Stored program control: Difference between revisions

Revision as of 00:43, 23 June 2020

Revision as of 20:58, 18 November 2021

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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 [[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, 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: 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]] [[PRX (telephony)|PRX]] were large-scale systems in the [[public switched telephone network]].

SPC enables sophisticated [[calling feature]]s. As SPC 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 of their 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}})

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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 [[Time-division multiplexing|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, by implementing 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, ''An Experimental Electronic Switching System'', Bell Laboratory Record, 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 standard in all automatic exchanges.

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