A Primer on Asynchronous Modem Communication

Citation preview

A Prim er on Asynchronous Modem Com m unicat ion by W in fie ld St a n t on a n d Th om a s Spe n ce r Last Updat ed: May 1998 This paper cover s t he basic RS- 232 signaling, er r or cor r ect ion, com pr ession, and m odulat ion st andar ds. I n t he near fut ur e, t he asy nchr onous m odem w ill cont inue t o be an im por t ant m eans of t r ansfer r ing infor m at ion t o and fr om t he per sonal com put er . By som e est im at es, m or e t han 60 per cent of people who access t he I nt er net do so w it h a conv ent ional dial- up m odem and a connect ion t o an analog t elephone line ( w w w .boar dw at ch.com ) . You m ay be connect ing t o a bullet in boar d syst em ( BBS) t hr ough a piece of com m unicat ions soft w ar e or you m ay be using t he Window s NT® Rem ot e Access Ser vice ( RAS) t o dial out t o y our I nt er net Ser v ice Pr ov ider ( I SP) . Whet her y ou use a sim ple file t r ansfer pr ot ocol such as XMODEM or anot her pr ot ocol such as TCP/ I P over t he RAS Point - t o- Point ( PPP) pr ot ocol, t he under ly ing asy nchr onous com m unicat ions issues ar e t he sam e. Most asynchr onous m odem com m unicat ion im plem ent ed t oday confor m s t o t he RS- 232 signaling st andar d ( RS st ands for r ecom m ended st andar d) , and t he m ost im por t ant st andar d layer ed ont o t he RS232 signaling is t he m odulat ion st andar d. Ot her st andar ds such as er r or cor r ect ion and dat a com pr ession also com e int o play. Asy nchr onous com m unicat ion is a m et hod of ser ial dat a t r ansfer t hat is com m on t o a num ber of pr ot ocols, such as XMODEM, YMODEM, and Ker m it . For t he pur pose of t his paper , t he set of r ules com m on t o asy nchr onous com m unicat ion w ill occasionally be r efer r ed t o as t he asynchronous prot ocol. One of t he m ain t hings t o lear n about t he asy nchr onous pr ot ocol is how t he m odem and t he com put er use t he differ ent leads in t he ser ial cable t o com m unicat e accor ding t o t he RS- 232 st andar d.

BASI C TERMI NOLOGY I t is necessar y t o fir st clar ify som e of t he basic t er m s im por t ant t o t his t opic. For exam ple, w hat ar e DTE and DCE? What ar e t he differ ences bet w een t he baud rat e and t he speed of dat a t r ansfer m easur ed in bit s per second ( bps) ? What does a m odem act ually do, and w her e did it get it s nam e? And finally , w hat ex act ly is m eant by a m odulat ion st andar d, and w hat ar e t he k ey elem ent s t hat m ak e up such a st andar d? These ar e t he quest ions t hat w ill be addr essed in t he follow ing sect ions. Most discussions of net wor king pr ot ocols ar e fr am ed against t he pr ot ocol layer s defined by t he Open Sy st em s I nt er connect ( OSI ) m odel. Alt hough a det ailed discussion of t hat m odel is bey ond t he scope of t his paper , it is im por t ant t o k now t hat t he asy nchr onous pr ot ocol consist s of j ust t he applicat ion, dat a link , and phy sical lay er s of t he OSI m odel. Also, t he funct ions of t he dat a link and phy sical lay er ov er lap, because t hey ar e per for m ing funct ions t hat ar e supposed t o be per for m ed by t he ot her layer accor ding t o t he OSI m odel. For ex am ple, t he dat a- link lay er w or k s by changing elect r ical v olt age lev els of cir cuit s ( w ir es and pins) in t he phy sical lay er , w hich is a funct ion of t he phy sical lay er in t he OSI m odel. Dat a t er m inal equipm ent ( DTE) is t he com binat ion of a com put er , a ser ial por t , and applicat ion soft w ar e t hat com m unicat es w it h a second applicat ion—t he rem ot e applicat ion—ov er a t elephone line. The r em ot e applicat ion is also a configur at ion of a com put er , a ser ial por t , and an applicat ion t hat m ak es up t he second DTE. Most com put er s use a chip called a univ er sal asy nchr onous r eceiv er / t r ansm it t er ( UART) t o conv er t t he com put er 's sy nchr onous par allel dat a int o asy nchr onous ser ial dat a r eady for t r ansm ission t o t he m odem . The dat a com m unicat ions equipm ent ( DCE) is t he m odem . A DCE is also k now n as dat a cir cuit t er m inat ing equipm ent , and it s pur pose is t o link t he DTE t o t he com m unicat ion line.

Var ious asy nchr onous st andar ds define t he int er face or signaling t hat goes on bet w een t he DTE and t he DCE, and bet w een t he t w o DCEs. The connect ion bet w een t w o m odem s is an or dinar y t elephone line. Because t he connect ion is a dial- up connect ion, it is also called a swit ched line. For t his r eason, t he t elephone lines ar e k now n as a public sw it ched t elephone net w or k ( PSTN) . MCI , Spr int , and AT&T ar e all PSTNs. Som et im es t he t elephone lines ar e sim ply r efer r ed t o as t he plain old t elephone sy st em ( POTS) . An or dinar y phone line has a phy sical lim it at ion in t r ansm it t ing dat a. The bandw idt h of a an or dinar y phone line lies w it hin t he r ange of appr ox im at ely 300 t o 3,000 Hz—a r ange t hat cov er s t he m ost fr equent ly used spok en t ones, but t hat is far less t han t he full r ange of hum an hear ing. PSTN bandw idt h is com par able t o less t han half t he play able not es on a st andar d piano. This bandw idt h lim it at ion for t elephone lines m eans t hat t he m ax im um of 2,400 dist inct t ones per second can be t r ansm it t ed on t he line. This dat a t r ansfer r at e is m easur ed as t he baud r at e. A baud is a single change of st at e in t he analog signal per second. This change m ay be in fr equency , am plit ude, or phase of t he car r ier signal. Ther efor e, 2400 baud is t he m ax im um t hr oughput a PSTN w ill allow w it hout t he aid of som e m odulat ion schem e t o incr ease t he bps dat a t hr oughput for t he sam e m ax im um baud r at e. Som et im es a sim ple t elephone line is a v er y fast phy sical link t hat is split at each end int o m any slow er lines, each of w hich oper at es as an indiv idual phone line. For ex am ple, a st at ist ical m ult iplexer is one piece of equipm ent t hat split s a high- speed ( T1) t r unk line int o m any slow er ( 9600 bps) lines so t hat each of t he slow er lines appear s t o have a dedicat ed m odem at t ached t o it . Anot her v ar iat ion on t his sam e pr inciple is t he im plem ent at ion of a hunt group. Using a hunt gr oup ov er a high bandw idt h line, one phone num ber is r out ed t o a ser ies of 5, 10, or m or e differ ent inbound ex t ensions, each of w hich has an indiv idual m odem . The hunt gr oup finds t he fir st av ailable m odem fr om t he bank of m odem s and connect s t he caller t o t hat m odem . This pr ocess cont inues for each successiv e caller unt il all av ailable m odem s ar e in use. All subsequent caller s get a busy signal unt il one of t he m odem s is fr ee t o t ake a call. Modem s t ak e digit al input fr om t er m inals or com put er s and m ap t he discr eet bit s ont o an analog sine w ave ( t he car r ier w ave) . The car r ier and t he m odulat ed signal ar e t r ansm it t ed t o t he r em ot e m odem , w hich st r ips off t he signal r iding t he car r ier , conv er t s it back int o a digit al bit st r eam , and t hen sends t he r e- digit ized dat a st r eam t o t he r em ot e com put er or t er m inal. The pr ocess is called m odulat iondem odulat ion, fr om w hich t he m odem t ak es it s nam e. Ther e ar e five or six m aj or schem es called m odulat ion st andar ds for m apping or unm apping t he digit al signal ont o or fr om t he car r ier fr equency. A discussion of t hese m odulat ion st andar ds is beyond t he scope of t his paper ; how ev er , t he m odulat ion st andar d at t r ibut es discussed in t he nex t sect ion ar e im por t ant t o under st and.

