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 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. modems sense this change by watching don 't have to use fancy high-speed The advantage of this approach is its I : the re1ative queue length· of their 110 modem engines at ali. With a guaranteed ability to "map around" bad spots in tele I ; buffers. The modems exchange contraI 4-to-l compression, they can get by with phone tines. If discrete portions of the information to swap the assignment of low-cost V.22bis (standard 2400-bps) telephone line are of poor quality, the their high-now-speed channels. technology that now exists. The draw modems recognize this and simply do not I i A second approach used by a few fmns back to this is that you need a similar place canier tones in those areas. In I makes use of the technically easier to de modem at the other end of the link to theory, this lets the modems operate at sign partions of V.32's modulation. The achieve 9600-bps speeds. An unexpected full speed under good line conditioÍls and key technical problem with V.32 is echo benefit is built-in compatibility with s10wly fàll back under poorer line condi- 104 B YTE • JUNE 1988 • mGH-SPEED MODEMS • Mlcrocom Racal-Vadlc Telcor Teleblt Telenetics USRobotlcs Ven-Tel AX19624c 9600VP Acceleratar TrallBlazer Plus 96OOEN.32 CourlerHST EC18K-34 2496MA Rockwell RockweII Rockwell Custem Custom Custem Custom sets sets getS TMS32010 TMS32010 and TMS32020 TMS32010 TMS32025 f~ 19,200 ,,9600 9600 19.200 9600 ,19.200 19.200 , Yes Yes Yes Yes Yes Yes PEP (19.200 to 24(0) (MNP classes 1-6) (MNP classes 4&5) (MNP classes 1-3) (MNP classes 1-5) MNP below 2400 , Yes Yas 'Yas Yes ·No 'Ves 'Yes Statistical Dynamic V.22bls with PEP Echo cancellation Asymmetric PEP compression TCMOAM "'-,"--""'.-,.,:"',.- ... -- . V.29 with statis-V.29 with dynamic :TeJcor Acce!erator, , ;PEP modems, . V.32 ',: .úSRobotics HST. "~, PEP moctems. tical duplexing . 'duplex (Vadie),· , '" ,; V.22bis, V.22. ' . -V.22bis, V.22. '~,V.22bis. V.22. " V.22bis. V.22. 212A. V.22bis, V.22, - "212A.' 103, and .. ,', 212A. -and 103 "212A, and 103 -'.:212A, and 103 ' 1llld 103 212A. 103. V.27, V.29 (synchronous and V.29FT (syn- .' ha1f-duplex) chronous onty) $1399 $995 $895 $1345 $2295 $995 $1399 tions. The modems automatically seek class 4. It features an AT command set, a operating at speeds greater than 2400 and fmd the maximum operating speed Concord command set, and operates bps. The unit features an AT command ,achievable under constantly cbanging either asynchronously or synchronously set with some additional extensions, flow line conditions. at 4800 or 9600 bps. lt supports 2-wire control, and compatibility with other dia} or leased-line operation. modems at O to 300, 1200, and 2400 bps. A Fast Field of Contenders Data Race Race BMX-VM: The BMX- Hayes V-Series Smartmodem 9600: The 13 modems we tested for tbis review VM operates in half-duplex V.29 made This Hayes modem uses a modified V.32 all use at least one of the modulation ap and uses a high-speed line-tumaround design that provides full-duplex trans proaches discussed above. Wben you tecbnique to simulate full-duplex opera- mission at O to 300, 1200, and 2400 bps, look at the features for each modem in tion. The modem has an AT command plus half-duplex transmission at 4800 and table 1 and the results of the testS we nm, set, a BMX command set, enor detec- 9600 bps. At the higher speeds, a ping remember that high-speed data commu- tionlcorrection, data compression, and pong protocol is used to simulate full . nications must be tailored to individual flow control. lt supports V.29 and V .27, duplex operation. This product is com situations. Before you purcbase a high and an option adds V.22bis, V.22, BeU patible with aIl earlier Hayes products speed modem (or a set ofmodems), take 212A, and Belll03 compatibility. and with modems that support the a carefu) look at precisely how the fea Data Race Roce VM I: The Race VM 1 V.22bis, V.22, BeU212A, ând BeIl103 tures and performance