GMP snooping, which can be used t o cont rol I P m ult icast t raffic, was t he first m echanism t hat was developed t o cont rol m ult icast s. I t is described in t he sect ions t hat follow. The GARP Mult icast Regist rat ion Prot ocol ( GMRP) is a m ore effect ive and reliable m et hod t hat applies t o all m ult icast t raffic, not j ust I P- based m ult icast s. GMRP is described lat er in t he sect ion called " The GARP Mult icast Regist rat ion Prot ocol." GARP is described in t he sect ion, " Generic At t ribut e Regist rat ion Prot ocol."

IP Multicasts The TCP/ I P prot ocol suit e includes m echanism s t hat m ake it possible t o set up m ult icast conferences. I P is a prot ocol t hat has broad scope. An I P dest inat ion can be far away in a locat ion t hat is reached by passing t hrough m any rout ers. The scope of an I P m ult icast can be one of t he following: • • •

A single LAN A privat e net work t hat includes m any LANs A group of part icipant s connect ed by t he I nt ernet

A class of I P addresses has been set aside for use as m ult icast I P addresses. Syst em s t hat want t o receive t raffic sent t o a part icular m ult icast I P address form an I P m ult icast group.

M a pping I P M u lt ica st Addr e sse s t o Et h e r n e t M u lt ica st Addr e sse s

An I P dat agram sent t o a m ult icast I P address m ust be forwarded ont o every LAN t hat cont ains an I P m ult icast group m em ber. When an I P dat agram wit h a 4- byt e I P m ult icast address arrives at a rout er, t he rout er m ust st ick a 6- byt e MAC fram e header on t he dat agram t o t ransm it it ont o a LAN. What MAC address should it use? For Et hernet LANs, t here is a sim ple rule t hat m aps a m ult icast I P address int o a m ult icast MAC address in a very elegant m anner. Figure 14.3 shows t he Et hernet MAC address for an I P dat agram whose dest inat ion is a m ult icast I P address. The first 3 byt es of t he MAC address always are X'01- 00- 5E.

Figu r e 1 4 .3 . W r a ppin g a m u lt ica st I P da t a gr a m in a n Et h e r n e t fr a m e .

The last t hree ( decim al) num bers of t he m ult icast I P address are 2.5.9. These t ranslat e int o hex as X'02- 05- 09, which is t he value in t he last 3 byt es of t he m apped MAC address shown on t he right .

N ot e Act ually, t he first 25 bit s of t he MAC address always are X'01- 00- 5E, followed by a 0 bit . The last 23 bit s of t he m ult icast I P address get copied int o t he last 23 bit s of t he MAC address.

I P m ult icast addresses st art wit h a num ber in t he range 224–239 and are called Class D addresses.

M a pping I P M u lt ica st Addr e sse s t o FD D I M u lt ica st Addr e sse s The sam e m apping as shown in Figure 14.3 is done for FDDI , but t he address probably will appear in a m onit or t race in non- canonical order, which reverses t he bit s in each byt e. ( See Chapt er 2, "LAN MAC Addresses," for m ore on non- canonical order.) For FDDI , t he address X'01- 00- 5E- 02- 05- 09 t hat appeared in Figure 14.3 would look like X'80- 00- 7A- 40- A0- 90.

Tok e n Rin g a n d M u lt ica st Addr e sse s Unfort unat ely, t here is no m apping for Token Ring, which has very lim it ed support for m ult icast s. All m ult icast dat agram s are placed in fram es wit h funct ional ( reversed- byt e) address X'C0- 00- 00- 04- 00- 00 or, even worse, in fram es wit h broadcast MAC address. A Token Ring st at ion t hat want s t o part icipat e in an I P m ult icast group m ust absorb, exam ine, and discard t raffic t hat has been sent t o ot her m ult icast groups.

IGMP Snooping I GMP defines t he procedures t hat enable an I P end syst em t o not ify rout ers on it s LAN t hat it want s t o j oin or leave a part icular I P m ult icast group. 1. A rout er periodically sends an I GMP query asking whet her st at ions want t o j oin ( or cont inue t o be part of) a m ult icast group. 2. A host t hat want s t o j oin a group or renew it s group m em bership responds wit h an I GMP m em bership report . Mem bership report s are addressed t o t he m ult icast address of t he group being j oined or renewed. 3. A rout er list ens for t hese report s and uses t hem t o det erm ine whet her at least one st at ion on t he LAN want s t o receive t raffic sent t o t hat m ult icast address. Many swit ch product s enable t he swit ch adm inist rat or t o t urn on I GMP snooping, which causes t he swit ch t o eavesdrop on t hese m essages. The swit ch t hen m akes ent ries in t he filt ering t able, which channels fram es addressed t o a group t o t he port s t hat lead t o group m em bers. Specifically, a port is added t o t he out put list of a filt ering t able ent ry for a m ult icast address if at least one part icipant is reached via t hat port . Figure14.4 shows t he effect of I GMP snooping. The m ult icast group m em bers in t he figure include st at ions A, B, and C. St at ion A has sent a fram e t o t he m ult icast group.

Figu r e 1 4 .4 . Usin g I GM P sn oopin g t o cu t ou t u n n e ce ssa r y m u lt ica st t r a ffic.

1. By snooping, swit ch X has discovered t hat port s 1 and 4 are t he only ones t hat lead t o m ult icast m em bers. Because t he fram e arrives on port 1, swit ch X forwards t he fram e only t hrough port 4. Table 14.1 shows t he filt ering t able ent ry at swit ch X. 2. Swit ch Y has discovered t hat port s 1, 3, and 5 are t he only ones t hat lead t o m ult icast m em bers. Because t he fram e arrives on port 1, swit ch Y forwards t he fram e only t hrough port s 3 and 5. 3. Swit ch Z has discovered t hat port s 1 and 4 are t he only ones t hat lead t o m ult icast m em bers. Because t he fram e arrives on port 1, swit ch Y forwards t he fram e only t hrough port 4.

Ta ble 1 4 .1 . En t r y a t Sw it ch X Le a r n e d via I GM P Sn oopin g D e st in a t ion Addr e ss St a t u s 01- 00- 5E- 02- 05- 09 Mult icast

Pr ot ocol Com m e n t IP

Re ce ive Por t Any

Tr a n sm it Por t s 1, 4

Learned

Figure 14.5 shows t he prot ocol com ponent s t hat are used for I GMP snooping.

Figu r e 1 4 .5 . Com pon e n t s u se d for I GM P sn oopin g.

Problems with IGMP Snooping A swit ch t hat locat es m ult icast recipient s via I GMP snooping t akes on som e CPUint ensive work. Mem bership report s are sent t o t he address of t he group t hat t he syst em want s t o j oin. Thus, t he swit ch m ust exam ine every m ult icast fram e and check whet her it cont ains an I P dat agram . I f so, t he swit ch m ust exam ine t he I P header t o find out whet her t he dat agram cont ains an I GMP m em bership report m essage. When a dat agram cont ains a m em bership report , t he swit ch m ust creat e or updat e a filt ering t able ent ry for t he m ult icast address and t hen m ake sure t hat t he port on which t he fram e arrived is included in t he list of out put port s. A swit ch also should forward each m ult icast fram e t ransm it t ed by a rout er t o ot her rout ers on t he LAN t hat lead t o m ult icast recipient s. For exam ple, in Figure 14.6, t he st at ions shown in black belong t o t he m ult icast group for address M, and

Figu r e 1 4 .6 . For w a r din g a m u lt ica st fr a m e .

•

•

•

Rout er 1 t ransm it s a fram e wit h dest inat ion m ult icast address M ont o t he LAN t hat is surrounded by a dot t ed line. The fram e m ust be forwarded t o t he m em ber st at ions at t ached t o swit ch A and swit ch B. The fram e also m ust be forwarded t o rout er 3, which is at t ached t o a different LAN cont aining a group m em ber.

To ident ify which rout ers lead t o recipient s, a swit ch m ust eavesdrop on rout er- t orout er prot ocol m essages. Unfort unat ely, lot s of different rout er- t o- rout er prot ocols are used t o exchange t his inform at ion ( including DVMRP, Dense Mode PI M, Sparse Mode PI M, and MOSPF) . This st art s t o get com plicat ed!

N ot e DVMRP st ands for Dist ance Vect or Mult icast Rout ing Prot ocol. PI M st ands for Prot ocol I ndependent Mult icast , and MOSPF st ands for Mult icast Ext ensions t o Open Short est Pat h First .

I n addit ion t o being com put e- int ensive, I GMP snooping has ot her drawbacks. I t does not provide an easy way t o ident ify rout ers t hat need t o receive fram es sent t o a part icular m ult icast address. I t provides no m et hod for ident ifying net work m onit ors t hat want t o receive m ult icast fram es for t he purpose of t roubleshoot ing. Furt herm ore, it works only for I P m ult icast s.

The GARP Multicast Registration Protocol (GMRP) The GARP ( Generic At t ribut e Regist rat ion Prot ocol) Mult icast Regist rat ion Prot ocol ( GMRP) is a general Layer 2 st andard t hat does not have t he problem s associat ed wit h I GMP snooping. I t works efficient ly and can support m ult icast s on behalf of any prot ocol, not j ust I P. GMRP is described in t he 802.1D bridge prot ocol st andard. A syst em t hat want s t o receive t raffic sent t o a part icular m ult icast group uses GMRP t o regist er wit h adj acent swit ches. GMRP let s host s, swit ches, rout ers, and net work m onit ors act ively request t he t raffic t hat t hey want . When GMRP has been im plem ent ed, a swit ch will not forward m ult icast fram es t hrough a port t hat does not lead t o regist ered recipient s. GMRP m akes it possible t o channel t he t ransm ission of fram es addressed t o a m ult icast group ont o t hose subLANs t hat cont ain or lie on a pat h t o t he syst em s t hat need t o receive t he fram es.

N ot e The disadvant age of GMRP is t hat int erfaces in all syst em s t hat will part icipat e in m ult icast s m ust be updat ed t o versions t hat support GMRP. I n cont rast , I GMP snooping needs t o be support ed only in net work swit ches. Swit ch perform ance could be prot ect ed by using ASI Cs t o handle t he ext ra processing burden.

GMRP Procedures A syst em uses GMRP t o regist er it s wish t o receive m ult icast t raffic. The syst em can ask t o receive t he following: • • •

Mult icast fram es addressed t o a list of specific m ult icast MAC address. All m ult icast fram es ( as long as t here is no m anual ent ry prevent ing som e of t his t raffic from being forwarded) . This service is called " all groups" and is likely t o be request ed by a rout er or a net work m onit or. All m ult icast fram es for which t here are no ot her filt ering t able ent ries t hat specify whet her t hey should be forwarded. This is called t he " all unregist ered groups" service.

Figure 14.7 illust rat es what happens when an end syst em regist ers t o receive fram es sent t o m ult icast address M. As illust rat ed in t he figure, adj acent swit ch A adds an ent ry t o it s filt ering dat abase indicat ing t hat fram es t o address M m ust be propagat ed t hrough port 1, which leads t o t he end syst em .

Figu r e 1 4 .7 . Pr opa ga t in g GM RP r e gist r a t ion r e qu e st s.

Swit ch A t hen sends regist rat ion m essages out it s ot her act ive port s, indicat ing t hat it needs t o receive m ult icast s sent t o M. The inform at ion propagat es across t he LAN t o all t he ot her swit ches. Each swit ch forwards fram es addressed t o M t hrough t he port at which t he regist rat ion m essage arrives. A syst em also uses GMRP m essages t o announce t hat it want s t o cancel regist rat ions.

N ot e The act ive port s on a swit ch are t hose t hat have not been blocked by t he Spanning Tree Prot ocol. The fact t hat t he t opology is a t ree ensures t hat regist rat ion and deregist rat ion m essages will not cycle back t o t he originat ing swit ch.

Generic Attribute Registration Protocol GMRP is one specific applicat ion of a m ore general prot ocol: t he Generic At t ribut e Regist rat ion Prot ocol ( GARP) . GARP provides m echanism s t hat enable a st at ion t o regist er a piece of inform at ion wit h LAN syst em s t hat propagat e t hat inform at ion t hrough t he LAN. The creat ors of GARP had m ult icast regist rat ion in t he back of t heir m inds as t he first way t hat GARP would be applied. They m ade GARP general so t hat it could be used t o regist er ot her t ypes of request s.

N ot e The GARP VLAN Regist rat ion Prot ocol ( GVRP) is a relat ed m em ber of t he GARP fam ily. GVRP enables st at ions t o j oin and leave VLANs.

Like ot her GARP- based prot ocols, GMRP is a Layer 2 prot ocol and can be im plem ent ed in NI C drivers. For exam ple, when a syst em j oins a m ult icast via I GMP, t he NI C driver could aut om at ically send a GMRP regist rat ion m essage t o adj acent swit ches. No snooping would be required. Up- t o- dat e NI Cs support GMRP. I t m ight be possible t o upgrade an older NI C t o perform GMRP by m eans of a device driver soft ware updat e, if t he vendor has m ade t his soft ware available. The discussion t hat follows provides a rough idea of how t he prot ocol works. For det ails, see 802.1D. •

• • •

A syst em t hat is direct ly connect ed t o a swit ch regist ers for a group by sending t wo j oin m essages. A syst em m ust refresh it s m em bership by sending fresh j oins periodically. The st at ion quit s by sending a leave m essage. I f t he leave m essage is lost , t he swit ch will t hink t hat t he st at ion st ill wishes t o part icipat e. To correct t his sit uat ion, aft er an ext ended period t he swit ch sends a leaveall m essage, announcing t hat if it does not receive fresh j oins soon, it will t erm inat e all regist rat ions on t he port .

Figure 14.8 illust rat es t he procedure.

Figu r e 1 4 .8 . Re gist e r in g a nd de r e gist e r in g a cr oss a sin gle st a t ion lin k .

Figure 14.9 illust rat es what happens when m ult iple st at ions connect t o a swit ch via a single swit ch port . St at ions A, C, and D want t o receive m ult icast s sent t o group M. A hub connect s t hem t o a swit ch port .

Figu r e 1 4 .9 . M u lt iple st a t ion s con n e ct e d t o on e sw it ch por t .

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•

I f st at ion A has sent t wo j oin m essages, t here is no need for t he ot hers t o do so. The hub will send ident ical m ult icast fram es t o all of t hem . I f st at ion A decides t o quit and sends a leave m essage, ot her st at ions send j oins t o keep t he flow of m essages com ing.

N ot e The GARP prot ocol is designed so t hat only a couple of j oins will be sent even if t wo or m ore st at ions concurrent ly decide t hat t hey need t o send j oins. The reason for t his is t hat each st at ion delays for a random t im e before sending a j oin, which spreads out t he t ransm ission t im es. Because all st at ions hear t he j oins as t hey are sent , no m ore are needed when t wo of t hem have been heard. All st at ions t hen can becom e quiet .

GMRP/GARP Encapsulation and Format Fram es cont aining GMRP m essages ( which also are called GMRP prot ocol dat a unit s, or GMRP PDUs) are sent t o m ult icast address X'01- 80- C2- 00- 00- 20 and are absorbed by any adj acent bridge port t hat support s GMRP. A receiver knows t hat a GARP m essage cont ains GMRP inform at ion because t he m essage is cont ained in a fram e addressed t o t he special GMRP m ult icast MAC address.

GMRP m essages are sim ply GARP m essages t hat carry GMRP at t ribut es. GARP m essages are encapsulat ed in t he sam e way as bridge prot ocol dat a unit s ( BPDUs) . That is, t hey are wrapped in fram es t hat cont ain a 3- byt e Logical Link Cont rol ( LLC) header wit h t he following: DSAP= X'42 SSAP= X'42 Cont rol field = X'03 ( unnum bered inform at ion) Figure 14.10 displays t he form at of a GARP m essage. A GARP m essage st art s wit h a 2- byt e prot ocol ident ifier field equal t o X'00- 01. This is followed by one or m ore m essage blocks. The m essage closes wit h a 1- byt e end m ark equal t o X'00.

Figu r e 1 4 .1 0 . GARP a n d GM RP m e ssa ge s.

Each m essage block consist s of an at t ribut e t ype code, a list cont aining one or m ore ( event , value) pairs, and an end m ark. Separat e codes are included for " j oin" and " leave" event s. A value can be eit her of t he following: • •

Gr ou p—The values are MAC m ult icast addresses. Se r vice r e qu ir e m e n t —The value is an I D code t hat corresponds t o eit her " all groups" or " all unregist ered groups."

Figure 14.11 shows t he prot ocol- layer com ponent s t hat support GARP and GMRP. GMRP is sim pler and m ore accurat e t han I GMP snooping and was designed t o operat e ent irely at Layer 2. GMRP also was cust om - designed t o do t he j ob of opt im izing t he flow of m ult icast fram es t hrough a LAN.

Figu r e 1 4 .1 1 . Com pon e n t s u se d for GARP a n d GM RP.

Summary Points • •

I t is a good idea t o prot ect t he bandwidt h available wit hin a collision dom ain by blocking off m ult icast t raffic t hat is not addressed t o a st at ion in t hat dom ain. I t is m ore difficult t o block off m ult icast t raffic t han t o block off unicast t raffic.

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I P m ult icast addresses are m apped int o Et hernet or FDDI MAC addresses. The first 25 bit s of t he MAC address hold a fixed- bit pat t ern. The last 23 bit s of t he MAC address are copied from t he last 23 bit s of t he m ult icast I P address. I GMP snooping is one m et hod used t o discover which part s of a LAN need t o receive m ult icast s sent t o a part icular group. To perform I GMP snooping, bridges eavesdrop on t he I GMP m essages t hat st at ions use t o j oin and leave m ult icast groups. I GMP snooping is CPU- int ensive. I t requires a Layer 2 swit ch t o exam ine t he cont ent s of a lot of Layer 3 t raffic. The GARP Mult icast Regist rat ion Prot ocol ( GMRP) is a Layer 2 prot ocol t hat enables st at ions t o regist er wit h bridges and ident ify t he m ult icast t raffic t hat t hey want t o receive. GMRP is built on t op of a m ore general Layer 2 prot ocol called t he Generic At t ribut e Regist rat ion Prot ocol ( GARP) . GARP provides m echanism s t hat enable a st at ion t o regist er a piece of inform at ion wit h ot her LAN syst em s and t hen propagat e t hat inform at ion t hrough t he LAN.

References GMRP and GARP are described in: •

I EEE 802.1D. " Local and Met ropolit an Area Net works—Com m on Specificat ions—Media Access Cont rol ( MAC) Bridges." 1998. Chapt er 10–13.

The rout er- t o- rout er m essages t hat t rack t he locat ions of m ult icast recipient s are described in t he I ETF Request for Com m ent s ( RFC) docum ent s t hat follow: •

• • •

•

RFC 1075—Wait zm an, D., C. Part ridge, and S.E. Deering. " Dist ance Vect or Mult icast Rout ing Prot ocol." RFC 1075, 1988. RFC 1585—Moy, J. " MOSPF: Analysis and Experience." RFC 1585, 1994. RFC 2117—Est rin, D., D. Farinacci, A. Helm y, D. Thaler, S. Deering, M. Handley, V. Jacobson, C. Liu, P. Sharm a, and L. Wei. " Prot ocol I ndependent Mult icast - Sparse Mode ( PI M- SM) : Prot ocol Specificat ion." RFC 2117, 1997. RFC 2337—Farinacci, D., D. Meyer, and Y. Rekht er. " I nt ra- LI S I P Mult icast Am ong Rout ers over ATM Using Sparse Mode PI M." RFC 2337, 1998. RFC 2362—Est rin, D., D. Farinacci, A. Helm y, D. Thaler, S. Deering, M. Handley, V. Jacobson, C. Liu, P. Sharm a, and L. Wei. " Prot ocol I ndependent Mult icast - Sparse Mode ( PI M- SM) : Prot ocol Specificat ion." RFC 2362, 1998.

Several draft docum ent s t hat will updat e t he RFCs list ed here are in progress.

Chapter 15. Link Aggregation Link aggregat ion, som et im es called t runking, com bines a set of links so t hat t hey behave like a single link. A com bined set of links is called an aggregat ed link, a virt ual link, or a t runk. A port t hat part icipat es in link aggregat ion is called an aggregat ion port .

The capabilit y t o aggregat e links affords im port ant benefit s: •

•

A high- capacit y link can be set up bet ween a pair of syst em s by com bining a group of links. The procedure im proves net work availabilit y. The failure of one under- lying link in a group of links causes a reduct ion in available bandwidt h, but t he aggregat ed link cont inues t o funct ion.

I n t he past , it was com m on t o inst all ext ra LAN links for backup purposes. A backup link would be idle unless it s prim ary link failed. This m eant t hat an organizat ion had t o invest in bandwidt h t hat was used only in em ergencies. Today, one or m ore backup links can be aggregat ed wit h a prim ary link, and t he ext ra bandwidt h can be used all t he t im e. Because link aggregat ion is a very useful capabilit y, several vendors have creat ed propriet ary aggregat ion solut ions over t he past few years. These solut ions have been based on vendor- defined prot ocol m essage exchanges. This m eant t hat aggregat ion would work only bet ween syst em s obt ained from t he sam e vendor. I n 1998, t he I EEE 802.3ad Link Aggregat ion Task Force was form ed t o define a st andard procedure for t he Et hernet environm ent . Many vendors have pledged support for t he em erging 802.3ad Et hernet link aggregat ion st andard.

N ot e At t he t im e of t his writ ing, t he 802.3ad st andard had not been finalized. The m at erial in t his chapt er was included because link aggregat ion is an im port ant funct ion and because t he prot ocol m echanism s t hat are used appear t o be st able. Som e product t est ing already has st art ed.

A m ult ivendor link aggregat ion st andard is a useful addit ion t o t he Et hernet prot ocol fam ily. I t provides users wit h scalable bandwidt h and good availabilit y opt ions. Because t he prot ocol funct ions can be perform ed at t he device driver level, t he prot ocol can be m ade available quickly in new drivers, or by updat ing som e exist ing drivers. Alt hough Et hernet is t he focus of at t ent ion for 802.3ad, link aggregat ion has been int roduced int o ot her part s of t he net work. Som e vendors have im plem ent ed propriet ary link aggregat ion for FDDI links. I n addit ion, t he I ETF has defined an aggregat ion m et hod for point - t o- point WAN links called t he PPP m ult ilink prot ocol.

N ot e The PPP m ult ilink prot ocol is described in RFC 1990.

Using Link Aggregation

Figure 15.1 shows an exam ple of how aggregat ed links are used t o prevent net work bot t lenecks. At t he t op, a pair of full- duplex 1Gbps links bet ween large swit ches has been aggregat ed int o a virt ual full- duplex 2Gbps link. On t he right , four full- duplex 100Mbps links leading t o a server farm have been aggregat ed int o a virt ual fullduplex 400Mbps link. On t he left , t wo 100Mbps full- duplex links leading t o a com m unit y of 10Mbps workst at ions have been aggregat ed.

Figu r e 1 5 .1 . Aggr e ga t e d lin k s.

Link Aggregation Features Link aggregat ion can group t oget her a set of full- duplex Et hernet links connect ing syst em s such as: • • • • •

A A A A A

pair of Layer 2 swit ches Layer 2 swit ch and a server pair of servers Layer 2 swit ch and a rout er pair of rout ers

The links share t he t raffic load t hat is sent bet ween t he syst em s.

N ot e All t he links in an aggregat ed group are required t o operat e at t he sam e speed.

A crucial feat ure of t he prot ocol is t hat it com bines links bet ween any pair of syst em s t hat are capable of using t he negot iat ion procedure defined by t he link aggregat ion st andard: •

•

When an adm inist rat or has enabled aggregat ion, t he prot ocol aut om at ically groups duplicat e links t o a com m on dest inat ion int o one or m ore virt ual links. I f links are added or rem oved, or if t hey fail, t he prot ocol will regroup t he links aut om at ically.

An adm inist rat or opt ionally can cont rol and guide link aggregat ion t hrough m anual configurat ion choices. Link aggregat ion is backward com pat ible. I f an older syst em t hat does not support t he prot ocol is connect ed t o a newer syst em t hat does, t he negot iat ion procedure sim ply will fail and links will not be aggregat ed. For som e t ypes of t raffic, it is im port ant t o deliver fram es in t he sam e order in which t hey were t ransm it t ed. The link aggregat ion prot ocol includes a sim ple m echanism t hat assures t hat select ed st ream s of fram es will be delivered in t he sam e order in which t hey were sent . The sect ions t hat follow describe link aggregat ion concept s. Alt hough links bet ween m any t ypes of syst em s can be aggregat ed, t o keep t he discussion concret e, t his chapt er focuses m ainly on aggregat ing links t hat connect swit ches. Link aggregat ion operat es in exact ly t he sam e way for ot her syst em s.

Link Aggregation Concepts and Procedures Figure 15.2 shows som e of t he com ponent s of t he environm ent in which virt ual links exist . The figure shows swit ches t hat consist of a chassis, a CPU m odule, and a set of slot s for line cards. A line card is an input / out put ( I / O) card t hat can be inst alled in a chassis.

Figu r e 1 5 .2 . Aggr e ga t e d lin k e nvir on m e n t .

A cust om er obt ains a set of int erface port s t hat support a part icular com m unicat ions t echnology by inst alling t he appropriat e t ype of line card—for exam ple, Et hernet , Token Ring, FDDI , ATM, or T1. A line card t ypically provides several com m unicat ions port s and cont ains an applicat ion specific int egrat ed circuit ( ASI C) chip t hat processes t he t ype of t raffic t hat t he line card support s. The chassis on t he left includes a 1Gbps Et hernet line card, a Token Ring line card, t wo 100Mbps Et hernet line cards, and som e ot her line cards. The separat e links t hat m ake up a virt ual link are called link segm ent s. Each link segm ent m ust be a fullduplex link. I n t he figure, four links on a 100Mbps Et hernet line card on t he left connect t o a line card at t he second swit ch. Link aggregat ion has been enabled on t he 100Mbps line cards, so t he four link segm ent s will be aggregat ed int o a single virt ual link. The m axim um num ber of link segm ent s ( or equivalent ly, t he num ber of port s) on a device t hat can be aggregat ed depends on how a vendor has im plem ent ed t he device, and t his differs for each product . For m any devices, t raffic is processed by an oncard ASI C, and all t he port s t hat are aggregat ed m ust be on t he sam e card. Som e product s allow port s on adj acent line cards t o be aggregat ed. Ot hers lim it t he t ot al num ber of port s t o som e value ( for exam ple, 4) and require t hat t hey be physically adj acent on a line card.

The rest rict ions are product - specific. Based on com pet it ive pressure, a vendor m ight m odify a product 's archit ect ure t o rem ove som e aggregat ion rest rict ions t o m ake t he product m ore at t ract ive t o cust om ers.

Virtual Link MAC Address A virt ual link m ust behave j ust like a real link. This m eans t hat a virt ual link in an end syst em ( such as a server) m ust have it s own MAC address. User fram es sent from t he server ont o a virt ual link carry t he MAC address in t heir source MAC address field. User fram es sent t o t he server across a virt ual link cont ain t he MAC address in t heir dest inat ion MAC address field. Som e im plem ent at ions aut om at ically use t he sm allest MAC address of t he aggregat ion port s part icipat ing in t he virt ual link as t he virt ual link's MAC address. Ot hers allow an adm inist rat or t o configure a MAC address. Figure 15.3 illust rat es t he relat ionship bet ween a virt ual MAC and it s under- lying real links. Fram es carrying t raffic passed down from higher layers include t he virt ual MAC address in each fram e's source address field.

Figu r e 1 5 .3 . Re la t ion sh ip be t w e e n a vir t u a l M AC a n d t h e M ACs for it s lin k se gm e nt s.

However, low- level prot ocol m essages are used t o set up and m aint ain t he virt ual link. These m essages are exchanged across each of t he underlying link segm ent s. The source address in one of t hese fram es is t he real MAC address of t he port from which it was sent . These prot ocol m essages are st udied lat er in t his chapt er, in t he sect ion " The Link Aggregat ion Cont rol Prot ocol." A MAC address also is needed for a virt ual link in a swit ch. The reason for t his is t hat t here are som e m essages t hat will be originat ed and received by t he swit ch. For exam ple, SNMP m essages, BPDUs, and GMRP m essages t hat are sent and received across a virt ual link need well- defined and consist ent source and dest inat ion MAC addresses.

Link Aggregation Sublayer Structure Link aggregat ion adds yet anot her opt ional sublayer t o t he dat a link layer, as shown in Figure 15.4. The link aggregat ion sublayer schedules out going user fram es t hat have been passed t o t he virt ual MAC ont o t he real links. I n t he figure, fram es are dist ribut ed across four real links. The sublayer also collect s and orders incom ing user fram es for delivery t o higher layers.

Figu r e 1 5 .4 . Lin k a ggr e ga t ion a r ch it e ct u r e .

The link aggregat ion sublayer is responsible for cont rolling t he aggregat ion process. I t decides which links it should at t em pt t o aggregat e, based on configurat ion inform at ion in t he local syst em . The sublayer t hen carries out a separat e negot iat ion across each real link by sending, receiving, and processing link aggregat ion cont rol prot ocol ( LACP) m essages.

What a System Needs to Know Links connect ed t o swit ch A in Figure 15.5 are candidat es for aggregat ion. Swit ch A is connect ed t o swit ch B by four links and t o swit ch C by t wo links.

Figu r e 1 5 .5 . Sw it ch e s ca pa ble of a ggr e ga t ion .

An adm inist rat or could configure every swit ch m anually and describe exact ly how links should be com bined. However, it is bet t er t o have a prot ocol t hat can do t he j ob aut om at ically. Not e t hat even if an adm inist rat or want s t o cont rol t he configurat ion, an aut om at ic prot ocol st ill is needed t o handle changes t o t he t opology caused by line failure and recovery. Aut om at ion also is needed t o handle cabling changes and t he addit ion of new swit ches. I n order t o aggregat e t he lines bet ween swit ch A and swit ch B in Figure 15.5: •

• •

•

Swit ch A m ust be configured in a way t hat perm it s it t o aggregat e t he four links shown on t he left . Swit ch A m ust check t hat no st ruct ural rules will be broken by aggregat ing t hese links. ( For exam ple, t here m ight be a requirem ent t hat links m ust belong t o t he sam e line card and m ust be adj acent .) Swit ch A m ust check which of t hese links are act ive and det erm ine whet her t hey all connect t o t he sam e device ( swit ch B, in t his case) . Swit ch A m ust learn whet her swit ch B is willing and able t o aggregat e t hese links.

The link aggregat ion cont rol prot ocol enables a syst em t o gat her inform at ion t hat it needs by exchanging m essages wit h it s neighbors across each link segm ent . Aft er links have been aggregat ed, t he st at us of each link segm ent is m onit ored by m eans of periodic m essage exchanges.

Link Aggregation Parameters Most of t he param et ers used in link aggregat ion m essages were borrowed from t he Spanning Tree Prot ocol. Recall t hat a Spanning Tree bridge syst em ident ifier consist s of t he following com ponent s: • •

A 2- byt e priorit y num ber. A unique MAC address assigned t o t he syst em . The syst em 's sm allest MAC address norm ally is used.

Syst em s ot her t han bridges ( for exam ple, rout ers or servers) t hat need t o aggregat e links can be assigned unique syst em ident ifiers in exact ly t he sam e way.

Sim ilarly, t he port s of any part icipat ing syst em can be assigned ident ifiers wit h t he sam e st ruct ure as t he Spanning Tree Prot ocol port ident ifiers. That is, a syst em port I D consist s of t he following com ponent s: • •

A port priorit y value The port num ber

One addit ional num ber called a key m ust be assigned t o each port . Port s wit h t he sam e key value have t he pot ent ial t o be aggregat ed. Port s wit h t he sam e key are said t o belong t o a key group. Assigning a set of local port s t o a key group does not guarant ee t hat all of t hem act ually can be aggregat ed successfully. The prot ocol m ust com ply wit h t hese point s: • •

•

Apply device- specific rules t o det erm ine which of t he port s can be aggregat ed. For exam ple, t here m ight be a lim it on t he num ber of port s t hat can be aggregat ed, or a requirem ent t hat t he port s be adj acent . Det erm ine whet her all t he port s in a key group really connect t o t he sam e rem ot e syst em . Check which of t he rem ot e port s also belong t o a com m on key group.

Prot ocol m essages carry flag param et ers t hat show whet her a port can be added t o a part icular group, and whet her it already has been added t o t he aggregat ion group.

N ot e A lazy adm inist rat or could sim ply assign all port s t o t he sam e key group. The prot ocol will find out what can be aggregat ed and t hen will go ahead and group all links t hat it possibly can. As t he prot ocol discovers groups t hat can be aggregat ed, it assigns it s own operat ional keys t o t hem .

The Link Aggregation Control Protocol This sect ion out lines t he m aj or feat ures of t he prot ocol st ruct ure. Virt ual links are creat ed, m onit ored, and updat ed by m eans of an exchange of prot ocol m essages. Link aggregat ion prot ocol m essages are exchanged across each link segm ent . They belong t o real links rat her t han virt ual links. The source address in each of t he fram es is t he source MAC address of t he real port from which it was sent .

Slow Protocols Link aggregat ion belongs t o a fam ily called slow prot ocols. The charact erist ics of an Et hernet slow prot ocol are list ed here: • •

No m ore t han five fram es are t ransm it t ed in any 1- second period. Prot ocol m essages are carried in fram es addressed t o a special m ult i- cast address assigned t o t he slow prot ocols. The slow prot ocols m ult icast address is X'01- 80- C2- 00- 00- 02.

•

The Et herType field cont ains a value assigned t o slow prot ocols. The Et herType value is X'88- 09.

The first byt e of t he inform at ion field in a slow prot ocol fram e cont ains a code t hat ident ifies t he subt ype of slow prot ocol PDU t hat follows.

Link Aggregation Control Protocol Messages Aggregat ion inform at ion is carried in Link Aggregat ion Cont rol PDU ( LACPDU) m essages, whose subt ype value is X'01. LACP m essage param et ers include • • • • • • • • •

Local syst em ident ifier ( priorit y and syst em MAC) Local port ident ifier ( priorit y and port num ber) Key assigned t o t he port Local st at e flags Part ner syst em ident ifier ( when it is known) Part ner port ident ifier ( when it is known) Part ner key assigned t o t he port Part ner st at e flags Tim er inform at ion

Figure 15.6 illust rat es how t his inform at ion can be used t o aggregat e links bet ween a pair of swit ches.

Figu r e 1 5 .6 . Aggr e ga t in g lin k s be t w e e n br idge s.

1. 1.At t he t op of t he figure, swit ch 1 learns t hat six port s in key group 1 lead t o neighbor swit ch 2. 2. 2.However, at swit ch 2, t he six port s have been configured int o t wo separat e key groups. This is because swit ch 2 is incapable of aggregat ing m ore t han four port s int o a group. 3. 3.Swit ch 1 and swit ch 2 agree t o aggregat e t he first four port s. 4. 4.Swit ch 1 aut om at ically assigns a new key value t o t he rem aining t wo port s, creat ing a new key group. Aft er a fresh exchange of m essages, t hese t wo link segm ent s are aggregat ed and used as a backup link. Ongoing prot ocol m essages updat e t he configurat ion, adding or rem oving link segm ent s from an aggregat ed link.

Frame Distribution and Conversations When a flow of fram es is split across m ult iple link segm ent s, som e m ight arrive out of order. Som et im es being out of order m at t ers—for exam ple, for fram es belonging t o I BM SNA sessions. I n ot her cases, order does not m at t er at all. For exam ple, delivering I P dat a out of order does not cause a problem . The t rick is t o preserve order when it m at t ers. A flow of fram es t hat m ust be delivered wit hout changing t heir order is called a conversat ion. The fram es wit hin a given conversat ion are delivered in order by t he sim ple expedient of sending t he fram es t hrough t he sam e port , across t he sam e link segm ent . The receiver m ust t hen be sure t o deliver fram es t hat were received by a

given port in t he order in which t hey arrived. ( However, when t hese fram es are delivered, t hey can be int erleaved wit h fram es received from ot her port s.) As shown in Figure 15.7, a link segm ent can carry several conversat ions. For exam ple, fram es for conversat ions C2, C6, and C7 have been assigned t o t he t op link segm ent ; C3, C5, and C8 are sent across t he m iddle link segm ent ; and fram es for conversat ions C1 and C4 are sent across t he bot t om link segm ent .

Figu r e 1 5 .7 . Con ve r sa t ion s a ssign e d t o lin k se gm e n t s.

Ot her t raffic t hat does not belong t o any conversat ion can be spread across t he t hree links using any algorit hm t he vendor want s t o apply. Many different param et ers can be used t o define a conversat ion subflow, including com binat ions of any t he following param et ers: • • • • • •

Source MAC address Dest inat ion MAC address The recept ion physical port num ber The t ype of dest inat ion MAC address ( individual, m ult icast , or broadcast ) The t ype of higher- layer prot ocol dat a t hat is carried in t he fram e ( such as SNA) Higher- layer prot ocol inform at ion ( such as a dest inat ion I P or I PX net work ident ifier)

Sw it ch in g a Con ve r sa t ion t o a D iffe r e n t Lin k Se gm e n t A crit ical part of t he prot ocol is t he capabilit y t o swit ch a conversat ion from one link segm ent t o anot her. This is vit al because a conversat ion's assigned segm ent m ight fail, or conversat ions m ight need t o be redist ribut ed t o im prove t he load balance. The procedure used t o carry out a t ransit ion first " flushes" t he old link and t hen swit ches over. The st eps are list ed here: 1. Choose a new link segm ent t o be used for t he conversat ion. 2. St op t ransm it t ing fram es for t he conversat ion. Discard subm it t ed fram es t hat belong t o t he conversat ion unt il t he changeover procedure is com plet e. 3. St art a t im er t hat last s long enough t o allow all fram es belonging t o t he conversat ion t hat already have been t ransm it t ed t o arrive. 4. I f t he link segm ent previously assigned t o t he conversat ion st ill is funct ioning, send a special m arker prot ocol m essage t hrough it s port . This m arker t ells

t he part ner t hat no m ore fram es for t he conversat ion will arrive on t his link segm ent . 5. Wait unt il eit her a m arker response prot ocol m essage is received or t he t im er expires. The arrival of a m arker response speeds up t he changeover. When a response has been received, t here is no need t o wait any longer. 6. Resum e fram e t ransm ission for t he conversat ion on t he new link segm ent . The recipient will know t hat a conversat ion has been swit ched successfully when it receives a fram e wit h t he conversat ion's defining charact erist ics at a different port . Not e t hat t here is a cost t o a swit chover: A few fram es m ight be discarded in St ep 2. Marker and m arker response prot ocol m essages have slow prot ocol subt ype X'02. They include fields ident ifying t he originat ing syst em and port as well as a t ransact ion I D.

Summary Points •

• • • • •

• • •

• •

Link aggregat ion, which also is called t runking, com bines a set of links so t hat t hey behave like a single link, which is called a virt ual link. Link aggregat ion im proves net work capacit y and availabilit y. The link aggregat ion st andard applies only t o full- duplex Et hernet links. All links in an aggregat ed group m ust operat e at t he sam e speed. The link aggregat ion cont rol prot ocol aut om at ically groups links and regroups t hem aft er a link addit ion, delet ion, or failure. Product - specific rest rict ions som et im es govern t he way t hat link segm ent s can be aggregat ed. A virt ual link m ust have a MAC address. Som e im plem ent at ions aut om at ically use t he sm allest MAC address assigned t o an aggregat ion port part icipat ing in t he virt ual link. The link aggregat ion sublayer is responsible for cont rolling and execut ing t he aggregat ion process. I t decides which links it should at t em pt t o aggregat e based on configurat ion inform at ion in t he local syst em . A key value is assigned t o each port . Port s t hat have t he sam e key value have t he pot ent ial t o be aggregat ed. Fram es t hat m ust be delivered in order are assigned t o a conversat ion. All t he fram es for a part icular conversat ion are t ransm it t ed t hrough t he sam e port . The receiver t hen m ust deliver t hem in t he order in which t hey arrived. Conversat ions are defined using param et ers such as source or dest inat ion MAC address or t ype of prot ocol dat a being carried.

References Link aggregat ion is described in •

I EEE 802.3ad. " Aggregat ion of Mult iple Link Segm ent s." 2000.

The I ETF has defined a wide area link aggregat ion prot ocol in

•

Sklower, K., B. Lloyd, G. McGregor, D. Carr, and T. Coradet t i. "The PPP Mult ilink Prot ocol ( MP) ." RFC 1990, 1996.

Chapter 16. VLANs and Frame Priority The int roduct ion of efficient bridges ( Layer 2 swit ches) has m ade it easy t o build very large LANs. There are som e advant ages t o let t ing a LAN grow very large inst ead of cut t ing it int o separat e sm aller LANs t hat are separat ed by rout ers: • • •

Fewer rout ers are needed. High- capacit y rout ers are m ore cost ly t han bridges. Many adm inist rat ors feel t hat rout ers are m ore com plicat ed t o configure t han bridges. Bridges forward t raffic fast er t han rout ers ( alt hough current Layer 3 swit ches which are rout ers are fairly fast ) .

N ot e Keep in m ind t hat by definit ion, a LAN cont ains only hubs ( Layer 1 wiring concent rat ors) and bridges ( Layer 2 swit ches) . When you go t hrough a rout er, you have left a LAN.

Building a big physical LAN can pay off in t erm s of perform ance gains and equipm ent savings. Wit h t he help of t ranslat ional swit ches ( which are discussed in Chapt er 17, " Source- Rout ing, Translat ional, and Wide Area Bridges" ) , a big LAN can be const ruct ed using a com binat ion of Et hernet , Token Ring, FDDI , and ATM t echnologies. Big LANs also cause som e problem s. I n t he past , m ost LAN t raffic was local. Traffic could be isolat ed wit hin sm all collision dom ains t hat were bridged t o t he rest of t he LAN. For exam ple, a bridge could prevent t raffic bet ween I PX st at ions and a local I PX server from leaving it s collision dom ain. Today t he capabilit y t o isolat e t raffic using convent ional swit ches is eroding. Users access far- away servers or belong t o proj ect t eam s whose m em bers are locat ed far apart . Relat ively lit t le t raffic can be bot t led up wit hin a local collision dom ain. Furt herm ore, collision dom ains are shrinking and gradually disappearing. They are being replaced by individual, full- duplex connect ions bet ween com put ers and Layer 2 swit ches. The m et hods ( described in Chapt er 12, " Et hernet Bridges and Layer 2 Swit ches" ) used t o keep t raffic isolat ed wit hin a collision dom ain are less relevant and are incapable of cont rolling m uch of t he t raffic on a big m odern swit ched LAN. Broadcast s are an especially t horny problem . Every higher- layer prot ocol causes st at ions t o init iat e a spat t ering of broadcast s. When t here are hundreds of st at ions

on a LAN, t his adds up t o a subst ant ial t raffic burden. Each broadcast is sent t hrough every port on t he LAN, delaying ot her t raffic. Flooding of ordinary unicast fram es is another source of t raffic t hat spreads across a LAN. Recall t hat if a fram e's dest inat ion MAC address is not in a swit ch filt ering t able, t he swit ch t ransm it s t he fram e t hrough every port except for t he one on which t he fram e arrived. Except for m anually ent ered MAC addresses, t he MAC address of any syst em t hat is quiet for a while is rem oved from all LAN filt ering t ables. Securit y is anot her issue t hat concerns som e LAN adm inist rat ors. Som e sensit ive LAN t raffic should not be flooded t o all port s on a swit ch. However, if t he com m unit y of users who exchange sensit ive dat a is physically dispersed, it is difficult t o cont rol where t heir fram es are sent . The concept of a virt ual LAN ( VLAN) was int roduced as a way t o cont rol t he t raffic flows on a physical LAN. A VLAN is m ade up of a group of syst em s ( such as t he com put ers in a workgroup) t hat need t o com m unicat e wit h one anot her. Traffic bet ween t hese syst em s st ays bot t led up wit hin t heir virt ual LAN. This can cut back overall LAN t raffic significant ly and can im prove securit y. The groundwork for support ing virt ual LANs in a st andardized fashion was laid by t he I EEE in 802.1Q, " Virt ual Bridged Local Area Net works." This chapt er describes t he concept s and prot ocols int roduced in t hat st andard, as well as m any vendor- init iat ed VLAN feat ures. This chapt er also discusses fram e priorit izat ion because t he priorit y and VLAN prot ocols are closely relat ed. Bot h are im plem ent ed using t he sam e prot ocol header.

N ot e Cust om ers want ed VLANs before VLAN st andards were available. Vendors built propriet ary solut ions, and t he solut ions varied a lot . St andards now are available, but a lot of dissim ilarit y st ill exist s in t he ways t hat product s work. I nst ead of list ing every feat ure t hat any product m ight offer, t his chapt er st art s wit h a st andards- based orient at ion and t hen present s part icularly useful enhancem ent s t hat are available in product offerings.

Virtual LAN Concepts VLAN prot ocols enable an adm inist rat or t o part it ion a LAN int o m ult iple virt ual LANs. Each VLAN is assigned a num ber t hat ident ifies it uniquely wit hin t he LAN. Mult iple VLANs share t he swit ches and links in an underlying physical LAN. However, each VLAN logically behaves like a separat e LAN. All of a VLAN's fram e t raffic ( including it s broadcast s and m ult icast s) is confined t o t he VLAN. This t ranslat es t o im proved perform ance because t he t ot al am ount of t raffic on t he physical LAN can be reduced.

VLANs are const ruct ed around a core set of swit ches t hat support t he needed funct ions and prot ocols. These are called VLAN- aware swit ches, or m ore sim ply VLAN swit ches.

N ot e VLAN swit ches are Layer 2 swit ches—t hat is, bridges. They are called swit ches in t his chapt er because m ost vendors refer t o t hem t hat way.

Many older swit ches in t he inst alled base are not VLAN- aware, and not all new swit ch product s support VLANs. Furt herm ore, t he swit ches t hat do support VLANs do not necessarily do it in exact ly t he sam e way. Before t he com plet ion of t he I EEE st andard, vendors invest ed a great deal of effort ( and m oney) in creat ing propriet ary VLAN solut ions. Today, a vendor t ypically has t o support it s propriet ary VLAN prot ocols along wit h t he st andard I EEE VLAN prot ocols because m any cust om ers st ill use older equipm ent in t he field t hat runs t he propriet ary solut ion. The I EEE st andard defines basic VLAN funct ionalit y. Vendors legit im at ely enhance t his in various ways. St andards- based product s int eroperat e at t he basic level, but you m ight lose som e at t ract ive add- on capabilit ies when you connect het erogeneous equipm ent .

Types of Virtual LANs A virt ual LAN can be creat ed t o serve any purpose. I t can be long- lived or t em porary. For exam ple, a virt ual LAN m ay consist of: • •

•

• •

All users in a com m on workgroup or proj ect t eam A com m unit y consist ing of a dat abase server and a set of syst em s t hat access t he server on a regular basis Syst em s t hat send and receive t raffic for a part icular prot ocol or fam ily of prot ocols ( for exam ple, I P, I PX, Net BI OS, DECnet , or SNA) A com m unit y of users t hat belong t o a m ult icast group A set of syst em s whose com m unicat ion m ust be isolat ed because of securit y concerns

A single LAN can cont ain a m ixt ure of workgroup, com m unit y, prot ocol, and securit ybased VLANs. Each virt ual LAN is assigned a num ber called a VLAN ident ifier. Every fram e handled by a VLAN swit ch is associat ed wit h som e VLAN.

N ot e

The scope of a VLAN ident ifier is an ent ire physical LAN. A syst em anywhere in t he LAN can be configured t o be a m em ber of any VLAN t hat has been defined for t he LAN.

A fram e is forwarded based on it s VLAN ident ifier and it s dest inat ion MAC address. For now, t he easiest way t o underst and how t his works is t o visualize a separat e filt ering t able for each VLAN ident ifier. A fram e's dest inat ion MAC address is looked up in t he swit ch filt ering t able t hat cont ains ent ries for it s VLAN.

VLAN Awareness and Ports Any end syst em or swit ch t hat perform s VLAN funct ions and prot ocols is said t o be VLAN- aware. A VLAN- aware swit ch can sim ply be called a VLAN swit ch. An end syst em becom es VLAN- aware when appropriat e device driver soft ware is inst alled and configured. Most current VLANs include few VLAN- aware end syst em s. A VLAN environm ent can be const ruct ed around a single VLAN swit ch. For exam ple, swit ch A is t he only syst em in Figure 16.1 t hat is VLAN- aware.

Figu r e 1 6 .1 . VLAN s de fin e d a t a sw it ch .

VLANs 2 and 3 in t he figure have been configured by ident ifying t he port s on swit ch A t hat lead t o syst em s assigned t o each of t he VLANs. Specifically, t he following is t rue: • •

Every syst em reached via swit ch A port s 0, 2, 3, and 7 belongs t o VLAN 2. Every syst em reached via swit ch A port s 1, 4, 5, and 6 belongs t o VLAN 3.

No com put er is aware t hat it belongs t o a VLAN. I n fact , swit ch B is not aware t hat it part icipat es in VLAN 2. As part of it s norm al operat ion, swit ch B t ransm it s broadcast s, m ult icast s, and flooded fram es t hrough port 5 t o swit ch A. This t raffic arrives at port 7. Swit ch A ident ifies every fram e arriving at port 7 as a VLAN 2 fram e and forwards it accordingly. From t he point of view of swit ch B, t he syst em s in VLAN 3 do not even exist . Swit ch A does not forward any VLAN 3 fram es t o swit ch B, and swit ch B only observes and learns t he MAC addresses of t he VLAN 2 syst em s at t ached t o swit ch A. Not e t hat t he ent ire st ruct ure of t he VLANs in Figure 16.1 is described by enum erat ing t he port s at swit ch A t hat part icipat e in each VLAN. I n general, assigning port s t o a VLAN is a key configurat ion st ep. Lat er sect ions cont ain m ore inform at ion about t his.

VLAN Switches Figure 16.2 shows t wo VLANs t hat are spread across t hree swit ches. Swit ches A and B bot h are VLAN- aware. Each support s m em bers of t wo VLANs.

Figu r e 1 6 .2 . VLAN m e m be r s.

Swit ch C does not have t o be a VLAN swit ch t o part icipat e in t he configurat ion shown in Figure 16.2 because all of it s st at ions are assigned t o VLAN 3. Swit ch A will m ake sure t hat VLAN 3 fram es are forwarded t o swit ch C, and will forward fram es sent from swit ch C t o VLAN 3 syst em s. However, if swit ch C is a not VLAN swit ch, an adm inist rat or will have lim it ed flexibilit y in m aking m oves and changes. For exam ple, if t he owner of st at ion W is m oved t o an office t hat is cabled int o swit ch C, t he st at ion cannot be configured int o it s old VLAN ( VLAN 2) unless swit ch C is changed t o a VLAN swit ch.

Trunks and Tags A link t hat carries t raffic bet ween a pair of VLAN swit ches is called a t runk link. Traffic belonging t o any num ber of VLANs can share a t runk link. I n Figure 16.2, VLAN 2 and VLAN 3 share t he t runk link bet ween swit ch A and swit ch B.

Figu r e 1 6 .7 . Be lon gin g t o m u lt iple VLAN s.

When a fram e arrives at a VLAN swit ch, t he swit ch m ust det erm ine which VLAN t he fram e belongs t o before t he fram e can be forwarded. For exam ple, if a unicast or m ult icast fram e from a VLAN 3 syst em at swit ch B arrives at swit ch A, swit ch A m ust look up t he dest inat ion address in t he VLAN 3 MAC t able. Swit ch A needs a way t o associat e a fram e t hat arrives from swit ch B wit h t he right VLAN. The solut ion is illust rat ed in Figure 16.3. Fram es t hat are t ransm it t ed across a t runk bet ween VLAN swit ches include an ext ra subheader called a t ag. A t ag includes a fram e's VLAN ident ifier and also cont ains a priorit y level for t he fram e. A fram e whose header cont ains a t ag t hat ident ifies a VLAN is called a VLAN- t agged fram e or a t agged fram e.

N ot e Som et im es t ags are used only t o indicat e t he priorit y of fram es wit hout ident ifying VLAN m em bership. These fram es are called priorit y- t agged. The VLAN ident ifier field in a priorit y- t agged fram e is set t o 0, which st ands for " t he null VLAN." A fram e whose t ag cont ains a nonzero VLAN ident ifier is called VLANt agged or j ust t agged. I t is assum ed t hat a priorit y is assigned t o a VLAN- t agged fram e, even if t he priorit y field happens t o be 0.

Thus, fram es can be unt agged, priorit y- t agged, or VLAN- t agged.

Access Links A link t hat connect s a VLAN- unaware syst em t o a VLAN swit ch is called an access link. Assum ing t hat all t he end syst em s in Figure 16.3 are VLAN- unaware, all t he links t hat connect end syst em s t o t he VLAN swit ches are access links.

Figur e 1 6 .3 . A t r unk link , t a gge d fr a m e s, a n d a cce ss lin k s.

Because swit ch C also is VLAN- unaware, t he link bet ween swit ch C and swit ch A in Figure 16.3 also is an access link.

Using a Collision Domain as a Trunk The t runk link in Figure 16.3 is a point - t o- point link, which is t he m ost com m on t ype of t runk link. However, a collision dom ain also can be used as a t runk link.

Figure 16.4 shows a set of VLAN bridges t hat are connect ed by an FDDI ring t hat is being used as a t runk. Every syst em at t ached t o a t runk m ust be VLAN- aware.

Figu r e 1 6 .4 . An FD D I r in g u se d a s a t r u n k lin k .

Hybrid Links I t would not be unusual for a LAN adm inist rat or t o add som e VLAN- unaware applicat ion servers t o t he FDDI ring in Figure 16.4. This would t urn t he ring int o t he hybrid link, shown in Figure 16.5.

Figu r e 1 6 .5 . A h ybr id lin k .

Form ally, a hybrid link is a collision dom ain t hat connect s VLAN bridges t o one anot her and also includes VLAN- unaware syst em s. The VLAN- unaware syst em s all m ust be assigned t o a single default VLAN. I n t his exam ple, VLAN 4 is t hat default VLAN. Any collision dom ain could be used as a hybrid link even a t wist ed- pair Et hernet collision dom ain built around a bunch of int erconnect ed hubs. This would be a bit of a m ess, t hough, and it also would negat e som e of t he desirable feat ures supposedly gained by using VLANs.

N ot e Every VLAN- unaware syst em in a hybrid link " sees" all of t he fram es t hat are t ransm it t ed across t he subLAN by every bridge. However, a t agged fram e cont ains an Et herType code t hat is not recognized by an ordinary end st at ion. The st at ion will not be capable of int erpret ing a t agged fram e and will discard it ( unless t he st at ion's owner is using m onit or soft ware t hat capt ures all fram es t hat are in t ransit .)

Communicating Between VLANs The purpose of a VLAN is t o cont rol and separat e som e t ypes of t raffic from ot her t ypes of t raffic. I n Figure 16.3, even t hough t he syst em s on VLAN 2 and VLAN 3 share swit ches and share t he bandwidt h on t he t runk bet ween swit ch A and swit ch B, a syst em on VLAN 2 cannot com m unicat e wit h a syst em on VLAN 3. I n Figure 16.6, swit ch A has been replaced by a Layer 2/ 3 swit ch in ot her words, a com bined bridge and rout er. Syst em s in VLAN 2 and VLAN 3 now can com m unicat e wit h each ot her by sending t raffic t hrough t he rout er m odule at bridge/ rout er A.

Figu r e 1 6 .6 . Rou t in g t r a ffic be t w e e n VLAN s.

Because t he t erm swit ch becom es am biguous when bot h bridges and rout ers are present , t he Layer 2 swit ches in t he figure are labeled as bridges. Not e t hat if st at ion X want s t o send dat a t o server S:

1. St at ion X wraps t he dat a in a fram e whose dest inat ion is t he MAC address of it s local rout er, bridge/ rout er A. 2. Bridge B t ags t he fram e and forwards it t o bridge/ rout er A. 3. The rout er rem oves t he fram e header and t railer, and creat es a new t agged fram e whose dest inat ion is t he MAC address of server S. This fram e's t ag indicat es t hat it belongs t o VLAN 3. 4. The rout er forwards t he fram e t o bridge B. 5. Bridge B t hen rem oves t he t ag and delivers t he fram e t o server S. This is a roundabout way t o get from X t o S! However, t his oft en is exact ly what a LAN adm inist rat or want s. The adm inist rat or m ight want t o place t ight cont rols on who can access what . A rout er can do a good j ob of securit y screening. However, if st at ions on VLAN 2 really should be allowed t o reach server S direct ly, t here are ways t o do it . The usual solut ion is shown in Figure 16.7. A syst em can be a m em ber of t wo or m ore VLANs. Server S in Figure 16.7 is VLAN- aware and belongs t o bot h VLAN 2 and VLAN 3. I t is sm art t o add a server t o every VLAN whose syst em s need t o access t he server. This allows t he t raffic t o be delivered direct ly, wit hout being rout ed.

N ot e Som e vendors support " rout e once, swit ch m any" m echanism s t hat enable a swit ched pat h t o be t em porarily opened up bet ween syst em s t hat belong t o different VLANs aft er som e init ial fram es have been rout ed bet ween t hem .

Independent and Shared Learning One way t o im plem ent VLANs at a swit ch is t o set up a separat e filt ering t able for each VLAN. The swit ch st ores t he MAC addresses learned for a VLAN in t hat VLAN's filt ering t able. This is called independent VLAN learning ( I VL) . As was t he case in Figure 16.7, syst em s m ight belong t o m ore t han one VLAN. I n t his case, it is m ore efficient t o use a single filt ering t able t hat cont ains MAC addresses t hat have been learned across a group of VLANs—or for all VLANs. This is called shared VLAN learning ( SVL) . Shared learning elim inat es som e unnecessary flooding of fram es. For exam ple, if independent learning is used for t he VLANs in Figure 16.7, t he MAC address of VLAN- aware server S m ight be included in t he VLAN 3 filt ering t able at swit ch B, but m ight not yet be in t he VLAN 2 filt ering t able. The t op of Figure 16.8 represent s independent swit ch B filt ering t ables for VLANs 2 and 3 in Figure 16.7. When a separat e VLAN 2 t able is used and st at ion X sends a fram e t o server S, t he following occurs:

Figu r e 1 6 .8 . I n de pe n de n t a n d sh a r e d filt e r in g t a ble s.

1. The dest inat ion MAC address of server S is not in t he VLAN 2 filt ering t able. 2. The fram e m ust be t ransm it t ed t hrough all VLAN 2 port s at swit ch B ot her t han t he port connect ed t o st at ion X. 3. When t he fram e arrives at swit ch A, it is forwarded t hrough all t he swit ch port s belonging t o VLAN 2 ot her t han t he port of arrival. Fram es addressed t o server S are flooded in t his m anner unt il server S sends a fram e t o a dest inat ion in VLAN 2 and it s MAC address is ent ered int o t he VLAN 2 filt ering t able. However, if shared learning is used for bot h VLANs in Figure 16.7, t hen as soon as server S t ransm it s a fram e t o any dest inat ion, it s MAC address is ent ered int o t he j oint filt ering t able at swit ch B. This t able is depict ed at t he bot t om of Figure 16.8. The ent ry for server S can be used im m ediat ely for all of t he VLANs t o which it belongs.

The GARP VLAN Registration Protocol Maint aining a big VLAN can be a lot of work. I t would be very helpful if a VLAN syst em could t ell it s neighbors which VLAN t raffic it want s t o receive.

I n fact , t his is exact ly what a very valuable m em ber of t he VLAN prot ocol fam ily, t he GARP VLAN Regist rat ion Prot ocol ( GVRP) , accom plishes. Using GVRP, a VLAN- aware end syst em or swit ch regist ers wit h a neighboring swit ch t o j oin ( or leave) a VLAN. Each syst em periodically updat es it s regist rat ions aut om at ically. St ale regist rat ions age out aft er a t im eout int erval.

N ot e GARP, which st ands for t he Generic At t ribut e Regist rat ion Prot ocol, was int roduced in Chapt er 14, " Swit ches and Mult icast Traffic." GARP provides a general regist rat ion m echanism for use in a bridged LAN.

GVRP propagat es VLAN regist rat ion inform at ion t hrough an ent ire LAN. Lat er sect ions present som e GVRP exam ples and describe t he prot ocol.

N ot e A LAN t hat consist s ent irely of VLAN- aware end syst em s and VLAN- aware swit ches will operat e very efficient ly if all end syst em s and swit ches support GVRP. Every end syst em could regist er and report it s locat ion, and t his inform at ion would be propagat ed t o every swit ch. The result would be t hat every unicast fram e would be delivered only t o it s int ended dest inat ion. No flooding would be required.

VLANs and the Spanning Tree Protocol A physical LAN eit her has a t ree st ruct ure or configures it self int o a t ree by m eans of t he Spanning Tree Prot ocol ( STP) . All VLANs are superim posed on t he current act ive t ree st ruct ure. This m eans t hat VLAN fram es cannot be forwarded t hrough port s t hat have been blocked by t he Spanning Tree Prot ocol. I n a big, com plicat ed LAN cont aining m any swit ches and backup links, it is possible t hat one or m ore VLANs could be chopped int o noncom m unicat ing pieces when t he Spanning Tree Prot ocol m odifies t he LAN t opology. Fort unat ely, when GVRP is enabled, GVRP m essages aut om at ically updat e VLAN m em bership inform at ion aft er a change in t he Spanning Tree t opology. GVRP repairs broken VLAN pat hs.

Implementing VLANs The previous sect ions int roduced som e m aj or VLAN concept s and feat ures. Now it is t im e t o focus on how VLANs are configured and how swit ches process VLAN fram es. A VLAN is creat ed by

• • • •

Assigning a VLAN ident ifier Defining VLAN m em bers List ing t he swit ch port s t hrough which m em bers of t he VLAN can be reached For each VLAN port , indicat ing whet her fram es belonging t o a VLAN m ust be t ransm it t ed from t he port wit h or wit hout t ags

There are m any different ways t o define VLAN m em bership, including t he following: • • •

•

• •

D e fa u lt —I nit ially, all port s in a swit ch belong t o t he sam e default VLAN. Por t - ba se d—At each swit ch, list t he port s t hat belong t o t he VLAN. Por t - a n d pr ot ocol- ba se d—At each swit ch, list port s t hat belong t o t he VLAN and t he t ypes of prot ocol t raffic t hat can t raverse t he VLAN. I P a ddr e ss pr e fix - ba se d—At each swit ch, list port s t hat belong t o t he VLAN and specify an I P address prefix. The VLAN carries fram es for I P syst em s whose I P addresses begin wit h t he prefix, I P broadcast and m ult i- cast fram es, and Address Resolut ion Prot ocol ( ARP) fram es t hat carry queries and responses relat ed t o t he I P addresses. Por t - a n d GVRP- ba se d—Creat e a VLAN by ent ering t he VLAN ident ifier at one or m ore swit ches. Allow com put ers and swit ches t o j oin and leave t he VLAN via GVRP prot ocol int eract ions. Por t - a n d M AC a ddr e ss- ba se d—At each swit ch, m anually ent er ( port , MAC address) com binat ions t hat describe t he syst em s t hat belong t o t he VLAN.

These VLAN t ypes are described in t he sect ions t hat follow. The t ypes t hat are support ed vary from vendor t o vendor.

The Default VLAN When a VLAN bridge init ializes for t he first t im e, all port s aut om at ically are m em bers of t he default VLAN. The VLAN ident ifier of t he default VLAN is 1. I f no ot her VLANs are configured on a LAN, t he default VLAN equals t he whole LAN. Som e adm inist rat ors delet e t he default VLAN t o assure t hat no t raffic will be forwarded unless it explicit ly can be associat ed wit h an int ent ionally defined VLAN.

Static Port-Based VLANs The m ost universally support ed m et hod of defining a VLAN at a swit ch is t o ent er t he VLAN I D int o a configurat ion screen and list t he swit ch port s t hat belong t o t he VLAN. This is called a port - based VLAN. End syst em s at t ached t o a sim ple port - based VLAN do not need t o know what VLAN t hey belong t o. The VLAN m em bership of a fram e sent by a st at ion is det erm ined by t he access port at which t he fram e ent ers t he swit ch. Figure 16.9 displays a LAN consist ing of a single swit ch wit h t wo port - based VLANs. There is no need t o t ag any fram e; t he swit ch easily can separat e t he t raffic for each VLAN.

Figu r e 1 6 .9 . Por t - ba se d VLAN s a t a sw it ch .

The benefit s of split t ing t he port s int o t hese t wo VLANs include •

•

Broadcast and m ult icast t raffic originat ed by a syst em st ays wit hin it s own VLAN. A fram e sent t o a MAC address t hat is not in t he swit ch's filt ering t able is flooded only t hrough port s t hat belong t o it s own VLAN.

Not e t hat t he syst em s in Figure 16.9 t hat belong t o VLAN 2 cannot com m unicat e wit h syst em s in VLAN 3. ( A rout ing m odule would have t o be added t o t he swit ch t o allow com m unicat ion bet ween t he VLANs.)

Ta ggin g in St a t ic Por t - Ba se d VLAN s Port 6 in Figure 16.10 belongs t o t wo VLANs. Now t here is a problem : When a fram e from t he server arrives at port 6, how does port 6 know which VLAN t he fram e belongs t o?

Figu r e 1 6 .1 0 . Ta ggin g fr a m e s t o a n d fr om a por t a ssocia t e d w it h t w o VLAN s.

For port - based VLANs, t he answer is t hat fram es bet ween t he swit ch and t his server need t o be t agged. NI Cs and device drivers t hat m ake a syst em VLAN- aware are available t oday. Som e NI C vendors enable an adm inist rat or t o configure VLAN- aware syst em s from a rem ot e adm inist rat or console. New NI Cs also have securit y feat ures t hat m ake it possible t o do t his safely.

Con figu r in g Tr u n k s for Por t - Ba se d VLAN s Figure 16.11 shows a LAN t hat includes VLAN- aware swit ches and t wo t runk links.

Figur e 1 6 .1 1 . A LAN w it h four VLAN s.

Trunk port s are configured in a vendor- specific way. For m any product s, a t runk port is aut om at ically enrolled in all VLANs t hat have been configured at it s swit ch; t his is t he case in Figure 16.11. For som e product s, you define every VLAN at every swit ch, whet her t here are m em bers at t he swit ch or not . I n addit ion, each t runk belongs t o every VLAN. The result is t hat all broadcast s, m ult icast s, and flooded t raffic for any VLAN are forwarded across all t runks t o every swit ch, whet her or not t hat swit ch has m em bers in t he VLAN. This negat es som e of t he benefit s t hat VLANs are supposed t o provide. A different approach is t aken in Figure 16.11. I n t he figure, a VLAN is defined at a swit ch only if t here are m em bers at t ached t o t hat swit ch. The t runks at t he swit ch aut om at ically are added t o each of t he defined VLANs. Thus, in Figure 16.11, t runk port 4 at swit ch A aut om at ically has been added t o VLANs 1, 2, and 3. Sim ilarly, t runk port s 3 and 12 at swit ch B belong t o VLANs 1, 2, and 3, while t runk port 2 at swit ch C belongs t o VLANs 3 and 4. Not e t hat t he t runk configurat ion is not perfect . Swit ches B and C st ill will forward som e t raffic unnecessarily. The following m anual changes fix t his:

• •

Rem ove port 12 at swit ch B from VLANs 1 and 2. This prevent s it from forwarding unwant ed broadcast s, m ult icast s, and floods for VLANs 1 and 2 t o swit ch C. Rem ove port 2 at swit ch C from VLAN 4. This prevent s it from forwarding unwant ed broadcast s, m ult icast s, and floods for VLAN 4 t o swit ch B.

Figure 16.12 shows t he result of t hese changes.

Figu r e 1 6 .1 2 . A m or e e fficie n t VLAN con figu r a t ion .

M a in t a in in g VLAN Con figu r a t ion M a n u a lly As a LAN grows and t he num ber of st at ions and swit ches increases, t he swit ch configurat ion chore can becom e quit e burdensom e. Vendors provide funct ions t hat aut om at e a lot of t he work. However, t he best way t o underst and t hese funct ions is t o look at what life is like wit hout t hem . Consider what happens when t he user at st at ion X is assigned t o a new workgroup and m ust be t ransferred t o VLAN 4. The ( unaided) adm inist rat or m ust m ake t he following changes at swit ch A:

1. Rem ove VLAN 3. 2. Creat e VLAN 4. Add port 1 t o VLAN 4. ( Add port 4 t o VLAN 4 if t he product does not add it aut om at ically.) The adm inist rat or m ust m ake t he following changes at swit ch B: 1. Rem ove port 3 from VLAN 3. There no longer is any reason t o forward VLAN 3 fram es t o swit ch A. 2. Creat e VLAN 4. ( Add port 3 and port 12 if t he product does not add t hem aut om at ically.) The adm inist rat or m ust m ake t he following changes at swit ch C: 1. Rem ove port 2 from VLAN 3. 2. Add port 2 t o VLAN 4. This sim ple exam ple shows t hat raw VLANs are not necessarily easy t o configure! The propriet ary vendor solut ions enabled swit ches t o exchange enrollm ent inform at ion wit h one anot her and aut om at ed m any of t hese st eps. However, a st andard was needed t o allow swit ches from different vendors t o int erwork. The GARP VLAN Regist rat ion Prot ocol is t he st andard prot ocol designed t o aut om at e VLAN configurat ion chores.

M a in t a in ing VLAN Con figu r a t ion w it h GVRP GVRP is m ost effect ive in a net work m ade up ent irely of VLAN- aware syst em s. At t he t im e of writ ing, relat ively few end syst em s are VLAN- aware. However, GVRP st ill has great value, even when support ed by only a backbone of VLAN- aware swit ches. Going back t o Figure 16.11, when GVRP is used, VLAN inform at ion associat ed wit h a t runk port defines t he VLANs for which t he t runk port wishes t o receive t raffic. Each t runk port regist ers wit h it s neighbor, announcing t he t raffic t hat it wishes t o receive. Each swit ch propagat es t hese regist rat ions t o it s neighbors. The result is t hat a swit ch port regist ers a VLAN ident ifier wit h a neighbor 1. I f t here are local m em bers of t hat VLAN at t he swit ch 2. I f t he swit ch has received regist rat ions t hat announce t hat t here are m em bers of t he VLAN t hat are at t ached t o other swit ches reached t hrough it s port s For exam ple, in Figure 16.11 1. Port 2 at swit ch C regist ers wit h port 12 at swit ch B, asking port 12 t o forward all fram es t hat belong t o VLANs 3 and 4. Swit ch C has m em bers t hat belong t o t hese VLANs. 2. Port 3 at swit ch B regist ers wit h port 4 at swit ch A, asking for all fram es t hat belong t o VLANs 1, 2, and 3 because it has local m em bers. I t also asks for all fram es belonging t o VLAN 4 because swit ch C has request ed t hese fram es. I n Figure 16.13, each t runk port is labeled wit h t he ident ifiers of VLANs whose fram es t he port has been asked t o forward. Not e t hat t hese num bers apply t o

out going t raffic only. This forwarding inform at ion is st ored in dynam ic filt ering t able ent ries at each swit ch.

