Data Communications Networking Devices - 4th Ed.pdf

4.2 MODEMS _____________________________________________________________________ 399Table 4.14V.32 differential encoding for use with trellis-coded alternative at 9600 bpsInputs Previous outputs OutputsQ1 n Q2 n Y1 n 1 Y2 n 1 Y1 n Y2 n0 0 0 0 0 00 0 0 1 0 10 0 1 0 1 00 0 1 1 1 10 1 0 0 0 10 1 0 1 0 00 1 1 0 1 10 1 1 1 1 01 0 0 0 1 01 0 0 1 1 11 0 1 0 0 11 0 1 1 0 01 1 0 0 1 11 1 0 1 1 01 1 1 0 0 01 1 1 1 0 1Table 4.15 indicates the trellis coding signal points while Figure 4.31 illustrates theconstellation pattern formed by a plot of all 32 signal points. When a V.32 modemoperates at 4800 bps, as previously explained the device operates upon dibitsinstead of quadbits. When this occurs, the bits Q1 n and Q2 n are differentiallyencoded into Y1 n and Y2 n according to Table 4.12 which results in a constellationpattern of four signal points indicated in Figure 4.31 by the letters A, B, C, and D.Since the V.32 recommendation was promulgated in 1984 numerous modemmanufacturers have used the V.32 modulation scheme as a platform for addingenhancements to this modem. These enhancements include error detection andcorrection capability, data compression, callback security, and other features.14 400 bpsCurrently, the only standardized 14 400 bps modems are the ITU V.32 bis andV.33 recommendations.V.32 bis modemThe V.32 bis recommendation was promulgated in 1981 and represents the ITUstandard for modems operating on the switched telephone network at data rates upto 14 400 bps. Although this modem has been available for over a decade, until 1992the average retail price of this modem exceeded $1000 and limited its widespreaduse. Since then, manufacturers have reduced the retail price of a V.32 bis modem tounder $100, which has signi®cantly increased its acquisition by personal computer