of a system will employs error detectionlcorrection, data modulation. In addition' to having true match your needs. That said, let~s look at compression, and flow control to offer Hayes AT commands, the modem rea themodems. full-duplex asynchronops communica- tures errar control,' flow control, and Case 4696IVS: The Case 4696NS is a tion. lt features an AT command set, a adaptive data compression. full- or half-duplex V.29 modem with Race command set, and supports connec- Microcom AX/9624c: The Microcom statisticaJ duplexing that operates at tion to either dial or leased 2-wire lines. AX/9624c supJ)9rts the V.22bis, V.22, speeds of 300, 1200, 2400, and 9600 An option is available for V.22bis, V.22, BeU212A, DeU 103, V .27, and V.29Ff bps. Data compression and error conec DeU 212A, and DeU 103 compatibility. (fast train) modulation standards and pro tion are provided through the six classes HaIf-duplex synchronous operation is vides its own SX command set, as weU as of MNP (Microcom Networking Proto possible when the modem is operating in an AT command sete Fast train is a tech col) that this product supports. This unit its V.29 mode at 4800, 7200, and 9600 nique within the V.29 standard that pro provides the Microcom SX and the AT bps. vides for an abbreviated handshaJcjng be- command sets, and it is compatible with Fastcomm Turbo 2496: The Turbo tween sending and receiving modems. Microcom's AX/9624c and other con 2496 uses a design based upon the V.29 This modem supports MNP classes 1 ventional V:22bis, V.22, BeU212A, and recommendation and simulates a full- through 6 and provides data compression Bell103 modems. duplex ~nous channel by using a anel enor correction. The AX/9624c is Concord 296 TreUis: This is a true proprietary modem-to-modem protocol compatible with the Case 4696NS. . full-duplex V.32 product with MNP with error detection and correction when C01IIituIed JUNE 1988 • BYTE 105 .--.,-....,..,""'.. .,.,. . ..,... ------• IDGH-SPEED MODEMS High':Speed .Modem Modulation · i aw high-speed modems work is a H complete mystery to many people. A 4=+SOO 4=+1800 The e1ectrical fundamemals are straight 0° +9QO +2700 +180° forward. The real triclts imolved lie in I I I I consistently and reliably demodulating lhe signal ovcr a broad I'BDgC of receiver conditions caused by telephone line COD f\i f\ [\ f\i f\ i f\ ditions tbat C8D vary in real time OD a givencall. J V V \J\KJ\N\J EDvision a sinusoid of fixed fre quency whose phase is cbanged at dis Baud interval 1 Interval2 Jnterval3 crete time imervals (the baud rate). Ifwe are careful to sample lhe waveform at B (1) (2) (3) these periodic and discrete baud inter jsinewt jsinewt jSlnewt vaIs, we C8D measure lhe phase changes that represem the data. ,..---- • • • • Figure A shaws a sinusoidal signal C- •I I tbat has been phase-modulated to pro A I : +1 • • _. .. I ICOS B cos duce +90, + 180, anel +270 degree rel ~-'cos VI wt three A+B=C B i -, +1 iwt wt ative phase shifts over successive I -1 I • • baud interYals. If we preassign .digital I I A · I ---- I meanings to the relative phase changes .. ----- • ~.. (O = 0,0; +90 = 0,1; + ISO = 1,1; and • • +270 = 1,0), we have encoded digital iDformatiOD and are sending data faster Figure A: AplUlse-modulated sinusoid. Figure B: (1) The sum oftwovectors. than the fundamental data transfer me. (2) The sum ofinteger orthogonal components. (3) Amplitude scaling of Bere, 2 bits of information (a dibit) is orthogonal components. exchanged on each baud intervalo If we increase the encoding density (tribits and quadbits), we C8D send even -is another sinusoid of the same fre Another form of DPSK modulatioD more infonnation per baud intervalo The quency at 4S degrees (see figure BI). tbat is used commoDly offers 4800-bps baud rate is restricted by the fixed band Also note tbat ifwe scale the two orthog communication. There are two popular width of the telephone liDe; the rate of anal signals with discrete multipliers (anel similar) schemes .. One used in the iDformatiOD exchange is not.