Figu r e 1 6 .1 3 . V LAN s w h ose fr a m e s w ill be for w a r de d a cr oss t r u n k s.

Not e t hat t he fact t hat swit ch C want s t o receive VLAN 4 fram es has propagat ed up t o swit ch A. However, because no syst em at swit ch A belongs t o VLAN 4, t his does not cause any VLAN 4 fram es t o be sent across from swit ch A t o swit ch B. Now look at what happens when st at ion X m ust be t ransferred t o VLAN 4: 1. The adm inist rat or delet es VLAN 3 from swit ch A. 2. The adm inist rat or creat es VLAN 4 and adds port 1 t o it . 3. Port 4 at swit ch A aut om at ically t ransm it s a GVRP regist rat ion st at ing t hat it wishes t o receive fram es sent t o VLAN 4. 4. Port 12 at swit ch B aut om at ically t ransm it s a GVRP regist rat ion st at ing t hat it wishes t o receive fram es sent t o VLAN 4. Swit ches B and C learn t hat t hey need t o forward VLAN 4 fram es t oward swit ch A.

When you consider t he effect of m oves and changes coupled wit h t he m odificat ions caused by t he aut om at ic self- reconfigurat ion of a LAN via t he Spanning Tree Prot ocol, t he argum ent for using a prot ocol such as GVRP is especially com pelling. For a LAN in which every syst em is VLAN- aware, it is possible t o use VLANs wit hout doing any m anual configurat ion at swit ches. I nst ead, each end syst em can be configured wit h t he VLAN( s) t o which it belongs. End syst em s regist er wit h neighboring swit ches, and t he swit ches creat e and rem ove VLANs based on t hese regist rat ions. Som e disadvant ages arise when doing t his. A collision dom ain at t ached t o one swit ch port m ight cont ain hundreds of end syst em s. I t is easier t o configure t he swit ch port t han all of t hese syst em s. I ssues of securit y and cont rol also m ust be considered. However, vendors are dealing wit h t hese problem s by support ing secure, cent ralized configurat ion of endpoint syst em s.

Port- and Protocol-Based VLANs Som e product s support port - and prot ocol- based VLANs. These VLANs are defined by list ing • •

The set of port s t hat belong t o t he VLAN One or m ore t ypes of prot ocol t raffic t hat can be carried across t he VLAN

A fram e belongs t o t he VLAN if bot h of t he following are t rue: • •

The fram e arrives at a swit ch port t hat belongs t o t he VLAN. The fram e carries t raffic for one of t he list ed prot ocols.

An ident ical set of port s can be used t o define of t wo dist inct VLANs. For exam ple • •

VLAN 1 0 Port s 1, 2, 3, 4, 5. Prot ocols I P and ARP VLAN 2 0 Port s 1, 2, 3, 4, 5. Prot ocol SNA

More oft en, a pair of VLANs overlaps at a few port s. For exam ple, VLAN 2 in Figure 16.14 support s I PX t raffic, while VLAN 3 carries I P t raffic. Port s 2 and 3 at swit ch A belong t o bot h VLANs.

Figu r e 1 6 .1 4 . Por t - a n d pr ot ocol- ba se d VLAN s.

Alt hough t he st at ions at t ached t o port s 2 and 3 belong t o bot h VLAN 2 and VLAN 3, t hese st at ions do not need t o support t agging. When a fram e arrives at port 2 or 3, swit ch A can assign it t o a VLAN based on it s prot ocol, so t ags are not required. I f VLANs 2 and 3 were m erged int o a single VLAN, I PX broadcast s and m ult icast s would have t o be forwarded t o swit ch B. Furt herm ore, a fram e exchanged bet ween an I PX st at ion and t he local I PX server would be flooded t o swit ch B if it s dest inat ion MAC addresses had t im ed out and had been rem oved from t he filt ering t able. Using t he prot ocol t ype t o separat e t he t raffic flows can cut back subst ant ially on overhead t raffic while not im posing a burdensom e am ount of adm inist rat ion.

I PX a n d I P VLAN s A few point s m ust be kept in m ind when configuring port - and prot ocol- based VLANs. As was not ed in Chapt er 4, " The Classical Et hernet CSMA/ CD MAC Prot ocol," I PX m essages can be encapsulat ed int o LAN fram es in several different ways. Your swit ch product m ight count each encapsulat ion as a separat e prot ocol. I t is im port ant t o include all of t he encapsulat ions used in your LAN in an I PX VLAN definit ion. Ot herwise, som e I PX fram es m ight be discarded by a swit ch and vanish.

The I P prot ocol relies on t he Layer 2 Address Resolut ion Prot ocol ( ARP) t o t ranslat e I P dest inat ion addresses int o MAC addresses. Hence, an I P VLAN m ust include t he ARP prot ocol as well. An I P VLAN usually corresponds t o an I P subnet . This is a set of syst em s whose I P addresses st art wit h a prefix t hat is not used anywhere else in t he I P net work. The only way for t raffic t o leave an I P subnet is via a rout er. An I P subnet VLAN is configured by specifying t he following at each swit ch: • • •

The set of port s t hat belong t o t he VLAN One or m ore t ypes of prot ocol t raffic The I P address prefix t hat defines t he subnet

Traffic is carried bet ween I P subnet VLANs by forwarding it t o a rout er.

MAC-Address-Based Secure VLANs The m ost t edious—but also t he m ost secure—way t o define VLAN m em bership at a swit ch is t o list t he port and MAC address of every VLAN endpoint syst em . Som e product s support t his opt ion, which is used t o define VLANs t hat have ext ra securit y. A swit ch can be configured t o discard any incom ing fram e whose source MAC address is not associat ed wit h t hat port . A unicast fram e will not be delivered unless it s dest inat ion address and exit port num ber are enrolled at one of t he swit ches. Unicast fram es never need t o be flooded t o m ult iple endpoint syst em s. An endpoint syst em never sees a fram e t hat it is not int ended t o receive. Figure 16.15 illust rat es how a swit ch delivers fram es for a secure VLAN. The st at ions t hat are displayed in t he figure are m em bers of t he secure VLAN.

Figu r e 1 6 .1 5 . D e live r in g fr a m e s on a se cu r e VLAN .

1. A st at ion t hat has address MAC- 2 and is at t ached t o port 1 at swit ch B t ransm it s a fram e t o dest inat ion address MAC- 8. 2. Port 1, MAC- 2 is a legit im at e m em ber of t he VLAN, so t he fram e is accept ed. 3. The dest inat ion MAC address ( MAC- 8) is not current ly in a filt ering t able at swit ch B. 4. Swit ch B forwards t he fram e t o swit ches A and C, but does not forward it t hrough any of it s local access port s. 5. Swit ch A discards t he fram e. MAC- 1 is t he only local MAC address on t he secure VLAN. 6. Swit ch C delivers t he fram e t hrough port 6 t o MAC- 8.

Processing VLAN Frames Earlier sect ions have int roduced som e of t he m aj or VLAN concept s and VLAN t ypes. Now it is t im e t o focus on how VLANs are configured and how swit ches process VLAN fram es.

The cost of reducing broadcast and flooding overhead and cont rolling user access t o part s of a LAN is t hat som e ext ra processing m ust be perform ed by VLAN swit ches. This processing is cont rolled by configurat ion choices m ade by a LAN adm inist rat or. The purpose of each configurat ion choice can be bet t er underst ood aft er exam ining t he st eps t hat are followed when a swit ch processes a fram e. These st eps include 1. Exam ine each incom ing fram e and assign it t o a VLAN. The rules cont rolling t he assignm ent process are called ingress rules. 2. Check t he filt ering t able t o det erm ine t he out put port or port s. 3. For each out put port , verify t hat t he port is a m em ber of t he fram e's VLAN and check whet her t he fram e needs t o be t agged. The out put processing procedures are called egress rules.

Assigning Frames to VLANs via Ingress Rules The associat ion of a fram e t o a VLAN is easy if t he arriving fram e is t agged and t he t ag cont ains a VLAN ident ifier. However, t his does not guarant ee t hat t he fram e will be forwarded. The fram e m ight have arrived at a port t hat is not on t hat VLAN's m em ber list . An adm inist rat or m ust decide whet her t he port should be st rict and discard t he fram e, or relaxed and forward t he fram e. I f t he port is configured t o be st rict , a t agged fram e is discarded if t he port is not on t he m em ber list for t he fram e's VLAN. An arriving fram e t hat has no t ag—or t hat has j ust a priorit y t ag—m ight be discarded. A port opt ionally can be configured t o rej ect all incom ing fram es t hat are not VLANt agged. I f an arriving fram e t hat is not VLAN- t agged can be adm it t ed, t he swit ch t ries t o classify t he fram e based on VLAN configurat ion inform at ion. The swit ch t ries t o m ake t he m ost specific m at ch: • •

•

I f t he arrival port belongs t o one or m ore VLANs, t he fram e is checked t o see whet her it m eet s t he requirem ent s for m em bership in any of t hese VLANs. I f not , t he fram e is discarded. I f t he fram e has m et t he m em bership crit eria for exact ly one of t he port 's VLANs, t he fram e is assigned t o t hat VLAN. I f t he fram e has m et t he m em bership crit eria for m ore t han one of t he port 's VLANs, t he fram e is assigned t o t he VLAN whose m em bership requirem ent is m ost specific and st ringent . The rat ings of m at ches, from m ost t o least specific, are: o A VLAN defined by ( port num ber, MAC address) com binat ions o A VLAN defined by a port list and an I P address prefix o A VLAN defined by a port list and a list of support ed prot ocols o A VLAN defined by a port list

Figure 16.16 out lines t he decision process. A vendor m ight vary t he procedure slight ly, based on special feat ures of it s swit ches.

Figu r e 1 6 .1 6 . Th e in gr e ss de cision pr oce ss.

Forwarding VLAN Frames A VLAN swit ch follows som e basic rules when processing fram es t hat need t o be forwarded: 1. When a fram e arrives at a swit ch, it s VLAN I D is det erm ined by t he ingress rules. 2. I f t he dest inat ion is a broadcast address, t he fram e is forwarded t hrough every swit ch port belonging t o t hat VLAN, except for t he source port . 3. I f t he dest inat ion is a unicast or m ult icast address, t he set of filt ering t able ent ries for t hat VLAN is searched for a m at ching dest inat ion MAC address. Table 16.1 shows t wo sam ple ent ries in a shared filt ering t able. 4. For a MAC address- based secure VLAN, if t he dest inat ion is a unicast or m ult icast address and is not found in t he filt ering t able, t he fram e is forwarded t hrough t he appropriat e t runk port s. 5. For ot her VLANs, if t he dest inat ion is a unicast or m ult icast address and is not found in t he filt ering t able, t he fram e is forwarded t hrough t he swit ch port s belonging t o t hat VLAN and t he appropriat e t runk port s.

6. For each exit port , t he configurat ion inform at ion for t hat VLAN port is checked t o see whet her t he fram e m ust be t ransm it t ed in t agged or unt agged form at . Depending on t he result , a t ag m ight have t o be added or rem oved. This would require t he fram e check sequence t o be recom put ed.

Ta ble 1 6 .1 . Sa m ple VLAN Sha r e d Filt e r ing Ta ble En t r ie s En t r y N um be r

D e st ina t ion M AC Addr e ss

VLAN

Tr a n sm it Por t ( s)

St a t u s

1

00- 60- 08- 1E- AE- 42

2

5

Learned

2

08- 00- 20- 5B- 31- 6D

2, 3

6

Learned

A VLAN filt ering t able cont ains •

•

•

•

St at ic ent ries for unicast and m ult icast MAC addresses, m anually configured by an adm inist rat or Dynam ic ent ries for unicast MAC addresses, learned by observing t he source MAC addresses of fram es arriving at a VLAN port Opt ionally, dynam ic ent ries for m ult icast addresses, learned via GMRP regist rat ions Opt ionally, dynam ic ent ries for unicast MAC addresses, learned via GVRP regist rat ions

N ot e Not all valid fram es arriving at a swit ch need t o be forwarded. A swit ch is t he dest inat ion for som e fram es, such as SNMP m essages or Bridge Prot ocol Dat a Unit s.

Configuring VLAN Ports I n t he previous sect ions, you saw t hat an adm inist rat or m ust m ake several decisions t hat influence how t ight ly t he scope of a VLAN is cont rolled. Several configurat ion decisions are m ade separat ely for each port : • •

• •

Will t his port rej ect incom ing fram es t hat are not t agged? Will t his arrival port rej ect an incom ing fram e if t he ingress port does not belong t o t he VLAN t hat m at ches t he fram e's VLAN classificat ion? For a specified VLAN, m ust fram es sent by t his port be t agged? Will t his port accept incom ing GVRP regist rat ions for a specified VLAN?

Table 16.2 list s t he form al nam es of t hese VLAN param et ers and sum m arizes t he m eaning of each. For t he first t wo param et ers, a separat e value is assigned t o each port . For t he rest , a separat e value is assigned for each ( port , VLAN) com binat ion for which t he port belongs t o t hat VLAN.

Ta ble 1 6 .2 . VLAN Por t Con figu r a t ion Pa r a m e t e r s For m a l N a m e of Pa r a m e t e r

Applie s t o:

Va lid Va lue s

Accept able fram e t ypes

I ncom ing fram es at a specific port

Adm it only VLAN- t agged fram es. Adm it all fram es.

Enable ingress filt er

I ncom ing t agged fram es at a specific port

I f yes, check whet her t he ingress port belongs t o a VLAN t hat m at ches t he fram e's VLAN classificat ion. Discard fram es for which t here is no m at ch. I f no, do not check for a m at ch.

Transm it wit h t ag fram es header

Out going fram es for Yes m eans t hat for t his VLAN m ust be t agged a specified ( port , before being t ransm it t ed from t he port . VLAN) pairing

GVRP regist rat ion allowed

Yes m eans t hat t he port will accept I ncom ing regist rat ions for t his VLAN. regist rat ions for a ( port , VLAN) pairing

VLANs and Multicasts I n a LAN environm ent t hat support s VLANs, m ult icast s are isolat ed wit hin VLANs. When a m ult icast fram e ent ers a bridge port , it is subj ect t o t he sam e ingress processing as ot her fram es and eit her is assigned t o a VLAN or is discarded. The sam e holds t rue for t he GMRP fram es t hat are used t o j oin or leave m ult icast groups. When a GMRP fram e arrives at a port , t he fram e eit her is assigned t o a VLAN or is discarded. The j oin or leave request s in t he fram e apply t o t he VLAN t hat has been associat ed wit h t he fram e.

VLAN Protocol Elements The I EEE com m it t ee t hat worked on VLANs t ook advant age of t he fact t hat a new header was about t o be added t o MAC fram es. The t ag header act ually is used for t riple dut y: • • •

I t cont ains a 12- bit VLAN ident ifier. I t includes a 3- bit priorit y value. Som e LANs are m ade up Et hernet s, Token Rings, and FDDI rings t hat are connect ed by bridges t hat t ranslat e bet ween fram e form at s. A t ag header can include inform at ion t hat assist s in t his t ranslat ion.

VLAN Identifiers VLAN ident ifiers are 12- bit quant it ies. Hence, t hey can represent decim al num bers ranging from 0 t o 4095. Som e of t hese values are reserved:

•

• • •

0 does not correspond t o a VLAN. I t is called t he null VLAN I D and is used in a priorit y- t agged fram e t o indicat e t hat no VLAN I D has been assigned. 1 is t he default VLAN I D. Values ranging from 2 t o 4094 are assigned t o VLANs as needed. 4095 is reserved.

Vendors t ypically add t heir own special rules for using VLAN I Ds. A vendor m ight lim it t he usable range of num bers. Every bridge port m ust belong t o one or m ore VLANs. When a VLAN bridge init ializes for t he first t im e, all port s aut om at ically are m em bers of VLAN 1.

Tagged Frame Formats A receiver m ust be capable of easily det ect ing whet her a t ag has been sandwiched int o a fram e header. The t ag really m ust st and out , so t he VLAN prot ocol designers had t o find a good place t o put t he t ag. The solut ion was t o int roduce a new Et herType value ( X'81- 00) signaling t hat a fram e has a t ag header.

For m a t of a Ta gge d Et h e r n e t Fr a m e Figure 16.17 shows how t his works for Et hernet fram es:

Figu r e 1 6 .1 7 . An Et h e r n e t fr a m e t h a t con t a in s a t a g h e a de r .

•

• •

The Et herType field, which follows t he source MAC address, cont ains X'81- 00. This is t he beginning of a t ag header. The next 2 byt es cont ain t ag cont rol inform at ion ( TCI ) . This includes t he 3- bit priorit y field, a 1- bit flag called t he Canonical Form at I ndicat or ( CFI ) , and t he 12- bit VLAN ident ifier field. The Et hernet lengt h or t ype value t hat was in t he fram e before it was t agged follows t he t ag cont rol inform at ion.

A t ag header adds 4 byt es t o t he lengt h of an Et hernet fram e. I f t he value of t he Canonical Form at I ndicat or flag is 1, anot her field cont aining 2 t o 30 byt es will be insert ed in t he fram e aft er t he lengt h/ t ype field.

N ot e This addit ional field appears when different t ypes of LANs have been com bined using special t ranslat ional bridges. The ext ra field oft en cont ains source rout ing inform at ion. The nam e of t his field is t he em bedded rout ing inform at ion field ( E- RI F) . The det ails are explained in Chapt er 17.

To accom m odat e ext ra t ag header byt es, a t agged Et hernet fram e is allowed t o be longer t han an unt agged fram e. A legacy LAN int erface will not be capable of m aking m uch sense out of t agged fram es. Som e of t he fram es will be larger t han t he norm al m axim um fram e size. All will have an Et herType t hat is unfam iliar. To play it safe, adm inist rat ors oft en rest rict t agging t o fram es t hat pass bet ween a pair of VLAN- aware swit ches. List ing 16.1 shows a Sniffer t race of a t agged Et hernet fram e carrying an ARP m essage. Not e t hat t he Et hernet t ype field carries X'81- 00, signaling t hat a t ag follows. The priorit y value is 0. The VLAN ident ifier is 2.

List in g 1 6 .1 Tr a ce of a Ta gge d Et h e r n e t Fr a m e DLC:

----- DLC Header ----DLC: DLC: Frame 1 arrived at 17:29:35.0000; frame size is 60 (003C hex) bytes. DLC: Destination = 3COM BD7C18 DLC: Source = 3COM BD7C19 DLC: 8021Q: ----- 802.1Q Packet ----8021Q: 8021Q: Tag Protocol Type = 8100 8021Q: Tag Control Information = 0002 8021Q: User Priority = 0 8021Q: VLAN ID = 2 8021Q: Ethertype = 0806 (ARP) 8021Q: ARP: ----- ARP/RARP frame ----ARP: ARP: Hardware type = 1 (10Mb Ethernet) ARP: Protocol type = 0800 (IP) ARP: Length of hardware address = 6 bytes ARP: Length of protocol address = 4 bytes ARP: Opcode 2 (ARP reply) ARP: Sender's hardware address = 006008BD7C18 ARP: Sender's protocol address = [10.0.0.1] ARP: Target hardware address = 006008BD7C19 ARP: Target protocol address = [10.0.0.2] ARP: ARP: 14 bytes frame padding ARP:

For m a t of a Ta gge d Tok e n Rin g Fr a m e Figure 16.18 shows how t he t ag header is sandwiched int o a Token Ring fram e. I f t he fram e cont ains a rout ing inform at ion field, it appears in it s norm al posit ion aft er t he source MAC address.

Figu r e 1 6 .1 8 . A Tok e n Rin g fr a m e t h a t con t a in s a t a g h e a de r .

An 8- byt e LLC/ SNAP header follows, ending in t he special Et herType prot ocol ident ifier X'81- 00, which indicat es t hat t he next 2 byt es cont ain t ag cont rol inform at ion. The fram e's original 5- byt e SNAP subheader, which ident ifies t he t ype of prot ocol dat a t he fram e carries, follows t he t ag cont rol inform at ion.

For m a t of a Ta gge d FD D I Fr a m e Figure 16.19 shows how t he t ag header is sandwiched int o an FDDI fram e. I f t he fram e cont ains a rout ing inform at ion field, it appears in t he norm al posit ion aft er t he source MAC address.

Figu r e 1 6 .1 9 . An FD D I fr a m e t h a t con t a in s a t a g h e a de r .

An 8- byt e LLC/ SNAP header follows, ending in t he Et herType prot ocol ident ifier X'8100, which indicat es t hat t he next 2 byt es cont ain t ag cont rol inform at ion. This is followed by t he fram e's original SNAP subheader. The m ain difference bet ween t he FDDI form at and t he Token Ring form at is t hat , like an Et hernet fram e, an FDDI fram e opt ionally can include a 2- t o 30- byt e field t hat carries som e source rout ing inform at ion.

Assigning Priorities to Frames Tradit ional bridges t reat ed all fram es t he sam e way—first com e, first served. But t he im port ance of user t raffic varies. Som e carries m ission- crit ical int er- act ive dat a. Som e consist s of background file t ransfers t hat can t olerat e delays. The capabilit y t o priorit ize t raffic has been high on t he cust om er wish list for quit e a long t im e. Each vendor has m et t his need in a different way. Som e product s allow cust om ers t o give a t ype of t raffic eit her high or low priorit y. Som e support m any priorit y classes. Depending on t he product , cust om ers m ay be able t o assign t raffic t o a priorit y class according t o t he following:

• •

•

Prot ocol ( such as I P, I PX, TCP, UDP, DECnet , or SNA) Applicat ion ( as defined by a TCP or UDP port num ber, or an I PX socket num ber) Fields in an I P header t hat st at e precedence and t ype of service value

IEEE 802 Priority The t rouble wit h im plem ent ing priorit ies based on feat ures such as I P t ype of service or TCP applicat ion port num ber is t hat a swit ch m ust exam ine t he upper- layer prot ocol dat a in every fram e t o find out which t ype of priorit y handling t he fram e deserves. By int roducing a t ag header, t he I EEE 802.1Q VLAN and priorit y st andard m akes it possible t o est ablish t he priorit y once—eit her at t he source host or at t he port of ent ry int o t he swit ch adj acent t o t he host —and writ e it int o t he t ag header added t o t he fram e. As Figures 16.17, Figures 16.18, and Figure 16.19 showed, a fram e's priorit y is announced in a 3- bit field in t he t ag. When t ranslat ed int o decim al, t he priorit y values range from 0 ( lowest priorit y) t o 7 ( highest priorit y) . A LAN adm inist rat or m ight wish t o priorit ize t he LAN's t raffic wit hout breaking t he LAN int o sm aller VLANs. When priorit y- t agged fram es are used, t he t ag's VLAN I D field is set t o t he null value ( 0) , and only t he priorit y bit s cont ain usable inform at ion.

Forwarding Prioritized Frames Bridges/ swit ches were int ended t o be sim ple devices ( at least , originally) . A fram e arrives, t he bridge checks it s filt ering t able and t hen forwards t he fram e accordingly. Priorit ies m ake swit ch archit ect ure a lit t le m ore sophist icat ed. A swit ch m ust m aint ain separat e out put queues for each priorit y class t hat it support s. Figure 16.20 depict s a swit ch t hat support s four priorit y classes. Out going fram es are lined up in four queues at each port .

Figu r e 1 6 .2 0 . Pr ior it y qu e ue s in a sw it ch .

N ot e Figure 16.20 reflect s a logical queuing st ruct ure. An act ual im plem ent at ion m ight m ake m ore efficient use of m em ory by st oring a single copy of a fram e and m aint aining a list of t he port s t hrough which it m ust be sent .

Basic rules t hat can govern t he t ransm ission of priorit ized fram es are list ed here: • •

Fram es in t he highest - priorit y class queue are t ransm it t ed first . A fram e in a lower- priorit y class queue is t ransm it t ed when all higher- level queues are em pt y.

There is a flaw in t hese rules, however. I f t here is a st eady st ream of high- priorit y t raffic, low- priorit y t raffic m ight never get sent ! Vendors overcom e t his flaw in different ways: •

Som e vendors prom ot e a fram e t o t he next higher- level queue aft er a t im eout period. Thus, a low- level fram e t hat is blocked will work it s way up t he ladder.

•

Som e vendors set m axim um t ransm ission t im e lim it s for each priorit y level. When t he t im e lim it is reached, t he right t o t ransm it is given t o t he next level down unt il t he lowest level is reached. The process t hen repeat s, st art ing wit h t he highest priorit y level.

N ot e A fram e cannot be queued in a swit ch for an unlim it ed am ount of t im e. There is a t im eout period aft er which a fram e is discarded.

Mapping 802.1 Priorities to Priority Classes Supervising a lot of queues adds t o t he com plexit y of t he soft ware in a swit ch. Som e swit ches do not priorit ize at all and have a single queue for each port . Som e have t wo priorit y classes, high and low. The exam ple in Figure 16.20 has four priorit y classes. However, few swit ches would be expect ed t o set up eight classes and support eight queues. When a fram e t hat is t agged wit h a priorit y value arrives at a swit ch, t he swit ch m ust figure out which queue t he fram e should j oin. To do t his, t he swit ch m aps t he fram e's priorit y value t o one of it s support ed priorit y classes. The I EEE has published a t able t hat recom m ends t he way t hat t his should be done. Table 16.3 shows t he m apping recom m ended in Table 8–2 of I EEE 802.1Q. For exam ple, if t he swit ch does not priorit ize ( t hat is, uses only one class) t hen obviously all fram es m ust be placed in t he sam e exit queue, which has priorit y class 0. The colum n headed " 1 Class" shows t his m apping. I f t he swit ch support s t wo classes ( 0 = low, 1 = high) , t hen fram es wit h priorit y values 0 t o 3 go int o t he low queue ( class 0) , and fram es wit h priorit y values 4 t o 7 go int o t he high queue ( class 1) . The colum n headed " 2 Classes" shows t his m apping. When a swit ch support s four or m ore classes, som e of t he m appings are st range. For exam ple, if t he swit ch support s eight classes • • •

Priorit y value 0 m aps t o class 2 Priorit y value 1 m aps t o class 0 Priorit y value 2 m aps t o class 1

There act ually is som e logic behind t his. Som e incom ing t agged fram es m ight be arriving from a swit ch t hat does not priorit ize. That swit ch sends all of it s fram es wit h a single default priorit y value of 0. Fram es wit h 0 priorit y values are given a sm all boost , j ust in case t he 0 m eans " default " inst ead of " low." The m apping of t he next couple of priorit y values is adj ust ed t o work around t his choice. Ta ble 1 6 .3 . M a ppin g Pr ior it y Va lu e s t o Pr ior it y Cla sse s N u m be r of Sw it ch Pr ior it y Cla sse s

Pr ior y Va lu e

1 2 3 4 5 6 7 8 Cla ss Cla sse s Cla sse s Cla sse s Cla sse s Cla sse s Cla sse s Cla sse s

0

0

0

0

1

1

1

1

2

1

0

0

0

0

0

0

0

0

2

0

0

0

0

0

0

0

1

3

0

0

0

1

1

2

2

3

4

0

1

1

2

2

3

3

4

5

0

1

1

2

3

4

4

5

6

0

1

2

3

4

5

5

6

7

0

1

2

3

4

5

6

7

The priorit y class used in a part icular bridge does not affect t he priorit y value in a fram e's t ag. Norm ally, t he fram e is forwarded wit hout any change t o t his value. However, Figure 16.21 illust rat es a case for which t he priorit y value would change. Swit ch B has only one priorit y level. All fram es arriving at swit ch A from swit ch B have priorit y 0.

Figu r e 1 6 .2 1 . Ch a n gin g t h e pr ior it y va lu e ba se d on sw it ch con figu r a t ion .

The adm inist rat or at swit ch A has m anually configured t he arrival port so t hat it will change incom ing fram e priorit ies from 0 t o 4. Not e t hat according t o Table 16.3, t hese fram es will be placed in Class 2 queues at swit ch A.

GARP VLAN Registration Protocol Details The convenience and power of having a generic regist rat ion prot ocol was confirm ed when t he second GARP applicat ion was int roduced. The GARP VLAN Regist rat ion Prot ocol ( GVRP) enables syst em s t o regist er VLAN m em berships wit h an adj acent port . A syst em sends a j oin m essage t o regist er. Bridges propagat e t heir st at ic, port - based VLAN m em berships and dynam ic VLAN regist rat ions in order t o assure t hat fram es t ransm it t ed ont o a VLAN can reach any VLAN part icipant . GVRP enables a syst em t o regist er at a port and request t hat fram es for one or m ore VLANs be forwarded t o t he syst em . Earlier, in t he sect ion " Maint aining VLAN Configurat ion wit h GVRP," you saw how GVRP sim plifies and aut om at es a large port ion of t he VLAN configurat ion effort . I n fact , a VLAN can even be creat ed dynam ically by an end- syst em GVRP regist rat ion. The VLAN is delet ed when eit her all part icipat ing syst em s have sent leave m essages or t heir regist rat ions have t im ed out . The GVRP regist rat ion prot ocol is sim ilar t o t he GMRP prot ocol, which was described in Chapt er 14: • •

A GVRP m essage cont ains a list of j oin and leave request s. Each request cont ains t he ident ifiers of t he VLANs t o be j oined or left .

When a st at ion regist ers and j oins a VLAN at a port and t he port is not current ly part icipat ing in t he VLAN, t he following occurs: • •

The bridge adds a dynam ic VLAN ent ry for t hat port . The bridge propagat es regist rat ion inform at ion t hrough all port s ( ot her t han t he source of t he regist rat ion) t hat are act ive for t he current spanning t ree.

The result of t his is t hat all port s on pat hs leading t o t he regist ered syst em learn t hat t hey need t o forward fram es belonging t o t he regist ered VLAN t oward t he regist ered syst em .

GVRP Protocol Messages GVRP m essages are GARP prot ocol dat a unit s t hat are sent t o m ult icast MAC address X'01- 80- C2- 00- 00- 21, which has been set aside for GVRP use. ( Recall t hat t he m ult icast address used for GMRP was X'01- 80- C2- 00- 00- 20.) The m essages cont ain VLAN j oins

and leaves. Figure 16.22 shows t he GVRP m essage form at , which is very sim ilar t o t he GMRP m essage form at shown earlier in Chapt er 14 ( see Figure 14.10) .

Figu r e 1 6 .2 2 . GARP a n d GVRP m e ssa ge s.

N ot e I n Figure 16.22, t he 4- byt e m essage block lengt h covers a 1- byt e lengt h field, a 1byt e event ident ifier, and a 2- byt e at t ribut e value field t hat cont ains a VLAN ident ifier.

Summary Points

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Building a big physical LAN can pay off in equipm ent savings and in t he capabilit y t o include high- bandwidt h backbone links. Broadcast s and flooded unicast fram es can generat e a lot of LAN t raffic. Virt ual LANs were int roduced as a way t o cont rol t he t raffic flows on a physical LAN. Syst em s t hat support VLAN prot ocols are said t o be VLAN- aware. VLANs can be used t o confine t raffic wit hin a workgroup, give select ed st at ions access t o a server, carry dat a for specified prot ocols, or isolat e t raffic for reasons of securit y. A VLAN can be defined by list ing swit ch port s t hat part icipat e in t he VLAN. A link bet ween a pair of VLAN swit ches is called a t runk. Traffic belonging t o several VLANs can share a t runk. A link t hat connect s one or m ore VLAN- unaware syst em s t o a VLAN swit ch is called an access link. A hybrid link is a subLAN ( such as an FDDI ring) t hat connect s VLAN bridges t o one anot her and t hat also connect s t o VLAN- unaware syst em s. VLAN- t agged fram es include a header t hat cont ains a VLAN ident ifier and a priorit y value. The VLAN ident ifier in a priorit y- t agged fram e cont ains a null VLAN ident ifier. A rout er forwards t raffic bet ween VLANs. Traffic bet ween client s and a VLAN- aware server can be expedit ed by enrolling t he server in m ult iple VLANs. When independent learning is used, a MAC address observed and learned in one VLAN is not shared wit h ot her VLANs. When shared learning is used, a MAC address observed and learned in one VLAN is shared wit h ot her VLANs. Using GVRP, a VLAN- aware end syst em can regist er wit h a neighboring swit ch t o j oin ( or leave) a VLAN. GVRP propagat es VLAN regist rat ion inform at ion t hrough t he ent ire LAN. All VLANs are superim posed on a single act ive Spanning Tree st ruct ure. When a VLAN bridge init ializes for t he first t im e, all port s aut om at ically are m em bers of a default VLAN whose ident ifier is 1. A st at ic port - based VLAN is t he m ost universally support ed t ype of VLAN. The adm inist rat or configures t he VLAN I D and t he list of swit ch port s t hat belong t o t he VLAN. Adding a GVRP regist rat ion capabilit y t o a port - based VLAN eases VLAN m aint enance chores. A port - and- prot ocol VLAN is defined by list ing t he set of port s t hat belong t o t he VLAN and one or m ore t ypes of prot ocol t raffic t hat can be carried on t he VLAN. A MAC address- based VLAN is defined by enum erat ing ( port , MAC address) pairings for t he syst em s t hat belong t o t he VLAN. This m et hod is used t o im prove t he securit y on a VLAN. When a fram e arrives at a swit ch port , a set of ingress rules is applied in order t o associat e t he fram e wit h a VLAN. The fram e is forwarded aft er looking for a m at ching MAC address and VLAN ident ifier in a filt ering t able. I n a LAN environm ent t hat support s VLANs, m ult icast s are isolat ed wit hin VLANs. VLAN ident ifiers are 12- bit quant it ies and can represent decim al num bers ranging from 0 t o 4095. The fact t hat a fram e cont ains a t ag header is signaled by t he use of a special Et herType value.

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The priorit y values in a t ag can range from 0 t o 7. When a fram e arrives at a swit ch, t he t ag priorit y value m ust be m apped t o a priorit y class support ed by t he swit ch. The swit ch priorit y classes correspond t o out put queues. The GARP VLAN Regist rat ion Prot ocol ( GVRP) enables a syst em t o regist er at a port and request t hat fram es for one or m ore VLANs be forwarded t o t he syst em . GVRP is sim ilar t o GMRP. A bridge propagat es GVRP regist rat ion inform at ion t hrough all port s ( ot her t han t he source of t he regist rat ion) t hat are act ive for t he current spanning t ree.

References The I EEE priorit y st andard originally was published as 802.1p. I t lat er was incorporat ed int o t he j oint VLAN and priorit y st andard: • •

I EEE 802.1Q. " Virt ual Bridged Local Area Net works." 1998. The GARP VLAN Regist rat ion Prot ocol also is described in 802.1Q.

The Generic At t ribut e Regist rat ion Prot ocol ( GARP) is described in Chapt er 12 of t he st andard: •

I EEE 802.1D. " Media Access Cont rol ( MAC) Bridges." 1998.

VLAN im plem ent at ions vary great ly from vendor t o vendor. The best way t o get a sense of t he capabilit ies t hat are available is t o check t he configurat ion m anuals for product s from vendors such as Cisco, 3Com , Nort el, and Cablet ron.

Chapter 17. Source-Routing, Translational, and Wide Area Bridges The last few chapt ers have discussed local area t ransparent bridges. Transparent bridges are charact erized by t heir capabilit y t o learn t he MAC addresses t hat are reached t hrough each port and by t he use of t he Spanning Tree Prot ocol t o rest ruct ure t he LAN t opology aut om at ically aft er a com ponent failure. This chapt er int roduces source- rout ing bridges, which I BM designed for it s Token Ring net works. Alt hough t ransparent bridging can be used in Token Rings, sourcerout ing oft en is preferred. Som e LAN adm inist rat ors inst all bridges t hat behave like t ransparent bridges for som e t raffic and follow source- rout es for ot her t raffic. Bridges t hat can do t his are called ( not surprisingly) source- rout ing t ransparent ( SRT) bridges. They are popular at sit es t hat are m igrat ing from source- rout e bridging t o t ransparent bridging. Norm ally, t he t erm s rout ing and rout es apply t o Layer 3 net working. Here t he t erm s are used at Layer 2. I t is unfort unat e t hat I BM int roduced t his t erm inology because it has caused som e confusion.

Bridging is a Layer 2 funct ion; fram e t raffic is forwarded based on dest inat ion MAC addresses. Rout ing is a Layer 3 funct ion. Layer 3 t raffic is forwarded based on Layer 3 net work addresses ( such as I P addresses) t hat are locat ed wit hin net work- layer prot ocol dat a unit s. Source- rout ed t raffic act ually is bridged t raffic. The advent of Layer 2/ Layer 3 swit ch product s has led som e vendors t o writ e product docum ent at ion t hat j um bles t he funct ions perform ed at each layer. To avoid confusion, t he t erm s, bridge ( inst ead of Layer 2 swit ch) and rout er ( inst ead of Layer 3 swit ch) , are used in t his chapt er. ( See Chapt er 18, " Rout ing and Layer 2/ 3 Swit ches," for a det ailed explanat ion of t he difference bet ween Layer 2 bridging and Layer 3 rout ing.) Translat ional bridging is anot her t echnology t hat is explored in t his chapt er. Translat ional bridges relay fram es bet ween Et hernet , Token Ring, and FDDI LANs, creat ing a big, int erconnect ed LAN. To do t his, t hey t ranslat e bet ween fram e form at s and perform m any ot her conversion funct ions. The designers of FDDI borrowed a lot from Token Ring t echnology. For exam ple, t he form at of an FDDI inform at ion fram e is alm ost ident ical t o t he form at of a Token Ring fram e. Furt herm ore, FDDI fram es opt ionally can be source- rout ed. Connect ing a Token Ring t o an FDDI ring is quit e st raight forward. However, Et hernet and Token Ring appear t o have been designed on different planet s. Connect ing Et hernet t o Token Ring or FDDI is a difficult proposit ion. I n spit e of t he problem s, t here have been cust om ers for t ranslat ional bridges for as long as t here have been m ult iple LAN t echnologies. A fram e can ent er a t ranslat ional bridge form at t ed for one MAC t ype ( such as Token Ring) and leave t he bridge form at t ed for a different MAC t ype ( such as Et hernet ) . The chapt er ends wit h a brief discussion of rem ot e bridges. These bridges unit e LANs locat ed at different sit es int o a single " LAN." Fram es are relayed bet ween sit es by forwarding t hem across a wide area connect ion such as a leased line or a fram e relay circuit .

Source-Routed Bridged LANs I n a source- rout ing bridged LAN, a fram e t hat m ust pass t hrough one or m ore bridges t o reach it s dest inat ion carries it s t ravel direct ions in a rout ing inform at ion field ( RI F) in t he fram e header. Figure 17.1 shows how t his works in a Token Ring LAN. Each Token Ring and each source- rout ing bridge in t he figure has been assigned a num eric ident ifier. By est ablished convent ion, t hese are writ t en using hexadecim al charact ers. To describe t he dot t ed line rout e from st at ion X t o server Y shown in t he figure, t he inst ruct ions in t he RI F would say: 1. 2. 3. 4.

Ent er ring X'201. Cross bridge X'8, and ent er ring X'203. Cross bridge X'6, and ent er ring X'214. Cross bridge X'3, and ent er ring X'180. The dest inat ion MAC address is on t his ring.

Figur e 1 7 .1 . Follow in g a sou r ce r ou t e .

A fram e t raveling from server Y t o st at ion X would follow t he sam e rout e in t he reverse direct ion. Before a source rout e can be used, it m ust be discovered. The source and dest inat ion NI Cs and int erm ediat e source- rout ing bridges cooperat e t o det erm ine a pat h bet ween a source and a dest inat ion. The process is called rout e discovery and is described a lit t le lat er ( in t he sect ion " Rout e Discovery" ) . Unlike a t ransparent ly bridged LAN, t here can be m ult iple act ive pat hs t hrough a source- rout ed LAN.

N ot e The m et hod used t o set up a source rout e m akes it very likely t hat t he pat h t hat is t he least busy at set up t im e is chosen. This m eans t hat t raffic get s spread across all

of t he pat hs t hat are available. Thus, t he t echnology support s bot h redundant pat hs and load balancing.

Figure 17.2 illust rat es a source- rout ed Token Ring LAN. Four pat hs are shown bet ween st at ion X and server Y.

Figu r e 1 7 .2 . A sou r ce - r ou t e d Tok e n Rin g LAN .

I n t he figure, " SRB" st ands for source- rout ing bridge.

Ring Identifiers and Bridge Identifiers Each ring in a source- rout ed Token Ring LAN m ust be assigned a unique ring num ber. Each bridge is assigned a bridge num ber, but bridge num bers do not have t o be unique. Bridge num bers are used t o dist inguish bet ween t wo or m ore " parallel" bridges t hat connect t wo rings t o one anot her. The num bers assigned t o a set of parallel bridges m ust be different . Figure 17.3 illust rat es parallel bridges. There are t hree pat hs bet ween st at ion X and server Y. The bridge num bers m ake it possible t o dist inguish bet ween t hese pat hs, indicat ing which bridge should be crossed.

Figu r e 1 7 .3 . Pa r a lle l. Br idge s.

Route Discovery Rout e discovery is a t wo- st ep process. The source of a com m unicat ion sends an init ial t est fram e addressed t o t he t arget ed dest inat ion around it s own ring.

N ot e Eit her of t wo special t ypes of fram es TEST or XI D is t ransm it t ed t o t est whet her a dest inat ion is on t he sam e ring as t he source. I f present on t he ring, t he dest inat ion responds wit h t he sam e t ype of fram e.

I f t he dest inat ion does not respond, t he source sends out an explorer fram e whose j ob is t o discover a pat h t o t he dest inat ion. Figure 17.4 illust rat es t he process.

Figu r e 1 7 .4 . illu st r a t e s t h e pr oce ss.

There are t wo t ypes of explorers: • •

The t radit ional all- rout es- explorer is flooded across all of t he rings in t he LAN. Launching a single explorer can cause m any explorers t o arrive at t he dest inat ion syst em . A Spanning Tree explorer is sent t hrough t he act ive t ree t hat has been est ablished via a Spanning Tree Prot ocol. Only one explorer can arrive at t he dest inat ion syst em .

As an explorer fram e progresses t hrough t he LAN, each bridge t hat is crossed adds an ent ry t o a list of rings and bridges t hat have been t raversed by t he fram e. The inform at ion is recorded in t he rout ing inform at ion field in t he fram e header. The dest inat ion syst em sends t he explorer fram e ( or fram es) t hat it receives back t o t he source. Each explorer uses it s RI F t o find it s way back, crossing rings and bridges in t he reverse order. For an all- rout es- explorer, t he rout e in t he first explorer t hat arrives back at t he source norm ally is t he one t hat is cached and used. The select ed rout e is placed int o t he headers of subsequent fram es sent t o t he dest inat ion.

Most of t he int elligence required t o m ake source- rout ing work is em bedded in t he source and dest inat ion NI Cs. Source- rout ing bridges are sim ple devices t hat do as t hey are t old. When a source- rout ing bridge receives an inform at ion fram e t hat cont ains a RI F, it checks t he RI F t o see if t he following com binat ion is on t he rout e: Arrival ring I D, I D of t his bridge, I D of ot her at t ached ring. I f so, t he RI F passes t he fram e t o t he exit ring.

N ot e Et hernet end syst em s cannot part icipat e in source- rout e bridging. An Et hernet NI C is not capable of perform ing t he rout e discovery prot ocol or specifying a rout e t o be followed.

All- Rou t e s- Ex plor e r Fr a m e s I f t here are m ult iple pat hs t o a dest inat ion, m ult iple all- rout es- explorer fram es will be generat ed. Figure 17.5 shows how t his occurs. When an all- rout es- explorer fram e arrives at a bridge port , t he following happens:

Figu r e 1 7 .5 . Ge n e r a t in g m ult iple e x plor e r fr a m e s.

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The bridge exam ines t he RI F t o see whet her t he ring at t ached t o t he ot her port already is list ed in t he RI F. I f not , t he bridge updat es t he RI F and forwards t he explorer ont o t hat ring.

The all- rout es- explorer sent from st at ion X t o server Y in Figure 17.5 will discover all four of t he rout es shown in Figure 17.2. That is:

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Ring Ring Ring Ring

1, 1, 1, 1,

bridge bridge bridge bridge

A, D, A, D,

ring ring ring ring

2, 4, 2, 4,

bridge bridge bridge bridge

C, E, B, B,

ring ring ring ring

3 3 4, bridge E, ring 3 2, bridge C, ring 3

Not e t hat when Bridge A sees a fram e t hat has arrived on t he rout e Ring 1, Bridge D, Ring 4, Bridge B, Ring 2 Bridge A will not forward t he explorer ont o Ring 1 because Ring 1 already is on t he rout e.

Spa n n in g Tr e e Ex plor e r Fr a m e s Spanning Tree explorer fram es are used on som e source- rout ed LANs. These explorer fram es are forwarded only t hrough port s t hat are act ive in t he Spanning Tree t opology. This m eans t hat t he unique pat h t hat is unblocked at t he t im e t he explorer is sent out is t he only one t hat is discovered and cached for com m unicat ion bet ween t he t wo syst em s. Spanning Tree explorers are used in big LANs when t here is concern about t he am ount of t raffic t hat is generat ed by all- rout es- explorer fram es. However, t his rest rict ion also causes m uch of t he load- balancing benefit t hat source- rout ing provides t o be lost .

N ot e I BM int roduced a propriet ary Spanning Tree Prot ocol t hat was used solely t o prevent all- rout es- explorer fram es from clogging t he LAN.

However, t oday m any sit es t hat use source- rout ing rely on t he I EEE Spanning Tree Prot ocol for t his purpose inst ead.

Source-Routing Bridge Protocol Elements A look at som e of t he source- rout ing prot ocol det ails provides m ore insight int o how source- rout ing bridges work. An exam inat ion of t he Token Ring inform at ion fram e form at exposes som e of t hese det ails. Figure 17.6 displays t he form at of a Token Ring inform at ion fram e t hat carries t raffic for prot ocols t hat are ident ified by an Et herType, such as I P, I PX, AppleTalk, DECnet , DEC LAT, and Banyan VI NES. ( The LLC field would be different for SNA t raffic.)

Figur e 1 7 .6 . A Tok e n Ring infor m a t ion fr a m e .

Fram es t ransm it t ed ont o source- rout ed LANs cont ain a rout ing inform at ion field. Fram es t ransm it t ed ont o t ransparent ly rout ed LANs do not cont ain a RI F. A sim ple m et hod is used t o indicat e whet her a fram e cont ains a rout ing inform at ion field. Because a source address always m ust be an individual address, t he individual/ group bit in a source address is unused and was recruit ed for sourcerout ing. This bit is set t o 1 t o indicat e t hat a RI F follows. A RI F consist s of a 2- byt e int roducer called a cont rol field, followed by t he rout e. The rout e is m ade up of a sequence of 2- byt e rout e descript ors. Each rout e descript or cont ains a 12- bit ring ident ifier followed by a 4- bit bridge ident ifier. The final rout e descript or in a RI F cont ains t he dest inat ion ring ident ifier followed by a 0.

N ot e Ring and bridge ident ifiers are expressed in hexadecim al not at ion on configurat ion screens, report s, and t races.

The sam e RI F form at is used for FDDI source rout ing. For sim plicit y, t he discussion will cont inue t o focus on Token Ring.

Tok e n Rin g Fr a m e Tr a ce List ing 17.1 shows part of a t race of a Token Ring fram e. The rout ing cont rol field indicat es t hat t he RI F consist s of 8 byt es. This includes t he init ial 2- byt e rout ing cont rol port ion plus t hree 2- byt e rout e descript ors. A direct ion flag in t he rout ing cont rol field indicat es whet her t he rout e should be followed in t he forward or reverse direct ion. I t s value is 0 in t he t race, which corresponds t o t he forward direct ion. The next value in t he cont rol field announces t he size of t he biggest inform at ion field t hat can be carried on t he pat h.

N ot e The m axim um inform at ion field size was discovered by t he explorer fram e t hat m apped t he pat h. I t s value init ially was set t o t he biggest size t hat t he sending device could support . The value is reduced along t he way if bridges on t he pat h are lim it ed t o sm aller sizes.

List in g 1 7 .1 A Tok e n Rin g Fr a m e Con t a in in g a Rou t ing I nfor m a t ion Fie ld DLC: Source = Station IBM2 C9656B DLC: RI: ----- Routing Indicators ----RI: RI: Routing control = 08 RI: 000. .... = Non-broadcast RI: ...0 1000 = RI length is 8 RI: Routing control = 40 RI: 0... .... = Forward direction RI: .100 .... = Largest frame is 8130 RI: .... 000. = Extended frame is 0 RI: .... ...0 = Reserved RI: Ring number 416 via bridge 1 RI: Ring number 43A via bridge 0 RI: Ring number 43E RI: LLC: ----- LLC Header ----LLC: LLC: DSAP Address = AA, DSAP IG Bit = 00 (Individual Address) LLC: SSAP Address = AA, SSAP CR Bit = 00 (Command) LLC: Unnumbered frame: UI (X'03) LLC: SNAP: ----- SNAP Header ----SNAP: SNAP: Type = 0800 (IP) SNAP: IP: ----- IP Header -----

. . . The rout e descript ors in List ing 17.1 provide t he following direct ions: 1. 2. 3. 4.

Follow ring X'416. Cross bridge X'1, and exit ont o ring X'43A. Cross bridge X'0, and exit ont o ring X'43E. Look for MAC address I BM1 X'0C892E ( X'08- 00- 5A- 0C- 89- 2E in I BM form at , and X'10- 00- 5A- 30- 91- 74 in I EEE form at ) .

The fram e cont ains t he sam e LLC and SNAP fields t hat are found in 802.3 LLC fram es. The DSAP and SSAP are followed by an LLC cont rol field whose value is X'03. This indicat es t hat t his is an unnum bered inform at ion fram e. Figure 17.7 illust rat es why bridge X'1 on ring X'416 would decide t o forward t he source- rout ed fram e in List ing 17.1, but bridge X'2 on ring X'416 would not . Each bridge checks whet her it is on t he rout e.

Figu r e 1 7 .7 . For w a r din g a sou r ce - r ou t e d fr a m e .

Rou t e D iscove r y Pr ot ocol D e t a ils Rout e discovery is a t wo- st age process: 1. Check whet her t he dest inat ion is on t he sam e ring as t he source. 2. I f not , send out an explorer t o find t he dest inat ion. Many special fram es are used in Token Rings and FDDI rings. To execut e st ep 1, a short TEST or XI D ( exchange ident ifier) fram e is sent out . The first bit of it s source address field is set t o 0, which m eans t hat no RI F is included. This keeps t he fram e on t he local ring.

N ot e These fram e t ypes are ident ified by t he cont rol field in a Token Ring or FDDI LLC header. The LLC cont rol field of a TEST fram e is X'E3 or X'F3, and t he LLC cont rol field of an XI D field is X'AF or X'BF. Recall t hat t he LLC cont rol field for unnum bered inform at ion is X'03.

I f t he dest inat ion does not respond, t he source issues a TEST or XI D for which t he first bit of t he source address has been set t o 1, indicat ing t hat a RI F follows. The source insert s t he 2- byt e RI F rout ing cont rol field. The first bit of t he rout ing cont rol field is set t o 1 t o indicat e t hat t his fram e is an explorer. This causes t he fram e t o be flooded, and it causes bridges along a rout e t o insert ring and bridge num bers.

Source-Routing Transparent Bridges Source- Rout ing Transparent ( SRT) bridging was int roduced t o enable a single Token Ring or FDDI LAN t o carry bot h source- rout ed and t ransparent ly bridged t raffic. I t provides a convenient way t o m erge source- rout ed Token Ring ( or FDDI ) LANs wit h t ransparent ly bridged Token Ring ( or FDDI ) LANs, or t o m igrat e from one bridging t echnology t o t he ot her. A Source- Rout ing Transparent bridge perform s source rout ing for fram es t hat cont ain rout ing inform at ion fields, and t ransparent bridging for fram es t hat do not carry rout ing inform at ion. Source- Rout ing Transparent bridging applies exclusively t o LAN t echnologies t hat support source rout ing nam ely, Token Ring and FDDI . Source- Rout ing Transparent bridges perform t he I EEE Spanning Tree Prot ocol t o discover an opt im al t ree st ruct ure. Transparent ly rout ed fram es cannot be forwarded t hrough blocked port s. However, source- rout ed fram es can be forwarded t hrough any port .

Translational Bridges One aspect of t he m ove t oward building bigger LANs and using fewer rout ers is t hat som e users are bridging Et hernet , Token Ring, and FDDI LANs t o each ot her. This is

being done in spit e of som e m aj or incom pat ibilit ies bet ween t hese LANs, including differences in t he following: • • • • •

Fram e sizes Fram e form at The bit order of t he byt es in t he fram es The MAC addresses t hat are used for LAN m ult icast s Transparent and source- rout ing bridging

No st andard defines exact ly how t ranslat ional bridging should be done, alt hough som e st andards offer part ial solut ions t hat work in part icular sit uat ions. These part ial solut ions are augm ent ed by as m any propriet ary solut ions as t here are vendors. Making t ranslat ional bridges work properly requires som e effort when FDDI is involved. Devices t hroughout a LAN m ust be configured carefully t o prevent incom pat ibilit ies t hat will cause t ranslat ions t o fail. The placem ent of t he bridges t hat perform t ranslat ions and t he t ypes of t raffic t hat each bridge will process m ust be planned and im plem ent ed carefully. While considering how m uch t ranslat ion should be done, it is good t o keep in m ind t hat t he processing required t o t ranslat e a fram e from one MAC t ype t o anot her usually exceeds t he processing required t o rout e t he fram e. Furt herm ore, it is not easy t o t roubleshoot t ranslat ion errors. Vendors recom m end t he use of rout ing for prot ocols such as I P, I PX, AppleTalk, and DECnet . However, LANs st ill carry unrout able t raffic, such as I BM's classic Syst em s Net work Archit ect ure ( SNA) and Digit al Equipm ent Corporat ion's ( DEC) Local Area Transport ( LAT) . Som et im es using an exist ing solid net work infrast ruct ure t o carry t his t raffic is t he best solut ion t hat is available. Barring som e com pelling need t hat cannot be m et in any ot her way, t ranslat ional bridging should be reserved for t he hard cases t hat cannot be rout ed.

MAC Frame Sizes One of t he problem s encount ered when t ranslat ing bet ween Et hernet , Token Ring, and FDDI fram es is t hat t hese MAC prot ocols were designed independent ly, and t heir m axim um fram e sizes differ great ly. Table 17.1 list s t he m axim um sizes of convent ional, unt agged fram es. Tagging can add from 4 t o 34 ext ra byt es. The m axim um fram e size t hat is support ed on a part icular real Token Ring oft en is m anually set t o a value below t he levels shown in t he t able. Translat ional bridges cannot perform m iracles. I f a 4,000- byt e Token Ring fram e ent ers a swit ch, it cannot be forwarded t hrough a st andard Et hernet port . A fram e is not placed on an out put queue if t he fram e is bigger t han t he m axim um size support ed by t hat out put port . Et hernet , Token Ring, and FDDI LAN fram es cannot be fragm ent ed, and t here is no way for a fram e t hat is t oo big t o be forwarded using Layer 2 processes.

W a r n in g Vendor docum ent at ion t hat blurs t he dist inct ion bet ween funct ions t hat are perform ed at Layer 2 and at Layer 3 has caused confusion about fragm ent at ion.

Ta ble 1 7 .1 . M a x im um Fr a m e Size s for Et h e r n e t , Tok e n Rin g, a n d FD D I Un t a gge d Fr a m e s M AC Type Et hernet

M a x im um Fr a m e Size ( Byt e s) 1518

Token Ring

Com m e n t s

9018 byt es for nonst andard Jum bo fram es. The act ual m axim um size t hat is used is4Mbps configured by an adm inist rat or based on16Mbps t he size of t he ring, t he m edium used, and t he desired t oken holding t im e.

4Mbps

4550

16Mbps

18200

FDDI

4500

This is t he lim it for Basic m ode FDDI , t he version t hat act ually is im plem ent ed.

Translational Bridging Between Token Ring and FDDI Token Ring fram es and FDDI fram es are alm ost ident ical, and bot h can be forwarded using source- rout e bridging or t ransparent bridging. Figure 17.8 shows a LAN t hat is m ade up of Token Rings connect ed by an FDDI backbone.

Figu r e 1 7 .8 . Tok e n Rin gs con n e ct e d by a n FD D I ba ck bon e .

A syst em can com m unicat e wit h any ot her syst em on t he LAN if som e condit ions are m et :

•

•

All syst em s m ust use t he sam e t ype of bridging. Eit her t hey all m ust use source- rout e bridging, or t hey all m ust use t ransparent bridging. All MAC addresses t hat appear wit hin t he inform at ion fields of FDDI fram es m ust have t he sam e form at preferably noncanonical so t hat no t ranslat ion from t he Token Ring form at s is needed.

To underst and t he last condit ion, recall t hat Chapt er 2, " LAN MAC Addresses," explained t hese point s: •

•

For Token Ring, address byt es appear in a bit - reversed ( non- canonical) order, bot h in t he MAC header and wit hin t he inform at ion field. For FDDI , address byt es appear in a bit - reversed order in t he MAC header. The order used for addresses t hat appear in t he inform at ion field m ay be bit reversed ( non- canonical) or not bit - reversed ( canonical) . I t depends on how t he FDDI device driver is configured.

I t saves a lot of grief if t he FDDI syst em s use t he non- canonical address form at wit hin t he inform at ion field, j ust as Token Ring syst em s do. Ot herwise, t he t ranslat ional bridges have quit e a lot of work t o do. The reason for t his is t hat m any fram es carry addresses wit hin t he inform at ion field. Chapt er 2 point ed out t hat Address Resolut ion Prot ocol ( ARP) fram es carry addresses wit hin t heir inform at ion fields. This is far from being t he only prot ocol for which t his is t he case. For exam ple, Net Ware I PX fram es carry MAC addresses in t heir net worklayer headers. Messages exchanged bet ween rout ers for prot ocols such as RI P or OSPF cont ain m any addresses.

Translational Bridging Between Transparent Token Ring and Ethernet The price and perform ance of Fast Et hernet and Gigabit Et hernet have m ade Et hernet an at t ract ive LAN backbone t echnology. Figure 17.9 shows Token Rings t hat are int erconnect ed across an Et hernet . Transparent bridging is used everywhere, and any syst em can com m unicat e wit h any ot her syst em .

Figu r e 1 7 .9 . Con n e ct in g t r a n spa r e n t Tok e n Rin gs t o a n Et h e r ne t .

Each t ranslat ional bridge has t o • •

Translat e bet ween Et hernet and Token Ring fram e form at s Perform conversions bet ween t he non- canonical address form at s used in Token Rings and t he canonical address form at s used in Et hernet fram es

This LAN is const ruct ed using t ransparent bridges. The Spanning Tree Prot ocol would be used t o generat e a loop- free t opology because t he LAN cont ains a looped pat h. There are drawbacks t o a t ot ally bridged solut ion: • •

The form at t ranslat ions add a lot of overhead. Available bandwidt h is wast ed because redundant links m ust be blocked by t he Spanning Tree Prot ocol.

Therefore, it m akes sense t o use Layer 2/ 3 swit ches and t o rout e as m uch of t he t raffic as possible.

Translational Bridging Between Source-Routing Token Rings and Ethernet The sit uat ion is m ore com plicat ed if source- rout ing bridges are used in a Token Ring LAN t hat is connect ed t o an Et hernet , but som e vendors offer propriet ary product s t hat can overcom e t he problem s. Figure 17.10 illust rat es what happens. Thanks t o t he act ions of t he t ranslat ional bridge:

Figu r e 1 7 .1 0 . Con n e ct in g sou r ce - r ou t e d Tok e n Rin gs t o ethernet.

• •

Token Ring syst em s t hink t hat t he ent ire Et hernet is a single Token Ring. Et hernet syst em s t hink t hat t he Token Ring LAN is an Et hernet .

For t raffic from t he Token Ring t o t he Et hernet , t he t ranslat ional bridge •

• •

Responds t o explorer fram es ( The RI F appears t o lead t o t he dest inat ion MAC address, but act ually it leads t o t he bridge.) Caches RI Fs Translat es incom ing Token Ring fram es t o Et hernet form at and forwards t he fram es

For t raffic from t he Et hernet t o t he Token Ring, t he bridge • • •

Translat es from Et hernet form at t o Token Ring form at Searches it s cache for a RI F t hat leads t o t he dest inat ion MAC address I f a usable RI F is not found, launches explorer fram es t o locat e t he dest inat ion and adds t he RI F t o t he cache

For eit her direct ion of t ransm ission, t he bridge m ust t ranslat e bet ween t he noncanonical address form at s used in Token Ring inform at ion fields and t he canonical address form at s used in Et hernet fram e inform at ion fields.

Tunneling Token Ring Across Ethernet Som et im es t he only reason t o connect Token Rings t o an Et hernet is t o use t he backbone bandwidt h of t he Et hernet . For exam ple, syst em s on one Token Ring m ight need t o reach an SNA host on anot her Token Ring, and m ight have no need t o reach any syst em connect ed t o t he Et hernet . I n t his case, t here is no reason t o t ranslat e bet ween Et hernet and Token Ring MAC form at s because Token Ring and Et hernet syst em s do not want t o com m unicat e wit h one anot her. Several vendors offer propriet ary t unneling solut ions t hat suit t his sit uat ion. Figure 17.11 illust rat es how it works. The following is t rue for t unneling bridges:

Figur e 1 7 .1 1 . Tunne ling Tok e n Rin g a cr oss Et h e r n e t .

•

•

The bridges carry each Token Ring fram e inside t he inform at ion field of an Et hernet fram e. The source and dest inat ion MAC addresses on t hese Et hernet fram es are t he source and dest inat ion MAC addresses of t unneling bridges.

The procedure is very sim ple if t he Token Rings in Figure 17.11 use t ransparent bridging. I n t his case, if st at ion X sends an inform at ion fram e t o server Y, t he following t akes place: 1. The fram e is forwarded t o bridge A, which checks it s filt ering t able. 2. I f server Y's MAC address is in bridge A's filt ering t able, t he ent ry indicat es t hat t he fram e m ust be t unneled t o bridge C. Bridge A wraps t he Token Ring fram e in an Et hernet fram e addressed t o bridge C and forwards it . 3. I f server Y's MAC address is not in bridge A's filt ering t able, bridge A wraps t he Token Ring fram e in Et hernet fram es t hat are addressed t o bridge B and bridge C, and forwards t hem . 4. Bridge C forwards t he fram e ont o t he ring t hat leads t o server Y. The procedure is slight ly m ore com plicat ed if t he Token Rings use source rout ing. The Token Rings are fooled int o t hinking t hat t he Et hernet is a single ring. This procedure is 1. St at ion X sends out an explorer fram e. 2. Bridge A caches t he rout e t o st at ion X. I t wraps t he explorer in Et hernet fram es addressed t o bridges B and C. 3. Bridges B and C unwrap t he explorer and add a " fake" Et hernet ring num ber and t heir own bridge num bers t o t he rout e. They t hen forward t he explorer

int o t heir Token Rings. They also creat e filt ering t able ent ries for st at ion A's MAC address. 4. When server Y ret urns t he explorer t o bridge C, bridge C caches t he rout e t o server Y and wraps t he explorer in an Et hernet fram e addressed t o bridge A. 5. Bridge A creat es a filt ering t able ent ry t hat indicat es t hat server Y is reached via t he t unnel t o bridge C. 6. Bridge A forwards t he fram e back t o st at ion X. All syst em s now are ready t o com m unicat e. This solut ion has low overhead and is fast . I t disposes of t he heavy- dut y t ranslat ions t hat t ranslat ional bridges m ust perform . I t also avoids t he pit falls t hat result from glit ches in t ranslat ing bet ween canonical and non- canonical address form s. The disadvant ages of t unneling Token Ring across Et hernet are t hese: • •

Et hernet fram e sizes are sm all. An ext ra prot ocol header usually is insert ed int o t he inform at ion field of t he Et hernet fram e t o indicat e t hat t he cont ent is t unneled Token Ring t raffic, m aking t he payload even sm aller. This lim it s t he m axim um size of t he Token Ring fram es. Solut ions are propriet ary. All part icipat ing bridges m ust be supplied by one vendor, and t here is no guarant ee t hat t he vendor will support t he solut ion int o t he fut ure.

Structuring a LAN Around a High-Speed Ethernet or FDDI Backbone with Tagging Anot her opt ion is available t o a LAN adm inist rat or who wishes t o bridge bet ween Et hernet , Token Ring and FDDI LANs. The I EEE 802 VLAN com m it t ee int roduced VLAN t ag feat ures t hat solve som e t agging of t he problem s encount ered in t ranslat ional bridging bet ween Et hernet , Token Ring, and FDDI . The I EEE solut ion support s bot h t ransparent LANs and LANs t hat include sourcerout ed port ions. The solut ion requires fram es t o be t ranslat ed, but it offers t wo advant ages: • •

I t is defined in a st andard ( 802.1Q) . The t ag inform at ion explicit ly st at es whet her MAC addresses cont ained in t he inform at ion field are in canonical or non- canonical form at .

Using Tags in a Transparently Bridged LAN Figure 17.12 shows a t ransparent ly bridged LAN t hat cont ains Et hernet , Token Ring, and FDDI port ions t hat are int erconnect ed by VLAN- aware t ransparent bridges.

Figu r e 1 7 .1 2 . V LAN - a w a r e br idge s in a t r a n spa r e n t ly br idge d LAN w it h a n Et h e r n e t ba ck bon e .

The VLAN- aware bridges insert t ag headers int o fram es t hat originat e in a Token Ring or FDDI syst em . The fram es are sent across t he Et hernet in t he form at shown in Figure 17.13.

Figu r e 1 7 .1 3 . Ta g in for m a t ion in a n Et h e r n e t fr a m e in a t r a n spa r e n t ly br idge d LAN

The Canonical Form at I ndicat or ( CFI ) flag in t he t ag cont rol inform at ion is im port ant . • •

I f t he CFI is 0, t his is an ordinary t agged Et hernet fram e. The CFI is set t o 1 t o indicat e t hat t his fram e has been t ranslat ed from Token Ring or FDDI form at t o Et hernet form at , and t hat an ext ra em bedded rout ing inform at ion field ( E- RI F) follows t he lengt h/ t ype field.

The E- RI F field is considered t o be part of t he t ag inform at ion. I n Figure 17.13, t he ent ire E- RI F is j ust a 2- byt e rout ing cont rol field. The values carried for a t ransparent ly bridged net work are shown in Figure 17.14:

Figu r e 1 7 .1 4 . Th e Rou t in g Con t r ol fie ld for a fr a m e t h a t or igin a t e d a t a t r a n spa r e nt Tok e n Rin g or FD D I Syst e m .

•

• •

The init ial 3- bit field ident ifies t he rout ing t ype, which in t his case is t ransparent . A largest fram e ( LF) field indicat es, in t his case, t hat t he biggest inform at ion field is 1470 byt es. This rest rict ion assures t hat t he Token Ring payloads will fit wit hin t he Et hernet fram e payload areas. The final bit is a flag called t he Non- Canonical Form at I dent ifier ( NCFI ) . NCFI = 0 m eans t hat any MAC addresses carried in t he inform at ion field are in non- canonical form . NCFI = 1 m eans t hat t hey are in canonical form .

The NCFI bit t akes t he m yst ery out of t he m ost confusing part of t he t ranslat ion process.

Using Tags to Translate Source-Routed Frames Tagging also can help a high- speed Et hernet or a t ransparent FDDI ring t o be used as a conduit bet ween source- rout ed Token Ring com m unit ies. Figure 17.15 depict s source- rout ed Token Rings t hat are connect ed across an Et hernet or a t ransparent FDDI ring by VLAN- aware bridges.

Figu r e 1 7 .1 5 . V LAN - a w a r e br idge s con n e ct in g sou r ce - r ou t e d Tok e n Rin g LAN s a cr oss Et h e r n e t or t r a n spa r e n t FD D I .

Source- rout ed Token Ring fram es are t ranslat ed t o Et hernet form at and are sent across t he Et hernet in t he t agged form at shown in Figure 17.16.

Figu r e 1 7 .1 6 . An Et h e r n e t fr a m e t h a t ca r r ie s t h e in for m a t ion fie ld of a sou r ce - r ou t e d Tok e n Rin g fr a m e .

The CFI value of 1 indicat es t hat an em bedded rout ing inform at ion field ( E- RI F) follows t he lengt h/ t ype field. I n t his case, t he E- RI F lives up t o it s nam e. I t includes t he rout e descript ors t hat were in t he rout ing inform at ion field of t he original sourcerout ed Token Ring fram e before it was t ranslat ed. Source- rout ed Token Ring fram es are t ranslat ed and sent across a t ransparent FDDI LAN in t he t agged fram e form at shown in Figure 17.17.

Figu r e 1 7 .1 7 . A t r a n spa r e n t FD D I fr a m e t h a t ca r r ie s t h e in for m a t ion fie ld of a sou r ce - r ou t e d Tok e n Rin g fr a m e .

For eit her t ype of backbone, t he rout ing cont rol byt es have t he form at shown in Figure 17.18. The rout ing t ype indicat es whet her t his is an ordinary rout ed fram e ( 000) , an all- rout es explorer ( 100) , or a Spanning Tree explorer ( 110) . The next field indicat es t he lengt h of t he E- RI F. The direct ion bit indicat es whet her t he rout e should be followed in forward or reverse order.

Figu r e 1 7 .1 8 . Th e Rou t in g Con t r ol fie ld for a fr a m e t h a t or igin a t e d a t a sou r ce - r ou t in g syst e m .

I f t he backbone is an Et hernet , t he code in t he largest fram e field indicat es t hat t he biggest payload is 1470 byt es. I f t he backbone is an FDDI LAN, t he value can range up t o 4399 byt es. As before, NCFI = 0 m eans t hat any MAC addresses carried in t he inform at ion field are in non- canonical form . NCFI = 1 m eans t hat t hey are in canonical form .

Remote Bridges By definit ion, a " local" area net work was int ended t o be a net work facilit y covering a lim it ed space, such as a set of offices, a building, or perhaps a cam pus. But as earlier discussion has shown, t he evolut ion of LANs has been fueled by t he desire of users t o bend t he rules t o suit t heir special needs. Rem ot e bridges were designed t o unit e LANs locat ed at different sit es int o a single LAN. Rem ot e bridges have been used for several reasons. Som e prot ocol t raffic cannot be rout ed, so if local client s needed t o cross a wide area link t o reach a rem ot e server, bridging was t he only alt ernat ive. Frequent ly used nonrout able prot ocols have included I BM SNA, Digit al Equipm ent Corporat ion Local Area Transport ( LAT) , and I BM/ Microsoft Net BI OS over Net BEUI . Som e net work adm inist rat ors feel t hat rout ers are t oo expensive, t oo slow, or t oo com plicat ed t o configure. They prefer t o bridge all prot ocol—seven rout able ones—on principle. Figure 17.19 shows a pair of LANs connect ed t oget her by rem ot e bridges t o form one large " LAN."

