Data Communications Networking Devices - 4th Ed.pdf

372 _________________________________ WIDE AREA NETWORK TRANSMISSION EQUIPMENTIn a 14 400 bps TCM modem, the signal point mapper uses the three encodedbits to select one of eight 2 3 ) subsets consisting of 16 points developed from thefour data bits. This encoding process ensures that only certain points are valid. Atthe receiving modem, the decoder compares the observed sequence of signal pointsand selects the valid point closest to the observed sequence. The encoder makes thisselection process possible by generating redundant information that establishesdependencies between successive points in the signal constellation. At the receivingmodem, the decoder uses an alogorithm that compares previously received datawith currently received data. The convolutional decoding algorithm then enablesthe modem to select the optimum signal point. Because of this technique, a TCMmodem is twice as immune to noise as a conventional QAM modem. In addition,the probability of an error occurring when a TCM modem is used is substantiallylower than when an uncoded QAM modem is used.TCM modem developmentsThe ®rst TCM modems marketed in 1984 operated at 14.4 kbps. Since then, anumber of modems that use more complex TCM techniques have been introduced.These modems operate synchronously at 19.2 kbps, 24.4 kbps, 28.8 kbps and33.6 kbps. By 1998, the application of TCM to modems enabled network users tomore than double the transmission speed on analog circuits from that achievableonly a few years before.Echo cancellationPrior to 1984, full-duplex transmission was achieved on the switched telephonenetwork by splitting the available bandwidth on the line into two separate channels,each of which was used to transmit and receive data simultaneously. While thismethod of frequency division of the channel enabled full-duplex transmission atdata rates up to 2400 bps, higher data rates using this modulation techniquerequired more bandwidth than was available on the line. This constraint resulted inmodem engineers developing a new modulation technique to achieve 9600 bps fullduplextransmission on the switched telephone network that was based upon theuse of echo cancellation technology. Two other techniques used by modemdesigners that simulate full-duplex transmission are asymmetric transmission and`ping-pong' transmission. Both of these transmission techniques are discussed laterin this section.Using echo cancellation, both the sending and receiving modem use the samefrequency, which would normally result in the occurrence of interference betweentransmitted and received signals. By the use of echo cancellation technology itbecomes possible for the modem's receiver to cancel out the effect of its owntransmitted signal, enabling the modem to obtain the ability to distinguish thereceived signal.Echo cancellation is used in the ITU V.32, V.32 bis, V.33 and V.34 modems thatare described later in this section. The V.32 modem uses QAM to encode 4 bits intoone signal change or baud, operating at 2400 baud to provide 9600 bps full-duplex