TABLE OF CONTENTS Digital transmission consists of converting data into a waveform suitable for the path along which they are to be sent allied to the adherence to some protocol or data structure that allows the receiving device correctly to interpret the data.
In this chapter the fundamentals of digital transmission are introduced along with descriptions of the coding and error-correction techniques used in practical applications.
The generic term for the path down which information is sent is the channel. In a transmission application, the channel may be a point-to-point cable, a network stage or a radio link.
In digital circuitry there is a great deal of noise immunity because the signal has only two states, which are widely separated compared with the amplitude of noise. In transmission this is not always the case. In real channels, the signal may originate with discrete states which change at discrete times, but the channel will treat it as an analog waveform and so it will not be received in the same form. Various frequency-dependent loss mechanisms will reduce the amplitude of the signal. Noise will be picked up as a result of stray electric fields or magnetic induction and the receiving circuitry will contribute some noise. As a result, the received voltage will have an infinitely varying state along with a degree of uncertainty due to the noise. Different frequencies can propagate at different speeds in the channel; this is the phenomenon of group delay. An alternative way of...