· The price - (+ 1 ar -I) before summingthe 1\\0, we U .S. is called the Bell208 discipline; its we pay for higher encodiDg densities is can p1ace the resultant vector at any one counterpart in Europe is the CCI'IT the complexity of the modem '8 hard of four discrete POSitioDS, each of wbich V.27 discipline. ware-being able to differentiate be , differs from the other by even 9(klegree Instead ofa simple 4-point constella tween smaller discretesignal 'differ- -multiples (see figure B2). tion like tbat shawn in figure B2, :the • ences. Hence, we have taken a complicated V.27 -discipline -offers an 8-point-con In the example shown, the modem modulation problem (changing the stellation, 'where the individual points of need oDly difIerentiate between 9O-de phase of a single sinusoid) and reduced the CODStellatiOD forma circleabout the gree shifts. If we carried lhe example it to electrical functions that are éasy to origiD separated by 4S-degree angles. one step further ·(tribit encoding), the perform (iDverting a signal and sum V.27 is a half-duplex technique using a modem \\Ould have to slice the phase do ming two signals). Figure B2 represents single-carrier frequency, centered at -main intD eight pieces (2' = 8). Such a precisely what takes place inside the fa 1800 Hz tbat is modulated at 1600 baud modem wo&ld need to be able to distin miliar Bell 212A 1200-bps modem using tribit encoding. Thus, .we gelo guish between 4S-degree pbase differ differential phase shift keyiDg (DPSK). 4800-bps communication -in one direc ences. Although higher-speed modems op .tionata time (1600 baud x·3 bitslbaud). Modems don't actually shift a single erate similarly, the encodiDg density is Figure B3 shows the constellation pat sinusoid. They mate use of vector alge greater. In the DPSK example shawn, tem associated with newer 2400-bps bra by SUmmiDg quadrature- compo we restricted the amplitude values . dial-up modems that utilize quadrature nents. Suppose we have two signals (A placed UpoD our orthogonal components amplitude-modulation (QAM) technol . ánd B) of identical frequency that are 90 to integer values (+ 1 and -I). If we ogy to achieve ·full-duplex communica degrees out of phase with each other (a allow the quadrature components to be tion acconting to 'the ·CCl'IT V~22bis sine wave and a cosine wave). We say scaled by fractional values before sum recommendation: This is -a -16-point these signals are orthogODal or in quad ming the two signals, we can combine constellation, which implies that the rature to each other. amplitude modulation with phase modu modems excbange quadbits (24 = 16) of Vector algebra (remember physics?) lation and acbieve an even greater eD informatiOD at each baud interval. -tells us that the sum of the two (signal C) coding density. V.22bis techllology ·isfull-duplex. - 106 B YTE • JUNE 1988 ------,I , ,- I lDOH-SPEED MODEMS if Racal-Vadic 96OOVP: The 9600VP of those that support the USRobotics high fers full-duplex asynchronous operation speed technology (HST). When operat at 300, 1200, and 9600 bps. Its modified ing at interface speeds of 4800 bps· to V.29 design offers errar controI, selec 19,200 bps, the Courier HST operates in tive retransmission, and data compres the HST mode with errar controI and will sion, and it adjusts its speed dynamically eilher fall back or spring forward as line -Each modem ttansmits its own carrier, to optimize to current line conditions. conditions permito The unit supports ·1 and tbey are separated in frequency The unitprovides an AT -compatible MNP classes 1 through 5, data compres 11 by prearrangement. The origiDating command set, nonvolatile teIepbone sion, flow controI, and has an AT com modem transmits at 1200 Hz; ·the BD number storage, and compatibility with mand set with extensions. IJ swering -modem tnmsmits at 2400 Hz. the standard communication protocoIs Ven-Tel EC18K-34: This modem is The carriers are modulated at 600 báud when operating in the Be1l212A or Bell also known as the Pathfinder 18K and 11 using the 16-point