M odu la t ion St a n da r d At t r ibu t e s The fr equency of t he car r ier sine w av e, how m any car r ier w av es t her e act ually ar e, t he phy sical m odulat ion t echnique, and t he speed at w hich t he t w o m odem s can m odulat e or dem odulat e t he digit al signal ar e all at t r ibut es of t he m odulat ion st andar d. For t w o m odem s t o com m unicat e, t hey m ust be able t o fir st negot iat e t o a com m on m odulat ion st andar d. Not ice t hat t he m odulat ion t echnique is not equiv alent t o t he m odulat ion st andar d. The follow ing is a list of som e of t he m ost com m only used m odulat ion t echniques: l

l

l

l

Fr e qu e n cy sh ift k e yin g ( FSK) : This t echnique is built on t he pr inciple of fr equency m odulat ion ( FM) of t he car r ier signal. Ph a se sh ift k e yin g ( PSK) : This t echnique is built on t he m et hod of phase m odulat ion ( PM) of t he car r ier signal. Qu a dr a t u r e a m plit u de m odu la t ion ( QAM ) : This t echnique inv olv es changing t he am plit ude and fr equency of t he car r ier w ave at t he sam e t im e. Tr e llis codin g m odu la t ion ( TC or TCM ) : This is a v ar iant on QAM t hat includes a m et hod for for war d er r or cor r ect ion; t hat is, er r or cor r ect ing infor m at ion is alr eady car r ied wit h t he dat a,

r at her t han r equir ing r et r ansm ission t o cor r ect er r or s. Any one of t he m odulat ion st andar ds can use t he sam e m odulat ion t echnique as anot her st andar d; how ev er , t he t w o m odulat ion st andar ds st ill m ay not be com pat ible if t hey do not specify t he sam e am plit udes or fr equencies.

ASYNCHRONOUS DATA STREAM The follow ing gr aphic illust r at es t he bit s in an asy nchr onous dat a st r eam .

Each char act er or byt e is m apped or coded ont o t he car r ier signal as a ser ies of pulses. Each char act er is pr eceded by a st ar t pulse or st ar t bit , and is follow ed by a st op pulse or st op bit . Each pulse is t ypically about 0.0001 seconds long or wide. The st op bit is usually 1, 1.5, or 2 bit t im es wide. A dat a packet in it s sim plest for m consist s of t he st ar t bit , sev en or eight dat a bit s, and one or t w o st op bit s. I f par it y is used t her e ar e only 7 dat a bit s. The am ount of idle t im e bet w een t he char act er s v ar ies. Anot her w ay of say ing t his is t hat pack et s ar e spaced asynchronously along t he car r ier w ave, giving t he pr ot ocol it s nam e. Som e of t he t er m inology fr om t he ear liest day s of t he t elecom m unicat ions indust r y st ill sur v iv es t oday . An im plem ent at ion of t he t elegr aph used a st y lus t o place a m ark on a piece of paper . Lift ing t he st y lus r esult ed in a space. Today, w hen t he line is idle ( no dat a m apped ont o t he car r ier w ave) , w e say t he v olt age lev el is in a m ark st at e. A st ar t bit is a change t o t he space st at e. The st at e is alt er ed fr om a m ark t o a space or a space t o a m ark t o code t he dat a bit s ont o t he carrier wave. A dat a bit represent ed by a space is int er pr et ed as a logical zer o ( 0) . A dat a bit r epr esent ed by a m ar k is int er pr et ed as a logical one ( 1) . Bit s are som et im es dropped because of noise. A parit y check can be added t o t he dat a packet t o recover fr om er r or s in which a single bit is dr opped or added. Many com m unicat ion soft war e packages allow you t o configur e par it y set t ings, or y ou m ay also be able t o specify par it y infor m at ion t hr ough t he ser ial por t configur at ion opt ions in an oper at ing sy st em such as t he Micr osoft ® Window s® oper at ing syst em . Par it y , t he num ber of st op bit s, and t he num ber of dat a bit s, can all be set as ser ial por t par am et er s. I f a par it y bit is used, t hen an ex t r a bit is added t o each char act er t o m ak e t he t ot al num ber of ones ( 1s) in t he char act er eit her odd or even, depending on w hich par it y t ype is used. I f you configur e your com m unicat ion soft w ar e t o use par it y bit s, t hen t her e ar e sev en dat a bit s. I f par it y is not used ( set t o Pa r it y N on e ) , t hen t her e ar e eight dat a bit s.

Som e par t s of t he phy sical cabling in RS- 232 dial- up connect ions ar e t wo- wire; for exam ple t he connect ion bet w een t he com put er and t he m odem . Older m odem s and m odulat ion st andar ds can only be oper at ed in half- duplex t r ansm ission m ode ov er phy sical t w o- w ir e connect ions. Half duplex m eans t hat t r ansm ission can only happen in one dir ect ion at a t im e. When a m odem r eceiv ing in half- duplex m ode w ant s t o send dat a, it signals t he sending m odem . When t he sender det ect s t he t urn around r equest , it st ar t s r eceiv ing and t he fir st m odem sends. Not sending and r eceiv ing in bot h dir ect ions at once is obv iously m uch slow er t han full duplex m ode, and t he line t ur n ar ound r equest s delay t he dat a t r ansm ission ev en m or e. I t is m uch fast er t o t r ansm it in bot h dir ect ions sim ult aneously , in full- duplex m ode. The car r ier fr equency defines t he t ot al bandw idt h of t he connect ion. Modem s can split t he car r ier fr equency int o t w o separ at e bandw idt hs or channels t o pr ovide t w o- w ay full- duplex t r ansm ission over a t w o- w ir e phy sical connect ion.

ASYNCHRONOUS vs. SYNCHRONOUS TRANSMI SSI ON The follow ing gr aphic illust r at es an asy nchr onous v er sus a sy nchr onous dat a st r eam .

I n asy nchr onous t r ansm ission, dat a is t r ansm it t ed one char act er at a t im e. Each char act er is br ack et ed by a st ar t bit and one or m or e st op bit s. Asy nchr onous t r ansm ission is also called st art - st op t r ansm ission. The asy nchr onous char act er s ar e not ev enly spaced along t he t r ansm ission m edium . I n gaps bet w een char act er s, t he line is idle; not hing is t r ansm it t ed. The char act er s ar e t r ansm it t ed independent ly w it h r egar d t o t im ing signals. I n sy nchr onous t r ansm ission an ex t r a signal, t he clock signal, is added t o t he RS- 232 leads. Usually , t he t w o sy nchr onous m odem s supply t he clock signal, alt hough t he DTE ( PC) can also supply t he clock signal. Configur ing a synchr onous por t for ext ernal clock m eans t he m odem supplies t he clock signal. I n synchr onous t r ansm ission, char act er s ar e gr ouped t oget her in lar ge blocks of dat a, som et im es 2,000 t o 4,000 char act er s t o a block. Ther e ar e no spaces bet ween char act er s in t he block, wher eas t her e ar e alw ays spaces bet w een asynchr onous char act er s, and t her efor e, som e idle line t im e. A cont r ol char act er , called a SYN char act er , always pr ecedes t he dat a block. Usually t wo or t hr ee SYN char act er s pr ecede t he dat a. The block st ar t s w it h a st ar t - of- t ex t ( STX) char act er . An end- of- t ex t ( ETX) char act er t er m inat es t he block.

