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SPC was introduced in [[electronic switching system]]s in the 1960s. The 101ESS [[Private branch exchange|PBX]] was a transitional switching system in the Bell System to provide expanded services to business customers that were otherwise still served by an electromechanical central office switch. Examples of SPC based 3rd generation switching systems include: Western Electric [[1ESS switch]], British GPO [[TXE]] (various manufacturers), Metaconta 11 (ITT Europe) and the AKE, ARE and pre-digital (1970s) versions of the [[AXE telephone exchange]] by [[Ericsson]] and [[Philips]] [[
== History ==
These systems could considered to be the 3rd generation of public telephony switching technology, following on from 1st generation manual switch boards and 2nd generation step-by-step, rotary, crossbar and similar electromechanical technologies.
However, later crossbar systems used computer control of their switching matrices and would be considered full SPC systems. Examples include the Ericsson ARE 11 (local) and ARE 13 (transit) and the ITT Metaconta 11, once found throughout Western Europe and in many countries around the world.
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SPC technology using analog switching matrices began to be phased out in the 1980s and had disappeared from most modern networks by the late 1990s.
Viable, fully digital switches emerged in the 1970s, with early systems like the French [[Alcatel-Lucent|Alcatel]] E10 and Candian Nortel DMS going live during that decade. Other widely adopted systems hit the market in the early 1980s. These included Ericsson AXE 10, which would become the world's most popular switching platform, the [[Western Electric]] [[5ESS]] used through the US and in many other countries, the German designed Siemens ESWD, the ITT System 12 (later rebranded [[Alcatel (mobile device brand)|Alcatel]] S12) and [[NEC]] NEAX all of which were widely used around the world. The British developed [[System X telephone exchange]] and many other successful smaller systems also emerged in the early 1980s
Some digital switches (notably the 5ESS and very early versions of Ericsson AXE 10) continued to use analog concentrator stages, using SPC like technologies, rather
As at 2017, we move onto a 5th generation of telephony switching as [[Time Division Multiplexing|TDM]] and specialist hardware based digital circuit switching is being replaced by soft switches and voice over IP [[VoIP]] technologies.
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=== Distributed control ===
Distributed SPC is both more available and more reliable than centralized SPC. The control function are shared by many processors within the exchange. It uses low cost [[microprocessor]]<nowiki/>s. Exchange control may decomposed either horizontally or vertically for distributed processing.
* In vertical decomposition the whole exchange is divided into several blocks and a processor is assigned to each block. This processor performs all tasks related to that specific block. Therefore, the total control system consists of several control units coupled together. For redundancy, processors may be duplicated in each block.
* In horizontal Decomposition each processor performs only one or only some exchange functions.
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