Figu r e 1 7 .1 9 . Tw o sit e s con n e ct e d by r e m ot e br idge s

Links Between Bridges LANs can be bridged across a fram e relay, ATM, or X.25 circuit ; a leased line; a m icrowave connect ion; or even a dialup I SDN or POTS line. Som e product s can aggregat e m ult iple links bet ween a pair of rem ot e bridges so t hat t hey behave like a single line. I n som e inst ances, m ult iple sit es are bridged t oget her. Figure 17.20 shows t hree sit es connect ed by rem ot e bridges. The link bet ween bridge B and bridge C has been blocked by t he Spanning Tree Prot ocol t o prevent a loop.

Figu r e 1 7 .2 0 . Th r e e sit e s con n e ct e d by r e m ot e br idge s.

To Bridge or Not to Bridge The absolut e need t o bridge t raffic across wide area links is decreasing. Today, Net BI OS can run over I P and, hence, can be rout ed. The use of t he DEC LAT prot ocol is in decline as well. However, I BM SNA st ill is t he source of a subst ant ial am ount of unrout able t raffic. At t he sam e t im e, obj ect ions against using rout ers are dim inishing. Today's rout ers are fast , and oft en less effort is required t o set up a rout er t han t o configure a bridge t o keep unnecessary t raffic off t he wide area link. This m ust be done by placing m anual ent ries int o bridge filt ering t ables, or by defining VLANs t hat cont rol and reduce t raffic. Com bined Layer 2/ 3 swit ches ( bridge/ rout ers) can rout e m ost of t he t raffic and bridge what ever unrout able t raffic rem ains. Unlike bridged t raffic, rout ed t raffic can and will use all available links. For exam ple, if t he bridges in Figure 17.20 are replaced by Layer 2/ 3 swit ches, rout ed t raffic can be sent across t he link connect ing bridge/ rout ers B and C. Because wide area bandwidt h st ill is cost ly, it m akes sense t o use it at all t im es—not j ust for backup. A rout er forwards exact ly t he t raffic t hat needs t o cross a link, and no m ore. LAN broadcast s and local m ult icast s are not forwarded by rout ers. I n cont rast , a bridge forwards a m ass of ext ra t raffic. This includes broadcast s and flooded fram es whose dest inat ion MAC addresses have not yet been learned by t he

bridge. For exam ple, if all prot ocols used at sit es A, B, and C in Figure 17.20 are bridged, t he following would be t rue: •

• • •

ARP broadcast s used t o m ap I P addresses t o MAC addresses would be propagat ed t o rem ot e sit es. Net BI OS nam e regist rat ions would be broadcast t o rem ot e sit es. Net Ware Service Advert isem ent Prot ocol ( SAP) m essages would be broadcast t o rem ot e sit es. I f bridge A had not yet learned t hat syst em M was locat ed on it s local LAN A, a fram e addressed t o M would be flooded across t he rem ot e links.

Anot her fact or t o consider is t he delay t hat crossing a wide area link adds t o LAN com m unicat ion. This delay m ust be t aken int o account when configuring t he t im ers used for t he Spanning Tree Prot ocol.

Frame Formats on Wide Area Links An Et hernet , Token Ring, or FDDI MAC fram e is bridged across a wide area link by placing t he MAC fram e int o t he inform at ion field of a WAN fram e before sending it across t he link. The MAC fram e is ext ract ed at t he ot her end and t hen is forwarded ont o t he rem ot e LAN.

N ot e The encapsulat ion of an Et hernet , Token Ring, or FDDI MAC fram e inside a WAN fram e act ually is an inst ance of t unneling.

Figure 17.21 illust rat es what an encapsulat ion looks like.

Figu r e 1 7 .2 1 . En ca psu la t in g a LAN fr a m e w it h in a W AN fr a m e .

I n Figure 17.21, t he WAN fram e header is followed by a header t hat indicat es t he t ype of dat a t hat follows. This dat a m ight be a bridged Et hernet , Token Ring, or FDDI fram e, or a Bridge Prot ocol Dat a Unit ( BPDU) used for t he Spanning Tree Prot ocol. Ot her encapsulat ed t raffic in t he m ix could include SNA fram es and rout ed prot ocol dat a unit s. Not e t hat for t he encapsulat ion in t he upper part of Figure 17.21, t he LAN and WAN fram es each cont ain a fram e check sequence ( FCS) field. One of t hese is excess baggage, and for som e wide area encapsulat ions, t he LAN FCS can be dropped before t he Layer 2 fram e is placed int o a WAN fram e's inform at ion field. This is illust rat ed in t he lower part of Figure 17.21. I n t his case, t he WAN fram e check sequence is validat ed when t he fram e arrives at t he rem ot e end of a WAN link. I f t he fram e has not been corrupt ed, t he MAC fram e's FCS is recalculat ed and added before t he fram e is forwarded ont o a LAN segm ent . Encapsulat ion is done a lit t le different ly for each wide area t echnology. Som et im es m ore t han one encapsulat ion can be used for a part icular wide area t echnology. Table 17.2 list s som e encapsulat ion m et hods.

Ta ble 1 7 .2 . W AN En ca psu la t ion For m a t s W AN Te chn ology Point - t o- Point line ( includes leased lines; swit ched 56K, I SDN B- channel connect ions; and ordinary dialup)

En ca psu la t ion M e t h od

Com m e n t s

Propriet ary version of High- level Dat a Link Cont rol ( HDLC)

Propriet ary versions of HDLC are designed by vendors.

Point - t o- Point Prot ocol ( PPP)

Defined in a series of I ETF st andards docum ent s. The basic st andard is RFC 1662.

Fram e Relay

I ETF Mult iprot ocol over Fram e Relay

Oft en called t he RFC 1490 encapsulat ion by vendors, alt hough t he current st andard is RFC 2427.

ATM

I ETF Mult iprot ocol Encapsulat ion over ATM Adapt at ion Layer 5

Defined in I ETF RFC 2684. Sim ilar, but not ident ical t o t he encapsulat ion used for fram e relay.

ATM LAN Em ulat ion

Defined in t he ATM Forum LANE specificat ion.

X.25 and I SDN LAPD packet s I ETF Mult iprot ocol I nt erconnect on X.25 and I SDN in t he Packet Mode

Defined in I ETF RFC 1356. Sim ilar, but not ident ical t o, t he encapsulat ion used for fram e relay.

To provide an idea of what an encapsulat ed fram e looks like, List ing 17.2 shows a Token Ring fram e t hat is encapsulat ed wit hin a fram e relay fram e. The fram e relay

header cont ains a fram e relay dat a link connect ion ident ifier ( DLCI ) . The DLCI is 32, in t his case. The fram e relay header also includes som e flags t hat are used t o report congest ion and t o indicat e whet her t he fram e m ay be discarded when congest ion occurs. The header aft er t he address field, which st art s wit h X'03- 00- 80, was defined by t he I ETF. This is followed by t he organizat ionally unique ident ifier ( OUI ) X'00- 80- C2, which indicat es t hat a bridged MAC fram e is enclosed. The t ype field value, X'00- 09, m eans t hat t his is a Token Ring fram e and t hat it s fram e check sequence has been st ripped off. ( A t ype field value of X'00- 03 would indicat e t hat a com plet e Token Ring fram e t hat included a fram e check sequence was included in t he payload.)

List in g 1 7 .2 A Tok e n Rin g Fr a m e En ca psu la t e d W it h in a Fr a m e Re la y Fr a m e FRELAY: ----- Frame Relay ----FRELAY: FRELAY: Address word = 0801 FRELAY: 0000 10.. 0000 .... = DLCI 32 FRELAY: .... ..0. .... .... = Response FRELAY: .... .... .... 0... = No forward congestion FRELAY: .... .... .... .0.. = No backward congestion FRELAY: .... .... .... ..0. = Not eligible for discard FRELAY: .... .... .... ...1 = Not extended address FRELAY: FRELAY: ----- Multiprotocol over Frame Relay ----FRELAY: FRELAY: Control, pad(s) = 0300 FRELAY: NLPID = 0x80 (SNAP) FRELAY: SNAP: ----- SNAP Header ----SNAP: SNAP: Vendor ID = IEEE (OUI X'00-80-C2) SNAP: Type = 0009 (802.5) SNAP: Pad = 11 SNAP: TRING: ----- Token Ring Header ----TRING: TRING: Physical Control Field : TRING: Access Control = 40 TRING: 010. .... = Priority Bits TRING: ...0 .... = Token Bit TRING: .... 0... = Monitor Bit TRING: .... .000 = Reservation Bits TRING: AC: Frame priority 2, Reservation priority 0, Monitor count 0 TRING: TRING: Frame Control = 40 TRING: 01.. .... = Frame Type (LLC) TRING: ..00 .... = Reserved TRING: .... 0000 = Reserved TRING: FC: LLC frame, PCF attention code: None TRING: Destination = Station 0000000060C0 TRING: Source = Station 000000006108 TRING: LLC: ----- LLC Header -----

LLC: . . .

Som e t rends m ight m ake wide area swit ching grow in popularit y inst ead of fading away. New fiber opt ic t ransm ission t echniques are expanding t he bandwidt h t hat can be carried on a t iny opt ical fiber at an unprecedent ed rat e. Tom orrow's wide area bandwidt h price will be a fract ion of t oday's price. The m ove t o st andardize on TCP/ I P is gradually eroding t he use of unrout able prot ocols and prot ocols t hat have a high densit y of broadcast t raffic. I P version 6 cut s back on I P broadcast t raffic. I n addit ion, ATM LANE, which is discussed in Chapt er 21, " ATM LAN Em ulat ion," cut s down on t he am ount of broadcast ing and is designed t o blur t he difference bet ween wide area and local area com m unicat ion.

Summary Points • •

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Transparent bridges learn t he MAC addresses t hat are reached t hrough each port and can part icipat e in t he Spanning Tree Prot ocol, which rest ruct ures t he LAN t opology aut om at ically aft er a com ponent failure. I n a source- rout ing bridged LAN, a fram e carries it s t ravel direct ions in a rout ing inform at ion field ( RI F) in t he fram e header. Source rout es t hat cross one or m ore bridges are discovered by explorer fram es. An all- rout es- explorer fram e is flooded across all of t he rings in t he LAN. When all- rout es- explorers are used, m ult iple act ive pat hs can carry t raffic t o a dest inat ion ring. Spanning Tree explorer fram es are forwarded t hrough port s t hat are act ive in t he Spanning Tree t opology. They are used in big LANs when t here is concern about t he am ount of t raffic t hat is generat ed by all- rout es- explorer fram es. A rout ing inform at ion field cont ains a sequence of ring ident ifiers and bridge num bers. Source- rout ing t ransparent bridging enables a single Token Ring or FDDI LAN t o carry bot h source- rout ed and t ransparent ly bridged t raffic. Source- rout ing t ransparent bridging provides a convenient way t o m igrat e from source- rout ed t o t ransparent ly bridged Token Ring or FDDI LANs. Translat ional bridges relay fram es bet ween Et hernet , Token Ring, and FDDI LANs, creat ing a big, int erconnect ed LAN. Translat ional bridges m ust overcom e m aj or incom pat ibilit ies in fram e form at s, bit order, and t ransparent versus source- rout ed bridging. I t is not difficult t o t unnel Token Ring fram es across an Et hernet backbone. Tagged fram es sim plify form at t ranslat ion. They enable an Et hernet or FDDI fram e t o carry a Token Ring fram e's payload and rout ing inform at ion field across a t ransparent Et hernet or FDDI backbone. Rem ot e bridges unit e Et hernet or Token Ring LANs locat ed at different sit es int o a single LAN. Broadcast t raffic can degrade a wide area link bet ween t wo sit es. An Et hernet , Token Ring, or FDDI fram e is bridged across a wide area link by placing t he MAC fram e int o t he inform at ion field of a WAN fram e.

References Source- rout ing is described in Annex C of •

I EEE 802.1D ( I SO/ I EC 15802- 3) . " Media Access Cont rol ( MAC) Bridges." 1998.

Som e of t he issues t hat arise in t ranslat ing bet ween Et hernet fram es t hat cont ain a t ype field and 802.3, 802.5, and FDDI fram es are discussed in •

ANSI / I EEE St andard 802.1H ( I SO/ I EC Technical Report 11802- 5) . " Media Access Cont rol ( MAC) Bridging of Et hernet V2.0 in Local Area Net works." 1997.

VLANs and E- RI Fs are described in •

I EEE 802.1Q. " Virt ual Bridged Local Area Net works." 1998.

I nsight int o source- rout ing bridge configurat ion param et ers can be gained by exam ining t he I ETF docum ent : •

RFC 1525. " Definit ions of Managed Obj ect s for Source Rout ing Bridges." E. Decker, K. McCloghrie, P. Langille, and A. Rij singhani. 1993.

Several I ETF RFC docum ent s discuss encapsulat ions for MAC fram es t hat are t ransm it t ed across a wide area link t hat connect s rem ot e bridges: •

•

•

• •

RFC 2427. " Mult iprot ocol I nt erconnect over Fram e Relay." C. Brown and A. Malis. 1998. RFC 2684. " Mult iprot ocol Encapsulat ion over ATM Adapt at ion Layer 5." D. Grossm an and J. Heinanen. 1999. RFC 1356. " Mult iprot ocol I nt erconnect on X.25 and I SDN in t he Packet Mode." A. Malis, D. Robinson, and R. Ullm ann. 1992. RFC 1662. " PPP in HDLC- like Fram ing." W. Sim pson, ed. 1994. RFC 1990. " The PPP Mult ilink Prot ocol ( MP) ." K. Sklower, B. Lloyd, G. McGregor, D. Carr, and T. Coradet t i. 1996.

Descript ions of wide area encapsulat ions, along wit h several det ailed exam ples, can be found in •

Feit , Dr. Sidnie. Wide Area High Speed Net works. I ndianapolis, I N: Macm illan Technical Publishing, 1999.

Chapter 18. Routing and Layer 2/3 Switches This book is prim arily concerned wit h com m unicat ion bet ween devices t hat are connect ed t o t he sam e LAN. But it also is im port ant t o underst and t he role and operat ion of rout ers, which carry t raffic t o and from LANs. This is especially t rue

because of t he increasing use of Layer 2/ 3 swit ches, which bridge som e t ypes of t raffic and rout e ot her t ypes. This chapt er provides an overview of t he funct ions t hat are perform ed by rout ers. I t also explains what Layer 2/ 3/ 4 and applicat ion- layer swit ches are. Because of it s widespread use, t he I P prot ocol is used as t he basis of exam ples t hat describe t he t ypes of funct ions t hat a rout er perform s. Recall t hat t he I P prot ocol dat a unit , which consist s of a header and payload dat a, is called a dat agram .

Features of Routing A net work- layer prot ocol consist s of a net work- addressing plan plus a set of procedures perform ed t o deliver dat a t o a dest inat ion net work address. Rout ers forward dat a t o it s net work dest inat ion, t raversing m ult iple links along t he way. Several vendors have designed successful propriet ary net work- layer prot ocols. I BM SNA, Digit al Equipm ent Corporat ion's DECnet , Novell's rout able I PX/ SPX, and rout able AppleTalk all have had m any loyal adherent s. I n t he end, however, I P, an open st andard, won out . Because t here is a large inst alled base of equipm ent t hat uses legacy prot ocols, t hough, m any of t oday's rout ers forward one or m ore of t he propriet ary t raffic flows as well as I P t raffic.

Router Benefits Rout ers provide t ot al flexibilit y in building a net work t opology. As illust rat ed in Figure 18.1, rout ers can m ove dat a across any series of LAN and wide area links. You can build as m uch redundancy int o a rout ed net work as you want ( or can afford) .

Figur e 1 8 .1 . A r out e d ne t w or k .

The I P net work layer includes a m ult inet work, global addressing plan. This has m ade t he worldwide connect ion of com put ers across t he I nt ernet possible.

Network Addresses versus MAC Addresses A global net work- layer address plan is hierarchical, in som e ways resem bling t he t elephone num ber syst em . I n t he Unit ed St at es, t elephones whose 10- digit num bers st art wit h t he sam e 6- digit prefix are connect ed t o t he sam e swit ch. I P net work addresses t hat st art wit h t he sam e prefix belong t o t he sam e LAN. An I P syst em decides whet her a dest inat ion is on it s LAN by com paring it s own address prefix wit h t he dest inat ion's address prefix.

N ot e The address prefix consist s of t wo part s—a net work num ber associat ed wit h a whole net work, and a subnet num ber associat ed wit h syst em s on a LAN.

I f t he source and dest inat ion address prefixes m at ch, bot h syst em s are on t he sam e LAN. The next st ep is t o discover t he dest inat ion's Layer 2 MAC address. This st ep is responsible for a lot of t he Layer 2 broadcast s t hat flood t hrough a LAN. An I P syst em discovers t he MAC address of a dest inat ion on it s LAN by broadcast ing an Address Resolut ion Prot ocol ( ARP) m essage t hat asks t he owner of a specified I P address t o respond. For exam ple, in Figure 18.2, st at ion X broadcast s an address resolut ion request asking t he owner of an enclosed I P address t o send back t he owner's MAC address. When server Y answers, st at ion X caches t he MAC address. St at ion X t hen is able t o wrap I P dat agram s addressed t o server Y int o fram es direct ed t o t his dest inat ion MAC address.

Figu r e 1 8 .2 . ARP M AC a ddr e ss r e solu t ion .

Exiting a LAN I f a dest inat ion I P address prefix does not m at ch t he source address prefix, a dat agram m ust leave it s own LAN t o reach it s dest inat ion. The way t hat t he dat agram is launched is 1. St at ion X looks up t he I P address of it s local default rout er. ( This eit her was m anually configured, was obt ained from a configurat ion server, or was announced by a rout er broadcast .) 2. I f necessary, st at ion X m akes an ARP query t o obt ain t he default rout er's MAC address.

3. St at ion A wraps t he dat agram in a fram e whose dest inat ion is t he rout er's MAC address and sends it t o t he rout er. 4. The rout er processes t he dat agram , refram es it and forwards it onward. Figure 18.3 shows a fram ed dat agram ent ering a rout er, being processed and refram ed, and leaving. The dat agram in t his exam ple is being forwarded from an Et hernet LAN t o a Token Ring LAN.

Figur e 1 8 .3 . A r out e r r e fr a m in g a da t a gr a m .

The rout er accept s t he incom ing Et hernet fram e because t he fram e's dest inat ion MAC address m at ches t he MAC address of port 1 at t he rout er. Aft er validat ing t he fram e check sequence, t he rout er uses Et herType t o ident ify t he kind of prot ocol dat a unit t hat is enclosed—an I P dat agram , in t his case. The I P dat agram header includes t he dat agram 's dest inat ion I P address. The rout er processes t he dat agram , looking up t he dest inat ion I P address in it s I P rout ing t able t o det erm ine t he exit port t hrough which t he dat agram should be forwarded—in Figure 18.3, t his is port 2.

The dat agram t hen is given a com plet ely new fram e header and t railer. The fram e form at depends on t he t ype of segm ent or link t hat is connect ed t o port 2, which in t his case is a Token Ring segm ent . The fram e check sequence is calculat ed and placed in t he fram e t railer. The source address for t his new fram e is t he MAC address of rout er port 2. The dest inat ion MAC address is t he MAC address of t he next - hop syst em , which is eit her t he dest inat ion or anot her rout er on t he pat h t o t he dest inat ion. Figure 18.4 shows a bird's- eye view of t he rout ed pat h bet ween st at ion X on an Et hernet and server Y on a Token Ring.

Figu r e 1 8 .4 . Rou t in g a da t a gr a m be t w e e n LAN s.

I n cont rast , Figure 18.5 illust rat es how a Layer 2 swit ch forwards a fram e. I n Figure 18.5,bot h t he ent rance and t he exit port s connect t o Et hernet segm ent s. The swit ch looks up t he dest inat ion MAC address in t he filt ering t able. The fram e is forwarded based on t he result .

Figu r e 1 8 .5 . A sw it ch for w a r din g a fr a m e .

Not e t hat t he ent ire fram e is unchanged—it st ill has it s original source and dest inat ion MAC addresses and fram e check sequence. This t ransact ion is a lot sim pler t han t he rout ing t ransact ion.

N ot e Som e product s give t he cust om er t he opt ion of accelerat ing fram es t hrough t he swit ch by using cut - t hrough operat ion, which m eans t hat t he out put port is looked up as soon as a fram e's dest inat ion address has been received. The swit ch st art s t o t ransm it t he init ial part of t he fram e before t he com plet e fram e has arrived and, hence, before t he fram e check sequence can be verified.

The sim plicit y of a Layer 2 swit ch enables it t o process m assive num bers of fram es per second. However, if t he exit port connect s t o a Token Ring LAN and t he fram e form at m ust be t ranslat ed, t he am ount of processing rises dram at ically.

Routing Procedures The key st ep in t he Layer 3 rout ing process is a net work address t able lookup. A rout er m aint ains a t able t hat m at ches address prefixes t o out put port s. The specific processing st eps t hat are carried out differ slight ly for each of t he net work- layer prot ocols. A rough out line t hat includes m ost of t he st eps perform ed by an I P rout er follows: •

• • • • •

• •

Aft er discarding t he fram e header and t railer, t he dat agram is passed t o an I P m odule for processing. The I P header cont ains a checksum field. The checksum is recalculat ed, and t he dat agram is discarded if t he out com e does not m at ch t he value in t he field. The I P header includes a field t hat lim it s t he num ber of hops t hat a dat agram m ay t raverse. This is decrem ent ed, and if t he value is 0, t he dat agram is discarded. The header checksum is recalculat ed and placed int o t he checksum field. A rout ing t able lookup m aps t he dest inat ion I P address in t he I P header t o t he correct out put port . That port leads eit her t o a next - hop rout er or t o t he final dest inat ion. Opt ionally, a list of securit y screening rules is applied t o t he dat agram . I f t he dat agram fails any t est , it is discarded. Opt ionally, a set of rules is applied t o assign a priorit y t o t he dat agram . The dat agram is wrapped in a fresh fram e, which is sent across t he next link, eit her t o a next - hop rout er or t o t he final dest inat ion.

This is quit e a lot of processing com pared t o what a Layer 2 swit ch ( bridge) does.

Building Routing Tables There is a very direct way t o get a rout ing t able int o a rout er—an adm inist rat or can t ype it in. This m et hod is feasible for very sm all, sim ple net works. For m ore com plex net works ( such as t he one in Figure 18.1) , it is best t o let t he rout ers do t he brainwork. Rout ers exchange inform at ion about t he addresses t hat t hey can reach. Each rout er uses t his inform at ion t o build up it s rout ing t able. Rout ing inform at ion is updat ed whenever a change, such as a broken link, occurs. The rout er- t o- rout er exchange of rout ing t able inform at ion is carried out according t o a rout ing prot ocol. A rout ing prot ocol defines t he cont ent and form at of rout ing inform at ion m essages, and t he event s t hat cause m essages t o be sent . Over t he years, m any different rout ing prot ocols have been creat ed and used for t he exchange of I P rout ing inform at ion. The ones used in privat e net works t oday include t hese: •

RI P ve r sion 1 —Obsolet e, but st ill used. I t is adequat e for very sm all and sim ple net works. Rout er- t o- rout er report s can generat e a lot of LAN

• • • •

broadcast t raffic. Swit chover of t raffic from a failed pat h t o a good pat h can t ake m inut es. RI P ve r sion 2 —An im proved version t hat support s m ore flexible addressing and aut hent icat ion of rout er- t o- rout er m essages. Broadcast s used for LAN rout er- t o- rout er report s opt ionally can be replaced wit h m ult icast s. Swit chover from a failed pat h st ill is slow. Ope n Shor t e st Pa t h Fir st ( OSPF) —Scales t o very large net works and enables rout ers t o build det ailed net work m aps. The current version is 2. Rout er- t o- rout er report s consum e negligible am ount s of t raffic. Swit ch- over from a failed pat h can occur wit hin a few seconds. I n t e r n e t Ga t e w a y Rou t in g Pr ot ocol ( I GRP) —The old version of Cisco's propriet ary prot ocol. Rout er- t o- rout er report s can generat e a lot of LAN broadcast t raffic. Provides rout ers wit h det ailed t hroughput , delay, and reliabilit y inform at ion. En h a n ce d I n t e r n e t Ga t e w a y Rou t in g Pr ot ocol ( EI GRP) —The current version of Cisco's propriet ary prot ocol. Rout er- t o- rout er report s consum e negligible am ount s of t raffic. Swit chover occurs fairly quickly. EI GRP provides rout ers wit h det ailed t hroughput , delay, and reliabilit y inform at ion.

Different divisions in a com pany m ight choose different rout ing prot ocols. Rout er vendors accom m odat e t his by enabling a rout er t o run several prot ocols at t he sam e t im e, as is done by rout er A in Figure 18.6. Rout er A is able t o share inform at ion wit h rout ers in ot her divisions. All of t he inform at ion t hat rout er A learns from it s neighbors cont ribut es t o building up it s rout ing t able. Not e t hat execut ing t hese prot ocols and updat ing t he rout ing t able consum es CPU cycles.

Figu r e 1 8 .6 . Usin g m u lt iple r ou t in g pr ot ocols.

Vendors also support rout er- t o- rout er prot ocols t hat provide inform at ion used t o forward t raffic across t he I nt ernet : •

•

Ex t e r ior Ga t e w a y Pr ot ocol ( EGP) —Obsolet e, but st ill occasionally used t o report t he locat ion at which a part icular net work connect s t o t he I nt ernet . Bor de r Ga t e w a y Pr ot ocol ( BGP) —A coarse- grained but im port ant prot ocol for discovering pat hs across t he I nt ernet t hat lead t o a part icular net work. The current version is 4.

I f t his is not enough, t here also are several choices of rout er- t o- rout er prot ocols t hat are used t o build up inform at ion used for m ult icast rout ing. There are several t o choose from : • • • •

Dist ance Vect or Mult icast Rout ing Prot ocol ( DVMRP) Dense Mode Prot ocol I ndependent Mult icast Sparse Mode Prot ocol I ndependent Mult icast Mult icast OSPF ( MOSPF)

Today, j ust about every rout er support s I P, but a rout er m ight also support addit ional prot ocols such as I PX, AppleTalk, or DECnet .

Other Router Functions Rout ers oft en are used as t he guardian at t he door, keeping unwant ed or dangerous t raffic out of part of a net work. To perform t his funct ion, a rout er is configured wit h a set of securit y rules t hat rest rict t he t raffic t hat is perm it t ed t o pass t hrough t he rout er. Rout ers som et im es are given t he j ob of priorit izing t raffic and sort ing dat agram s int o queues t hat get m ore or less preferent ial t reat m ent . For m any product s, a securit y or priorit y rule can be based on fields in t he t ransport layer ( Layer 4) header.

Legacy Router Architecture Legacy rout ers have a m ain CPU t hat carries out t he m aj or I P processing st eps one dat agram at a t im e. I n big net works, rout ing t ables are large, m aking t able searches a com put e- int ensive act ivit y. Applying a large num ber of securit y or priorit y rules can bring a legacy rout er t o it s knees. I ronically, earlier at t em pt s t o priorit ize and speed up select ed t raffic som et im es caused all t raffic t o be forwarded m ore slowly. At busy t im es, dat agram s t hat are wait ing t o be processed or t o be t ransm it t ed ont o t he next link pile up in m em ory. When m em ory usage get s close t o capacit y, som e dat agram s need t o be dum ped. This is t he m ost com m on cause of dat agram loss.

Layer 3 Switch Architecture Som et im es t he com m unicat ions t rade press t alks about Layer 3 swit ches as if t hey are exot ic devices. They are not . They sim ply are rout ers, and t hey do all of t he chores t hat have been described up t o t his point . They j ust do t heir work am azingly fast er t han t he old syst em s. New t echnology has boost ed Layer 3 swit ch t hroughput t o t ens of m illions of packet s per second. The reason t hat Layer 3 swit ches are fast is not because som eone waved a m agic wand and t urned rout ers int o hardware aut om at ons. The difference bet ween a convent ional rout er and one of t he new swit ching rout ers is like t he difference bet ween having a j ob done by a single craft sm an versus running a set of efficient assem bly lines. All of t he old t asks st ill are done, but now a lot of addit ional specialized hardware has been int roduced, and a lot of t he work is done in parallel. The change t o a parallel archit ect ure was m ade possible by t he int roduct ion of lowcost applicat ion specific int egrat ed circuit ( ASI C) chips. A Layer 3 swit ch has lot s of busy ASI C chips working away, usually one or m ore per line card. I n part icular, t he CPU- int ensive j ob of searching t he rout ing t able is perform ed in ASI Cs.

N ot e Chapt er 12, " Et hernet Bridges and Layer 2 Swit ches," not ed t hat ASI C chips also are key com ponent s of t he archit ect ure of Layer 2 swit ches.

A Layer 3 swit ch also st ill has a cent ral processor. I t does chores such as • •

•

•

Originat e out going rout er- t o- rout er m essages and process t he incom ing m essages for prot ocols such as RI P or OSPF, and updat e t he rout ing t able ( or t ables, if m ult iple t ypes of net work prot ocols are support ed) Load copies of t he rout ing t able int o ASI Cs, and updat e t hese copies as needed Provide an adm inist rat ive user int erface for syst em configurat ion and m anagem ent Run an SNMP agent process and support rem ot e m onit oring ( RMON)

Som e Layer 3 swit ches perform securit y and priorit y processing in t he m ain CPU.

Tip I f you are going t o apply securit y or priorit y rules, it is im port ant t o check t hat t hese funct ions are perform ed in ASI Cs. I f not , perform ance will be disappoint ing.

Vendors describe t he perform ance of t heir syst em s in t erm s of packet lat ency and num ber of packet s per second forwarded. Packet is a helpfully vague word: I t can st and for fram e when Layer 2 t raffic is being discussed, or any t ype of net work- layer prot ocol dat a unit ( such as an I P dat agram ) when Layer 3 is t he focus of at t ent ion. The lat ency is t he am ount of t im e t hat elapses bet ween t he t im e a syst em receives a packet and t he t im e t hat t he packet is forwarded t hrough a port . The num ber of packet s per second can vary great ly depending on t he size of t he packet s t hat are being processed. I n a second, a syst em can forward a lot m ore 64- byt e packet s t han 4000- byt e packet s. A set of num bers for various packet sizes plus a result for a feasible m ix of sizes is a lot m ore useful t han a single m easurem ent . Keep in m ind t hat t he perform ance benchm arks t hat are published by vendors usually do not include any securit y or priorit y processing.

W a r n in g Som e vendors provide only ASI C processing for I P t raffic and process all ot her rout able prot ocols in t he CPU. I f you need t o rout e m ult iple prot ocols, t he product archit ect ure will m ake a big difference in t he perform ance t hat is delivered.

Bridge/Routers

Rout ers t hat include a bridging funct ion have been around for a long t im e. The bridging funct ion was int roduced so t hat rout ers could forward t he unrout able t ypes of t raffic. A cust om er can choose t o bridge ot her rout able t raffic as well.

Layer 2/3 Switches Bridges m orphed int o Layer 2 swit ches, rout ers int o Layer 3 swit ches, and bridge/ rout ers ( brout ers) int o Layer 2/ 3 swit ches. As before, a cust om er configures which prot ocols should be rout ed and which should be bridged. The difference is t hat all t he act ual processing is done a lot fast er. Configuring a Layer 2/ 3 swit ch can be a challenge. All t he norm al rout er configurat ion has t o be done, along wit h Layer 2 chores such as VLAN configurat ion.

Route Once, Switch Many MAC addresses have m eaning only on a single local area net work. Rout ing is needed t o direct t raffic t o globally defined net work- layer addresses. However, even for a Layer 3 swit ch, rout ing st ill requires a few m icroseconds of processing. Considering t he m ult igigabit s of bandwidt h t hat are becom ing available for bot h local and wide area net working, t his is t oo m uch processing t im e. For several years, vendors have t ried t o cut t he am ount of processing perform ed by rout ers t o a m inim um using propriet ary " rout e once, swit ch m any" t echniques. The int roduct ion of a st andard int o t his arena would be welcom e. The I ETF is working on a draft for a st andard called Mult iprot ocol Label Swit ching Archit ect ure ( MPLS) . The basic idea is st raight forward: 1. Classify all of t he packet s processed by a rout er int o num bered Forwarding Equivalence Classes. 2. Assign an incom ing packet t o it s Forwarding Equivalence Class, and prefix a label t hat carries t he corresponding num eric ident ifier. 3. Swit ch t he packet t o a dist ant rout er using t he label. No ot her packet processing needs t o be perform ed. This archit ect ure has several at t ract ive feat ures: • • • •

I t can be used wit h any Layer 3 prot ocol. I t can run on t op of any t ype of physical link. Traffic can be assigned t o a Forwarding Equivalence Class based on fact ors such as priorit y or securit y, as well as dest inat ion. Then it can be handled appropriat ely. Label swit ching is a very sim ple funct ion and could be perform ed by inexpensive devices.

Figure 18.7illust rat es a net work t hat has MPLS rout ers at it s core.

Figu r e 1 8 .7 . An M PLS n e t w or k .

Layer 4 and Application-Layer Switching Figure 18.8 shows t he layered prot ocol m odel for TCP/ I P. Transport layer ( Layer 4) and applicat ion layer swit ching current ly are applied t o TCP/ I P t raffic.

Figu r e 1 8 .8 . TCP/ I P Pr ot ocol la ye r s.

Layer 3, t he net work layer, is supposed t o include all of t he funct ions needed t o get dat a from it s source t o it s dest inat ion. However, som e of t oday's swit ches peek int o t he Layer 4 t ransport - layer header or even int o applicat ion dat a during t he rout ing process. The inform at ion t hat a swit ch get s from t he upper layers is used in different ways by each vendor. All of t he procedures are propriet ary. I P carries t wo t ypes of Layer 4 t raffic. The Transm ission Cont rol Prot ocol ( TCP) is used for applicat ions such as t he World Wide Web, file t ransfer, and em ail, which require reliable sessions. The User Dat agram Prot ocol ( UDP) is used for applicat ions such as SNMP net work m anagem ent and server queries t hat t ranslat e com put er nam es t o I P addresses. A Layer 4 TCP or UDP header cont ains a num ber t hat ident ifies t he applicat ion in use. For exam ple, 80 ident ifies t he World Wide Web, 20 and 21 ident ify file t ransfer, 25 ident ifies em ail, 161 corresponds t o SNMP, and 53 corresponds t o address queries.

N ot e Thanks t o Berkeley Universit y researchers who coined t he t erm —t hese applicat ion ident ifiers are called port num bers. This t it le deserves a t op rank in t he list of bad net work t erm inology.

Net work people use t he t erm port for a hardware int erface t o a swit ch, rout er, or com put er. A Berkeley port num ber has not hing t o do wit h hardware; it is an ident ifier used t o nam e a client or server applicat ion t hat is t he endpoint of an exchange of dat a.

Filtering for Security Rout ers have been peeking int o Layer 4 for years in order t o perform t heir securit y filt ering funct ion. They frequent ly grant or deny access t o a sit e or t o a LAN based on t he source or dest inat ion applicat ion ident ified in t he t ransport - layer header. I n addit ion, by wat ching for Layer 4 m essages t hat open TCP sessions, a rout er can allow int ernal users t o open sessions t o t he out side world, while blocking any at t em pt t hat an out sider m ight m ake t o get in.

Prioritizing Traffic Layer 4 inform at ion can be used in a st raight forward way t o priorit ize t raffic. For exam ple, a user could configure a Layer 2/ 3/ 4 swit ch t o give preference t o World Wide Web TCP t raffic and UDP SNMP t raffic. The priorit ies are up t o t he cust om er. Som e product s assure t hat specified am ount s of bandwidt h are m ade available t o select ed business- crit ical applicat ions.

Load Balancing The advent of m am m ot h Web sit es com bined wit h a need t o be available 7 days per week and 24 hours per day has inspired m any ingenious product solut ions. A load balancer m akes a group of servers ( called a server farm ) look like a single server t o t he out side world. A great advant age of t his schem e is t hat if a server fails, t he workload is carried by t he rem aining servers. I t also allows for easy growt h. New servers can be rolled out as t raffic increases. Figure 18.9 shows a load balancer t hat front s an assort m ent of World Wide Web and file t ransfer servers. To t he out side world, t he ent ire sit e appears t o be a single com put er wit h a single I P address. The load balancer act ually is assigning sessions t o com put ers in t he farm based on crit eria such as

Figu r e 1 8 .9 . A loa d ba la n ce r .

• •

Applicat ion ( World Wide Web or file t ransfer) Least busy server or server wit h best response t im e

Som e product s t ake t his a st ep furt her: They perform applicat ion- layer swit ching for t he World Wide Web applicat ion. This m eans t hat t hey read t he Uniform Resource Locat or ( URL) request ed by t he client . The request t hen is rout ed based on t he URL. For exam ple, a URL t hat point s t o a " Buy" t ransact ion can be rout ed t o a powerful server t hat is dedicat ed t o t hese im port ant t ransact ions.

Summary Points • • • •

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A net work- layer prot ocol consist s of a net work addressing plan plus a set of procedures t hat are carried out t o deliver dat a t o a dest inat ion net work address. A rout ed net work can provide m ult iple pat hs t o a dest inat ion. A global net work- layer address plan is hierarchical. I P addresses t hat st art wit h t he sam e net work and subnet num bers belong t o t he sam e LAN. An I P syst em discovers t he MAC address of a dest inat ion on it s LAN by broadcast ing an ARP m essage. An I P dat agram t hat is addressed t o a dest inat ion t hat is not on t he originat or's LAN m ust be t ransm it t ed t o a rout er.

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A rout er perform s a rout ing t able lookup t o det erm ine t he port t hrough which a dat agram should be forwarded. A new fram e header and t railer m ust be wrapped around t he dat agram before it is forwarded. Except for very sm all net works, rout ers exchange inform at ion wit h one anot her t o build up t heir rout ing t ables. Som et im es rout ers perform securit y filt ering and priorit ize t raffic. Securit y and priorit y rules can be based on t he cont ent of a Layer 4 header. Layer 3 swit ches are rout ers t hat perform CPU- int ensive chores such as searching t he rout ing t able in ASI C chips. Packet lat ency is t he am ount of t im e t hat elapses bet ween t he t im e a swit ch or rout er receives a packet and t he t im e t hat t he packet is forwarded t hrough a port . Bridge/ rout er ( brout er) product s were t he predecessors of Layer 2/ 3 swit ches. Vendors have int roduced m any propriet ary " rout e once, swit ch m any" t echniques. The draft I ETF Mult iprot ocol Label Swit ching Archit ect ure ( MPLS) is an at t em pt at st andardizing " rout e once, swit ch m any" t echnology. A server farm is a set of servers t hat have t he appearance of a single server t o t he out side world. Load balancers som et im es are called Layer 4 swit ches because t hey assign t raffic t o a server in a server farm by exam ining Layer 4 headers and evaluat ing which server is least busy. Som e load balancers are called applicat ion- layer swit ches because t hey exam ine applicat ion- layer dat a t o decide which server( s) can process a part icular request .

References A good descript ion of rout er int ernals can be found in t he I ETF st andard: •

RFC 1812. " Requirem ent s for I P Version 4 Rout ers." F. Baker. 1995.

Part III: Other Old and New Technologies 19 Token Ring and FDDI Overview 20 ATM Overview 21 ATM LAN Em ulat ion 22 Fibre Channel

Chapter 19. Token Ring and FDDI Overview I BM has been connect ing syst em s across net works for a long t im e, and I BM net working always has had a dist inct ive personalit y. Much of it s early focus was on dat a ent ry and dat a display at rem ot e t erm inals. I BM equipm ent was ( and st ill is) used in banks and governm ent offices, and t he m ost im port ant design crit erion always has been predict able response t im e.

The free and easy—and unpredict able—Et hernet LAN environm ent did not m at ch t his requirem ent . I BM st art ed working on a m ore cont rollable LAN t echnology—Token Ring—in t he 1970s. A 4Mbps Token Ring specificat ion was com plet ed in t he early 1980s, was subm it t ed t o t he I EEE for st andardizat ion, and was rat ified by 1985. Operat ion at 16Mbps was st andardized in 1989. Up t o t his point , all versions ran on I BM Type 1 shielded t wist ed- pair ( STP) cabling. Unshielded t wist ed- pair ( UTP) cabling was added lat er. Perform ance got a boost when support for full- duplex t ransm ission was added in t he 1990s. This feat ure was called Dedicat ed Token Ring ( DTR) because all of t he available bandwidt h on a link could be dedicat ed t o a single st at ion connect ed t o a DTR concent rat or or swit ch. Challenged by 100Mbps and 1000Mbps Et hernet , several Token Ring vendors form ed t he High Speed Token Ring Alliance ( HSTRA) and wrot e a specificat ion for 100Mbps full- duplex Token Ring. NI C and swit ch product s followed quickly.

N ot e At t he t im e of writ ing, t he m arket for high- speed Token Ring product s is t epid, and t he sponsorship of t he High Speed Token Ring Alliance has shrunk t o t wo com panies: I BM and Madge. Convent ional 16Mbps half- duplex connect ions and 32Mbps full- duplex links appear t o offer sufficient bandwidt h for t he deskt op connect ions at m ost Token Ring sit es. When higher- speed deskt op, server, or infrast ruct ure connect ions are needed, som e organizat ions opt for 100Mbps Token Ring swit ches, while ot hers convert t o 100Mbps and 1000Mbps Et hernet or ATM t echnology. Token Ring cont inues t o be used at m any sit es, but it is not winning fresh cust om ers. The t echnology is in it s sunset years.

Features of Classic Half-Duplex Token Ring A classic Token Ring consist s of a set of st at ions connect ed in a ring configurat ion. The st at ions share t he bandwidt h provided by t he ring. The physical cabling of a Token Ring looks like a ring of st ars, as shown at t he t op of Figure 19.1. Bat ches of st at ions are cabled t o concent rat ors in wiring closet s, and t he concent rat ors are connect ed t o one anot her. The sect ion of cable bet ween a concent rat or and a st at ion is called a lobe. Trunk cables run bet ween concent rat ors.

Figu r e 1 9 .1 . Tok e n Rin g t opology.

I t is easier t o visualize som e aspect s of t he Token Ring prot ocols by looking at a st ret ched- out ring, like t he one shown at t he bot t om of Figure 19.1. Mult iple rings can be int erconnect ed by source rout e bridges or t ransparent bridges. A classic Token Ring operat es at 4Mbps or 16Mbps. Shielded t wist ed- pair or unshielded t wist ed- pair cabling is used for t he st at ion cables. Copper or fiber opt ic segm ent s can be used for t he links bet ween concent rat ors, which are called t runks. Token Ring fram es can be big—as large as 18,000 byt es for 16Mbps Token Ring. The use of fram es whose size is 4K or m ore gives Token Ring a definit e advant age in t hroughput over Et hernet . The classical Token Ring prot ocol is half- duplex because at m ost one st at ion on a ring can t ransm it at any given t im e. Throughput can be kept at a high level on a half- duplex Token Ring because no collisions occur.

Basic Ring Operation The idea behind Token Ring operat ion is st raight forward. A special prot ocol dat a unit called a t oken circulat es from st at ion t o st at ion around t he ring. A st at ion t hat has dat a t o send capt ures ( absorbs) t he t oken and t ransm it s a fram e followed by som e filler bit s called an int erfram e gap. The st at ion can cont inue t o t ransm it fram es for a period of t im e. On com plet ion, it forwards a fresh t oken t o t he next st at ion. A fram e is forwarded t hrough each act ive NI C connect ed t o t he ring. Each NI C ot her t han t he originat or repeat s t he fram e bit s ont o t he next sect ion of cable. A st at ion wat ches t o see whet her it s own MAC address ( or a broadcast address) appears in t he dest inat ion address field. I f so, t he dest inat ion copies t he ent ire fram e int o m em ory, but also cont inues t o repeat t he bit s ont o t he next segm ent . Event ually, a fram e ret urns t o it s originat or. The originat or does not repeat t he bit s, and t his rem oves t he fram e from t he ring. When it s last fram e has been rem oved, t he originat or t ransm it s a t oken t o t he next st at ion. Figure 19.2 illust rat es t he procedure. The direct ion in which dat a is t ransm it t ed is called t he downst ream direct ion. A st at ion receives bit s from it s upst ream neighbor on t he ring and sends bit s t o it s downst ream neighbor.

Figu r e 1 9 .2 . Ba sic Tok e n Rin g ope r a t ion .

Opt ionally, som e ext ra t hroughput can be squeezed out of a 16Mbps Token Ring by using early t oken release. I nst ead of wait ing for it s last fram e t o ret urn, t he sender t ransm it s a t oken right aft er t he int erfram e gap t hat follows it s final fram e.

N ot e Token Ring int erfram e gaps can be very short . One byt e is t he m inim um allowed at 4Mbps, alt hough 2 byt es are recom m ended. Five byt es is t he m inim um at 16Mbps.

Alt hough t he basic idea is very sim ple, quit e a lot is involved in keeping t he ball rolling around a Token Ring. For exam ple, a st at ion m ight crash aft er t ransm it t ing a fram e. I t will fail t o absorb it s fram e and generat e a fresh t oken. A t oken m ight be lost due t o bit errors. To solve t hese problem s, one st at ion act s as a ring m onit or and perform s recovery procedures. However, t hat raises t he quest ion of what t o do when t he m onit or crashes. The sect ions t hat follow provide t he solut ion. A st at ion needs t o j oin t he ring gracefully when it powers up, and leave it gracefully when it powers down. Unlike Et hernet , t he operat ional param et ers are not et ched in st one, so a syst em can get param et ers from a configurat ion server at st art up t im e.

Token Ring designers want ed t o im prove LAN reliabilit y by m aking it easy t o pinpoint t rouble spot s, so error diagnost ic and report ing capabilit ies were added. On an Et hernet , t here is one t ype of m essage: an inform at ion fram e. Many addit ional fram e t ypes are needed on a Token Ring t o support t he various ring m aint enance funct ions. This has led t o t erm inology t hat is different from t hat used in t he Et hernet world: •

•

The fram es t hat cont ain user dat a are called LLC fram es. As you m ight expect , t hese fram es cont ain an LLC header. The fram es t hat carry various t ypes of Token Ring prot ocol m essages are called MAC fram es.

Active Monitor and Standby Monitor Roles A st at ion on a ring called t he act ive m onit or wat ches over t he t oken prot ocol. I f a t oken has been lost , t he act ive m onit or generat es a new one. I f a fram e repeat edly circles t he ring, t he m onit or rem oves it . I n a classic Token Ring, t he act ive m onit or's clock is t he ring's m ast er clock. Ot her st at ions adj ust t heir bit t im ing t o t he t im ing of incom ing bit s.

N ot e Many concent rat or product s built during t he last few years act ually repeat signals and provide t im ing t o t he st at ions on a Token Ring.

The act ive m onit or is chosen by an elect ion prot ocol. I t periodically announces t hat it is st ill doing it s j ob by sending Act ive Monit or Present ( AMP) fram es. The ot her st at ions at t ached t o t he ring act as st andby m onit ors. I f t he " Act ive Monit or Present " m essages cease, t he st andby m onit ors part icipat e in an elect ion process and choose a new act ive m onit or.

Functions Performed by the Active Monitor The first t hing a newly elect ed act ive m onit or st at ion does is clean old dat a out of t he ring by sending a Ring Purge fram e. A Ring Purge fram e is like a yell t hat says "Quiet down! " Each syst em t hat receives t he Ring Purge fram e reset s all of it s t im ers and becom es quiet , sim ply repeat ing bit s t hat it is sent . Sending a Ring Purge fram e around t he ring is called " purging t he ring." Aft er t he Ring Purge fram e has circled t he ring, t he act ive m onit or rem oves it and st art s norm al operat ion by t ransm it t ing a t oken. The act ive m onit or det ect s t he fact t hat t he t oken has been lost when a t im er expires. The act ive m onit or rest ores norm alcy by purging t he ring and t ransm it t ing a fresh t oken.

A Token Ring fram e header includes a m onit or bit t hat init ially is set t o 0. When a fram e passes t hrough t he act ive m onit or's NI C, t his bit is set t o 1. This enables t he act ive m onit or t o det ect whet her an incom ing fram e is com ing around for t he second t im e. I f so, t he act ive m onit or rem oves t he fram e from t he ring, purges t he ring, and init iat es a new t oken.

The Claim Token Election Process St at ions det ect t hat t here is no act ive m onit or via a t im eout . A st at ion t hat det ect s t hat t he act ive m onit or is not funct ioning properly part icipat es in a m onit or elect ion by t ransm it t ing a special fram e called a Claim Token fram e. The st at ion, which is called a t ransm it t er, repeat s it s Claim Token fram e periodically, sending fill bet ween t he fram es. Several st at ions m ight det ect t he problem and becom e t ransm it t ers at t he sam e t im e. Furt herm ore, ot her st at ions t hat find out what is going on by receiving a Claim Token fram e m ight have been configured t o cont end t he elect ion by becom ing t ransm it t ers. They will st art t o t ransm it t heir own Claim Token fram es. Any rem aining st at ions st ay out of t he fray, repeat ing t he Claim Tokens fram es t hat t hey receive. The t ransm it t er wit h t he highest MAC address wins t he elect ion. The procedure is st raight forward: A t ransm it t er t hat receives a Claim Token fram e originat ing from a st at ion wit h a bigger MAC address is knocked out of t he race and becom es a repeat er. Event ually, only one t ransm it t er is left . When t hat t ransm it t er st art s t o receive it s own Claim Token fram es, it knows t hat it has won.

The Beacon Process The beacon process ident ifies t he locat ion of a serious fault and init iat es som e act ions t hat m ight heal t he fault . A st at ion st art s t o send periodic Beacon MAC fram es under t he following circum st ances: • • •

A loss of signal occurs, and not hing is arriving from it s upst ream link. Bit s are arriving, but t he dat a does not conform t o t he expect ed prot ocol. For exam ple, t he dat a m ight be a long st ream of fill. The upst ream st at ion has been sending a st ream of Claim Tokens for t oo long.

Ot her st at ions repeat t he Beacon fram es around t he ring. The Beacon fram es cont ain t he cause of t he beacon and t he MAC address of t he originat or's upst ream neighbor. I f t he upst ream neighbor receives several Beacons point ing t o it s address, it rem oves it self from t he ring and t est s it s NI C t o see if t here is a problem . I f t he t ransm it t er receives it s own Beacon fram e, t he fram es are being delivered and t he problem has been resolved. The st at ion t hen init iat es t he claim t oken process t o elect an act ive m onit or. On t he ot her hand, if t he problem has not been resolved when a t im eout expires, t he st at ion t hat st art ed t he beaconing rem oves it self from t he ring and t est s it s NI C. A failure t hat is caused by a fault y link or a NI C t hat cannot recognize t hat it should rem ove it self m ust be solved by adm inist rat or act ion. Monit oring t he beacon process enables an adm inist rat or t o isolat e t he fault dom ain, which consist s of t hree part s:

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The st at ion downst ream t o t he fault , which report s t he problem The st at ion upst ream t o t he fault The equipm ent bet ween t he upst ream and downst ream syst em s ( cables, concent rat ors, or repeat ers)

The Neighbor Notification Process A st at ion m ust know t he MAC address of it s upst ream neighbor t o describe a fault dom ain. I n t he beaconing process described previously, t he beacon t ransm it t er included t he MAC address of t he upst ream neighbor in it s Beacon fram es. The neighbor not ificat ion process enables each st at ion t o discover t he address of it s upst ream neighbor. The act ive m onit or periodically st art s a neighbor not ificat ion process by broadcast ing an Act ive Monit or Present ( AMP) fram e. The process uses som e flag bit s in t he t railers of Token Ring fram es. These are called " A" ( for address recognized) bit s and " C" ( for fram e- copied) bit s and are set t o 0 when a fram e is t ransm it t ed by it s originat or. The first st at ion t o receive t he fram e does t he following: • •

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Set s t he A and C bit s t o 1, and repeat s t he rest of t he Act ive Monit or Present fram e unchanged. ( The rem aining st at ions will repeat t he Act ive Monit or Present fram e around t he ring.) St ores t he source MAC address in t he fram e, which is t he address of it s upst ream neighbor. Wait s for a brief t im eout period and broadcast s a St andby Monit or Present ( SMP) fram e.

On receiving a St andby Monit or Present fram e whose A and C bit s are 0, a st at ion does t he following: •

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Set s t he A and C bit s t o 1, and repeat s t he rest of t he St andby Monit or Present fram e unchanged St ores t he source MAC address in t he fram e, which is t he address of it s upst ream neighbor Wait s for a brief t im eout period and broadcast s it s own St andby Monit or Present fram e

When t he act ive m onit or receives a St andby Monit or Present fram e whose A and C bit s are 0, t he cycle is com plet e. Every st at ion now knows t he address of it s upst ream neighbor.

Special Servers A ring opt ionally can include t hree servers: a Ring Param et er Server ( RPS) , Configurat ion Report Server ( CRS) , and a Ring Error Monit or ( REM) . •

An init ializing st at ion sends a request for param et er values t o a Ring Param et er Server. A st at ion can operat e wit h default param et ers if no response t o a request for param et ers arrives.

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A st at ion sends a m essage t o a Configurat ion Report Server when t he st at ion det ect s t hat t he MAC address of it s upst ream neighbor has changed. The Configurat ion Report Server can send a m essage t o a st at ion t elling t he st at ion t o change it s param et ers, can ask a st at ion t o rem ove it self from t he ring, and can ask a st at ion t o provide st at us inform at ion. A st at ion periodically report s error count s t o a Ring Error Monit or.

St at ions send m essages t o t he servers by addressing t he fram es t o well- known m ult icast funct ional addresses. For t his reason, t hese servers also are known as t he Token Ring funct ional servers. Funct ional servers can be spread across different syst em s or can all be resident at one st at ion. However, a nat ural place for t he servers is at a sm art concent rat or or a bridge ( swit ch) .

Hard and Soft Errors Som e fault s disable a ring, while ot hers are t ransient , causing dat a t o be corrupt ed for a short t im e. The Token Ring prot ocol form ally charact erizes t wo t ypes of errors: •

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Hard errors are defined as fault s t hat prevent fram es and/ or t okens from circulat ing around t he ring. Soft errors are defined as fault s t hat cause dat a corrupt ion but t hat do not prevent fram es and t okens from circulat ing around t he ring.

Hard errors are dealt wit h im m ediat ely by t he beacon process. Soft errors are count ed and report ed. Each st at ion m aint ains a set of soft error count ers and occasionally report s t heir values t o t he Ring Error Monit or.

Joining a Ring When a st at ion is inact ive, it s lobe cable is bypassed by t he concent rat or. A st at ion m ust act ively perform an insert ion procedure before it can part icipat e in t he ring. Five st eps are involved: 1. Te st t he ca ble —The st at ion t ransm it s a series of Lobe Media Test fram es ont o it s cable. These fram es are sent t o t he null address, X'00- 00- 00- 00- 0000, and are wrapped back t o t he st at ion by t he concent rat or. This enables t he st at ion t o find out whet her it s cable is fault y. I f t he t est is successful, t he st at ion places a DC current on it s cable t hat causes t he concent rat or t o open it s relay and insert t he st at ion on t he ring.

N ot e This is called a phant om signal because it does not int erfere wit h t he t ransm ission of bit s on t he sam e wire.

2. M a k e su r e t h a t t he a ct ive m on it or is pr e se n t —The st at ion set s a t im er and wat ches for evidence t hat t here is an act ive m onit or on t he ring. Specifically, it looks for an Act ive Monit or Present , St andby Monit or Present , or Ring Purge fram e. ( I f none arrives in t he t im eout period, t he st at ion init iat es an elect ion by sending Claim Token fram es.) 3. Ch e ck for du plica t e a ddr e ss—The st at ion m akes sure t hat no ot her st at ion on t he ring has t he sam e address by sending a Duplicat e Address Test fram e. I f t here is a duplicat e, t he st at ion rem oves it self from t he ring.

N ot e For Token Ring net works, locally adm inist ered MAC addresses oft en are preferred t o t he globally unique m anufact urer- assigned MAC addresses. Perform ing a duplicat e address t est is an especially wise precaut ion when locally adm inist ered addresses are used.

4. Le a r n a ddr e ss of upst r e a m n e igh bor —Aft er t he st at ion has verified t hat it s address is unique on t he ring, it st art s t o part icipat e in t he nearest upst ream neighborhood not ificat ion processes. 5. Ge t configu r a t ion pa r a m e t e r s—The st at ion m ight t ransm it a Request I nit ializat ion fram e t o t he Ring Param et er Server funct ional m ult icast address. I f t here is a Ring Param et er Server, it responds wit h an I nit ialize St at ion or Change Param et ers fram e. This funct ion rarely is used. The lobe m edia t est m ent ioned earlier is used any t im e t hat t he st at ion needs t o t est it s lobe pat h as part of an error recovery process. Rem oval from t he ring is a lot sim pler: The st at ion st ops applying power t o it s phant om circuit s, which causes t he st at ion t o be bypassed.

Physical Components for Half-Duplex Token Ring Figure 19.3 illust rat es som e of t he com ponent s of a Token Ring. A Token Ring st at ion m ust be hooked up t o a concent rat or t o com m unicat e; t wo st at ions cannot be direct ly connect ed t o one anot her. A classic st at ion is connect ed t o a concent rat or by t wo t wist ed pairs.

Figu r e 1 9 .3 . A st a n da lon e con ce n t r a t or .

The st at ion receives dat a on one pair and t ransm it s on t he ot her. The t wo wire pairs t hat m ake up t he lobe cable connect t o a t runk coupling unit ( TCU) in t he concent rat or. When a st at ion is inact ive, t he TCU bypasses t he st at ion's cables. When a st at ion t ransm it s a phant om power current , t he TCU closes a relay so t hat ring dat a can flow t o and from t he st at ion. Originally, shielded t wist ed- pair cable was used. The cable t ypically was connect ed t o a st at ion by a 9- pin D- connect or, and t o a wall out let by a special I BM connect or.

N ot e The I BM connect ors were well designed, durable, reliable, and virt ually " idiot - proof."

Today, t he use of unshielded t wist ed- pair cable and RJ- 45 connect ors at bot h t he st at ion and wall out let is com m on. Adj acent concent rat ors are connect ed by a t runk cable. A pair of opt ical fibers can be used for a t runk if a very long cable run is needed. Concent rat ors are known by a lot of nam es. I BM has called it s concent rat ors Mult ist at ion Access Unit s ( abbreviat ed as MAU or MSAU) and Cont rolled Access Unit s ( CAUs) . Som e vendors sim ply call t heir product s Token Ring hubs. What ever t he nam e m ight be, t oday t he m ain difference bet ween different concent rat or product s is whet her t he product 's port s are passive or act ive, and how sm art t he product s are: • • •

Passive port s j ust allow signals t o pass t hrough. This m eans t hat a single sect ion of cable ext ends from a st at ion NI C, t hrough t he TCU, and t o t he NI C of t he next st at ion on t he ring. Act ive port s ret im e and repeat t he signal. Sm art port s ret im e t he signal, repeat t he signal, and perform diagnost ics, cut t ing off st at ions whose NI Cs t ry t o ent er t he ring at t he wrong speed or t hat m isbehave in som e ot her way.

Sm art concent rat ors support SNMP m anagem ent variables and SNMP rem ot e m onit oring ( RMON) . RMON m akes an enorm ous am ount of inform at ion on LAN act ivit y available t o a net work adm inist rat or. Som e concent rat or product s even support t roubleshoot ing via packet capt ure. Figure 19.3 shows a st andalone concent rat or. I nt ernal wiring com plet es t he ring. A concent rat or has ring in and ring out port s t hat are used t o connect t o ot her concent rat ors. As shown in Figure 19.4, cabling runs from t he ring out port of t he first concent rat or t o t he ring in port of t he second. Several concent rat ors can be chained t oget her using ring out and ring in port s.

Figu r e 1 9 .4 . Tok e n Rin g com pon e n t s.

I EEE 802.5 suggest s a m axim um of 250 st at ions per half- duplex ring. I BM proposed a lim it of 260. The act ual num ber t hat can be support ed depends on t he qualit y of t he cable and equipm ent . For exam ple, far fewer st at ions can be support ed when Cat egory 3 unshielded t wist ed- pair cable is used. The m axim um dist ance bet ween a st at ion and a concent rat or also I t depends on whet her Cat egory 3 UTP, Cat egory 5 UTP, or STP is whet her t he hub port s repeat signals. The cable dist ance bet ween hub can be a lot bigger when a hub wit h act ive, repeat ing port s is

is quit e variable: used, and on a st at ion and a used. Typical

t wist ed- pair dist ances for 16Mbps repeat ed port s are 100 m et ers for Cat egory 3 unshielded t wist ed–pair cable, 225 m et ers for Cat egory 5 cable, and 400 m et ers for I BM Type 1 shielded- t wist ed pair cable. ( Great er dist ances are possible at 4Mbps.) Sim ilar lengt h rest rict ions apply t o t wist ed- pair t runk cables. The lengt h of a t runk cable can range up t o 2000 m et ers when opt ical fiber is used. Modern concent rat ors are very sophist icat ed. Up- t o- dat e product s build an ASI C int o each port , giving t he port t he int elligence t hat enables it t o •

•

Sense t he speed at which an at t ached stat ion is operat ing, and refuse ent ry t o t he ring if t he speed is not com pat ible I dent ify and bypass lobes t hat are responsible for a fault

Classic Token Ring Protocol Elements The sect ions t hat follow out line t he m aj or feat ures of t he classic Token Ring prot ocol, including • • • • • • • •

Token Ring MAC addresses The Token Ring form at LLC fram e form at s MAC fram e form at s Param et ers Rout ing inform at ion Tim ers Types of MAC prot ocol fram es

Addresses The I EEE LAN addresses described in Chapt er 2, " LAN MAC Addresses," are used in Token Rings. However, t hese addresses are used in a special m anner: • •

Each address byt e is writ t en in reverse order. The first bit of a dest inat ion address indicat es whet her it is an individual address ( 0) or a group address ( 1) . The second bit indicat es whet her it is a universal ( 0) or local ( 1) address. The first bit of a source address is set t o 1 t o indicat e t hat t hat a rout ing inform at ion field ( RI F) follows.

The broadcast address, X'FF- FF- FF- FF- FF- FF, denot es all st at ions on t he LAN and is used t o broadcast an LLC fram e. A second address, X'C0- 00- 00- FF- FF- FF, is used as t he broadcast address for Token Ring MAC fram es, which carry t he m essages ( such as Act ive Monit or Present or Beacon fram es) t hat operat e t he prot ocol. St at ions use t he funct ional addresses m ent ioned in Chapt er 2 t o send prot ocol m essages t o t he Token Ring funct ional servers and t o ot her syst em s t hat play a special role in t he LAN. Funct ional addresses st art wit h t he bit pat t ern: 1 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 Frequent ly used funct ional addresses are reviewed in Table 19.1.

Ta ble 1 9 .1 . Tok e n Rin g Fu n ct ion a l M AC Addr e sse s Fu n ct ion N a m e

M AC Addr e ss

Act ive Monit or

X'C0- 00- 00- 00- 00- 01

Ring Param et er Server ( RPS)

X'C0- 00- 00- 00- 00- 02

Ring Error Monit or ( REM)

X'C0- 00- 00- 00- 00- 08

Configurat ion Report Server ( CRS)

X'C0- 00- 00- 00- 00- 10

Source Rout e Bridge

X'C0- 00- 00- 00- 01- 00

Token Format Figure 19.5 displays t he form at of t he t oken, which is only 3 byt es long. The st art ing and ending delim it ers ( which also appear in fram es) m ake it easy t o det ect where t he t oken ( or a fram e) begins and ends. The delim it ers include special sym bols ( called J and K) whose encoding on t he cable is different from t he encoding of 0s and 1s. ( Appendix A, "Physical- Layer Encodings for Et hernet , Fibre Channel, FDDI , and Token Ring," has t he det ails.) The I and E bit s are explained lat er, in t he sect ion called " Ending Delim it er and Fram e St at us Fields."

Figur e 1 9 .5 . Tok e n for m a t .

Th e Acce ss Con t r ol Fie ld The access cont rol field t hat appears in t he t oken also appears in fram es. The t oken bit is 0 in a t oken and 1 in a fram e. The m onit or bit st art s out as 0 and is changed t o

1 when a fram e reaches t he act ive m onit or NI C. I f a fram e wit h t he m onit or bit equal t o 1 arrives at t he m onit or, t he act ive m onit or knows t hat t he fram e has cycled all t he way around t he ring. I f t his happens, t he act ive m onit or purges t he ring. The access cont rol field cont ains 3 priorit y bit s. The Token Ring prot ocol support s eight levels of priorit y ( num bered from 0 t o 7) . When a st at ion receives t he t oken, it is allowed t o send only fram es whose priorit y level is at least t hat of t he t oken. A st at ion wit h high priorit y fram es t o send set s t he reservat ion bit s t o t he desired priorit y level. This causes t he next t oken t hat is released t o be set t o t his request ed priorit y.

General Frame Format Bot h user dat a ( LLC) fram es and Token Ring prot ocol ( MAC) fram es have t he form at shown in Figure 19.6.

Figu r e 1 9 .6 . Tok e n Rin g fr a m e for m a t .

Fr a m e Con t r ol Fie ld The first 2 bit s of t he fram e cont rol field in Figure 19.7 dist inguish LLC fram es ( 01) from MAC prot ocol fram es ( 00) . The m eaning of t he rem aining bit s depends on t his set t ing. For an LLC fram e, t he next 3 bit s are reserved and t he last 3 bit s are a user priorit y t hat has been passed down by a higher- layer prot ocol. For a MAC prot ocol fram e, t he last 6 bit s depend on t he t ype of prot ocol funct ion t hat t he fram e perform s.

Figu r e 1 9 .7 . For m a t of t h e fr a m e con t r ol fie ld.

Rou t in g I n for m a t ion Fie ld The rout ing inform at ion field was discussed in Chapt er 17, " Source- Rout ing, Translat ional, and Wide Area Bridges," but it is reviewed here wit h a few m ore det ails filled in. A source address m ust always be an individual ( unicast ) address, so t he first bit in a source address is set t o 1 t o signal whet her t he fram e includes a rout ing inform at ion field ( RI F) . A rout ing inform at ion field lays out a pat h from t he source t o t he dest inat ion t hat passes across a series of rings and bridges. As illust rat ed in Figure 19.8, t he first 2 byt es of t he RI F m ake up t he rout ing cont rol port ion, which includes

Figu r e 1 9 .8 . For m a t of a r ou t ing in for m a t ion fie ld.

• • •

•

The rout ing t ype. 0XX m eans t hat a specific rout e is provided in t he rout ing descript or fields. 10X is an all- rout es explorer. 11X is a Spanning Tree explorer. ( Bit s labeled " X" can be eit her 0 or 1.) The lengt h of t he ent ire RI F, including t he rout ing cont rol field. The direct ion bit , which indicat es whet her t he rout e should be followed in a forward ( 0) or reverse ( 1) order. The largest fram e field, which cont ains a code t hat announces t he size of t he biggest inform at ion field t hat can be carried along t he specified pat h.

The final rout ing cont rol bit is reserved.

N ot e As not ed in Chapt er 16, " VLANs and Fram e Priorit y," and Chapt er 17, in an em bedded rout ing inform at ion field ( E- RI F) , t he final bit is used as a noncanonical form at ident ifier ( NCFI ) .

The rem ainder of t he rout ing inform at ion field cont ains a list of rout ing descript ors. Each rout ing descript or is m ade up of a 12- bit ring ident ifier and 4- bit bridge num ber. A RI F can be 2 t o 30 byt es in lengt h.

En din g D e lim it e r a n d Fr a m e St a t u s Fie lds The ending delim it er, which follows t he fram e check sequence field, is displayed at t he t op of Figure 19.9.

Figu r e 1 9 .9 . Th e e n din g de lim it e r a n d fr a m e st a t u s fie lds.

When t he t oken holder sends m ult iple fram es, t he int erm ediat e fram e bit ( t he I - bit , which is second from t he right ) is set t o 1 in all of t he fram es except t he last . The bit is set t o 0 in t he last ( or only) fram e. The error bit is set t o 0 when a t oken or fram e is t ransm it t ed. A st at ion set s t he value t o 1 if it has det ect ed an error ( such as an incorrect fram e check sequence) while processing t he fram e. The fram e st at us field, which is shown at t he bot t om of Figure 19.9, is locat ed aft er t he ending delim it er. The field includes t he useful A and C flag bit s. These bit s are set t o 0 by t he t ransm it t ing st at ion. • •

The " A" st ands for address recognized. A st at ion on t he ring t hat recognizes t he dest inat ion address as eit her it s own unicast address or as t he address of a group t o which it belongs set s bot h A bit s t o 1. The " C" st ands for copied. I f a st at ion on t he ring copies t he fram e int o it s m em ory, it set s bot h of t he C bit s t o 1.

The A and C bit s are duplicat ed because t hey are not covered by t he fram e check sequence calculat ion. The fact t hat a pair has consist ent values gives som e assurance t hat t he bit s have not been corrupt ed.

The rem aining bit s in t he fram e st at us field are not used and are set t o 0.

Abort Sequence Som et im es a t ransm it t ing st at ion experiences an error condit ion and wishes t o break off before t he ent ire fram e has been sent . This is done by t ransm it t ing t he 2- byt e abort sequence t hat is shown in Figure 19.10. The unexpect ed arrival of back- t oback st art and end delim it ers warns a receiving st at ion t hat t he previous fram e was not valid.

Figu r e 1 9 .1 0 . Th e a bor t se qu e n ce .

Token Ring MAC Protocol Frames Figure 19.11 shows t he form at of t he inform at ion field in a MAC prot ocol fram e.

Figu r e 1 9 .1 1 . Tok e n Rin g M AC pr ot ocol fr a m e in for m a t ion fie ld.

The cont ent is a m essage t hat form ally is called a vect or. A vect or is int roduced by lengt h, class, and ident ifier fields. Two 4- bit codes in t he class field ident ify t he source and dest inat ion syst em t ype ( such as a pair of st at ions or a st at ion and a Ring Error Monit or server) . The vect or ident ifier num ber indicat es t he t ype of m essage t hat is enclosed

Furt her param et ers are carried in subvect or fields in t he vect or. The param et ers t hat are included depend on t he m essage t ype. Table 19.2 list s t ypes of m essages and t heir ident ifiers.