constellation sbown 103 modes. Speed conversion and five featores a high-speed mode that supports Jf and provide 2400-bps (600 baud x 4 types of flow controI are availabIe. . interface speeds to 19,200 bps. The unit '\ bitslbaud) information interchange. This modem offers true full-dupIex uses PEP at the higher speeds;-making it To achieve 9600-bps communication . data transmission during interactive ses companõle witb the TrailBlazer Plus and rates, several techniques are in use to sions that have relatively Iow data other PEP modems. It is also compatible day. The oldest one stems flom the throughput requirements and a balf with modems that support the V.22bis, I, CCITT V.29 recommendation. V.29 duplex link while transferring large V.22, Bell212A, andBelll03 standards. .1 was originally wriuen to provide 9600- amounts of data in one direction. The dy In the high-$peed mode, the modem uses d . bps communications over 4-wire leased namic dupla technology will automati data compression, error detection, and lines using a syncbronous data formato cally select the appropriate duplexing error correction, and it adjusts its operat There is no reason why the core teclmol . method baseei on data traftic patterns. ing characteristics to compensate for line ogy cannot be used in asyncbronous JIelcor Accelerotor 2496MA:' The Ac changes. An AT command set with exten· dial-up environments, and a number of celerator uses a proprietary data-com sions is supported, as is MNP at speeds manufacturers have elected to do so. pression and error-detection technique of 2400 bps and below. V.29 uses the same generic 16-point with its V.22bis design to increase data [Editor's note: Cermetek. NEC, anti QAM constellation sbown for V.22bis tbroughput. It supports full-duplex inter Universal Dato Svnems also mak.e 9600- in figure B3. However, the carrier as face speeds of 1200, 2400, 4800, and bps modems. Although none cou/d supply sigmnent is ·changed. Instead of using 9600 bps, along with severa! interface us with unirs in time for this review, we two discrete carrier frequencies, V.29 and flow-control protocoIs. The unit is will ewzluate them in an upcoming issue.] places a single carrier frequency in the configured by an.AT command set with center ofthe voice band at 1700 Hz. The some unique extensions. When nOl com .MeasuriDg Modem Performance modulatioD rate is increased from 600 municating witb another Accelerator, We used Telequality Associates' SNR baud to 2400 baud, which "uses the this modem wiU communicate with (signal-to-noi~ ratio) Map technology to modem to take up virtuaIly alI the avaiI V.22bis, V.22, Be1l212A, and Belll03 measure the performance ofthe modems. abIe bandwidth the telephone fine bas to modems. (For more information on SNR Map tech offer. This means that only one of the Telebit TrailBIazer Plus: The Trail nology, see the text box 4CHow Testing two modems can send data at a given Blazer PIus features companõility with . Was Conducted" on page 108.) The per time, but they operate much 1àster- modems that support the V.22biS, V.22, formance parameter we collected was 9600 bps (2400 baud x 4 bitslbaud). Bell 212A, and Bell 103 standards, plus throughput efficiency as a percentage of The newest modulation technique its own Packetized Ensem.ble Protocol the data rate driving the modem. For pur (V.32) is quite similar to V.29 in lD8D)l (pBP). Supported interface speeds are poses of comparison, our test setup drove ; I respects. h uses a single carrier fre 300, 1200, 2400, 4800, 9600, and all modems at 9600 bps, aIthough many , qyency at 1800 Hz instead of 1700 Hz, a 19,200 bps. This unit features automatic of the units tested offer higher nominal ! 