Other examples of SPC-based third-generation switching systems include the British GPO TXE (various manufacturers), Metaconta 11 (ITT Europe), and the AKE, ARE.
Examples include the Ericsson ARE 11 (local) and ARE 13 (transit), as well as the North Electric NX-1E & D Switches, and the ITT Metaconta 11, once found throughout Western Europe and in many countries around the world.
SPC technology using analog switching matrices was largely phased out in the 1980s and had disappeared from most modern networks by the late 1990s.
The addition of time-division multiplexing (TDM) decreased subsystem sizes and dramatically increased the capacity of the telephone network.
Viable, fully digital switches emerged in the 1970s, with early systems, such as the French Alcatel E10 and Canadian Nortel DMS series going into production during that decade.
The principal feature of stored program control is one or multiple digital processing units (stored-program computers) that execute a set of computer instructions (program) stored in the memory of the system by which telephone connections are established, maintained, and terminated in associated electronic circuitry.
An immediate consequence of stored program control is automation of exchange functions and introduction of a variety of new telephony features to subscribers.
In the 1980s SPC displaced electromechanical switching in the telecommunication industry, hence the term lost all but historical interest.
Today, SPC is an integral concept in all automatic exchanges, due to the universal application of computers and microprocessor technology.
Although many present day exchange design continue to use centralized SPC, with advent of low cost powerful microprocessors and VLSI chips such as programmable logic array (PLA) and programmable logic controllers (PLC), distributed SPC became widespread by the early 21st century.
An important requirement of this configuration is ability of standby processor to reconstitute the state of exchange system when it takes over the control; means to determine which of the subscriber lines or trunks are in use.
In small exchanges, this may be possible by scanning the status signals as soon as the standby processor is brought into action.
The shared secondary storage need not to be duplicated and simple unit level redundancy would suffice.
In synchronous duplex mode of operation hardware coupling is provided between two processors which execute same set of instructions and compare the results continuously.
When system is operating normally, the two processors have same data in memories at all times and simultaneously receive information from exchange environment.
It is possible that a comparator fault occurs only due to transient failure which is not shown even when check out program is run.
In such case three possibilities exists: When a processor is taken out, it is subjected to extensive testing to identify a marginal failure.