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.
History 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. 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. 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. Viable, fully digital switches emerged in the 1970s, with early systems, such as the French Alcatel E10 and Canadian Nortel DMS series going into production during that decade. Other widely adopted systems became available in the early 1980s. These included Ericsson AXE 10, which became the world's most popular switching platform, the Western Electric 5ESS used through the US and in many other countries, the German designed Siemens ESWD, the ITT System 12 (later rebranded Alcatel S12) and NEC NEAX all of which were widely used around the world. The British developed System X (telephony), and other smaller systems also emerged in the early 1980s. 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 time-division multiplexing (TDM) and specialist hardware-based digital circuit switching is replaced by softswitches and voice over IP VoIP technologies. The principal feature of stored program control is one or multiple digital processing units (stored-program computers) that execute a set of computer instructions (program) stored in the 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; it implements 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. 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 an 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.
Types Stored program control implementations may be organized into centralized and distributed approaches. Early electronic switching systems (ESS) developed in the 1960s and 1970s almost invariably used centralized control. Although many present day exchange design continue to use centralized SPC, with advent of low cost powerful microprocessors and VLSI chips such as programmable logic array (PLA) and programmable logic controllers (PLC), distributed SPC became widespread by the early 21st century.
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