Data Communications Networking Devices - 4th Ed.pdf

370 _________________________________ WIDE AREA NETWORK TRANSMISSION EQUIPMENTbps modems with respect to the ratio of noise to the strength of the signal N/S)onthe circuit. From this illustration, it should be apparent that 14 400 bps modemsusing conventional quadrature amplitude modulation should only be used on highqualitycircuits.Modems that transmit data at 16 000 bps are very similar to 14 400 bps devices,with the major difference being in the baud rate. Thus, most 16 000 bps modemsencode data into 6-bit symbols and transmit the signals 2667 times per second. Thismethod also employs a total of 64 signal points; however, the baud rate is increasedfrom 2400 to 2667 to obtain the higher data transfer rate.Trellis coded modulationDue to the susceptibility of conventional QAM modems to transmissionimpairments, a new generation of modems based upon Trellis coded modulationTCM) was developed. Such modems tolerate more than twice as much noisepower as conventional QAM modems, permitting bidirectional data rates up to33.6 kbps to be obtained over the switched telephone network.To understand how TCM provides a higher tolerance to noise and other lineimpairments, to include phase jitter and distortion, let us consider what happenswhen a line impairment occurs when conventional QAM modems are used. Herethe impairment causes the received signal point to be displaced from its appropriatelocation in the signal constellation. The receiver then selects the signal point in theconstellation that is closest to what it received. Obviously, when line impairmentsare large enough to cause the received point to be closer to a signal point that isdifferent from the one transmitted, an error occurs. To minimize the possibility ofsuch errors, TCM employs an encoder that adds a redundant code bit to eachsymbol interval.In actuality, at 14 400 bps the transmitter converts the serial data stream into 6-bit symbols and encodes two of the six bits employing a binary convolutionalencoding scheme as illustrated in Figure 4.19. The encoder adds a code bit to thetwo input bits, forming three encoded bits in each symbol interval. As a result ofthis encoding operation, three encoded bits and four remaining data bits are thenFigure 4.19Trellis coded modulation