2400-baud modulation rale, an.d a core error detection and correction, flow con I/Ospeeds, 16-point constellation. V.32 differs trol, an AT command set, and support of Figures la and 1b provide raw test data from V.29 by offering an optional 32- MNP classes 1 through 3. in graphical form for two modems. We paint constellation (quintbit encoding) , Telenetics 96OOEIV.32: This is a V.32 annotated the graphs to show lhe peak or which is trellis-encoded. modem with an AT command set and maximum throughput efficiency mea The.fifth bit is a logical derivative of error controI. It offers full-duplex asyn sured, the mean or average efficiency re the other Cour. It represents an integrity chronous or syncbronous operation at corded, lhe range, and the standard devi· check similar to the ninth bit that is used 4800 and 9600 bps. An option switch lets ation of the test data. in mM PC and PC XT machines for you enable/disable its trellis-encoded ldeally, we like to see performance memory-parity purposes or banuning 9600 The can where both the peak and average efficien L modulation at bps. modem r code techniques used in more advanced monitor call progress electronically with cies are high and close to one another. We ; i error-correction-coding memory stor terse or verbose responses or via a built also desire a value approaching 40 for the I age systems. Trellis encoding gives the in speaker. Front-panel controIs provide range, indicative of a low headroom de L modem superior signal-to-noise perfor the ability to manually answer, originate, mando (Headroom is a term used in con . ,I and calls, well force ventional modem engine testing that de· I mance. V.32 is true full-duplex 2-wire disconnect as as ! modem technology. Beho cancellation 4800-bps OperatioD and select errar con 'scribes the margin between signal I is used to separate the transmitted and troI. The unit also provides nonvolatile strength and line noise required to oper received data streams of analog wave stomge for ten 4O-cbaracter telephone ate properly.) Last, we like to see small forms tbal are propagating simulta numbers. values of standard deviation, indicating a neously ~ugh the 2-wire Iink. USRobotics Courier HST: The Courier very consistent modem. HST is compatible with V.22bis, V.22, Let's look at figure la, which shows Bell 212A, and Bell 103 modems, plus continlled JUNB198S·· B YTE 107 lflGH-SPEED MODEMS How Testing' 'Was Conducted igure A provides a simplified block ,;the desiIedSNR. The bybrid and CO dramatically if asked to traffic two-way F diagram of our test system. Modem · . simulator deliver this sigual (i.e .• scaled 9600-bps full-duplex data. ~. ·#1 and modem 412 represent the target . signalsummedwith the appropriate We tested the modems under two dif modems undertest.ln each alSe.<1he '.level ofnoise) to the otber modem. ferem operating environments. One pro target modems were a pair of identical -The RS-232C ports of the modems vided -26-dBm received signaJ models from ODe manufacturer. are connected to data-pattem genemtors strength over a C2 line simulator, pro The centraloftu:e (CO) simulator ,and checkers. Serial data is given to, viding typical pbone line conditions. provides operatiDg loop current to simu say, modem #1 for ttansmission, and the The other presented 40-dBm signal late the De COnditiODS of a typical con · 8-bitbinary data stream output from strength over a 3002 simulator, provid nection 'to the telephone network. The 'modem #2 is checked. The tester is ca ing poor phone line conditions. CO simulator also provides CODVen pable of both one-way data flow ar two- At each signallevel, we decreased the tional ringing voltage to trigger the · wav simulraneous data fIow. SNR by adding noise to the received sig modem's autmnatic answer function. . Some of tbe modems evaluated were nal until the modem's throughput effi· Additionally, the CO simwator is true .full-duplex devices; others merely ciency dropped to less than 10 percent of connected to a hybrid (2-/4-wire con simnlatOO ful1-duplex operation. We de the tester's nominal9600-bps feed rate. . verter) that separates .. the ttansmitted cided to test the modems by sending data At each point in SNR space, the _. signal from the teeeived signal, u tele in on1y one direction for the simulated modemsexchanged .aminimum of .. phone linesdo.