Q. Explain Standby mode of operation in SPC organization?

Standby mode of operation is the simplest of dual processor configuration operations. Generally one processor is active and other is on standby, both software and hardware wise. The Standby processor is brought online only when active processor fails. An significant requirement of this configuration is the ability of standby processor to reconstitute the state of exchange system when it takes over the control, which implies to determine which of the subscribers and trunks are busy or free, which of the paths are connected through switching network etc. In small exchanges, this may be possible by scanning all status signals as soon as standby processor is brought into operation. In such a case, only calls which are being established at the time of failure of active processor are disturbed. In large exchanges, it isn't possible to scan all the status signals within a reasonable time. The active processor copies the status of system periodically, say every five seconds, into a secondary storage. When a switch over takes place, online processor loads the most recent update of system status from the secondary storage and continues the operation. In this case, only calls that changed status between the last update and failure of the active processor are disturbed. Figure demonstrates a standby dual processor configuration with a common backup storage. Shared secondary storage need not be duplicated and simple unit level redundancy would suffice.