Stored program control: Difference between revisions

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{{Short description|Technology for telephone exchanges}}

'''Stored program control''' ('''SPC''') is 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]], 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

|access-date= 2012-06-17

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== 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, Illinois, 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 enables 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. Some such switches in countries like Russia or Ukraine may still be in service.{{citation needed|date=December 2019}})

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 fifth 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 principal 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.

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|page=359

|date=October 1958

}}</ref> The world’s first electronic switching system for 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 aan 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 for large-scale exchange 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 devices.