In centralized control, all the control equipment is replaced by a single processor which must be quite powerful. It must be capable of processing 10 to 100 calls per second, depending on the load on the system, and simultaneously performing many other ancillary tasks. A typical control configuration of an ESS using centralized SPC is shown in Fig. A centralized SPC configuration may use more than one processor for redundancy purposes. In almost all the present day electronic switching systems using centralized control, only a two-processor configuration is used. A dual processor architecture may be configured to operate in one of three modes:
(i) Standby mode
(ii) Synchronous duplex mode
(iii) Load Sharing mode
In load sharing operation, an incoming call is assigned randomly or in a predetermined order to one of the processors which then handles the call right through completion. Thus both the processors are active simultaneously and share the load and the resources dynamically.