· The isolatedttansmit , .full-duplex modems and in both direc 81,920 bytes. We used proprietary signalis th.en impaired by the telephone tioDS for the ttue ful1-duplex units. methods to ensure that the resulting data channel simulator. which has program Testing in ODe directiOD corresponds was within a 4 percem accuracy band mable ftequency response, group de1ay, · on1y to the conditions of a file transfer with a 90 percent con:fidence leveI. We and gaiD characteristics that·can simu- -where 108 BYTE· JUNE1988 I 11 ; . :-.1,." .' InOH-SPEED MODEMS r~ . Figure la: The SNR Maps here anti in figure 1b show I: that the Telenetics modem ran with greater efficiency thon lhe Jlen-Tel modem I· under typicalline conditions. The Telenetics modem. I lhough, showed a much I narrower range of operation. Shown here are ~Il lhe row leSt resuIts for the Standard deviation 11.4 percant .. Telenetics 96OOEIV.32, a ~'. l, t. true V. 32 modem. (In ideal performance. mIlXÍmum anti " :. average efficiencies are I; high anti close to one I another, lhe stllIUÜlrd ~ Ii deviation'is a small value, I If anti the range is a large ; I: value.) I!I' 11 Figure Ib: The raw test I results for lhe Ven- TeZ EC18K-34 modem. a mulIicarrier unit, show that ir operated with less efficiency buz over a broader ~ Maximum efficiency 73.6 percant range tJum lhe Telenetics --"'---", ,'-', modem. \ , ..... - '---',' '\ ~:IIf .~'~-#." r------..:..---;----~-~,="-t--.....,-- Average efficiency \ : : 56.4 percent \~, : Standard deviation 17.5 percent -f------...:.T-----ri- Range: 28 observations the resulting test data taken from the fi you operate a 'conventional modem typica1line conditions. There are a num Telenetics 9600EN .32 modem (a true below its native headroom needs, data ber of differences between the operating V.32 unit) operating under typicalline errors resulto Modems that have error characteristics of these two modems. conditions (simulated C2 cbannel witb a correction don't make data errors. Their First, both the peak and average through received signal leve1 of .-26 decibels tbroughput degrades, due to the extra put efficiencies measured for the Ven-Tel below 1 milliwatt [dBm]). The modem time they spend in retransmission to cor modem are lower tban for the Telenetics was almost 100 percent efficient in trans rect the flawed data. modem. With a maximum efficiency of ferring data at 9600 bps as long as the From. the sbape of the curve, you can 73.6 percent, the Ven-Tel modem deliv SNR accompanying the received signal see that error correction is not a panacea, ered a peak communications speed of was above the modern's native headroom since modems exhibit extremely sharp 7066 bps under 9600-bps feed-rate needs of24 dB. roll-off characteristics. By forcing the conditions. In this example, the 9600E/V.32 modems to operate just a few decibels The Ven-Tel modem showed a much needed operating conditions where the below their headroom needs, they make a broader range of operation than the received signal leveI was 24 dB greater fast transition from perfect throughput to Telenetics modem, meaning that its head- tban channel noise. As long as these con no throughput at all. 100m needs are smaller. This confirms ditions were met, the modem' s receiver Figure Ib shows test data for the Ven the advertised benefits of multicarrier operated flawlessly. Once the modern's TeI EC18K-34 modem, which employs modulation. The modem can brown out headroom needs were violated,its .re multicarrier modulation technology .The instead of black out under poor line ceiver performance rell dramatically. Ven-Tel modem is also operating under CCI1IIinued JUNE1988 • B YTE 109 " HIGH-SPEED MODEMS (Concord, Telcor, and Telenetics) had Table 2: When fed dma aI a rale of9600 bps. lhe true ftdI-duplex modems consistently better throughput than their from Concord, Telcor, and Telenetics consistently showed bener throughput pseudo-duplex cousms. Among the lat thluz lhe pseudo-duplex modeIs. With lhe exceprion oflhe Telenetics modem, ter, the USRobotics, Microcom, and the true fuIl-duplex