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Stored program control (SPC) is 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. Early 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, while the Western Electric 1ESS switch and the AXE telephone exchange by Ericsson were large-scale systems in the public switched telephone network. SPC enabled sophisticated calling features. As SPC exchanges evolved, reliability and versatility increased. 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. ==Introduction== The principle 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, 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.〔A. E. Joel, ''An Experimental Electronic Switching System'', Bell Laboratory Record, October 1958 p.359〕 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 a standard feature in all electronic 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. Nonetheless, private automatic branch exchanges (PABX) and smaller exchanges do use electronic switching devices. The two types of space division electronic switching systems are using electromechanical switching network and the other using electronic switching network. The second type is fully electronic. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「stored program control」の詳細全文を読む スポンサード リンク
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