Abstract: An automatic telephone exchange, such as a PABX, includes a number of group control units (GCU1 to GCU4) each of which is a simplified version of the system of U.S. Pat. No. 4581733. These are connected via respective PCM highways to a central switch, which is an electronic time space switch, and a central controller. The central controller does the major part of the call setting and control, all calls being set up via the central switch.

Abstract: In a small business-type telephone exchange, calls are set up in TDM manner using PCM to convey speech and data if the exchange serves any data terminals. The exchange has subscriber line ports (SLC) each serving two lines and a smaller number of trunk line ports (TLC) each serving a trunk to a local exchange. Each port has a local processor (SIP) with analogue-digital and digital to analogue conversion circuitry (CODEC-FILTER-TSAC). The exchange also has a central processor (CCP) with its associated memories (ROM, RAM, EAROM).The ports and the central processor are interconnected by an intelligence bus (PCM BUS) and a signalling bus (SIG BUS). The central processor (CCP) and the port processors (SIP) co-operate in call setting with all communications between the processors over the signalling bus. During operation the central processor (CCP) polls the ports via the signalling bus (SIG BUS) for ports needing the services of the central processor and also to pass call control information to those ports.