big question in advanced design is how to increase the data rate and keep the error rate reasonable. One important item not accounted for in Shannon's formula is intersymbol interference. A major problem of a pulse in a band-limited channel is that the pulse tends not to die out immediately, and a subsequent pulse is interfered with by "tails" from the preceding pulse, as illustrated in Figure 10.13. 10.9.5 Modem Selection Considerations The critical parameters that affect data transmission have been discussed; these are amplitude-frequency response (sometimes called "amplitude distortion"), envelope delay distortion, and noise. Now we relate these parameters to the design of data modems to Figure 10.13 Pulse response through a Gaussian band-limited channel (GBLC). "Gaussian" refers to a channel limited by thermal noise that has a Gaussian distribution. See Ref. 1. 12 Thermal noise. 10.9 DIGITAL TRANSMISSION ON AN ANALOG CHANNEL establish some general limits or "boundaries" for equipment of this type. The discussion that follows purposely avoids HF radio considerations. As stated earlier in the discussion of envelope delay distortion, it is desirable to keep the transmitted pulse (bit) length equal to or greater than the residual differential EDD. Since about 1.0 msec is assumed to be reasonable residual delay after equalization (condi- tioning), the pulse length should be no less than approximately 1 msec. This corresponds to a modulation rate of 1000 pulses per second (binary). In the interest of standardization [CCITT Rec. V.22 (Ref. 11)], this figure is modified to 1200 bps. The next consideration is the usable bandwidth required for the transmission of 1200 bps. The figure is approximately 1800 Hz using modulation such as FSK (frequency shift keying) or PSK (phase shift keying). Since delay distortion of a typical voice channel is at its minimum between 1700 Hz and 1900 Hz, the required band, when centered around these points, extends from 800 Hz to 2600 Hz or 1000 Hz to 2800 Hz. Remember, the delay should n