Telecommunication engineering is concerned with the transmission of in- formation between two distant points. Intuitively we may say that a signal contains information if it teils us something we did not already know. This definition is too imprecise for telecommunications studies, and we shall devote a section oftbis chapter to a formal description ofinformation. For the present it is sufficient to say that a signal that contains information varies in an unpredictable or random manner. We have thus specified a primary characteristic of the signals in telecommunications systems; they are random in nature. These random signals can be broadly subdivided into discrete signals that have a fixed number of possible values, and continuous signals that have any value between given Iimits. Whichever type of signal we deal with, the tele- communication system that it uses can be represented by the generalized model of Fig. 1. 1. The centrat feature of this model is the transmission medium or channel. Some examples of channels are coaxial cables, radio links, optical fibres and ultrasonic transmission through solids and liquids. It is clear from these examples that the characteristics of channels can vary widely. The common feature of all channels, however, is that they modify or distort the waveform of the transmitted signal. In some cases the distortion can be so severe that the signal becomes totally unrecognizable. In many instances it is possible to minimize distortion by careful choice of the transmitted signal waveform.