nated Q1, Q2, Q3, and Q4. Figure 10.17 shows the signal space diagram of the V.29 modem. There are 16 points in the diagram representing the 16 quadbit possibilities. Other modems of the CCITT V series are designed to operate at 14,400 bps, 28,800 bps, 33.6 kbps, and up to 56 kbps over the standard voice channel. The actual transmission rates that are achievable also have a lot to do with how far out (e.g., distance) from the local switch the modem is attached on the subscriber loop and the condition of the loop. Their waveforms become extremely complex. Table 10.4 Amplitude-Phase Relationships for the CCITT Rec. V.29 Modem Relative Signal Absolute Phase (°) Element Amplitude 0, 90, 180, 270 45, 135, 225, 315 0° Absolute Figure 10.17 Signal space diagram for the CCITT Rec. V.29 modem when operating at 9600 bps (Ref. 12). DATA COMMUNICATIONS 10.9.6 Equalization Of the critical circuit parameters mentioned in Section 10.9.3, two that have severely deleterious effects on data transmission can be reduced to tolerable limits by equal- ization. These two are amplitude-frequency response (amplitude distortion) and EDD (delay distortion). The most common method of performing equalization is the use of several networks in tandem. Such networks tend to flatten response and, in the case of amplitude response, add attenuation increasingly toward channel center and less toward its edges. The overall effect is one of making the amplitude response flatter. The delay equalizer operates in a similar manner. Delay increases toward channel edges parabolically from the center. To compensate, delay is added in the center much like an inverted parabola, with less and less delay added as the band edge is approached. Thus the delay response is flattened at some small cost to absolute delay, which has no effect in most data systems. However, care must be taken with the effect of a delay equalizer on an amplitude equalizer and, conversely, of an amplitude equalizer on the delay equalizer. Their design and adjustment must be such that the 