Dur ing a t r ansm ission st r eam , block s ar e ev enly spaced along t he m edium w it h r espect t o t im e. I f no dat a is being t r ansm it t ed, SYN char act er s ar e t r ansm it t ed per iodically t o k eep t he t w o m odem s synched up. Gener ally , m odem s can achiev e a higher bit r at e using sy nchr onous t r ansm ission, but such t r ansm ission r equir es m or e com plex and t her efor e m or e expensive har dwar e because of t he need for a clocking signal. Also, if you send a lar ge block of dat a synchr onously and an er r or occur s, t her e is a gr eat er speed penalt y for r e- t r ansm it t ing t he lar ge pack et , as opposed t o j ust r e- t r ansm it t ing a single cor r ect ed pack et w hen using asy nchr onous t r ansm ission. How ev er , in asy nchr onous t r ansm ission, t he added need for a st ar t bit and one or m or e st op bit s per char act er m ay add 10 t o 20 per cent t o t he size of t he t r ansm ission. Nonet heless, asy nchr onous m odem s cur r ent ly enj oy w ider use.

EI A STANDARDS The EI A issued RS- 232- C in 1969. I t t hen issued t he RS- 449, RS- 422, and RS- 423 st andar ds in 1970, expect ing t hat t hey w ould r eplace RS- 232- C. The RS- 4xx st andar ds w er e designed t o suppor t higher m odulat ion r at es t han RS- 232. How ev er , equipm ent m anufact ur er s cont inued t o use RS- 232- C, m ak ing it t he de fact o st andar d. I n 1987, t he EI A issued t he RS- 232- D and RS- 530 st andar ds. The EI A st andar ds define t he phy sical- lay er cir cuit s and basic signaling. Ot her st andar ds, such as dat a com pr ession or er r or cor r ect ion st andar ds, ar e coded in t he dat a- link layer . I n it s sim plest for m , asy nchr onous com m unicat ion r equir es j ust t he signals of t he phy sical RS- 232 lay er and t he basic m odulat ion st andar d. Get t ing t w o differ ent m odem s t o com m unicat e som et im es m eans negot iat ing dow n t o t he RS- 232 st andar d and an older m odulat ion st andar d. The RS- 232- D st andar d defines ex t r a t est signals bet w een m odem s, for m ally specifies t he DB- 25 pin connect or , and defines a prot ect ed ground conduct or for ext r a shielding fr om st r ay r adio fr equency ( RF) signals. I n Eur ope, t he I TU- T V.24 st andar d is t he m ost w idely used. ( I TU- T st ands for I nt er nat ional Telecom m unicat ions Union - Telecom m unicat ions St andar ds Sect or .) I TU- T V.24 is near ly ident ical t o t he RS- 232- C specificat ion. Ther efor e, in pr act ical t er m s, you ar e not likely t o encount er m odem devices claim ing t o confor m t o t he RS- 4xx st andar d. Alm ost all m odem devices and connect or s w ill confor m t o eit her RS- 232C, RS- 232D, or I TU- T V.24 st andar ds, and t hese t hr ee st andar ds can be consider ed t o be equiv alent for m ost real world configur at ions.

ASYNCHRONOUS CI RCUI TS AND PI N ASSI GNMENTS

This diagr am show s t he signals used in asy nchr onous com m unicat ion and t he connect or pin num ber t he cir cuit s ar e assigned t o by t he RS- 232C st andar d. Two t ypes of physical connect or s or plugs ar e used. The left colum n show s t he pin assignm ent s for 9- pin cabling, and in t he r ight colum n, pin assignm ent s for 25- pin cables and connect or s ar e show n. The signals do t he sam e t hing for bot h t ypes of physical

connect or s. Handshaking bet w een a com put er ( DTE) and a m odem ( DCE) is done by r aising and low er ing volt age lev els on t hese pins. Each of t he r epr esent ed cir cuit s m ay or m ay not hav e a cor r esponding indicat or light on t he m odem . This light , if pr esent , is k now n as a light - em it t ing diode ( LED) . RS- 232- C defines nine elect r ical cir cuit s t hat ar e used for t he pr ot ocol handshak ing. The follow ing sect ions descr ibe w hat t he nine cir cuit s do.

Ca r r ie r D e t e ct ( CD ) When t he local m odem det ect s a car r ier signal fr om t he r em ot e m odem , it r aises t he v olt age lev el on t his lead, indicat ing t o t he com put er t hat a car r ier signal is being r eceiv ed fr om t he r em ot e m odem . This signal is also called t he Receiv ed Line Signal Det ect or ( RLSD) . When a car r ier is det ect ed, t he CD indicat or on t he local m odem goes on solid. I f t he CD LED goes off, t he connect ion has been dr opped. The car r ier signal is a t one or fr equency . You can hear it usually j ust aft er y ou hear t he or iginat ing m odem dial. I t is t he sine w av e signal t hat digit al dat a fr om t he t w o com put er s is m apped ont o by t he m odem 's m odulat ion funct ion. Wit hout a car r ier t her e can be no connect ion. I f t his signal is dr opped dur ing a connect ion, y our com m unicat ion soft w ar e w ill usually display a lost carrier m essage. I f a carrier is not det ect ed at t he st ar t of a connect ion, t he calling or or iginat ing m odem display s no carrier. I f t he CD indicat or is on for a lit t le w hile and t hen a car r ier is dr opped, it pr obably m eans t he t w o m odem s could not negot iat e t o a com m on set of st andar ds. This lead is essent ial t o asy nchr onous com m unicat ion. I f t he m odem does not hav e a panel LED t o indicat e CD, t ur n on t he speak er and list en for a car r ier . Clear t o Send ( CTS) w ill not be signaled t o t he ser ial por t if CD is not det ect ed. Ther e should be a CTS LED, especially if t her e is no indicat or for a car r ier .

Re ce ive D a t a ( RD ) When t he local m odem dem odulat es dat a r eceived fr om t he r em ot e DCE ( m odem ) , t he dem odulat ed dat a is sent t o t he com put er ser ial por t by r aising and low er ing t he v olt age lev el on t his lead. The m odem LED m ay be variously labeled as " RD," " Rx," or " RxD," depending on t he m odem m anufact urer. When t he RD indicat or is blinking r apidly, it m eans you ar e r eceiving dat a fr om t he r em ot e m odem . When RD goes off, t he r em ot e m odem has st opped sending. This lead is essent ial t o asy nchr onous com m unicat ion.

Tr a n sm it D a t a ( TD ) The com put er sends dat a t o t he local m odem by changing t he v olt age lev el on t he Transm it Dat a cir cuit , pin 3 on 9- pin RS232- C connect or s. The Tr ansm it Dat a LED m ay be labeled as " TD," " SD," " Tx," or " Tx D," depending on t he m anufact ur er . I f t he com put er is sending dat a, t he TD indicat or on t he m odem should be blink ing r apidly . At t he sam e t im e, t he RD indicat or ( LED) on t he r em ot e m odem should be blink ing r apidly t o show dat a is being sent t o t he r em ot e com put er . This lead is essent ial t o asy nchr onous com m unicat ion.

D a t a Te r m in a l Re a dy ( D TR) The com put er t ur ns t his cir cuit on ( asser t s DTR) w hen it is r eady t o be connect ed t o t he phone line. I f t he m odem does not get t his " r eady" signal, no at t em pt t o dial can occur and no com m ands fr om t he com put er w ill be r eceived. I f t he m odem indicat or panel has a DTR LED, it should be on solid befor e dialing can be at t em pt ed. Som et im es t he LED is labeled TR ( Ter m inal Ready ) inst ead of DTR.

