Data Communications Networking Devices - 4th Ed.pdf

396 _________________________________ WIDE AREA NETWORK TRANSMISSION EQUIPMENTFigure 4.29V.32 channel derivationsynchronous converter built into the modem. A V.32 modem uses a carrierfrequency of 1800 Hz and has a modulation rate of 2400 bauds to support a datatransfer rate of 9600 bps. At 9600 bps, the V.32 recommendation speci®es twoalternative modulation schemesÐnon-redundant coding and trellis coding.The use of non-redundant coding results in a 16-point constellation pattern,while trellis coding results in a 32-point constellation pattern. Readers should notethat while all V.32 modems must be capable of `interworking' using the 16-pointconstellation pattern not all V.32 modems will include trellis coding. Users thatrequire the better immunity to impairments thus afforded by trellis coding shouldensure that the V.32 modem they are considering supports that coding alternative.Non-redundant codingUnder the non-redundant coding technique, the data to be transmitted at 9600 bpsis divided into groups of four consecutive data bits. The value of the ®rst two bits isused in conjunction with the value of the two bits last output to generate the valuefor the next two output bits. These two output bits are then used with the value ofbits three and four of the quadbit to select an appropriate signal point. Table 4.12indicates the dependencies of the output dibit selection upon the value of the inputdibit and the previous output dibits. Note that the value of the two input bits Q1 nand Q2 n ) and the value of the previous dibit outputs Y1 n 1 and Y2 n 1 ) are used todetermine the phase quadrant change where the signal point will be located;however, this does not actually locate the point in the quadrant. To perform thelatter operation the dibit outputs Y1 n and Y2 n ) are used in conjunction with thevalues of the third and fourth bits in the quadbit input to select a position in thequadrant.To select a quadrant position, the value of the dibit output Y1 n and Y2 n )isused in conjunction with the value of the third and fourth input Q3 and Q4) bits.Table 4.13 indicates the selection of the non-redundant coding axis position basedupon the values of the dibit output and the second dibit input.To illustrate the use of Tables 4.12 and 4.13, assume a quadbit input has thevalue 0001 and the previous dibit output was 01. Since Q1 n and Q2 n have the value00 while Y1 n 1 and Y2 n 1 have the value 01, from Table 4.12 the outputs Y1 n and