ot exceed duration of a bit (baud). At 1200 bps, the duration of a bit is 0.833 msec. Turning to Figure 10.10, the delay distortion requirement is met over the range of 800-2800 Hz. Bandwidth (Hz) limits modulation rate. However, the modulation rate in bauds and the data rate in bits per second need not necessarily be the same. This is a very impor- tant concept. The "baud rate" is the measure of transitions per second. We then must turn to bit packing much as we did in Section 9.3 (Digital LOS Microwave). There we briefly discussed PSK and especially quadrature phase shift keying (QPSK). Let's review it here. The tone frequency for this example is 1800 Hz. The phase of that tone can be retarded. For binary phase shift keying, we assign binary 0 to 0° (no phase retardation) and binary 1 to 180° retardation as illustrated in Figure 10.14. Suppose we divide the circle in Figure 10.14 into quadrants with the unretarded phase at 0° as before (unretarded), then with 90°, 180°, and 270° of retardation. Let us assign two binary digits (bits) to each of the four phase possibilities. These could be as follows: Phase Change Equivalent (Degrees) Binary Number We could call it a 4-ary scheme. More commonly it is called quadrature phase shift keying (QPSK). This scheme is illustrated in Figure 10.15. Figure 10.14 A spatial representation of a data tone with BPSK modulation. Commonly a circle is used when we discuss phase modulation representing the phase retardation relationship up to 360°. DATA COMMUNICATIONS Figure 10.15 A spatial representation of a data tone with QPSK modulation. Nearly all data modems operating at 2400 bps use QPSK modulation. The phases do not have to start at 0°. The only requirement is that each contiguous phase be separated from its neighbor by 90° in the case of QPSK. Of course we can carry the idea still further by breaking the circle up into 45° phase segments. There are now eight phase segments, each segment represent a distinct grouping of 3 bits. This is shown in Figure 10.16. It is called 8-ar