Ta ble 1 9 .2 . Type s of M AC Fr a m e Ve ct or s N u m e r ic I de n t ifie r

Nam e

D e scr ipt ion

X'00

Response

Sent t o acknowledge receipt of a MAC fram e or t o report an error in a received MAC fram e.

X'02

Beacon

Signals t hat dat a has st opped flowing from t he upst ream st at ion.

X'03

Claim Token

Used t o elect an act ive m onit or.

X'04

Ring Purge

Transm it t ed by t he act ive m onit or t o clear t he ring before generat ing a new t oken.

X'05

Act ive Monit or Present

Transm it t ed by t he act ive m onit or t o verify it s act ive st at us and st art t he neighbor not ificat ion process.

X'06

St andby Monit or Present

Transm it t ed by st at ions during t he neighbor not ificat ion process.

X'07

Duplicat e Address Test

Transm it t ed by a st at ion t hat want s t o j oin t he ring. The m essage is used t o check whet her t he st at ion has a duplicat e address.

X'08

Lobe Media Test

Transm it t ed t o check t he cable before at t em pt ing t o j oin t he ring. I t is sent t o an allzeroes address.

X'0B

Rem ove Ring St at ion

Sent t o a st at ion by t he Configurat ion Report Server t o force t he st at ion's rem oval from t he ring.

X'0C

Change Param et ers

Sent t o a st at ion by t he Configurat ion Report Server t o set t he st at ion's operat ing param et ers.

X'0D

I nit ialize St at ion

Sent t o a st at ion by t he Ring Param et er Server t o init ialize t he st at ion's operat ing param et ers.

X'0E for

Request St at ion

Sent by a m anagem ent ent it y t o ask

/

Addresses

t he list of addresses recognized by t he st at ion.

X'0F

Request St at ion St at e

Sent by a m anagem ent ent it y t o ask for a st at ion's operat ional st at e inform at ion.

X'10

Request St at ion At t achm ent s

Sent by a server or net work m anagem ent syst em t o ask what funct ions are act ive at t he st at ion.

Ta ble 1 9 .2 . Type s of M AC Fr a m e Ve ct or s N u m e r ic I de n t ifie r

Nam e

D e scr ipt ion

X'20

Request I nit ializat ion

Sent t o t he Ring Param et er Server as part of t he j oin process.

X'22

Report St at ion Addresses

Responds t o Request St at ion Addresses.

X'23

Report St at ion St at e

Responds t o Request St at ion St at e.

X'24

Report St at ion At t achm ent s

Responds t o Request St at ion At t achm ent s.

X'25

Report New Act ive Monit or

Sent t o t he Configurat ion Report Server by a st at ion t hat has becom e t he act ive m onit or.

X'26

Report SUA Change

Sent t o t he Configurat ion Report Server by a st at ion whose st ored upst ream neighbor's address has changed.

X'27

Report Neighbor Not ificat ion I ncom plet e

Sent t o t he Ring Error Monit or by t he act ive m onit or t o report an error in t he neighbor not ificat ion process.

X'28

Report Act ive Monit or error

Sent t o t he Ring Error Monit or by an act ive m onit or if it det ect s anot her act ive m onit or or if anot her st at ion init iat es t he claim t oken process.

X'29

Report Error

Sent t o t he Ring Error Monit or by a st at ion t hat is report ing error count s.

Timers The Token Ring prot ocol relies on a lot of t im ers, which are described briefly in Table 19.3. The full det ails are in Sect ion 3.4 of t he I EEE 802.5 specificat ion.

Ta ble 1 9 .3 . Tok e n Rin g Tim e r s Type of Tim e r

D e scr ipt ion

Act ive Monit or Used by t he act ive m onit or. Est ablishes t he int erval bet ween Act ive Monit or Present MAC fram es. Beacon Repeat

Set whenever a st at ion receives a Beacon MAC fram e. When it expires, t he st at ion can st art t o t ransm it Claim Token MAC fram es.

Beacon Transm it

Est ablishes t he lengt h of t im e t hat a beaconing st at ion should t ransm it Beacon MAC fram es.

Ta ble 1 9 .3 . Tok e n Rin g Tim e r s Type of Tim e r

D e scr ipt ion

Claim Token

Set when t he st at ion st art s t o part icipat e in t he claim t oken process. I f t he t im er expires, it indicat es t hat t he process has failed.

Error Report

Cont rols error report ing. The t im er is set when an init ial error occurs. Errors t end t o com e in burst s, and t he st at ion collect s errors unt il t he t im er expires. The st at ion t hen report s all of it s error count ers in a Report Error MAC fram e and reset s t he error count ers t o 0.

I nsert Delay

A t im er t hat cont rols reinsert ion. Aft er a beaconing st at ion has rem oved it self from t he ring, it perform s a self- t est and rej oins t he ring. This t im er m akes sure t hat t he st at ion refrains from sending dat a for a period t o allow t im e for reinsert ion int o t he ring t o com plet e.

Join Ring

A t im eout on t he j oin process. Enables t he st at ion t o det ect whet her a j oin has failed.

No Token

A t im eout t hat indicat es t hat a t oken has not appeared wit hin t he expect ed t im e period.

Queue PDU

Tim e allowed for queuing an appropriat e fram e aft er an event such as receiving an Act ive Monit or Present or Claim Token MAC fram e.

Rem ove Hold

Tim e period aft er insert ion when t he st at ion cannot request rem oval. This assures t hat insert ion will com plet e before a rem oval is request ed.

Rem ove Wait

Tim e period aft er a rem oval request during which t he st at ion m ust cont inue t o repeat fram es.

Request I nit ialize

Tim eout used t o det ect t hat a response t o t he last Request I nit ializat ion MAC fram e was not received.

Ret urn t o Repeat

Tim eout t hat t riggers a ret urn t o repeat st at e aft er a procedure during which a st at ion t em porarily st ops repeat ing fram es.

Ring Purge

Tim eout used t o det ect a failed ring purge.

Signal Loss

Tim eout on t he longest valid period during which no signals m ay arrive. Expirat ion det ect s a physical failure.

St andby Monit or

Tim eout on receiving an Act ive Monit or Present MAC fram e. I ndicat es t hat t he act ive m onit or is dead.

Valid Transm ission

Tim eout used by t he act ive m onit or t o det ect t he absence of fram es or t okens.

Wire Fault Delay

Tim eout t hat allows a j oin or reinsert ion t o com plet e. This prevent s a false wire fault from being diagnosed.

Wire Fault

Defines a sam pling period for filt ering wire fault .

Token Ring Frame Traces The sect ions t hat follow display Net work Associat es Sniffer t races of several t ypes of Token Ring fram es: • • • • • • •

An LLC fram e carrying an I P dat agram An all- rout es explorer fram e An Act ive Monit or Present fram e A St andby Monit or Present fram e A Claim Token fram e A Ring Purge fram e A Report Soft Error fram e

Sim ple LLC D a t a Fr a m e List ing 19.1 displays a sim ple LLC inform at ion fram e t hat carries an I P dat agram . Only t he dat a link and LLC headers are shown. The fram e st at us field cont ains t he address recognized ( A) and fram e copied ( c) bit s. The first line of t he t race report s t hat t his fram e was observed before it had been received and repeat ed by it s dest inat ion, so t hese bit s st ill are 0. The priorit y and reservat ion bit s in t he access cont rol field are set t o 0, which is t he norm . Use of t he priorit y feat ure is fairly rare. The fram e cont rol field indicat es t hat t his is an LLC fram e. The LLC header is X'AAAA- 03, indicat ing t hat a SNAP subheader follows. The SNAP indicat es t hat t he inform at ion field carries an I P dat agram .

List in g 1 9 .1 A Sim ple LLC Tok e n Rin g Fr a m e DLC: FS: Addr recognized indicators: 00, Frame copied indicators: 00 DLC: AC: Frame priority 0, Reservation priority 0, Monitor count 0 DLC: FC: LLC frame, DLC: Destination = Station 3Com2 063841 DLC: Source = Station 3Com2 115176 DLC: LLC: ----- LLC Header ----LLC: LLC: DSAP Address = AA, DSAP IG Bit = 00 (Individual Address) LLC: SSAP Address = AA, SSAP CR Bit = 00 (Command) LLC: Unnumbered frame: UI LLC: SNAP: ----- SNAP Header ----SNAP: SNAP: Type = 0800 (IP) SNAP: IP: ----- IP Header -----

All- Rou t e s- Ex plor e r LLC Fr a m e List ing 19.2 shows an all- rout es- explorer fram e.

The fram e cont rol field shows t hat t his is an LLC fram e. The dest inat ion and source addresses are locally defined rat her t han universal MAC addresses. The source address is shown as: X'40- 00- 00- 00- 90- 52 However, t he act ual hexadecim al t race of t he source address field is t his: X'C0- 0000- 00- 90- 52 The difference is t hat t he first bit of t he source address has been t ransm it t ed as 1, t o indicat e t hat a rout ing inform at ion field follows. This changes X'4 ( 0100) t o X'C ( 1100) . The rout ing inform at ion field consist s of 10 byt es: t he 2- byt e rout ing cont rol port ion and four 2- byt e rout ing descript ors. • • •

The fact t hat t his is an explorer fram e ( used t o discover a rout e t o t he dest inat ion) is indicat ed by t he first 3 bit s of t he rout ing cont rol field. The value of t his subfield ( 100) indicat es t hat t his is an all- rout es- explorer. The direct ion bit ( 0) shows t hat t he rout e st ill is being built in t he forward direct ion. This also is confirm ed by t he 0- valued A and C bit s in t he fram e st at us field. The largest fram e field shows t hat 8130- byt e fram es can be carried on t his rout e.

The fram e carries an ordinary Net BI OS payload ( a Net BI OS " nam e recognized" m essage) .

List in g 1 9 .2 An All- Rou t e s- Ex plor e r Fr a m e DLC: FS: Addr recognized indicators: 00, Frame copied indicators: 00 DLC: AC: Frame priority 0, Reservation priority 0, Monitor count 0 DLC: FC: LLC frame, DLC: Destination = Station 400000000851, BORIS851 DLC: Source = Station 400000009052, KREMLIN1 DLC: RI: ----- Routing Indicators ----RI: RI: Routing control = 8A RI: 100. .... = All-routes broadcast, non-broadcast return RI: ...0 1010 = RI length is 10 RI: Routing control = 40 RI: 0... .... = Forward direction RI: .100 .... = Largest frame is 8130 RI: .... 000. = Extended frame is 0 RI: .... ...0 = Reserved RI: Ring number 00F via bridge A RI: Ring number 101 via bridge B RI: Ring number 00E via bridge 2 RI: Ring number 200 RI: LLC: ----- LLC Header ----LLC: LLC: DSAP Address = F0, DSAP IG Bit = 00 (Individual Address) LLC: SSAP Address = F0, SSAP CR Bit = 00 (Command) LLC: Unnumbered frame: UI LLC: NETB: ----- NETBIOS Name Recognized -----

Act ive M on it or Pr e se n t Fr a m e List ing 19.3 shows an Act ive Monit or Present fram e. The dest inat ion address X'C000- FF- FF- FF- FF is used for broadcast s of MAC prot ocol fram es ( as opposed t o LLC inform at ion fram es) . The address recognized and fram e copied bit s in t he fram e st at us field show t hat t his fram e was capt ured right aft er it was sent by t he act ive m onit or. The first NI C t hat receives t he fram e will change t hese bit s from 0 t o 1 because t he fram e has been broadcast . I n t he inform at ion field, t he vect or ident ifier st at es t hat t his is an Act ive Monit or Present fram e. This is a st at ion- t o- st at ion m essage ( which is indicat ed in t he fram e's vect or class byt e) . The physical drop num ber is a physical locat ion param et er t hat could be assigned t o a st at ion by a configurat ion server. This param et er is rarely, if ever, set and has default value 0 in t he t race. The final field cont ains t he address of t he sender's upst ream neighbor. I n t his case, t his is t he address of t he st at ion t hat is upst ream from t he act ive m onit or.

List in g 1 9 .3 An Act ive M on it or Pr e se n t M AC Fr a m e MAC: Active Monitor Present DLC: ----- DLC Header ----DLC: DLC: Frame 413 arrived at 12:14:10.806; frame size is 32 (0020 hex) bytes. DLC: FS: Addr recognized indicators: 00, Frame copied indicators: 00 DLC: AC: Frame priority 0, Reservation priority 0, Monitor count 0 DLC: FC: MAC frame, PCF attention code: Active monitor present DLC: Destination = BROADCAST C000FFFFFFFF, TR_Broadcast DLC: Source = Station Madge2411430 DLC: MAC: ----- MAC data ----MAC: MAC: MAC Command: Active Monitor Present MAC: Source: Ring station, Destination: Ring station MAC: Subvector type: Physical Drop Number 00000000 MAC: Subvector type: Upstream Neighbor Address Madge24113FE MAC:

St a ndby M on it or Pr e se n t Fr a m e List ing 19.4 shows a St andby Monit or Present fram e, which is used in t he neighbor not ificat ion procedure and enables a st at ion t o discover it s upst ream neighbor's MAC address.

The address recognized and fram e copied bit s in t he fram e st at us field show t hat a prior st at ion has recognized t he fram e's dest inat ion address ( which is t he MAC broadcast address) and has copied t he fram e. The m onit or bit is 1, indicat ing t hat t he fram e has passed t hrough t he act ive m onit or st at ion's NI C. The fram e cont rol field ident ifies t his as a St andby Monit or Present fram e. I n t he inform at ion field, t he vect or ident ifier st at es t hat t his is a St andby Monit or Present fram e. This is a st at ion- t o- st at ion m essage. The subvect ors report t he physical drop num ber ( if one had been assigned) and t he address of t he sender's upst ream neighbor.

List in g 1 9 .4 St a n dby M on it or Pr e se n t M AC Fr a m e MAC: Standby Monitor Present DLC: ----- DLC Header ----DLC: DLC: Frame 414 arrived at 12:14:10.827; frame size is 32 (0020 hex) bytes. DLC: FS: Addr recognized indicators: 11, Frame copied indicators: 11 DLC: AC: Frame priority 0, Reservation priority 0, Monitor count 1 DLC: FC: MAC frame, PCF attention code: Standby monitor present DLC: Destination = BROADCAST C000FFFFFFFF, TR_Broadcast DLC: Source = Station Madge24113FE DLC: MAC: ----- MAC data ----MAC: MAC: MAC Command: Standby Monitor Present MAC: Source: Ring station, Destination: Ring station MAC: Subvector type: Physical Drop Number 00000000 MAC: Subvector type: Upstream Neighbor Address Madge2411430 MAC:

Cla im Tok e n Fr a m e List ing 19.5 shows a Claim Token fram e. Claim Token fram es are used t o elect an act ive m onit or. The first subvect or report s t he physical drop num ber. The last variable report s t he address of t he sender's upst ream neighbor.

List in g 1 9 .5 A Cla im Tok e n M AC Fr a m e MAC: Claim Token DLC: ----- DLC Header ----DLC: DLC: Frame 86 arrived at 11:12:05.555; frame size is 32 (0020 hex) bytes. DLC: FS: Addr recognized indicators: 11, Frame copied indicators: 11 DLC: AC: Frame priority 0, Reservation priority 0, Monitor count 0 DLC: FC: MAC frame, PCF attention code: Claim Token DLC: Destination = BROADCAST C000FFFFFFFF, TR_Broadcast DLC: Source = Station IBM2 DEC785 DLC: MAC: ----- MAC data ----MAC: MAC: MAC Command: Claim Token

MAC: MAC: MAC: MAC:

Source: Ring station, Destination: Ring station Subvector type: Physical Drop Number 00000000 Subvector type: Upstream Neighbor Address IBM2 DEDC89

Rin g Pu r ge Fr a m e List ing 19.6 displays a Ring Purge fram e. The act ive m onit or t ransm it s a Ring Purge fram e t o clear out t he ring prior t o launching a new t oken. Again, t he sender includes t he address of it s own upst ream neighbor in it s inform at ion field.

List in g 1 9 .6 A Rin g Pu r ge M AC Fr a m e MAC: Ring Purge DLC: ----- DLC Header ----DLC: DLC: Frame 31 arrived at 19:34:37.775; frame size is 32 (0020 hex) bytes. DLC: FS: Addr recognized indicators: 00, Frame copied indicators: 00 DLC: AC: Frame priority 0, Reservation priority 0, Monitor count 0 DLC: FC: MAC frame, PCF attention code: Ring purge DLC: Destination = BROADCAST C000FFFFFFFF, TR_Broadcast DLC: Source = Station IBM2 118D9D DLC: MAC: ----- MAC data ----MAC: MAC: MAC Command: Ring Purge MAC: Source: Ring station, Destination: Ring station MAC: Subvector type: Physical Drop Number 00000000 MAC: Subvector type: Upstream Neighbor Address IBM2 12F7EC MAC:

Re por t Soft Er r or Fr a m e List ing 19.7 displays a Report Soft Error fram e. I t s dest inat ion is t he funct ional address of t he Ring Error Monit or. The m essage report s t he current values of a st at ion's error count ers. The sender reset s it s count ers t o 0 aft er t ransm it t ing t he report .

List in g 1 9 .7 A Re por t Soft Er r or M AC Fr a m e MAC: Report Soft Error DLC: ----- DLC Header ----DLC: DLC: Frame 47 arrived at 19:34:39.880; frame size is 48 (0030 hex) bytes. DLC: FS: Addr recognized indicators: 00, Frame copied indicators: 00 DLC: AC: Frame priority 0, Reservation priority 0, Monitor count 0 DLC: FC: MAC frame DLC: Destination = Functional address C00000000008, RingError Mon. DLC: Source = Station IBM2 118D9D DLC:

MAC:

----MAC: MAC: MAC: MAC: MAC: errors MAC: MAC: MAC: errors MAC: MAC: MAC: MAC:

MAC data ----MAC Command: Report Soft Error Source: Ring station, Destination: Ring Error Monitor Subvector type: Isolating Error Counts 0 line errors, 0 internal errors,

1

burst

0 AC errors, 0 abort delimiters transmitted Subvector type: Non-Isolating Error Counts 0 lost frame errors, 0 receiver congestion, 0 FC 0 frequency errors, 1 token errors Subvector type: Physical Drop Number 00000000 Subvector type: Upstream Neighbor Address IBM2 118E9A

Dedicated Token Ring The developm ent of Dedicat ed Token Ring ( DTR) accom plished t hree im port ant goals: • • •

I t int roduced full- duplex operat ion t o Token Rings. I t enabled vendors t o offer st andards- based Token Ring swit ches. I t set t he st age for 100Mbps Token Ring.

Swit ching and full- duplex operat ion great ly increased t he backbone capacit y of Token Ring LANs. DTR enabled m ult iple st at ions and classic Token Rings t o be at t ached t o a swit ch, and allowed t raffic t o flow freely bet ween t hem . Swit ches could be connect ed t o st at ions and t o one anot her by full- duplex links. DTR shored up t he inst alled base of Token Ring syst em s. Fort unat ely for cust om ers, on som e Token Ring NI C product s t he upgrade could be carried out by replacing a device driver and perform ing a m icrocode updat e. The DTR prot ocol allows for backward com pat ibilit y and support s t he classic halfduplex Token Ring prot ocol as well as full- duplex operat ion. Full- duplex Dedicat ed Token Ring is a point - t o- point prot ocol. Dat a is not passed around a ring, and t here is no t oken. Just as was t he case for full- duplex Et hernet , t he com plicat ed prot ocol elem ent s have been t ossed out t he window. St at ions connect t o a device t hat vendors call a swit ch. The specificat ions call it a DTR concent rat or, but neit her t erm t ot ally hit s t he nail on t he head. A DTR product provides swit ching funct ions but oft en also provides t he capabilit y t o connect a select ed set of half- duplex st at ions int o a virt ual ring. I n t he spirit of com prom ise, DTR concent rat ors t hat also support virt ual rings are called Concent rat ors/ Swit ches in t he sect ions t hat follow.

N ot e The full- duplex m ode of operat ion is referred t o as t ransm it im m ediat e ( TXI ) m ode. St andards groups never overlook an opport unit y t o int roduce a new acronym , so half- duplex t oken passing is called t oken passing ( TKP) m ode.

A DTR Concent rat or/ Swit ch is t he ideal place for a net work m anagem ent funct ion, and DTR product s usually support SNMP m anagem ent variables and SNMP RMON m onit oring variables. A DTR Concent rat or/ Swit ch can accum ulat e inform at ion from several Token Rings.

C-Ports A new t ype of port , called a C- Port ( for concent rat or port ) , was defined for use in DTR Concent rat or/ Swit ches. Clearly, a different t ype of port was needed t o support full- duplex DTR st at ions. But a C- Port also is backward com pat ible and t hus can behave like a classic hub port . A classic Token Ring st at ion's lobe cable can be plugged int o t he port . An Et hernet port always is ready for act ion, whet her it is locat ed in a st at ion or in a hub. Classic Token Ring is different . A classic Token Ring hub port wait s passively unt il t he st at ion at t he end of t he lobe cable init iat es a ring insert ion process. A C- Port in a Token Ring Concent rat or/ Swit ch m ust be capable of playing a passive role and allowing a st at ion t o connect t o it . However, it also needs t o be capable of playing an act ive role so t hat it can init iat e t he insert ion process when it is linked t o a port at a classic hub or at a peer DTR swit ch. This act ive role is called st at ion em ulat ion m ode. Figure 19.12 illust rat es t he versat ilit y of C- Port s; each C- Port is m arked wit h it s role.

Figu r e 1 9 .1 2 . C- Por t r ole s.

The behavior of a Concent rat or/ Swit ch port depends on a product 's archit ect ure and configurat ion: •

•

A DTR st at ion m ay connect in full- duplex m ode. I n t his case, t he st at ion links t o a bridging funct ion in t he Concent rat or/ Swit ch. Mult iple C- Port s can be com bined so t hat t he connect ed st at ions appear t o share a logical ring. The bridging funct ion int erconnect s t hese logical rings wit h one anot her and wit h swit ched st at ions.

Concent rat or/ Swit ch product s usually include bot h source- rout ing and t ransparent rout ing bridge capabilit ies.

Full-Duplex Protocol I n full- duplex m ode, m any part s of t he classic Token Ring are discarded. Dat a does not circle around a ring, and t here is no act ive or st andby m onit or role. Som e of t he fram e param et ers are unused as well. For exam ple, consider t hese point s: • •

The priorit y, m onit or, and reservat ion bit s in t he access cont rol field are set t o 0 and are ignored. There is no t oken, so t he t oken bit always is set t o 1, m eaning " not a t oken." The int erm ediat e fram e and error bit s in t he ending delim it er are set t o 0 and are ignored.

Som e feat ures have been preserved. The Ring Param et er Server, Configurat ion Report Server, and Ring Error Monit or Server funct ions are ret ained. For exam ple, t he servers can provide t he following funct ions: •

• • •

A Configurat ion Report Server can reset t he operat ing param et ers in a st at ion or a C- Port . A Configurat ion Report Server can rem ove a st at ion from t he net work. A Ring Param et er Server can init ialize t he operat ing param et ers in a st at ion or a C- Port . A st at ion or C- Port can report errors t o t he Ring Error Monit or.

One classical funct ion is ret ained by int roducing a replacem ent m echanism . Det ect ing a failed cable and isolat ing a fault have great value. For a classic st at ion, a long silence on an input cable alert s t he st at ion t o t he fact t hat t here is an upst ream fault . But t he full- duplex prot ocol does not have t he cont inuous chat t er of an ordinary ring. Fram es are sent t o or from a st at ion on an as- needed basis, and a fram e sent by a st at ion does not cycle back t o t he st at ion. A Heart beat MAC fram e has been int roduced t o avoid a com plet e lull in incom ing t raffic. Heart beat s are sent periodically by bot h C- Port s and DTR st at ions. This enables a long silence t o be properly diagnosed as a hard fault . The syst em t hat det ect s t he fault t ransm it s a Beacon MAC fram e. The Heart beat fram e has anot her use as well: I t replaces neighbor not ificat ion as a way of providing an ongoing check of t he neighbor's MAC address.

Join in g a St a t ion t o a C- Por t A classic st at ion t hat is connect ed t o a C- Port follows it s usual insert ion process. CPort s are backward com pat ible and support t his classic insert ion process. A DTR st at ion or a DTR C- Port in st at ion em ulat ion m ode also is backward com pat ible. I t can connect t o a convent ional Token Ring hub using t he classic insert ion process. A DTR st at ion capable of using t he new DTR prot ocol init iat es a connect ion t o a CPort by t ransm it t ing a Regist rat ion Request MAC fram e. This is answered by a Regist rat ion Response MAC fram e. The st at ion and C- Port use t hese fram es t o announce whet her t hey are configured t o operat e in half- duplex or full- duplex m ode. I f t hey are incom pat ible, t he st at ion will rem ain in bypass m ode but will be able t o send an error m essage report ing t he problem t o one or m ore of t he Token Ring funct ional servers. A st at ion t hat want s t o operat e in 100Mbps full- duplex m ode can regist er a request t o t rade up t o t his rat e of speed. The response code t hat com es back m ay accept t his speed or indicat e t hat a lower speed m ust be used. I f regist rat ion is successful, t he st at ion t ransm it s Lobe Media Test fram es t o check t hat t he lobe can operat e at an accept able bit error rat e. A C- Port repeat s t est fram es back t o t he st at ion, j ust as a hub port did for classic Token Ring.

N ot e I n classic Token Ring, t est fram es were addressed t o X'00- 00- 00- 00- 00- 00. A new funct ional address, X'C0- 00- 00- 00- 40- 00, has been defined as t he dest inat ion for t he DTR Lobe Media Test fram es.

Next , t he st at ion request s perm ission t o j oin by sending an I nsert Request fram e. Then t he Concent rat or/ Swit ch checks t o m ake sure t hat t he st at ion's MAC address is unique on it s logical ring. I f all is well, t he C- Port sends back an I nsert Response fram e cont aining an accept ance. A st at ion connect ed via t wist ed- pair cabling places a phant om DC current on it s cable, and t he j oin is com plet ed.

D TR M AC Pr ot ocol Fr a m e s Table 19.4 list s t he MAC fram es used in t he DTR prot ocol. Som e classic Token Ring fram es have been dropped, but several new fram es have been added.

Ta ble 1 9 .4 . M AC Fr a m e s Use d in t h e D TR Pr ot ocol N u m e r ic I de n t ifie r

Nam e

D e scr ipt ion

X'00

Response

Sent t o acknowledge receipt of a MAC fram e or t o report an error in a received MAC fram e.

X'02

Beacon

Signals an int errupt ion in t he dat a flow on t he link.

X'05

C- Port Heart beat

Sent periodically by a C- Port in port m ode. Enables t he st at ion t o det ect whet her t he link is working, and t o learn or verify t he MAC address of it s neighboring C- Port .

X'06

St at ion Heart beat

Sent periodically by a st at ion. Enables it s C- Port t o det ect whet her t he link is working, and t o learn or verify t he MAC address of it s neighbor.

X'08

Lobe Media Test

Transm it t ed t o check t he cable before at t em pt ing t o j oin t he ring or during hard error recovery.

X'0B

Rem ove DTR St at ion

Sent by a Configurat ion Report Server or net work m anagem ent syst em t o a st at ion's unicast address t o force it s rem oval from t he net work.

X'0C

Change Param et ers

Sent by t he Configurat ion Report Server t o a C- Port or a st at ion t o set it s operat ing param et ers.

X'0D

I nit ialize St at ion

Sent by t he Ring Param et er Server t o a C- Port or a st at ion t o init ialize it s operat ing param et ers.

X'0E

Request St at ion Addresses

Sent t o a st at ion by any m anagem ent ent it y t o ask for t he list of addresses recognized by t he st at ion.

Ta ble 1 9 .4 . M AC Fr a m e s Use d in t h e D TR Pr ot ocol N u m e r ic I de n t ifie r

Nam e

D e scr ipt ion

X'0F

Request St at ion St at e

Sent t o a st at ion by any m anagem ent ent it y t o ask for operat ing st at e inform at ion.

X'10

Request St at ion At t achm ent s

Sent by any m anagem ent ent it y t o ask a st at ion or C- Port which funct ions it perform s.

X'11

Regist rat ion Request

Sent by a st at ion t o regist er it s operat ing param et ers wit h t he C- Port .

X'12

Regist rat ion Response

Sent by t he C- Port t o accept or rej ect a regist rat ion.

X'13

I nsert Request

Sent by a st at ion aft er a successful lobe m edia t est , asking t he C- Port t o com plet e t he insert ion process.

X'14

I nsert Response

Sent by t he C- Port t o accept or rej ect t he insert ion request .

X'15

Regist rat ion Query

Sent by a C- Port whose st at ion is using t he classical Token Ring prot ocol t o invit e t he st at ion t o upgrade t o full- duplex m ode.

X'22

Report St at ion Addresses.

Responds t o Request St at ion

/

Addresses

/

X'23

Report St at ion St at e

Responds t o Request St at ion St at e.

X'24

Report St at ion At t achm ent s

Responds t o Request St at ion At t achm ent s.

X'29

Report Error

Sent t o t he Ring Error Monit or t o report error count s. I t can be sent by a st at ion, a C- Port in st at ion em ulat ion m ode, or a C- Port in port m ode.

High Speed Token Ring High Speed Token Ring ( HSTR) is a 100Mbps im plem ent at ion of Dedicat ed Token Ring. I n fact , it s official t it le is 100Mbps Dedicat ed Token Ring. Com binat ion 4/ 16/ 100Mbps NI Cs and m at ching Concent rat ors/ Swit ches have been available for several years. The HSTR physical coding and t ransm ission specificat ion was adopt ed direct ly from CDDI and FDDI , as were 100BASE- TX and 100BASE- FX Et hernet . The copper im plem ent at ion runs across t wo pairs of Cat egory 5 unshielded t wist ed- pair cabling or t wo pairs of I BM Type 1 shielded t wist ed–pair cabling. The fiber opt ic im plem ent at ion runs on a pair of m ult im ode fibers. Fiber opt ic lobes can be used.

N ot e A 4B/ 5B encoding is used for bot h copper and fiber opt ic cables. Appendix A has t he det ails.

High Speed Token Ring operat es only in full- duplex m ode. The MAC prot ocol used is t he full- duplex DTR prot ocol. Unt il t he int roduct ion of High Speed Token Ring, swit ch- t o- swit ch links were lim it ed t o full- duplex 16Mbps. The only way t o increase t his capacit y was t o use parallel links. This was cost ly because it occupied several port s on each swit ch and also was inconvenient . The Token Ring com m unit y welcom ed t he j um p t o 100Mbps. Originally, t here were hopes for a furt her j um p t o gigabit speed, but t hese were drowned in t he rising t ide of Et hernet successes. Et hernet won t he LAN backbone, and m arket dem and could not support t he m ore advanced Token Ring product .

Fiber Optic HSTR Lobes I n classic Token Ring, a fiber opt ic cable can be used as a t runk link t hat connect s a pair of concent rat ors. However, t he procedure used t o j oin a st at ion t o a ring relied on t he use of a phant om current across a copper wire t o open t he TCU relay, and fiber opt ic lobes were not support ed. However, it was very desirable t o support fiber for t he 100Mbps im plem ent at ion. Hence, an alt ernat ive cabling coupling unit and relay m anagem ent t echnology were needed. This was engineered in a way t hat also would work for lower speeds of at t achm ent . The t echnical problem was solved by defining special, nondat a opt ical signals t hat a st at ion t ransm it s across an opt ical fiber t o request insert ion or t o ask for a ret urn t o bypass st at e.

Fiber Distributed Data Interface Fiber Dist ribut ed Dat a I nt erface ( FDDI ) LANs were int roduced in t he m id- 1980s. The t echnology was developed by t he Am erican Nat ional St andards I nst it ut e ( ANSI ) X3T9.5 st andards com m it t ee. FDDI LANs were designed t o operat e at high speed, be reliable, and have a large diam et er. This m ade FDDI a popular choice for LAN backbones. Dat a is t ransm it t ed around a ring at 100Mbps. Syst em s can be connect ed by dual rings, and aft er a break a usable ring pat h is est ablished aut om at ically. A dual- ring fiber opt ic net work can have a circum ference of 100 kilom et ers. FDDI t ruly is a legacy t echnology t hat is being replaced by high- speed Et hernet swit ches.

I n place of cost ly 100Mbps FDDI NI Cs, t here are now cheap 100Mbps Et hernet cards. I nst ead of FDDI 's dual ring and elaborat e ring recovery procedures, t here are reliable swit ches and aggregat ed links. FDDI 's large, awkward connect ors have been replaced by sm all and reliable SC connect ors. Long dist ances at t ainable t hrough t he use of opt ical fiber now are at t ainable by ot her LAN t echnologies using opt ical fiber. Several vendors offer LAN swit ches or rout ers t hat include port s t hat can be connect ed t o FDDI , Et hernet , Token Ring, and ATM segm ent s. This enables t he t echnologies t o int erwork and also assist s in m igrat ion bet ween t echnologies. FDDI is sket ched briefly in t he sect ions t hat follow.

FDDI Topology I n an FDDI LAN, t he nodes t hat can be t he source or dest inat ion of dat a fram es are called st at ions. A st at ion has a MAC layer, one or m ore MAC addresses, and one or m ore FDDI port s. The st at ions share t he bandwidt h provided by t he ring. As was t he case for Token Ring, cabling m anagem ent oft en is sim plified by connect ing st at ions t o a concent rat or ( hub) unit . A concent rat or can play it s role invisibly. However, it will need a MAC layer and a MAC address if it is going t o be configured or supervised via net work m anagem ent m essages. I n t his case, it will have t he st at us of a st at ion, t oo. Norm ally, m ost st at ions are connect ed t o concent rat ors t hat are at t ached t o a cent ral ring called t he t runk. Concent rat ors can be cascaded, m aking t he physical t opology of an FDDI net work look like a ring of t rees, as shown in Figure 19.13.

Figu r e 1 9 .1 3 . FD D I t opology.

The pat h followed by a fram e passes t hrough every act ive st at ion. For exam ple, in Figure 19.13: 1. A fram e ent ering concent rat or A flows t o concent rat or B. 2. The fram e flows in and out of each st at ion NI C at t ached t o concent rat or B and ret urns t o concent rat or A. 3. The fram e advances t o concent rat or C. 4. I t flows in and out of each of each st at ion NI C at t ached t o concent rat or C and ret urns t o concent rat or A. 5. 5. The fram e cont inues around t he t runk ring. Like a Token Ring, an FDDI ring is m ade up of a series of point - t o- point links t hat connect a st at ion t o a st at ion, a st at ion t o a concent rat or, or a concent rat or t o anot her concent rat or. Originally, only opt ical fiber links were used, but copper links were added lat er. Separat e fibers or wires are used in t he t ransm it and receive direct ions. Figure 19.14 provides a m ore det ailed view of t he pat h t hrough t he t ree of concent rat ors shown at t he bot t om of Figure 19.13, and it can be seen t hat dat a ent ers each device on one line and exit s on a different line.

Figu r e 1 9 .1 4 . A pa t h t h r ou gh a n FD D I LAN .

The pat h m ust be adj ust ed when an addit ional st at ion—or a lower- level concent rat or—is added t o t he configurat ion. FDDI adapt ers m ake t hese adj ust m ent s aut om at ically. Pat h cont rol is an im port ant feat ure of FDDI .

Primary and Secondary Rings The t runk can consist of t wo rings ( a dual ring) . For a dual ring, during norm al operat ion t raffic flows on one of t he rings, which is called t he prim ary ring. The second ring provides t he backup capabilit y and is called t he secondary ring. I f a t runk link on t he prim ary ring fails, t he syst em s adj acent t o t he break aut om at ically reconfigure t he pat h t hrough t he ring and creat e a new pat h t hat includes a m ixt ure of prim ary and secondary ring links.

N ot e The pat h lengt h around t he ring is lim it ed t o, at m ost , 200 kilom et ers. The dual–ring LAN circum ference is rest rict ed t o 100 kilom et ers t o ensure t hat t he pat h lengt h st ays wit hin t he 200km lim it when a break causes bot h prim ary and secondary links t o be used. The st andard does not set a hard lim it on t he num ber of st at ions t hat can be at t ached t o an FDDI ring. I t does suggest a default m axim um of 1000 port at t achm ent s. This oft en is st at ed as 500 st at ions.

The m axim um t im e for a bit t o circle a m axim um - lengt h fiber opt ic ring t hat is loaded wit h st at ions would be about 2 m illiseconds.

The direct ion of t ravel on t he secondary ring is t he opposit e of t he direct ion on t he prim ary ring. When a secondary ring is used, fram es t ravel in opposit e direct ions on segm ent s of t he prim ary and secondary rings. For t his reason, t he prim ary and secondary rings are said t o be count er- rot at ing. I n Figure 19.15, t he prim ary ring is shown as t he solid out er ring, and t he secondary ring is shown as t he dot t ed inner ring.

Figu r e 1 9 .1 5 . D u a l FD D I r in gs.

The t op of Figure 19.15 shows a dual ring t hat connect s servers and concent rat ors. The bot t om part of t he figure shows anot her com m on set up: The dual ring is used as a net work backbone and connect s t o rout ers and bridges. Syst em s t hat connect t o bot h t runk rings are called dual- at t ached. Ot her st at ions are called single- at t ached.

Single-Attached Stations and S Ports The syst em s in Figure 19.13 and Figure 19.14 are single- at t ached. Figure 19.16 shows a det ailed view of single- at t ached port connect ions. A port t hat is used for a single- at t ached connect ion is called ( not - surprisingly) an S Port . Dat a flows int o t he port on one line and out on t he ot her line.

Figu r e 1 9 .1 6 . Sin gle - a t t a ch e d st a t ion por t s.

Dual-Attached Stations and A and B Port Types The syst em s in Figure 19.17 are dual- at t ached and connect t o t he t wo rings via t wo separat e port s. Each of t hese port s has a different t ype:

Figu r e 1 9 .1 7 . D u a l- a t t a che d st a t ion por t s.

• •

Type A Port —The receive line is at t ached t o t he prim ary ring, and t he t ransm it line is at t ached t o t he secondary ring. ( I n brief: prim ary in, secondary out .) Type B Port —The t ransm it line is at t ached t o t he prim ary ring, and t he receive line is at t ached t o t he secondary ring. ( I n brief: prim ary out , secondary in.)

Figure 19.18 shows an ent ire ring t o which all st at ions are dual- at t ached. The heavy out er line is t he prim ary ring. The prim ary ent ers each node at an A Port and exit s at a B Port . The t hin inner line is t he secondary ring. The secondary ent ers each node at a B Port and exit s at an A Port .

Figu r e 1 9 .1 8 . A dua l- t r u n k r in g.

Concentrator M Ports One m ore t ype of port t hat is used in an FDDI ring is illust rat ed in Figure 19.19. The concent rat or in t he figure is at t ached t o dual rings via t ype A and t ype B Port s. I t also is connect ed t o single- at t ached st at ions. The concent rat or port s t hat at t ach t o t hese st at ions are called M Port s.

Figu r e 1 9 .1 9 . M Por t s in a con ce n t r a t or .

Wrapping an FDDI Ring The capabilit y of a dual FDDI ring t o recover from failed wiring or a failed node is viewed as one of it s big advant ages. Figure 19.20 shows a ring t hat has suffered a failure and has recovered.

Figu r e 1 9 .2 0 . A w r a ppe d r in g.

The problem is solved by creat ing a ring t hat cont ains som e segm ent s from t he prim ary ring and som e from t he secondary ring. This is im plem ent ed by changing t he pat h at st at ion 2 and at st at ion 3. •

•

At st at ion 2, dat a ent ers t he A Port as usual but exit s via t he ot her ( secondary) link at t hat port ( inst ead of t hrough t he B Port ) . At st at ion 3, dat a flows int o t he B Port ( via t he secondary link) and t hen exit s as usual via t he ot her ( prim ary) link at t hat port .

This is called wrapping t he ring, which m eans t hat t he secondary ring segm ent at t ached t o t he A Port in st at ion 2 is used t o exit t hat st at ion. This causes t raffic t o t raverse t he secondary ring unt il it ent ers t he B Port in st at ion 3.

FDDI Media and Encoding Originally, m ult im ode and single- m ode opt ical fiber were used for all FDDI node- t onode connect ions. Lat er, copper links were added, and t he copper im plem ent at ion was called Copper Dist ribut ed Dat a I nt erface ( CDDI ) .

The copper wiring st andard is called Twist ed Pair- Physical Medium Dependent ( TPPMD) and operat es over Cat egory 5 unshielded t wist ed- pair cable or I BM Type 1 150 ohm shielded t wist ed- pair cable. The physical m edia and t he encoding m et hod used for FDDI and CDDI were adopt ed for 100BASE- FX ( fiber opt ic) and 100BASE- TX ( t wist ed- pair) Et hernet , and were discussed briefly in Chapt er 7, " The Et hernet 100Mbps Physical Layer." The encoding and t ransm ission m echanism s are described in det ail in Appendix A. Just a few of t he basic fact s are reviewed here. Dat a is t ransform ed before it is sent ont o an FDDI or CDDI line. Each byt e is broken int o t wo 4- bit nibbles. Each nibble is t ranslat ed t o a 5- bit pat t ern. The encoding schem e is called 4B/ 5B. Ext ra 5- bit pat t erns are used ( alone or com bined in pairs) as special cont rol codes. Special cont rol codes represent t he following: • •

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I dle ( 11111) —The idle pat t ern is sent cont inuously bet ween fram es. St art - of- st ream delim it er ( 11000 10001) —A new fram e is int roduced by t he st art - of- st ream delim it er. End- of- st ream delim it er ( 01101 00111) —An end- of- st ream delim it er is sent at t he end of a fram e.

FDDI Protocol Elements As is t he case on a Token Ring, st at ions on an FDDI ring pass a t oken around. The holder of t he t oken has t he right t o t ransm it dat a for a period of t im e. The t oken t hen is passed t o t he next st at ion. FDDI allocat es bandwidt h t o each st at ion in a different m anner t han t he Token Ring prot ocol. Each st at ion is guarant eed a reserved t im e quot a and is perm it t ed t o t ransm it fram es for t hat t im e period when it receives t he t oken. Use of t he reserved t im e is called synchronous t ransm ission. There also is an ext ra am ount of t im e t hat is loosely shared by t he st at ions. During ring init ializat ion, t he st at ions agree on a Target Token Rot at ion Tim eout ( TTRT) . This is t he average t im e t hat any st at ion expect s t o wait before it receives t he use of t he t oken.

N ot e During norm al operat ion, t wice t he Target Token Rot at ion Tim eout is t he longest t im e t hat any st at ion will have t o wait .

I f a t oken arrives early, a st at ion can use t he ext ra slack t im e in addit ion t o it s reserved t im e. This ext ra t im e is said t o be used for asynchronous t ransm ission. Unlike t he synchronous t ransm ission t im e, t he asynchronous t im e is unpredict able.

An ordinary t oken is called an unrest rict ed t oken. There also is a rest rict ed t oken. A st at ion uses a rest rict ed t oken when it want s t o have an unint errupt ed dialogue wit h anot her st at ion during t im e current ly available for asynchronous t ransm ission. The rest rict ed t oken enables t he st at ion t o pass cont rol back and fort h t o it s part ner, enabling t hem t o send fram es t o one anot her.

Claim Frames and the Target Token Rotation Timeout To get an FDDI ring ready for act ion, t he st at ions m ust perform t hese act ions: • •

Decide which one will t ransm it t he first t oken Det erm ine t he value of t he Target Token Rot at ion Tim eout

The st at ions elect an init ial t oken t ransm it t er and choose t he Target Token Rot at ion Tim eout winner by t ransm it t ing Claim fram es. Each Claim fram e includes •

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The value t hat t he st at ion would like t o use as t he Target Token Rot at ion Tim eout . This is a param et er t hat is preconfigured at each st at ion. The sending st at ion's MAC address.

I f a st at ion receives a Claim fram e whose t arget t im eout is larger t han it s own, it discards t he fram e and t ransm it s it s own Claim fram e. I f t he t im e values are t ied, t he sender wit h t he bigger MAC address wins. Event ually, only one winning st at ion is t ransm it t ing claim fram es, and t he lowest t arget t im eout value has been discovered. This becom es t he operat ional Target Token Rot at ion Tim e for t he ring. The winning st at ion t hen t ransm it s a t oken, and dat a t ransm ission can get st art ed.

Station Management On a Token Ring, all of t he ring m anagem ent funct ions are perform ed using MAC fram es. An act ive m onit or st at ion has t he j ob of wat ching for errors and init iat ing recovery chores. The work is organized different ly on an FDDI ring. There is no act ive m onit or, and every st at ion part icipat es in ring m anagem ent and recovery using a com ponent called st at ion m anagem ent ( SMT) . The st at ion m anagem ent ent it y • • •

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Perform s all of t he st eps required t o init ialize FDDI funct ions at t he st at ion Cont rols t he opt ical bypass of an inact ive st at ion Test s t he int egrit y of t he link t o an adj acent node, and perform s ongoing m onit oring of link errors Checks whet her t he ot her end of a st at ion port 's line is com pat ible. For exam ple –An M Port m ust connect t o an S Port ( see Figure 19.19 –An A Port m ust connect t o a B Port ( see Figure 19.17 and Figure 19.18)

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I nsert s an act ive st at ion int o t he ring and rem oves it from t he ring I nit iat es t he sending of Claim fram es Perform s t he wrap ( and unwrap) funct ion, changing t he port used t o t ransm it or receive as needed I s in charge of t he st at ion's m anagem ent inform at ion dat abase, which cont ains it s configurat ion param et ers, st at us values, and perform ance st at ist ics

Beacon Frames Beacon fram es are used t o recover from serious fault s, such as a failed link, a reconfigured ring, or a failed claim process. Any st at ion t hat det ect s a fault ( for exam ple, because of a t im eout ) t ransm it s a cont inuous st ream of Beacon fram es t o it s downst ream neighbor. The downst ream neighbor m ight be sending it s own Beacons. I f so, it st ops and inst ead repeat s t he incom ing Beacons. Event ually, only t he st at ion t hat is im m ediat ely downst ream from t he fault is init iat ing Beacons. This st at ion sends Beacons t hat carry it s upst ream neighbor's MAC address. The Beacons propagat e around t he ring t o t he st at ion t hat is upst ream from t he fault . This st at ion rem oves it self from t he ring and perform s self- t est s. I f t hese succeed, it will t ry t o reinsert . St at ion m anagem ent operat es t he recovery processes. I f a ring wrap is needed, st at ion m anagem ent init iat es and supervises t he pat h change.

FDDI Frames There are four t ypes of FDDI fram es: • • • •

LLC fram es t hat carry user dat a. MAC fram es t hat perform im port ant init ializat ion and recovery chores. St at ion m anagem ent ( SMT) fram es used t o ident ify upst ream neighbors and t o perform t est and m anagem ent funct ions. The void fram e, which causes receivers t o reset t heir t im ers t o st art up values. A void fram e is circulat ed before t he ring is reinit ialized wit h Claim fram es. The void fram e cont ains no payload.

Table 19.5 list s and describes t he t wo MAC fram es.

Ta ble 1 9 .5 . M AC Fr a m e s Fr a m e Type

D e scr ipt ion

Claim

Elect s t he st at ion t hat will send t he first t oken, and est ablishes t he operat ional Target Token Rot at ion Tim e for t he ring

Beacon

Used t o announce a serious fault and ident ify t he st at ions t hat are

Ta ble 1 9 .5 . M AC Fr a m e s Fr a m e Type

D e scr ipt ion upst ream and downst ream of t he fault

Table 19.6 list s t he t ypes of st at ion m anagem ent fram es. Not e t hat in cont rast t o t he Token Ring prot ocol, FDDI neighbor inform at ion is propagat ed using SMT fram es. The FDDI st andards were writ t en before t he exist ence of SNMP. Som e of t he funct ions form erly perform ed using SMT fram es now are perform ed using SNMP.

Ta ble 1 9 .6 . St a t ion M a na ge m e nt Fr a m e s Fr a m e Type

D e scr ipt ion

Neighbor I nform at ion

Enables each MAC t o det erm ine it s upst ream neighbor's MAC address. Also used t o det ect duplicat e MAC addresses. Transm it t ed every 2 t o 30 seconds.

Echo

Used for loopback t est ing. Can check t hat t he t arget port , MAC layer, and st at ion m anagem ent funct ion are operat ional.

St at us I nform at ion

Used for Request and Response fram es t hat ret rieve basic st at us inform at ion.

Param et er Managem ent

Used t o get or set st at ion m anagem ent param et ers.

Request Denied

I ndicat es t hat a request was inappropriat e or had an incorrect form at .

St at us Report ing

Not ifies FDDI m anagers of st at ion event s and condit ions via a st at us report prot ocol.

FDDI Formats The FDDI t oken and fram es were pat t erned on t he Token Ring t oken and fram es, but t here are som e differences. For exam ple, t here is no m onit or st at ion on an FDDI ring, and no priorit y or reservat ion bit s are used, so no access cont rol field is present . Anot her difference is t he use of special charact ers t o delim it fram es.

FDDI Token Format Figure 19.21 shows t he form at of an ordinary ( unrest rict ed) t oken. A t oken m ust be preceded by four or m ore idle sym bols. The st art ing and ending delim it ers are m ade up of pairs of special sym bols. These are represent ed by 5- bit code- groups t hat never are used t o represent dat a byt es.

Figu r e 1 9 .2 1 . Th e FD D I t ok e n .

N ot e The st art ing delim it er code- groups are called J and K. The 10 bit s t hat act ually are t ransm it t ed are 1 1 0 0 0 1 0 0 0 1. A code- group called T is repeat ed t wice t o form t he ending delim it er for a t oken. The 10 bit s t hat act ually are t ransm it t ed are 0 1 1 0 1 0 1 1 0 1.

The fram e cont rol field has a value of X'80 t o indicat e t hat t his is an ordinary t oken. ( The value X'C0 is used for a rest rict ed t oken.)

FDDI Frame Format An FDDI fram e is alm ost ident ical t o a Token Ring fram e. A fram e can be up t o 4500 byt es in lengt h. Figure 19.22 displays t he form at of an FDDI fram e.

Figu r e 1 9 .2 2 . FD D I fr a m e for m a t .

The sender t ransm it s a pream ble consist ing of at least 16 idles before t he st art of a fram e. However, because of differences bet ween t he delays bet ween nodes, t he num ber of idles can shrink as t he fram e is repeat ed around t he ring. Most of t he fields in t he FDDI fram e should be fam iliar: • •

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The st art ing delim it er is t he sam e as t he one used for t he t oken. The ending delim it er consist s of a single special T sym bol inst ead of a repeat ed pair. The fram e cont rol field announces t he fram e's t ype ( for exam ple, LLC or MAC Claim fram e) . For a user dat a fram e, t he inform at ion field st art s wit h an LLC header. The fram e st at us field consist s of t hree special sym bols t hat are used very m uch like t he st at us bit s at t he end of a Token Ring fram e. These sym bols are used t o report error st at us, address recognized, and fram e copied.

Summary Points

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A classic half- duplex Token Ring consist s of a set of st at ions connect ed in a ring. The st at ions share a 4Mbps or 16Mbps bandwidt h. The physical wiring of a classic Token Ring looks like a ring of st ars. St at ions connect t o concent rat ors in wiring closet s. A t oken circulat es around t he ring. A st at ion wit h dat a t o send capt ures t he t oken, t ransm it s one or m ore fram es, and t hen sends a fresh t oken. I f early t oken release is used, a sender t ransm it s a t oken right aft er t he int erfram e gap t hat follows it s final fram e. Fram es t hat cont ain user dat a are called LLC fram es. Fram es t hat carry various t ypes of prot ocol m essages are called MAC fram es. The act ive m onit or, which is chosen by an elect ion process, wat ches over t he t oken prot ocol. Ot her st at ions at t ached t o t he ring act as st andby m onit ors. St andby m onit or st at ions elect a new act ive m onit or by t ransm it t ing Claim Token fram es. A Ring Purge fram e clears dat a out of t he ring and is used in several error recovery procedures. The Beacon process ident ifies t he locat ion of a fault and init iat es som e act ions t hat m ight correct t he fault . A hard error is a fault t hat prevent s fram es and/ or t okens from circulat ing around t he ring. A soft error is a fault t hat does not prevent fram es and t okens from circulat ing around t he ring. The fault dom ain for a hard error consist s of t he st at ion downst ream t o a fault ( which report s t he problem ) , t he st at ion upst ream t o t he fault , and t he equipm ent bet ween t he upst ream and downst ream st at ions. The neighbor not ificat ion process enables each st at ion t o discover t he address of it s upst ream neighbor. A ring can include a Ring Param et er Server, a Configurat ion Report Server, and a Ring Error Monit or. These are called funct ional servers. To j oin a ring, a st at ion at t ached via t wist ed- pair cabling m ust t est t he cable, physically get on t he ring by t ransm it t ing a phant om current , check t hat an act ive m onit or is present , check t hat t here is no duplicat e address on t he ring, learn t he address of it s upst ream neighbor, and get it s configurat ion param et ers. An act ive concent rat or port repeat s signals. Up- t o- dat e product s provide act ive port s t hat can isolat e bad lobes, bypass st at ions t hat are operat ing at t he wrong speed, and st at ions whose NI Cs have ot her problem s. The Dedicat ed Token Ring prot ocol provides full- duplex swit ched operat ion and also support s backward com pat ibilit y wit h classic Token Ring. A DTR swit ch C- Port can connect t o a full- duplex st at ion, a half- duplex st at ion, a half- duplex hub, or a C- Port at anot her swit ch. To connect t o a C- Port , a full- duplex st at ion regist ers, perform s a lobe m edia t est , and perform s a duplicat e address check. High Speed Token Ring is a 100Mbps im plem ent at ion of Dedicat ed Token Ring. I t s physical coding and t ransm ission specificat ion were adopt ed from CDDI and FDDI . FDDI rings were designed t o operat e at 100Mbps, be reliable, and have a large circum ference. A dual- ring fiber opt ic net work can have a circum ference of 100 kilom et ers and can connect t o 1000 port s. A node t hat can be t he source or dest inat ion of dat a fram es is called a st at ion. The pat h followed by a fram e passes t hrough every act ive st at ion. Separat e fibers or wire pairs are used in t he t ransm it and receive direct ions. Cabling m anagem ent is sim plified by connect ing st at ions t o a concent rat or.

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Many FDDI LANs look like a ring of t rees. The cent ral ring is called t he t runk. A t runk can consist of t wo rings: a prim ary and a secondary ring. Syst em s t hat connect t o t wo t runk rings are called dual- at t ached. I n t he absence of a fault , fram es flow from a B Port t o an A Port on a prim ary ring, and from an A Port t o a B Port on a secondary ring. FDDI runs on m ult im ode fiber, single- m ode fiber, Cat egory 5 UTP, and I BM Type 1 STP. Dat a is encoded using 4B/ 5B t ranslat ion before it is t ransm it t ed. Every st at ion has a fixed ( synchronous) am ount of t ransm ission t im e and also can use varying am ount s of slack ( asynchronous) t im e t hat occurs when a t oken arrives earlier t han expect ed. Claim fram es det erm ine what t he value for t he t arget rot at ion t im e is and who will send t he first t oken. Beacon fram es ident ify t he st at ions t hat are upst ream and downst ream from a serious fault . St at ion m anagem ent perform s init ializat ion and error recovery funct ions.

References The classic Token Ring prot ocol is described in •

ANSI / I EEE St d 802.5 ( I SO/ I EC 8802- 5) . " Token Ring Access Met hod and Physical Layer Specificat ions." 1998.

Dedicat ed Token Ring and t he use of fiber opt ic lobes are described in •

ANSI / I EEE St d 802.5r and 802.5j . " Token Ring Access Met hod and Physical Layer Specificat ions, Am endm ent 1: Dedicat ed Token Ring Operat ion And Fibre Opt ic Media." 1998.

At t he t im e of writ ing, t he 100Mbps st andard st ill is in draft form : •

802.5t draft . " Token Ring Access Met hod and Physical Layer Specificat ions, 100 Mbit / s Dedicat ed Token Ring Operat ion." 1998.

The High Speed Token Ring Alliance Web sit e is ht t p: / / www.hst ra.com / . For a real classic, see •

I BM Token Ring Net work Archit ect ure Reference. Third Edit ion ( SC3D- 3374D2) . New York: I BM Corporat ion, Sept em ber 1989.

Maj or FDDI st andards include •

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ANSI X3.166. "Fiber Dist ribut ed Dat a I nt erface ( FDDI ) Physical Layer Medium Dependent ( PMD) ." 1989, revised 1995. ANSI X3.148. "Fiber Dist ribut ed Dat a I nt erface ( FDDI ) —Token Ring Physical Layer Prot ocol ( PHY) ." 1988, revised 1994.

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ANSI X3.139. "Fiber Dist ribut ed Dat a I nt erface ( FDDI ) —Token Ring Media Access Cont rol ( MAC) ." 1987, revised 1997. ANSI X3.184. " Fiber- Dist ribut ed Dat a I nt erface ( FDDI ) —Single- Mode Fiber Physical Layer Medium Dependent ( SMF- PMD) . " Original 1993, lat revision 1998. ANSI X3.263 ( I SO/ I EC CD 9314- 10) . " FDDI Twist ed Pair—Physical Medium Dependent ( TP- PMD) ." 1995. ANSI X3.229. " Fibre Dist ribut ed Dat a I nt erface ( FDDI ) —St at ion Managem ent ( SMT) ." 1994. ANSI X3.184. " Fiber- Dist ribut ed Dat a I nt erface ( FDDI ) —Single- Mode Fiber Physical Layer Medium Dependent ( SMF- PMD) ." 1993, revised 1998. ANSI X3.278. " Fibre Dist ribut ed Dat a I nt erface ( FDDI ) —Physical Layer Repeat er Prot ocol ( PHY- REP) ." 1997.

Chapter 20. ATM Overview Asynchronous Transfer Mode ( ATM) is a t echnology t hat was designed t o upgrade t he world's public wide area t elecom m unicat ions net works t o a new, higher- speed archit ect ure. The prim ary ATM st andards body is t he Telecom m unicat ions St andardizat ion Sect or of t he I nt ernat ional Telecom m unicat ion Union ( I TU- T) . The Am erican Nat ional St andards I nst it ut e ( ANSI ) and t he European Telecom m unicat ions I nst it ut e ( ETSI ) cont ribut ed t o t he ATM effort . The ATM Forum , whose m em bers are equipm ent vendors and service providers, published int eroperabilit y agreem ent s t hat m ade it possible t o build pract ical product s. The Forum m ade ATM work by solving t he t echnical problem s t hat arise in real- world use. ATM was designed t o cure t he short com ings of t he old, rigid t elecom m unicat ions st ruct ure: • • •

Lack of scalabilit y I nflexible bandwidt h opt ions I ncapabilit y t o handle m ixed dat a and voice t raffic gracefully

These short com ings result ed from t he age of t hese net works and from t he fact t hat t he legacy t elecom m unicat ions infrast ruct ure was built for t elephone service. A t elephone call is digit ized at 64,000 bit s per second. This num ber becam e t he basic building block for t he ent ire syst em . The ot her building blocks are carrier lines designed t o m ult iplex t elephone call circuit s. A T1 carrier provides 24 call circuit s, and a T3 carrier provides 672 call circuit s. The whole syst em depends on t im e- division m ult iplexing. A periodic t im e slot is set aside for each circuit , and com plicat ed m ult iplexing pat t erns are used t o com bine t he 672 calls t hat m ake up a T3 carrier. Taking apart a T3 bundle and swit ching it s calls int o ot her bundles is a m aj or chore t hat is done under t he gun of t ight t im e slot const raint s. Mult iplexing and dem ult iplexing are m essy t asks.

ATM was m et wit h ent husiasm because t he t echnology overcom es all of t hese problem s: •

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I t is scalable and can run at t he high speeds provided by t oday's fiber opt ic t echnologies. I t is flexible; a circuit can be configured wit h what ever bandwidt h m eet s it s t ransm ission requirem ent s. I t can provide a variet y of circuit t ypes t hat support anyt hing from t im esensit ive voice or video t raffic t o background dat a t ransfers.

Today, every m aj or fram e relay service runs on t op of ATM, and ATM is em bedded in I nt ernet backbones. ATM is a t echnology t hat is suit able for local area swit ches as well as wide area swit ches. This led t o an early euphoric belief t hat ATM would displace all local area and wide area t echnologies, and lead t o a sim plified, hom ogeneous com m unicat ions environm ent . Cheap 100Mbps Et hernet and Gigabit Et hernet put at least a t em porary hold on t hat dream for t he local environm ent . As for t he wide area, cust om er use of ATM oft en begins wit hin a service provider's t elecom m unicat ions net work. I t is hidden from m ost end users, who current ly prefer t o connect t o a fram e relay service via a leased line. The fut ure success or failure of ATM will be based on t he price of equipm ent and of service, and on how well t he t echnology m eet s t he changing needs of users. This chapt er present s a brief sket ch of ATM t echnology and point s out som e of it s st rong point s and it s weaknesses. Chapt er 21, " ATM LAN Em ulat ion," out lines ATM's LAN Em ulat ion prot ocols and explains how ATM is used t o creat e high- speed " LAN" com m unit ies t hat span great dist ances.

ATM Concepts An ATM net work is m ade up of endpoint syst em s t hat have ATM NI Cs and swit ches t hat m ove t raffic bet ween t he endpoint s. An endpoint could be a workst at ion, a server, or a rout er. I t also m ight be an Et hernet , Token Ring, or FDDI swit ch t hat has an ATM int erface.

N ot e An SNMP agent or a net work server locat ed wit hin an ATM swit ch also can play t he role of an endpoint syst em .

Figure 20.1 illust rat es a big ATM net work. As t he figure shows, ATM net working is swit ch- based. Syst em s are connect ed t o swit ches, and t he swit ches are connect ed t o each anot her.

Figu r e 2 0 .1 . An ATM n e t w or k .

ATM prot ocols operat e at Layers 1 and 2. Unlike t he LAN prot ocols t hat have been exam ined in t his book, ATM is connect ion- orient ed. This m eans t hat a circuit m ust be set up bet ween t he com m unicat ing part ies before t hey can t ransm it dat a t o one anot her. A circuit can be long- t erm ( behaving like a leased line) or short - t erm ( behaving like a t elephone call) . A long- t erm circuit is called a perm anent virt ual circuit ( PVC) . A short - t erm circuit is called a swit ched virt ual circuit ( SVC) . Set t ing up a swit ched virt ual circuit is like m aking a t elephone call. The caller ident ifies t he called ATM syst em by providing it s ATM num ber ( which act ually is called an ATM address) . ATM num bers used in public net works have t he sam e st ruct ure as t elephone num bers. There is a lot of flexibilit y in privat e net work num bering syst em s. A privat e net work ATM address consist s of 20 hexadecim al byt es.

Quality of Service An Et hernet , Token Ring, or FDDI LAN is like a broad roadway. Syst em s shoot fram es ont o t he roadway. I n an ATM local or wide area net work, t he roadway is divided int o lanes, and groups of lanes provide different cat egories of service. This is illust rat ed in figure 20.2, which shows one direct ion of t raffic. A lot of payload is

hauled t hrough t he t wo t ruck lanes, but t raffic in t he t hree aut om obile lanes can whiz along at a higher speed. At rush hour, of course, t he t ruck and aut o lanes can get congest ed. Furt herm ore, accident s t hat prevent som e t raffic from get t ing t hrough are not uncom m on. The express lanes provide a high- priorit y service.

Figu r e 2 0 .2 . D iffe r e n t se r vice ca t e gor ie s on a r oa dw a y.

I n t he ATM world, a dual highway is analogous t o a com m unicat ions link. A set of lanes wit h com m on charact erist ics is called a pat h. An individual lane is called a channel.

N ot e A circuit is m ade up of t wo channels ( highway lanes) one for each direct ion of t raffic.

I nst ead of different service cat egories for groups of highway lanes, ATM offers different service cat egories for circuit s: •

The Const ant Bit Rat e ( CBR) service provides circuit s wit h a const ant bandwidt h.

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Real- t im e Variable Bit Rat e ( rt - VBR) service delivers a specified average bandwidt h and support s applicat ions such as com pressed voice or video, which are delay- sensit ive. Non- real- t im e Variable Bit Rat e ( nrt - VBR) service delivers a specified average bandwidt h and is suit able for dat a applicat ions, which are not st rongly sensit ive t o delay. Unspecified Bit Rat e ( UBR) is a best - effort service. I t m ight behave like I nt ernet service good at som e t im es of t he day and sluggish at ot her t im es. Available Bit Rat e ( ABR) is a best - effort service t hat is im plem ent ed in a sm art way. The service provides cont inuing feedback t hat indicat es how m uch bandwidt h is available for t he sender's use. By t hrot t ling back when necessary, senders avoid congest ing t he net work. This prevent s t raffic from m aking it halfway t hrough t he net work and t hen being t hrown away. The result is an im provem ent in overall t hroughput for everyone.

A circuit is described by it s service cat egory, t raffic param et ers, and Qualit y of Service param et ers. Traffic param et ers include it em s such as t he guarant eed average bandwidt h and peak bandwidt h for a circuit . Qualit y of Service param et ers include it em s such as delay, variat ion in delay ( which also is called j it t er) , and percent age of dat a loss.

N ot e The t rade press uses t he t erm Qualit y of Service as a blanket t erm encom passing t he com binat ion of service cat egory, t raffic param et ers, and Qualit y of Service param et ers. This m akes it easier for writ ers t o t alk about t he t opic wit hout going int o a lot of det ails.

Concurrent Circuits You can ident ify a st ream of t raffic by ident ifying t he group of lanes and t he lane num ber—for exam ple, aut om obile lane 3. Each ATM circuit is labeled wit h a pair of num bers t hat are called t he Virt ual Pat h I dent ifier ( VPI ) and t he Virt ual Channel I dent ifier ( VCI ) . The VPI is like t he group num ber, and t he VCI is like t he lane num ber wit hin t he group. An ATM int erface can support m any circuit s at t he sam e t im e. Figure 20.3 illust rat es channels for t wo circuit s at an ATM int erface. Each pair of channels is inside a pat h.

Figu r e 2 0 .3 . Ch a n n e ls for t w o vir t u a l cir cu it s t h a t sha r e a n a cce ss lin e .

N ot e By long t radit ion, when several circuit s share a single physical line wit hout being given fixed t im e slot s, t hey are called virt ual circuit s. Each circuit is set up wit h what ever service cat egory, bandwidt h, and Qualit y of Service is needed at a given t im e. The circuit s share an access line t hat is real and t hat has a real physical bandwidt h capacit y.

N ot e The ATM t echnology is very flexible. A virt ual circuit can be set up t o be one- way only so t hat t here would be a channel in one direct ion, but not t he ot her. I t also is possible t o im plem ent one- way point - t o- m ult ipoint circuit s. Dat a sent by a source would be replicat ed at a swit ch and sent out on several channels.

ATM Architecture ATM operat es at Layer 1 and Layer 2. The st ruct ure is shown in Figure 20.4

Figu r e 2 0 .4 . ATM La ye r s.

The ATM dat a link layer ( Layer 2) is called t he ATM Adapt at ion Layer ( AAL) . The ATM prot ocol suit e includes several prot ocols t hat can be used at Layer 2. The one t hat current ly is used for dat a com m unicat ions is called AAL5, and t hat is t he one t hat will be discussed here. AAL5 fram es can be very big. The inform at ion field in an AAL5 fram e can cont ain up t o 65,535 byt es. However, real im plem ent at ions generally st ick t o sizes of 9232 byt es or less. A higher- layer prot ocol in an endpoint device passes dat a t o t he ATM Adapt at ion Layer and ident ifies t he pat h and channel on which t he fram e should be sent . The ATM Adapt at ion Layer encapsulat es t he dat a int o a fram e and t hen slices t he fram e int o 48- byt e chunks. I t passes each chunk, along wit h it s VPI and VCI ident ifier, t o t he physical layer. The physical layer is divided int o t wo sublayers. The upper sublayer is called t he ATM Layer. The ATM Layer in an endpoint device adds a 5- byt e header t o each 48- byt e payload, form ing a cell. I t also queues cells in t he order in which t hey will be t ransm it t ed. The lower sublayer is called t he ATM Physical Layer. This sublayer is responsible for put t ing cells ont o a m edium at t he sending end and ext ract ing cells from t he m edium at t he receiving end.

I t oft en is m ist akenly st at ed t hat ATM has cells inst ead of fram es. This is t ot ally wrong. ATM has cells at Layer 1 inst ead of 1- byt e t im e slot s. I t has fram es at Layer 2 j ust like any ot her Layer 2 dat a prot ocol. Earlier chapt ers discussed Layer 2 and Layer 3 swit ching. ATM sw it ches operat e at Layer 1—only t he endpoint syst em s deal wit h fram es. I n Figure 20.4, not e t hat t he ATM Adapt at ion Layer appears only in t he endpoint syst em s. The Layer 1 ATM swit ches have t he j ob of m oving cells t hrough t he net work as fast as t hey can.

The ATM Cell Header Figure 20.5 shows t he form at of an ATM cell header for a cell t hat is sent bet ween an endpoint syst em and it s adj acent ATM swit ch.

Figu r e 2 0 .5 . Ce ll h e a de r se n t be t w e e n a n e ndpoin t syst e m a n d a n ATM sw it ch .

Current ly, t he first 4 bit s are unused. Recall t hat a virt ual circuit is ident ified by it s Virt ual Pat h I dent ifier ( VPI ) and Virt ual Channel I dent ifier ( VCI ) . The VPI occupies t he next 8 bit s. When expressed as a decim al num ber, t he VPI value can range from 0 t o 255.

The VCI occupies 16 bit s. Hence, when expressed as a decim al num ber, t he VCI value can range from 0 t o 65,535. I n all, t here are 16,777,216 possible ( VPI , VCI ) pairings. Most ATM cells carry user dat a, but t here also are som e diagnost ic, t est , and net work m anagem ent cells t hat belong t o a special cat egory called operat ions, adm inist rat ion, and m aint enance ( OAM) . The 3- bit payload t ype reveals whet her a cell cont ains user dat a or OAM inform at ion as well as som e ot her inform at ion t hat is useful: •

•

•

The first bit of t he payload t ype field indicat es whet her t he cell carries user dat a ( 0) or OAM dat a ( 1) . For user dat a, t he second bit is set t o 1 if congest ion is experienced along t he pat h t o t he dest inat ion. The t hird bit is im port ant for AAL5 fram es. I t is set t o 1 t o indicat e t hat a cell cont ains t he final segm ent of an AAL5 fram e.

