nnel terminates into the digital network. In this case, CCITT and Bellcore (now Telcordia) recommend 1020 Hz. The reason for this is explained in Chapter 6. 3 Any signal-passing device, active or passive, can display filter-like properties. A good example is a subscriber loop, particularly if it has load coils and bridged taps. Load coils and bridged taps are discussed in Chapter 5. 3.3 THE THREE BASIC IMPAIRMENTS AND HOW THEY AFFECT THE END-USER propagation of the medium and, of course, the length of the medium. The value can vary from 10,000 mi/sec (16,000 km/sec) to 186,000 mi/sec (297,600 km/sec). The former value is for heavily loaded 4 subscriber pair cable. This latter value is the velocity of propagation in free space, namely, radio propagation. The velocity of propagation also tends to vary with frequency because of the electrical characteristics associated with the network. Again, the biggest culprit is filters. Consid- ering the voice channel, therefore, the velocity of propagation tends to increase toward band center and decrease toward band edge. This is illustrated in Figure 3.4. The finite time it takes a signal to pass through the total extension of the voice channel or through any network is called delay. Absolute delay is the delay a signal experiences while passing through the channel end-to-end at a reference frequency. But we have learned that propagation time is different for different frequencies with the wavefront of one frequency arriving before the wavefront of another frequency in the passband. A modulated signal will not be distorted on passing through the channel if the phase shift changes uniformly with frequency, whereas if the phase shift is nonlinear with respect to frequency, the output signal is distorted with respect to frequency. In essence we are dealing with phase linearity of a circuit. If the phase-frequency relationship over a passband is not linear, phase distortion will occur in the transmitted signal. Phase distortion is often measured by a parameter called envelope de