Sign a l Gr ou n d ( SG) Signal Gr ound ( SG) pr ovides a r efer ence level or benchm ar k volt age for t he ot her leads ( cir cuit s) . Asy nchr onous signals ( ev ent s, st at e changes) ar e sent bet w een t he ser ial por t and t he m odem by v olt age changes in cir cuit s. For exam ple, t he com put er sends or asser t s DTR t o t he m odem by changing

t he v olt age on pin 4 t o t he v olt age lev el t hat indicat es " ON" . The v olt age lev el, act ually a r ange, is dict at ed by t he RS232- C st andar d. However , for t he m odem t o m ake sense of t he volt age change, a const ant benchm ar k v olt age is needed; t his is t he v olt age lev el on pin 5, SG, w hich is consider ed t o be t he zer o volt age level. Ther e is no LED indicat or for SG. Not e t hat t her e is anot her gr ound, called prot ect ive ground, w hich is lik e t he t hir d pr ong on a t hr eepr ong appliance plug. Pr ot ect iv e gr ound is pin 1 in 25- pin connect or s, and if it is pr esent , SG is usually st r apped or connect ed t o pr ot ect ive gr ound ( chassis gr ound) . This pair ing is not possible on a 9- pin connect or because t he DB- 9 does not have a pr ot ect ive gr ound lead.

D a t a Se t Re a dy ( D SR) The m odem t ur ns t his cir cuit on ( asser t s DSR t o t he com put er ) w hen it is r eady and phy sically connect ed t o t he phone line. I f t he t er m inal does not get t his r eady signal, any at t em pt t o dial w ill fail. I f t he m odem indicat or panel has a DSR LED, it should be on solid. The com put er applicat ion w ill not be able t o dial if t his signal is not pr esent fr om t he m odem . Usually , t he m odem indicat or LED is labeled MR ( Modem Ready) inst ead of DSR.

Re qu e st To Se n d ( RTS) The RTS cir cuit is t ur ned on by t he ser ial por t t o t ell t he m odem t hat it act ually has dat a queued for t r ansm ission. I f t he m odem has CTS ( Clear t o Send) t ur ned on, act ual t r ansm ission w ill st ar t . CTS off, signaled by t he m odem t o t he com put er , t ells t he ser ial por t t hat t he m odem t em por ar ily cannot accept any dat a for t r ansm ission. When t he asy nchr onous applicat ion w r it es a buffer dest ined for t he r em ot e applicat ion, t his causes t he ser ial por t t o r aise ( t ur n on) RTS. RTS and CTS ar e used for flow cont rol bet w een DTE and DCE. ( See t he Flow Cont r ol sect ion of t his docum ent for m or e infor m at ion.) Usually, t her e is no LED indicat or on t he m odem fr ont panel for RTS. Do not confuse Request To Send ( RTS) wit h Send Dat a ( SD, or alt er nat ively TxD, TD, or Tx) . SD is alw ay s a panel LED and blink s r apidly t o show t hat t he m odem is act ually r eceiv ing dat a fr om t he DTE. RTS is a st at e pr epar at or y t o act ually sending t o t he m odem . I t is lik e a quer y by t he DTE, ask ing if t he m odem is ready t o receive dat a.

Cle a r To Se n d ( CTS) The CTS cir cuit is t ur ned on by t he m odem t o signal t he ser ial por t t hat t he m odem w ant s t o w r it e t o t he com put er . I f t he ser ial por t has Request To Send ( RTS) t ur ned on ( r esponds w it h RTS) , t he m odem w ill act ually t r ansfer dat a t o t he ser ial por t . This is, of cour se, dat a r eceived fr om t he r em ot e m odem . Dat a is dem odulat ed as it is t r ansfer r ed t o t he ser ial por t . Dat a is accum ulat ed by t e by by t e in a buffer by t he ser ial por t . When t he buffer is full, it is t r ansfer r ed t o an applicat ion buffer and is t hen r eady t o be filled again by t he m odem . I f t he ser ial por t cannot accept dat a fr om t he m odem ( DCE) , it t ur ns RTS off and t he m odem w ait s for RTS on. I f RTS is off m ost of t he t im e, it usually m eans t he applicat ion is t oo slow . The m odem is r eceiving fr om t he r em ot e DCE fast er t han t he local applicat ion can r ead and pr ocess dat a. Som et im es t her e is a CTS indicat or on t he m odem panel. I f t her e is an LED for CTS, it should eit her be on solid ( especially w hen t he local DTE is supposed t o be r eceiving lot s of dat a) or blink occasionally. Do not confuse Clear To Send ( CTS) wit h Receive Dat a ( RD, or alt ernat ively Rx or RxD) . RD is always a panel LED and blink s r apidly t o show t hat t he m odem is act ually w r it ing dat a t o t he ser ial por t . CTS is a st at e pr epar at or y t o act ually w r it ing t o t he com put er by m odem . I t is like a quer y by t he m odem ( DCE) , asking if t he DTE is r eady t o r eceive dat a.

Rin g I n dica t or ( RI ) The RI cir cuit is t ur ned on by t he m odem t o signal t he ser ial por t t hat t her e is an inbound call. The line r ings for one second, and t hen t her e is a four second pause. The RI cir cuit is t ur ned on for each r ing. I t serves as a wake- up signal t o t he DTE. The DTE r esponds by asser t ing DTR t o t ell t he m odem t hat it is r eady t o be connect ed over t he t elephone com pany line. I f Car r ier ( CD) is det ect ed, t hen dat a t r ansm ission st ar t s.

The follow ing gr aphic illust r at es t he sequence of ev ent s t hat occur s in RS- 232 com m unicat ion.

The RTS/ CTS and DTR/ DSR pins ar e t he m ain handshak ing signals. Pin assignm ent s for bot h 9- pin and 25- pin connect or s ar e giv en in t he t ex t below . The 9- pin is alw ay s st at ed fir st . The num ber s in t he diagr am above shows t he or der or sequence of t he signaling when a m odem r eceives a call and dat a t r ansfer occur s in full- duplex m ode. The signals ar e discussed below in t he or der of t he above sequence.

D SR When t he m odem is pow er ed on, it asser t s DSR t o t he ser ial por t by r aising t he volt age on pin 6. As st at ed ear lier , t her e usually is a m odem LED labeled MR ( Modem Ready) , w hich should be on.

RI a n d D TR When an incom ing call is r eceiv ed, t he m odem r aises t he RI v olt age ( pin 9 or 22) . When t he ser ial por t det ect s t he RI v olt age, it asser t s DTR, pin 4 or 20. When t he m odem r aises RI , it m ust t hen det ect DTR fr om t he com put er . The RI m odem LED will go on. Som et im es t her e is a DTR or TR ( Ter m inal Ready) LED on t he m odem panel. I t should light shor t ly aft er t he RI LED light s. Opt ionally , t he com put er can also t ur n on DTR w hen it is boot ed, pr ior t o r eceiv ing t he RI signal.

CD The calling m odem t r ansm it s a car r ier t one t o t he answ er ing m odem . When t he answ er ing m odem det ect s t he car r ier t one, it k now s t he cir cuit t o t he r em ot e m odem is com plet e. The answ er ing m odem signals it s pr esence t o t he com put er by r aising t he v olt age on pin 1 or 8. When t he ser ial por t det ect s RI and CD, it k now s t her e is an incom ing call.

RTS To send dat a, t he com put er asser t s RTS by r aising t he v olt age on pin 7 or 4. Ev en t hough in t his ex am ple t he local DTE did not init iat e t he call, it m ust fir st signal t he r em ot e DTE t hat it is r eady t o r eceiv e dat a. At t his point , t he com put er applicat ion w ill usually t r ansm it a logon or hello m essage t o t he or iginat ing end ( Held, page 88) . Raising RTS on t he r eceiv ing DTE w ill r esult in t ur ning on CD at t he r em ot e or calling m odem ( McNam ar a, page 57) . This will occur as soon as CTS gives t he " OK."

CTS

The m odem r esponds t o RTS by asser t ing CTS. I t does t his by r aising t he volt age on pin 8 or 5. When t he com put er det ect s CTS, it k now s t hat t he m odem is r eady t o send t he ready t o receive m essage t o t he r em ot e DTE.

