Stored program control exchange

Stored program control (SPC) was a telecommunications technology used for telephone exchanges controlled by a computer program stored in the memory of the switching system. SPC was the enabling technology of electronic switching systems (ESS) developed in the Bell System in the 1950s and could be considered the third generation of switching technology.

Second generation exchanges such as Strowger, panel, rotary, and 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.

SPC was introduced in electronic switching systems in the 1960s. The 101ESS 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. Examples of SPC based 3rd generation switching systems include: Western Electric 1ESS switch, 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 were large-scale systems in the public switched telephone network. SPC enabled sophisticated calling features. As SPC exchanges evolved, reliability and versatility increased.

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