Data Communications Networking Devices - 4th Ed.pdf

392 _________________________________ WIDE AREA NETWORK TRANSMISSION EQUIPMENTAT&T as the 2048A and 2048C models, which were designed for four-wire leasedline operation. The 2048C has a start-up time less than one half of the 2048A,which makes it more suitable for operations on multidrop lines. This is because arequirement by any remote modem in a multidrop network for retraining can besatis®ed quicker than by the use of a 2048A modem. Other third-party vendorsintroduced 208B type modems with rapid equalization times that provide enhancedthroughput capability for switched network transmission. This primarily occursduring interactive transmission since the half-duplex operation of the 208B on theswitched network results in numerous line turnarounds. By shortening the timerequired for the modems to re-equalize each time the direction of transmissionchanges the modem can begin to modulate data quicker after a line turnaround, ineffect, boosting data throughput.Both Bell 208 type modems and ITU V.27 modems pack data three bits at a time,encoding them for transmission as one of eight phase angles. Unfortunately, sinceeach type of modem uses different phase angles to represent a tribit value, theycannot talk to each other. Table 4.9 lists the V.27 modulation phase shifts withrespect to each of the eight possible tribit values.Table 4.9bit patternTribit valuesV.27 modulation phase drift versusPhase changedegrees)001 0000 45010 90011 135111 180110 225100 270101 315During the 1970s to the mid-1980s, the primary use of 208 and V.27 modemswas to support the transmission requirements of synchronous remote batchterminals RBTs) that were used to communicate with mainframe computers. Theuse of RBTs allowed users located remote from a mainframe to enter jobs forprocessing as well as pull system output Sysout) to the RBT's printer. In addition,many RBTs had tape and disk storage that allowed jobs to be batched fortransmission as well as for Sysout to be queued to tape or disk for printing if theprinter was being used for local processing.From the early 1980s, personal computers began to be used as RBT replacements.Several manufacturers introduced synchronous communications adaptercards for installation in the system unit of PCs which allow them to be connected tosynchronous modems, such as Bell System 208 and ITU V.27 devices. Othermanufacturers offer internal 208 and V.27 type modems which permit PCs tocommunicate synchronously at 4800 bps over the PSTN or on a leased line by