witching stage. It is also sometimes called of a matrix in which the crosspoints are electronic gates controlled memory switching. The size of exchange is limited by the available by a connection store. Each incoming timeslot on each incoming technology since RAM size and speed requirements are propor- circuit is switched individually to the appropriate outgoing circuit, tional to the number of channels provided across the exchange. The Stored program control 31/15 RECEIVE SPEECH STORES TRANSMIT Battery feed to line. Overvoltage protection. Ringing current injection and ring trip detection. Cyclic Supervision (on-hook/off-hook detection). Cyclic Codec for analogue to digital conversion. write Hybrid for two-wire to four-wire conversion. Write Test access to line and associated line circuit. SWITCH The connection between the concentration stage and the main Select switching stage is a pcm multiplex and very suitable for connection over an external transmission line. This is not true of analogue exchanges where the connection consists of a pair of wires per link (channel). Digital exchanges are, therefore, particularly suited to provision I/C address Crosspoint(row) O/G address as a single large central switch serving a number of remote concen- address trators. CONTROL STORES Stored program control Figure 31.19 T S - T switch 31.6.1 Processors continuous evolution of l.s.i. performance makes this technique The functions of a digital exchange switch, described above, need increasingly attractive. Larger exchanges can be constructed by processor control to interpret the connections required in terms of interconnecting separate TS stages. signals received and connection store contents to be maintained. One other possible configuration is space-time-space (STS). This One advantage of central control by processors, is that facility was used for some early designs when the only available time changes may be effected by changes to a few processors instead of switching technology was to insert expensive time delay elements