Stored program control

Stored program control (SPC) is a telecommunications technology for telephone exchanges. Its characteristic is that the switching system is controlled by a computer program stored in a memory in the switching system. SPC was the enabling technology of electronic switching systems (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. Proposed and developed in the 1950s, SPC was introduced in production electronic switching systems 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, Illinois, in a 1960 trial of electronic switching, followed 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. Pre-digital (1970s) versions of the AXE telephone exchange by Ericsson and Philips PRX were large-scale systems in the public switched telephone network (PSTN). SPC enables sophisticated calling features. As such exchanges evolved, reliability and versatility increased. Second-generation exchanges such as Strowger, panel, rotary, and 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 in the switching matrices, and may be considered SPC systems as well.