TABLE OF CONTENTS In this section we look at the general principles of transmission and the range of practical systems used in a telecommunications network.
The purpose of a transmission system is to provide a link between two distant points or nodes. The link may be unidirectional, as in the case of radio broadcast where transmission is only from the transmitter to the radio receivers, or bidirectional, as in the case of a telephone connection with its two-way conversation. In telecommunication networks the links between nodes are either 2- or 4-wire circuits, as described in Chapter 1. With 4-wire circuits, the separate 'Go' and 'Return' paths are considered as transmission channels, as explained in the generalised view of a multiplexed transmission system in Fig. 3.14 of Chapter 3.
In general, a transmission system takes as an input the signal to be conveyed, which may be a single channel or a multiplexed composite of channels, converts that signal into a format suitable for the transmission medium, propagates the transmission signal to the far end, where after conversion from the transmission signal a reproduction of the input signal is produced (Fig. 3.13 of Chapter 3). The actual conveyance of the channel over the transmission system is achieved by 'modulating' an electrical bearer signal which travels over the transmission medium. It is the modulation or modification of the bearer signal that constitutes the transmitted information, not the bearer itself indeed, the bearer signal is usually called a 'carrier' for...