RD a n d TD At t his t im e, t he com put er w ill r eceiv e dat a on t he RD pin ( pin 2 or 3) and t r ansm it dat a on t he TD pin ( pin 3 or 2) . The m odem TD and RD LEDs should blink r apidly. Not ice t hat t he per ceiv ed or der is differ ent for t he sending m odem , and t he ex act handshak ing sequence for your m odem m ay var y slight ly fr om t he sequence pr esent ed her e. The sequence w ill var y bet w een a half- duplex and full- duplex t r ansm ission. Also, som e lat it ude ex ist s in t he RS- 232 st andar d for specific im plem ent at ion in bot h t he har dw ar e and com m unicat ion soft w ar e ( McNam ar a, pages 53, 57, 80- 86) .

DATA FLOW Gener ally t he r at e of dat a t r ansfer fr om t he com put er t hr ough t he ser ial por t t o t he m odem ( k now n as t he DTE r at e or t he local t r ansm ission r at e) is m uch fast er t han t he t r ansm ission speed ov er t he m odem t o- m odem connect ion ( r efer r ed t o as t he link r at e) . This is because cur r ent v er sions of t he UART chip ar e capable of sending ser ial dat a t o t he m odem at a r at e of 115.2 Kbps, w hile t he t ypical m odem t r ansfer r at e is m uch less t han t hat . Wit h t his k ind of differ ence in dat a t r ansfer r at es, t he pot ent ial ex ist s t hat t he com put er w ill over w helm t he m odem w it h dat a ( som et im es r efer r ed t o as dat a overrun) , causing eit her lost dat a or t he need for r et r ansm issions. How ev er , t he r ev er se of t his sit uat ion can also occur if a new er high- speed m odem is connect ed w it h an older com put er t hat has an ear lier v er sion of t he UART chip inst alled. Som e of t he ear lier UARTs wer e only capable of a m axim um speed of 28.8 Kbps or slower . Ther efor e, it is im por t ant t hat y our m odem is capable of handling t he speed of t he dat a t r ansfer fr om t he ser ial por t of t he com put er and vice ver sa.

One of t he ways t hat com put er s and m odem s t ake car e of differ ences bet ween dat a t r ansfer r at es is t hr ough t he use of buffer s. Bot h t he com m unicat ion applicat ion on t he DTE side and t he m odem on t he DCE side can use buffer s as a t em por ar y st or age locat ion for dat a. However , pr oblem s can st ill occur ev en w it h buffer s in place. When t he asy nchr onous applicat ion does a w r it e, t he by t es ar e queued in one or m or e buffers. The com put er oper at ing sy st em t hen em pt ies t he applicat ion buffer s by sending or w r it ing t he by t es out t he

ser ial por t t o a buffer in t he m odem . The asy nchr onous applicat ion in a single w r it e can fill a lar ge buffer ; for exam ple, 1024 byt es. Because t he m odem sends or t r ansm it s dat a one byt e at a t im e over t he line, t he m odem buffer can be ov er w helm ed. Applicat ion dat a in t he m odem buffer can be ov er w r it t en and lost . When t he applicat ion is r eading dat a, t he m odem can t r ansm it t oo fast , causing dat a in t he applicat ion buffer s t o be ov er w r it t en. To k eep t his fr om happening, for dat a going in eit her dir ect ion, flow cont rol is im plem ent ed. When t he m odem 's buffer is so full t hat addit ional dat a w r it t en int o t he buffer causes dat a t o be ov er w r it t en and lost , t he m odem has t o signal t he ser ial por t t o st op sending dat a. For inbound dat a, if t he applicat ion can't pr ocess t he dat a fast enough ( t hat is, it em pt ies t he m odem buffer t oo slow ly ) , t hen t he ser ial por t has t o signal t he m odem t o st op sending. This handshak ing or signaling is called flow cont r ol. Typically, flow cont r ol w ill be im plem ent ed in one of t hr ee w ays: RTS/ CTS or har dw ar e flow cont rol; XON/ XOFF or soft ware flow cont rol; or ENQ/ ACK or Enquire / acknowledge flow cont rol.

RTS/ CTS or H a r dw a r e Flow Con t r ol Wit h RTS/ CTS flow cont r ol, w hen t he ser ial por t w ant s t o send dat a t o t he m odem , it signals Request To Send ( RTS) t o t he m odem . I f t he m odem can accept t he dat a ( t hat is, t he buffer not t oo full) , it r et ur ns a Clear To Send ( CTS) t o t he ser ial por t . I f t he m odem cannot accept t he dat a, it t ur ns off t he CTS signal. On a 9- pin connect or , RTS is signaled by r aising or low er ing v olt age lev els on pin 7; CTS is signaled by r aising or low er ing t he volt age on pin 8. For exam ple, if t he m odem cannot r eceive any m or e dat a, it dr ops t he v olt age on pin 8, t ur ning off CTS. Som et im es t he applicat ion cannot pr ocess it s buffer s fast enough. To keep t he m odem fr om over - w r it ing dat a, t he applicat ion can t ell t he ser ial por t t o dr op RTS ( low er t he volt age level on pin 7) . The DCE ( m odem ) senses t hat RTS is dr opped and st ops filling t he applicat ion buffer . When t he applicat ion has em pt ied som e of it s buffer s, it r aises RTS again and t he m odem st ar t s filling t he buffer .

XON / XOFF or Soft w a r e Flow Con t r ol When a m odem is r eceiving dat a fr om t he local t er m inal or ser ial por t t oo fast for t he m odem t o pr ocess t he dat a, t he m odem w ill send an ASCI I 19 ( CTRL+ S) or DC3 ( device cont r ol 3) char act er t o t he ser ial por t . This is t he default XOFF char act er . I t is sent on pin 3 ( RD) . When t he m odem buffer is no longer full ( t hat is, dat a has been t r ansm it t ed t o t he r em ot e m odem ) , t he m odem sends t he XON char act er —an ASCI I 17 ( CTRL+ Q) or DC1 char act er t o t he ser ial por t , and t he ser ial por t st ar t s sending dat a again. The char act er s used for XON/ XOFF can oft en be changed in t he applicat ion and t he m odem , if desir ed. One pr oblem wit h XON/ XOFF is t hat t he CTRL+ S and CTRL+ Q char act er s can be char act er s em bedded in a dat a file. The dat a is int er pr et ed as DC1 and DC3 char act er s, causing t r ansm ission er r or s. Modem s and DTEs using XON/ XOFF flow cont r ol do not need all nine RS- 232C cir cuit s or pins show n in t he RS- 232 Signals diagr am in t his docum ent . XON/ XOFF can, and oft en does, oper at e w it h j ust t hr ee pin- out s, CD, RD, and TD, alt hough it is adv isable t o also hav e at least SG. A cable t hat w or k s w it h an XON/ XOFF applicat ion m ay not w or k w it h an applicat ion t hat uses har dw ar e flow cont r ol, because applicat ions t hat use har dw ar e flow cont r ol need all nine signals or pins.

EN Q/ ACK or En qu ir e / Ack n ow le dge Flow Con t r ol I n ENQ/ ACK flow cont r ol, w hen t he ser ial por t w ant s t o send dat a, it t r ansm it s an ASCI I ENQ ( enquir e) char act er . I f t he m odem can accept dat a, it ACKs t he ser ial por t ( t r ansm it s back t he ACK, posit ive acknowledgm ent char act er ) . I f it can't accept t he dat a ( buffer full) , t he device or m odem sends back a NAK, or negat ive acknowledgm ent , in r esponse t o t he ENQ. ENQ/ ACK flow cont r ol is used in som e t ypes of Hewlet t - Packar d com put er s and devices.