I f an ATM swit ch get s congest ed, it m ight be forced t o t hrow away som e cells. Cells whose cell loss priorit y bit is set t o 1 have a lower priorit y and are m ore eligible for discard. The header error cont rol field cont ains t he result of a calculat ion perform ed on t he previous 4 byt es. The header error cont rol field is recalculat ed at every swit ch along t he way and at t he dest inat ion. I f t he calculat ed value does not m at ch t he value in t he field, t he header has been corrupt ed. I n som e cases, t he header error cont rol value act ually can be used t o correct a single- bit error. Correct ion does not work for som e physical layers because of t he way dat a is encoded ont o t he m edium . I n any case, if t he problem was caused by a m ult ibit error, or if correct ion does not work, t he cell is discarded.

VPI and VCI Numbers The designers of ATM t hought big and envisioned net works t hat would include hundreds or t housands of swit ches. Each part icipat ing syst em m ust be assigned an address t hat ident ifies where it is locat ed. However, t he VPI and VCI num bers t hat ident ify a circuit are com plet ely separat e from t he ATM address. They have not hing t o do wit h locat ion. They are chosen for convenience at each end when set t ing up a perm anent or swit ched virt ual circuit . As long as a channel has been assigned t o a suit able t ype of pat h, and as long as t he com binat ion of pat h and channel num bers is unique at t he endpoint , any choice is all right . Thinking in t erm s of m illions of circuit s, it would be im possible t o t ry t o pick a new ( VPI , VCI ) com binat ion t hat was not being used anywhere else in t he net work. I nst ead, t hese num bers have only local significance and can be m apped t o a new pairing at every swit ch in t he net work.

Figure 20.6 illust rat es how t his works. Endpoint syst em A ident ifies t he circuit using VPI = 0 and VCI = 40. ATM swit ch 1 knows t hat t he circuit t hat ent ers wit h ident ifiers ( 0,40) m ust leave wit h ident ifiers ( 2,83) . ATM swit ch 2 knows t hat ident ifiers ( 2,83) m ust be m apped t o ( 0,53) because endpoint syst em B recognizes t he circuit using t hese num bers.

Figu r e 2 0 .6 . Pa ir s of ( VPI , VCI ) n u m be r s u se d a lon g a cir cu it .

Processing a Cell Cell headers are processed by special- purpose chips and can be processed very quickly. At a swit ch, t he m ost im port ant processing st ep is a t able lookup t hat m aps t he incom ing port , VPI , and VCI t o an out going port , VPI , and VCI . Table 20.1 shows a few sam ple m appings. For exam ple, a cell arriving at swit ch port 1 wit h VPI = 0 and VCI = 45 will be forwarded t hrough port 6 wit h new VPI = 2 and VCI = 62. Table 20.1. Mapping I ncom ing Cells t o Out going Cells I n com in g

Ou t goin g

VPI

VCI

Port

VPI

VCI

1

0

45

6

2

62

1

1

72

3

3

75

2

0

38

3

5

100

Aft er an endpoint passes a cell t o an adj acent ATM swit ch, t he size of t he VPI field in t he cell header is increased t o 12 bit s, as is shown in Figure 20.7 This m eans t hat wit hin t he net work, pat h num bers can range from 0 t o 4095.

Figu r e 2 0 .7 . H e a de r of a ce ll se n t be t w e e n a pa ir of ATM sw it ch e s.

Interleaving Cells The use of cells m akes it easy t o build scalable swit ches t hat forward t raffic at very high speeds. Unlike t he old t elephony swit ches, ATM swit ches do not have t o dem ult iplex and rem ult iplex com plicat ed pat t erns of call slot t im es. Endpoint syst em s also gain a big advant age by breaking fram es int o cells. Fram es vary in size from very sm all t o very large. Wit h a convent ional Layer 2 or Layer 3 swit ch, when t ransm ission of a fram e st art s, all ot her t raffic m ust wait in line unt il t hat t ransm ission is com plet e. Figure 20.8 shows t raffic t hat is lined up, wait ing for t he t ransm ission of a large fram e t o com plet e. The sm all fram es at t he t op arrived in t heir high- priorit y queue aft er t ransm ission of t he large fram e had begun. Not hing can be done t o get t he high- priorit y fram es ont o t he wire unt il t he big fram e is out of t he way.

Figu r e 2 0 .8 . Fr a m e de la y.

The variable sizes of fram es m ake fram e delay t im es very difficult t o cont rol and predict . Figure 20.9 shows cells t hat are queued up for t ransm ission on t hree channels. Channel 1 is a high- priorit y channel. ( For exam ple, it m ight be used for voice.) Channel 2 is used for int eract ive dat a, and channel 3 is used for background bulk dat a t ransfer.

Figu r e 2 0 .9 . I nt e r le a ve d t r a n sm ission of ce lls.

When dat a is segm ent ed int o cells, it is a lot easier t o ensure t hat a channel get s access t o t he bandwidt h t hat it has been allocat ed. The use of cells bot h m inim izes

delay and m akes delay t im es far less variable. The high- priorit y channel 1 cells are not delayed because t hey can be int erleaved int o t he t raffic im m ediat ely. Furt herm ore, if t here is a lull in channel 1 and channel 2 t raffic, t he ext ra cells do not have t o be wast ed; t hey can be filled wit h som e of t he dat a sent on channel 3. This is t he idea behind t he unspecified bit rat e ( UBR) service. A UBR channel usually is assigned som e m inim um guarant eed rat e, but also get s t o use left over bandwidt h from ot her channels. Using circuit s and cells offers several advant ages: • •

•

•

•

A physical link can be shared by several virt ual circuit s. Each circuit can be assigned t he proport ion of t he bandwidt h t hat t he user want s it t o have. Traffic across a circuit is seen only by it s source and dest inat ion syst em s, which enhances securit y. Using cells avoids t he variable delays t hat plague t ransm ission m et hods based on variable- sized fram es. Cells are easy t o swit ch, and swit ch product archit ect s do not have t o worry about com plicat ed buffer m em ory m anagem ent . Every cell is 53 byt es in lengt h.

Som e disadvant ages also arise, however: •

• •

Wit h 5 byt es of header for each 48 byt es of payload, cell headers add a 10.4% overhead t o dat a t raffic. Most local area net works adm inist rat ors are unfam iliar wit h ATM t echnology, m aking t he chore of configuring an ATM LAN endpoint syst em or swit ch difficult . Convert ing a syst em t o ATM requires t he inst allat ion of a relat ively cost ly NI C.

AAL5 Frame Format Figure 20.10 shows t he form at of an AAL5 fram e. At first view, t he form at is surprising because t here is no fram e header. There is a good reason for t his. I n an ordinary LAN, a header is used t o ident ify a fram e's source and dest inat ion. I n a circuit - based environm ent , t he header is not needed. A circuit m ust be set up before any dat a can be t ransferred. The circuit is ident ified by it s pat h and channel num bers, which appear in each cell header. No circuit ident ifier appears in a fram e header.

Figu r e 2 0 .1 0 . For m a t of a n AAL5 fr a m e .

The t railer field cont ains som e im port ant housekeeping inform at ion. As was t he case for Et hernet , Token Ring, and FDDI , t he AAL5 t railer cont ains t he result of a 4- byt e cyclic redundancy check funct ion calculat ion. ( This result is st ored in t he fram e check sequence field in an Et hernet , Token Ring, or FDDI fram e.) Recall t hat t he AAL layer slices a fram e int o 48- byt e segm ent s. The pad field is needed because t he ent ire fram e m ust be a m ult iple of t he cell payload size, 48 byt es. The payload lengt h value enables t he receiver t o figure out how m any pad byt es have been insert ed and t o verify t hat none of t he cells in t he fram e have been lost . The end of a fram e is m arked by set t ing a flag in t he header of it s final cell. The user- t o- user byt e is reserved for use by a higher- layer prot ocol. The com m on part indicat or ( CPI ) current ly is j ust a fill byt e, bringing t he t railer t o a t ot al lengt h of 8 byt es. This brief overview has provided a general idea of t he way t hat ATM works. Chapt er 21 looks at t he way t hat ATM has been adapt ed for use in local area net works.

Summary Points •

• • • •

•

•

ATM originally was designed t o upgrade t he world's public wide- area t elecom m unicat ions net works. ATM is scalable, flexible, and capable of handling voice, video, and dat a t raffic. ATM can be used in local area net works as well as wide area net works. An ATM net work is m ade up of a set of ATM swit ches, and com put ers, rout ers, and Layer 2 LAN swit ches t hat have ATM int erfaces. ATM prot ocols operat e at Layers 1 and 2. Layer 2 is called t he ATM Adapt at ion Layer. The physical layer is divided int o t wo sublayers, called t he ATM Layer and t he ATM Physical Layer. Several service cat egories have been defined for ATM. These include Const ant Bit Rat e ( const ant bandwidt h, such as for uncom pressed voice) , Real- t im e

•

• • •

• •

•

Variable Bit Rat e ( for exam ple, com pressed voice or video) , Non- real- t im e Variable Bit Rat e ( for exam ple, LAN dat a) , Unspecified Bit Rat e ( best effort ) , and Available Bit Rat e ( best effort wit h feedback) . An ATM circuit is labeled wit h a Virt ual Pat h I dent ifier ( VPI ) and a Virt ual Channel I dent ifier ( VCI ) . Mult iple ATM circuit s can share t he bandwidt h on a physical link. An ATM cell has a 5- byt e header t hat cont ains t he circuit 's VPI and VCI . Som e diagnost ic, t est , and net work m anagem ent cells belong t o a special cat egory called operat ions, adm inist rat ion, and m aint enance. VPI and VCI num bers can be changed at each swit ch. A device t ransm it s a st ream of cells. Cells for m ult iple circuit s are int erleaved in t he st ream . An AAL5 fram e does not have a header. I t s t railer cont ains a CRC value and t he lengt h of t he payload.

References The prot ocol feat ures im plem ent ed by endpoint syst em s and t heir adj acent swit ches are described in t hese ATM Forum docum ent s: • •

" ATM User- Net work I nt erface Specificat ion Version 3.1." 1994. " ATM Forum Traffic Managem ent Specificat ion Version 4.0." 1996.

The I TU- T I series includes a very large num ber of docum ent s t hat relat e t o ATM. For a det ailed descript ion of ATM net works and ATM prot ocols, see •

Feit , S. Wide Area High Speed Net works. I ndianapolis, I N: Macm illan Technical Publishing, 1998.

Chapter 21. ATM LAN Emulation A set of syst em s connect ed t o an ATM swit ch behaves like a bunch of t elephones connect ed t o a t elephone swit ch: One inst rum ent t alks t o anot her by m aking a call. This is very different from t he way a LAN swit ch works. A st at ion on a LAN fires off an individual fram e t o anot her syst em —or t o all syst em s—any t im e it wishes. I t does not have t o go t hrough a call set up first . I n spit e of t his vast difference in archit ect ures, t he ATM Forum was det erm ined t o int egrat e ATM swit ches int o convent ional LANs. The solut ion is called LAN Em ulat ion ( LANE) . LANE sat isfies t hese ATM Forum obj ect ives: •

•

I t enables a set of syst em s t hat have ATM NI Cs and are connect ed t o ATM swit ches t o em ulat e a convent ional LAN. I t enables Et hernet or Token Ring syst em s connect ed t o ordinary hubs and swit ches t o int eract wit h ATM syst em s as if t hey all belong t o a convent ional LAN.

LANE overcom es anot her hurdle t hat is t he m ost im port ant of all: I t provides backward com pat ibilit y wit h exist ing higher- layer prot ocols and applicat ions. LANE m akes it possible for exist ing net work prot ocols and net work applicat ions t o run on t op of ATM wit hout m aking changes t o any of t he soft ware. To t he net work layer and above, t he environm ent appears t o be an ordinary Et hernet or Token Ring.

N ot e At t he t im e of writ ing, LAN Em ulat ion is not support ed for FDDI .

Because wide area ATM links can run at speeds t hat m eet or exceed local area connect ions, it is possible t o build a high- perform ance LAN t hat spans m ult iple sit es. Client s at one sit e t hat access ATM- at t ached servers at anot her sit e can experience rem ot e bandwidt h capacit y and response t im e t hat rivals local bandwidt h capacit y and response t im e. See figure 21.1 illust rat es a high- speed em ulat ed LAN t hat spans a wide area ATM connect ion.

Figur e 2 1 .1 . An e m ula t e d LAN t h a t spa n s a w ide a r e a lin k .

The goal of t his chapt er is t o build a good concept ual underst anding of LANE, sufficient t o com prehend • •

The value t hat LANE can add t o a net work Product descript ions and product docum ent at ion

Lot s of opt ions and prot ocol det ails have been om it t ed so t hat t he discussion can concent rat e on t he essent ials. Even so, LANE can appear t o be quit e com plicat ed at first view. However, aft er you sort out t he roles of it s various special servers, it t urns out t o be pret t y st raight forward.

Emulated LAN Environments An em ulat ed LAN ( ELAN) is m ade up of com put ers, rout ers, and bridges t hat have ATM int erfaces and are connect ed t o ATM swit ches. A single ELAN can span several int erconnect ed ATM swit ches, and t he sam e set of swit ches can host several ELAN com m unit ies. Each ELAN is ident ified by a unique nam e.

There is a close relat ionship bet ween VLANs and ELANs. An ELAN can be part of a larger VLAN t hat includes ATM syst em s and convent ional LAN syst em s. I n an environm ent t hat uses VLANs, an adm inist rat or can configure t he ELAN t o be part of a part icular VLAN. An ATM NI C is assigned a unique 6- byt e MAC LAN address, j ust like an Et hernet or Token Ring NI C. Having a unique MAC address is an essent ial requirem ent t o part icipat ing in an em ulat ed LAN. The circuit - based environm ent is m olded int o a LAN- like environm ent t hrough t he use of t hree special em ulat ion servers. These em ulat ion servers m ake it possible for a MAC fram e t o get from it s source t o it s dest inat ion. See figure 21.2 shows an Et hernet environm ent t hat includes ATM ELAN com ponent s. I n t he figure, Et hernet swit ches A, B, and C and a pair of cent ral applicat ion servers are connect ed t o a backbone ATM swit ch. The syst em s at t ached t o t he Et hernet swit ches and t o t he hubs in t he Et hernet collision dom ain can com m unicat e wit h one anot her and wit h t he applicat ion servers t hat are at t ached t o t he ATM swit ch.

Figur e 2 1 .2 . An e m ula t e d Et h e r n e t LAN e n vir on m e n t .

The Et hernet swit ches relay t raffic bet ween t he Et hernet environm ent and t he ATM environm ent . They are called proxies because t hey relay t raffic t o and from ATM syst em s on behalf of t he convent ional LAN st at ions. See figure 21.3 shows a Token Ring environm ent t hat includes ATM ELAN com ponent s. Token Ring swit ches A and C, source rout e bridge B, and a pair of cent ral applicat ion servers are direct ly connect ed t o a backbone ATM swit ch. The ordinary LAN syst em s at t ached t o t he Token Ring swit ches and t o source rout e bridge B and it s at t ached rings can com m unicat e wit h one anot her and wit h t he applicat ion servers t hat are at t ached t o t he ATM swit ch.

Figur e 2 1 .3 . An e m ula t e d Tok e n Ring e nvir onm e nt .

Token Ring swit ches A and C and source rout e bridge B are called proxies because t hey relay t raffic bet ween t he Token Ring environm ent and t he ATM environm ent .

LAN Emulation Clients

One of t he im port ant funct ions t hat LANE perform s is t o hide t he underlying ATM layers from t he higher prot ocol layers. This is done by a special device driver t hat is called a LAN Em ulat ion Client ( LEC) or a LANE Client . See Figure 21.4 shows t he locat ion of t he LANE Client in an endpoint syst em 's prot ocol st ack. The higher- layer prot ocol soft ware sends and receives dat a using send and receive calls t hat are part of a st andard library of Et hernet or Token Ring device driver calls.

Figu r e 2 1 .4 . A LAN Em u la t ion Clie n t in a n e ndpoin t syst e m .

N ot e There are LANE Client device drivers for every m aj or operat ing syst em .

To t he upper layers, t he LANE Client looks j ust like an ordinary Et hernet or Token Ring device driver. However, t he LANE Client t ransm it s dat a t hrough an ATM int erface and also perform s several fairly com plicat ed prot ocol procedures.

For exam ple, a LANE Client m ust enroll it s syst em in an ELAN. The enrollm ent process t ies t he client t o t wo em ulat ion servers—one t hat assist s wit h unicast com m unicat ion and anot her t hat handles broadcast s and m ult icast s. When an upper layer t ells t he client t o t ransm it a st ream of unicast fram es t o a dest inat ion MAC address, t he client 1. Sends a query t o t he first em ulat ion server asking for t he ATM address of t he dest inat ion. 2. Opens an ATM circuit t o t he dest inat ion. 3. Exchanges fram es wit h t he peer syst em . When an upper layer t ells t he client t o t ransm it a broadcast or m ult icast fram e, t he client forwards t he fram e t o t he second em ulat ion server, which relays t he fram e t o all of t he ot her m em bers of t he ELAN. The sect ions t hat follow fill in t he det ails.

LAN Emulation Servers Three special em ulat ion servers are used t o help LAN Em ulat ion client s m im ic t he behavior of connect ionless LAN syst em s: •

•

•

The first ( t he configurat ion server) is like t he principal of an elem ent ary school, who assigns each incom ing st udent t o a classroom ( an ELAN) . The second ( t he LANE Server) is like t he classroom t eacher, who t akes at t endance and keeps t rack of everyone who is t here. This t eacher ( like a LAN Em ulat ion Server) has got everybody's num ber. The t hird ( t he broadcast er) is a gossip. Tell him som et hing, and everybody else will know it im m ediat ely.

More form ally, t he t hree servers are t he •

•

•

LAN Em u la t ion Configu r a t ion Se r ve r or LAN E Configu r a t ion Se r ve r ( LECS) — Assigns a LAN Em ulat ion Client t o a specific em ulat ed LAN. LAN Em u la t ion Se r ve r or LAN E Se r ve r ( LES) — I s in charge of one em ulat ed LAN and keeps t rack of t he MAC addresses and ATM addresses of it s m em bers. A LANE client st ays connect ed t o t his server so t hat t he client can ask for t ranslat ions from MAC address t o ATM address any t im e t hat it needs t hem . Br oa dca st a n d Unk n ow n Se r ve r ( BUS) —Delivers broadcast and m ult icast fram es t o st at ions in t he em ulat ed LAN. A client st ays connect ed t o t his server so t hat t he client can send and receive broadcast and m ult icast fram es at any t im e.

See figure 21.5 displays anot her view of t he Et hernet ELAN in See figure 21.2 t hat includes t he LANE client s and servers. See figure 21.6 displays anot her view of t he Token Ring ELAN in Figure 21.3 t hat includes t he LANE client s and servers.

Figu r e 2 1 .5 . Clie n t s a n d se r ve r s in a n Et h e r n e t ELAN .

Figu r e 2 1 .6 . Clie n t s a n d se r ve r s in a Tok e n Rin g ELAN .

N ot e I n t he figures, t he t hree em ulat ion servers are locat ed in t he ATM swit ch. I n fact , em ulat ion servers can be locat ed in a swit ch, rout er, or endpoint syst em . The servers also can be spread across different syst em s.

The LAN Emulation Configuration Server A LANE Configurat ion Server does not have a lot t o do, but it s j ob is im port ant . The server assigns each LANE client t o an appropriat e ELAN. The client int eract ion wit h t he configurat ion server is st raight forward. The client opens an ATM circuit t o t he server and announces it s own ATM address. Opt ionally, t he client can provide addit ional param et ers t hat can help t he server det erm ine t he ELAN t o which t he client should be assigned. These param et ers can include •

The t ype of ELAN t hat t he client want s t o j oin ( Et hernet or Token Ring)

• • • •

The nam e of an ELAN t hat t he client would like t o j oin The m axim um fram e size t hat t he client can support The client 's MAC address A Layer 3 address

The configurat ion server responds wit h • •

The ATM address of a specific LANE Server The t ype of ELAN ( Et hernet or Token Ring) t o which t he client has been assigned The nam e of t he ELAN t o which t he client has been assigned The m axim um fram e size for t he ELAN

• •

A single configurat ion server can support an ent ire net work, but it also is possible t o divide t he j ob am ong several servers. See figure 21.7 illust rat es a client 's int eract ion wit h a LANE Configurat ion Server.

Figu r e 2 1 .7 . Ask in g t h e con figur a t ion se r ve r for t h e ATM a ddr e ss of a LAN E Se r ve r .

N ot e The special connect ion used bet ween t he client and t he configurat ion server is called a Configurat ion Direct Virt ual Channel Connect ion.

Se t t in g Up a Con figu r a t ion Se r ve r When you underst and how client s int eract wit h a configurat ion server, it is easy t o underst and what an adm inist rat or m ust do t o init ialize t he server. The adm inist rat or m ust ent er one or m ore pairings: Nam e of ELAN ATM address of ELAN Server

Next , t he crit eria for assigning client s t o an ELAN m ust be ent ered. The crit eria t hat can be used are up t o each vendor, but t he following are reasonable alt ernat ives: • • •

•

Ent er a single ELAN nam e, and assign all client s t o t hat ELAN by default . Assign a client t o an ELAN when t he client asks t o j oin t hat ELAN by nam e. Assign a client t o an ELAN based on t he client 's ATM address. The ATM addresses of all t he client s t hat belong t o each ELAN m ust be ent ered. Set up t wo ELANs: one for Et hernet and one for Token Ring. Assign a client t o an ELAN based on t he t ype of ELAN request ed.

Fin din g t h e Con figu r a t ion Se r ve r To get st art ed, a client m ust figure out how t o cont act it s configurat ion server. Three m et hods can be used: •

Use a " well- known" ATM group address t hat has been reserved for configurat ion servers: X'47007900000000000000000000- 00A03E000001- 00

• •

Obt ain t he configurat ion server's address during syst em init ializat ion. This is done via t he I nt egrat ed Local Managem ent I nt erface ( I LMI ) prot ocol, which is described lat er, in t he sect ion " I nit ializat ion wit h I LMI ." When t he client syst em is inst alled, set up a perm anent virt ual circuit t hat aut om at ically is act ivat ed whenever t he client boot s. This circuit m ust have VPI = 0 and VCI = 17.

Opening a connect ion t o t he well- known ATM address of t he configurat ion server is t he m et hod t hat current ly is preferred. When t he client cont act s t he configurat ion server, t he client ret rieves t he ATM address of it s LANE Server.

N ot e The form at of t he preceding 20- byt e configurat ion server address follows an ATM address display convent ion. The first 13 byt es are grouped t oget her. A hyphen separat es t his group from t he next group of 6 byt es. Anot her hyphen int roduces t he final byt e.

The LAN Emulation Server Arm ed wit h t he address of it s LANE server, t he client now is prepared t o j oin an ELAN. The first st ep is t o open an ATM connect ion t o t he LANE Server.

N ot e This connect ion is called t he client 's cont rol direct virt ual channel connect ion.

The client and LANE Server t hen exchange a j oin request and response. The client can include it s MAC address wit h it s j oin request , or, alt ernat ively, t he client can announce it s MAC address in a separat e regist er request aft er t he j oin has com plet ed. I f t he client is locat ed in an Et hernet or Token Ring bridge or swit ch, it indicat es t hat it is a proxy. I t regist ers it s own address but also indicat es t hat it represent s ot her syst em s whose MAC addresses will not be regist ered. Traffic will be relayed t o t hose syst em s via an ATM connect ion t o t he proxy.

N ot e Som e client s have m ore t han one MAC address. A client can announce m ult iple MAC addresses in a series of regist er request s.

I n any case, when t he j oin and regist er st eps are com plet e: • • •

•

The server knows t he client 's ATM address and MAC address( es) . The client has been given t he aut horit at ive values of t he LAN t ype, t he m axim um fram e size, and t he nam e of t he ELAN. The client m ight have been assigned a unique 2- byt e ident ifier called t he LAN Em ulat ion Client I D ( LECI D) . The client includes t his ident ifier ( if assigned) in all it s inform at ion fram es. ( However, som e LAN Servers do not assign t hese ident ifiers.) The server knows whet her t he client act s as a proxy for som e ordinary LAN syst em s.

N ot e A client syst em t hat is a Token Ring source rout e bridge can regist er it s rout e descript or inst ead of—or in addit ion t o—a MAC address. Hence, in Token Ring LANE environm ent s, t he LANE Server m aint ains a list of source rout e bridge rout e descript ors as well as MAC addresses and ATM addresses.

See figure 21.8 illust rat es a j oin/ regist er t ransact ion.

Figu r e 2 1 .8 . Join in g a n ELAN .

Addr e ss Re solu t ion The LANE Server builds up a dat abase t hat cont ains t he t ranslat ions from MAC address t o ATM address for syst em s t hat have j oined it s ELAN. This put s it in a perfect posit ion t o assist LANE client s t hat need t o m ap a dest inat ion MAC address t o a dest inat ion ATM address t o open up an ATM connect ion and exchange fram es across t he connect ion. The process of discovering t he ATM address t hat corresponds t o a MAC address is called address resolut ion. Aft er j oining an ELAN, t he client holds on t o it s init ial connect ion t o it s ELAN LANE Server. The client sends address resolut ion request s, which are called LAN Em ulat ion ARP request s ( LE ARP request s) , across t his connect ion and receives LE ARP responses from t he server on t his connect ion. The LANE Server opt ionally can open a second connect ion back t o t he client .

N ot e The connect ion opened by t he server is called t he cont rol dist ribut e virt ual channel connect ion.

This can be an ordinary point - t o- point connect ion. However, a very efficient way for t he server t o set t his up is t o use a point - t o- m ult ipoint connect ion. Figure 21.9 illust rat es t he way a point - t o- m ult ipoint cont rol dist ribut e connect ion is used. Client A request s t he ATM address corresponding t o a given MAC address. The server checks and discovers t hat t he MAC address is not list ed in it s regist rat ion dat abase. The server forwards t he request on t he point - t o- m ult ipoint connect ion. A syst em t hat knows t he answer will respond.

Figu r e 2 1 .9 . Addr e ss r e solu t ion .

Not e t hat in See figure 21.9, t he syst em whose MAC address is not in t he LANE Server's dat abase is not an ATM syst em ; it is at t ached t o an Et hernet swit ch. The swit ch act s as a proxy for it s at t ached syst em s and responds t o t he request wit h it s own ATM address. Client A t hen can open an ATM connect ion t o t he proxy, which will relay t raffic t o and from t he t rue t arget syst em . A LANE Server t hat does not open t hese back connect ions t o it s client s can relay LE ARP request s t o it s client s using t he ordinary cont rol connect ions. I n t his case, a LAN Em ulat ion server t hat needs t o resolve an unknown MAC address m ight not bot her t o forward a request t o every client , but t he server would be sure t o forward t he request t o client s t hat act as proxies.

The Broadcast and Unknown Server A client prepares t o send broadcast s and m ult icast s by linking t o t he Broadcast and Unknown Server ( BUS) . To get t he ATM address of t he BUS, t he client sends an LE ARP query t o it s LANE Server, asking for t he ATM address of t he syst em corresponding t o t he broadcast address MAC address X'FF- FF- FF- FF- FF- FF.

When t he client receives t he ATM address of t he BUS, t he client opens an ATM connect ion t o t he BUS. The client uses t his connect ion t o t ransm it broadcast fram es and m ult icast fram es.

N ot e This connect ion is called t he m ult icast send virt ual channel connect ion.

The client can forward ot her MAC fram es t o t he BUS on t his connect ion. This m ight be done for prot ocols t hat involve very lit t le t raffic. For exam ple, a TCP/ I P Dom ain Nam e server query t hat t ranslat es a com put er nam e such as www.abc.com t o an I P address t ypically is perform ed using one query fram e and one response fram e. I t would not m ake a lot of sense t o open up an ATM circuit for t his sm all exchange of dat a. I n addit ion, som et im es t he local LANE Server will not have a dest inat ion MAC address in it s dat abase and will be incapable of t ranslat ing it t o an ATM address im m ediat ely. The client can ask t he BUS t o flood t he fram e. Aft er t he client has connect ed t o t he BUS, t he BUS opens up a separat e connect ion t hat it uses t o deliver broadcast , m ult icast , or flooded fram es t o t he client .

N ot e This connect ion is called t he m ult icast forward virt ual channel connect ion.

See figure 21.10 illust rat es t hese procedures.

Figu r e 2 1 .1 0 . Con t a ct in g t h e BUS.

The bus can deliver broadcast s, m ult icast s, and flooded fram es t o it s client s very efficient ly by using a point - t o- m ult ipoint connect ion as it s m ult icast forward- delivery vehicle. See figure 21.11 illust rat es t he use of a point - t o- m ult ipoint delivery connect ion.

Figu r e 2 1 .1 1 . BUS de live r y via a poin t - t o- m u lt ipoin t con n e ct ion .

Not e t hat t he BUS in See figure 21.11 is locat ed at an ATM swit ch, which is a nat ural place t o put it . However, recall t hat t he BUS Server also can be locat ed in a rout er or an end st at ion. I n t he figure, client D t ransm it s a broadcast fram e by sending it t o t he BUS on t he m ult icast send connect ion. The BUS delivers t he fram e by sending it out t he m ult icast forward connect ion. Not e t hat t he fram e will be delivered t o all client s, including client D.

ATM LANE Protocol Elements This sect ion describes t he m aj or elem ent s t hat are used in t he LANE prot ocol, including • •

• •

Privat e ATM addresses ATM address init ializat ion via t he I nt egrat ed Local Managem ent I nt erface ( I LMI ) The form at of LANE dat a fram es The funct ions and form at of t he cont rol fram es t hat are used in t he LANE prot ocols

Privat e ATM addresses and I LMI are general ATM prot ocol elem ent s t hat are used bot h out side of and wit hin LANE environm ent s. The sect ion also includes a t race of a LAN Em ulat ion Et hernet fram e.

ATM Addresses Several form at s are defined for ATM addresses, but t hey all have one t rait in com m on: They are hierarchical. For exam ple, global t elephone num bers t hat ident ify a count ry, region, and subscriber num ber are used as ATM addresses.

N ot e These st andard num bers are called E.164 addresses.

The t wo address form at s m ost likely t o be used in a privat e ATM net work are shown in See Figure 21.12. Each consist s of 20 byt es. The init ial byt e indicat es t he t ype of address t hat follows. The address form at at t he t op of See figure 21.12 st art s wit h one of t he st andard E.164 num bers. This m ight be a general num ber t hat is used t o reach t he sit e across a wide area net work. This num ber is followed by 4 byt es t hat can be used t o ident ify a specific swit ch at t he sit e.

Figu r e 2 1 .1 2 . Addr e ss for m a t s.

The next 6 byt es cont ain an End Syst em I dent ifier ( ESI ) . A MAC address norm ally is used as t he End Syst em I dent ifier.

The select or ( SEL) byt e at t he end of each address plays no role in rout ing a call t o an ATM endpoint syst em ; it is an ext ra byt e whose use has not been st andardized. The select or byt e can be used t o select a specific soft ware com ponent t hat should process incom ing ATM fram es. For exam ple, it can different iat e am ong m ult iple LANE Servers t hat are collocat ed at t he sam e node. The address form at at t he bot t om of See figure 21.12 st art s wit h 12 byt es t hat an organizat ion can use for hierarchical addressing. For exam ple, subfields could ident ify an area, a building, and t he specific swit ch t o which t he endpoint is connect ed.

Initialization with ILMI ATM m akes resourceful use of Sim ple Net work Managem ent Prot ocol m essages t o im plem ent a set of procedures called t he I nt egrat ed Local Managem ent I nt erface ( I LMI ) . I LMI procedures are not specific t o LANE; t hey are general ATM procedures. At init ializat ion, t he following occurs: 1. An endpoint syst em obt ains t he first 13 byt es of it s ATM address from it s swit ch. 2. The syst em com plet es it s ATM address by regist ering it s MAC address at t he swit ch. 3. The syst em and it s swit ch exchange configurat ion set t ings and select a com m on set of param et er values t hat bot h support . 4. Opt ionally, a syst em also can find out t he addresses of t he LANE Configurat ion Server via I LMI . The swit ch cont inues t o use I LMI t o poll t he st at ion periodically t o check t hat t he link and t he st at ion are st ill funct ioning. I LMI m essages are SNMP request s and responses t hat are carried in AAL5 fram es.

N ot e SNMP m essages m ost frequent ly are carried using UDP and I P. However, in t his case, t hese prot ocols are not needed. The SNMP m essages are packed direct ly int o AAL5 payloads.

LANE Data Frames See figure 21.13 shows t he form at of an Et hernet fram e t hat is carried in an AAL5 fram e. The AAL5 fram e st art s wit h a 2- byt e field t hat cont ains eit her t he sending client 's unique LANE Client ident ifier or X'00- 00. An Et hernet fram e follows im m ediat ely. The Et hernet fram e does not include a fram e check sequence; error checking is t aken care of by t he CRC in t he AAL5 fram e t railer.

Figu r e 2 1 .1 3 . A LAN E Et h e r n e t fr a m e .

Et hernet fram es sent bet ween a pair of ATM syst em s can be bigger t han ordinary Et hernet fram es. I n fact , t hey can be as large as Token Ring fram es. For an 802.3 LLC fram e, t he lengt h field of an oversized fram e m ust be set t o 0. The t rue size is det erm ined from t he lengt h field in t he AAL5 t railer. Using large fram es m akes good sense when an ATM syst em com m unicat es wit h anot her ATM syst em . Of course, large fram es cannot be used when an ATM syst em com m unicat es wit h a convent ional Et hernet LAN syst em . List ing 21.1 shows a Net work Associat es Sniffer t race of a LANE Et hernet fram e arriving at an endpoint syst em . The first part of t he t race displays inform at ion about t he ATM circuit t hat carried t he fram e in t he t race. The fact t hat t here is a call reference value is a clue t o t he fact t hat t he fram e was carried on a swit ched virt ual circuit . Each call is assigned a call reference num ber when it is opened. The Virt ual Pat h I dent ifier for t he circuit is 0, and t he Virt ual Channel I dent ifier is 42. Moving t o t he AAL5 fram e, it st art s wit h t he LAN em ulat ion client ident ifier of t he sender, which is X'00- 10 ( decim al 16) . The encapsulat ed Et hernet fram e follows. The

fram e in t he t race cont ains an LLC and SNAP, but a fram e wit h an Et herType field is equally accept able. The inform at ion port ion has been om it t ed from t he list ing because it is not of any part icular int erest . The t railer cont ains a CRC value and also indicat es t hat t he fram e payload size was 318 byt es.

List in g 2 1 .1 Tr a ce for a LAN E Et h e r n e t Fr a m e ATM: ----- ATM Header ----ATM: ATM: Frame 3 arrived at 15:26:51.6041; frame size is 336 (0150 hex) bytes. ATM: Call reference = 175 ATM: Link = DCE ATM: Virtual path id = 0 ATM: Virtual channel id = 42 ATM:

LE8023: ----- LAN EMULATION 802.3 ----LE8023: LE8023: LE Header 0010 LE8023: ETHER: ----- Ethernet Header ----ETHER: ETHER: Destination = Multicast 01000CCCCCCC ETHER: Source = Station Cisco140DC20 ETHER: 802.3 length = 302 ETHER: LLC: ----- LLC Header ----LLC: LLC: DSAP Address = AA, DSAP IG Bit = 00 (Individual Address) LLC: SSAP Address = AA, SSAP CR Bit = 00 (Command) LLC: Unnumbered frame: UI LLC: SNAP: ----- SNAP Header ----SNAP: SNAP: Vendor ID = Cisco1 SNAP: Type = 2000 (CDP) SNAP: CDP: ----- Cisco Discovery Protocol (CDP) Packet ----. . . (message contents omitted) . . . ATM: ATM: ----- AAL5 Trailer ----ATM: ATM: 10 pad octets ATM: UU = 0 ATM: CPI = 0 ATM: Length = 318 ATM: CRC = 57C64FAE See figure 21.14 shows t he form at of an encapsulat ed Token Ring fram e. Like t he Et hernet fram e, it st art s wit h t he sender's LANE Client ident ifier or X'00- 00. The Token Ring fram e follows. However, a Token Ring access cont rol field serves no

purpose in an em ulat ed LAN, so t he access cont rol byt e is j ust a placeholder and it s cont ent is ignored. The fram e cont rol field cont ains t he binary value:

Figur e 2 1 .1 4 . A LAN E Tok e n Ring Fr a m e .

0 1 0 0 0 p p p Because only user dat a ( LLC) fram es m ake sense for LANE, t he first 5 bit s are 01000. The last 3 bit s are a user priorit y t hat has been passed down by a higher- layer prot ocol. The rem ainder of t he fram e follows t he usual Token Ring form at , except t hat , as was t he case for Et hernet , t he CRC appears in t he AAL5 t railer inst ead of a fram e check sequence field.

LANE Control Frames The configurat ion direct connect ion bet ween a client and a LANE Configurat ion Server, and t he cont rol direct and cont rol dist ribut e connect ions bet ween a client and it s LANE Server are special—t hey are used t o set up and m aint ain t he LANE

environm ent . Ordinary fram es are not sent on t hese connect ions. These connect ions carry special cont rol fram es t hat cont ain a variet y of request s and responses. See Table 21.1 list s and briefly describes t he LANE cont rol fram e t ypes. The t able includes som e cont rol fram e t ypes t hat have not been discussed elsewhere in t his chapt er. I n t he t able, "LE" st ands for LAN Em ulat ion.

Ta ble 2 1 .1 . LAN E Cont r ol Fr a m e s Type of Con t r ol Fr a m e

D e scr ipt ion

LE Configure Request

Sent by a client t hat wishes t o j oin an ELAN and needs t o find out t he ATM address of it s LANE Server.

LE Configure Response

Provides t he address of a LANE Server ( and som e opt ional param et ers) .

LE Join Request

Sent t o a LANE Server by a client t hat wishes t o j oin an ELAN.

LE Join Response

Sent by a LANE Server. I f t he client has been accept ed, t he server provides t he nam e of t he ELAN, it s t ype, t he m axim um fram e size, and opt ionally, t he client 's LANE ident ifier.

LE Regist er Request

Sent by a client t o regist er a MAC address or a Token Ring rout e descript or.

LE Regist er Response

Acknowledges t he regist rat ion.

LE Unregist er Sent by a client t o wit hdraw a regist rat ion ( for exam ple, if it is about Request t o det ach from t he net work) . LE Unregist er Acknowledges t he unregist rat ion. Response LE ARP Request

Sent by a client t hat wishes t o know t he ATM address corresponding t o a given MAC address. I f t he LANE server does not know t he answer, it will forward t he request t o client s.

LE ARP Response

Provides t he ATM address corresponding t o a given MAC address.

Ready I ndicat ion

Sent by a caller as soon as it is ready t o receive fram es on a newly est ablished connect ion.

Ready Query

Sent by t he called part y if it has not yet received an expect ed Ready I ndicat ion.

LE Flush Request

Sent by a client t o clear out a connect ion pipeline. The client wait s for a response before sending m ore fram es ont o t he connect ion.

LE Flush Response

Sent in response t o a Flush.

Ta ble 2 1 .1 . LAN E Cont r ol Fr a m e s Type of Con t r ol Fr a m e

D e scr ipt ion

LE NARP Request

Sent by a client t o announce t hat it s MAC- address/ ATM- address pairing has changed.

LE Topology Request

Sent t o it s LANE Server by a LANE Client in a t ransparent bridge. I t announces t hat t he client has sent a Configurat ion BPDU t o t he BUS and indicat es whet her a Spanning Tree t opology change is occurring. The server forwards t he m essage t o ot her client s.

LAN Em u la t ion Con t r ol Fr a m e For m a t See Figure 21.15 shows t he form at of a LAN Em ulat ion cont rol fram e. See Table 21.2 describes t he fields in a cont rol fram e. Each t ype of m essage uses a different subset of t he fields, and unused fields are set t o zeroes.

Figu r e 2 1 .1 5 . For m a t of a LAN Em u la t ion con t r ol fr a m e .

Ta ble 2 1 .2 . Fie lds in a LAN Em ula t ion Con t r ol Fr a m e Fie ld

D e scr ipt ion

Marker

X'FF- 00 indicat es t hat t his is a cont rol fram e.

Prot ocol

X'01 is used for t he ATM LANE prot ocol.

Version

I dent ifies t he ATM LANE prot ocol version. Two versions ( X'01 and X'02) have been defined.

Op- Code

I dent ifies t he t ype of cont rol fram e, such as a configurat ion, j oin, or ARP request .

St at us

Set t o X'00- 00 in request s and successful responses. Report s a problem ot herwise.

Transact ion- I d

Used t o m at ch a response t o it s request .

Request erLECI D

I s t he LAN Em ulat ion Client I D of t he client m aking a request . ( I t is X'00- 00 if t he client 's ident ifier is unknown.)

Ta ble 2 1 .2 . Fie lds in a LAN Em ula t ion Con t r ol Fr a m e Fie ld

D e scr ipt ion

Flags

Used t o indicat e m iscellaneous fact s, such as whet her t he sender is a proxy client in a LAN swit ch or rout er.

Source

I s eit her a MAC address associat ed wit h t he m essage source or a Token Ring rout e descript or t hat ident ifies a client t hat is a source rout e bridge.

Target

I s eit her a MAC address associat ed wit h a t arget syst em or a Token Ring rout e descript or t hat ident ifies a dest inat ion t hat is a source rout e bridge.

Source ATM Address

I s t he ATM address of t he source.

Target ATM Address

I s t he ATM address of t he t arget syst em ( if any) .

LAN Type

X'00 is unspecified, X'01 is Et hernet , and X'02 is Token Ring.

Trace of a Join Response Control Frame List ing 21.2 shows a Net work Associat es Sniffer t race of a Join Response cont rol fram e. The fram e arrived on t he circuit wit h VPI = 0 and VCI = 33. The fram e st art s wit h t he cont rol fram e m arker, X'FF- 00. Prot ocol 1 is t he LANE prot ocol, and t he version shown is 1. The opcode ident ifies t his as a Join Response fram e, and t he st at us code shows t hat t he j oin was successful. The Src LAN Dest field carries t he request ing client 's MAC address. ( I f t he t ag in t his field were X'00- 02 inst ead of X'00- 01, t he cont ent would be a source rout e bridge rout e descript or.) The client 's ATM address also is shown. The client 's MAC address and ATM address have been regist ered at t he server. The LAN t ype is Et hernet . The m axim um fram e size is 1516, and t he nam e of t he ELAN is elan43. The m essage does not include a client ident ifier, which m eans t hat all client s in t his ELAN will place ident ifier X'00- 00 at t he t op of t heir fram es.

List in g 2 1 .2 Tr a ce of a Join Re spon se ATM: ATM: Frame 24 arrived at 12:31:24.8874; frame size is 144 (0090 hex) bytes. ATM: Call reference = 10449 ATM: Link = DCE ATM: Virtual path id = 0 ATM: Virtual channel id = 33 ATM: LECTRL: ----- LAN EMULATION CONTROL FRAME ----LECTRL: LECTRL: Marker = FF00 LECTRL: Protocol = 01

LECTRL: Version = 01 LECTRL: Opcode = 0102 (JOIN_RESPONSE) LECTRL: Status = 0 (Success) LECTRL: Trans ID = 2091 LECTRL: Request ID = 21 LECTRL: Flags = 0000 LECTRL: Src LAN Dest = 000100603E1AF000 LECTRL: Tag = 0001(MAC address) LECTRL: MAC Addr = 00603E1AF000 LECTRL: Tar LAN Dest = 0000000000000000 LECTRL: Tag = 0000(not present) LECTRL: MAC Addr = 000000000000(not present) LECTRL: SRC ATM ADDR = 47:0091:8100 0000 0060 3E5C EE01:Cisco21AF000:01 LECTRL: LAN-TYPE = 01 (Ethernet/IEEE 802.3) LECTRL: MAX FRAME SIZE= 01 (1516) LECTRL: NUMBER TLVs = 0 LECTRL: ELAN NAME SIZE= 6 LECTRL: TARG ATM ADDR = 0000000000000000000000000000000000000000 LECTRL: ELAN NAME = "elan43" ATM: ATM: ----- AAL5 Trailer ----ATM: ATM: 28 pad octets ATM: UU = 0 ATM: CPI = 0 ATM: Length = 108 ATM: CRC = 9BD3AA61 (Correct)

LANE Version 2 At t he t im e of writ ing, LANE version 1 is im plem ent ed in product s. However, t he ATM Forum has published version 2 st andards for client s and servers. Version 2 adds som e desirable feat ures t o LANE, including • • •

The int roduct ion of Qualit y of Service for dat a connect ions The capabilit y t o set up m ult iple inst ances of each server t ype An im proved m et hod of handling m ult icast t raffic

A LANE client can regist er t he accept able service cat egories t hat it is willing t o accept in incom ing calls. A caller can set up m ult iple connect ions t o a dest inat ion wit h each call carrying t raffic for a different service cat egory. For exam ple, separat e calls could be used for voice, video, and bulk dat a. One short com ing of version 1 LANE is t hat t he Configurat ion, LANE, and BUS servers are single point s of failure. Also, som e syst em s in a widely dist ribut ed ELAN m ust m aint ain long- dist ance connect ions t o t heir servers t o part icipat e in t he ELAN. At t he t im e of writ ing, vendors use propriet ary m et hods t o overcom e t hese problem s. Version 2 of LANE defines st andard prot ocols t hat enable servers t o com m unicat e wit h one anot her and coordinat e t heir act ivit ies. An ELAN can be served by m ult iple

LANE Servers t hat coordinat e t heir MAC- address/ ATM- Address dat abases wit h one anot her. A version 2 BUS has access t o a LANE Server's address dat abase. There can be m ult iple connect ed BUSes, each responsible for flooding fram es t o syst em s t hat have connect ed t o t he BUS. All BUSes for t he LAN are list ed in t he LANE Server dat abase. When a BUS m ust flood a fram e t o all nodes, it t ransm it s t he fram e t o it s connect ed client s and t o t he ot her BUSes. A BUS can be " int elligent " as well. A client som et im es passes a fram e t o t he BUS in spit e of t he fact t hat t he fram e's dest inat ion MAC address is regist ered. A version 1 BUS would flood t his fram e t o all client s. An int elligent BUS can forward t he fram e t o it s recipient inst ead of flooding t he fram e. Mult icast s also are handled inefficient ly in LANE version 1 because a m ult icast fram e is forwarded t o all client s. I n version 2, a client can regist er it s m em bership in a m ult icast group. I t t hen can be assigned t o a Select ive Mult icast Server ( SMS) . A Select ive Mult icast Server opens a point - t o- m ult ipoint connect ion t o it s client s and forwards a m ult icast fram e addressed t o a group t o t he m em bers of t hat group.

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•

• •

LANE enables syst em s t hat have ATM NI Cs and are connect ed t o ATM swit ches t o em ulat e a convent ional LAN. LANE enables Et hernet or Token Ring syst em s connect ed t o ordinary hubs and swit ches t o int eract wit h ATM syst em s as if t hey all belong t o a convent ional LAN. LANE m akes it possible for exist ing net work prot ocols and net work applicat ions t o run on t op of ATM wit hout m aking changes t o any of t he soft ware. An em ulat ed LAN ( ELAN) is m ade up of com put ers, rout ers, and bridges t hat have ATM int erfaces and are connect ed t o ATM swit ches. An ATM NI C has a 6- byt e MAC LAN address. A convent ional LAN swit ch t hat also has an ATM int erface is called a LANE proxy because it relays t raffic bet ween t he LANE environm ent and norm al LAN syst em s. A LANE Client is a special device driver t hat hides t he underlying ATM com m unicat ions from higher- layer prot ocols and applicat ions. A LANE Client connect s t o a LANE Configurat ion Server t o obt ain t he address of a LANE Server. A LANE Client connect s t o it s LANE Server t o regist er it s MAC address and t o find out t he nam e of it s ELAN, t he ELAN t ype, and t he m axim um fram e size. The client also m ay obt ain a unique 2- byt e LANE Client I D. The LANE Server uses it s address dat abase t o help LANE Client s m ap dest inat ion MAC addresses t o ATM addresses. A LANE Client sends and receives broadcast s, m ult icast s, and flooded fram es via it s Broadcast and Unknown Server ( BUS) . Privat e ATM addresses are 20 byt es in lengt h. An Et hernet or Token Ring fram e is carried inside an AAL5 fram e t hat st art s wit h a 2- byt e field cont aining t he sender's unique client ident ifier or X'00- 00. The FCS field is om it t ed because t he AAL5 t railer cont ains a CRC value.

• •

Cont rol fram es are sent on t he configurat ion direct connect ion bet ween a client and a LANE Configurat ion Server, and t he cont rol direct and cont rol dist ribut e connect ions bet ween a client and it s LANE Server. Version 2 of LANE support s Qualit y of Service for dat a connect ions, t he capabilit y t o set up m ult iple inst ances of each server t ype, and an im proved m et hod of handling m ult icast t raffic.

References The defining docum ent s for LANE were writ t en by t he ATM Forum . I n t he t it les list ed here, LUNI st ands for LANE User Net work I nt erface and relat es t o t he prot ocols used bet ween LANE Client s and t he special servers. LNNI st ands for LAN Em ulat ion Net work- t o- Net work I nt erface and relat es t o t he prot ocols t hat enable t he servers t o com m unicat e and cooperat e wit h one anot her. • •

•

AF- LANE- 0021.000. " LAN Em ulat ion over ATM, Version 1.0." 1995. AF- LANE- 0084.000. " LAN Em ulat ion over ATM Version 2—LUNI Specificat ion." 1997. AF- LANE- 0112.000. " LAN Em ulat ion over ATM Version 2—LNNI Specificat ion." 1999.

The I ETF has specified procedures t o be used t o carry I P t raffic across an ATM circuit in t he following docum ent : •

RFC 2225. " Classical I P and ARP over ATM." M. Laubach and J. Halpern. 1998.

Chapter 22. Fibre Channel Fibre channel t echnology was invent ed t o t urn a bold t hought int o realit y. The designers believed t hat t he dist inct ion bet ween t he LAN prot ocols and t he prot ocols used t o connect a com put er t o st orage peripherals was art ificial. They reasoned t hat if a com m on t echnology, physical int erface, and prot ocol fram ework could be est ablished for bot h, everyone would benefit : •

•

•

St andardized hardware and drivers would lead t o lower prices because of econom ies of scale. Syst em cost s would decrease because m any kinds of devices could be accessed t hrough one high- speed port . Technical im provem ent s would be fost ered by creat ing a big m arket for a st andard hardware int erface.

Furt herm ore, fibre channel was designed wit h a dream wish list of feat ures. I t replaces a rat 's nest of cables wit h a clean, swit ch- based backbone. I t support s highspeed t ransm ission. I t offers m ult iple classes of service—including bot h connect ionorient ed service t hat delivers a guarant eed bandwidt h across t he net work and various t ypes of connect ionless service. I t also offers several qualit y of service capabilit ies.

Current ly, fibre channel adapt ers are available for every m aj or com put er plat form . There is growing use of fibre channel t echnology t o int erconnect disk and t ape st orage resources, and t o link up com put ers in environm ent s t hat have st ringent perform ance and reliabilit y requirem ent s. At t he sam e t im e, dem and for Gigabit Et hernet hardware, which uses fibre channel com ponent s, also is increasing. The result is t hat t he cost of bot h fibre channel and Gigabit Et hernet t echnology is decreasing.

N ot e The choice of using t he spelling fibre inst ead of fiber was int ent ional. A fibre channel port does not have t o be connect ed t o opt ical fiber; a link can be copper or opt ical fiber. The word fibre proj ect s an im age of high- speed and broad bandwidt h, what ever t he m edium m ight be.

Fibre channel t echnology was designed t o do everyt hing—and t o do it any way you want . I t is described in a large and rapidly growing bookshelf of st andards docum ent s. This chapt er present s som e of t he im port ant ideas and out lines som e applicat ions. But of necessit y, m any det ails, fine point s, and opt ions had t o be left out in order t o get a descript ion t hat was in any way int elligible. Fibre channel st andards are published by t he Am erican Nat ional St andards I nst it ut e ( ANSI ) . Much of t he work is perform ed by t he T11 st andards body of t he Nat ional Com m it t ee for I nform at ion Technology St andards ( NCI TS) , which is responsible for device- level int erfaces.

Features of Fibre Channel Fibre channel t echnology provides a high- speed infrast ruct ure t hat blends device com m unicat ion wit h LAN com m unicat ion. I t can be used as a new infrast ruct ure for t radit ional low- level, device- based com m unicat ions such as t he Sm all Com put er Syst em I nt erface ( SCSI ) , t he I nt elligent Peripheral I nt erface ( I PI ) , and t he High- Perform ance Parallel I nt erface ( HI PPI ) . I t also support s t he t ransport of LAN fram es carrying high- level prot ocol dat a. Fibre channel's applicabilit y is broad, basically consist ing of " t hings t hat need t o be connect ed t o ot her t hings." A single high- speed int erface and physical m edium can be used t o at t ach a syst em t o a disk cont roller or a video device, or t o carry TCP/ I P t raffic t o a peer applicat ion. Specifically, fibre channel st andards deal wit h net working for • •

•

Com put er LANs St orage area net works ( SANs) int erconnect ing m any different t ypes of disk or t ape devices Audio/ video t ransm ission net works

•

Real- t im e syst em s, such as avionics syst em s t hat cont rol every aspect of space rocket flight

Many organizat ions wit h large ( and growing) dat a bases and dat a warehouses have been responsive t o fibre channel product s t hat bind t oget her diverse st orage resources. The t echnology support s graceful increm ent al growt h of resources. As a bonus, it allows st orage peripheral unit s t o be added or rem oved wit hout disrupt ion, providing a built - in hot - swapping capabilit y.

Transmission Speeds The speeds t hat fibre channel support s are high and keep clim bing higher Table 22.1 shows som e current ly defined levels. The fract ional speeds are used in som e older syst em s. At t he t im e of writ ing, m any " full- speed" swit ch product s are on t he m arket , and higher- speed product s are in t he pipeline. The m egabit per second ( Mbps) speed rat e indicat es t he num ber of bit signals per second on t he m edium . The bit rat e includes a 20 percent overhead because a byt e is t ranslat ed t o a 10- bit code- group pat t ern prior t o t ransm ission. ( This is t he sam e 8B/ 10B t ranslat ion t hat was borrowed by 1000BASE- X.) Fibre channel t hroughput oft en is st at ed in m egabyt es per second ( MBps) . This is nat ural, given t he frequent applicat ion of t he t echnology t o st orage devices, which m easure t hroughput in byt es per second rat her t han bit s per second.

Ta ble 2 2 .1 . Fibr e Ch a n n e l Spe e ds Tit le

M e ga bit s pe r Se cond ( I n clu din g Ove r h e a d)

M e ga byt e s pe r Se cond ( W it h out Ove r he a d)

Eight h Speed

133

12.5

Quart er Speed

266

25

Half Speed

531

50

Full Speed

1062.5

100

Double Speed

2126

200

Quadruple Speed

4252

400

Distances, Media, and Connectors Support ed dist ances range from 10 m et ers t o 10 kilom et ers—or furt her wit h t he help of a single- m ode fiber ext ender. Fibre channel runs on a special 150- ohm shielded t wist ed- pair cable ( which was reused for 1000BASE- CX Et hernet ) , 75- ohm t winax coaxial cable, video coaxial

cables, and m ult im ode and single- m ode opt ical fiber. Opt ical fiber is required for t he higher 200MBps and 400MBps speeds. A DB- 9 connect or is used for shielded t wist ed- pair cable, and BNC and ThreadedNeil- Councilm an Coaxial Cable Connect or ( TNC) connect ors are used for coax. An SC connect or is preferred for t he opt ical fiber im plem ent at ions.

Fibre Channel Equipment and Topology The com m unicat ing devices in a fibre channel net work are very diverse. They can be com put ers, disk drives, print ers, scanners, video out put syst em s, and ot her syst em s. Because of t he m ult iplicit y of end- syst em t ypes, t he devices usually are referred t o by t he generic t erm nodes. A fibre channel adapt er m ust be inst alled in each node t hat part icipat es in a fibre channel net work. St andard fibre channel t opologies include • • • •

A sim ple point - t o- point connect ion bet ween t wo nodes. A set of nodes connect ed in a ring configurat ion t hat is called a loop, or, m ore form ally, an arbit rat ed loop. As was t he case for classic Token Rings, a loop oft en is im plem ent ed using a hub. Loop bandwidt h is shared. Dat a t ransm ission is half- duplex. A set of nodes connect ed t o a swit ch. Like a LAN swit ch, a fibre channel swit ch can support m ult iple sim ult aneous com m unicat ions bet ween pairs of nodes. Nodes linked by int erconnect ed swit ches and arbit rat ed loops.

Figure 22.1 illust rat es point - t o- point , loop, and swit ched t opologies. The loop in t he figure is im plem ent ed by connect ing syst em s t o a fibre channel hub. I t is becom ing rout ine t o use fibre channel loop connect ions as t he com m unicat ions backplane in disk st orage unit s.

Figu r e 2 2 .1 . St a n da r d fibr e ch a n n e l t opologie s.

Nodes t hat belong t o a loop share bandwidt h ( as is done by syst em s on a Token Ring) . Only one node on a loop can t ransm it dat a at any given t im e. A st andalone loop is called a privat e loop. A loop t hat is at t ached t o a swit ch is called a public loop. A node com m unicat es t hrough it s port via separat e t ransm it and receive wires or opt ical fibers. This is illust rat ed in Figure 22.2. The t op of t he figure shows a connect ion bet ween a pair of nodes. The m iddle shows connect ions bet ween nodes and a swit ch. The bot t om shows som e nodes connect ed int o a loop.

Figu r e 2 2 .2 . Fibr e ch a n n e l t r a n sm it a n d r e ce ive lin e s.

Figure 22.3 shows a net work t hat consist s of int erconnect ed swit ches and loops. I n t he figure, several disk devices are connect ed t o public loops.

Figu r e 2 2 .3 . A fibr e ch a n n e l n e t w or k con sist in g of in t e r con n e ct e d sw it ch e s a n d loops.

Swit ches m ake up t he backbone of a fibre channel net work. A loop can have at m ost one act ive connect ion t o a swit ch, so t raffic never t ravels from one swit ch t o anot her across an int ervening loop. The figure includes a RAI D disk syst em t hat happens t o cont ain a convent ional SCSI int erface rat her t han a fibre channel int erface. A SCSI - t o- fibre- channel bridge is used t o connect t he RAI D array int o t he fibre channel net work. SCSI com m ands are part of t he fibre channel prot ocol fam ily, so legacy disk syst em s can be bridged int o a fibre channel net work quit e easily. Fibre channel net works have t he pot ent ial t o be quit e scalable. Mult iple swit ches can be int erconnect ed t o form large net works. However, a lot of work rem ains t o be done on st andards ( current ly at draft st at us) t hat deal wit h pat h select ion and a hierarchical net work st ruct ure. Many fibre channel vendors support individual 10km single- m ode fiber opt ic links, and som e have announced longer links.

At t he t im e of writ ing, work is proceeding on a fibre channel backbone st andard t hat will enable fibre channel swit ches t o be connect ed across a wide area net work. I nt erfaces are being defined for ATM and SONET.

Fabrics Fibre channel st andards docum ent at ion uses t he t erm fabric t o m ean " t hat t o which nodes at t ach." This t erm is used in a very confusing m anner. When writ t en in lower case, fabric st ands for any infrast ruct ure t hat connect s fibre channel nodes t o one anot her. For exam ple, a point - t o- point link, an arbit rat ed loop, or a net work wit h all of t he infrast ruct ure in Figure 22.3, including swit ches and hubs, can be called a fabric. When writ t en in upper case, t echnically, a Fabric provides swit ched connect ivit y bet ween nodes based on 3- byt e dest inat ion addresses t hat have been assigned t o nodes. I t can consist of one swit ch, m any swit ches int erconnect ed by links, or int erconnect ed swit ches along wit h som e at t ached arbit rat ed loops.

N ot e Unlike an Et hernet LAN or a t ransparent Token Ring LAN, a Fabric t opology can cont ain any num ber of pat hs bet ween nodes.

Vendors call t heir product s hubs, swit ches, and ( see t he following sect ion) loop swit ches, but t hey also t oss t he t erm fabric int o t heir docum ent at ion.

Loop Switches An arbit rat ed loop hub is a sim ple, lim it ed device t hat support s half- duplex dat a com m unicat ion bet ween at t ached nodes. Vendors have creat ed a hybrid device called a loop swit ch t hat boost s bandwidt h and allows m ore com plex at t achm ent s. A loop swit ch is designed t o im prove perform ance for nodes t hat have loop int erfaces t hat cannot be at t ached t o a swit ch. As shown in Figure 22.4, individual devices, arbit rat ed loops, and hubs can be connect ed t o a loop swit ch. Each device believes t hat it is connect ed t o a real arbit rat ed loop. However, t he loop swit ch in t he figure delivers a full 100 m egabyt es per second bandwidt h t hrough each of it s port s. Bandwidt h is not shared, and t raffic act ually is swit ched bet ween t he port s.

Figu r e 2 2 .4 . A loop sw it ch .

I f all at t ached devices are updat ed t o swit ch- at t achable adapt ers, a loop swit ch can be convert ed t o a swit ch.

Fibre Channel Ports Fibre channel port s have several different roles: •

• • •

Nodes in t wo devices m ight be connect ed t o form a point - t o- point link bet ween t he devices. A port in a node m ight connect t he node t o a swit ch. A port in a node m ight connect t he node t o a loop. A swit ch port m ight connect t he swit ch t o a node, t o a loop, or t o anot her swit ch.

A short hand is used t o describe port roles. The short hand t it le indicat es whet her t he port is in a node or in a swit ch and t he capabilit ies t hat t he port has. Table 22.2 list s and describes t hese t it les.

Ta ble 2 2 .2 . Fibr e Cha nne l Por t s Type of Por t

D e scr ipt ion

N_Port

Any node port .

NL_Port

A node port t hat is connect ed t o an arbit rat ed loop.

F_Port

A Fabric ( swit ch) port t hat can be connect ed t o a node.

FL_Port

A swit ch port t hat can be connect ed t o an arbit rat ed loop.

L_Port

An N_Port or F_Port t hat can perform arbit rat ed loop funct ions.

E_Port

A swit ch expansion port . I t can be connect ed t o a port in anot her swit ch t o form an int erswit ch link.

G_Port

A swit ch port t hat can be connect ed t o a node or t o anot her swit ch.

GL_Port

A swit ch port t hat can be connect ed t o a node, a loop, or anot her swit ch.

B_Port

A port on a bridge t hat connect s t o an E_Port on a swit ch.

I t is not unusual for every port in a fibre channel swit ch product t o be a GL_Port t hat is capable of connect ing t o anyt hing: a node, a loop, or anot her swit ch. Figure 22.5 illust rat es N- Port s, F_Port s, NL_port s, and FL_Port s. The hub in t he figure act s as a wiring concent rat or. The port s in a hub can be passive, but m any hub product s have port s t hat repeat bit s in order t o increase t he m axim um lengt h of t he lobe cables t hat lead t o at t ached nodes.

Figu r e 2 2 .5 . Type s of por t s.

Fibre Channel Names and Addresses I n convent ional LAN t echnologies, each adapt er is assigned a MAC address t hat ident ifies it uniquely on a global basis. This MAC address does not indicat e where a syst em is locat ed. On legacy LANs, every fram e was placed on a shared m edium and was seen by every st at ion, so locat ion did not m at t er. The fact t hat t his was not a perfect archit ect ure was m ade clear in earlier chapt ers, which have described t he great effort s t hat have gone int o prevent ing fram es from being flooded across a LAN. Fibre channel t echnology was built from t he ground up t o deliver a fram e t o it s int ended dest inat ion and t o no ot her node. To do t his, it int roduces 3- byt e addresses t hat pinpoint t he locat ion of every node port . However, fibre channel also ret ains t he use of globally unique ident ifiers t hat are like MAC addresses. Wit hin fibre channel, t hese serve as nam es t hat ident ify a port adapt er or a syst em . These nam es are used by several higher- layer prot ocols and also are useful for net work m anagem ent and t roubleshoot ing. Many fibre channel vendors creat e unique nam es in t he t im e- honored m anner. A vendor regist ers wit h t he I EEE, get s a 3- byt e OUI , and ident ifies a specific adapt er by adding an addit ional 3- byt e num ber. These basically are MAC addresses.

N ot e Support ing t hese ident ifiers m akes it easy t o run higher- layer prot ocols t hat rely on MAC addresses on a syst em t hat has a fibre channel adapt er.

I n addit ion t o I EEE MAC address num bering, several ot her num bering schem es are used t o ident ify st orage unit s and ot her devices. All of t he num bering schem es have been int egrat ed int o an 8- byt e nam ing convent ion t hat is described in t he next sect ion. The sect ion aft er t hat describes fibre channel addresses.

World Wide Port and Node Names Globally unique nam es can be assigned t o N_Port s, F_Port s, and nodes. These are 8byt e quant it ies called world wide port nam es and world wide node nam es. Each nam e consist s of • •

A 4- bit int roducer t hat ident ifies a net work address aut horit y ( NAA) The nam e t hat has been assigned by following t he rules of t he aut horit y

Table 22.3 describes som e of t he t ypes of world wide port nam es. Not e t hat I EEE nam es t op t he list . These 8- byt e nam es cont ain a 6- byt e ident ifier t hat has t he sam e form as a MAC address. A com pany applies for a 3- byt e OUI and adds anot her 3- byt e port ion so t hat a port ( or node) can be given a unique nam e. Fibre channel is different from t he ot her LAN prot ocols, so t he t erm MAC does not quit e fit . On t he ot her hand, a t it le such as " unique address for a field replaceable hardware elem ent " does not lend it self t o a graceful acronym . Zero- bit s are used t o pad t he front of a nam e t hat does not fill t he 60 bit s aft er t he 4- bit int roducer. Hence, t welve 0- bit s precede a 6- byt e I EEE address. The I EEE ext ended address allows t he vendor t o use t hese bit s t o ident ify separat e port s wit hin a single unit .

Ta ble 2 2 .3 . W or ld W ide Por t or N ode N a m e s Type of N a m e

D e scr ipt ion

I EEE ( 0001)

Twelve 0- bit s followed by a 6- byt e I EEE address

I EEE Ext ended ( 0010)

A 12- bit address uniquely ident ifying a F_Port or an N_Port cont ained in a unit , followed by t he 6- byt e address of t he unit

Locally Assigned ( 0011)

Assigned by a vendor or via configurat ion

I EEE Regist ered Nam e ( 0101)

A 3- byt e I EEE- assigned com pany ident ifier, followed by a 36bit vendor assigned ident ifier

Ta ble 2 2 .3 . W or ld W ide Por t or N ode N a m e s Type of N a m e CCI TT I ndividual Address ( 1100)

D e scr ipt ion A 60- bit CCI TT address

Port Identifiers (Addresses) Port s have 3- byt e addresses. These addresses are hierarchical, and each byt e is a level in t he hierarchy: Byt e 1—A dom ain ident ifier. This ident ifies one swit ch or can be used t o ident ify a set of int erconnect ed swit ches. I f t wo swit ches belong t o t he sam e dom ain, t here m ust be a pat h bet ween t hem t hat only passes t hrough swit ches in t hat dom ain. Byt e 2—An area ident ifier. An area is eit her a set of port s wit hin and at t ached t o one swit ch, or an arbit rat ed loop at t ached t o a swit ch port . Byt e 3—A port ident ifier. This ident ifies a port at t ached t o a swit ch, a port at t ached t o an arbit rat ed loop at t ached t o a swit ch, or a port at t ached t o a privat e arbit rat ed loop.

N ot e The addresses of port s at t ached t o a privat e arbit rat ed loop st art wit h X'00- 00.

An N_Port 's 3- byt e address form ally is called an N_Port ident ifier. Fram es are delivered t o t he port based on t his address. Fram es are rout ed t o t he dom ain, t hen t arget ed t o an area, and finally t ransm it t ed t o t he port wit h a given ident ifier. An N_Port obt ains it s address from t he swit ch t o which it is at t ached during an init ializat ion process t hat is called a login. This is a sensible arrangem ent . An N_Port address indicat es an act ual locat ion. I f a node is disconnect ed from one swit ch and plugged int o anot her, it will receive a fresh address t hat reflect s it s new locat ion. Wherever it is, a port st ill ret ains it s built - in world wide port nam e. This ident ifies t he port adapt er uniquely, independent ly of it s locat ion.

Fibre Channel Levels The t opics covered in fibre channel specificat ions are cat egorized using five levels, labeled FC- 0, FC- 1, FC- 2, FC- 3, and FC- 4. Som e ( but not all) of t he levels correspond t o prot ocol layers.

The levels are • •

FC- 0—This level corresponds t o t he lower part of t he physical layer. I t s specificat ion defines t he charact erist ics of fibre channel physical m edia and connect ors, and t he signals used t o place bit s ont o each m edium . FC- 1—This is t he part of t he physical layer t hat perform s an 8B/ 10B encoding of dat a byt es and defines t he usage of special code- groups.

N ot e The physical m edia, connect ors, signals, and 8B/ 10B dat a encoding used for t he 1000BASE- LX, SX, and CX im plem ent at ions of Gigabit Et hernet were borrowed from fibre channel FC- 0 and FC- 1.

• • •

FC- 2—The FC- 2 level corresponds t o t he dat a link layer. I t defines t he way t hat dat a is packaged int o fram es and m oved from one node t o anot her. I t also defines special com m and signals t hat provide cont rol funct ions, such as flow cont rol and error recovery. FC- 3—FC- 3 is not a prot ocol layer. I t is a placeholder for funct ions t hat are applied across several port s in a node. None have been defined at t he t im e of writ ing, but one candidat e is dat a st riping ( for RAI D disk arrays) . FC- 4—This is t he highest level. I t defines t he way t hat t he various applicat ions t hat ride on t op of fibre channel m ap ont o t he fibre channel environm ent .

Classes of Service A fibre channel net work support s several classes of service. These are designed t o m eet t he needs of different t ypes of dat a t ransfers. The classes include • • • •

•

Class 1—A service t hat provides dedicat ed point - t o- point connect ions. A connect ion bet ween t wo nodes has a guarant eed bandwidt h. Fram es are delivered in order. Buffered Class 1—A proposed connect ion- based service t hat enables port s t hat operat e at different dat a rat es t o com m unicat e across a connect ion. The flow of dat a is regulat ed so t hat t he fast er port will not overload t he slower port . Dedicat ed Sim plex Class 1—A connect ion- based service t hat support s dat a flow in one direct ion only. Acknowledgm ent s are sent back via t he connect ionless Class 2 service. Class 2—An acknowledged connect ionless m ult iplexing service in a swit ched net work. Som e fabric product s deliver fram es in order, while ot hers deliver fram es out of order. The net work provides t he sender wit h a not ificat ion of delivery or nondelivery, as long as t he fram e has not been corrupt ed during t ransm ission across a link. Class 3—Like Class 2, Class 3 is a connect ionless m ult iplexing service. Som e fabrics product s deliver fram es in order, while ot hers deliver fram es out of order. Unlike Class 2, Class 3 does not provide any not ificat ion of delivery or nondelivery.

