Data Communications Networking Devices - 4th Ed.pdf

402 _________________________________ WIDE AREA NETWORK TRANSMISSION EQUIPMENTAs indicated in Table 4.16, the V.32 bis modem supports two operating ratesabove the maximum operating rate of the V.32 modem. Since those operating ratesre¯ect the data transfer capability of each modem without the effect of datacompression, the addition of that feature to each modem signi®cantly increases thedifference in throughput achievable through the use of each modem. For example,a V.32 modem that supports V.42 bis data compression, which provides an averagecompression ratio of 4:1, results in a throughput of 38.4 kbps when the modemoperates at 9600 bps. In comparison, a V.32 bis modem operating at 14 400 bpsemploying V.42 bis compression provides a throughput of 57.6 kbps when theaverage compression ratio is 4:1. Thus, the operating rate difference of 4800 bpsbetween a V.32 and a V.32 bis can expand to a throughput difference of almost 20kbps when the effect of compression is considered.Both V.32 and V.32 bis modems employ trellis coding at operating rates of 9600bps and above. Each modem operates at 2400 baud and packs either four, ®ve, orsix data bits plus the Trellis Coded Modulation TCM) bit into each signal change.The constellation pattern thus increases from 32 signal points for a V.32 modemoperating at 9600 bps to 64 and 128 signal points for a V.32 bis modem operating at12 000 and 14 400 bps, respectively.At an operating rate of 14.4 kbps the V.32 bis modem divides data to betransmitted into groups of six consecutive bits. The ®rst two bits in time Q1 n andQ2 n ) in each group are differently coded into Y1 and Y2 in a similar manner to thatpreviously described for the coding used by a V.32 modem. However, the outputsbased on similar inputs and similar previous output are changed. Table 4.17provides a summary of the differential encoding used by a V.32 bis modemoperating at 14.4 kbps. You can note the differences between the coding of bits Y1 nTable 4.17V.32 bis differential dncoding for use with trellis codingInputs Previous Outputs OutputsQ1 n Q2 n Y1 n 1 Y2 n 2 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 1