Data Communications Networking Devices - 4th Ed.pdf

4.2 MODEMS _____________________________________________________________________ 387the 202 series and are designed for transmission at 1200 and 1800 bps over theswitched network and leased lines, respectively. At speeds in excess of 1400 bps, the202T requires line conditioning when interfaced to either two-or four-wirecircuits, whereas for a two-wire circuit, conditioning is required at speeds in excessof 1200 bps when an optional reverse channel is used.V.23 modemsThe V.23 standard is for modems that transmit at 600 or 1200 bps over the PSTN.Both asynchronous and synchronous transmission is supported by using FSKmodulation; and, an optional 75 bps backward or reverse channel can be used forerror control. Figure 4.26 illustrates the channel assignments for a V.23 modem. Incomparing Figure 4.26 with Figure 4.25, it is obvious that Bell System 202 andV.23 modems are incompatible with each other.Figure 4.26V.23 channel assignmentsThe V.23 reverse channel uses a frequency of 390 Hz to represent a binary 1 anda frequency of 450 Hz to represent a binary 0. In this modem's ®rst mode ofoperation, which governs transmission up to 600 bps, a binary 1 is represented by a1300 Hz tone while a binary 0 is represented by a 1700 Hz tone. In the modem'ssecond mode of operation, which governs data transmission at 1200 bps, a binary 1is represented by a frequency of 1300 Hz while a binary 0 is represented by afrequency of 2100 Hz.Unlike the Bell System 202 that has been essentially replaced by more modernmodems, the 75 bps reverse channel of the V.23 modem has prolonged its life as itmakes the modem well suited to support many applications, including Videotexttransmission requirements in Europe. When used in this manner, a V.23 modem isbuilt into television sets and special terminal devices. The reverse channel whichoperates at 75 bps is used to transmit screen selection information to the computercontrolling the Videotext system. Since the screen selection information mayconsist of a few alphanumeric characters, the 75 bps data rate is normally more thansuf®cient to support the transmission of data to the computer. The primary channelwhich operates at 1200 bps is then used as a reverse channel with respect to the