• • •

I nt erm ix—This is an opt ional variat ion on Class 1 service. I t allows a node in a Class 1 connect ion t o exchange Class 2 or Class 3 fram es wit h it s connect ion part ner or wit h ot her nodes. Bandwidt h allocat ed t o t he Class 1 connect ion but unused for t hat connect ion can be ut ilized for t he connect ionless fram es. Class 4 ( Fract ional Service) —This connect ion- orient ed service enables a node port t o split up it s bandwidt h and use it for m ult iple virt ual circuit s. Virt ual circuit s can connect t o different dest inat ions. Class F ( Fabric) —This connect ionless service can be used for cont rol and coordinat ion of t he int ernal behavior of a Fabric. Fabric circuit s originat e or t erm inat e int ernally t o t he fabric.

N ot e At t he t im e of writ ing, m ost fibre channel swit ch vendors support Class 2 and Class 3 service.

Generic Services Several generic services can be provided in a fibre channel net work. The services t hat are im plem ent ed wit hin a part icular fibre channel net work depend on t he net work's specific needs. Generic services t hat have been defined include •

•

Direct ory service—A direct ory responds t o queries from client s t hat need address inform at ion about t he nodes and port s in a region of t he Fabric address space. At m inim um , a direct ory m aps world wide port nam es t o N_Port addresses. The direct ory can include ot her inform at ion, such as m appings bet ween MAC addresses or I P addresses and N_Port addresses. Alias service—Oft en t here are m ult iple N_Port s t hat provide access t o t he sam e resource. A user can get t he sam e dat a by connect ing t o any of t hese port s. Anot her way of saying t his is t hat t hese port s m ake up a hunt group. This is im plem ent ed by allowing t he port s in t he hunt group t o share an assigned alias address. A hunt group call is answered by t he first free N_Port in t he group. An aut horized client can ask an alias server t o add a list of N_Port s t o an alias group, rem ove a list of N_Port s from a group, or ret urn param et ers for t he alias along wit h t he group of N_Port s t hat are in t he group. Anot her applicat ion of t he alias service is t o support m ult icast ing. A client can add a list of N_Port s t o a m ult icast group or rem ove t hem from t he group.

• •

Tim e service—A t im e server ( which is opt ional) can provide a current t im e value in response t o a query. Mult iple t im e servers t hat synchronize t heir values can be provided in t he net work. Securit y- key service—A securit y- key server enables a pair of client s t o be aut hent icat ed and t o obt ain a t em porary key t hat is used t o encrypt t he dat a t ransferred bet ween t hem .

To im plem ent t his, each client is assigned a secret key, and a copy of t his key is st ored at t he server. On request , t he server generat es a t em porary encrypt ion key and dist ribut es it t o t he t wo client s by encrypt ing it wit h each client 's secret key. •

Managem ent service—This is a st andard SNMP- based net work m anagem ent service.

A server m ay be locat ed wit hin a swit ch or in a node.

FC-4 Application Mappings FC- 4 includes m appings for ordinary dat a net working and for dat a t ransfers t hat previously were done across a com put er- t o- device channel. Mappings t hat have been defined or are in progress include •

• • • •

Encapsulat ion of I P and ARP in fibre channel fram es. This m apping was defined by t he I ETF in RFC 2625. Sm all Com put er Syst em I nt erface ( SCSI ) I nt elligent Peripheral I nt erface ( I PI ) Single Byt e Com m and Code Set s ( SBCCS) High Perform ance Parallel I nt erface ( HI PPI ) Fram ing Prot ocol

Two exam ples of FC- 4 funct ionalit y are present ed lat er in t he sect ions, " Encapsulat ion of I P Dat agram s" and " SCSI over Fibre Channel."

Fibre Channel Data Transfer Dat a is t ransferred across a fibre channel net work in fram es. Each fram e has a 20byt e header and a t railer t hat cont ains a CRC code used t o check whet her t he fram e has been corrupt ed during t ransm ission. The m axim um size of a fram e payload is 2112 byt es.

8B/10B Encoding and Ordered Sets The encoding used for fibre channel already has m ade it s appearance in t his book. The I EEE 802.3 Gigabit Et hernet working group adopt ed t he fibre channel 8B/ 10B encoding for 1000BASE- LX, 1000BASE- SX, and 1000BASE- CX. This encoding is described in Appendix A, " Physical- Layer Encodings for Et hernet , Fibre Channel, FDDI , and Token Ring." Before dat a is t ransm it t ed, each 8- bit byt e is t ranslat ed int o a 10- bit pat t ern called a code- group t hat cont ains a balanced sprinkling of 0s and 1s. The dat a code groups used for fibre channel are list ed in Table B.3 of Appendix B, " Tables." Ext ra 10- bit pat t erns are used t o define special code- groups. Com binat ions of codegroups ( called ordered set s) are int roduced by a special code group and are used t o • •

Mark t he beginnings and ends of fram es Provide idle fill bet ween fram es

• •

Signal t hat t he receiver is ready ( used for flow cont rol) Signal link reset s and offline or nonoperat ional condit ions

Each ordered set used for fibre channel consist s of four code- groups. Fibre channel ordered set s are list ed in Appendix B in Tables 22.4, Table 22.5, and Table 22.6: •

•

•

Table B.4 displays ordered set s t hat are used as st art - of- fram e and end- offram e delim it ers. Table B.5 displays ordered set s used as idles and receiver ready signals. These are called prim it ive signals. Table B.6 displays t he rem aining ordered set s. These are called prim it ive sequences.

Sequences of Frames The basic unit of dat a t ransfer is a sequence of fram es: •

•

A sequence consist s of one or m ore dat a fram es t ransm it t ed in one direct ion from one N_Port t o anot her N_Port . The sending N_Port is called t he sequence init iat or, and t he receiver is called t he sequence recipient .

While a sequence is being back t o t he sender. These delivery, report a delivery send low- level com m ands

t ransm it t ed, an associat ed set of link cont rol fram es flows link cont rol fram es are used t o acknowledge successful problem , perform flow cont rol and buffer m anagem ent , or t o t he originat ing N_Port .

The sequence init iat or assigns an ident ifier t o a sequence. This ident ifier is placed in t he header of every fram e sent by t he init iat or and in every response from t he recipient . A sequence oft en is used t o convey a big block of dat a t hat does not fit int o a single fram e. The block is cut int o subblocks t hat can fit int o fram e payloads. As shown in Figure 22.6, t here is a sequence count field in t he fram e header, and t he sequence num bers are used t o reassem ble t he blocks in t he correct order and t o assure t hat all subblocks have arrived.

Figu r e 2 2 .6 . Se gm e n t in g a la r ge block of da t a .

The sequence count num ber can be reset t o 0 at t he st art of each new sequence, or can cont inue t o increase ( unt il it wraps around from 65535 t o 0) . The sender knows t hat it s sequence has arrived safely when t he receiver acknowledges t he fram es in t he sequence. Fram es can be acknowledged ( ACKed) in several ways: • • •

Each fram e can be acknowledged separat ely. Several fram es can be acknowledged as a group. A single acknowledgm ent can be sent at t he com plet ion of a sequence. This m akes good sense when t he sequence is used t o carry a big block t hat has been broken int o subblocks.

The ACK st rat egy t hat will be used is est ablished when a com m unicat ion is set up bet ween a pair of nodes.

The upper- layer prot ocol for t he com m unicat ion est ablishes a policy on what should be done if fram es are not t ransm it t ed successfully. For exam ple, recovery m ight require one or m ore received sequences t o be discarded followed by act ion init iat ed by t he upper- layer prot ocol. Alt ernat ively, t he upper- layer prot ocol m ight specify t hat t he receiver should aut om at ically request ret ransm ission aft er discarding dam aged sequences.

Exchanges An exchange corresponds t o a dat a t ransact ion. Form ally, an exchange consist s of one or m ore sequences. Two sequences in an exchange cannot be sent concurrent ly—t hat is, an exchange is a sequence of sequences. I f all of t he sequences are t ransm it t ed in t he sam e direct ion, t he exchange is called unidirect ional. An exchange also can t ake t he form of a t wo- way conversat ion, wit h som e sequences flowing in each direct ion.

N ot e A SCSI read of a specified num ber of disk byt es is an exam ple of an exchange.

Figure 22.7 illust rat es unidirect ional and bidirect ional exchanges.

Figu r e 2 2 .7 . Fr a m e s, se qu e n ce s, a n d e x ch a n ge s.

A port can part icipat e in m ult iple exchanges, int erleaving t he fram es t hat belong t o each exchange. A fram e header includes an exchange ident ifier field. Placing a value in t his field m akes it possible for a receiver t o associat e t he fram e wit h t he correct exchange.

Relative Offsets When dat a is read from a disk, t he reader specifies a st art ing point for t he read. The byt es t hat are read are m easured from t his point . A relat ive offset field in t he fibre channel header is used for applicat ions t hat need t o specify an origin and count byt es from t hat point . When a relat ive offset ( RO) is used, t he origin offset appears in t he header of t he first fram e t hat is t ransm it t ed. I n successive fram es: RO( n+ 1) - RO( n) + Lengt h- ofPayload( n) I n ot her words, t he relat ive offset of t he current fram e's payload is t he previous payload's relat ive offset plus t he lengt h of t hat payload.

Logins and Addresses When an N_Port connect ed t o a swit ch init ializes, it perform s a login t o t he swit ch in order t o est ablish it s operat ing param et ers. The param et ers t hat will apply depend on t he bot h port and swit ch capabilit ies.

N ot e Alt hough t he login prot ocol is t he st andard way t o est ablish t he environm ent , a vendor could require t hat anot her m et hod, such as st at ic preconfigurat ion, be used for it s product .

Every fram e needs a source and dest inat ion address. To get st art ed, an N_Port sends it s login fram es t o t he special well- known Fabric address X'FF- FF- FE. The N_Port does not yet have a unique 3- byt e N_Port address, so it uses X'00- 00- 00 as it s source address. During login, t he N_Port announces param et ers such as t hese: •

• • • •

I t s 8- byt e world wide port nam e and world wide node nam e. ( Not e t hat t he node nam e applies t o t he whole node, which m ight have several port s.) The t ot al num ber of buffers it has available for receiving dat a from t he swit ch. The largest fram e payload size t hat it can receive. The t ot al num ber of concurrent sequences for which t he N_Port can act as receiver. The classes of service t hat t he node wishes t o use, and special service charact erist ics such as o Whet her t he node would like t o int erm ix Class 2 and Class 3 fram es wit h Class 1 connect ion fram es o Whet her Class 2 or 3 fram es m ust be delivered in order, or if out - oforder delivery is accept able

The swit ch F_Port responds wit h it s own set of param et ers, describing it s buffer resources, support ed service classes, and ot her feat ures. This is also t he t im e when t he swit ch assigns a 3- byt e address ( t he N_Port ident ifier) t o t he N_Port . Fram es are rout ed t o t he port based on it s port address. A node port m ight already have been assigned an N_Port ident ifier on a previous login. I n any case, during login, t he F_Port eit her confirm s t he exist ing address or assigns a new one t o t he port . Aft er an N_Port has logged ont o it s swit ch, it is eligible t o com m unicat e wit h ot her port s. I f t he N_Port want s t o set up a Class 1 or Class 2 com m unicat ion wit h a peer, it perform s a sim ilar login process t o t he rem ot e N_Port t o est ablish a class of service and t o exchange com m unicat ion param et ers. At t he end of a com m unicat ion, one of t he nodes frees up resources by perform ing a logout .

Well-Known Addresses Several well- known addresses are used for generic fibre channel dest inat ions. One generic address was already m ent ioned in t he previous sect ion—nam ely, t he dest inat ion address used by an N_Port t hat wishes t o send a login m essage t o an adj acent swit ch port . The well- known addresses are list ed in Table 22.4.

Ta ble 2 2 .4 . W e ll- Kn ow n Fibr e Ch a n ne l Addr e sse s Addr e ss

D e scr ipt ion

X'FF- FF- F0 t o X'FF- FF- F7

Reserved

X'FF- FF- F8

Alias server

X'FF- FF- F9

Qualit y of service facilit at or—Class 4

X'FF- FF- FA

Managem ent server

X'FF- FF- FB

Tim e server

X'FF- FF- FC

Direct ory server

X'FF- FF- FD

Swit ch cont roller

X'FF- FF- FE

Swit ch F- Port

X'FF- FF- FF

Broadcast address

Buffers and Credit The qualit y of t oday's dat a t ransm ission t echnology and m edia is far bet t er t han in earlier years. Bit error rat es have been falling st eadily. Current ly, t he m ost com m on reason for dat a loss is congest ion in a swit ch or dest inat ion syst em . Congest ion causes buffer m em ory t o be exhaust ed. When t here is no place t o put incom ing dat a, it m ust be discarded. The fibre channel dat a t ransm ission prot ocol m anages buffers very carefully. Dat a t ransfer is m odeled as t he m ovem ent of dat a from m em ory buffers in one node t o m em ory buffers in anot her node. Dat a in an out put buffer is packaged int o a fram e and t ransferred t o t he dest inat ion input buffer. The buffer size corresponds t o t he m axim um size of a fram e payload. A buffer credit syst em is used t o prevent buffer overflow. During t he login process, each part y announces t he num ber of buffers t hat it has allocat ed t o receive fram es from it s peer. I t also announces t he biggest fram e payload t hat a buffer can hold. The current credit is t he num ber of free buffers t hat are available at t he receiver. A t ransm it t er m ust st ay wit hin it s credit lim it . The receiver sends acknowledgm ent s t o t he t ransm it t er t o indicat e t hat dat a has been received and rem oved from buffers. This allows t he t ransm it t er t o send m ore dat a.

Two t ypes of buffer credit exist : • •

En d- t o- e nd cr e dit —This is credit offered by a peer N_Port . Buffe r - t o- buffe r cr e dit —This is credit offered across a single link. This can be a point - t o- point link t hat is a direct connect ion bet ween a pair of N_Port s, or a link bet ween an N_Port and it s adj acent swit ch port .

See Figure 22.8 illust rat es credit m anagem ent for a Class 1 or Class 2 com m unicat ion. Bot h end- t o- end and buffer- t o- buffer credit are illust rat ed. The t ransm it t ing port

Figu r e 2 2 .8 . Cr e dit for a Cla ss 1 con n e ct ion .

•

•

•

Decreases it s end- t o- end credit and buffer- t o- buffer credit by 1 each t im e it t ransm it s a fram e. I ncreases it s buffer- t o- buffer credit by 1 each t im e it get s a receiver ready signal. The receiver ready signal is an ordered set . I ncreases it s end- t o- end credit by 1 for each fram e acknowledged by t he peer P_Node. An acknowledgm ent is t ransm it t ed as an ACK fram e. An ACK m ight cover m ult iple received fram es.

A dedicat ed connect ion is set up for Class 1, and resources always are available all t he way t o t he dest inat ion. However, t his is not t he case for t he connect ionless Class

2 service. I f t he fabric cannot deliver a fram e, t he fabric ret urns a fabric busy fram e, or t he dest inat ion port ret urns a port busy fram e. No end- t o- end acknowledgem ent is provided for Class 3 service. Only buffer- t obuffer flow cont rol is provided.

Fibre Channel Frame Format The prot ocols described in earlier chapt ers of t his book were byt e- orient ed. Fibre channel t ransm ission is word- orient ed. Before encoding, a word corresponds t o 4 byt es. Aft er 8B/ 10B encoding, a word consist s of four 10- bit code- groups. Som e code- groups represent byt es, while ot hers represent ordered set s. The form at of a fibre channel fram e is shown at t he t op of See Figure 22.9. As was t he case for 1000BASE- X, idle ordered set s are t ransm it t ed bet ween fram es.

Figu r e 2 2 .9 . For m a t of a fibr e ch a n n e l fr a m e .

Also, like 1000BASE- X, a fram e is bounded by special ordered set s t hat represent st art and end delim it ers. A fram e ends wit h a 4- byt e cyclic redundancy check field, j ust as is t he case for convent ional LAN fram es. I f t he num ber of byt es in t he payload field is not a m ult iple of 4, fill byt es are insert ed before t he CRC. The lower part of Figure 22.9 shows t he form at of t he 24- byt e fram e header. The fram e header fields are explained in t he following list . Acronym s for t he field nam es are displayed for reference purposes because t hey oft en appear in vendor m anuals. However, t he acronym s will not be used in t his t ext . •

•

•

•

•

Rout in g con t r ol ( R_ CTL, 1 byt e ) —I nform at ion t hat indicat es t he purpose for which t he fram e is being used. For exam ple, t he field can indicat e t hat t he fram e carries upper- layer prot ocol dat a, video dat a, or cont rol inform at ion. Or, t he field m ight indicat e t hat t his is a link cont rol fram e. Link cont rol fram es are used t o report acknowledgem ent s or unsuccessful delivery, t o perform flow cont rol and buffer m anagem ent , or t o send low- level com m ands t o an N_Port . D e st ina t ion ide n t ifie r ( D _ I D , 3 byt e s) —The N_Port address ident ifying t he port t o which t he fram e is being sent . Sour ce ide n t ifie r ( S_ I D , 3 byt e s) —The N_Port address ident ifying t he port t hat sent t he fram e. Type ( 1 byt e ) —A code t hat ident ifies t he t ype of prot ocol dat a carried in a dat a fram e. The specific m eaning of t he code depends on t he value in t he rout ing cont rol field. Sam ple payloads include link service m essages ( such as, logon, logout , request credit from peer, or t est ) , LLC/ SNAP dat a fram es, SCSI , I PI - 3, HI PPI , SNMP, and propriet ary vendor video dat a. Fr a m e cont r ol ( F_ CTL, 3 byt e s) —A field t hat is packed wit h useful inform at ion. For exam ple, t here are subfields t hat indicat e o The num ber of fill byt es in t he fram e o Whet her t he source of t he fram e is t he originat or or responder for t he exchange o Whet her t he source of t he fram e is t he init iat or or recipient of t he sequence o Whet her t his is t he first , last , or an int ernal fram e in t he sequence o For a Class 1 connect ion, whet her t he sender want s t o t erm inat e t he connect ion o Whet her t he sender has finished it s current t ransm ission and is passing t he role of sender t o it s part ner o Whet her t he offset field is m eaningful and cont ains a value t hat describes t he relat ive offset of t he fram e's payload o Se qu e n ce ide n t ifie r ( SEQ_ I D , 1 byt e ) —An ident ifier assigned t o t he current sequence by t he init iat or of t he fram e sequence. o D a t a fie ld con t r ol ( D F_ CTL, 1 byt e ) —I nform at ion t hat indicat es whet her addit ional opt ional headers appear at t he beginning of t he payload. For exam ple, a special net work header is included when an I P dat agram is enclosed in a fibre channel fram e. The sect ion t hat follows describes t he form at of t his net work header.

o

o

o

Se qu e n ce cou n t ( SEQ_ CN T, 2 byt e s) —A sequence num ber assigned t o each fram e in a sequence. At t he end of t he sequence, num bering m ay rest art at 0 or cont inue t o increase. Or igina t or e x cha n ge ide n t ifie r ( OX_ I D , 2 byt e s) —A value assigned by t he originat or of an exchange. I t is used t o different iat e bet ween m ult iple concurrent exchanges. Re sponde r e x cha nge ide n t ifie r ( RX_ I D , 2 byt e s) —A value assigned by t he responder of an exchange.

Examples of FC-4 Fibre Channel Use The subsect ions t hat follow sket ch t he way t hat t wo upper- layer services are m apped ont o an underlying fibre channel net work. The exam ples are • •

Transm ission of I P and ARP over fibre channel SCSI over fibre channel

Encapsulation of IP Datagrams I n t he world of TCP/ I P, a user ident ifies a host by a nam e such as www.yahoo.com. The user's com put er get s t he host 's I P address from a direct ory server t hat m aps nam es t o I P addresses. The dest inat ion I P address is placed int o t he header of every dat agram sent t o t he dest inat ion host . An I P dat agram is delivered t o a dest inat ion t hat is at t ached t o a LAN by wrapping t he dat agram in a fram e addressed t o t he dest inat ion MAC address: • •

I f t he source and dest inat ion are at t ached t o t he sam e LAN, t he source t ransm it s an Address Resolut ion Prot ocol ( ARP) broadcast , asking t he dest inat ion wit h t he t arget I P address t o respond and supply it s MAC address. I f t he source and dest inat ion are not at t ached t o t he sam e LAN, t he dat agram is forwarded t o a rout er t hat is at t ached t o t he dest inat ion LAN. I n t his case, t he rout er t ransm it s an Address Resolut ion Prot ocol ( ARP) broadcast , asking t he dest inat ion t o respond and supply it s MAC address.

I P dat agram s and ARP m essages can be carried in fibre channel fram es. I n a fibre channel net work, however, a fram e is delivered based on t he dest inat ion's 3- byt e N_Port ident ifier rat her t han on it s MAC address. Two address t ranslat ion st eps are required before a fram e can be t ransm it t ed across a fibre channel LAN: 1. I P address t o I EEE world wide port nam e ( MAC address) 2. I EEE world wide port nam e t o 3- byt e N_Port address I n fibre channel LANs, ARP broadcast s are used t o m ap I P addresses t o world wide port nam es. A new fibre channel ARP ( FARP) m essage was creat ed t o resolve 8- byt e world wide port nam es t o 3- byt e port ident ifiers.

N ot e

FARP opt ionally can be im plem ent ed t o perform bot h st eps at once, direct ly m apping a 4- byt e I P address t o a 3- byt e port address.

A dat agram or ARP m essage t hat is carried in t he payload of a fibre channel fram e ( or in t he payloads of a sequence of fram es) m ust be int roduced by a 16- byt e net work header and an 8- byt e LLC/ SNAP header. The net work header cont ains t he dest inat ion and source world wide port nam es. Recall t hat a world wide port nam e is an 8- byt e field t hat cont ains •

•

A 4- bit net work address aut horit y ( NAA) ident ifier. For I P and ARP, t his is binary 0001, which is t he code assigned t o t he I EEE. A 60- bit net work address whose form at depends on t he net work address aut horit y. I EEE net work addresses consist of 12 bit s of 0- padding, followed by a 6- byt e MAC address.

N ot e At first glance, using t his net work header m ight seem st range. Why not j ust place an I P dat agram int o t he payload field? However, including t hese em bedded MAC addresses m akes it easy t o bridge fram es bet ween a fibre channel environm ent and a convent ional LAN.

The left side of Figure 22.10 shows a net work header followed by an LLC/ SNAP header and an I P dat agram t hat is t o be encapsulat ed wit hin one or m ore fibre channel fram es.

Figu r e 2 2 .1 0 . En ca psu la t in g a n I P da t a gr a m in a fibr e ch a n n e l fr a m e .

The right side of Figure 22.10 shows how a big dat agram has been cut int o subblocks and is carried in a sequence of fram es. The first subblock cont ains t he net work header and LLC/ SNAP. The net work header is not repeat ed in t he ot her subblocks. The fibre channel encapsulat ion used for ARP m essages is ident ical t o t he one used for I P, except for t he fact t hat t he LLC/ SNAP field is X'AA- AA- 03- 00- 00- 00- 08- 06. FARP request s and replies are encapsulat ed direct ly int o fibre channel fram es. No net work header or LLC/ SNAP header is needed.

N ot e There is yet anot her opt ion t hat can be used for address m appings. I P addresses, world wide port nam es, and port ident ifiers could be m apped using a direct ory service, if one were provided and all nodes were capable of using it .

SCSI over Fibre Channel The SCSI int erface bet ween com put ers and at t ached devices has been a popular choice for m any com put er syst em s for quit e a few years. The prot ocol includes m any com m ands t hat cont rol t he flow of dat a bet ween a com put er and an at t ached device. I n t he fibre channel im plem ent at ion, dat a, com m ands, and responses t o com m ands are carried bet ween a pair of P_Nodes. SCSI com m ands, dat a, and responses are carried in fibre channel fram es. Every SCSI operat ion st art s wit h a com m and and ends wit h a response. The work done as t he result of a com m and is called a t ask. Figure 22.11 illust rat es a SCSI device writ e t ask. This is im plem ent ed as a fibre channel exchange.

Figu r e 2 2 .1 1 . A SCSI de vice w r it e .

Figure 22.12 shows t he form at of a fibre channel fram e header t hat int roduces a SCSI com m and fram e. Several fields in t he header have specially assigned values. For exam ple, t he t ype is X'08 and t he fram e cont rol field is X'29- 00- 00.

Figu r e 2 2 .1 2 . For m a t of t h e fibr e ch a n n e l h e a de r for a SCSI com m a nd fr a m e .

Figure 22.13 shows t he form at of t he payload of a SCSI com m and fram e. ( Not e t hat t he fields are displayed wit h a 1- byt e widt h, as com pared t o t he 4- byt e widt h used in t he display of t he fram e header.) Fields include

Figu r e 2 2 .1 3 . SCSI com m a n d fr a m e pa yloa d.

• • • • • • • • • • •

Logica l u nit n u m be r —Picks a specific physical or virt ual device at t he t arget . Ta sk a t t r ibu t e —I ndicat es t he t ype of queue m anagem ent t hat is request ed. There are several possibilit ies for exam ple, first in, first out or a choice m ade by t he SCSI device based on perform ance opt im izat ion. Te r m ina t e t a sk —Ends t he t ask. Cle a r a u t o con t inge n t a lle gia n ce —Ret urns t he drive t o norm al condit ion. Ta r ge t r e se t —Clears t he com m and queue for all init iat ors. Cle a r t a sk se t —Clears t he queue for all init iat ors. Abor t t a sk se t —Clears t he com m ands from t his init iat or out of t he queue. Re a d da t a —I ndicat es whet her dat a will be t ransferred t o t he init iat or. W r it e da t a —I ndicat es whet her dat a will be t ransferred from t he init iat or. Com m a nd de scr ipt or block —Cont ains param et ers appropriat e t o t he specific t ype of com m and. D a t a le n gt h —Equals t he m axim um num ber of dat a byt es t hat will be t ransferred as a result of t his com m and.

Arbitrated Loops An arbit rat ed loop is m ade up of a series of nodes connect ed int o a ring. Oft en a hub is used t o organize t he cabling.

Like a Token Ring, inact ive nodes in an arbit rat ed loop are bypassed, and a node m ust perform an insert ion process in order t o j oin t he loop. A loop som et im es is built int o t he backplane of a disk enclosure or som e ot her device chassis. The unit s t hat are slot t ed int o t he enclosure becom e act ive nodes on t he loop. There is an im port ant benefit of t he arbit rat ed loop st ruct ure. When a loop is used t o link disk unit s t oget her, connect ivit y t o all rem aining disk unit s is preserved when one or m ore disk unit s are rem oved. A privat e ( st andalone) arbit rat ed loop can have up t o 126 act ive node port s ( NL_Port s) . More nodes can be physically at t ached t o t he loop, but t hey would be inact ive and t heir port s would be bypassed. However, a bypassed node can becom e act ive when one of t he act ive nodes leaves t he loop. I f a swit ch port ( FL_Port ) is connect ed int o a loop and becom es act ive, t he loop becom es a public loop. A public loop has one addit ional port , t he swit ch port ( FL_Port ) . Thus, t here can be at m ost 127 act ive port s on a public loop. Figure 22.14 shows t he node- t o- node connect ions in a public arbit rat ed loop. One of t he nodes in t he figure is inact ive. The circuit ry aut om at ically causes t he node t o be bypassed.

Figu r e 2 2 .1 4 . A public a r bit r a t e d loop.

N ot e Som e loop im plem ent at ions support high availabilit y by connect ing t he sam e set of devices t oget her using t wo separat e loops. Of course, each device would need t wo port s.

L_Port Addresses A loop port ( L_Port ) does not log in t o a swit ch. I t m ust obt ain it s 3- byt e address via a different procedure: •

•

•

I t obt ains a 1- byt e arbit rat ed loop physical address ( AL_PA) t hrough an init ializat ion process. I f t he port is at t ached t o a privat e loop, t he 3- byt e port address consist s of X'00- 00 followed by t he AL_PA. For a public loop, t he fabric port address always has t he form X'wx- yz- 00. I n t his case, t he first 2 byt es of each loop port address are defined t o be X'wx- yz. The t hird byt e is t he AL_PA.

An AL_PA is a single byt e, and 1 byt e t ranslat es t o decim al num bers ranging from 0 t o 255. However, only 127 of t hese can be used as arbit rat ed loop physical addresses. The reason for t his is t hat a byt e can be used for an address only if it s 10- bit encoding has an equal num ber of 0s and 1s. There are 134 byt es t hat m eet t his requirem ent , but 7 of t hem are reserved, leaving 127 useful values. A low address is desirable because it t ranslat es t o a high priorit y. The best AL_PA address of all, X'00, can be held only by a fabric port . The AL_PA addresses of loop port s are assigned during loop init ializat ion. The init ializat ion process is described in t he next sect ion. Som e port s do not care what physical address t hey are assigned. Ot hers m ake a bid for a specific address: •

A loop port m ay " rem em ber" t he physical address it got from a previous init ializat ion. I f so, it will m ake an at t em pt t o get it back. I f t he loop is at t ached t o a swit ch and t he port not only got a prior AL_PA but also com plet ed a login t o t he swit ch, it has a bet t er chance of get t ing it s old address back. ( I n t his case, t he address is called a fabric- assigned AL_PA.) However, anot her port also m ight be ent it led t o t ry t o grab t hat address, and t he first port m ight have t o set t le for a different address.

•

A loop port physical address m ight have been hard- coded—for exam ple, int o t he backplane of a disk enclosure. The port will t ry t o get t his hard- coded address, but in case of a conflict , it will have t o set t le for anot her address.

Arbitrated Loop Initialization A loop init ializat ion process occurs • •

Every t im e a node is insert ed int o t he loop During error recovery

During init ializat ion, a port discovers t hat it is connect ed t o a loop and realizes t hat it needs t o obt ain an arbit rat ed loop physical address. A port kicks off t he procedure by t ransm it t ing a series of ordered set s called loop init ializat ion prim it ive sequences. Successive port s t ransm it t hese ordered set s onward unt il t hey arrive back at t he init iat or node. The nodes are now ready t o t ransm it init ializat ion fram es, and 1. Select a loop m ast er 2. Make sure t hat each port get s a unique 1- byt e physical address 3. Generat e a list of t he physical addresses arranged according t o each node's posit ion around t he loop

Figure 22.15 shows t he form at of t he m essages used during loop init ializat ion. Seven different init ializat ion fram e t ypes are used in t he process. The first fram e t ype is

Figu r e 2 2 .1 5 . For m a t of loop in it ia liza t ion m e ssa ge s.

•

Loop I n it ia liza t ion Se le ct M a st e r ( LI SM ) —Select s a loop m ast er based on t he com binat ion of t he 3- byt e source port address and 8- byt e world wide port nam e. A fabric port uses X'00- 00- 00 as it s source address in t his procedure, so it always wins ( when present ) . All of t he loop node port s use source address X'00- 00- EF, so on a privat e loop, t he port wit h t he num erically sm allest world wide port nam e wins.

The m ast er is in charge of launching t he rem aining m essages around t he loop:

• •

•

• •

•

Loop I nit ializat ion Fabric Assigned ( LI FA) —Gat her all fabric- assigned arbit rat ed loop physical addresses by circulat ing a bit m ap. A device can claim an address whose value in t he bit m ap is 0 by set t ing t he bit t o 1. Loop I n it ia liza t ion Pr e viou sly Acqu ir e d ( LI PA) —Recirculat e t he bit m ap and gat her ot her previously assigned physical addresses. Loop I n it ia liza t ion H a r d Assigne d ( LI H A) —Recirculat e t he bit m ap and gat her hardware- assigned physical addresses. Loop I n it ia liza t ion Soft Assigne d ( LI SA) —Recirculat e t he bit m ap and let each node t hat has not yet been capable of claim ing an address choose t he first unused value in t he bit m ap. Loop I n it ia liza t ion Re por t Posit ion ( LI RP) —Circulat e a posit ion m ap. Each node records it s physical address in t he first em pt y slot . Loop I n it ia liza t ion Loop Posit ion ( LI LP) —Circulat e t he com plet ed posit ion m ap.

I f t here is a conflict in a fabric- assigned, previously assigned, or hardware- assigned physical address, t he node t hat get s t he bit m ap first set s t he bit , and any ot her node t hat want ed t hat address m ust pick up a new address at t he LI SA st age.

Arbitrated Loop Data Transfer The prot ocols t hat m anage dat a t ransfer on a loop are a lot sim pler t han Token Ring prot ocols. The reason for t his is t hat several 4- byt e ordered set signals are used t o m anage t he loop. These signals t ake t he place of m echanism s such as t okens, purges, claim fram es, and act ive m onit ors. A series of 4- byt e idle ordered set s is propagat ed bet ween fram e t ransm issions. A node t hat want s t o com m unicat e replaces t he idles wit h an arbit rat e signal t hat includes it s own 1- byt e AL_PA physical address.

N ot e A node's arbit rat e signal can be bum ped off and replaced by an arbit rat e signal from a higher priorit y node ( t hat is, one wit h a num erically lower AL_PA physical address) .

I f an arbit rat e signal circles t he loop and ret urns t o t he init iat or node, t he init iat or t hen can st art a com m unicat ion by sending open signals t hat include t he 1- byt e physical address of t he dest inat ion. I f t he com m unicat ion is t o be t wo- way, t he open also includes t he sender's physical address. Aft er t he open, t he part ies are ready t o com m unicat e. When a part y want s t o t erm inat e t he com m unicat ion, it sends a close signal. I t can t hen m ake t he loop available t o ot her nodes or im m ediat ely send open signals ident ifying a new part ner. As described so far, t his prot ocol allows one node t o grab t he loop and m onopolize it . There is an opt ional fairness algorit hm t hat periodically opens up t he arbit rat ion process. Also, once a fair node has had its t urn, it cannot arbit rat e again unt il it receives an idle.

Table 22.5 describes t he special 4- byt e signals t hat are used in arbit rat ed loop prot ocols. For several of t he signals, t he last 2 byt es of t he signal cont ain t wo physical addresses.

Ta ble 2 2 .5 . Fibr e Cha n n e l Ar bit r a t e d Loop Signa ls Signa l

Acr on ym

D e scr ipt ion

Arbit rat e

ARBx

The node wit h address x asks t o use t he loop. The last 2 byt es of t he signal are copies of address x.

Arbit rat e ( F0)

ARB( F0)

A node t est s t he loop t o see if anot her node want s t o arbit rat e. I t insert s address X'F0, which is larger t han any valid address. Any ot her node will be capable of replacing X'F0 wit h it s own address.

Open fullduplex

OPNyx

The node wit h address x not ifies t he node wit h address y t hat it want s t o open a t wo- way circuit .

Open halfduplex

OPNyy

The originat or node not ifies t he node wit h address y t hat it want s t o open a one- way circuit . The t arget node cannot send any fram es t o t he originat or.

Open broadcast replicat e

OPNfr

The init iat or announces t hat all nodes should accept t he fram es t hat it will send. The 2 address byt es are X'FF- FF.

Open select ive OPNyr replicat e

The init iat or prepares a set of nodes t o receive a m ult icast . These m essages are sent t o each of t he dest inat ion nodes in t urn, asking t hem t o accept t he fram es t hat will follow. The " y" is t he address of one of t he t arget nodes, and t he " r" is X'FF.

Close

CLS

A part icipant closes an act ive circuit .

Dynam ic halfduplex

DHD

The init iat or of a one- way circuit announces t hat it has no m ore fram es t o send.

Mark

MRKt x

This is used for housekeeping funct ions, such as t im e synchronizat ion.

Offline

OLS

A node t ransm it s t his for a period of t im e t o announce t hat it is preparing t o go offline.

Not Operat ional

NOS

A node t ransm it s t his several t im es t o announce t hat it s port has det ect ed a loop failure or is offline.

Link Reset

LR

A node t ransm it s t his several t im es t o init iat e t he link reset prot ocol following a link t im eout .

Link Reset Response

LRR

A node t ransm it s t his several t im es t o indicat e t hat it s port has received and recognized link reset signals.

Loop I nit ializat ion

LI P

A node init ializes t he loop, eit her t o j oin t he loop or t o recover from a det ect ed problem .

Loop Port

LPEyx

Node x deact ivat es t he bypass circuit at node y. ( Node x

Ta ble 2 2 .5 . Fibr e Cha n n e l Ar bit r a t e d Loop Signa ls Signa l

D e scr ipt ion

Acr on ym

Enable

should be a net work m anagem ent node.)

Loop Port Enable port s All

LPEfx

Node x reset s t he bypass circuit s on all on t he loop.

Loop Port Bypass

LBEyx

Node x closes t he bypass circuit at y and prevent s t he port from act ively part icipat ing in t he loop.

Summary Points •

•

• • •

• • •

•

•

•

• •

•

• •

Fibre channel provides a com m on infrast ruct ure t hat can be used t o net work com put ers and peripherals t oget her. Fibre channel support s prot ocol dat a carried in LLC/ SNAP fram es, SCSI , I PI , and HI PPI , am ong ot hers. Defined speeds range from 12.5MBps t o 400MBps. Fibre channel can connect one pair of syst em s across a point - t o- point link, a set of syst em s j oined in a loop, or a large swit ched net work ( which is called a Fabric) . Every port has a built - in world wide port nam e. For exam ple, m any port s are assigned I EEE ident ifiers or I EEE field replaceable hardware unit num bers. FC- 0 defines cables, connect ors, and t he signals sent on t he m edium . FC- 1 defines t he 8B/ 10B encoding used t o t ranslat e dat a byt es int o 10- bit code- groups. FC- 2 is t he fibre channel dat a link layer; it defines t he way t hat dat a is packaged int o fram es and specifies t he cont rol signals t hat are used t o m anage dat a t ransfer. FC- 3 is a placeholder for funct ions t hat are applied across several port s in a node. FC- 4 defines a series of m appings of various applicat ions ont o fibre channel for exam ple, I P and ARP prot ocol dat a unit s or SCSI dat a and cont rol fram es. Fibre channel provides several classes of service. The m ost im port ant are t he Class 1 connect ion- orient ed service; Class 2, which is an acknowledged connect ionless service; and Class 3, which is an unacknowledged connect ionless service. Dat a is t ransferred in fram es. Groups of relat ed fram es are organized int o sequences. One or m ore sequences t hat represent som e t ype of t ransact ion are organized int o an exchange. A port at t ached t o a swit ch m ust perform a login before it can com m unicat e. I t s environm ent param et ers are est ablished by t he login. A port t hat want s t o open Class 1 or Class 2 com m unicat ion wit h a t arget port m ust log in t o t he t arget t o est ablish t he param et ers for t hat com m unicat ion. Each port announces t he num ber of receive buffers ( t he credit ) t hat it is m aking available t o a com m unicat ion. A node m ust keep t rack of it s part ner's current credit . End- t o- end credit is updat ed by acknowledgem ent fram es. An arbit rat ed loop is m ade up of a series of nodes connect ed int o a ring. Oft en a hub or loop swit ch is used t o organize t he cabling.

•

•

• •

A st andalone loop is called a privat e loop. A loop t hat is at t ached t o a swit ch is called a public loop. A loop init ializat ion process occurs whenever a node is insert ed int o t he loop or when error recovery act ion is needed. During loop init ializat ion, t he nodes select a loop m ast er, m ake sure t hat each port is assigned a unique 1- byt e physical address, and generat e a list of physical addresses arranged according t o each node's posit ion around t he loop. A loop node request s perm ission t o t ransm it by replacing idle signals wit h arbit rat e signals.

References Approved fibre channel references include •

•

•

•

•

• • •

ANSI X3.230. " I nform at ion Technology—Fibre Channel Physical and Signaling I nt erface ( FC- PH) ." 1994. ANSI X3.272. " I nform at ion Technology—Fibre Channel—Arbit rat ed Loop ( FCAL) ." 1996. ANSI X3.289. " I nform at ion Technology—Fibre Channel—Fabric Generic ( FCFG) ." 1996. ANSI X3.297. " I nform at ion Technology—Fibre Channel—Physical and Signaling I nt erface- 2 ( FC- PH- 2) ." 1997. ANSI X3.303. " Fibre Channel—Physical and Signaling I nt erface- 3 ( FC- PH- 3) ." 1998. ANSI NCI TS 321- 1998. " Fibre Channel—Swit ch Fabric ( FC- SW) ." 1998. ANSI NCI TS TR- 20- 1998. " Fibre Channel—Fabric Loop At t achm ent ( FC- FLA) ." 1998. ANSI NCI TS 332- 1999, " Fibre Channel—Arbit rat ed Loop ( FC- AL- 2) ." 1999.

A large num ber of ot her references are under developm ent by ANSI com m it t ees. Am ong t hese are • • • • • • • • •

" Fibre Channel Link Encapsulat ion ( FC- LE) ." " Fibre Channel Fabric Generic Requirem ent s ( FC- FG) ." " Fibre Channel Swit ch Fabric- 2 ( FC- SW- 2) ." " Fibre Channel Backbone ( FC- BB) ." " Fibre Channel Virt ual I nt erface ( FC- VI ) ." " Fibre Channel Fram ing and Signaling I nt erface ( FC- FS) ." " SCSI Fibre Channel Prot ocol2 ( FCP- 2) ." " Fibre Channel Privat e Loop SCSI Direct At t ach ( FC- PLDA) ." " FCA I P Profile."

Online inform at ion is available at t he Fibre Channel I ndust ry Associat ion Web sit e, ht t p: / / www.fibrechannel.com / ; t he Fibre Channel Loop Com m unit y Web sit e, ht t p: / / www.fcloop.org; and t he T11 com m it t ee Web sit e, ht t p: / / www.t 11.org/ . The I ETF has published a docum ent t hat st andardizes t he way t hat I P is im plem ent ed over fibre channel:

•

RFC 2625. " I P and ARP over Fibre Channel." M. Raj agopal, R. Bhagwat , and W. Rickard.

A t ut orial can be found at t he Universit y of New Ham pshire int eroperabilit y lab Web sit e: ht t p: / / www.iol.unh.edu/ t raining/ index.ht m l

Part IV: Appendixes A Physical Layer Encodings for Et hernet , Fibre Channel, FDDI , and Token Ring B Tables C St andards Bodies and References D Acronym List Glossary

Appendix A. Physical Layer Encodings for Ethernet, Fibre Channel, FDDI, and Token Ring This appendix exam ines t he different ways t hat dat a is encoded ont o a m edium for Et hernet , fibre channel, FDDI , and Token Ring LANs. A det ailed acquaint ance wit h encodings is not really needed t o inst all, m anage, or t roubleshoot a LAN. I f an Et hernet or Token Ring chip or a digit al signal processor m isbehaves, a LAN adm inist rat or uses a t est er or SNMP t o t rack down t he bad NI C, pull it out , and replace it . Many NI Cs com e wit h lifet im e replacem ent guarant ees. Hub and swit ch product s usually are backed by good m ult iyear guarant ees. A LAN adm inist rat or can get along very well wit hout t he m at erial in t his appendix, which is why it has been included as opt ional reference m at erial. However, som e insight s can be gained from a look int o physical layer int ernals. For exam ple, t his appendix provides a bet t er underst anding of how collisions are det ect ed, explains t he delay t hat occurs wit hin Class I hubs, and clarifies t he full- duplex underpinnings of Gigabit Et hernet . One t hing t o wat ch for is t he way t hat encodings are designed t o accom plish t he following: •

•

Reduce crosst alk on copper wires by keeping t he signal frequency as low as possible Prevent an opt ical fiber from overheat ing due t o a high densit y of st rong light pulses

This is called prom ot ing dc balance. The t erm dc act ually st ands for direct current . When applied t o bot h copper and opt ical fiber, " prom ot ing dc balance" t ranslat es roughly t o " t ransm ission of alm ost equal num bers of high and low signals."

Code-Groups and Special Signals The 0s and 1s in a st ream of digit al dat a m ust be t ranslat ed int o physical sym bols t hat can be im pressed ont o a m edium . I t is nat ural t o visualize t he bit s in a fram e being t ransm it t ed across a m edium by fait hfully copying it s 0 and 1 bit s int o physical represent at ions of 0 and 1for exam ple, as dist inct volt age levels or opt ical power levels. The problem wit h using a high volt age for 1 and a low volt age for 0 is t hat a receiver has t rouble keeping t rack of where bit s begin and end when it receives a long st ring of 1s or a long st ring of 0s. Over t he years, several ways of overcom ing t his problem have been int roduced. For exam ple, for 10BASE5, 10BASE2, and 10BASE- T Et hernet , each 0 or 1 is t ranslat ed int o a Manchest er encoded sym bol. Manchest er encoding ( which is described in t he next sect ion) represent s bit s as changes in volt age levels. Manchest er encoding works very well, but it requires a t ransit ion bet ween high and low volt age levels for each bit . This is feasible at speeds of up t o 16Mbps, but is difficult t o im plem ent at higher speeds. The physics of support ing a large num ber of t ransit ions per second bet ween high and low volt age levels t ranslat es int o a requirem ent t hat t he m edium support high frequencies and have a big bandwidt h capacit y ( in m egahert z) . Engineers have developed several ways t o cram m ore dat a ont o a cable using fewer t ransit ions bet ween high and low volt age levels. One t echnique t ranslat es a set of bit s t o a bigger set of bit s t hat has a good dist ribut ion of 0s and 1s. For exam ple, 4B/ 5B coding t ranslat es each 4- bit pat t ern t o a 5- bit pat t ern cont aining a good m ix of 0s and 1s. Aft er t ranslat ion, a 0 can be sent as a " low" and a 1 as a " high" because t he bit st ream will cont ain enough t ransit ions t o m aint ain accurat e bit clocking. Anot her t echnique t hat cram s m ore bit s per second ont o a copper cable is t o use m ore t han t wo volt age levels. Three levels were used for 100BASE- TX and 100BASET4. Five levels were int roduced for 100BASE- T2 and 1000BASE- T. Table A.1 sum m arizes various t ypes of encodings t hat have been used over t he years. These encodings are described in t he sect ions t hat follow. Som e of t he encodings t hat are st udied include special cont rol signals in addit ion t o signals t hat correspond t o 0 or 1 dat a bit s. Cont rol signals are delim it ers t hat m ark t he beginning and end of a fram e, or t hat represent idle sym bols t hat are used as filler bet ween fram es.

Ta ble A.1 . Sym bol Encodin gs

Type of LAN 10BASE5

Tr a n sla t ion None

10BASE2, and 10BASE- T Et hernet FOI RL and 10BASE- FL Et hernet

None

Sym bols

D e scr ibe d As

0= High- t o- low opt ical volt age.

Manchest er

1= Low- t o- high volt age.

Encoding

0= High- t o- low opt ical power.

Manchest er encoding for bit s

1= Low- t o- high opt ical power. Special pulse for idle. 10BASE- FB Et hernet

None

0= High- t o- low opt ical power.

Manchest er encoding for bit s

1= Low- t o- high opt ical power. Addit ional synchronous signals for idle and fault . 100BASE- FX Et hernet and FDDI

4B/ 5B Map 4- bit nibbles t o 5- bit code- groups

0= No t ransit ion

Non ret urn t o zero invert ed ( NRZI )

1= Alt ernat es bet ween high and low opt ical power. Special code- groups for idle, st art and end of st ream , and error condit ions. 100BASE- TX Et hernet and CDDI

4B/ 5B Map 4- bit nibbles t o 5- bit code- groups

Three volt age levels: low, m iddle, and high. . 0= No t ransit ion.

MLT- 3, som et im es called NRZI - 3

1= St ep t ransit ions: low- t om iddle, m iddle- t o- high, high- t o- m iddle, m iddle- t olow, and so on. Special code- groups for idle, st art and end of st ream , and error condit ions 100BASE- T4 Et hernet

8B/ 6T Map 8- bit byt es t o 6- sym bol code- groups

Three sym bols, labeled [ - , 0, + ] or [ - 1, 0, 1] are used. Each sym bol is t ransm it t ed

8B/ 6T

Ta ble A.1 . Sym bol Encodin gs Type of LAN

Tr a n sla t ion

Sym bols

D e scr ibe d As

as a different volt age level. 100BASE- T2 Et hernet

Map 4- bit nibbles t o 2- sym bol codegroups

Five sym bols labeled [ - 2, 1, 0, 1, 2] are used. Each sym bol is t ransm it t ed as a different volt age level.

PAM5

1000BASE- SX, 1000BASE- LX, and 1000BASECX Et hernet and Fibre Channel

8B/ 10B Map 8- bit byt es t o 10- bit code- groups

1= High opt ical power or high volt age. 0= Low opt ical power or low volt age. Special code- groups for idle, ext ensions, st art and end of packet , negot iat ion, and error condit ions.

8B/ 10B

1000BASE- T Et hernet

8B1Q4 Map 8- bit byt es t o 4- sym bol code- groups. One sym bol is sent on each pair.

Five sym bols labeled [ - 2, 1, 0, 1, 2] are used. Each sym bol is t ransm it t ed as a different volt age level. Special code- groups for idle, ext ension, st art and end of st ream , and error condit ions.

4D- PAM5

4Mbps or 16Mbps Token Ring

None

0= Transit ion at st art and m iddle. 1= Transit ion in m iddle.

Different ial Manchest er encoding, wit h special " J" and " K" sym bols

100Mbps Dedicat ed Token Ring

4B/ 5B

Sam e as 100BASE- FX for opt ical fiber and 100BASETX for t wist ed- pair cable.

NRZI for fiber opt ic, MLT- 3 for copper

Ethernet 10BASE5, 10BASE2, and 10BASE-T Manchester Encoding Dat a is t ransm it t ed ont o 10Mbps coaxial cable and 10Mbps t wist ed- pair LAN m edia using a m et hod called Manchest er encoding. For each 0 and 1 t ransm it t ed across t he m edium : • • •

The encoded represent at ion of t he bit has a volt age t ransit ion at it s m idpoint . For a 0- bit , t he first half is high and t he second half is low. For a 1- bit , t he first half is low and t he second half is high.

Figure A.1 shows t he Manchest er pat t ern for 1 0 1 1 1 0 0 1.

Figu r e A.1 . M a n ch e st e r e n codin g.

The fact t hat t here is a t ransit ion wit hin each bit m akes it easy for a receiver t o synchronize it s t im ing wit h t he t ransm it t er. I f t wo signals collide on t he cable, t he t ransit ion pat t ern is broken, enabling syst em s t o det ect t he collision.

Ethernet FOIRL, 10BASE-FL, and 10BASE-FB Manchest er encoding also is used for 10Mbps Et hernet across fiber opt ic cable. However, som e special signals have been added t o t he fiber opt ic specificat ions t o support som e useful feat ures. As not ed in Chapt er 6, " The Et hernet 10Mbps Physical Layer," whenever dat a is not being t ransm it t ed by a FOI RL or 10BASE- FL int erface, t he int erface sends special idle pulses ont o t he send fiber. The idle signal for 10BASE- FB is called a synchronous idle and is m ade up of special clocked sym bols. Two new Manchest er sym bols are defined: •

•

M a n che st e r Code Viola t ion Ze r o ( M V0 ) A clocked sym bol t hat is low for a bit durat ion. M a n che st e r Code Viola t ion One ( M V1 ) A clocked sym bol t hat is high for a bit durat ion.

A synchronous idle is a repeat ing sequence of sym bols: MV1 MV1 MV0 MV0 A different repeat ing com binat ion of MV0 and MV1 sym bols is used t o not ify t he rem ot e part ner t hat a fault ( such as j abber, invalid dat a, low light , or loss of clocking) has been det ect ed. The rem ot e fault pat t ern is t his: MV1 MV1 MV1 MV0 MV0 MV0

Figure A.2 shows what t he signals look like.

Figu r e A.2 . Syn ch r on ou s idle a n d r e m ot e fa u lt pa t t e r n s.

100BASE-X Ethernet, FDDI, and CDDI The t it le 100BASE- X encom passes 100BASE- TX and 100BASE- FX. The 100BASE- X t it le was coined because even t hough 100BASE- TX t ransm it s ont o t wist ed- pair cable and 100BASE- FX t ransm it s ont o opt ical fiber, t hese t echnologies have a lot in com m on. The physical layers for 100BASE- FX and 100BASE- TX were derived from t he FDDI and CDDI physical layers. Figure A.3 shows a division of t he physical layer int o a part t hat is independent of t he m edium and a part t hat is dependent on t he m edium . The m edium - independent part s of 100BASE- TX and 100BASE- FX are t he sam e.

Figu r e A.3 . A vie w of t he ph ysica l la ye r .

For FDDI , CDDI , 100BASE- TX, and 100BASE- FX, dat a is prepared for t ransm ission by t ranslat ing each 4- bit quant it y int o a 5- bit pat t ern. This is called 4B/ 5B t ranslat ion.

N ot e A byt e is m ade up of t wo 4- bit quant it ies. Recall t hat each 4- bit half- byt e is called a nibble.

Before exam ining exact ly how t he t ranslat ion is done, it is helpful t o exam ine t he physical signals t hat are used for 100BASE- FX and FDDI , and for 100BASE- TX and CDDI . The lim it at ions of t he t wo signaling m et hods are t he reason t hat each nibble m ust be t ranslat ed before it is sent .

NRZI Signals for 100BASE-FX and FDDI 100BASE- FX and FDDI operat e over t wo opt ical fibers. Dat a is sent on one fiber and received on t he ot her.

Values of 0 and 1 are im pressed ont o a fiber using non- ret urn- t o- zero, invert - on- one encoding. This works as follows: • •

The 1s are alt ernat ively represent ed by a high or low signal. No change of signal level t akes place at a 0.

Figure A.4 displays an NRZI encoding of t he bit st ream 11010001.

Figu r e A.4 . N RZI - e n code d da t a .

The problem wit h NRZI is t hat when a long st ring of 0s is t ransm it t ed, no t ransit ions occur. This causes t he receiver t o lose bit t im e synchronizat ion. The cure is 4B/ 5B t ranslat ion, which convert s each 4- bit nibble int o a 5- bit codegroup t hat cont ains at least t wo 1s. Transm it t ing t hese 5- bit code- groups guarant ees t hat t here will be t wo or m ore t ransit ions whenever a nibble is sent . This enables t he receiver t o m aint ain it s bit clocking.

MLT-3 Signals for 100BASE-TX and CDDI Mult i- Level 3 encoding ( MLT- 3) is an efficient signaling m et hod t hat was int roduced for CDDI and adopt ed for 100BASE- TX. I t has a lower bandwidt h requirem ent t han t he NRZI signaling used for FDDI and 100BASE- FX. This is helpful because Cat egory 5 cable support s far less bandwidt h t han opt ical fiber. Like NRZI , MLT- 3 m akes a t ransit ion for each 1 and st ays t he sam e at each 0. However, t he t ransit ions are m ade at t hree different signal levels. The signal changes one st ep at a t im e, as follows: 1. 2. 3. 4.

Low t o m iddle Middle t o high High t o m iddle Middle t o low

The result is t hat t he num ber of t ransit ions bet ween low and high volt age decreases. ( You go all t he way from low t o high and back t o low half as oft en.) This t ranslat es t o a lower frequency, which m akes it possible t o fit 100Mbps ont o Cat egory 5 cable wit h bandwidt h t o spare and cut s down on crosst alk.

Figure A.5 shows an MLT represent at ion of t he bit st ring 11010001. I nst ead of being called low, m iddle and high, t he levels oft en are represent ed using t hese sym bols: [ - , 0, + ] or [ - 1, 0, and 1]

Figu r e A.5 . M LT- 3 e n code d da t a .

MLT- 3 present s t he sam e problem as NRZI . No t ransit ions t ake place when a long st ring of 0s occurs, and t his can cause a receiver t o lose bit t im ing. The sam e solut ion t hat was applied t o NRZI is used here. Each 4- bit nibble is convert ed t o a 5- bit code- group using 4B/ 5B t ranslat ion. The com binat ion of 4B/ 5B t ranslat ion and MLT- 3 signals enables a sender t o t ransm it 100Mbps across a 31.25MHz line.

4B/5B Code-Groups for FDDI, CDDI, and 100BASE-X Table A.2 shows how t he 4B/ 5B t ranslat ion is done. The encoding choices have been m ade so t hat no 5- bit code has m ore t han t wo consecut ive 0s. When t wo nibbles are t ranslat ed in sequence, a few of t he 10- bit result s cont ain t hree consecut ive 0s. For exam ple, X'24 t ranslat es t o 1010 0 0 1010. However, no sequence of nibbles result s in m ore t han t hree consecut ive 0s.

Ta ble A.2 . 4 B/ 5 B D a t a M a ppin gs Hex

Bin a r y " N ibble " ( 4 B)

Five Bit Code - Gr ou p ( 5 B)

0

0000

11110

1

0001

01001

2

0010

10100

Ta ble A.2 . 4 B/ 5 B D a t a M a ppin gs Hex

Bin a r y " N ibble " ( 4 B)

Five Bit Code - Gr ou p ( 5 B)

3

0011

10101

4

0100

01010

5

0101

01011

6

0110

01110

7

0111

01111

8

1000

10010

9

1001

10011

A

1010

10110

B

1011

10111

C

1100

11010

D

1101

11011

E

1110

11100

F

1111

11101

Only 16 of t he 32 possible 5- bit pat t erns are needed t o represent dat a nibbles. Som e of t he rem aining pat t erns have been assigned t o special code- groups. Table A.3 describes t hese special code- groups. Code- groups I , J, K, T, and R are called cont rol codes. Code- group H is a code violat ion t hat is used t o signal a collision or error condit ion. Code- group Q is used for som e special physical connect ion- m anagem ent signals needed for FDDI and CDDI . The idle code- group ( I ) is sent cont inuously bet ween fram es and enables a syst em t o check t he int egrit y of it s receive pair on an ongoing basis. The t wo- code st art - ofst ream delim it er ( JK) is sent in place of t he first pream ble byt e. The end- of- st ream delim it er ( TR) m arks t he com plet ion of t he t ransm ission of a fram e.

Ta ble A.3 . Spe cia l 5 B Code - Gr ou ps Nam e

Code Gr ou p

Use for Et h e r n e t

Use for FD D I / CD D I

I

11111

I dle sym bol, sent as filler bet ween fram es

I dle sym bol, sent as filler bet ween fram es

J

1 1 0 0 0

First half of a st art - ofst ream delim it er

First half of a st art - of- st ream delim it er.

Ta ble A.3 . Spe cia l 5 B Code - Gr ou ps Nam e

Code Gr ou p

Use for Et h e r n e t

Use for FD D I / CD D I

K

1 0 0 0 1

Second half of a st art - ofst ream delim it er

Second half of a st art - of- st ream delim it er.

T

0 1 1 0 1

First half of an end- ofst ream delim it er

One T is used as a dat a fram e end delim it er. Two T sym bols are used as a t oken end delim it er.

R

0 0 1 1 1

Second half of an endof- st ream delim it er

H

0 0 1 0 0

Used t o indicat e a collision or error condit ion connect ion

Q

0 0 0 0 0

Used for special physical m anagem ent signals. Used for special physical connect ionm anagem ent signals.

Ethernet 100BASE-T4 100BASE- T4 runs on four pairs of Cat egory 3 t wist ed- pair cable. An efficient encoding is needed t o pack 100Mbps ont o Cat egory 3 cable.

Ternary Symbols and 8B/6T Encoding As wit h 100BASE- TX, t hree different signal levels are used for 100BASE- T4. I nst ead of following fixed st eps from low- t o- m iddle, m iddle- t o- high, and so on, however, t he volt age levels are used as sym bols t hat can appear in any order. These are called t ernary sym bols. As was t he case for MLT- 3, t he volt age levels correspond t o t hese t ernary sym bol labels: [ - , 0, + ] or [ - 1, 0, and 1] . Each 8- bit byt e is m apped t o a unique code- group consist ing of six t ernary sym bols. There are 2< + z6.175> 8< $z$> = 256 different 8- bit pat t erns, but t here are 3< + z6.175> 6< $z$> = 729 different 6- sym bol code- groups, so m ore t han enough code- groups are available. The code- groups t hat have been select ed t o represent dat a byt es are t hose t hat cont ain an equal or alm ost equal num ber of posit ive and negat ive sym bols. The t ranslat ion is called 8B/ 6T. Table A.4 shows j ust a few of t he 8B/ 6T- byt e t ranslat ions. The full t ranslat ion t able appears in Appendix B, " Tables" ( see Table B.2) .

Ta ble A.4 . Som e Sa m ple 8 B/ 6 T Byt e Tr a n sla t ion s H EX

Bin a r y Byt e

6 T Code - Gr ou p

00

0 0 0 0 0 0 0 0

+ - 0 0 + -

01

0 0 0 0 0 0 0 1

0 + - + - 0

02

0 0 0 0 0 0 1 0

+ - 0 + - 0

03

0 0 0 0 0 0 1 1

- 0 + + - 0

04

0 0 0 0 0 1 0 0

- 0 + 0 + -

05

0 0 0 0 0 1 0 1

0 + - - 0 +

06

0 0 0 0 0 1 1 0

+ - 0 - 0 +

07

0 0 0 0 0 1 1 1

- 0 + - 0 +

08

0 0 0 0 1 0 0 0

- + 0 0 + -

09

0 0 0 0 1 0 0 1

0 - + + - 0

0A

0 0 0 0 1 0 1 0

- + 0 + - 0

0B

0 0 0 0 1 0 1 1

+ 0 - + - 0

0C

0 0 0 0 1 1 0 0

+ 0 - 0 + -

0D

0 0 0 0 1 1 0 1

0 - + - 0 +

0E

0 0 0 0 1 1 1 0

- + 0 - 0 +

0F

0 0 0 0 1 1 1 1

+ 0 - - 0 +

58

0 1 0 1 1 0 0 0

+ + + 0 - -

59

0 1 0 1 1 0 0 1

+ + + - 0 -

5A

0 1 0 1 1 0 1 0

+ + + - - 0

I n ve r t in g 6 T D a t a Code - Gr ou ps Each 6T dat a code- group displayed in Table A.4 cont ains eit her an equal num ber of + and - sym bols, or has one m ore + t han - sym bol. This also holds t rue for t he ent ire dat a code- group t able in Appendix B. I f t hese sym bols were sent as is, dc balance would be lost over t im e. The dc balance for a wire pair is rest ored by follow ing rules t hat invert som e of t he code- groups sent on t hat t wist ed- pair cable: 1. Com put e t he weight of each code group by adding it s sym bols. Every codegroup has a weight of 0 or 1. For exam ple, t he weight of ( + + + - - 0) is 1. 2. Bet ween fram es, set t he cum ulat ive weight for t he t wist ed- pair cable t o 0.

3. I f t he weight of t he current dat a code- group is 0, do not change t he cum ulat ive weight . 4. I f t he weight of t he current dat a code- group is 1 and t he cum ulat ive weight is 0, set t he cum ulat ive weight t o 1. 5. I f t he weight of t he current dat a code- group is 1 and t he cum ulat ive weight is 1, set t he cum ulat ive weight t o 0 and reverse t he signs of t he sym bols in t he code- group.

100BASE-T4 Transmission For 100BASE- T4, dat a is t ransm it t ed across t hree pairs of wires. The sender list ens for incom ing dat a on t he rem aining pair t o det ect a collision. Figure A.6 illust rat es t he dat a flows. The diagram at t he t op shows t he flow when t he syst em on t he left receives dat a. The diagram on t he bot t om shows t he flow when t he syst em on t he left is t he sender.

Figu r e A.6 . 1 0 0 BASE- T4 da t a flow s.

Each byt e is t ranslat ed int o a 6T t ernary code- group and is sent across one of t he pairs. Byt es are assigned t o t he t hree pairs in round robin fashion. Figure A.7 illust rat es how t his is done. ( Not e t hat dat a flows from right t o left in t his figure.) I n t he figure, t he first byt e is t ranslat ed t o a 6T code- group and is sent across pair 4. The code- group for t he next byt e is sent across pair 1, and t he codegroup for t he byt e aft er t hat is sent across pair 3. The sequence t hen cycles back t o pair 4. The sender list ens t o it s pair 2 t o check for collisions.

The vert ical dot t ed lines on t he right side of Figure A.7 indicat e t hat t he t ransm ission t im es of each 6T code- group are st aggered. Thus, as is shown by t he vert ical dot t ed lines on t he left , a 6T code- group arriving on pair 4 is followed by a code- group on pair 1, and finally one on pair 3. This enables incom ing byt es t o be collat ed int o t heir original order at t he receiving end. The next sect ion shows how t his get s synchronized at t he beginning of a fram e.

Figu r e A.7 . Tr a n sm it t in g byt e s of da t a for 1 0 0 BASE- T4

A t ot al of 331/ 3Mbps m ust be t ransm it t ed across each wire. Aft er convert ing each 8bit quant it y t o six sym bols, t he rat e becom es 6/ 8 ( 331/ 3) = 25 m illion sym bols per second. Because of a reduced num ber of high- low t ransit ions required in t he codegroups t hat are used, t he bandwidt h t hat act ually is required is only 12.5MHz per wire.

Special Symbols and Alignment As was t he case wit h t he 4B/ 5B encoding used for 100BASE- X, som e 6T pat t erns are used for special purposes. For 100BASE- T4, special pat t erns are used t o m ark t he beginning and end of each fram e. Figure A.8 shows how a st art - of- st ream , dat a, and end- of- fram e pat t ern is encoded and sent across t hree wires. Dat a flows from right t o left in t his figure; t he left m ost bit s are t ransm it t ed first . The t op of t he figure shows t hat byt es sent across each pair arrive at different t im es. t he lower part of t he figure shows t he special sym bol pat t erns t hat are sent at t he beginning and end of each t ransm ission. These pat t erns are dist inct from t he codegroups t hat represent dat a byt es: •

•

•

Sixt een st art up sym bols are t ransm it t ed on pair 4, which will carry t he codegroup t hat represent s t he first byt e of dat a. Eight een st art up sym bols are sent on pair 1, which will carry t he code- group for t he second dat a byt e. Twent y st art up sym bols are sent on pair 3, which will carry t he code- group for t he t hird dat a byt e.

•

For each pair, t he pat t ern ( + - - + ) precedes t he first dat a code- group t hat is sent across t he pair.

The different lengt hs t hat are used for t he st art up pat t erns set up t he st aggered arrival t im es for t he 6T code- groups. The arrival t im es are st aggered by t wo sym bol t im es. This enables t he receiver t o keep t he incom ing code- groups in order and collat e t hem correct ly. Specifically, code- group 1 arrives t wo sym bol t im es before code- group 2, which arrives t wo sym bol t im es before code- group 3, which arrives t wo sym bol t im es before code- group 4, and so fort h. The lengt h of t he fram e being t ransm it t ed det erm ines which wire pair carries t he last dat a code- group. The last dat a code- group in Figure A.8 happens t o be sent on pair 3. The special pat t ern ( + + - - 00) signals t he end of dat a. Ot her special t erm inat ing pat t erns appear on pairs 4 and 1. The lower part of Figure A.8 shows t he nam es t hat have been given t o groupings of st art and end pat t erns. These pat t erns and t he acronym s and nam es t hat have been assigned t o t hem are shown in Table A.5.

Ta ble A.5 . Spe cia l 6 T Pa t t e r ns Nam e

Codin g

D e scr ipt ion

SOSA

+ - + - + -

St art - of- st ream - A. Used t o represent a pream ble byt e.

SOSB

+ - + - - +

St art - of- st ream - B. Used as a st art - of- st ream delim it er.

P3

+ -

Pream ble 3. Cont ains pream ble bit s.

P4

+ - + -

Pream ble 4. Cont ains pream ble bit s.

EOP1

+ + + + + +

End- of- packet - 1. Signals t he end of a fram e.

EOP2

+ + + + - -

End- of- packet - 2.

EOP3

+ + - - 0 0

End- of- packet - 3.

EOP4

- - - - - -

End- of- packet - 4.

EOP5

- - 0 0 0 0

End- of- packet - 5.

Bad- Code

- - - + + +

Sent when t here has been a t ransm ission error.

Figu r e A.8 . Tr a n sm it t in g a fr a m e u sin g 1 0 0 BASE- T4 .

N ot e Special dc balance rules are applied t o t he special 6T pat t erns. For exam ple, SOSA, SOSB, and Bad- Code never are invert ed.

Translating Between 100BASE-X and 100BASE-T4 in a Hub The 4B/ 5B encoding used for 100BASE- TX or 100BASE- FX is very different from t he 8B/ 6T encoding used for 100BASE- T4. A Class I I hub t hat support s eit her 100BASE- X or 100BASE- T4 exclusively can forward each incom ing sym bol wit hout furt her processing. However, a Class I hub t hat support s bot h 100BASE- X and 100BASE- T4 int erfaces m ust convert bet ween t hese encodings. For exam ple, if dat a is arriving on a 100BASE- X port , t he port m ust accum ulat e 10 sym bols, convert t hem t o 8 bit s, and t hen reconvert t hem t o a 6T code. This is what causes t he ext ra delay in a Class I hub.

Ethernet 100BASE-T2

100BASE- T2 never was im plem ent ed in product s. Nonet heless, t he t echnology is of som e int erest because m uch of it was borrowed for 1000BASE- T, so it will be described here briefly. Like 100BASE- T4, 100BASE- T2 was designed t o run on Cat egory 3 or bet t er wiring. However, unlike 100BASE- T4, 100BASE- T2 requires only t wo t wist ed- pair cables and support s full- duplex links as well as half- duplex operat ion. A st at ion wit h a 100BASE- T2 int erface could be connect ed t o a half- duplex environm ent by linking it t o a hub, or it could run in full- duplex m ode when connect ed t o anot her st at ion or a swit ch. A 100BASE- T2 link act ually always physically operat es in a full- duplex m anner. Aft er an aut oconfigurat ion negot iat ion t hat est ablishes t he propert ies of t he link, st eady st ream s of sym bols flow across each wire pair in bot h direct ions. I dle sym bols are sent bet ween dat a t ransm issions. The st art and end of each fram e are m arked by special st art - of- st ream and end- of- st ream delim it ers.

Quinary Symbols and PAM5x5 Encoding Three volt age levels were used for 100BASE- T4. Five were needed for 100BASE- T2. The sym bols corresponding t o t he five different volt age levels are labeled [ - 2, - 1, 0, + 1, + 2] and represent quinary sym bols. Transm ission of dat a using several volt age levels is called pulse am plit ude m odulat ion ( PAM) . Because t here are five volt age levels, t he t ransm ission m et hod is called 5- level pulse am plit ude m odulat ion, or PAM5. A pair of quinary sym bols such as ( + 1, - 2) or ( 0, - 1) is used t o represent 4 dat a bit s ( a nibble) . Because t here are 16 four- bit pat t erns and 25 quinary sym bol pairs, a m apping of 4 bit s int o 2 sym bols can be done easily. The ext ra pairs of quinary sym bols can be used t o represent idles, fram e delim it ers, and error codes. Each out going 4- bit quant it y is scram bled and t hen is t ranslat ed t o a pair of quinary sym bols using a com plicat ed m apping algorit hm . The com bined m apping and t ransm ission procedure is called PAM5x5.

