s Figure 31.18 Simple space switch and connection store run in synchronism, the outgoing circuit car- in accordance with the addresses stored in the space switch connec- ries a similar multiplex to the incoming circuit but with the timeslots tion store. in a different order as determined by the contents of the connection A complete tandem exchange might use a time switch, like Figure store. In practice, the connection store may have a different speed 31.17, to connect each incoming circuit to a row in a matrix, like or a different number of channels (words) from the incoming circuit. Figure 31.18. The verticals of the matrix could be the outgoing Practical PCM systems employ additional channels for signalling circuits but are more usually highways connected to a time switch and framing purposes. These can be extracted for use by the ex- associated with each outgoing trunk group. This arrangement is change control by providing additional RAM addresses from which shown in Figure 31.19 and is known as time-space-time (TST) they are read out on to a circuit connected to the exchange control switching. processor. Each incoming circuit, therefore, requires 32 X 8 bit If the internal exchange bit rate is high enough to interleave all the RAM addresses. incoming channels on to a single highway, no space switch is For large exchanges, each time switch serves a group of say 32 required. Each cross-office timeslot corresponds to a unique incoming circuits using 1024 RAM addresses. The connection store outgoing circuit/timeslot combination SO no outgoing time switch is then runs at 32 times the incoming multiplex data rate to interleave required either. Allocation of highway timeslots to outgoing chan- all 30 X 32 speech channels on a single high speed highway to the nels is achieved by gating on a fixed pattern of strobe clock pulses. next switching stage in the exchange. This is sometimes known as TS switching, the output gating being A simple TDM space switch is shown in Figure 31.18 consisting regarded as a 'space' (S) s