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CHAPTER 7 MULTIPLEXED COMMUNICATION SYSTEMS In the early stages of communications, each circuit between two points carried only one set of telegraph signals or one telephone conversation. With increasing demand for such services, the need arose for terminal equipment which would permit the transmission of a number of completely separate communications over a single circuit, i.e. over one pair of wires or an individual radio link. The procedure involved is known as multiplexing and may be achieved by either time division (time sharing) or frequency division. The major difference between the two systems is that in frequency division multiplexing (FDM) the signals of all channels are transmitted simultaneously in different bands of frequencies whereas with time division multiplexing (TDM) samples of all channels are transmitted sequentially within the same frequency band, i.e. only one channel is being transmitted at any given time. Examples of both systems, their development and application are discussed in the following paragraphs. 7.1 Time Division Multiplexing One of the earliest examples of time division multiplexing is the Murray Multiplex system which found wide application for the transmission of a number of telegraph channels over the same circuit. This equipment consisted essentially of an electrically-driven switch or distributor which connected each of four telegraph channels in turn to the outgoing line. At the receiving end, a similar switch connected the line in turn to the equipment associated with each channel. When the two rotating switches are driven in synchronism the separate channels each use the same transmission medium but for only a fraction of the total available time. The rotating switch assembly of a Murray Multiplexer is shown in Fig. 29 (facing p. 161). This method of handling a number of telegraph channels on a single circuit was introduced locally in 1922 and used extensively in telegraphic networks until such time-sharing systems were progressively replaced by voice frequency telegraphy (VFT)