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{{Short description|Technology for telephone exchanges}}
'''Stored program control''' ('''SPC''') is a telecommunications technology
Early exchanges such as [[Strowger switch|Strowger]], [[Panel switch|panel]], rotary, and [[Crossbar switch|crossbar]] switches were constructed purely from electromechanical switching components with analog control electronics, and had no computer software control. Stored program control was invented by [[Bell Labs]] scientist [[Erna Schneider Hoover]] in 1954 who reasoned that computer software could control the connection of telephone calls.<ref name=tws2Q312>{{cite news▼
|author= Alpha Doggs
|title= Phone switching pioneers to be inducted in National Inventors Hall of Fame
|work= Network World
|date=
|url= http://www.networkworld.com/community/node/25111
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}}</ref><ref name=tws2Q315>{{cite news
|title= Erna Schneider Hoover
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|date= 2012-06-17
|url= http://www.maximumpc.com/article/features/15_most_important_women_tech_history
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}}</ref><ref name=tws2Q311>{{cite news
|title= Erna Schneider Hoover
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|date= 2012
|url= http://www.ieeeghn.org/wiki/index.php/Erna_Schneider_Hoover
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}}</ref>
== History ==
Proposed and developed in the 1950s, SPC was introduced in production [[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. The first central office switch with SPC was installed at Morris, Illinois, in a 1960 trial of electronic switching, followed by the first Western Electric [[1ESS switch]] at Succasunna, NJ in 1965. Other examples of SPC-based third-generation switching systems include the British GPO [[TXE]] (various manufacturers), Metaconta 11 (ITT Europe), and the AKE, ARE. Pre-digital (1970s) versions of the [[AXE telephone exchange]] by [[Ericsson]] and [[Philips]] [[PRX (telephony)|PRX]] were large-scale systems in the [[public switched telephone network]] (PSTN).
SPC enables sophisticated [[calling feature]]s. As such exchanges evolved, reliability and versatility increased.
The principle feature of stored program control is one or multiple digital processing units ([[stored-program computer]]s) that execute a set of computer instructions (''program'') stored in the [[computer memory|memory]] of the system by which telephone connections are established, maintained, and terminated in associated electronic circuitry.▼
▲
Later crossbar systems also used computer control in the switching matrices, and may be considered SPC systems as well. 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. By the 1980s, SPC technology dominated the telecommunications industry.
Viable, fully digital switches emerged in the 1970s, with early systems, such as the French [[Alcatel-Lucent|Alcatel]] E10 and Canadian Nortel DMS series going into production during that decade. Other widely adopted systems became available in the early 1980s. These included Ericsson AXE 10, which became 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 (telephony)]], and other 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 than direct connections to the digital line cards containing the [[CODEC]].
Early in the 21st century the industry began using a fifth generation of telephony switching, as [[time-division multiplexing]] (TDM) and specialist hardware-based digital circuit switching is replaced by [[softswitch]]<nowiki/>es and voice over IP [[VoIP]] technologies.
▲The
An immediate consequence of stored program control is automation of exchange functions and introduction of a variety of new telephony features to subscribers.
A telephone exchange must run continuously without interruption at all times
|title=An Experimental Electronic Switching System |last=Joel |first=A. E. |journal=Bell |volume=36 |issue=10 |page=359 |date=October 1958 }}</ref> The world’s first electronic switching system for The attempts to replace the electromechanical switching matrices by semiconductor cross
==Types==
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===Centralized control===
In centralized control, all control equipment is replaced by a central processing unit. It must be able to process 10 to 100 [[calls per second]], depending on the load to the system.{{citation needed|date=July 2015}} Multiprocessor configurations are commonplace and may operate in various modes, such as in load-sharing configuration, in synchronous duplex-mode, or one processor may be in stand-by mode.
====Standby mode====
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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:
*Continue with both processors: This is based on the assumption that the fault is transient and may not appear again.
*Take out the active processor and continue with the other.
*Continue with active processor but remove other processor from service.
====Load-sharing mode====
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There is an inter processor link through which the processors exchange information needed for mutual coordination and verifying the 'state of health’ of the other. If the exchange of information fails, one of the processors which detect the same takes over the entire load including the calls that are already set up by the failing processor. However, the calls that were being established by the failing processor are usually lost. Sharing of resources calls for an exclusion mechanism so that both the processors do not seek the same resource at the same time. The mechanism may be implemented in software or hardware or both. Figure shows a hardware exclusion device which, when set by one of the processors, prohibits access to a particular resource by the other processor until it is reset by the first processor.
=== 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]]s. Exchange control may decomposed either horizontally or vertically for distributed processing.<ref>{{Cite book|title=Telecommunication switching systems and networks|last=Thiagarajan.|first=Viswanathan|date=1992|publisher=Prentice Hall of India Private Ltd|isbn=0876927134|___location=New Delhi|oclc=29022605}}</ref>
== See also ==
* [[List of telephone switches]]
* [[Stored-program computer]]
== References ==
{{reflist}}
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