in order to correctly demultiplex the stream of pulses it receives and extract the signals concerning each channel individually. Since the receiver must be continuously synchronised in both frequency and phase this reference point is repeated peri- odically. The easiest way to understand this is to cyclically represent the time division multiplexing process as illustrated in Figure 21.2. Figure 21.2 depicts a rotating switch S scanning a number of Figure 21.2 Cyclical representation of time division channels connected to its inputs. The channels are scanned one by multiplexing one in the order they are encountered by the switch: channel 1 is sampled at time t1, channel 2 at time t2, and finally channel n at time to, after which the process is repeated. The resulting output signal is composed of the samples of the different channels, shifted over a exactly the same frequency and phase, it demultiplexes the input time period given by Equation 21.1. signal and distributes the extracted signals over the respective channels. Both systems synchronise each time they pass the refer- ence point. (21.1) Time division multiplexing has become very important, not only in transmission but also in switching, particularly in connection to If the channels presented at the input are pulse amplitude modu- the digital systems. lated signals, we obtain a signal as depicted in Figure 21.1. At the In the early days of electrical communications a medium such as other side of the connection we find an identical switch executing copper wire carried a single information channel. For economic the opposite operation, i.e. scanning the composite input signal at reasons, in terms of both cost and equipment, it was necessary to Channel 1 Channel 2 Channel 3 1 frame Channel 123123123 Figure 21.1 Time division multiplexing 21/4 Time division multiplexing find ways of packing multiple channels onto one physical link. The Digital time division multiplex resulting system is referred to as a carrier. Digital signals are now structure transmitted from