I TU- T AND PROPRI ETARY STANDARDS The Consult at iv e Com m it t ee on I nt er nat ional Telegr aphy and Telephony ( CCI TT) w as r esponsible for fir st pr oposing int er nat ional st andar ds for dat a com m unicat ion ov er t elephone lines using m odem s. Their st andar ds begin w it h t he let t er " V" and ar e t her efor e know n as t he " V ser ies" st andar ds. Som et im es a

given st andard m ay be followed by t he word bis w hich is Fr ench for " second." This indicat es t he fir st r evision of t he or iginal st andar d. The w or d t er, or " t hir d," is used t o r epr esent t he second r evision of a par t icular st andar d. The CCI TT ev ent ually changed it s nam e and is now k now n as t he I nt er nat ional Telecom m unicat ions Union - Telecom m unicat ions St andar ds Sect or ( I TU- T) . I n addit ion t o t he official int er nat ional st andar ds, som e pr opr iet ar y st andar ds have becom e w idely accept ed by t he indust r y. A good exam ple of such a st andar d cr eat ed by a m odem m anufact ur er is t he Microcom Net working Prot ocol ( MNP) . MNP defines t en levels of dat a com pression and error correct ion schem es. MNP begins w it h t he sim plest level, Level 1, and pr ogr esses t o t he m ost advanced level, cur r ent ly Level 10. MNP Level 1 is obsolet e and no longer im plem ent ed by m anufact ur er s. Level 8 w as nev er im plem ent ed. Lev els 1 t hr ough 4 ar e now par t of t he public dom ain, and m odem m anufact ur er s can license m ost of t he ot her lev els for use. I n t he next sect ion, som e of t he m ost im por t ant and w idely used st andar ds for er r or cor r ect ion, com pr ession, and m odulat ion ar e discussed. Before 1989, t here was no st andard for error correct ion and det ect ion. Several vendors had developed sophist icat ed er r or cor r ect ion and/ or det ect ion schem es based on cyclic r edundancy checks. By 1989, t he Micr ocom algor it hm s had becom e so w idely used t hat MNP w as t he de fact o st andar d. When t he I TU- T issued t he V.42 er r or cor r ect ion/ det ect ion st andar d in 1989, it m ade MNP an opt ional st andar d w it hin V.42. The I TU- T V.42 er r or cor r ect ion/ det ect ion st andar d is called LAP- M ( Link Access Prot ocol- Modem ) .

Cyclic Re du n da n cy Ch e ck ( CRC) When t he fir st m odem has accum ulat ed a buffer full of dat a for t r ansm ission, it applies a for m ula t o t he dat a block , calculat ing a v alue D. A second for m ula is applied t o t he dat a and t he r esult is div ided int o t he fir st v alue. The final r esult is a w hole num ber quot ient and a r em ainder . Apply 1st form ula t o dat a, result = D Apply 2nd form ula t o dat a, result = G Calculat e D/ G = Q + R D is t he r esult of apply ing t he fir st for m ula t o t he dat a block . G is t he v alue of t he second for m ula. The r em ainder , R, is t he cy clic r edundancy check ( CRC) . I t is usually 16 bit s long and is appended t o t he end of t he dat a block w hen t he m odem , w hich is not sending at t hat t im e, t r ansm it s. When t he r eceiv ing m odem get s t he dat a block , it goes t hr ough t he sam e calculat ion. I f t her e w as a t r ansm ission er r or , t he second CRC v alue w ill not equal t he or iginal one, and t he r eceiv ing m odem t r ansm it s a negat iv e ack now ledgm ent ( NAK) . The or iginat ing m odem t hen r e- t r ansm it s t he dat a block . I f t he t w o CRCs m at ch, t he r eceiv ing m odem t r ansm it s a posit iv e ack now ledgm ent ( ACK) . The m ost com m only used for m ula, or polynom ial, is called CRC- 16.

M icr ocom N e t w or k ing Pr ot ocol ( M N P) MNP m odem s connect fir st w it h a low er class pr ot ocol, and t hen negot iat e up t o t he highest class. The higher t he class, t he gr eat er t he t hr oughput . I f t he connect ion is t o a non- MNP m odem , t he MNP m odem w ill negot iat e dow n t o a dum b asynchronous st andard, probably basic RS- 232- C. Please under st and t hat error correct ion as it is com m only im plem ent ed by indust r y r eally m eans r et r ansm it t ing good copies of dat a pack et s t hat w er e cor r upt ed in t he or iginal t r ansm ission. Ret r ansm ission of packet s t akes t im e. Ther efor e, in gener al, er r or cor r ect ion slow s dat a t r ansm ission r at es. Error cont rol is t he t er m used t o r efer ence a m et hod for r educing r et r ansm issions. Because MNP lev els include ot her speed opt im izat ion schem es besides er r or cont r ol, t he im pr oved efficiencies of speed m ent ioned in t he follow ing par agr aphs ar e only par t ially t he r esult of im plem ent ing er r or cont r ol. Class 1 and Class 2 ar e sim ple MNP st andar ds t hat allow m odem s t o oper at e at less t han 100 per cent of

t he baud r at e. For exam ple, a 2400 bps m odem r unning MNP Class 2 will only oper at e at 2000 bps, or at appr oxim at ely 85 per cent of efficiency. Class 3 is a synchronous pr ot ocol t hat gives about 115 per cent efficiency . Under Class 3, t he t w o m odem s com m unicat e sy nchr onously , but t he t er m inal dev ice ( DTE) is asynchr onous. MNP does not offer m uch per for m ance benefit unt il at least Class 4 oper at ion is achieved. MNP er r or cor r ect ion, using t he CRC algor it hm , act ually st ar t s in Class 4. Modem s t hat can use MNP er r or cont r ol adver t ise or docum ent t hem selves as being " MNP Class 4 Com pliant ." Besides er r or cont r ol, MNP Class 4 uses t w o per for m ance enhancing t echniques, Adapt iv e Pack et Assem bly ( APA) and Dat a Phase Opt im izat ion ( DPO) , bot h of w hich im pr ov e t hr oughput . APA: I f t he phone connect ion is error free ( very few CRC errors) , t hen MNP assem bles larger dat apacket s for t r ansm ission. I f er r or s st ar t occur r ing, t he packet size get s sm aller . D PO: Dat a Phase Opt im izat ion elim inat es adm inist r at iv e ov er head in pack et t r ansm ission if t he line is error free. A Class 4 m odem gives 120 per cent efficiency, so t hat a 2400 bps m odem r unning Class 4 w ould appear t o operat e at 2900 bps.