N ot e I f you are curious about t he m apping, see Chapt er 32 of I EEE 802.3 for t he det ails. But be forewarned: The det ails are very m essy.

The box at t he t op of Figure A.9 represent s t he conversion of 4 bit s t o t wo quinary sym bols ( An, Bn) . The lower part of t he figure shows t wo sym bols being t ransm it t ed across t he t wo pairs at t he sam e t im e. The label BI _DA st ands for " bidirect ional dat a A," and BI _DB st ands for " bidirect ional dat a B." When pair ( An, Bn) is received, it is m apped t o 4 bit s and is unscram bled.

Figu r e A.9 . En codin g a n d t r a n sm it t in g qu in a r y sym bols.

The rat e across each pair is 25 m illion sym bols per second, giving a t ot al of 50 m illion sym bols per second. Each pair of sym bols m aps t o 4 bit s, result ing in t he 100Mbps dat a rat e. The sam e num ber of sym bols is sent in t he reverse direct ion sim ult aneously.

1000BASE-X and Fibre Channel 1000BASE- X is a t it le t hat applies t o t hree t echnologies: 1000BASE- SX, 1000BASELX, 1000BASE- CX. These t echnologies were based on exist ing fibre channel t ransm ission st andards. The 8B/ 10B encoding m et hod int roduced for fibre channel is used for 1000BASE- SX, 1000BASE- LX, and 1000BASE- CX. As t he nam e 8B/ 10B suggest s, each byt e is t ranslat ed t o a 10- bit pat t ern t hat has a good dist ribut ion of 1s and 0s. Aft er t ranslat ion, a very sim ple t ransm ission m et hod is used t o send t he 10- bit pat t ern. On opt ical fiber, a 1 is t ransm it t ed as a high opt ical power level, and a 0 is a low opt ical power level. High and low volt ages are used on copper.

8B/10B Data Encoding The designers of fibre channel and 1000BASE- X want ed a bet t er balance of 0s and 1s t han was obt ained by t he 4B/ 5B coding used for FDDI and 100BASE- X. At gigabit speed, m aint aining bit clocking is a great er challenge t han at 100Mbps. On t he ot her hand, t oo m any t ransit ions produces a signal t hat has t oo high a frequency. This can heat up an opt ical fiber or creat e a m agnet ic dist urbance around copper. The 8B/ 10 encoding t hat is used provides a sufficient num ber of 1s while keeping t he average num ber of t ransit ions down. There are 256 8- bit pat t erns and 1024 10- bit pat t erns. This m eans t hat dat a codegroups can be chosen from a big pool of 10- bit pat t erns. To ensure t hat t here is a good dist ribut ion of 1s and 0s on t he m edium , t he 10- bit code- groups t hat have

been select ed cont ain t he sam e—or alm ost t he sam e—num ber of 1s and 0s. Nam ely, t hey cont ain one of t hese values: • • •

Five 1s and five 0s Four 1s and six 0s Six 1s and four 0s

I t is desirable t o avoid t oo m any bunched- up 1s or 0s, so t he following rules hold t rue: • • •

No dat a code- groups cont aining t he st rings 11111 or 00000 are used. Only four dat a code- groups cont aining t he st ring 1111 are used. Only one dat a code- group cont ains t he st ring 0000.

The beginning and end of each 10- bit code- group also was designed t o avoid long st rings of 0s and 1s being produced when t wo pat t erns occur in sequence. For exam ple, if t he pat t erns shown below were sent in sequence, t here would be six consecut ive 0s in t he bit st ream : 01101110 0 0 0 0 0 1110110 I n fact , t his will not happen. Joining t wo 10- bit code- groups never produces m ore t han five consecut ive 1s or five consecut ive 0s. The final problem t o be solved is t o be sure t hat over t im e, roughly t he sam e num ber of 0s and 1s is sent across t he m edium . There is no way t o predict exact ly what dat a byt es will occur in a user's fram es. Byt e aft er byt e of one dat a st ream m ight m ap t o pat t erns wit h four 1s, while anot her st ream m ight m ap t o a long st ring of pat t erns wit h six 1s. The solut ion was t o provide t wo different 10- bit code- groups t o represent each byt e. • •

I f one code- group has six 1s, t he ot her has four. I f one code- group has five ones, t he ot her also has five.

The basic idea is t hat t he code- group t hat is used t o represent t he current byt e is chosen based on t he dist ribut ion of 1s and 0s in t he previous code- group. The goal is t o balance out t he num ber of 1s and 0s. Table A.6 displays a few of t he t ranslat ions from byt es t o code- groups. Not e t hat in t he t able, each 10- bit code- group is writ t en as a 6- bit subblock, a space, and a 4- bit subblock.

N ot e The 8B/ 10 code was creat ed by com bining older 5B/ 6B and 3B/ 4B codes. The 6- bit subblock com es from t he 5B/ 6B code. The 4- bit subblock com es from t he 3B/ 4B code.

Suppose t hat t he current byt e is X'03 and has been t ranslat ed t o it s code- group wit h six 1s: 110001 1011 I f X'05 followed, t his would be balanced by choosing t he code- group wit h four 1s: 101001 0100 But suppose t hat X'04 now follows. Bot h of it s code- groups cont ain five 1s. However, t he prior code- group ended wit h 0100, which has a lot of 0s, so t he best choice is t he one t hat st art s wit h som e 1s: 110101 0100 As you m ight have guessed, a set of rules enables a com put er t o decide which codegroup should be select ed t o balance out t he flow of 0s and 1s.

Relationship between the Code-Group Columns At first glance, Table A.6 looks like gibberish, but t here is a m et hod t o it s m adness. The first code- group for each byt e appears in colum n 3, and t he second code- group appears in colum n 4. The code- groups in colum n 4 are com put ed from t he code- groups in colum n 3. Each 10- bit code- group is writ t en as a 6- bit subblock, followed by a 4- bit subblock. I f a subblock is unbalanced, t he corresponding subblock in t he second code- group is form ed by invert ing each of it s bit s. For exam ple, t he colum n 3 code- group for X'00 is 100111 0100. 100111 is unbalanced because it has four 1s and t wo 0s. When each 1 is invert ed t o 0 and each 0 t o 1, you get 011000 as t he first subblock of t he second codegroup. Sim ilarly, 0100 has one 1 and t hree 0s, so t he second subblock of t he colum n 4 code- group ends in 1011. Checking out anot her ent ry, t he first code- group for X'0E is 011100 1011. The init ial 6- bit subblock is balanced and, hence, will not be invert ed. The 4bit subblock is not balanced and m ust be invert ed. The result ing second code- group is 011100 0100. Except ions t o t his rule exist for som e subblocks wit h equal num bers of 0s and 1s. Because of t he bunching of 1s and 0s, 6- bit subblocks 111000 and 000111, and 4- bit sub- blocks 1100 and 0011 are viewed as unbalanced and becom e invert ed t o one anot her. Ta ble A.6 . Sa m ple 8 / B1 0 B M a ppin gs Hex Va lu e

Bin a r y Byt e

1 0 - bit Code - Gr oup Use d W h e n Cu r r e n t RD I s -

1 0 - bit Code - Gr oup Use d W h e n Cu r r e n t RD I s +

Ta ble A.6 . Sa m ple 8 / B1 0 B M a ppin gs Hex Va lu e

Bin a r y Byt e

1 0 - bit Code - Gr oup Use d W h e n Cu r r e n t RD I s -

1 0 - bit Code - Gr oup Use d W h e n Cu r r e n t RD I s +

00

00000000

100111 0100

011000 1011

01

00000001

011101 0100

100010 1011

02

00000010

101101 0100

010010 1011

03

00000011

110001 1011

110001 0100

04

00000100

110101 0100

001010 1011

05

00000101

101001 1011

101001 0100

06

00000110

011001 1011

011001 0100

07

00000111

111000 1011

000111 0100

08

00001000

111001 0100

000110 1011

00

00000000

100111 0100

011000 1011

01

00000001

011101 0100

100010 1011

02

00000010

101101 0100

010010 1011

03

00000011

110001 1011

110001 0100

04

00000100

110101 0100

001010 1011

05

00000101

101001 1011

101001 0100

06

00000110

011001 1011

011001 0100

07

00000111

111000 1011

000111 0100

08

00001000

111001 0100

000110 1011

09

00001001

100101 1011

100101 0100

0A

00001010

010101 1011

010101 0100

0B

00001011

110100 1011

110100 0100

0C

00001100

001101 1011

001101 0100

0D

00001101

101100 1011

101100 0100

0E

00001110

011100 1011

011100 0100

0F

00001111

010111 0100

101000 1011

Ru le s for Ch oosin g a n 8 B/ 1 0 B Code - Gr ou p

The current code- group always is rat ed as eit her negat ive or posit ive. More form ally, it is said t o have negat ive running disparit y ( RD–) or a posit ive running disparit y ( RD+ ) . Aft er t he current running disparit y has been calculat ed, t he code- group for t he next byt e is chosen from t he RD– or RD+ colum n based on t he result . At powerup, a t ransm it t er's running disparit y is set t o negat ive ( –) . Thus, t he first code- group t hat is t ransm it t ed is chosen from t he RD- colum n. The running disparit y of t he current code- group is com put ed in a t wo- st ep process. The running disparit y is com put ed at t he end of t he first subblock, and t hen again at t he end of t he second subblock. This final value is t he one assigned t o t he codegroup. The rules follow. Running disparit y at t he end of a 6- bit sub- block is posit ive if one of t he following is t rue: • • •

There are m ore 1s t han 0s. The subblock is 000111. The subblock is not 000111, but it does have an equal num ber of 0s and 1s, and t he previous code- group had posit ive running disparit y.

Running disparit y at t he end of a 6- bit sub- block is negat ive if one of t he following is t rue: • • •

There are m ore 0s t han 1s. The subblock is 111000. The subblock is not 111000, but it does have an equal num ber of 0s and 1s, and t he previous code- group had negat ive running disparit y.

Running disparit y at t he end of a 6- bit sub- block is negat ive if one of t he following is t rue: • • •

There are m ore 1s t han 0s. The subblock is 0011. The subblock is not 0011, but it does have an equal num ber of 0s and 1s, and t he prior 6- bit sub- block had posit ive running disparit y.

Running disparit y at t he end of a 4- bit sub- block is negat ive if one of t he following is t rue: • • •

There are m ore 0s t han 1s. The subblock is 1100. The subblock is not 1100, but it does have an equal num ber of 0s and 1s, and t he previous 6- bit sub- block had negat ive running disparit y.

8B/10B Data Code-Group Naming Convention Each 8B/ 10B dat a code- group has been assigned a nam e t hat is derived ( in a rat her peculiar m anner) from t he binary represent at ion of t he original byt e.

First , a byt e is split int o a 3- bit part and a 5- bit part . The nam e of t he byt e's dat a code- group is: D( decim al value of 5- bit part ) .( decim al value of 3- bit part ) For exam ple, t o get t he nam e of t he code- group for X'24, writ e t he binary for X'24 in t wo bat ches as: 001 00100 The code- group nam e is D4.1. Table A.7 cont ains m ore exam ples.

Ta ble A.7 . Ex a m ple s of 8 B/ 1 0 B D a t a Code - Gr ou p N a m e s D a t a Code - Gr ou p N a m e

H e x Va lu e

Bin a r y

D0.0

00

000 00000

D1.0

01

000 00001

D2.0

02

000 00010

D3.0

03

000 00011

D4.0

04

000 00100

D11.0

0B

000 01011

D12.0

0C

000 01100

D13.0

0D

000 01101

D14.0

0E

000 01110

D0.1

20

001 00000

D1.1

21

001 00001

D2.1

22

001 00010

D3.1

23

001 00011

D4.1

24

001 00100

D10.1

2A

001 01010

D11.1

2B

001 01011

D12.1

2C

001 01100

D13.1

2D

001 01101

D14.1

2E

001 01110

Special 8B/10B Code-Groups Many 10- bit pat t erns do not represent dat a byt es. Som e of t hese are used for special, nondat a code- groups. Table A.8 displays t hese code- groups; t heir nam es are int roduced by a K inst ead of a D.

Ta ble A.8 . Ta ble A.8 Spe cia l 8 B/ 1 0 B Code - gr ou ps Spe cia l Code Gr ou p N a m e

Hex Va lu e

Bina r y

1 0 - bit Code - Gr oup Use d W h e n Cu r r e n t RD I s –

1 0 - bit Code - Gr oup Use d W h e n Cu r r e n t RD I s +

K28.0

1C

000 11100

001111 0100

110000 1011

K28.1

3C

001 11100

001111 1001

110000 0110

K28.2

5C

010 11100

001111 0101

110000 1010

K28.3

7C

011 11100

001111 0011

110000 1100

K28.4

9C

100 11100

001111 0010

110000 1101

K28.5

BC

101 11100

001111 1010

110000 0101

K28.6

DC

110 11100

001111 0110

110000 1001

K28.7

FC

111 11100

001111 1000

110000 0111

K23.7

F7

111 10111

111010 1000

000101 0111

K27.7

FB

111 11011

110110 1000

001001 0111

K29.7

FD

111 11101

101110 1000

010001 0111

K30.7

FE

111 11110

011110 1000

100001 0111

Ordered Sets of 8B/10B Code-Groups The special code- groups are used t o form code sequences called ordered set s. Table A.9 displays t he layout s of t he ordered set s. Each is int roduced by a special codegroup and consist s of one, t wo, or four code- groups. •

• •

The / S/ and / T/ codes st art and t erm inat e a fram e. The / R/ code represent s ext ension byt es for half- duplex Gigabit Et hernet . The / V/ code signals a collision or ot her error. I dles are sent bet ween fram es. / I 1/ is sent if t he current running disparit y is posit ive. I t convert s t he running disparit y t o negat ive. When t he RD is negat ive, a st ream of I 2s is sent .

At init ializat ion, t he part ners send each ot her an alt ernat ing series of C1 and C2 ordered set s. These carry Aut o- Negot iat ion param et ers wit hin 16- bit unit s. ( See Chapt er 11, " Aut o- Negot iat ion," for inform at ion about Aut o- Negot iat ion.) The K28.5 code- group t hat int roduces idle sequences and configurat ion ordered set s has t his form : ( RD+ ) 0011111010 or ( RD- ) 1100000101 These code- groups st art wit h a special 7- bit st ring t hat is called a com m a. The com m a is defined as eit her of t he following: 0011111 or 1100000 This st ring is found only in K28.5, K28.1, and K28.7. Because of t he way t hat dat a code- groups have been designed, neit her 7- bit com m a st ring will ever occur as part of a 10B dat a st ream . Also, K28.1 and K28.7 current ly are unused. Hence, t he only t im e a com m a ever will appear is in a K28.5 code- group. This is very useful. When a receiver locat es a com m a pat t ern, it has found t he beginning of a K28.5 Aut o- Negot iat ion or idle code- group. I t now knows exact ly where each of t he succeeding code- groups begins. A receiver uses t he com m a t o lock ont o code- group boundaries during Aut o- Negot iat ion and t o resynchronize t he boundaries when idles are sent .

Ta ble A.9 . Ta ble A.9 Or de r e d Se t s of 8 B/ 1 0 B Code - Gr ou ps Nam e

N um be r of Code Gr ou ps

Pu r pose

En codin g

/ C/

Aut o- Negot iat ion

Alt ernat ing / C1/ and / C2/

/ C1/

Configurat ion 1

4

/ K28.5/ D21.5/ 16- bit - config

/ C2/

Configurat ion 2

4

/ K28.5/ D2.2/ 16- bit - config

/I/

I DLE

/ I 1/

I DLE 1

2

/ K28.5/ D5.6/

/ I 2/

I DLE 2

2

/ K28.5/ D16.2/

/ R/

Carrier Ext end

1

/ K23.7/

/ S/

St art of Packet

1

/ K27.7/

/ T/

End of Packet

1

/ K29.7/

/ V/

Error Propagat ion

1

/ K30.7/

Correct ing / I 1/ , preserving / I 2/

Fr a m e En ca psu la t ion

1000BASE-T The design of 1000BASE- T borrowed several feat ures from 100BASE- T2. • •

• •

A 1000BASE- T link physically operat es in a full- duplex m anner. Aft er an aut oconfigurat ion negot iat ion t hat est ablishes t he propert ies of t he link, st eady st ream s of sym bols flow across each wire pair in bot h direct ions. I dle sym bols are sent bet ween dat a t ransm issions. The st art and end of each fram e are m arked by special delim it ers.

1000BASE- T operat es bidirect ionally across four t wist ed pairs. A com plex encoding and lot s of ext ra elect ronic com ponent s have been int roduced t o accom plish fullduplex gigabit t ransm ission. Crosst alk can becom e a serious problem when signals are cont inuously being sent across a bundle cont aining four cables. One m echanism t hat helps is t o scram ble dat a before it is sent . Scram bling produces bit st ream s t hat do not generat e very high- frequency signals on any of t he four cables. A high frequency on one cable would produce elect rom agnet ic radiat ion t hat would affect t he ot her cables. Aft er scram bling, each byt e is t ranslat ed t o four sym bols. One sym bol is t ransm it t ed across each of t he four t wist ed pairs. Figure A.10 illust rat es t his procedure. The box at t he t op of Figure A.10 represent s t he conversion of an 8- bit byt e t o four sym bols labeled ( An, Bn, Cn, and Dn) . The " n" represent s sym bol t im e period num ber n. The lower part of t he figure shows one sym bol being t ransm it t ed across each of t he pairs at t he sam e t im e. When t he four sym bols ( An, Bn, Cn, and Dn) are received, t hey are m apped t o 8 bit s and are unscram bled.

Figu r e A.1 0 . Tr a n sm it t in g t h e sym bols t h a t m a k e u p a byt e .

The " BI _D" prefix in t he four labels at t he left ( BI _DA, BI _DB, BI _DC, and BI _DD) st ands for " bi- direct ional dat a."

Outline of 1000BASE-T Encoding Aft er scram bling, byt es are encoded using a m et hod called 8B1Q4. The t it le indicat es t hat each 8- bit pat t ern is t ranslat ed t o a code- group m ade up of four sym bols. Each sym bol can be one of t he following: [ - 2, - 1, 0, + 1, + 2] For exam ple, a code- group com binat ion such as ( 0, - 1, + 1, 0) or ( - 2, - 1, + 1, 0) can be used t o represent a dat a byt e. Each of t he sym bols [ - 2, - 1, 0, + 1, + 2] is represent ed on t he t wist ed- pair m edium as a different volt age level.

N ot e The five volt age levels are - 1V, - 5V, 0V, .5V, and 1V.

The physical t ransm ission m et hod is called 4- dim ensional 5- level pulse am plit ude m odulat ion ( 4D- PAM5) . The 4D part refers t o t he fact t hat four sym bols are sent at t he sam e t im e. The PAM5 part m eans t hat t here are five volt age levels.

Wit h five choices for each sym bol, 625 different four- sym bol code- group com binat ions can be produced. This provides a lot of flexibilit y in t he way t hat t he 256 8- bit dat a byt es can be represent ed. Lot s of ext ra code- groups exist , and som e of t hese are used t o represent idles, st art of- st ream delim it ers ( SSDs) at t he beginning of a fram e, end- of- st ream delim it ers ( ESDs) aft er a fram e, error indicat ions, and ( for short fram es and half- duplex t ransm ission) carrier ext ension byt es. Figure A.11 illust rat es t he form at of a st ream of codes t hat is sent across t he four pairs. I dle sym bols are sent bet ween fram es. A pair of special SSD codes replaces t he first t wo pream ble byt es. The rem ainder of t he pream ble and t he MAC fram e are t ranslat ed int o dat a codes. I f t here is no fram e ext ension, t wo CSReset code- groups follow. Two ESD code- groups t erm inat e t he fram e t ransm ission.

N ot e I f half- duplex t ransm ission was support ed and a fram e included an ext ension, t he CSReset code- groups would not be present . A series of CSExt end code- groups would represent t he fram e ext ension byt es. These would be followed by t wo ESD codegroups.

The t ranslat ion of a dat a byt e int o ( An, Bn, Cn, Dn) sym bols requires several st eps: • • •

Perform a calculat ion whose input s are a dat a byt e and a set of scram bling bit s t o produce a 9- bit word. The m et hod t hat is used is com plex, and t he 9bit word depends not only on t he value of t he source byt e, but also on where t hat byt e appears in t he dat a st ream . Perform a t able lookup t hat m aps t he 9- bit word t o a four- sym bolcode- group. Apply a form ula t hat adj ust s t he sign of som e of t he sym bols t o balance out posit ive ( + ) and negat ive ( - ) t ransm issions on each cable.

The operat ions on t he dat a m ust be reversed at t he receiving end.

Figu r e A.1 1 . 1 0 0 0 BASE- T t r a n sm ission

N ot e The code- groups t hat are used have been chosen carefully so t hat t hey are well separat ed from each ot her. Because of t he spread bet ween code- groups and t he encoding rules t hat are used, a receiver act ually is able t o correct som e invalid codegroups on arrival and replacing t hem wit h t he original valid ones. This is called forward error correct ion.

Table A.10 displays a few of t he m appings of 9- bit quant it ies t o four- sym bol groups. However, t he values shown are not t he final sym bols t hat will be t ransm it t ed; t hey are t he sym bols j ust before t he final sign correct ion is applied. The full set of m appings from 9- bit quant it ies t o four- sym bol groups can be found in Tables 40- 1 and 40- 2 of I EEE St andard 802.3ad. Table A.11 cont ains code- groups t hat are used for special funct ions. A fram e is int roduced by SSD1 SSD2. I t is t erm inat ed by ESD1, followed by one of t he ESD2 codes. Several pat t erns represent idles, CSReset ESD2, CSExt end, and a t ransm it error. The choice depends on t he current st at e of t he t ranslat ion algorit hm .

Ta ble A.1 0 . Sa m ple 1 0 0 0 BASE- T En codings Scr a m ble d 9 - bit W or d

An ,

Bn,

Cn ,

D n Pr ior t o Fina l Sign Adj u st m e n t

000000000

0,

0,

0,

0

000000001

- 2,

0,

0,

0

000000010

0,

- 2,

0,

0

Ta ble A.1 0 . Sa m ple 1 0 0 0 BASE- T En codings Scr a m ble d 9 - bit W or d

An ,

Bn,

Cn ,

D n Pr ior t o Fina l Sign Adj u st m e n t

000000011

- 2,

- 2,

0,

0

010000000

0,

0,

+ 1,

+1

010000001

- 2,

0,

+ 1,

+1

010000010

0,

- 2,

+ 1,

+1

010000011

- 2,

- 2,

+ 1,

+1

Ta ble A.1 1 . Spe cia l Code - Gr ou ps Pr ior t o Fina l Sign Adj u st m e nt Spe cia l Fun ct ion

Code - Gr oups

SSD1

( + 2, + 2, + 2, + 2)

SSD2

( + 2, + 2, + 2, - 2)

I dle

Any pat t ern of 0s and - 2s. For exam ple: ( 0, 0, 0, 0) ( - 2, 0, 0, 0) ( 0, - 2, 0, 0) ( - 2, 0, - 2, 0) ( - 2, 0, - 2, - 2) ( - 2, - 2, - 2, - 2)

CSReset ( Even)

( + 2, - 2, - 2, + 2) ( + 2, + 2, - 1, - 1) ( - 1, + 2, + 2, - 1) ( - 1, + 2, - 1, + 2)

CSReset ( Odd)

( + 2, - 2, + 2, - 1) ( + 2, - 2, - 1, + 2) ( - 1, - 2, + 2, + 2) ( + 2, - 1, - 2, + 2)

ESD1

( + 2, + 2, + 2, + 2)

ESD2

( + 2, + 2, + 2, - 2) ( + 2, + 2, - 2, + 2) ( + 2, - 2, + 2, + 2) ( - 2, + 2, + 2, + 2)

CSExt end ( Even)

( + 2, 0, 0, + 2) ( + 2, + 2, + 1, + 1) ( + 1, + 2, + 2, + 1) ( + 1, + 2, + 1, + 2)

CSExt end ( Odd)

( + 2, 0, + 2, + 1) ( + 2, 0, + 1, + 2) ( + 1, 0, + 2, + 2) ( + 2, + 1, 0, + 2)

CSExt end Error ( Even)

( - 2, + 2, + 2, - 2) ( - 1, - 1, + 2, + 2) ( + 2, - 1, - 1, + 2) ( + 2, - 1, + 2, - 1)

CSExt end Error ( Odd)

( + 2, + 2, - 2, - 1) ( - 2, + 2, - 1, + 2) ( - 1, + 2, + 2, - 2) ( + 2, - 1, + 2, - 2)

Transm it Error

( + 2, + 2, 0, + 1) ( 0, + 2, + 1, + 2) ( + 1, + 2, + 2, 0) ( + 2, + 1, + 2, 0)

This exposit ion has only skim m ed t he surface of t he 1000BASE- T encoding process, which is ext rem ely com plex. A com plet e descript ion would const it ut e a Mast er's t hesis in engineering!

Token Ring Different ial Manchest er encoding is used for classical 4Mbps and 16Mbps Token Rings. I t is described in t he subsect ion t hat follows. The encoding used for 100Mbps Token Ring is ident ical t o t hat used for 100BASE- X Et hernet . This helped Token Ring vendors roll out product s quickly and kept cost s down t hrough reuse of exist ing com ponent s.

Differential Manchester Encoding Different ial Manchest er encoding is a variat ion on t he Manchest er encoding used for 10Mbps Et hernet LANs. I t is used for 4Mbps and 16Mbps Token Ring LANs. Bit s are represent ed using t ransit ions bet ween high and low volt ages for t wist ed- pair cabling, or high and low light levels for opt ical fiber. • •

For a 0, t here are t ransit ions at bot h t he beginning and t he m iddle of t he bit . For a 1, t here is a t ransit ion only at t he m iddle of t he bit .

What 's " different " about Different ial Manchest er encoding is t hat t he coding used for a sym bol depends on t he previous sym bol. Figure A.12 shows t he t wo ways t hat t he st ring 1 0 1 1 0 0 0 1 can be encoded. For t he upper encoding, t he previous sym bol ended at a high level. For t he lower encoding, t he previous sym bol ended at a low level. Not e t he single t ransit ion locat ed in t he m iddle of each 1- bit ( eit her high- t o- low or low- t o- high) and t he t wo t ransit ions for each 0- bit ( one at t he st art and one in t he m iddle) . Two addit ional sym bols labeled J and K are used in Token Ring t ransm issions. The J and K sym bols appear only wit hin t he st art ing delim it er at t he beginning of a fram e and t he ending delim it er at t he end of a fram e. Figure A.13 displays t he st art ing and ending delim it er byt es, and shows how t he J and K sym bols are encoded wit hin t hem . • •

The J sym bol has no t ransit ions. The K sym bol has one t ransit ion at t he beginning of t he bit .

As was t he case for t he 0s and 1s, t he coding of a J or K depends on t he previous sym bol.

Figur e A.1 2 . D e lim it e r s cont a in in g J a n d K sym bols.

Figur e A.1 3 . D e lim it e r s cont a in in g J a n d K sym bols.

The st art ing delim it er pat t ern enables a receiver t o lock ont o byt e and bit boundaries, synchronize t im ing, and receive t he rest of t he fram e accurat ely. As Figure A.13 shows, t he value of t he last 2 bit s can vary. I n fact , each of t hese bit s report s som e im port ant inform at ion. They are called t he int erm ediat e fram e bit and t he error- det ect ed bit .

100Mbps Dedicated Token Ring 100Mbps High Speed Token Ring ( HSTR) High Speed Token Ring ( HSTR can operat e in a full- duplex swit ched environm ent . The dat a encoding and bit t ransm ission m et hods were borrowed from CDDI and FDDI , j ust as was t he case for 100BASE- X Et hernet . That is, each 4- bit nibble is t ranslat ed t o a 5- bit code- group. NRZI t ransm ission t hen is used for opt ical fiber and MLT- 3 for copper. The shielded t wist ed- pair wire t radit ionally used for Token Ring LANs is support ed in addit ion t o unshielded t wist ed- pair and fiber opt ic m edia.

Summary Points •

• •

• • • •

•

•

•

Manchest er encoding is used for 10BASE5, 10BASE2, 10BASE- T, FOI RL, 10BASE- FL, and 10BASE- FB. For Manchest er encoding, t here is a volt age ( or opt ical power) t ransit ion in t he m iddle of each bit . At high speeds, t his would produce a high- frequency signal. For 100BASE- FX Et hernet and FDDI , each nibble is t ranslat ed t o a 5- bit codegroup t hat has a sufficient num ber of 1- bit s. Non ret urn t o zero invert ed ( NRZI ) t ransm ission is used. This m eans t hat 1- bit s are t ransm it t ed by alt ernat ing bet ween high and low opt ical power levels, but no power change occurs for a 0- bit . For 100BASE- TX Et hernet and CDDI , each nibble is t ranslat ed t o a 5- bit codegroup t hat cont ains at least t wo 1- bit s. Each 1- bit is encoded as a st ep bet ween t hree volt age levels. No volt age change occurs for a 0- bit . For 100BASE- T4 Et hernet , each byt e is m apped t o a code- group m ade up of six sym bols chosen from [ - , 0, + ] . Byt es are t ransm it t ed in a st aggered order across t hree t wist ed pairs. For 1000BASE- SX, 1000BASE- LX, and 1000BASE- CX Et hernet and fibre channel, each byt e is t ranslat ed t o a 10- bit code- group t hat has a balanced dist ribut ion of 1s and 0s. A 1- bit is t ransm it t ed as a higher opt ical power or higher volt age. A 0- bit is t ransm it t ed as a low opt ical power or low volt age. For 1000BASE- T Et hernet , each byt e is t ranslat ed t o four sym bols chosen from [ - 2, - 1, 0, + 1, + 2] . Different ial Manchest er encoding is used for 4Mbps or 16Mbps Token Ring. For a 0, t here are t ransit ions at bot h t he beginning and t he m iddle of t he bit . For a 1, t here is a t ransit ion only at t he m iddle of t he bit . 100Mbps Token Ring on t wist ed- pair cabling or opt ical fiber uses t he sam e t ransm ission t echniques as 100BASE- TX and 100BASE- FX. Many of t he Et hernet t echnologies support full- duplex Et hernet . These include 10BASE- T, 10BASE- FL, 100- BASE- FX, 100BASE- TX, 100BASE- T2 ( not im plem ent ed) , 1000BASE- SX, 1000BASE- LX, 1000BASE- CX, and 1000BASE- T.

References Most of t he encodings described in t his appendix are defined in: I EEE St andard 802.3, 1998 Edit ion, " Carrier Sense Mult iple Access wit h Collision Det ect ion ( CSMA/ CD) Access Met hod and Physical Layer Specificat ions." The chapt ers in t he st andard t hat describe specific encodings are t he following:

• • • • • • •

Chapt er 9 describes FOI RL. Chapt er 14 describes 10BASE- T. Chapt ers 15, 17, and 18 describe 10BASE- F. Chapt er 24 Chapt er 25 Chapt er 26 describe 100BASE- X. Chapt ers 36, Chapt er 38, and Chapt er 39 describe 1000BASE- X. The 4B/ 5B t ranslat ions are found in Table 24- 1 of 802.3. The 8B/ 6T t ranslat ions are in Annex 23A of 802.3.

The encoding for 1000BASE- T is described in: I EEE St andard 802.3ab. " Physical Layer Param et ers and Specificat ions for 1000Mbps Operat ion Over 4- Pair of Cat egory 5 Balanced Copper Cabling, Type 1000BASE- T," 1999. This docum ent cont ains Chapt er 40 of t he 802.3 st andard. The 9- bit t o 4- sym bol m appings are found in Tables 40- 1 and 40- 2. FDDI is described in t he following: • • •

ANSI X3.139. " I nform at ion Syst em s Fiber Dist ribut ed Dat a I nt erface ( FDDI ) Token Ring Media Access Cont rol ( MAC) ," Original in 1987; last revision in 1997. ANSI X3.148. " I nform at ion Syst em s Fiber Dist ribut ed Dat a I nt erface ( FDDI ) Token Ring Physical Layer Prot ocol ( PHY) ," Original in 1988; last revision in 1994. ANSI X3.166. "Fiber Dist ribut ed Dat a I nt erface ( FDDI ) Physical Layer Medium Dependent ( PMD) ," Original in 1989; last revision in 1995.

The Copper Dist ribut ed Dat a I nt erface ( CDDI ) is described in: ANSI X3.263. " Fibre Dist ribut ed Dat a I nt erface ( FDDI ) Token Ring Twist ed Pair Physical Layer Medium Dependent ( TP- PMD) ," 1995. Several useful docum ent s can be found at ht t p: / / www.iol.unh.edu/ t raining/ index.ht m l For exam ple: •

•

Frain, John. " 1000BASE- X Physical Coding Sublayer ( PCS) and Physical Medium At t achm ent ( PMA) Tut orial," 1998. Nosewort hy, Bob. " 1000BASE- T PCS Funct ional Basics and Overview," 1998.

Different ial Manchest er encoding is described in Chapt er 5 of: ANSI / I EEE St andard 802.5. " Token Ring Access Met hod and Physical Layer Specificat ions," 1998.

Appendix B. Tables This appendix cont ains a m iscellaneous set of t ables t hat augm ent t he t ables in Appendix A, " Physical- Layer Encodings for Et hernet , Fibre Channel, FDDI , and Token Ring." They include

• • •

Four- bit binary " nibbles" and t heir decim al and hexadecim al equivalent s The full set of 8B/ 6T byt e t ranslat ions used for 100BASE- T4 dat a t ransm ission The full set of 8B/ 10B byt e t ranslat ions used for 1000BASE- X and fibre channel dat a t ransm ission Fibre channel ordered set s

•

Binary, Decimal, and Hexadecimal Characters The norm al m at hem at ical order for a st ring of binary bit s is t o place t he m ost significant bit on t he far- left end of t he st ring. ( This som et im es is big- endian order) [ 1903] > called big- endian order.) A st ring of 8 binary bit s is convert ed t o a pair of hexadecim al charact ers by breaking t he byt e int o t wo 4- bit groups ( called nibbles) and convert ing each t o a hexadecim al sym bol in t he range 0- F. For exam ple: Binary 1 1 0 0 = 8( 1) + 4( 1) + 2( 0) + 1( 0) = 12 = X'C Binary 0 0 1 0 = 8( 0) + 4( 0) + 2( 1) + 1( 0) = 2 = X'2 Hence: Binary 1 1 0 0 0 0 1 0 = X'C2 Table B.1 displays t he m apping bet ween set s of 4 binary bit s, t heir decim al value, and t heir hexadecim al represent at ion.

Ta ble B.1 . Bin a r y, D e cim a l, a nd H e x a de cim a l Re pr e se n t a t ion s Bin a r y

D e cim a l

H e x a de cim a l

0 0 0 0

0

0

0 0 0 1

1

1

0 0 1 0

2

2

0 0 1 1

3

3

0 1 0 0

4

4

0 1 0 1

5

5

0 1 1 0

6

6

0 1 1 1

7

7

1 0 0 0

8

8

1 0 0 1

9

9

1 0 1 0

10

A

Ta ble B.1 . Bin a r y, D e cim a l, a nd H e x a de cim a l Re pr e se n t a t ion s Bin a r y

D e cim a l

H e x a de cim a l

1 0 1 1

11

B

1 1 0 0

12

C

1 1 0 1

13

D

1 1 1 0

14

E

1 1 1 1

15

F

8B/6T Tables Table B.2 present s t he full set of 8B/ 6T t ranslat ions bet ween byt es and t ernary sym bols used for 100BASE- T4 dat a t ransm ission.

Ta ble B.2 . 1 0 0 BASE- T4 8 B/ 6 T D a t a Byt e Tr a n sla t ion Hex

6 T Code Gr ou p

Hex

6 T Code Gr ou p

Hex

6 T Code Gr ou p

Hex

6 T Code Gr ou p

00

+ - 0 0 + -

20

0 0 - + + -

40

+ 0 + 0 0 -

60

0 - 0 + + 0

01

0 + - + - 0

21

- - + 0 0 +

41

+ + 0 0 - 0

61

0 0 - + 0 +

02

+ - 0 + - 0

22

+ + - 0 + -

42

+ 0 + 0 - 0

62

0 - 0 + 0 +

03

- 0 + + - 0

23

+ + - 0 - +

43

0 + + 0 - 0

63

- 0 0 + 0 +

04

- 0 + 0 + -

24

0 0 + 0 - +

44

0 + + 0 0 -

64

- 0 0 + + 0

05

0 + - - 0 +

25

0 0 + 0 + -

45

+ + 0 - 0 0

65

0 0 - 0 + +

06

+ - 0 - 0 +

26

0 0 - 0 0 +

46

+ 0 + - 0 0

66

0 - 0 0 + +

07

- 0 + - 0 +

27

- - + + + -

47

0 + + - 0 0

67

- 0 0 0 + +

08

- + 0 0 + -

28

- 0 - + + 0

48

0 0 0 + 0 0

68

- + - + + 0

09

0 - + + - 0

29

- - 0 + 0 +

49

0 0 0 - + +

69

- - + + 0 +

0A

- + 0 + - 0

2A

- 0 - + 0 +

4A

0 0 0 + - +

6A

- + - + 0 +

0B

+ 0 - + - 0

2B

0 - - + 0 +

4B

0 0 0 + + -

6B

+ - - + 0 +

0C

+ 0 - 0 + -

2C

0 - - + + 0

4C

0 0 0 - + 0

6C

+ - - + + 0

0D

0 - + - 0 +

2D

- - 0 0 + +

4D

0 0 0 - 0 +

6D

- - + 0 + +

0E

- + 0 - 0 +

2E

- 0 - 0 + +

4E

0 0 0 + - 0

6E

- + - 0 + +

Ta ble B.2 . 1 0 0 BASE- T4 8 B/ 6 T D a t a Byt e Tr a n sla t ion Hex

6 T Code Gr ou p

Hex

6 T Code Gr ou p

Hex

6 T Code Gr ou p

Hex

6 T Code Gr ou p

0F

+ 0 - - 0 +

2F

0 - - 0 + +

4F

0 0 0 + 0 -

6F

+ - - 0 + +

10

+ 0 + - - 0

30

+ - 0 0 - +

50

+ 0 + - - +

70

- + + 0 0 0

11

+ + 0 - 0 -

31

0 + - - + 0

51

+ + 0 - + -

71

+ - + 0 0 0

12

+ 0 + - 0 -

32

+ - 0 - + 0

52

+ 0 + - + -

72

+ + - 0 0 0

13

0 + + - 0 -

33

- 0 + - + 0

53

0 + + - + -

73

0 0 + 0 0 0

14

0 + + - - 0

34

- 0 + 0 - +

54

0 + + - - +

74

- 0 + 0 0 0

15

+ + 0 0 - -

35

0 + - + 0 -

55

+ + 0 + - -

75

0 - + 0 0 0

16

+ 0 + 0 - -

36

+ - 0 + 0 -

56

+ 0 + + - -

76

+ 0 - 0 0 0

17

0 + + 0 - -

37

- 0 + + 0 -

57

0 + + + - -

77

0 + - 0 0 0

18

0 + - 0 + -

38

- + 0 0 - +

58

+ + + 0 - -

78

0 - - + + +

19

0 + - 0 - +

39

0 - + - + 0

59

+ + + - 0 -

79

0 - + + +

1A

0 + - + + -

3A

- + 0 - + 0

5A

+ + + - - 0

7A

- 0 + + +

1B

0 + - 0 0 +

3B

+ 0 - - + 0

5B

+ + 0 - - 0

7B

- - 0 + + 0

1C

0 - + 0 0 +

3C

+ 0 - 0 - +

5C

+ + 0 - - +

7C

+ + - 0 0 -

1D

0 - + + + -

3D

0 - + + 0 -

5D

+ + 0 0 0 -

7D

0 0 + 0 0 -

1E

0 - + 0 - +

3E

- + 0 + 0 -

5E

- - + + + 0

7E

+ + - - - +

1F

0 - + 0 + -

3F

+ 0 - + 0 -

5F

0 0 - + + 0

7F

0 0 + - - +

80

+ - + 0 0 -

A0

0 - 0 + + -

C0

+ - + 0 + -

E0

+ - 0 + + -

81

+ + - 0 - 0

A1

0 0 - + - +

C1

+ + - + - 0

E1

0 + - + - +

82

+ - + 0 - 0

A2

0 - 0 + - +

C2

+ - + + - 0

E2

+ - 0 + - +

83

- + + 0 - 0

A3

- 0 0 + - +

C3

- + + + - 0

E3

- 0 + + - +

84

- + + 0 0 -

A4

- 0 0 + + -

C4

- + + 0 + -

E4

- 0 + + + -

85

+ + - - 0 0

A5

0 0 - - + +

C5

+ + - - 0 +

E5

0 + - - + +

86

+ - + - 0 0

A6

0 - 0 - + +

C6

+ - + - 0 +

E6

+ - 0 - + +

87

- + + - 0 0

A7

- 0 0 - + +

C7

- + + - 0 +

E7

- 0 + - + +

88

0 + 0 0 0 -

A8

- + - + + -

C8

0 + 0 0 + -

E8

- + 0 + + -

89

0 0 + 0 - 0

A9

- - + + - +

C9

0 0 + + - 0

E9

0 - + + - +

Ta ble B.2 . 1 0 0 BASE- T4 8 B/ 6 T D a t a Byt e Tr a n sla t ion Hex

6 T Code Gr ou p

Hex

6 T Code Gr ou p

Hex

6 T Code Gr ou p

Hex

6 T Code Gr ou p

8A

0 + 0 0 - 0

AA

- + - + - +

CA

EA

0 + 0 + - 0

- + 0 + - +

8B

+ 0 0 0 - 0

AB

+ - - + - +

CB

+ 0 0 + - 0

EB

+ 0 - + - +

8C

+ 0 0 0 0 -

AC

+ - - + + -

CC

+ 0 0 0 + -

EC

+ 0 - + + -

8D

0 0 + - 0 0

AD

- - + - + +

CD

0 0 + - 0 +

ED

0 - + - + +

8E

0 + 0 - 0 0

AE

- + - - + +

CE

0 + 0 - 0 +

EE

- + 0 - + +

8F

+ 0 0 - 0 0

AF

+ - - - + +

CF

+ 0 0 - 0 +

EF

+ 0 - - + +

90

+ - + - - +

B0

0 - 0 0 0 +

D0

+ - + 0 - +

F0

+ - 0 0 0 +

91

+ + - - + -

B1

0 0 - 0 + 0

D1

+ + - - + 0

F1

0 + - 0 + 0

92

+ - + - + -

B2

0 - 0 0 + 0

D2

+ - + - + 0

F2

+ - 0 0 +

93

- + + - + -

B3

- 0 0 0 + 0

D3

- + + - + 0

F3

- 0 + 0 + 0

94

- + + - - +

B4

- 0 0 0 0 +

D4

- + + 0 - +

F4

- 0 + 0 0 +

95

+ + - + - -

B5

0 0 - + 0 0

D5

+ + - + 0 -

F5

0 + - + 0 0

96

+ - + + - -

B6

0 - 0 + 0 0

D6

+ - + + 0 -

F6

+ - 0 + 0 0

97

- + + + - -

B7

- 0 0 + 0 0

D7

- + + + 0 -

F7

- 0 + + 0 0

98

0 + 0 - - +

B8

- + - 0 0 +

D8

0 + 0 0 - +

F8

- + 0 0 0 +

99

0 0 + - + -

B9

- - + 0 + 0

D9

0 0 + - + 0

F9

0 - + 0 + 0

9A

0 + 0 - + -

BA

- + - 0 + 0

DA

0 + 0 - + 0

FA

- + 0 0 + 0

9B

+ 0 0 - + -

BB

+ - - 0 + 0

DB

+ 0 0 - + 0

FB

+ 0 - 0 + 0

9C

+ 0 0 - - +

BC

+ - - 0 0 +

DC

+ 0 0 0 - +

FC

+ 0 - 0 0 +

9D

0 0 + + - -

BD

- - + + 0 0

DD

0 0 + + 0 -

FD

0 - + + 0 0

9E

0 + 0 + - -

BE

- + - + 0 0

DE

0 + 0 + 0 -

FE

- + 0 + 0 0

9F

+ 0 0 + - -

BF

+ - - + 0 0

DF

+ 0 0 + 0 -

FF

+ 0 - + 0 0

8B/10B Translation Table Table B.3 displays t he 8B/ 10B ( 8- bit t o 10- bit ) t ranslat ions bet ween dat a byt es and 10- bit code- groups t hat are used for 1000BASE- SX, 1000BASE- LX, 1000BASE- CX, and fibre channel t ransm ission.

Table B.3 uses convent ions t hat were used in t he 802.3 st andards: •

•

•

The binary form of each 8- bit dat a byt e is writ t en wit h t he m ost significant bit at t he left . Each binary 10- bit code- group is writ t en wit h t he least significant bit at t he left . Each 10- bit code- group is writ t en as a 6- bit subblock followed by a 4- bit subblock.

The nam e assigned t o a dat a code- group is based on t he t wo part s of t he binary expression and is D( decim al value of 5- bit part ) .( decim al value of 3- bit part ) There are t wo ways t o t ranslat e each byt e, depending on whet her t he current running disparit y ( RD) is negat ive or posit ive. Appendix A explains how t he running disparit y is calculat ed.

Ta ble B.3 . 8 B/ 1 0 B Code - Gr ou ps D a t a Code Gr ou p N a m e

Hex Va lu e

Bin a r y

Code - Gr ou p Use d W h e n Cu r r e n t RD I s-

Code - Gr oup Use d W h e n Cu r r e n t RD I s+

D0.0

00

000 00000

100111 0100

011000 1011

D1.0

01

000 00001

011101 0100

100010 1011

D2.0

02

000 00010

101101 0100

010010 1011

D3.0

03

000 00011

110001 1011

110001 0100

D4.0

04

000 00100

110101 0100

001010 1011

D5.0

05

000 00101

101001 1011

101001 0100

D6.0

06

000 00110

011001 1011

011001 0100

D7.0

07

000 00111

111000 1011

000111 0100

D8.0

08

000 01000

111001 0100

000110 1011

D9.0

09

000 01001

100101 1011

100101 0100

Ta ble B.3 . 8 B/ 1 0 B Code - Gr ou ps D a t a Code Gr ou p N a m e

Hex Va lu e

Bin a r y

Code - Gr ou p Use d W h e n Cu r r e n t RD I s-

Code - Gr oup Use d W h e n Cu r r e n t RD I s+

D10.0

0A

000 01010

010101 1011

010101 0100

D11.0

0B

000 01011

110100 1011

110100 0100

D12.0

0C

000 01100

001101 1011

001101 0100

D13.0

0D

000 01101

101100 1011

101100 0100

D14.0

0E

000 01110

011100 1011

011100 0100

D15.0

0F

000 01111

010111 0100

101000 1011

D16.0

10

000 10000

011011 0100

100100 1011

D17.0

11

000 10001

100011 1011

100011 0100

D18.0

12

000 10010

010011 1011

010011 0100

D19.0

13

000 10011

110010 1011

110010 0100

D20.0

14

000 10100

001011 1011

001011 0100

D21.0

15

000 10101

101010 1011

101010 0100

D22.0

16

000 10110

011010 1011

011010 0100

D23.0

17

000 10111

111010 0100

000101 1011

D24.0

18

000 11000

110011 0100

001100 1011

D25.0

19

000 11001

100110 1011

100110 0100

D26.0

1A

000 11010

010110 1011

010110 0100

Ta ble B.3 . 8 B/ 1 0 B Code - Gr ou ps D a t a Code Gr ou p N a m e

Hex Va lu e

Bin a r y

Code - Gr ou p Use d W h e n Cu r r e n t RD I s-

Code - Gr oup Use d W h e n Cu r r e n t RD I s+

D27.0

1B

000 11011

110110 0100

001001 1011

D28.0

1C

000 11100

001110 1011

001110 0100

D29.0

1D

000 11101

101110 0100

010001 1011

D30.0

1E

000 11110

011110 0100

100001 1011

D31.0

1F

000 11111

101011 0100

010100 1011

D0.1

20

001 00000

100111 1001

011000 1001

D1.1

21

001 00001

011101 1001

100010 1001

D2.1

22

001 00010

101101 1001

010010 1001

D3.1

23

001 00011

110001 1001

110001 1001

D4.1

24

001 00100

110101 1001

001010 1001

D5.1

25

001 00101

101001 1001

101001 1001

D6.1

26

001 00110

011001 1001

011001 1001

D7.1

27

001 00111

111000 1001

000111 1001

D8.1

28

001 01000

111001 1001

000110 1001

D9.1

29

001 01001

100101 1001

100101 1001

D10.1

2A

001 01010

010101 1001

010101 1001

D11.1

2B

001 01011

110100 1001

110100 1001

Ta ble B.3 . 8 B/ 1 0 B Code - Gr ou ps D a t a Code Gr ou p N a m e

Hex Va lu e

Bin a r y

Code - Gr ou p Use d W h e n Cu r r e n t RD I s-

Code - Gr oup Use d W h e n Cu r r e n t RD I s+

D12.1

2C

001 01100

001101 1001

001101 1001

D13.1

2D

001 01101

101100 1001

101100 1001

D14.1

2E

001 01110

011100 1001

011100 1001

D15.1

2F

001 01111

010111 1001

101000 1001

D16.1

30

001 10000

011011 1001

100100 1001

D17.1

31

001 10001

100011 1001

100011 1001

D18.1

32

001 10010

010011 1001

010011 1001

D19.1

33

001 10011

110010 1001

110010 1001

D20.1

34

001 10100

001011 1001

001011 1001

D21.1

35

001 10101

101010 1001

101010 1001

D22.1

36

001 10110

011010 1001

011010 1001

D23.1

37

001 10111

111010 1001

000101 1001

D24.1

38

001 11000

110011 1001

001100 1001

D25.1

39

001 11001

100110 1001

100110 1001

D26.1

3A

001 11010

010110 1001

010110 1001

D27.1

3B

001 11011

110110 1001

001001 1001

D28.1

3C

001 11100

001110 1001

001110 1001

Ta ble B.3 . 8 B/ 1 0 B Code - Gr ou ps D a t a Code Gr ou p N a m e

Hex Va lu e

Bin a r y

Code - Gr ou p Use d W h e n Cu r r e n t RD I s-

Code - Gr oup Use d W h e n Cu r r e n t RD I s+

D29.1

3D

001 11101

101110 1001

010001 1001

D30.1

3E

001 11110

011110 1001

100001 1001

D31.1

3F

001 11111

101011 1001

010100 1001

D0.2

40

010 00000

100111 0101

011000 0101

D1.2

41

010 00001

011101 0101

100010 0101

D2.2

42

010 00010

101101 0101

010010 0101

D3.2

43

010 00011

110001 0101

110001 0101

D4.2

44

010 00100

110101 0101

001010 0101

D5.2

45

010 00101

101001 0101

101001 0101

D6.2

46

010 00110

011001 0101

011001 0101

D7.2

47

010 00111

111000 0101

000111 0101

D8.2

48

010 01000

111001 0101

000110 0101

D9.2

49

010 01001

100101 0101

100101 0101

D10.2

4A

010 01010

010101 0101

010101 0101

D11.2

4B

010 01011

110100 0101

110100 0101

D12.2

4C

010 01100

001101 0101

001101 0101

D13.2

4D

010 01101

101100 0101

101100 0101

Ta ble B.3 . 8 B/ 1 0 B Code - Gr ou ps D a t a Code Gr ou p N a m e

Hex Va lu e

Bin a r y

Code - Gr ou p Use d W h e n Cu r r e n t RD I s-

Code - Gr oup Use d W h e n Cu r r e n t RD I s+

D14.2

4E

010 01110

011100 0101

011100 0101

D15.2

4F

010 01111

010111 0101

101000 0101

D16.2

50

010 10000

011011 0101

100100 0101

D17.2

51

010 10001

100011 0101

100011 0101

D18.2

52

010 10010

010011 0101

010011 0101

D19.2

53

010 10011

110010 0101

110010 0101

D20.2

54

010 10100

001011 0101

001011 0101

D21.2

55

010 10101

101010 0101

101010 0101

D22.2

56

010 10110

011010 0101

011010 0101

D23.2

57

010 10111

111010 0101

000101 0101

D24.2

58

010 11000

110011 0101

001100 0101

D25.2

59

010 11001

100110 0101

100110 0101

D26.2

5A

010 11010

010110 0101

010110 0101

D27.2

5B

010 11011

110110 0101

001001 0101

D28.2

5C

010 11100

001110 0101

001110 0101

D29.2

5D

010 11101

101110 0101

010001 0101

D30.2

5E

010 11110

011110 0101

100001 0101

Ta ble B.3 . 8 B/ 1 0 B Code - Gr ou ps D a t a Code Gr ou p N a m e

Hex Va lu e

Bin a r y

Code - Gr ou p Use d W h e n Cu r r e n t RD I s-

Code - Gr oup Use d W h e n Cu r r e n t RD I s+

D31.2

5F

010 11111

101011 0101

010100 0101

D0.3

60

011 00000

100111 0011

011000 1100

D1.3

61

011 00001

011101 0011

100010 1100

D2.3

62

011 00010

101101 0011

010010 1100

D3.3

63

011 00011

110001 1100

110001 0011

D4.3

64

011 00100

110101 0011

001010 1100

D5.3

65

011 00101

101001 1100

101001 0011

D6.3

66

011 00110

011001 1100

011001 0011

D7.3

67

011 00111

111000 1100

000111 0011

D8.3

68

011 01000

111001 0011

000110 1100

D9.3

69

011 01001

100101 1100

100101 0011

D10.3

6A

011 01010

010101 1100

010101 0011

D11.3

6B

011 01011

110100 1100

110100 0011

D12.3

6C

011 01100

001101 1100

001101 0011

D13.3

6D

011 01101

101100 1100

101100 0011

D14.3

6E

011 01110

011100 1100

011100 0011

D15.3

6F

011 01111

010111 0011

101000 1100

Ta ble B.3 . 8 B/ 1 0 B Code - Gr ou ps D a t a Code Gr ou p N a m e

Hex Va lu e

Bin a r y

Code - Gr ou p Use d W h e n Cu r r e n t RD I s-

Code - Gr oup Use d W h e n Cu r r e n t RD I s+

D16.3

70

011 10000

011011 0011

100100 1100

D17.3

71

011 10001

100011 1100

100011 0011

D18.3

72

011 10010

010011 1100

010011 0011

D19.3

73

011 10011

110010 1100

110010 0011

D20.3

74

011 10100

001011 1100

001011 0011

D21.3

75

011 10101

101010 1100

101010 0011

D22.3

76

011 10110

011010 1100

011010 0011

D23.3

77

011 10111

111010 0011

000101 1100

D24.3

78

011 11000

110011 0011

001100 1100

D25.3

79

011 11001

100110 1100

100110 0011

D26.3

7A

011 11010

010110 1100

010110 0011

D27.3

7B

011 11011

110110 0011

001001 1100

D28.3

7C

011 11100

001110 1100

001110 0011

D29.3

7D

011 11101

101110 0011

010001 1100

D30.3

7E

011 11110

011110 0011

100001 1100

D31.3

7F

011 11111

101011 0011

010100 1100

D0.4

80

100 00000

100111 0010

011000 1101

Ta ble B.3 . 8 B/ 1 0 B Code - Gr ou ps D a t a Code Gr ou p N a m e

Hex Va lu e

Bin a r y

Code - Gr ou p Use d W h e n Cu r r e n t RD I s-

Code - Gr oup Use d W h e n Cu r r e n t RD I s+

D1.4

81

100 00001

011101 0010

100010 1101

D2.4

82

100 00010

101101 0010

010010 1101

D3.4

83

100 00011

110001 1101

110001 0010

D4.4

84

100 00100

110101 0010

001010 1101

D5.4

85

100 00101

101001 1101

101001 0010

D6.4

86

100 00110

011001 1101

011001 0010

D7.4

87

100 00111

111000 1101

000111 0010

D8.4

88

100 01000

111001 0010

000110 1101

D9.4

89

100 01001

100101 1101

100101 0010

D10.4

8A

100 01010

010101 1101

010101 0010

D11.4

8B

100 01011

110100 1101

110100 0010

D12.4

8C

100 01100

001101 1101

001101 0010

D13.4

8D

100 01101

101100 1101

101100 0010

D14.4

8E

100 01110

011100 1101

011100 0010

D15.4

8F

100 01111

010111 0010

101000 1101

D16.4

90

100 10000

011011 0010

100100 1101

D17.4

91

100 10001

100011 1101

100011 0010

Ta ble B.3 . 8 B/ 1 0 B Code - Gr ou ps D a t a Code Gr ou p N a m e

Hex Va lu e

Bin a r y

Code - Gr ou p Use d W h e n Cu r r e n t RD I s-

Code - Gr oup Use d W h e n Cu r r e n t RD I s+

D18.4

92

100 10010

010011 1101

010011 0010

D19.4

93

100 10011

110010 1101

110010 0010

D20.4

94

100 10100

001011 1101

001011 0010

D21.4

95

100 10101

101010 1101

101010 0010

D22.4

96

100 10110

011010 1101

011010 0010

D23.4

97

100 10111

111010 0010

000101 1101

D24.4

98

100 11000

110011 0010

001100 1101

D25.4

99

100 11001

100110 1101

100110 0010

D26.4

9A

100 11010

010110 1101

010110 0010

D27.4

9B

100 11011

110110 0010

001001 1101

D28.4

9C

100 11100

001110 1101

001110 0010

D29.4

9D

100 11101

101110 0010

010001 1101

D30.4

9E

100 11110

011110 0010

100001 1101

D31.4

9F

100 11111

101011 0010

010100 1101

D0.5

A0

101 00000

100111 1010

011000 1010

D1.5

A1

101 00001

011101 1010

100010 1010

D2.5

A2

101 00010

101101 1010

010010 1010

Ta ble B.3 . 8 B/ 1 0 B Code - Gr ou ps D a t a Code Gr ou p N a m e

Hex Va lu e

Bin a r y

Code - Gr ou p Use d W h e n Cu r r e n t RD I s-

Code - Gr oup Use d W h e n Cu r r e n t RD I s+

D3.5

A3

101 00011

110001 1010

110001 1010

D4.5

A4

101 00100

110101 1010

001010 1010

D5.5

A5

101 00101

101001 1010

101001 1010

D6.5

A6

101 00110

011001 1010

011001 1010

D7.5

A7

101 00111

111000 1010

000111 1010

D8.5

A8

101 01000

111001 1010

000110 1010

D9.5

A9

101 01001

100101 1010

100101 1010

D10.5

AA

101 01010

010101 1010

010101 1010

D11.5

AB

101 01011

110100 1010

110100 1010

D12.5

AC

101 01100

001101 1010

001101 1010

D13.5

AD

101 01101

101100 1010

101100 1010

D14.5

AE

101 01110

011100 1010

011100 1010

D15.5

AF

101 01111

010111 1010

101000 1010

D16.5

B0

101 10000

011011 1010

100100 1010

D17.5

B1

101 10001

100011 1010

100011 1010

D18.5

B2

101 10010

010011 1010

010011 1010

D19.5

B3

101 10011

110010 1010

110010 1010

Ta ble B.3 . 8 B/ 1 0 B Code - Gr ou ps D a t a Code Gr ou p N a m e

Hex Va lu e

Bin a r y

Code - Gr ou p Use d W h e n Cu r r e n t RD I s-

Code - Gr oup Use d W h e n Cu r r e n t RD I s+

D20.5

B4

101 10100

001011 1010

001011 1010

D21.5

B5

101 10101

101010 1010

101010 1010

D22.5

B6

101 10110

011010 1010

011010 1010

D23.5

B7

101 10111

111010 1010

000101 1010

D24.5

B8

101 11000

110011 1010

001100 1010

D25.5

B9

101 11001

100110 1010

100110 1010

D26.5

BA

101 11010

010110 1010

010110 1010

D27.5

BB

101 11011

110110 1010

001001 1010

D28.5

BC

101 11100

001110 1010

001110 1010

D29.5

BD

101 11101

101110 1010

010001 1010

D30.5

BE

101 11110

011110 1010

100001 1010

D31.5

BF

101 11111

101011 1010

010100 1010

D0.6

C0

110 00000

100111 0110

011000 0110

D1.6

C1

110 00001

011101 0110

100010 0110

D2.6

C2

110 00010

101101 0110

010010 0110

D3.6

C3

110 00011

110001 0110

110001 0110

D4.6

C4

110 00100

110101 0110

001010 0110

Ta ble B.3 . 8 B/ 1 0 B Code - Gr ou ps D a t a Code Gr ou p N a m e

Hex Va lu e

Bin a r y

Code - Gr ou p Use d W h e n Cu r r e n t RD I s-

Code - Gr oup Use d W h e n Cu r r e n t RD I s+

D5.6

C5

110 00101

101001 0110

101001 0110

D6.6

C6

110 00110

011001 0110

011001 0110

D7.6

C7

110 00111

111000 0110

000111 0110

D8.6

C8

110 01000

111001 0110

000110 0110

D9.6

C9

110 01001

100101 0110

100101 0110

D10.6

CA

110 01010

010101 0110

010101 0110

D11.6

CB

110 01011

110100 0110

110100 0110

D12.6

CC

110 01100

001101 0110

001101 0110

D13.6

CD

110 01101

101100 0110

101100 0110

D14.6

CE

110 01110

011100 0110

011100 0110

D15.6

CF

110 01111

010111 0110

101000 0110

D16.6

D0

110 10000

011011 0110

100100 0110

D17.6

D1

110 10001

100011 0110

100011 0110

D18.6

D2

110 10010

010011 0110

010011 0110

D19.6

D3

110 10011

110010 0110

110010 0110

D20.6

D4

110 10100

001011 0110

001011 0110

D21.6

D5

110 10101

101010 0110

101010 0110

Ta ble B.3 . 8 B/ 1 0 B Code - Gr ou ps D a t a Code Gr ou p N a m e

Hex Va lu e

Bin a r y

Code - Gr ou p Use d W h e n Cu r r e n t RD I s-

Code - Gr oup Use d W h e n Cu r r e n t RD I s+

D22.6

D6

110 10110

011010 0110

011010 0110

D23.6

D7

110 10111

111010 0110

000101 0110

D24.6

D8

110 11000

110011 0110

001100 0110

D25.6

D9

110 11001

100110 0110

100110 0110

D26.6

DA

110 11010

010110 0110

010110 0110

D27.6

DB

110 11011

110110 0110

001001 0110

D28.6

DC

110 11100

001110 0110

001110 0110

D29.6

DD

110 11101

101110 0110

010001 0110

D30.6

DE

110 11110

011110 0110

100001 0110

D31.6

DF

110 11111

101011 0110

010100 0110

D0.7

E0

111 00000

100111 0001

011000 1110

D1.7

E1

111 00001

011101 0001

100010 1110

D2.7

E2

111 00010

101101 0001

010010 1110

D3.7

E3

111 00011

110001 1110

110001 0001

D4.7

E4

111 00100

110101 0001

001010 1110

D5.7

E5

111 00101

101001 1110

101001 0001

D6.7

E6

111 00110

011001 1110

011001 0001

Ta ble B.3 . 8 B/ 1 0 B Code - Gr ou ps D a t a Code Gr ou p N a m e

Hex Va lu e

Bin a r y

Code - Gr ou p Use d W h e n Cu r r e n t RD I s-

Code - Gr oup Use d W h e n Cu r r e n t RD I s+

D7.7

E7

111 00111

111000 1110

000111 0001

D8.7

E8

111 01000

111001 0001

000110 1110

D9.7

E9

111 01001

100101 1110

100101 0001

D10.7

EA

111 01010

010101 1110

010101 0001

D11.7

EB

111 01011

110100 1110

110100 1000

D12.7

EC

111 01100

001101 1110

001101 0001

D13.7

ED

111 01101

101100 1110

101100 1000

D14.7

EE

111 01110

011100 1110

011100 1000

D15.7

EF

111 01111

010111 0001

101000 1110

D16.7

F0

111 10000

011011 0001

100100 1110

D17.7

F1

111 10001

100011 0111

100011 0001

D18.7

F2

111 10010

010011 0111

010011 0001

D19.7

F3

111 10011

110010 1110

110010 0001

D20.7

F4

111 10100

001011 0111

001011 0001

D21.7

F5

111 10101

101010 1110

101010 0001

D22.7

F6

111 10110

011010 1110

011010 0001

D23.7

F7

111 10111

111010 0001

000101 1110

Ta ble B.3 . 8 B/ 1 0 B Code - Gr ou ps D a t a Code Gr ou p N a m e

Hex Va lu e

Bin a r y

Code - Gr ou p Use d W h e n Cu r r e n t RD I s-

Code - Gr oup Use d W h e n Cu r r e n t RD I s+

D24.7

F8

111 11000

110011 0001

001100 1110

D25.7

F9

111 11001

100110 1110

100110 0001

D26.7

FA

111 11010

010110 1110

010110 0001

D27.7

FB

111 11011

110110 0001

001001 1110

D28.7

FC

111 11100

001110 1110

001110 0001

D29.7

FD

111 11101

101110 0001

010001 1110

D30.7

FE

111 11110

011110 0001

100001 1110

D31.7

FF

111 11111

101011 0001

010100 1110

Table B.4 list s ordered set s t hat are used for fibre channel fram e delim it ers. SOF st ands for st art of fram e, and EOF st ands for end of fram e. A st art of fram e always follows a series of idles, and t he running disparit y aft er an idle always is negat ive. When an end of fram e occurs, t he running disparit y could be negat ive or posit ive. Different ordered set s are used depending on t he running disparit y.

Ta ble B.4 . Or de r e d Se t s Use d for Fibr e Ch a n ne l D e lim it e r s Pu r pose

Ru n n in g D ispa r it y

Or de r e d Se t

SOF Connect Class 1

-

K28.5 D21.5 D23.0 D23.0

SOF I nit iat e Class 1

-

K28.5 D21.5 D23.2 D23.2

SOF Norm al Class 1

-

K28.5 D21.5 D23.1 D23.1

SOF I nit iat e Class 2

-

K28.5 D21.5 D21.2 D21.2

SOF Norm al Class 2

-

K28.5 D21.5 D21.1 D21.1

SOF I nit iat e Class 3

-

K28.5 D21.5 D22.2 D22.2

SOF Norm al Class 3

-

K28.5 D21.5 D22.1 D22.1

Ta ble B.4 . Or de r e d Se t s Use d for Fibr e Ch a n ne l D e lim it e r s Pu r pose

Ru n n in g D ispa r it y

Or de r e d Se t

SOF Act ivat e Class 4

-

K28.5 D21.5 D25.0 D25.0

SOF Norm al Class 4

-

K28.5 D21.5 D25.1 D25.1

SOF I nit iat e Class 4

-

K28.5 D21.5 D25.2 D25.2

SOF Fabric

-

K28.5 D21.5 D24.2 D24.2

EOF Term inat e

-

K28.5 D21.4 D21.3 D21.3

+

K28.5 D21.5 D21.3 D21.3

EOF Disconnect - Term inat e

-

K28.5 D21.4 D21.4 D21.4

Class 1 or Class 4

+

K28.5 D21.5 D21.4 D21.4

EOF Abort

-

K28.5 D21.4 D21.7 D21.7

+

K28.5 D21.5 D21.7 D21.7

-

K28.5 D21.4 D21.6 D21.6

+

K28.5 D21.5 D21.6 D21.6

EOF Disconnect - Term inat e-

-

K28.5 D10.4 D21.4 D21.4

I nvalid Class 1 or Class 4

+

K28.5 D10.5 D21.4 D21.4

EOF Norm al- I nvalid

-

K28.5 D10.4 D21.6 D21.6

+

K28.5 D10.5 D21.6 D21.6

EOF Rem ove Term inat e

-

K28.5 D21.4 D25.4 D25.4

Class 4

+

K28.5 D21.5 D25.4 D25.4

EOF Rem ove Term inat e

-

K28.5 D10.4 D25.4 D25.4

I nvalid Class 4

+

K28.5 D10.5 D25.4 D25.4

EOF Norm al

Table B.5 list s fibre channel ordered set s t hat are called prim it ive signals.

Ta ble B.5 . Or de r e d Se t s for Fibr e Cha n ne l Pr im it ive Signa ls Pu r pose

Ru n n in g D ispa r it y

Or de r e d Se t

I dle

-

K28.5 D21.4 D21.5 D21.5

Receiver Ready

-

K28.5 D21.4 D10.2 D10.2

Virt ual Circuit Ready Class 4

-

K28.5 D21.7 VC_I D VC_I D

Ta ble B.5 . Or de r e d Se t s for Fibr e Cha n ne l Pr im it ive Signa ls Pu r pose

Ru n n in g D ispa r it y

Or de r e d Se t

* VC_I D= Virt ual Circuit I dent ifier Table B.6 list s fibre channel ordered set s t hat are called prim it ive sequences.

Ta ble B.6 . Or de r e d Se t s for Fibr e Cha n ne l Pr im it ive Signa ls Pu r pose

Ru n n in g D ispa r it y

Or de r e d Se t

Offline

-

K28.5 D21.1 D10.4 D21.2

Not Operat ional

-

K28.5 D21.2 D31.5 D5.2

Link Reset

-

K28.5 D9.2 D31.5 D9.2

Link Reset Response

-

K28.5 D21.1 D31.5 D9.2

References Table B.2 is based on Table 23A- 1, t he 100BASE- T4 8B6T code t able, which is in Annex 23A of I EEE St andard 802.3, 1998 Edit ion. Table B.3 is based on Table 36- 1, t he Valid Dat a Code- groups t able, in Chapt er 36 of t he sam e st andard. Table B.4 is based on Table 24, Fram e Delim it ers, in t he 1994 ANSI specificat ion, " Fibre Channel Physical and Signalling I nt erface ( FC- PH) ," as updat ed by version 2 ( FC- PH- 2) in 1996. Table 24 is locat ed in Chapt er 11, Sect ion 4. Table B.5 is based on Table 25, Prim it ive Signals, and Table B.6 is based on Table 26, Prim it ive Sequences, in t he sam e fibre channel st andards docum ent s ( FC- PH and FC- PH- 2) .

Appendix C. Standards Bodies and References Adherence t o st andards is very im port ant in t he local area net work arena. I t is rare t o find a workplace com put er t hat is not at t ached t o a net work, and users t ake it for grant ed t hat com put ers, net worked peripherals, hubs, swit ches, and rout ers will int erwork wit hout a hit ch. Vendors support st andards while ret aining t heir right t o innovat e. Successful innovat ions usually find t heir way int o t he st andards at a brisk pace. I n several areas, st andards are driven by vendor associat ions. Som e of t hese vendor groups are sources of free t echnology inform at ion t hat can be accessed via t he I nt ernet .