modems operared at about the same rale over typical and Case modems turned in performances ap poor phone lines. This table shows only throughput. not efficiency, anti proaching those of the true full-duplex some ofthe modems can accept data at higher speeds (up to 19,200 bps). modems. With the exception of the Telenetics Typlcalllne throughput (bps) Poor IIne throughput (bps) modem, which refused to operate over (C2, - 26 dBm) (3002, - 40 dBm) our poor tine conditions, the true full Maximum Average Maximum Average duplex modems showed virtually no op erating distinction between running over Csse4696NS 8429 6422 4925 3274 good or bad lines. The Race BMX-VM, Concord296 Fastcomm, Hayes, Racal-Vadic. Telebit, TrelUs 8842 8448 8861 8237 and Ven-Tel modems were resistant to DataRace channel differences. however. thev were RaceBMX-VM 4953 4704 4829 4339 still not as efficient as the full-duplex units. DataRace The Case and Microcom modems both RaceVMI 5520 5136 O O ran over our poor line but exhibited a sig Fastcomm nificant 1055 of throughput efficiency. Turbo 2496 3475 2486 3341 2102 The Race VM I, Telenetics, and US Robotics modems simply fiUled to ex Hayes v-Serias change data under our poor line condi Smartmodem tions. Tbese are. indeed, grueling 9600 5002 4742 4973 4435 operating conditions rarely encountered Mlcrocom in dial-up America. However. other AX/9624c 8304 6115 3926 2592 modems tested were able to handIe the Racal-Yadlc poor line conditions successfully. 9600VP 6442 .5798 6461 5002 lo The surprise among the lot was TeJ cor's Accelerator 2496MA. This modem Telcor used conventional 2400-bps V.22bis en Accelerator gine technology and was robust and effi 2496MA 9091 8256 9082 8362 cient in handling our 9600-bps stteam of Teleblt pseudorandom data. While a number of TraJlBlazer Plus 7152 5568 7229 5078 other modems tested offered data-com pression technology, the Telcor modem Telenetlca 96OOEN.32 9283 8995 O O showed outstanding consistency in crunching IO-bit ASCD data. USRobotlcs :\ CourlerHST 8678 8083 O O j Some Caveats Ven-Tel Overall, these modems are complicated. EC18K-34 7066 5414 7190 4704 They have extensive hardware and soft ware options that can have a serious im pact on performance. To acbieve top per fonnance, you must carefully contraI the conditions. configuration of its multicanier assign following tbree parameters: errar correc The othei' modems we tested per ment to deal with signaI quality degrada tion, data compression, and raw data formed siJ:nilarly. These V.29- and V.32- tion. A trade-off was executed. Ven-Tel transfer Iate. based modems employed conventional purchased a greater operating range at the Almost all these modems offer built-in fallback tecbniques to contend with dete expense of consistency in throughput. error correction. If you work with a ter riorated line conditions. Although the minal emulator that employs data block digital feed rate to and from the modem Comparing Emciencies ing and errar correction, you can cboke was f:axed at 9600 bps, the modem saw Now that we have a bandle on how the one of these modems to death by setting line conditions degrading and ordered its data was analyzed, we can proceed to a the size of your feed blocks 100 small and engine to initiate speed fallback from side-by-side comparison of the modems. starving the modem's built-in buffer. 9600 bps to 7200 bps to 4800 bps. Table 2 shows performance in terms of We sent pseudorandom data, which is Figure Ib also shows that the Ven-Tel raw bps numbers. To consolidate the data bani to compress. Some of the modems modem recorded a larger standard devi in a meaningfu1 fashion, we used statis offered options to engage or disengage the ation. This indicates that its throughput tics to represent key aspects of perfor compression function so they wouldn't efficiency varied more than the Telenetics mance. Figure 2 shows the group's per choke when fed incompressible data. modem. You can see the effects of this in formance on typicallines and poor lines One modem (Hayes) used