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. Dual processor architecture may be configured to operate in one of three modes:
a. Standby mode
b. Synchronous duplex mode
c. Load Sharing mode
Standby mode of operation is the simplest of dual processor configuration operations. Normally, one processor is active and the other is on standby, both hardware and software wise. The Standby processor is brought online only when the active processor fails. An important requirement of this configuration is the ability of the standby processor to reconstitute the state of the exchange system when it takes over the control, i.e. to determine which of the subscribers and trunks are busy or free, which of the paths are connected through the switching network etc. In small exchanges, this may be possible by scanning all the status signals as soon as the standby processor is brought into operation. In such a case, only the calls which are being established at the time of failure of the active processor are disturbed. In large exchanges, it is not possible to scan all the status signals within a reasonable time. Here, the active processor copies the status of the system periodically, say every five seconds, into a secondary storage. When a switch over occurs, the online processor loads the most recent update of the system status from the secondary storage and continues the operation. In this case, only the calls which changed status between the last update and the failure of the active processor are disturbed. Fig. shows a standby dual processor configuration with a common backup storage. The shared secondary storage need not be duplicated and simple unit level redundancy would suffice.