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High-Speed Modems

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High-Speed Modems PRODUcr FOCUS , 3e : ê',I\~ \ ~\l..~" , PA~ :O High-Speed Modems John H. Humphrey and Gary S. Smock 11 I' ~ -,.êWe fly 10 Paris in 4 bours. seál A new crop of modems inexpeosive silicoo bnilding deaIs by Iacsimile .in Einntes. blocks. I I and can'l stand to wait more take transmission rates to a blazing .Today. V.32 modems .,..,'be- than 24 -hours 10 get iny>ortaol ginning to sbip in increasing mail. It'. 001 sl'IPrising that 9600 bps and beyond nnmbers. Tbey .,.., still bulky. bigh'Speed modems, able to ac· yower~consumptive, :and u­ curately .send 3 megabytes peosive. Most manufacturers across the countty .in less than an bour, 2-wire Jàciljfies . .lJave jumped tbe gun. designing in iso­ .,.., tbe fBstest growing segment of per­ .Echo canceIIation.mal<es extensivo use chronous (Le .• syncbronous/asyncbJ:on,. "SOnal romputer oommullications. õf bigh-speed digital signal processors cus) Ctlovertcrs to 8IIow tbe moiIem to Tbe advantages"Oflügh-~ mndems (DSPs). Botb sending 1I0d receiv.ing use asyncbrooous data. A nnmber of 'BI"C cloar: substantisl savings in .ccmmu­ .mndems tnmsmil simultaneous1y at iden­ smaIJer. lower-cost 9600-bps modems nications charges and:in time. l10til ,..". tical carrier frequencies. and tbe inbound .,.., also available. lhe product oI stopgap cen!ly. these bigh-speeiI units bave com­ and outbtlund data signals clasb and over­ desigo effurts. .:manded preminm prices. bUI'prices bave Jap •.interfering witb 'ooe anotber. fallen sharply during tbe past year. Tbe <.Tbe mãaem JenoWs'What signali it jusI CompetiIJg Modem Technlques 13 modems we n:view bere send data at a transmittca. tnmsmittel1 100 .millisec­ 'Ibllcbieve bigh-speeiI dia1-up communi­ .rate of at 1east 9600 bits per secónd.and onds agn.'tranSmitted 2 secondS1lgo. and cations for personal rompUtel"S. there.,.., range in price from ,ábout.$900go nver so 00. lt éreates scaled and inverted three rore requirements: an asyncbro­ $2()()(). .' '<"Pies of tbe waveforms tbal iVln!nsmil­ nous interface. fu11-duplex oJlCration. But as in any .fieJd 'cf rapid &"OWtb. ':it:d and aiiéls tbese ÍnIO lhe =ived data and a reliable and :inexpensive modem Standards' anil <UÜformity among liigh­ 'streatn 1O"CIIllCel themterferenceirom its engine capable of worldng under disl-up speed mndems have lagged bebind'in tbe ttransmitter. eaving: ooly lhe inCtlming line conditions. .rush tol'rovide tbe most :impressive per­ FgnaI for}tS ~i:ver to process: :This is V.32 acbieves tbe first two objectives formance for the most pa1atable póce. an incredi1íly complicsi.êd task that typi­ bul faIIs down .in tbe tbird arca. To get Unlike tbe bigbly standanIized 1200- and calJy.requires tbe "SerVice cfl>SP.s witb around tbe R&D .investmeru reguiIed to 2400-bps modems"WC·.re used to. "1Ilost '-performancéin the.range oI25 to 59 triiJ~ "l\evelop a !rue V.'32 modem.lllIIIIY manu· .high-speed modems work at top speed liooinstructions per secaod. .fucturers 'have elected ·to produce bigh­ ooly when oommunicating witb anotber As pubusbed today. V:32 doesnot pro­ speed modems that are nol Ctlmpaú"ble such modem. The inoompatibility-lS due v.ide niles regarding the use of asynébro­ witb V.32 but1ha1 dOl'rovide 9600-bps 10 thel'ush ror greater performance at'a DOUS data. wliich was léft for ~~er operarian. Tbese manufacturern:use ODe given price and to tbe use of severa! oom­ . study .......Basically. tbe CGl'l7I\,did nOI 'Of four modulation,approaébes tbatl'ro­ petiog-and confusing-standards ior foresce theTapid bui4lup 10 eJid'JIser de­ vide alternatives ,to tbe' V.32 StandaM. lrigh-speed data communications. mand fo, a móileratel,YiPóced. ~cbro­ (For .more .informatioo on tbe theory .and nous. 9600-bps. disl-up modemi·c,At tbe tecbnology·of tbe competing tecbn.iques. A Good but Sbortsigbted Start· .time V~ 32 -was adopted. lúgh;"peed see tbe text , box ·~gh-Speed .Modem Tbe ceotral force for standardizatiQo in mndems werep!"C1lotíiinaJjf]y themIm of · Modulatioo" 00 pagel06.) the communfcations industry is the corporate users, where pbysical size. ccrrr. headquarte~ in Geneva. Swit- price. and pawer Ctlnsnmption were of V.29 and V.32 Revisited zerland. lo 1976. tbe ccrrr drafted tbe secondarj importaoce lo solid -analog The flrst- and most popular-modula· fU"St standaM for 9600-bps rnodems. Tbis - performance. consistenLreliability. ·and >tioo approach involves changes to V.29 recommendatioo (V.29) was desigoed 10 - ,growtb flexibility. _ core engines. whicb .,.., avai1able 'in cbip speak SPCCillcalJy to modems operating llut user demand for .96QO-bps Ctlm-' sets from severa! so=. V.29 chips at 9600 bps aver 4-Wire.leased-1ine fucil- munication has been ~g faster-tban ·were oógina11y tooled to serve modem ities witb syncbronous data. tbe irik was drying on V.32. Mndem .manufacturers· needs for conventional 4- lo 1984. tIle ccrrr adopted anotber manufacturers. strugg\ing witb tbe tecb- wire. leased-Iine. syncbronous modems standaM (V .32). wbicb provides for a .nicaI impact of designing reliable ecbo- and for fax macbines. 