Formal Standards Bodies The form al LAN st andards bodies are present ed in t he sect ions t hat follow. Then t he vendor groups are described.

IEEE The I nst it ut e for Elect rical and Elect ronics Engineers ( I EEE) 802 com m it t ee is responsible for m ost of t he st andards t hat deal wit h local area net works. The I EEE Web sit e is ht t p: / / www.ieee.org/ I EEE LAN st andards docum ent s are not free. They can be ordered online at t he I EEE Web sit e. For a cat alog list ing, see ht t p: / / st andards.ieee.org/ cat alog/ cont ent s.ht m l. Prepaid subscript ions t hat enable a user t o download docum ent s from t he Web sit e are available. A few references can be downloaded from t he I EEE Web sit e free of cost . A list of assigned organizat ionally unique ident ifiers ( OUI s) is available at t his URL: ht t p: / / st andards.ieee.org/ regaut h/ oui/ oui.t xt Ot her useful m at erial can be reached from t his URL: ht t p: / / st andards.ieee.org/ regaut h/ oui/ t ut orials/

ANSI The Am erican Nat ional St andards I nst it ut e ( ANSI ) is responsible for publishing FDDI and fibre channel st andards. The ANSI Web sit e is ht t p: / / www.ansi.org/ ANSI docum ent s are not free. They can be purchased at t he Web sit e. ANSI st andards usually are republished as I SO or I TU- T st andards.

IETF The I nt ernet Engineering Task Force ( I ETF) is responsible for creat ing and im plem ent ing st andards for t he I nt ernet . Much of it s work is relat ed t o updat ing or enlarging t he TCP/ I P prot ocol suit e. The I ETF has published m any st andards relat ing to • •

Encapsulat ing I P t raffic in LAN fram es SNMP Managem ent I nform at ion Bases ( MI Bs) for LAN t echnologies

Prelim inary and final I ETF st andards are published in docum ent s called Request s for Com m ent s ( RFCs) . I ETF docum ent s are free and can be obt ained from t his Web sit e: ht t p: / / www.iet f.org/

RFC docum ent s can be locat ed easily from t his search page: ht t p: / / www.rfcedit or.org/ rfcsearch.ht m l

TIA/EIA The Elect ronic I ndust ries Alliance ( EI A) is t he parent organizat ion for a num ber of st andards groups, including t he Telecom m unicat ions I ndust ry Associat ion ( TI A) . Several im port ant TI A/ EI A st andards relat e t o prem ise cabling. The TI A is a nat ional t rade organizat ion and accredit ed ANSI st andards organizat ion whose m em bers provide com m unicat ions and inform at ion t echnology product s and services. I nform at ion is available at t he organizat ion's Web sit e: ht t p: / / www.t iaonline.org/ You can search for docum ent s at t he TI A Web sit e. However, t he docum ent s act ually are purchased from Global Engineering Docum ent s: ht t p: / / global.ihs.com /

ITU-T The I nt ernat ional Telecom m unicat ions Union—Telecom m unicat ion St andardizat ion Sect or ( I TU- T) is responsible for t he basic net working st andards relat ing t o ATM. The I TU's World Wide Web sit e is ht t p: / / www.it u.int / I TU- T docum ent s are not free. They can be purchased at t he Web sit e.

ISO The I nt ernat ional Organizat ion for St andardizat ion ( I SO) was creat ed t o prom ot e int ernat ional t rade and cooperat ive advances in science. The I nt ernat ional Organizat ion for St andardizat ion Web sit e is ht t p: / / www.iso.ch/ I SO docum ent s are not free. They can be purchased t hrough m em ber count ry organizat ions.

Vendor Groups Specific vendor groups of int erest include t he following: • •

The Gigabit Et hernet Alliance was form ed t o prom ot e indust ry cooperat ion in t he developm ent of Gigabit Et hernet . Several whit e papers are available at t heir Web sit e, ht t p: / / www.gigabit - et hernet .org/ St andards m ust be obt ained from I EEE. The High Speed Token Ring Alliance was form ed by leading Token Ring vendors. Their Web sit e provides som e t echnology whit e papers: ht t p: / / www.hst ra.com / whit epapers.ht m l St andards m ust be obt ained from t he I EEE.

•

The ATM Forum was form ed t o prom ot e t he use of ATM. The Forum originat ed m any ATM st andards, including t he ATM st andards relat ing t o LAN Em ulat ion. Free t echnical specificat ions are available at t his sit e: ht t p: / / www.at m forum .com /

•

The Nat ional Com m it t ee for I nform at ion Technology St andards ( NCI TS) was form ed t o produce m arket - driven, volunt ary consensus st andards for st orage devices, m ult im edia, program m ing languages, and securit y. I t s T11 com m it t ee is responsible for fibre channel. The lat est revisions of fibre channel draft docum ent s are available at t his sit e: ht t p: / / www.t 11.org/

Many vendor consort ium s have been form ed t o prom ot e int eroperabilit y t est ing for each of t he m aj or LAN t echnologies. The Universit y of New Ham pshire's I nt erOperabilit y Lab ( I OL) publishes a list of consort ium s at ht t p: / / www.iol.unh.edu/ consort ium s/ The I OL perform s t est ing t hat is sponsored by t he consort ium s. The I OL also publishes excellent t ut orials. Frequent ly asked quest ions ( FAQ) docum ent s relat ing t o LAN t echnologies can be reached from t he following Web page: ht t p: / / www.faqs.org/ faqs/ by- newsgroup/ com p/

Appendix D. Acronym List AAL5 ATM Adapt at ion Layer 5 ABR Available Bit Rat e ( ATM) ACK ACKnowledge ACR at t enuat ion t o crosst alk rat io AL_ PA arbit rat ed loop physical address ( fibre channel) AM P Act ive Monit or Present ( Token Ring) AN SI Am erican Nat ional St andards I nst it ut e ARE all- rout es explorer fram e ( source rout ing) ARP Address Resolut ion Prot ocol ASI C applicat ion specific int egrat ed circuit

ASN .1 Abst ract Synt ax Not at ion One ( SNMP) ATM Asynchronous Transfer Mode AUI at t achm ent unit int erface ( Et hernet ) AW G Am erican Wire Gauge BER bit error rat e BN C Bayonet Neil- Concelm an BOOTP Boot st rap Prot ocol ( TCP/ I P) BPD U Bridge Prot ocol Dat a Unit BPS bit s per second BUS Broadcast and Unknown Server ( LANE) CATV Com m unit y Ant enna Television CAU Cont rolled Access Unit ( Token Ring) CBR Const ant Bit Rat e ( ATM) CD D I Copper Dist ribut ed Dat a I nt erface CEN ELEC Com it e Europeen de Norm alisat ion Elect rot echnique CFI Canonical Form at I ndicat or ( VLANs) CM I P Com m on Managem ent I nform at ion Prot ocol CM I S Com m on Managem ent I nform at ion Service CRC cyclic redundancy check CRS Configurat ion Report Server ( Token Ring) CSM A/ CD Carrier Sense Mult iple Access wit h Collision Det ect ion ( Et hernet ) D A dest inat ion address D AS dual at t achm ent st at ion ( FDDI ) dB decibel D CE dat a circuit - t erm inat ing equipm ent

D I X Digit al, I nt el, and Xerox D M D different ial m ode delay D RD dest inat ion rout e descript or ( Token Ring) D SAP dest inat ion service access point D TE dat a t erm inal equipm ent D TR Dedicat ed Token Ring EI A Elect ronic I ndust ries Alliance ELAN em ulat ed LAN ( ATM) ELFEXT equal level far end crosst alk EM I elect rom agnet ic int erference E- RI F em bedded rout ing inform at ion field ESD end- of- st ream delim it er FC fibre channel FCS fram e check sequence FD D I Fibre Dist ribut ed Dat a I nt erface FEXT far end crosst alk FI D filt ering ident ifier FI FO first in first out FLP fast link pulse ( Aut o- Negot iat ion) FM I fiber opt ic m edium int erface ( Token Ring) FOI RL fiber opt ic int er- repeat er link ( Et hernet ) FTP foil t wist ed pair GARP Generic At t ribut e Regist rat ion Prot ocol GbE Gigabit Et hernet GBI C gigabit int erface convert er

Gbps gigabit s ( billions of bit s) per second GI D GARP I nform at ion Declarat ion GI P GARP I nform at ion Propagat ion GM I I gigabit m edia independent int erface GM RP GARP Mult icast Regist rat ion Prot ocol GVRP GARP VLAN Regist rat ion Prot ocol H D LC High- level Dat a Link Cont rol H I PPI High Perform ance Parallel I nt erface H SSD C High Speed Serial Dat a Connect or H SSI High Speed Serial I nt erface H STR High Speed Token Ring H z hert z I EC I nt ernat ional Elect rot echnical Com m ission I EEE I nst it ut e of Elect rical and Elect ronic Engineers I ETF I nt ernet Engineering Task Force I FG int erfram e gap I GM P I nt ernet Group Managem ent Prot ocol I LM I I nt egrat ed Local Managem ent I nt erface ( ATM) I P I nt ernet Prot ocol I PI I nt elligent Peripheral I nt erface I PG int erpacket gap ( Et hernet ) I SD N I nt egrat ed Services Digit al Net work I SO I nt ernat ional Organizat ion for St andardizat ion I V init ializat ion vect or I VL independent VLAN learning

k m kilom et ers LACP Link Aggregat ion Cont rol Prot ocol LACPD U Link Aggregat ion Cont rol Prot ocol Dat a Unit LAG Link Aggregat ion Group LAN local area net work LAN E LAN Em ulat ion LAT local area t ransport LB- LAN locally bridged local area net work LEC LAN Em ulat ion Client LECI D LAN Em ulat ion Client I D LECS LAN Em ulat ion Configurat ion Server LED light em it t ing diode LES LAN Em ulat ion Server LFSR linear feedback shift regist er LLC Logical Link Cont rol LM E Layer Managem ent Ent it y LSAP link service access point M AC m edia access cont rol M AN m et ropolit an area net work M AU m edium at t achm ent unit ( Et hernet ) M AU Mult ist at ion Access Unit ( Token Ring) M bps m egabit s ( m illions of bit s) per second M Bps m egabyt es per second M D I m edium dependent int erface M D I - X m edium dependent int erface crossover

M H z m egahert z M I B Managem ent I nform at ion Base ( SNMP) M I C m edium int erface connect or M I I m edium independent int erface M LT- 3 Mult i- Level 3 encoding M M m ult im ode M M F m ult im ode fiber M PLS Mult iprot ocol Label Swit ching Archit ect ure M POA Mult iprot ocol Over ATM M UTO Mult iuser Telecom m unicat ions Out let M SAU Mult ist at ion Access Unit ( Token Ring) M SD U MAC service dat a unit N AA net work address aut horit y N BF Net BEUI Fram e Prot ocol N CFI noncanonical form at indicat or N CI TS Nat ional Com m it t ee for I nform at ion Technology St andards N D I S Net work Driver I nt erface Specificat ion N EC Nat ional Elect rical Code N e t BEUI Net BI OS Ext ended User I nt erface N e t BI OS Net work Basic I nput / Out put Syst em N EXT near end crosst alk N I C net work int erface card N LP norm al link pulse ( 10BASE- T Et hernet ) ) N OS net work operat ing syst em N P next page

n r t - VBR non- real- t im e variable bit rat e ( ATM) N RZI non- ret urn- t o- zero, invert - on- one N TT Nippon Telephone and Telegraph N VP nom inal velocit y of propagat ion ORD opt ical receive dat a OSI Open Syst em s I nt erconnect OSPF open short est pat h first OTD opt ical t ransm it dat a OTD R Opt ical Tim e Dom ain Reflect om et er OUI organizat ionally unique ident ifier PAM pulse am plit ude m odulat ion PCI Peripheral Com ponent I nt erconnect PCS Physical Coding Sublayer PD U prot ocol dat a unit PH Y physical layer PI CS Prot ocol I m plem ent at ion Conform ance St at em ent PLS physical layer signaling PM A physical m edium at t achm ent PM D Physical Medium Dependent PM I Physical Medium I ndependent POTS Plain Old Telephone Service PPP Point - t o- Point Prot ocol PSELFEXT power sum equal level far end crosst alk PSN EXT power sum near end crosst alk PVC perm anent virt ual circuit

PVI D Port VLAN I dent ifier QoS Qualit y of Service RB rem ot e bridge RD rout e descript or ( bridge) REM Ring Error Monit or ( Token Ring) RF rem ot e fault RFC Request for Com m ent s ( I ETF) RFI radio frequency int erference RI rout ing inform at ion ( bridge) RI F Rout ing I nform at ion Field ( bridge) RI I Rout ing- I nform at ion I ndicat or ( bridge) RI P Rout ing I nform at ion Prot ocol RM AC Repeat er Medium Access Cont rol ( 100VG AnyLAN) RM ON rem ot e m onit or ( SNMP) RPS Ring Param et er Server ( Token Ring) r t - VBR Real- t im e Variable Bit Rat e ( ATM) SA source address SAI D securit y associat ion ident ifier SAN st orage area net work SAP service access point SC square corner SCSI sm all com put er syst em int erface ScTP Screened Twist ed- Pair SD U service dat a unit SEL select or byt e ( ATM)

SFD st art fram e delim it er SFF sm all form fact or SI LS St andard for I nt eroperable LAN/ MAN Securit y SM Single- m ode SM A sub m iniat ure t ype A SM F single- m ode fiber SM I St ruct ure of Managem ent I nform at ion ( SNMP) SM I B Securit y Managem ent I nform at ion Base SM P St andby Monit or Present ( Token Ring) SM S Select ive Mult icast Server SM T st at ion m anagem ent ( FDDI ) SN A Syst em s Net work Archit ect ure SN AP Subnet work Access Prot ocol SN M P Sim ple Net work Managem ent Prot ocol SQE signal qualit y error SRB source rout ing bridge SRL st ruct ural ret urn loss SRT source- rout ing t ransparent ( bridge) SSAP source service access point SSD st art - of- st ream delim it er SSTP screened/ shielded t wist ed pair ST st raight t ip ( fiber opt ic connect or) STE Spanning Tree Explorer ( bridge) STP shielded t wist ed pair ( cabling) STP Spanning Tree Prot ocol ( bridges)

SVC swit ched virt ual circuit SVL shared VLAN learning TBPS t erabit s ( 1000 billion bit s) per second TCI t ag cont rol inform at ion TCP Transm ission Cont rol Prot ocol TCU Trunk Coupling Unit ( Token Ring) TD R Tim e Dom ain Reflect om et er TI A Telecom m unicat ions I ndust ry Associat ion TKP Token Passing Prot ocol TN C Threaded- Neil- Councilm an Coaxial Cable Connect or TPI D Tag Prot ocol I dent ifier TTRT Target Token Rot at ion Tim eout ( FDDI ) TXI Transm it I m m ediat e Prot ocol ( Token Ring) UBR Unspecified Bit Rat e ( ATM) UD P User Dat agram Prot ocol ( TCP/ I P) UP unform at t ed page ( Aut o- Negot iat ion) USOC Universal Service Order Code UTP unshielded t wist ed pair VCI Virt ual Channel I dent ifier ( ATM) VCSEL Vert ical Cavit y Surface Em it t ing Laser VI D VLAN ident ifier VLAN virt ual LAN VPI Virt ual Pat h I dent ifier ( ATM) W AN wide area net work

Glossary Num erals A B C D E F G H I –K L M N O P–Q R S T U V W–Z

N u m e r a ls 1 BASE5

Obsolet e 1Mbps version of Et hernet over t wist ed- pair cables, also called St arLAN. 4 B/ 5 B An encoding used for 100BASE- X Et hernet , FDDI , CDDI , and som e ATM physical layers. 5 - 4 - 3 r u le Rest rict ion on a coax Et hernet t hat st at es t hat a fram e can t raverse at m ost five segm ent s, go t hrough at m ost four repeat ers, and cross at m ost t hree segm ent s t hat cont ain st at ions. 8 B1 Q4 An encoding used for 1000BASE- T Et hernet . 8 B/ 6 T An encoding used for 100BASE- T4 Et hernet . 8 B/ 1 0 B An encoding used for 1000BASE- CX, 1000BASE- FX, 1000BASE- TX, and fibre channel. Each byt e is t ranslat ed t o a 10- bit pat t ern before it is t ransm it t ed. 1 0 BASE2 10Mbps Et hernet over t hin 50- ohm coaxial cable. 1 0 BASE5 10Mbps Et hernet over t hick 50- ohm coaxial cable. 1 0 BASE- F A generic t erm for 10BASE- FB, 10BASE- FL, and 10BASE- FP Et hernet over fiber opt ic cable. 1 0 BASE- FB Specificat ion for 10Mbps Et hernet repeat er- t o- repeat er fiber opt ic backbone links. 1 0 BASE- FL Specificat ion for a 10Mbps fiber opt ic link t hat can be used for st at ion- t ost at ion, repeat er- t o- repeat er, and repeat er- t o- st at ion links.

1 0 BASE- FP An unim plem ent ed specificat ion for a passive opt ical device designed t o int erconnect m ult iple 10Mbs Et hernet st at ions. 1 0 BASE- T 10Mbps Et hernet over t wo pairs of Cat egory 3 or bet t er unshielded t wist edpair cabling. 1 0 BROAD 3 6 A broadband coax im plem ent at ion of Et hernet t hat uses frequency m odulat ed t ransceivers. 1 0 0 BASE- FX 100Mbps Et hernet over fiber opt ic cable. 1 0 0 BASE- T General t erm for 100Mbps Et hernet over t wist ed- pair cable. 1 0 0 BASE- T2 Unim plem ent ed I EEE 802.3 specificat ion for 100Mbps Et hernet over t wo pairs of Cat egory 3 or bet t er cabling. 1 0 0 BASE- T4 100Mbps half- duplex Et hernet over four pairs of Cat egory 3, 4, or 5 unshielded t wist ed- pair ( UTP) wire. 1 0 0 BASE- TX 100Mbps Et hernet over t wo pairs of Cat egory 5 unshielded t wist ed- pair or shielded t wist ed- pair. 1 0 0 BASE- X General t erm for 100BASE- FX and 100BASE- TX. 1 0 0 VG- AnyLAN A t echnology t hat com pet ed wit h 100Mbps Et hernet and was published as a separat e st andard ( 802.12) . 1 0 0 0 BASE- CX Gigabit Et hernet over specialt y shielded copper cable assem blies.

1 0 0 0 BASE- LH Nonst andard long- haul im plem ent at ion of Gigabit Et hernet over single- m ode opt ical fiber. 1 0 0 0 BASE- LX Gigabit Et hernet using long- wavelengt h laser devices over m ult im ode and single- m ode fiber. 1 0 0 0 BASE- SX Gigabit Et hernet using short - wavelengt h laser devices over m ult im ode fiber. 1 0 0 0 BASE- T Gigabit Et hernet over Cat egory 5 unshielded t wist ed- pair copper cabling. 1 0 0 0 BASE- X General t erm for 1000BASE- CX, 1000BASE- LX, and 1000BASE- SX.

A a cce ss link A link t hat connect s VLAN- unaware syst em s t o a VLAN swit ch. a ct ive m on it or A Token Ring st at ion t hat supervises t he ring and recovers from problem s such as loss of t he t oken and repeat edly circulat ing fram es. a da pt e r Net work int erface card. Addr e ss Re solu t ion Pr ot ocol ( ARP) A procedure t hat is used t o m ap a higher- layer prot ocol address int o a lowlayer prot ocol address ( for exam ple, t o m ap an I P address t o a MAC address) . a ge n t See [ Un k now n snm pa ge n t ] a ll- r ou t e s e x plor e r ( ARE) A Token Ring fram e t hat records a rout e t o a dest inat ion by t raversing every loop- free pat h t hrough a bridged Token Ring LAN.

Am e r ica n W ir e Ga uge ( AW G) I nverse m easure of t he t hickness ( in inches) of a wire. For exam ple, t he diam et er of 24 AWG wire is 1/ 24 inch. a pplica t ion - la ye r sw it ch A rout ing swit ch t hat t akes applicat ion- layer dat a ( such as a World Wide Web URL) int o account when m aking forwarding decisions. a pplica t ion - spe cific in t e gr a t e d cir cu it ( ASI C) ch ip A processing chip t hat cont ains program logic im plem ent ed as hardware. a r bit r a t e d loop A set of fibre channel port s connect ed int o a ring. a sym m e t r ic flow cont r ol For full- duplex Et hernet , t he capabilit y of one st at ion t o send PAUSE fram es t o it s part ner, who is not allowed t o send PAUSE fram es. a syn ch r onou s ba ndw idt h For FDDI , an ext ra period of t im e t hat a st at ion can use t o t ransm it dat a if t he t oken ret urns t o t he st at ion early. Asynchr onous Tr a nsfe r M ode ( ATM ) A m et hod of t ransm it t ing inform at ion t hat is organized int o cells. Also, a virt ual circuit service based on ATM t echnology. ATM Ada pt a t ion La ye r 5 ( AAL5 ) An ATM dat a link layer. ATM Br oa dca st a n d Unk now n Se r ve r ( BUS) A server t hat delivers broadcast and m ult icast fram es t o st at ions in an em ulat ed LAN. ATM I n t e gr a t e d Loca l M a na ge m e n t I n t e r fa ce ( I LM I ) A prot ocol based on SNMP, used t o init ialize an ATM st at ion and m onit or t he link connect ing t he st at ion t o a swit ch. ATM LAN Em u la t ion Con figu r a t ion Se r ve r ( LECS)

A server t hat assigns a LAN Em ulat ion Client t o a specific em ulat ed LAN. ATM LAN Em u la t ion ( LAN E) A set of m echanism s t hat enable an ATM st at ion t o em ulat e a convent ional Et hernet or Token Ring LAN st at ion. ATM LAN Em u la t ion Clie n t ( LEC) A special device driver t hat hides t he under- lying ATM layers from t he higher prot ocol layers. ATM LAN Em u la t ion Se r ve r ( LES) A server t hat is in charge of one em ulat ed LAN and t hat keeps t rack of t he ATM addresses and MAC addresses of it s m em bers. ATM la ye r Upper part of t he physical layer, responsible for adding a 5- byt e cell header t o each 48- byt e payload and queuing cells in t he order in which t hey will be t ransm it t ed. a t t a ch m e nt u n it in t e r fa ce ( AUI ) For 10Mbps Et hernet , t he int erface bet ween com m unicat ions elect ronics in a st at ion and a t ransceiver. This can be im plem ent ed as a cable or as an int ernal int erface wit hin a NI C. a t t e nu a t ion The loss of signal st rengt h t hat occurs as inform at ion t raverses a m edium . At t enuat ion is expressed in decibels. a t t e nu a t ion t o cr osst a lk r a t io ( ACR) A m easurem ent of t he signal t o noise rat io at t he receive end of a pair, expressed in decibels. Au t o- N e got ia t ion A prot ocol t hat enables Et hernet syst em s at t he ends of a t wist ed- pair or opt ical fiber segm ent t o negot iat e configurat ion param et ers such as speed, half- or full- duplex m ode, and use of flow cont rol. Ava ila ble Bit Ra t e ( ABR) A best - effort ATM service t hat provides cont inuing feedback t hat t ells t he sender how m uch bandwidt h is available for t he sender's use.

B ba ck off For CSMA/ CD Et hernet , a period of t im e t hat a st at ion m ust wait before ret ransm it t ing aft er part icipat ing in a collision. The period is a random ly select ed m ult iple of t he slot t im e. ba la n ce d ca ble A m et allic cable designed wit h sym m et ric t ransm ission elem ent s so t hat t heir induced m agnet ic fields will cancel each ot her out . ba se pa ge The first 16- bit m essage exchanged during Et hernet Aut o- Negot iat ion. Also called t he base link code word. ba se ba n d Com m unicat ion via pulses serially im pressed on a m edium . Only one signal can be on t he m edium . Ba yon e t N e il- Con ce lm a n ( BN C) A t ype of connect or used for t hinnet Et hernet . Be a con fr a m e A fram e t ransm it t ed by a st at ion on a Token Ring or FDDI ring t hat has det ect ed a ring failure. bit e r r or r a t e ( BER) The num ber of bit s received in error divided by t he t ot al num ber of bit s received. br idge A LAN device t hat operat es at Layer 2 ( t he dat a link layer) . A bridge int erconnect s t wo or m ore LAN segm ent s and select ively forwards LAN fram es bet ween segm ent s. br idge a ddr e ss A MAC address used t o uniquely ident ify an ent ire bridge. br idge ide n t ifie r

A priorit y num ber followed by t he bridge address. br idge por t cost A cost value assigned by an adm inist rat or. br idge por t ide n t ifie r The com binat ion of an assigned port priorit y and t he port num ber. Br idge Pr ot ocol D a t a Un it ( BPD U) m e ssa ge s Spanning Tree Prot ocol m essages t hat enable bridges t o agree on an init ial t ree- shaped t opology and, aft er t he failure of som e com ponent , change t he t opology t o repair broken pat hs. br idge r oot pa t h cost A value com put ed by adding pat h cost num bers for receive port s along t he pat h from t he root t o t he bridge. br idge r oot por t The port on a bridge t hat connect s t o it s best pat h t o t he root . br idge / r ou t e r A device t hat rout es som e t raffic and bridges t he rest . A Layer 2/ 3 swit ch. br oa dca st The act of sending a fram e addressed t o all st at ions. br out e r See [ br idge / r out e r ] bu ffe r A unit of m em ory, used for t he t em porary st orage of dat a. bu ffe r e d dist r ibu t or A special repeat er used for Gigabit Et hernet . Each syst em is connect ed t o t he repeat er by a full- duplex link. Also called a full- duplex repeat er. bu s A net work t opology t hat connect s devices via a shared cable.

C ca n on ica l for m a t The form at of a dat a fram e in which t he byt es of any MAC addresses conveyed in t he user dat a field have t he sam e bit ordering as in t he hexadecim al represent at ion. Ca non ica l For m a t I ndica t or ( CFI ) fla g A flag in a t ag header t hat indicat es whet her t he fram e cont ains an em bedded rout ing inform at ion field ( E- RI F) . ca r r ie r e x t e n sion For half- duplex Gigabit Et hernet , t he addit ion of nondat a sym bols t o t he end of fram es t hat are less t han 512 byt es in lengt h. Ca r r ie r Se n se M u lt iple Acce ss w it h Collision D e t e ct ion ( CSM A/ CD ) The m edium access prot ocol used for classical half- duplex Et hernet . A st at ion wait s unt il t he m edium is free before t ransm it t ing. I f t wo st at ions t ransm it concurrent ly, a collision occurs, and each st at ion pauses before ret ransm it t ing. See also [ Unk now n ba ck off] ca t e gor y A classificat ion syst em used t o different iat e bet ween grades of t wist ed- pair cable. ch a ssis An enclosure t hat cont ains slot s for a set of net work cards. ce r t ifica t ion t ool A high- end t est er t hat can det erm ine whet her inst alled cables m eet TI A Cat egory 5, 5E, or proposed Cat egory 6 or 7 requirem ent s. ch a n n e l For a cable, includes all t he cabling syst em com ponent s bet ween a st at ion and a hub or swit ch in a wiring closet . cla ddin g A cylinder of glass t hat surrounds t he core of a fiber opt ic cable. Cla im Tok e n fr a m e

A Token Ring fram e used in a procedure t hat elect s an act ive m onit or st at ion. Cla ss 1 A fibre channel service class t hat provides dedicat ed point - t o- point connect ions wit h guarant eed bandwidt h. Cla ss 2 A fibre channel service class t hat provides acknowledged connect ionless com m unicat ion. Cla ss 3 A fibre channel service class t hat provides unacknowledged connect ionless service. Cla ss I r e pe a t e r A 100Mbps Et hernet repeat er t hat t ranslat es bet ween dat a encodings. This int roduces a delay int o t he forwarding process, and t he result is t hat norm ally only one Class I repeat er can be used in a collision dom ain. Cla ss I I r e pe a t e r A 100Mbps Et hernet repeat er t hat at t aches only t o st at ions t hat use com pat ible dat a encodings. At m ost t wo Class I I repeat ers can be used in a collision dom ain. cla ssic Tok e n Rin g con ce n t r a t or A device t hat connect s t o m ult iple st at ions and form s all or part of t he backbone of a Token Ring. cla ssic Tok e n Rin g st a t ion A st at ion t hat operat es in t oken- passing m ode. coa x ia l ca ble se ct ion A single lengt h of coaxial cable. coa x ia l ca ble se gm e n t A single cable sect ion, or t wo or m ore sect ions j oined by connect ors, t erm inat ed at each end. code - gr ou p

A set of sym bols t hat represent dat a or cont rol inform at ion. For exam ple, for 100BASE- X Et hernet , FDDI , CDDI , and som e ATM physical layers, every 4- bit nibble is t ranslat ed t o a 5- bit pat t ern before it is t ransm it t ed, and special 5bit pat t erns represent idles and m ark t he st art and end of a fram e. collision I n Et hernet , t he disrupt ion of com m unicat ion t hat occurs when m ult iple syst em s t ransm it at t he sam e t im e. collision dom a in A set of Et hernet segm ent s connect ed by repeat ers. I f t wo syst em s t ransm it at t he sam e t im e, a collision will occur. com m a I n 1000BASE- X, a special 7- bit pat t ern ( eit her 0011111 or 1100000) t hat is part of an 8B/ 10B code- group and is used for t he purpose of code- group alignm ent . con ce n t r a t or A device used t o organize LAN cabling. Cables run from a syst em in a work area t o a concent rat or in a wiring closet or com put er room . Configu r a t ion BPD U A bridge prot ocol dat a unit used t o select a Spanning Tree root , est ablish param et ers, and m aint ain t he t ree t opology. con figu r a t ion r e por t se r ve r ( CRS) A Token Ring server t hat assist s in m onit oring and cont rolling t he st at ions on t he ring. con solida t ion poin t A wiring concent rat or t hat int roduces an ext ra connect or int o t he horizont al run. Con st a n t Bit Ra t e ( CBR) An ATM service t hat provides circuit s wit h a const ant bandwidt h. Coppe r D ist r ibu t e d D a t a I n t e r fa ce ( CD D I ) The copper ( t wist ed- pair) version of FDDI . cor e

The cent ral cylinder in a fiber opt ic m edium . C- Por t A port in a dedicat ed Token Ring concent rat or. cr e dit Buffer allocat ion syst em t hat provides flow cont rol for Class 1 and Class 2 fibre channel com m unicat ions. cr oss- con n e ct A physical connect ion bet ween pat ch panels or punch- down blocks. Crossconnect s are used t o m anage t he cable t opology. cr ossove r ca ble A cable whose wires cross so t hat a t ransm it pin at one end is connect ed t o a receive pin at t he ot her end. cr osst a lk A signal induced on a wire by a signal on a neighboring wire. cu t - t h r ou gh br idge A bridge t hat st art s t o t ransm it a fram e before it has received t he ent ire fram e. cyclic r e du n da n cy ch e ck ( CRC) A value used t o det ect t ransm ission errors. I t is calculat ed by applying a m at hem at ical form ula t o t he bit s in a fram e and is appended t o t he fram e.

D da t a cir cu it - t e r m in a t in g e qu ipm e n t ( D CE) Equipm ent t hat connect s dat a t erm inal equipm ent t o a service net work. da t a lin k la ye r The layer above t he physical layer, in which 0s and 1s are organized int o fram es. da t a t e r m in a l e qu ipm e n t ( D TE) A com m unicat ing syst em t hat can be t he source or dest inat ion of dat a.

D ECne t A set of propriet ary net working prot ocols designed by Digit al Equipm ent Corporat ion ( now part of Com paq) . D e dica t e d Tok e n Ring ( D TR) An updat ed version of Token Ring t hat can support full- duplex operat ion. de la y sk e w The difference bet ween t he propagat ion delays of t he slowest and fast est pairs in a four- pair cable. de sign a t e d br idge for a subLAN The bridge t hat t ransm it s fram es com ing from t he direct ion of t he root ont o t he subLAN. de signa t e d por t The single unblocked port t hat connect s a subLAN t o it s designat ed bridge. de st ina t ion se r vice a cce ss point a ddr e ss See [ Un k now n lsa p] de vice dr ive r A soft ware program t hat cont rols a device used in a com put er, such as a print er, CD- ROM, m onit or display, or net work int erface card. diffe r e n t ia l M a n che st e r e n coding The physical encoding m et hod used for 4Mbps and 16Mbps Token Ring. Bit s are represent ed using t ransit ions bet ween high and low signals. For a 0, t here are t ransit ions at t he beginning and t he m iddle of t he bit . For a 1, t here is a t ransit ion only at t he m iddle of t he bit . diffe r e n t ia l m ode de la y ( D M D ) For m ult im ode fiber, t he creat ion of m ult iple overlapping signals t hat can m ake it im possible for a receiver t o int erpret an incom ing signal correct ly. dow n st r e a m n e ighbor For Token Ring or FDDI , t he adj acent neighbor t o which a st at ion t ransm it s and forwards fram es. du a l a t t a ch m e n t st a t ion ( D AS)

A syst em t hat is connect ed t o bot h rings in an FDDI dual- ring configurat ion.

E e gr e ss r u le s The rules applied t o an out going fram e at a VLAN swit ch t o det erm ine whet her t he fram e can be t ransm it t ed t hrough a port and whet her t he fram e needs t o be t agged. e m be dde d r ou t ing infor m a t ion fie ld ( E- RI F) A field used in som e VLAN fram es t hat have passed t hrough a t ranslat ional bridge. e m ula t e d LAN ( ELAN ) A set of syst em s t hat have ATM NI Cs and perform prot ocols t hat em ulat e t he behavior of convent ional LAN syst em s. e qua l le ve l fa r e nd cr osst a lk ( ELFEXT) A m easurem ent of far end crosst alk t hat rem oves t he difference in at t enuat ion t hat result s from using different cable lengt hs. Et h e r ne t A fam ily of LAN prot ocols operat ing at 10Mbps, 100Mbps, and 1000Mbps. Et h e r Type code A 2- byt e code used t o ident ify t he t ype of prot ocol dat a carried by a fram e. e x cha n ge For fibre channel, t ransm ission of sequences of fram es t hat m ake up a t ransact ion of som e kind. e x plor e r fr a m e s For Token Ring, fram es used in t he rout e discovery prot ocol in a sourcerout ing LAN. Two t ypes exist : all- rout es- explorer fram es and Spanning Tree explorer fram es. e x t e n sion bit s For half- duplex Gigabit Et hernet , nondat a sym bols added t o t he end of fram es t hat are less t han 512 byt es in lengt h.

F

F_ Por t A fibre channel fabric port . fa br ic The infrast ruct ure of a fibre channel net work ( for exam ple, net worked swit ches) . fa r e nd cr osst a lk ( FEX T) The dist ort ion of an out going signal by an incom ing signal on an adj acent wire, m easured in decibels. Fa st Et h e r n e t A nicknam e for t he 100Mbps version of Et hernet . fa st lin k pu lse ( FLP) bu r st A sequence of pulses used t o t ransm it t he 16- bit m essages used for Et hernet Aut o- Negot iat ion on a t wist ed pair m edium . FC- 0 A fibre channel level t hat corresponds t o t he lower part of t he physical layer. FC- 1 A fibre channel level t hat perform s 8B/ 10B encoding of dat a byt es and defines t he usage of special code- groups. FC- 2 A fibre channel level t hat corresponds t o t he dat a link layer. FC- 3 A fibre channel level for services t hat are provided across m ult iple port s in a single node. FC- 4 A fibre channel level t hat defines t he way t hat t he various applicat ions t hat ride on t op of fibre channel m ap ont o t he fibre channel environm ent . FCS fie ld

A field at t he end of a fram e t hat cont ains t he result of a calculat ion ( called a cyclic redundancy check, or CRC) perform ed on t he rem aining bit s of t he fram e. fibe r opt ic in t e r - r e pe a t e r lin k ( FOI RL) An early im plem ent at ion of 10Mbps Et hernet across a fiber opt ic link bet ween t wo repeat ers. fibr e cha nn e l ( FC) A local area t echnology for high- speed com m unicat ion bet ween net worked com put ers and peripherals. Fibr e D ist r ibu t e d D a t a I n t e r fa ce ( FD D I ) A 100Mbps, local area net work based on opt ical fiber segm ent s and a t okenpassing prot ocol. filt e r in g da t a ba se A t able in a t ransparent bridge, used t o det erm ine how fram es should be forwarded. I t can include m anually ent ered st at ic ent ries, ent ries t hat t he bridge learned by observing t he fram es at each port , and ent ries learned via t he GARP Mult icast Regist rat ion Prot ocol ( GMRP) . filt e r in g t a ble See [ filt e r in g da t a ba se ] floodin g Act ion by a bridge t hat does not know t he port t hrough which a fram e's dest inat ion MAC address is reached. The bridge t ransm it s t he fram e t hrough all port s except for t he one on which it arrived. FLP bu r st See [ fa st lin k pu lse ( FLP) bur st ] FOI RL See [ fibe r opt ic int e r - r e pe a t e r lin k ( FOI RL) ] for w a r ding t a ble See [ filt e r in g da t a ba se ] fr a m e A dat a link layer prot ocol dat a unit .

fr a m e bu r st in g For half- duplex Gigabit Et hernet , t he capabilit y t o send a sequence of fram es before giving up cont rol of t he m edium . fr a m e ch e ck se que nce ( FCS) fie ld See [ FCS fie ld] Fr a m e Re la y A wide area virt ual circuit service. fu ll- du ple x A m ode of operat ion t hat enables a pair of syst em s connect ed by a link t o t ransm it fram es t o one anot her at t he sam e t im e. fu ll- du ple x r e pe a t e r See [ Un k now n buffe r e ddist r ibut or ] fu n ct iona l a ddr e sse s For Token Ring LANs, addresses t hat ident ify nodes t hat perform various t ypes of special services.

G GARP M u lt ica st Re gist r a t ion Pr ot ocol ( GM RP) A prot ocol t hat enables syst em s t o regist er t heir m ult icast m em berships wit h bridges. GARP VLAN Re gist r a t ion Pr ot ocol ( GVRP) A prot ocol t hat enables a VLAN- aware end syst em or swit ch t o regist er wit h a neighboring swit ch t o j oin ( or leave) a VLAN. Ge n e r ic At t r ibu t e Re gist r a t ion Pr ot ocol ( GARP) A prot ocol t hat provides a general regist rat ion m echanism for use in a bridged LAN. Giga bit Et h e r n e t 1000Mbps Et hernet . I t current ly is im plem ent ed in full- duplex m ode only. giga bit in t e r fa ce conve r t e r ( GBI C)

A t ransceiver used t o connect a Gigabit Et hernet port t o a specific m edium . giga bit m e dia in de pe n de n t in t e r fa ce ( GM I I ) A Gigabit Et hernet adapt er int erface t o which different t ypes of GBI C t ransceivers can be at t ached. This provides choice in t he m edia t o which a syst em can connect .

H h a lf- duple x A m ode of operat ion t hat allows only a single st at ion t o successfully t ransm it a fram e at a given t im e. hertz ( H2 ) Elect rical wave frequency in cycles per second. One hert z ( Hz) equals one cycle per second. H igh - Le ve l D a t a Lin k Con t r ol ( H D LC) A fram e encapsulat ion and prot ocol used across wide area point - t o- point links. h igh pe r for m a n ce pa r a lle l in t e r fa ce ( H I PPI ) A high- perform ance int erface st andard defined by ANSI , t ypically used t o connect supercom put ers t o peripherals and ot her devices. H igh Spe e d Se r ia l I nt e r fa ce ( H SSI ) A physical- layer int erface for serial com m unicat ions bet ween a DTE ( for exam ple, a rout er) and a DCE ( for exam ple, a DSU/ CSU) t hat support s up t o 52Mbps. H igh Spe e d Tok e n Rin g ( H STR) The 100Mbps version of Token Ring. hub A Layer 1 repeat er device used t o provide connect ivit y bet ween DTEs. h ybr id lin k A subLAN t hat connect s VLAN bridges t o one anot her and t hat also includes VLAN- unaware syst em s.

I –K

idle A special signal sent bet ween fram es, used wit h several LAN t ransm ission t echnologies. I GM P sn oopin g Eavesdropping on I GMP m essages by bridges t o ident ify t he port s t hat lead t o m em bers of a m ult icast group. im pe da nce A m easure ( in ohm s) of t he opposit ion t o t he flow of elect ricit y down t he wire. in de pe nde n t VLAN le a r n ing The developm ent of a separat e filt ering t able for each VLAN. in gr e ss r u le s The rules used by a swit ch t o assign an incom ing fram e t o a VLAN. I n t e r n e t En gin e e r ing Ta sk For ce ( I ETF) A group responsible for defining and im plem ent ing t he TCP/ I P suit e of prot ocols t hat is used on t he I nt ernet . I n t e r n e t Gr ou p M a n a ge m e n t Pr ot ocol ( I GM P) A prot ocol t hat TCP/ I P syst em s use t o j oin and leave an I P- based m ult icast group. I n t e r n e t Pr ot ocol ( I P) The Layer 3 I ETF prot ocol t hat rout es t raffic from it s source t o it s dest inat ion. in t e r fr a m e ga p The gap required bet ween t he end of an Et hernet fram e and t he st art of t ransm it t ing t he next fram e. For exam ple, for Et hernet , t he int erfram e gap is 96 bit t im es. ( Also called t he int erpacket gap.) in va lid fr a m e A fram e t hat eit her does not cont ain an int egral num ber of byt es, is t oo short or t oo long, or carries an invalid CRC value. j a bbe r

A condit ion wherein a st at ion t ransm it s for a period of t im e longer t han t he m axim um perm issible fram e lengt h, usually due t o a fault condit ion. j a m bit s I n Et hernet , a series of 32 random ly select ed bit s t hat are sent aft er a t ransm it t er det ect s a collision t o assure t hat all st at ions hear t he collision. j it t e r Tim e variat ion in t he rat e at which bit s, cells, or fram es are delivered. Ju m bo fr a m e A nonst andard Et hernet fram e im plem ent ed by som e Gigabit Et hernet vendors. I t s size ranges up t o 9018 byt es.

L LAN Em u la t ion ( LAN E) See [ LAN Em u la t ion ( LAN E) ] La ye r 2 sw it ch An up- t o- dat e im plem ent at ion of a bridge. La ye r 3 sw it ch An up- t o- dat e im plem ent at ion of a rout er. La ye r 4 sw it ch A swit ch t hat m akes forwarding decisions t hat t ake Layer 4 prot ocol inform at ion int o account . La ye r 2 / 3 sw it ch See [ Un k now n br idge _ r ou t e r ] ligh t e m it t in g diode ( LED ) A device used t o t ransm it signals across m ult im ode fiber opt ic cable. lin e ca r d An input / out put ( I / O) card t hat can be inst alled in a chassis. lin k

The t ransm ission pat h bet ween any t wo int erfaces connect ed by a cable segm ent . lin k a ggr e ga t ion The capabilit y t o com bine a group of links so t hat t hey behave like a single link. Lin k Aggr e ga t ion Cont r ol Pr ot ocol ( LACP) A prot ocol t hat enables link part ners t o discover t he set of links t hat connect t hem and convert groups of links t o an aggregat ed link. lin k a ggr e ga t ion conve r sa t ion A flow of fram es t hat m ust be delivered wit hout changing t heir order. All fram es are t ransm it t ed across t he sam e link segm ent . lin k code w or d A 16- bit Aut o- Negot iat ion m essage encoded int o a fast link pulse ( FLP) burst . lin k in t e gr it y t e st sign a l A periodic pulse sent across t wist ed- pair cable t o t est t he cable when t he dat a t ransm it t er is idle. lin k se gm e n t A link t hat is part of an aggregat ion. lin k pa r t ne r The device at t he opposit e end of a link segm ent from t he local st at ion. lin k se r vice a cce ss poin t ( LSAP) a ddr e ss An 802.2 address t hat appears in a LLC header, int ended t o ident ify a source or dest inat ion of a flow of dat a. LLC fr a m e A fram e cont aining a Logical Link Cont rol header and user dat a. loa d ba la nce r A device t hat m akes a group of servers have t he appearance of a single server. lobe ca bling

Two t wist ed pairs of cables used t o connect a Token Ring st at ion t o a t runk coupling unit ( TCU) in a concent rat or. loca l a r e a n e t w or k ( LAN ) A net work t hat connect s a set of com put ers so t hat t hey can com m unicat e wit h one anot her direct ly. loca l a r e a t r a n spor t ( LAT) A nonrout able Digit al Equipm ent Corporat ion prot ocol used for t erm inal access across a net work. Logica l Link Cont r ol ( LLC) subla ye r The port ion of t he dat a link layer t hat support s m edia- independent dat a link funct ions and uses t he services of t he MAC t o provide services t o t he net work layer. Logica l Link Cont r ol ( LLC) he a de r A header t hat cont ains a source and dest inat ion service point addresses and a cont rol field. Loca lly Adm in ist e r e d M AC a ddr e ss A MAC address t hat is assigned t o a net work int erface card by an adm inist rat or, replacing t he vendor's unique address. loop sw it ch A fibre channel device t hat connect s t o individual devices, arbit rat ed loops, and hubs. Mult iple devices can t ransm it and receive dat a concurrent ly.

M M AC a ddr e ss A Layer 2 address associat ed wit h a net work int erface card. I t is used for LAN source and dest inat ion addresses. M AC su bla ye r The port ion of t he dat a link layer t hat wraps out going dat a int o fram es, checks t he CRC for incom ing fram es, and includes t he prot ocols t hat cont rol access t o t he m edium . M a na ge m e n t I n for m a t ion Ba se d ( M I B) A collect ion of SNMP net work m anagem ent variables.

M a nche st e r e ncoding A m et hod of encoding bit s ont o a m edium . There is a volt age t ransit ion in t he m iddle of each bit . A 0 has a t ransit ion from high t o low, and a 1 has a t ransit ion from low t o high. m e dia a cce ss con t r ol a ddr e ss See [ M AC a ddr e ss] m e dia a cce ss con t r ol su bla ye r See [ M AC su bla ye r ] m e dia conve r t e r A device t hat connect s segm ent s for t wo different m edia ( such as t wist ed- pair t o fiber opt ic cabling) m aking t he t wo segm ent s behave like one ext ended segm ent . m e diu m de pe nde nt in t e r fa ce ( M D I ) por t A port t hat t ransm it s and receives via t he st andard pin connect ions. Com put er adapt ers or t ransceivers have MDI port s. m e diu m de pe nde nt in t e r fa ce cr ossove r ( M D I - X) por t A hub port t hat reverses t he role of t he send and receive pins, allowing a st raight - t hrough cable t o be used bet ween t he port and a st at ion m e dia in de pe nde nt in t e r fa ce ( M I I ) A 100Mbps adapt er int erface t o which different t ypes of t ransceivers can be at t ached. This provides choice in t he m edia t o which a syst em can connect . m e diu m a t t a chm e n t u n it ( M AU) A t ransceiver t hat is used t o connect a repeat er or dat a t erm inal equipm ent ( DTE) t o a t ransm ission m edium . m e diu m The m at erial on which t he dat a m ay be t ransm it t ed. STP, UTP, and opt ical fibers are exam ples of m edia. m e ssa ge pa ge ( M P) An Aut o- Negot iat ion m essage t hat cont ains a m essage code. M I B M odule

An organized set of relat ed SNMP definit ions wit hin a MI B docum ent . m oda l ba ndw idt h The worst - case 3dB bandwidt h t hat will be achieved on a part icular t ype of m ult im ode cable. m oda l dispe r sion Dispersion of t he arrival t im es of rays em it t ed int o a m ult im ode fiber. A large dispersion m akes it im possible for t he receiver t o int erpret incom ing signals correct ly. m ode - condit ionin g pa t ch cor d A special pat ch cord t hat consist s of a single- m ode fiber spliced t o an offcent er posit ion on a m ult im ode fiber. I t is used t o funnel an LX laser signal int o a m ult im ode fiber core. m on it or A device ( also called a probe) t hat can eavesdrop on LAN act ivit ies. m u lt ica st a ddr e ss An address used t o ident ify a group of syst em s t o which fram es will be forwarded. M u lt i- Le ve l 3 e n coding ( M LT- 3 ) A signaling m et hod t hat is used for 100BASE- TX and CDDI . I t uses t hree signal levels. m u lt im ode fibe r Opt ical fiber cable wit h a relat ively wide core ( 62.5 or 50 m icrons) t hat allows m ult iple rays of light t o follow different pat hs t hrough t he core. M u lt ipr ot ocol La be l Sw it ch ing Ar ch it e ct u r e ( M PLS) A " rout e once, swit ch m any" net work prot ocol. m u lt iu se r t e le com m u n ica t ion s ou t le t ( M UTO) An office wiring concent rat or.

N N _ Por t

A fibre channel node port . N _ Por t ide n t ifie r A 3- byt e address t hat ident ifies t he locat ion of a port . N a t ion a l Ele ct r ica l Code ( N EC) A specificat ion t hat applies t o all facilit y wiring in t he Unit ed St at es. n e a r e nd cr osst a lk ( N EXT) The dist ort ion of a weak incom ing signal by a st rong out going signal on a neighboring wire pair. NEXT is m easured in decibels. N e t w or k Addr e ssin g Au t hor it y An organizat ion, such as t he I EEE, responsible for adm inist ering t he dist ribut ion of unique nam es or addresses. n e t w or k dia m e t e r For a collision dom ain, t he lengt h of t he longest pat h bet ween t wo point s. N e t w or k D r ive r I n t e r fa ce Spe cifica t ion ( N D I S) A Microsoft specificat ion for a generic device driver for net work adapt ers. n e t w or k int e r fa ce ca r d ( N I C) A hardware com ponent inst alled in a syst em expansion slot t hat enables t he syst em t o send dat a ont o a m edium and receive dat a from t he m edium . A NI C also is called an adapt er. n e t w or k int e r fa ce The point of connect ion bet ween a st at ion ( such as a com put er) and a LAN m edium . n e x t pa ge I n Et hernet Aut o- Negot iat ion, any page t ransm it t ed subsequent t o a base page. n ibble A group of 4 dat a bit s t hat is half of a byt e. n om in a l ve locit y of pr opa ga t ion ( N VP)

The t ransm ission speed along a wire relat ive t o t he speed of light in a vacuum . n on - ca nonica l for m a t The form at of a dat a fram e in which t he oct et s of MAC addresses conveyed in t he user dat a field have t he sam e bit ordering as in t he bit - reversed represent at ion. n on - ca nonica l for m a t ide n t ifie r ( N CFI ) fla g A flag in an E- RI F. NCFI = 0 m eans t hat any MAC addresses carried in t he inform at ion field are in non- canonical form . n on - r e a l- t im e va r ia ble bit r a t e ( n r t - VBR) An ATM service delivers a specified average bandwidt h and is suit able for dat a applicat ions, which are not st rongly sensit ive t o delay. n on r e t u r n t o ze r o, inve r t on one s ( N RZI ) An encoding used for 100BASE- FX and FDDI . Ones are alt ernat ively represent ed by a high or low signal. There is no change of signal level at a 0. n or m a l link pu lse ( N LP) The periodic pulse used for a link int egrit y t est signal, which checks whet her t he link is working. N W AY An Aut o- Negot iat ion prot ocol int roduced by Nat ional Sem iconduct or t hat becam e t he basis of t he 802.3 Aut o- Negot iat ion prot ocol.

O Ope n Shor t e st Pa t h Fir st ( OSPF) A net work rout ing prot ocol. Ope n Syst e m s I nt e r con n e ct ( OSI ) m ode l A layered m odel for dat a com m unicat ions, defined by an I nt ernat ional Organizat ion for St andardizat ion com m it t ee. opt ica l t im e - dom a in r e fle ct om e t e r ( OTD R) An opt ical fiber t est t ool t hat locat es fiber fault s. or de r e d se t

A single special code- group or a com binat ion of special and dat a code- groups t hat represent a signal such as an idle or t he st art of a fram e. or ga n iza t iona lly un iqu e ide n t ifie r ( OUI ) A 3- byt e code assigned by t he I EEE, usually for use as a MAC address prefix.

P– Q pa ck e t A generic t erm used for a Layer 2 fram e or a Layer 3 prot ocol dat a unit . pa ge A 16- bit m essage used for Aut o- Negot iat ion. PAM 5 x 5 A coding t echnique t hat represent s 4 bit s as a pair of sym bols select ed from five signal levels. The levels are represent ed as ( - 2, - 1, 0, + 1, + 2) . pa r a lle l de t e ct ion I n Aut o- Negot iat ion, t he capabilit y t o det ect whet her a non- negot iat ing part ner has a 10BASE- T, 100BASE- T4, or 100BASE- TX int erface by exam ining t he signals arriving from t he part ner. pa ssive con ce n t r a t or A t ype of Token Ring concent rat or t hat cont ains no act ive elem ent s in t he signal pat h of any lobe port —t hat is, it does not repeat t he signals. pa t ch cor d A short , flexible cable t erm inat ed wit h connect ors. pa t ch pa ne l A cross- connect designed t o accom m odat e t he use of pat ch cords. I t facilit at es adm inist rat ion of m oves and changes. PAUSE ope r a t ion A flow cont rol prot ocol used across a full- duplex Et hernet link. A syst em sends a PAUSE fram e t o ask it s part ner t o st op sending fram es for a specified period of t im e. pe r m a n e n t lin k

I ncludes t he horizont al cabling and t he connect ors at each end of a horizont al cable. pe r m a n e n t vir t ua l cir cu it ( PVC) For ATM, a long- t erm preconfigured circuit . ph a n t om sign a l A low- level direct current t ransm it t ed by a Token Ring st at ion. I t causes t he concent rat or t o open it s relay and insert t he st at ion int o t he ring. ph ysica l a ddr e ss See [ Un k now n m a ca ddr e ss] ph ysica l la ye r ( PH Y) The layer t hat int erfaces wit h t he t ransm ission m edium and t ransm it s and receives bit s. Ph ysica l Coding Su bla ye r ( PCS) A sublayer t hat cont ains t he funct ions t o encode dat a int o code- groups t hat can be t ransm it t ed over t he physical m edium . Ph ysica l M e dium D e pe n de n t ( PM D ) su bla ye r The port ion of t he physical layer responsible for int erfacing t o t he t ransm ission m edium . ple n um ca ble Fire- ret ardant cable t hat can be carried in air duct s. Poin t - t o- Poin t Pr ot ocol ( PPP) An encapsulat ion and prot ocol defined by an I ETF t ask force and used across point - t o- point wide area links. por t - ba se d VLAN A VLAN defined by a list of bridge port s. All syst em s reached t hrough t hose port s belong t o t he VLAN. pow e r sum e qu a l le ve l fa r e nd cr osst a lk ( PSELFEXT) The sum of t he equal level far end crosst alk ( ELFEXT) effect s on a pair by t he ot her t hree pairs.

pow e r sum n e a r e nd cr osst a lk ( PSN EXT) The sum of t he NEXT effect s on a pair by t he ot her t hree pairs. pr e a m ble An int roduct ory set of signals t ransm it t ed before an Et hernet fram e, used t o synchronize t im ing on a LAN. pr im a r y FD D I r in g The ring around which fram es flow during norm al operat ion. pr ior it y cla sse s Priorit y levels corresponding t o t he out put queues at a port . pr ior it y- t a gge d fr a m e A fram e t hat cont ains a header t hat carries a null ( 0) VLAN ident ifier and a priorit y value. pr iva t e loop A st andalone fibre channel arbit rat ed loop. pr opa ga t ion de la y The am ount of t im e t hat elapses when a signal t ravels across a cable or across an ent ire pat h. pr ot ocol A set of rules t hat govern com m unicat ion. pr ot ocol da t a u n it ( PD U) I nform at ion delivered as a unit bet ween peer ent it ies t hat cont ains cont rol inform at ion and, opt ionally, dat a. pu blic loop A fibre channel arbit rat ed loop connect ed t o a swit ch. pu n ch - dow n block A t wist ed- pair wiring panel for which each wire is placed in a pin and t hen punched int o place, st ripping t he insulat ion and m aking a connect ion in t he process.

pu r gin g A Token Ring st at e t hat occurs when t he act ive m onit or has det ect ed a ring error and is ret urning t he ring t o an operat ional st at e by t ransm it t ing Ring Purge fram es. Qu a lit y of Se r vice pa r a m e t e r s ATM param et ers t hat relat e t o delay and reliabilit y requirem ent s. qu ina r y sym bols Sym bols corresponding t o five different volt age levels t hat are labeled [ - 2, - 1, 0, + 1, + 2] . Quinary sym bols are used for 1000BASE- T t ransm ission.

R r a ndom ba ck off See [ Un k now n ba ck off] r e a l- t im e va r ia ble bit r a t e ( r t - VBR) An ATM service t hat delivers a specified average bandwidt h and support s applicat ions such as com pressed voice or video, which are delay- sensit ive. r e gist r a t ion The exchange of inform at ion bet ween a dedicat ed Token Ring st at ion and it s concent rat or port t hat is required t o init iat e t ransm ission bet ween t he devices. r e m ot e br idge s Bridges t hat unit e separat e Et hernet , Token Ring, or FDDI LANs locat ed at different sit es int o a single LAN. Rem ot e bridges are connect ed by a wide area connect ion. r e m ot e m on it or ing ( RM ON ) St andards t hat enable an SNMP net work m anagem ent st at ion t o int erwork wit h a net work m onit oring com ponent in a syst em . r e pe a t e r A physical- layer device t hat accept s signals from one cable segm ent and t ransm it s t hem ont o one or m ore ot her cable segm ent s at full st rengt h. ( Also called a hub or repeat ing concent rat or.) r e sist a n ce

A m easurem ent ( in unit s called ohm s) of t he degree t o which a conduct or resist s t he flow of direct current in an elect ronic circuit . r e t u r n loss A m easure of t he relat ive am ount of a signal t hat is reflect ed back t o it s source. r in g e r r or m on it or ( REM ) A server t hat receives ring error dat a from Token Ring st at ions. r in g in por t A Token Ring concent rat or port t hat receives signals from t he m ain ring pat h on t he t runk cable. r in g ou t por t A Token Ring concent rat or port t hat t ransm it s signals t o t he m ain ring pat h on t he t runk cable. r in g pa r a m e t e r se r ve r ( RPS) A Token Ring server t hat is responsible for init ializing a set of operat ional param et ers in a st at ion on a Token Ring. Rin g Pu r ge fr a m e A fram e used t o clear dat a out of a Token Ring as part of an init ializat ion or recovery procedure. r ise r ca ble Cable t o be used in vert ical shaft s. r oot ( of a Spa n n in g Tr e e ) The bridge wit h t he sm allest bridge ident ifier. r ou t e de scr ipt or A subfield of t he rout ing inform at ion field t hat ident ifies a segm ent and bridge on t he net work pat h. The rout e bet ween a pair of com m unicat ing syst em s is represent ed by a series of descript ors. r ou t e discove r y A prot ocol t hat uses explorer fram es t o discover a pat h t o a dest inat ion in a source rout ing LAN.

r ou t e r A device t hat forwards Layer 3 prot ocol dat a unit s. A rout er can int erconnect m ult iple local area net works and WAN links wit h one anot her. Also called a Layer 3 swit ch. r ou t ing infor m a t ion fie ld ( RI F) A field in a Token Ring or FDDI fram e header in which inform at ion describing t he rout e bet ween t he com m unicat ing syst em s is st ored. Rout in g I nfor m a t ion Pr ot ocol ( RI P) A net work rout ing prot ocol. r ou t ing pr ot ocol A net work prot ocol t hat enables rout ers t o exchange inform at ion used t o build t heir rout ing t ables. r u n n ing dispa r it y A param et er having a value of + or - , represent ing t he im balance bet ween t he num ber of ones and zeros in a sequence of 8B/ 10B code- groups.

S scr a m bling fun ct ion A t ransform at ion applied t o out going bit s t o im prove t he balance bet ween t ransm it t ed 1s and 0s. The funct ion is reversed at t he receiving end. scr e e n e d t w ist e d- pa ir ( ScTP) Four UTP pairs, wit h a single foil or braided screen surrounding all four pairs in order t o m inim ize t he effect of EMI radiat ion. scr e e n e d/ sh ie lde d t w ist e d- pa ir ( SSTP) Four t wist ed pairs, where each pair is shielded and a shield surrounds all four pairs. se conda r y FD D I r ing A backup FDDI ring. Aft er a fault , segm ent s on t he secondary ring will be int egrat ed int o a usable pat h. se gm e n t

A copper or opt ical fiber bet ween t wo devices. Device t ypes include DTEs, repeat ers, bridges, and rout ers. sh a r e d VLAN le a r n ing The use of a single filt ering t able t hat cont ains MAC addresses t hat have been learned across a group of VLANs, or for all VLANs. sh ie lde d t w ist e d- pa ir ( STP) Cable for which each t wist ed pair of wires is shielded t o prevent elect rom agnet ic int erference. Som et im es a shield also is placed around t he ent ire cable bundle. signa l qua lit y e r r or ( SQE) m e ssa ge A signal sent across an AUI cable or an int ernal circuit from a t ransceiver t o it s parent DTE or repeat er. I t is used t o announce a collision or t o indicat e t hat im proper signals have been det ect ed on t he m edium . signa l qua lit y e r r or ( SQE) t e st An ongoing check t hat a t ransceiver is working. The t ransceiver sends an SQE signal across an AUI cable or int ernal circuit whenever it finishes t ransm it t ing a fram e. Sim ple N e t w or k M a na ge m e n t Pr ot ocol ( SN M P) A widely used net work m anagem ent prot ocol. sin gle - a t t a ch m e nt FD D I st a t ion A st at ion at t ached t o only t he prim ary ring of an FDDI LAN. sin gle - m ode fibe r An opt ical fiber wit h a narrow core ( 8 t o 9 m icrons) t hat allows light t o t ravel along only one pat h. slot t im e The m inim um num ber of bit t im es in a valid half- duplex Et hernet t ransm ission. I t is 512 bit t im es for t ransm ission rat es up t o 100Mbps, and 4096 bit t im es for 1000Mbps Et hernet . SN AP he a de r A fram e header field t hat ident ifies t he t ype of prot ocol dat a t hat is being carried.

SN M P a ge nt Soft ware com ponent in a m anaged device t hat part icipat es in SNMP. SN M P m a na ge r Soft ware com ponent in a net work m anagem ent st at ion t hat reads or updat es m anagem ent variables at a rem ot e device by sending request s t o t he device's SNMP agent . sou r ce - r out in g br idge ( SRB) A bridge t hat is used in a source- rout ing LAN. sou r ce - r out in g LAN A bridged LAN in which a fram e t hat needs t o be bridged cont ains a field t hat describes t he rout e t hat t he fram e will t raverse. sou r ce - r out in g t r a n spa r e n t ( SRT) br idge A bridge t hat can perform bot h source- rout ing and t ransparent bridging. sou r ce se r vice a cce ss poin t a ddr e ss See [ Un k now n lsa p] Spa n n in g Tr e e e x plor e r ( STE) fr a m e A t ype of source rout ing explorer fram e t hat t raverses t he LAN following only valid Spanning Tree pat hs. Spa n n in g Tr e e Pr ot ocol ( STP) A prot ocol t hat m akes it possible t o elim inat e point s of failure by inst alling backup bridges and backup links. The prot ocol aut om at ically reconfigures t he LAN t opology aft er a failure. squa r e cor n e r ( SC) A popular t ype of fiber opt ic connect or. st a r t - of- st r e a m de lim it e r A delim it er sent at t he beginning of a 100BASE- X, FDDI , or CDDI fram e. st a ndby m on it or s All Token Ring st at ions ot her t han t he act ive m onit or. Any st at ion is capable of t aking over t he act ive m onit or role if t he act ive m onit or fails.

St a r LAN A 1Mbps t wist ed- pair Et hernet LAN int roduced by AT&T. st a r t fr a m e de lim it e r ( SFD ) A special bit pat t ern sent before t he first byt e of an Et hernet fram e. st a t ion A device at t ached t o a LAN t hat is capable of t ransm it t ing and receiving dat a. st a t ion m a n a ge m e n t ( SM T) A soft ware com ponent in an FDDI st at ion t hat perform s init ializat ion, t est ing, and error recovery funct ions. st r a igh t t ip ( ST) A t ype of fiber opt ic connect or. st or e - a nd- for w a r d br idge A bridge t hat wait s unt il it has received an ent ire fram e before it st art s t o forward t he fram e. st r u ct u r a l r e t u r n loss ( SRL) A m easure of signal loss caused by difference in im pedance along a wire, expressed in decibels. Su bLAN I n an Et hernet LAN, a collision dom ain or point - t o- point link. Su bn e t w or k Acce ss Pr ot ocol ( SN AP) h e a de r See [ SN AP h e a de r ] syn ch r onou s ba n dw idt h For FDDI , a guarant eed reserved t im e quot a during which a st at ion can t ransm it fram es. sw it ch See also [ Un k n ow n la ye r 2 sw it ch ] See also [ Un k n ow n la ye r 3 sw it ch ]

See also [ Un k n ow n la ye r 4 sw it ch ] sw it che d vir t u a l cir cuit ( SVC) For ATM, a virt ual circuit set up on dem and. sym m e t r ic flow con t r ol For full- duplex Et hernet , t he capabilit y of a st at ion at eit her end of t he link t o send a PAUSE fram e t o it s part ner.

T t a gge d fr a m e A fram e t hat cont ains a header t hat carries a VLAN ident ifier and a priorit y value. Also called a VLAN t agged fram e. Ta r ge t Tok e n Rot a t ion Tim e ou t ( TTRT) The average t im e t hat an FDDI st at ion expect s t o wait before it receives t he use of t he t oken. TCP/ I P A popular fam ily of com m unicat ions prot ocols, originally designed for use on t he I nt ernet . t e r m in a t or For a coax Et hernet LAN, hardware at t ached t o t he ends of a coax segm ent . A t erm inat or absorbs signals and prevent s t hem from being reflect ed back int o t he cable. Tim e D om a in Re fle ct om e t e r ( TD R) A t ool used t o t rack t he locat ion of cable fault s. Tok e n Pa ssin g Pr ot ocol ( TKP) Use of t he classical Token Ring prot ocol, which perm it s a st at ion t hat has capt ured a t oken t o t ransm it dat a in a Dedicat ed Token Ring environm ent . Tok e n Ring A LAN t echnology originat ed by I BM and st andardized as I EEE 802.5. t ok e n

A special sequence of sym bols passed from st at ion t o st at ion t hat is used t o cont rol access t o a LAN m edium . t on e t e st se t A device t hat is at t ached t o one end of a cable and generat es a t one t hat is t ransm it t ed ont o a wire. I t is used t o discover t he m at ching end of a cable. Topology Ch a n ge N ot ifica t ion BPD U A BPDU t ransm it t ed t oward t he root by any bridge t hat has evidence t hat t he t opology inform at ion needs t o change. t r a n sce ive r A hardware com ponent t hat t ransm it s signals ont o a m edium and receives t hem from t he m edium . Also called a m edium at t achm ent unit ( MAU) . t r a n sit ion poin t A locat ion where flat undercarpet cabling connect s t o round cabling. t r a n sla t iona l br idge A bridge t hat connect s LAN com ponent s t hat use different MAC prot ocols ( for exam ple, Et hernet and Token Ring) . Tr a n sm it I m m e dia t e Pr ot ocol ( TXI ) For dedicat ed Token Ring, t he capabilit y t o t ransm it dat a wit hout wait ing t o capt ure a t oken. t r a n spa r e n t br idge A bridge t hat forwards a fram e based on inform at ion in it s filt ering dat abase. trap A m essage t hat an SNMP agent sends t o report a significant event ( such as a reboot or a serious error t hat occurs at a device) . t r u n ca t e d bin a r y e x pon e nt ia l ba ck off See [ Un k now n ba ck off] t r u n k cou plin g u n it ( TCU) A device t hat couples a Token Ring st at ion t o t he m ain pat h around t he ring. A TCU provides t he m echanism for insert ing a st at ion int o t he ring and rem oving it from t he ring.

t r u n k ing A t erm som et im es used for link aggregat ion. t u n n e lin g Encapsulat ing one t ype of fram e inside t he dat a field of anot her fram e. t w ist e d- pa ir lin k A t wist ed- pair cable plus connect ing hardware.

U unfor m a t t e d pa ge ( UP) A m essage t hat is part of an Aut o- Negot iat ion exchange. I t s cont ent depends on t he m essage code in a prior m essage page. u n ica st a ddr e ss An address t hat ident ifies a single net work int erface card. u n ive r sa lly a dm in ist e r e d M AC a ddr e ss An address assigned t o a net work int erface card by a m anufact urer t hat has obt ained a block of unique ident ifiers from t he I EEE. u n sh ie lde d t w ist e d- pa ir ca ble ( UTP) A cable in which wires are paired and t wist ed around each ot her. Usually four pairs are bundled int o a single cable. The cables are graded by cat egories ranging from 1 t o 7. Un spe cifie d Bit Ra t e ( UBR) A best - effort ATM service. u pst r e a m n e ighbor For Token Ring or FDDI , t he adj acent neighboring st at ion t hat t ransm it s and forwards fram es t o t he st at ion. Use r D a t a gr a m Pr ot ocol ( UD P) A connect ionless prot ocol in t he TCP/ I P fam ily, used t o send st andalone m essages.

V

Ve r t ica l Ca vit y Su r fa ce Em it t in g La se r ( VCSEL) A low- cost laser used for gigabit t ransm ission across m ult im ode fiber. Vir t ua l Cha n ne l I de nt ifie r ( VCI ) An ident ifier for an ATM virt ual channel. VCI s appear in ATM cell headers. vir t ua l LAN ( VLAN ) A group of syst em s ( such as t he com put ers in a workgroup) t hat need t o com m unicat e wit h one anot her, and prot ocols t hat rest rict t he delivery of virt ual LAN fram es t o m em bers of t he VLAN. Vir t ua l Pa t h I de n t ifie r ( VPI ) An ident ifier for an ATM virt ual pat h. VPI s appear in ATM cell headers. VLAN - a w a r e sw it ch A swit ch t hat is capable of part icipat ing in VLAN prot ocols. Also called a VLAN swit ch. VLAN t r u nk A link t hat carries t raffic bet ween a pair of VLAN swit ches. I t can carry t raffic for m ult iple VLANs.

W–Z w ide a r e a n e t w or k ( W AN ) Com m unicat ions facilit ies such as point - t o- point links or fram e relay service net works t hat carry dat a across large dist ances. w ir e m a p t e st in g A procedure t hat checks wires for proper pin t erm inat ion, cont inuit y, crossed pairs, short s, and split pairs. w or ld w ide por t na m e A unique 8- byt e global ident ifier assigned t o a fibre channel port . w or st pa ir - t o- pa ir e qu a l le ve l fa r e nd cr osst a lk The biggest equal level far end crosst alk ( ELFEXT) effect of one pair on anot her.