Stored program control: Difference between revisions

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{{Short description|Technology for telephone exchanges}}

{{Refimprove|date=December 2009}}

'''Stored program control''' ('''SPC''') wasis a telecommunications technology used for [[telephone exchangesexchange]]s. Its characteristic is that the switching system is controlled by a computer program stored in thea memory ofin the switching system. SPC was the enabling technology of [[electronic switching system]]s (ESS) developed in the [[Bell System]] in the 1950s, and may be considered the third generation of switching technology. Stored program control was invented in 1954 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= February 15, 2008-02-15

|url= http://www.networkworld.com/community/node/25111

|accessdateaccess-date= 2012-06-17

}}</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

|accessdateaccess-date= 2012-06-17

}}</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

|accessdateaccess-date= 2012-06-17

}}</ref>

== History ==

Proposed and developed in the 1950s, SPC was introduced in production [[electronic switching system]]s in the 1960s. The 101ESS [[Privateprivate branch exchange|PBX]] (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, ILIllinois, in a 1960 trial of electronic switching, followed shortly thereafter 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. and prePre-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 enabledenables sophisticated [[calling feature]]s. As SPCsuch exchanges evolved, reliability and versatility increased.

Second -generation exchanges such as [[Strowger switch|Strowger]], [[Panel switch|panel]], rotary, and [[Crossbar switch|crossbar]] switches were constructed purely from electromechanical switching components with [[combinational logic]] control, and had no computer software control. The first generation were the manual switchboards operated by attendants and operators.

Later crossbar systems also used computer control ofin theirthe 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.

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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 5thfifth generation of telephony switching, as [[Timetime-division multiplexing|TDM]] (TDM) and specialist hardware -based digital circuit switching is being replaced by [[softswitch]]<nowiki/>es and voice over IP [[VoIP]] technologies.

The principleprincipal 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.

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,; byit implementingimplements a [[fault-tolerant design]]. Early trials of electronics and computers in the control sub systems of an exchange were successful and resulted in the development of fully electronic systems, in which the switching network was also electronic. A trial system with stored program control was installed in [[Morris, Illinois]] in 1960. It used a [[flying-spot store]] with a word size of 18 bits for semi-permanent program and parameter storage, and a barrier-grid memory for random access working memory.<ref>A. E. Joel,{{cite ''journal
|title=An Experimental Electronic Switching System'',
|last=Joel
|first=A. E.
|journal=Bell LaboratoryLaboratories Record,
|volume=36
|issue=10
|page=359
|date=October 1958 p.359
}}</ref> The world’s first electronic switching system for permanent production use, the [[No.1 ESS]], was commissioned by AT&T at [[Succasunna, New Jersey]], in May 1965. By 1974, AT&T had installed 475 No. 1ESS systems. In the 1980s SPC displaced electromechanical switching in the telecommunication industry, hence the term lost all but historical interest. Today, SPC is standardan integral concept in all automatic exchanges, due to the universal application of computers and microprocessor technology.

The attempts to replace the electromechanical switching matrices by semiconductor cross -point switches were not immediately successful, particularly infor large-scale exchangesexchange systems. As a result, many space -division switching systems used electromechanical switching networks with SPC, while private automatic branch exchanges (PABX) and smaller public exchanges used electronic switching devices. Electromechanical matrices were replaced in the early 21st century by fully electronic onesdevices.

==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.

ThisWhen schemea is based on the assumption that the faultprocessor is transienttaken one and may not appear again. Hereout, the processor taken outit is subjected to extensive testing to identify a marginal failure in these cases.

====Load-sharing mode====

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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.<ref>{{Cite book|url=https://www.worldcat.org/oclc/29022605|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>

* 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.

* In horizontal Decompositiondecomposition each processor performs only one or only some exchange functions.

== See also ==