I TU- T V.4 2 – Lin k Acce ss Pr ot ocol- M ode m ( LAP- M ) V.42 is t he I TU- T r ecom m endat ion for er r or det ect ion and cor r ect ion for asynchr onous t r ansm ission. Wit hin V.42, LAP- M is t he pr im ar y st andar d; how ev er , because MNP w as in such w idespr ead use, t he I TU included MNP as an opt ion under t he V.42 er r or cor r ect ion and det ect ion specificat ion. This m eans t hat som e V.42 m odem s support MNP Class- 4 and som e do not . Most V.42 m odem s incorporat e up t o MNP Class 4. At t he st ar t of t he handshak e, t he or iginat ing LAP- M m odem t r ansm it s a bit pat t er n called t he or iginat or det ect ion pat t er n ( ODP) . The answ er ing m odem r esponds w it h a bit pat t er n called t he answ er det ect ion pat t er n ( ADP) . The ADP for m at specifies t hat V.42 is suppor t ed or t hat no er r or cor r ect ion w ill be used. I f V.42 ( LAP- M) is not available, som et im es MNP is negot iat ed or t he m odem s oper at e w it h no er r or cor r ect ion. LAP- M uses a differ ent CRC for m ula and has a differ ent header t han MNP. This m eans MNP and LAP- M ar e not com pat ible. This sect ion cont ains t hr ee w idely used m et hods of dat a com pr ession, ex am ples of how t his t echnology w or k s. The fir st and sim plest m et hod is k now n as Run Lengt h Encoding ( RLE) . RLE count s r epeat ed char act er s or bit pat t er ns, and t hen r eplaces t he r epeat ed char act er s w it h a r epr esent at iv e bit pat t er n and a m ult iplier equal t o t he num ber of t im es t he pat t er n or char act er is r epeat ed. The Lem ple- Ziv - Welch ( LZW) algor it hm inspect s t he dat a for r ecur r ing sequences of infor m at ion. LZW t hen builds a dict ionar y of r epeat ed sequences. A point er t o t he appr opr iat e dict ionar y ent r y can now r eplace each r epeat ed sequence in t he dat a st r eam . Huffm an encoding look s at t he dat a and count s w hich char act er s ar e m ost fr equent ly used. These char act er s ar e assigned t he shor t est bit v alues. At t he sam e t im e, t he least fr equent ly occur r ing char act er s ar e assigned t he longest bit v alues. St andards for dat a com pression have about t he sam e hist ory as for error correct ion. By 1989 or 1990, t he com pr ession algor it hm s in Micr ocom 's MNP had becom e a de fact o st andar d, par t ly because Micr ocom licensed MNP widely t o ot her vendors. By 1990, 100 or so vendors were incorporat ing MNP com pression in t heir pr oduct s. I n 1990, t he I TU- T issued it s ow n com pr ession st andar d, called V.42bis; it did not r ecognize MNP com pr ession as an alt er nat ive, as it did w it h er r or cor r ect ion. You m ay w ant t o t ur n off dat a com pr ession if you ar e t r ying t o send a pr eviously com pr essed file such as a ZI P file. Recom pr essing an alr eady com pr essed file m ay act ually incr ease t he t ot al packaged file size. This incr ease w ill dim inish t he speed benefit gained fr om t he init ial com pr ession.

M N P– Cla ss 5

MNP Class 5 adds dat a com pr ession t o MNP Class 4. MNP Class 5 uses t he Run Lengt h Encoding ( RLE) algor it hm for dat a com pr ession at it s fir st level. However , MNP Class 5 dat a com pr ession has a second lev el, called adapt iv e fr equency encoding, t hat is applied aft er t he init ial r un- lengt h encoding. I n adapt ive frequency encoding, a t ok en is subst it ut ed for t he t hr ee r epeat ed dat a by t es. An at t em pt is m ade t o use t he sm allest t ok ens so t hat less t han 8 bit s ar e needed for each subst it ut ion. The sm aller ASCI I values ar e cont inuously changed or adapt ed t o r epr esent t he r eplaced char act er s. The t oken consist s of t hr ee bit s of header and t he body , w hich cont ains t he bit s r epr esent ing t he r epeat ed by t e. Com pr ession incr eases efficiency by 1.3 t o 2 t im es. MNP Class 5 com pr ession w ill t ypically incr ease efficiency by 1 t o 1.6 t im es, giv ing a 200 per cent incr ease in efficiency , so t hat a 2400bps m odem w ill give a 4800 bps t hr oughput .

M N P– Cla ss 7 En h a n ce d D a t a Com pr e ssion MNP Class 7 dat a com pr ession st ar t s w it h r un lengt h encoding; how ev er , as t he t r ansm ission cont inues, MNP Class 7 builds t he t ables needed t o enable Huffm an encoding. Class 7 com pr ession gains about 40 per cent efficiency, w hich is added t o Class 5 com pr ession efficiencies. An MNP Class 7 m odem can negot iat e down t o Class 5. Class 5 pr edat es Class 7 and is m or e popular.

V.4 2 bis– I TU- T D a t a Com pr e ssion Re com m e n da t ion V.42bis is t he I TU- T r ecom m endat ion for dat a com pr ession, issued in 1990. V. 42bis uses t he Lim pelZiv - Welch com pr ession algor it hm . V.42bis com pr ession m at ches a st r ing in t he dat a st r eam t o a st r ing st or ed in a ser ies of t ables ( st r ing dict ionar ies) and subst it ut es a code w or d for t he st r ing. Then t he code w or d is t r ansm it t ed. V.42bis is about 10 per cent m or e efficient t han MNP Class 7. This sect ion list s som e of t oday 's m ost im por t ant m odulat ion st andar ds. The easiest w ay t o classify a m odulat ion st andar d is by t he dat a t r ansfer r at e, or speed, m easur ed in bit s per second ( bps) . The ear ly st andards covered dat a t ransfer rat es of about 300 bps t o 4800 bps. Most of t he low and m edium speed st andar ds ( 300 bps t o 4800 bps) ar e eit her Bell st andar ds or CCI TT st andar ds. Most of t hese st andar ds ar e now obsolet e, alt hough som e w ill be included in a new m odem 's capabilit ies as a fallback st andar d for backw ar d com pat ibilit y w it h older t echnology. Not e t hat w hen t w o m odem s agr ee t o connect w it h a specific st andar d, it is st ill possible for t hem t o dy nam ically negot iat e cer t ain par am et er s w it hin t hat st andar d. Micr ocom m odem s, for exam ple, can adj ust for line noise by changing t he negot iat ed fr equency on t he fly . Cur r ent ly t he m ost w idely im plem ent ed and used m odulat ion st andar ds cov er dat a t r ansfer r at es fr om 9600 bps t o about 33,600 bps. V.32, V.32bis , V.34, and V.34- 1996 ar e t he I TU- T high- speed asynchr onous st andar ds. Ther e ar e also pr opr iet ar y high- speed m odulat ion st andar ds ( 9600 bps and fast er ) , and t her e is a cur r ent ly em er ging st andar d for a speed of 56,000 bps. Som et im es t w o differ ent m odulat ion st andar ds use t he sam e phy sical m odulat ion t echnique, but ar e st ill not com pat ible because som e ot her par t of t he st andar d is differ ent . For exam ple, t wo st andar ds m ay use t he sam e m odulat ion t echnique, but use differ ent fr equencies w it hin t he car r ier band, m ak ing t hem incom pat ible. Ev en adher ence t o t he sam e st andar d by t w o m odem m anufact ur er s does not necessar ily m ean t hat t he t w o m odem s w ill be able t o com m unicat e successfully . Adher ence t o a st andar d sim ply m eans t hat t he st andar d is not v iolat ed. And ev en if t he st andar d is not v iolat ed, one m anufact ur er m ay follow only one por t ion of t he st andar d's r equir em ent s, w hile t he ot her m anufact ur er m ay follow a differ ent par t of t he r equir em ent s for t he sam e st andar d. Finally, em er ging st andar ds can be developed ar ound new t echnologies, but differ ent m anufact ur er s m ay im plem ent t he new t echnology in a differ ent w ay , causing t em por ar y incom pat ibilit ies unt il t he de fact o st andar ds becom e for m alized int o int er nat ionally accept ed specificat ions. A cur r ent ex am ple of t his is t he t echnology facilit at ing 56,000 bps speeds. Tw o cur r ent im plem ent at ions of 56 Kbps t echnology , t he X2

and 56K Flex m odem s, ar e bot h pr opr iet ar y and incom pat ible w it h each ot her . At t he sam e t im e, a new int er nat ional st andar d for 56 Kbps, t he V.90 st andar d, is cur r ent ly being r efined by t he I TU- T. I t m ay be quit e som e t im e befor e t he leader s in 56 Kbps t echnologies pr oduce m odem s t hat t r uly adher e t o t he sam e V.90 st andar d and can also com m unicat e w it h each ot her .

I TU- T V.3 2 – 9 6 0 0 Bps St a n da r d I TU- T's V.32 st andar d w as issued in 1989 for asynchr onous, full- duplex oper at ion at 9600 bps. Alt hough designed for asy nchr onous DTEs, t w o V.32 m odem s act ually com m unicat e sy nchr onously . A cir cuit conv er t s t he asy nchr onous dat a st r eam int o sy nchr onous block s, inv isible t o t he applicat ion. V.32 support s m odulat ion rat es of 2400, 4800, and 9600 bps. Dat a com pression and error correct ion can incr ease t he t hr oughput r at es t o 14.4, 19.2, and fast er .

