which are multiplexed in systems or a group of pulses as in pulse code modulated (pcm) groups usually of 24 or 30 speech channels on each link. The tandem systems. exchange has to be able to connect an incoming channel in one pcm Speech itself is analogue and SO too are most telephones. Anal- link to an outgoing channel in another PCM link. Received speech ogue to digital (A/D) conversion (modulation) is, therefore, necess- samples must, therefore, be switched in space to the appropriate link ary at some point in each connection if digital switching is to be and translated in time to the required channel time slot. employed. For customers connected to existing analogue local ex- A simple time switch is shown in Figure 31.17, which provides for changes this occurs when the call first encounters a pcm trans- time slot interchange between the 30 channels of an incoming pcm mission trunk on leaving the exchange. For customers connected to circuit and the 30 channels of an outgoing circuit. a digital local exchange it is necessary to provide the A/D conver- Each successive byte corresponding to a channel is written cycli- sion on each line. The cost of this conversion constitutes a major cally into the appropriate location in a 30 X 8 bit RAM under the part of the exchange cost but is amply compensated for by the control of a time slot counter synchronised with the incoming circuit overall economy of digital switching and the performance advant- clock. The contents of the store are read out to the outgoing circuit ages in the transmission network. under the control of a connection store. The words in the store, The European standard is for speech encoded in 8 bit bytes at which is itself addressed cyclically, identify which address in the 64kbit/s. The US standard is 7 bit coding at 56kbit/s. speech store is to be addressed. If the incoming time slot counter 31/14 Principles of switching systems Outgoing pcm SPACE SWITCH MATRIX row 1 row 2 Incoming pcm row n ADDRESS DECODERS timeslot 1 CONNECTION CONTROL STORES addres