Data Communications Networking Devices - 4th Ed.pdf

388 _________________________________ WIDE AREA NETWORK TRANSMISSION EQUIPMENTrequester providing the transmission path for data sent from the computer to theuser's screen.2400 bpsExamples of modems that operate at 2400 bps include the Bell System 201, theITU V.26 series, and the V.22 bis modem. The Bell System 201 and ITU V.26series modems are designed for synchronous bit serial transmission at a data rate of2400 bps, while the V.22 bis standard governs 2400 bps asynchronous transmission.Bell System 201B/CThe Bell System 201 series was originally introduced during the mid-1960s tosupport what was then considered to be high-speed synchronous data transmission.Although 201 modems are no longer manufactured by Western Electric,compatible modems are manufactured by that subsidiary of AT&T as well as byseveral third-party vendors.Members of the 201 series include the 201B and 201C models. Both of thesemodems use dibit phase shift keying modulation, with the phase shifts based uponthe dibit values listed in Table 4.6. The 201B modem is designed for half-or fullduplexsynchronous transmission at 2400 bps over leased lines. In comparison, the201C is designed for half-duplex, synchronous transmission over the PSTN. Amore modern version of the 201C is AT&T's 2024A modem, which is compatiblewith the 201C.Table 4.6 Bell System 201 B/C phase asopposed to bit patternDibit valuesPhase shiftdegrees)00 22510 31511 4510 135V.26 modemThe V.26 standard speci®es the characteristics for a 2400 bps synchronous modemfor use on a four-wire leased line. Modems operating according to the V.26standard employ differential phase shift keying, using one of two recommendedcoding schemes. The phase change based upon the dibit values for each of the V.26coding schemes is listed in Table 4.7. The use of each phase change pattern resultsin a modulation rate of 1200 baud providing a data signaling rate of 2400 bps.Under the V.26 standard a reverse channel can be used. This channel has the samespeci®cations as the V.23 reverse channel.