Here we deal with a family of impairments often overlooked by the telecommunication professional. There are two cases under consideration. The first is an analog switch and the second, of course, is the digital switch.
An analog switch is a source of noise, and to some extent, deteriorates the signal-to-noise ratio. The switch is lossy. This second impairment may even be desirable, because a telephone network, by definition, needs to be lossy. Loss helps us control singing and echo. An analog switch adds loss by its very nature. A digital switch, unless we do something about it, is loss-free. An analog switch distorts attenuation-frequency response of the signal passing through. A digital switch is clear of this impairment.
One more impairment which deserves consideration is return loss at input and output ports. This is especially true where a switch interfaces with subscriber loops. Here is the principal source of echo and singing. A digital switch with an analog input and A/D conversion isolates the line; and return loss, in general, is not really of major importance. In Chapter 9 we will present a major discussion of transmission factors in digital switching.
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