data compres the shape of the curve trace. A number of by graphing the differences in their effi Bion that was not user-defeatable. Per dips are found in the modem's efficiency ciencies. We think that figure 20ffers a haps, bad we sent spreadsheet or text curve. more complete basis than the raw bps ftles, we would have observed signifi These dips represent decision points rates for comparing the modems. cantly better throughput with the Hayes wbere the modem elected to change the AlI the true full-duplex machines co1lli1rlwl 110 BYTE· JUNE1988 • HlGH-SPEED MODEMS Figure.2: Again, thefulI- 60 & -40 ....c .rr: modem. However. tbe Telcor unit pro vided an intelligent compression algo rithm lha! simply ate up our data. Company Infonnation Mauy of lhe modems allow data UO rates in excess of lhe 9600-bps speed we used for our comparison. For those of Case Communications Inc. Fastromm Data Corp. you who can crauk up the feed rate (no! 7200 Riverwood Dr. 12347-E Sunrise ValIey Dr. alI computers and/or communications Colurnbia, MD 21046 Reston, VA 22091 paclolges will support fuster data rales), (301) 290-7710 (703) 620-3900 you can expect higber gross througbput. Inquiry 891. Inquiry 894. But for this review we wanted to measure both througbput and efficiency. . Concord Data Systems Hayes Microcomputer Products Inc. 45 Bartlett St. P.O. Box 105203 What the Future Bolds Marlbornugh, MA 01752 Atlanta, GA 30348 The outlook for the future seems clear. (617) 46().{)808 (404) 449-8791 The utility of error detectionlcorrection Inquiry 892. Inquiry 895. features is quite apparent, aod lhe CCITT is \YOrking 00 a new recommendation in Data Race Inc. Microcnm Inc. lhis area (V .42). As currently drafted, 12758 Cimarrnn Path I400A Providence Hwy. this provides a smooth transition from the Suite 108 Norwood, MA 02062 past to the future. San Antonio, TX 78249 (617)762-9310 V .42's main impetus revolvesaround a (512) 692-3909 Inquiry 896. new protocol, LAP M, which is similar Inquiry 893. to the byte-independent, bit-oriented, packe!-switching protocoIs used in X.25 lU BYTE -1UNE1988 .. IDGH-SPEED MODEMS , and ISDN (lntegrated Services Digital • Maximum Nerwork) communicatioos links. V .42 doeso 't De)!lect curtem modems, how PoorllneiSOO2. - 40 dBm) ever; during initial handsbaking, a Y.42 devicc will que')' the other modem &Dd use LAP M only if it is appropriate to doso. If ooe of the modems is DO! equipped with LAP M but bas MNP capability, both modems will begin to exchange data uoder MNP. Tnerefore. current users of modems equipped with MNP can take comfon in lhe knowledge lhat their han! ware is Dot likely (O become obsolete. !t·s a1so clear Ihat inteliigent data compression a1gnrithms will be increas ingJy imponant in data communicatioDS. If a lowly 2400-bps Y.22bis modem C8D 'I:: '60 o be souped up to consistently pass data aI -= 9600 bps, combining data-rompressioo 1 AO ." teehnology with a good V.32 engine of 'E fers the possibility of reliable aod cOSI· 20 ."'" effective dial-up communication ar effec :li u; tive rates of38,400 bps. o The wild can! in lhe data communica tioos picrure is ISDN. There are those in the relecommunicaoons and data commu nicaoons industries who pred.ict that lhe iDCreased bandwidth and line quality of ISDN will provide their own solutioDs to lhe data rate problem. These expens teU us lhat ISDN is ao inevitable pan of our communicaoons furure and that any pres eo! plans sbould be built arouod the fea tures offered bY ISDN. ISDN holds a tremeodous promise for corporate users shuttling mixed voice aod data signals between plant sites. But ISDN is not the only 10gicaJ a1ternative, aod it won't be implemeoted overnigbLlt took 100 years to wire America with cop per, put a telephone in virtually every household, and traio us to say "Hello" in response to a ringing bell. In the 1960s we staned to replace rotary dialing with tone dialing, and as we enter the 1990s, manufacturers are still churning OUI RacaJ-Vadic TeleDetics Corp. equipment with pulse- JUNE1988' BYTE 113