I TU- T V.3 2 bis– 1 4 .4 Kbps St a n da r d I TU- T's V.32 st andar d w as issued in 1991 for asynchr onous, full- duplex oper at ion at 14.4 Kbps. V.32bis is an ex t ension of t he V.32 t echnology . V.32bis suppor t s m odulat ion r at es of 2400, 4800, 9600 bps and 14.4 Kbps. Dat a com pr ession and er r or cor r ect ion can incr ease t he t hr oughput r at es.

V .FC This was an int er im m odem st andar d designed t o suppor t speeds up t o 28,800 bps. V.FC st ands for Ver sion Fast Class. This st andar d is pr opr iet ar y and is not com pat ible w it h t he I TU- T V. 34 at speeds fast er t han 14.4 Kbps.

I TU- T V.3 3 – 1 4 .4 Kbps St a n da r d I TU- T's V.33 st andar d w as issued in 1991 for synchr onous, full- duplex oper at ion at 9600 bps and 14,400 bps. Alt hough V.33 w as designed for synchr onous DTEs, conver sion cir cuit s can be added. The conv er sion of t he asy nchr onous dat a st r eam int o sy nchr onous block s is inv isible t o t he applicat ion. V.33 is an ex t ension of V.32 m odulat ion t echniques. V.33 suppor t s dat a r at es of 2400, 4800, 9600, and 14,400 bps, but pr opr iet ar y enhancem ent s by m odem vendor s, m ainly addit ion of dat a com pr ession, allows for fast er V.33 rat es.

I TU- T V.3 4 – 2 8 .8 Kbps St a n da r d By Sept em ber 1994, I TU- T's V.34 st andard was widely adopt ed over t he propriet ary 28.8 Kbps st andard V.FC ( V.Fast Class) . V.34 has been m or e widely adopt ed t han any ot her 28.8 Kbps st andar d.

I TU- T V.3 4 – 1 9 9 6 This is an em er ging st andar d for m odem speeds of 33.6 Kbps. While being for m ulat ed, t he st andar d has also been know n as t he v.34bis or t he v.34+ st andar d. The v.34- 1996 st andar d m odem is m or e r obust t han m odem s im plem ent ing t he v .34bis or v .34+ int er im st andar ds. Wit h t he v .34- 1996 st andar d m odem , user s ar e m uch m or e lik ely t o ex per ience a 28.8 Kbps connect ion on a noisy line w it h t he hope of at t aining possible speeds of 31.2 Kbps or 33.6 Kbps under opt im um condit ions. For t w o m odem s t o com m unicat e, t hey m ust fir st find a com m on m odulat ion st andar d. I f t w o m odem s connect br iefly and t hen t im e- out ( t hat is, t he CD LED com es on for sev er al seconds t hen goes off) , it is because t hey hav e failed t o find a com m on m odulat ion st andar d. The fir st t hing t he t w o m odem s negot iat e is t he m odulat ion st andar d; how ev er , som e m odem m anuals call t his " negot iat ing t he baud r at e," obscur ing t he fact t hat t her e is alw ay s t he possibilit y of a m odulat ion st andar ds com pat ibilit y issue. Check t he m odulat ion st andar ds suppor t ed in t he t w o m odem s' r efer ence guides. You m ay be able t o for ce a com m on st andar d if t he default negot iat ion fails. How ever , t he t w o m odem s m ust have a com m on m odulat ion st andar d, such as V.32 or V.34. Pr opr iet ar y m odulat ion st andar ds oft en offer fast er dat a r at es t han public st andar ds, such as Telebit PEP ( Pack et ized Ensem ble Pr ot ocol) and US Robot ics HST ( High Speed Asy nchr onous Tr ansm ission) .

How ever , a connect ion using a pr opr iet ar y m odulat ion st andar d can only be achieved if t he pr opr iet ar y st andar d is available in t he m odem s on bot h ends of t he connect ion. I f t he t w o m odem s ar e fr om differ ent vendor s or ar e differ ent m odels, t he best st r at egy for connect ing is t o look for a com m on I TU- T st andar d. Most m odem s w ill at t em pt t o negot iat e dow n fr om t he highest m odulat ion st andar d t hey ar e capable of t o t he low est m odulat ion st andar d t hat t hey ar e capable of. The follow ing ar e som e of t he m odulat ion st andards of last resort . Alm ost all m odem s suppor t V.22 ( 1200 bps) and V.22bis ( 2400 bps) . I f MNP Class- 5 dat a com pression can be added t o t he V.22 or V.22bis st andar ds, accept able t hr oughput m ay be achieved. For exam ple, Micr ocom QX ser ies achieves 19.2 kbps wit h V.22bis and MNP Class 4 and 5. Bell Sy st em st andar ds such as 103, 212, and 202( x ) r anging fr om 300 t o 1800 bps ( unless enhanced) ar e t oo slow t o be used for anyt hing ot her t han a t em por ar y connect ion solut ion. For exam ple, connect ing a Telebit m odem t o a US Robot ics m odem using t hese st andar ds allow s you t o connect t em por ar ily . Not e t hat Bell Sy st em st andar ds ar e not used in Eur opean m ar k et s. Ander son, Douglas, Pat r ick Dawson, and Michael Tr ibble. The Modem Technical Guide. Boulder, CO: Micr ohouse I nt er nat ional, 1996. Black, Uyless D. Dat a Link Prot ocols. Englew ood Cliffs, NJ: Pr ent ice Hall, 1993. Dvor ak, John C., Nick Anis, and Wer ner Feibel. Dvorak's Guide t o PC Connect ivit y. New York: Bant am Books, 1992. Held, Gilber t . The Com plet e Modem Reference, 2nd ed. New Yor k: John Wiley & Sons, 1994. Lewart , Cass R. The Ult im at e Modem Handbook. Upper Saddle River , NJ: Pr ent ice- Hall PTR, 1998. McNam ar a, John E. Technical Aspect s of Dat a Com m unicat ion, 3r d ed. Digit al Pr ess, 1998. Newt on, Harry. Newt on's Telecom Dict ionary, 9t h ed. New Yor k : Flat ir on Publishing, 1995. Richar d, Jack. " The 56K Modem Bat t le." Boardwat ch Magazine < www.boardwat ch.com > . Vol. XI : I ssue 3. Mar ch, 1998. Seyer , Mar t in D. RS- 232 Made Easy: Connect ing Com put ers, Print ers, Term inals, and Modem s, 2nd ed. Englew ood Cliffs, NJ: Pr ent ice- Hall, 1991. Cont r ibut or - Er ic Ber nst ein, EBCS, Com put er Service Engineer, er icber @oz.net . The inform at ion cont ained in t his docum ent represent s t he current view of Microsoft Corporat ion on t he issues discussed as of t he dat e of publicat ion. Because Microsoft m ust respond t o changing m arket condit ions, it should not be int erpret ed t o be a com m it m ent on t he part of Microsoft , and Microsoft cannot guarant ee t he accuracy of any inform at ion present ed aft er t he dat e of publicat ion. This docum ent is for inform at ional purposes only. MI CROSOFT MAKES NO WARRANTI ES, EXPRESS OR I MPLI ED, I N THI S DOCUMENT. © 1998 Microsoft Corporat ion. All right s reserved. Microsoft , Windows and Windows NT are eit her regist ered t radem arks or t radem arks of Microsoft Corporat ion in t he Unit ed St at es and/ or ot her count ries. Ot her product and com pany nam es herein m ay be t he t radem arks of t heir respect ive owners. TechNet Hom e © 1999 Microsoft Corporat ion. All right s reserved. Term s of Use.

Feedback