Explain the working of any one of centralized SPC?

Standby mode of operation is the easiest of dual processor configuration operations. Usually, one processor is active and another is on standby, software and hardware both wise. The Standby processor is brought online simply while the active processor fails. A significant requirement of this configuration is the capability of the standby processor to reconstitute the state of the exchange system while this takes over the control, that is to find out which of the subscribers and trunks are free or busy, which of the paths are associated by the switching network and so on. In small exchanges, it may be possible by scanning all the status signals immediately the standby processor is brought in operation. In this a case, only the calls that are being established at the time of failure of the active processor are disturbed. In huge exchanges, this is impossible to scan all the status signals in a reasonable time. Now, the active processor copies the status of the system periodically, as every five seconds, in a secondary storage. While a switch over arise, the online processor loads the latest update of the system status from the secondary storage and carries out the operation. In such case, only the calls that changed status in between the last update and the failure of the active processor are disturbed. Figure demonstrates a standby dual processor configuration along with a common backup storage. The shared secondary storage requires not be duplicated and easy unit level redundancy would suffice.

Standby Dual Processor Configuration

Here, P1 = active storage and P2 = stand by processor

FIG - Standby Dual Processor Configuration.