Editing Common control

{{more citations needed|date=January 2013}}

In [[telecommunications]], '''common control''' is a principle of switching [[telephone call]]s in an automatic [[telephone exchange]] that employs shared control equipment which is attached to the circuit of a call only for the duration of establishing or otherwise controlling the call.<ref>{{Cite book
|url=http://archive.org/details/bellsystem_No5CrossbarVol1EquipmentApplicationsMay63
|title=No. 5 Crossbar Volume 1: Equipment Applications May 63|last=Western Electric Co.
|date=1963-05-01
}}</ref>  Thus, such control equipment need only be provided in as few units to satisfy overall exchange traffic, rather than being duplicated for every subscriber line.

In contrast, ''direct control'' systems have subsystems for call control that are an integral part of the switching network. [[Strowger exchange]]s are usually direct control systems, whereas crossbar, and electronic exchanges (including all [[stored program control]] systems) are common control systems. Common control is also known as indirect control or register control.

== History ==
Early semi-mechanical installations with common control components existed, for example [[rotary system|rotary]] systems in Sweden and France in 1915, and the first [[panel switch]]es in Newark, New Jersey, also in 1915.  The first large-scale, fully automatic, common control switching system deployed in commercial production service was the ''ATlantic'' central office in Omaha, Nebraska, a panel system cut over on December 10, 1921. Other panel offices for Kansas City and New York City (the ''PENnsylvania exchange'') were in planning at the same time and opened shortly after.

In 1922, common control was introduced in [[Strowger switch|Strowger-type]] step-by-step systems,<ref>Automatic Electric Company, ''The Automatic Director in Strowger Metropolitan Telephone Systems'', in ''Automatic Telephone'', Volume 10(11-12), November 1922, p.116
</ref> resulting in the first installations of [[Director telephone system|Director]] systems in Havana, Cuba in 1924, and in London, England in 1927.

By the mid-1920s, common control ideas had extended to include [[Marker (telecommunications)|marker]] systems for testing for idle [[Trunking#Telecommunications|trunks]].{{citation needed|date=February 2019}}

During the 1960s, common control exchanges became [[stored program control exchange|stored program control]] exchanges,<ref name=bosse2007>J.G. Van Bosse, F.U. Devetak, ''Signaling in Telecommunication Networks'', 2nd edition (2007), p.111</ref> and by the 1970s they used [[common-channel signaling]] in which the channels that are used for [[signaling (telecommunications)|signaling]] are not used for [[message]] traffic (out of band signaling).<ref name=bosse2007/>

== References ==
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{{FS1037C MS188}}

[[Category:Telephone exchange equipment]]