rs became multi-user, so munications networking. Its central function, from the network the need grew for more users gaining remote access over a single managers viewpoint, is to concentrate many users (or information transmission line. Single users could gain access by simply attach- channels) on to a single transmission channel in order to maximise terminal to a modem that converted the digital information the efficiency of that channel: it is used in almost every aspect of by modulation into a voice frequency that could then be transmitted networking digital data, voice and video. This section will describe down a telephone to a modem at the receiving end that demodulated the advantages and disadvantages of different data multiplexing the signals back into digital form and hence onto the computer. techniques, why these different techniques evolved to solve particu- However the need for groups of co-located users to gain access to lar network engineering problems and how they fit in to modern computing resource meant that multiplexing techniques had to be networks. applied to maximise the utilisation of rented PTT lease line circuits Figure 40.1 shows the basic theoretical model for a multiplexer or costly dial up calls. with the composite line speed exactly equal to the aggregate speed The early forms of multiplexers, which evolved to meet this of the inputs. growth in computing power, were designed for use with asyn- Given a transmission channel, there are two ways the available chronous data as this was the most commonly available type of bandwidth can be used: firstly by dividing the available bandwidth terminal and printer. Typically running at speeds of up to 1200bit/s frequency spectrum into a subset of frequencies, each of which can and with code formats of 5 and 7 bits (Baudot and ASCII respec- then simultaneously use the transmission channel and allocate each tively). As computer manufacturers moved to synchronous data, as frequency band to an input channel that needs to be multiplexed; or a more effi