Data Communications Networking Devices - 4th Ed.pdf

406 _________________________________ WIDE AREA NETWORK TRANSMISSION EQUIPMENTrates up to 19.2 kbps. The AT&T approach requires signi®cantly less digital signalprocessing capability to implement than a V.34 modem, which requires the use of adigital signal processor capable of providing between 35 and 40 million instructionsper second mips). Similar to Rockwell, Paradyne Corporation, which until recentlywas part of AT&T now manufactures V.34 chip sets, although the V.32 terbomodulation capability is also included in many vendor high-speed modem productsto provide transmission compatibility with the base of previously manufacturedV.32 terbo modems. Since the V.32 terbo modem represents an extension of V.32bis technology and provides a maximum operating rate of 19 200 bps, we will focusour attention on that modem in this section.V.32 terboAs previously discussed, V.32 terbo represents a relatively simple change to the14.4 kbps V.32 bis standard. For this reason many people consider V.32 terbo as anenhancement of V.32 bis rather than as a competitor to more complex and higheroperating rate V.34 modems.Although the V.32 terbo modem was championed by AT&T, its speci®cationwas not formally adopted by any standards body. In spite of this, its technology wasincorporated into products from several modem manufacturers, both as the highestoperating rate of a modem and as one of several modulation methods supported bya V.34 modem to provide downward compatibility with V.32 terbo modems. Witha retail price only $30 more than a 14.4 kbps modem, the V.32 terbo modemprovides a respectable level of price performance. In addition, unlike V.34 modemswhich have dif®culty in reaching their highest-rated speed over the PSTN and fallback to a lower operating rate, a V.32 terbo modem in many cases can be expectedto provide a more consistent level of performance.OperationThe key to the design of the V.32 terbo modem is its modi®cation of the V.32 bisstandard to re¯ect two new data signaling rates: 16.8 and 19.2 kbps. All othercharacteristics of the V.32 terbo modem are identical to the V.32 bis standard. Thus,a V.32 terbo modem uses two-dimensional trellis coding and echo cancellation.Signal element codingAt 14 400, 16 800, and 19 200 bps, the scrambled data stream is divided into groupsof six, seven, or eight consecutive data bits, respectively. The ®rst two bits in timeQ1 and Q2 are differentially encoded and trellis encoded following recommendationV.32 bis to generate three trellis-encoded bits, referred to as Y0, Y1, and Y2.These trellis bits and all remaining information bits, Q3 through Q8, are mappedinto the coordinates of the signal elements to form the constellation pattern at aparticular operating rate. Figures 4.34 and 4.35 illustrate the signal space diagramand mapping for V.32 terbo operations at 16.8 kbps and 19.2 kbps, respectively. At14.4 kbps, the signal space diagram is the same as that of the V.32 bis recommendationfor operation at 14.4 kbps.