9600-bps modem to be used 00 dia11inks. amceling circuits. 'bave chosen to ofler "I'wo tecbniques. ping pong and statis­ V.32 inc1udes avant-garde communics, ~P8l!P alternativos to feed user demand tiaJl dup/exing.""'1lSed to build V.29- tions tecbnology known as ecbo canceUa- during tbe ínterim. perind required to de-- ibased.96OO-bps microcomputer modems. tion to acbieve full-:duPIex. operation aver Sigo. and cast ecbo-canceling circuits into' <X>IIlinM<d 102 'Jl YI.E °"JUNEl988 JUNE.l988 • BX r E "<Im ". ,I IDOH-SPEED MODEMS , j • ,:1 : tu O I :l . li.1'1 , li! l'able 1: The modems reviewed use a variety oftecJmiques to achieve high-speed dato. transfere P.rices VllT)I as weU • from a low 01$895 to a high 01$2295. lil Case Concord DataRace DataRace Fastcomm Hayes I 4696NS . 296 TrelUa RaceBMX-VM RaceVMI Turbo 2496 V-sertes Smart- modem 9600 I Chipset RockweII Custem Rockwell Rockwell Rockwell Custem I sets TMS32010 sets sets sets TMS32010 I I ,I Maxlmum '19.200 19.200 19.200 19.200 19.200 19.200 ~. lilti. 1;1 lnputapeed (bpa) Error Yes Yes Yes Yes Yes Yes i correction (MNP class 6) ]1' Data Yes "0pti0naI ,·Yes ,'tes No . '1Yes compressJon 1I Duplextng Statistical Echo V.29 V.27 Dynamic V.32HOX :1 technology ,cancelJation pingpong asymmetric pingpong ,I CompaUbfllty - ",' :V.29anciV.27·.~-::"·~:}'~··'lW~32· :~~"'<)~D~~ceBMX';;'Data'RacéRace" ,~Fastcommunits "';ifiayesV-8eries ! .statisticaJ ... , , ,curüts.'-V.29 -.units.V..29 )~(speedsabove~-Smartmodem9600. i! .Ii '~ng. ·~{V.22bis. V.22. ···(V.22bis. V.22, '2400bps),V.22bis. V.22bis. V.22, iij ·V.22bis.V22."212A, .'212A.~103 212A,and103 V.22,212A, 103, ' . 212A. and 103 ti' , :"and 103 - optional) optionaJ) lllld V.29 lil .1 II 11 Price $1395 $1795;$1995 $995;$1245 $1195;$1345 $1099 $1299 w/data compression w/options w/options 11 .... Both start with a core V.29 engine and cance11ation, which is required for fuI]­ existing 2400-bps modems operating build an isochronous converter onto it. duplex operation. One modification uses without compression. V.29 is intrinsicaIly half-duplex, how­ V.32 without echo cancellation and em­ ever, and a method to simulate full­ ploys the ping pong approach to simulare Multicarrier Technology duplex operation is needed. This is where full-duplex operation. The other method The fourth approach borrows from the two V.29 approaches differ. grafts a low-speed reverse cbannel onto spread specnum communications tech­ In the ping pong approach, data you the core V.32 high-speed center channel nology used by the military in secure send to the modem is buffered. The two to handle keyboard input. communications systems. This technol­ modems automaticaIly switch their car­ ogy breaks the data into discrete pieces riers on and of! rapidly..! exchanging data Please Squeeze the Data and spreads them across the available each time they have the Iink for transmis­ The third approach involves data com­ bandwidth on separate canier frequen­ sion. A form of ready/busy flow contrai pressione Although a number of the cies, keyed at different time intervals. • is used between the modem. anel your modems we tested offer some form of This requires the enemy to knaw which computer to prevent you from losing data compression, what ü we can get a spreading algorithm is being used to re­ data. - really big "squish"1 Data compressors cover the individual pieces ofthe commu­ Statistical duplexing uses a low-speed fmd clever ways to shrink 10-bit ASCll niqué: Without the correct algorithm, the reverse channel,which is added at ire­ data to, say, 4 or 5 pits for transmission, transmission looks Iike random noise. I quenciesabove or below the V.29 en­ then convert back to 10 bits at the other Modems using multicarriertechnology gine's pass bando The reverse channel is end. They look for repeated strings in the spread the telephone bandwidth with intended to handle data' at up to 300 bps data, convert them into unique conb'ol . hundreds of individual carrier tones, to characters, and explode them back to the each of which is modulated quite slawly. II1 1 and is there service data at keyboard I ! rates. Should a conversation change dy­ original string when received. Digital data is fed to the modem and buf­ namically (i.e., you were inputting and If the compressioD algorithm is effi­ fered. Individual bits are fed to the multi­ receiving file data, but now your partner cient enough to absolutely guarantee a pIe carriers and data is passed over the I is inputting and receiving fue data), the four-to-one advantage, modem makers link in n-Iength, bit-parallel fitshion.
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