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1ESS switch
The Number One Electronic Switching System (1ESS) was the first large-scale stored program control (SPC) telephone exchange or electronic switching system in the Bell System. It was placed into service in Succasunna, New Jersey, in May 1965.〔Ketchledge, R.: “The No. 1 Electronic Switching System” IEEE Transactions on Communications, Volume 13, Issue 1, Mar 1965, pp 38-41〕 The switching fabric was composed of a reed relay matrix controlled by wire spring relays which in turn were controlled by a central processing unit (CPU).
The 1AESS central office switch incorporated higher processing capacities based on the 1A Processor in 1976. It was a plug compatible upgrade with a faster processor that incorporated the existing instruction set for programming compatibility, and used smaller ''remreed'' switches, fewer relays, and featured disk storage.〔''1A Processor'', Bell System Technical Journal, 56(2), 119 (February 1977)〕
==Switching fabric==
The voice switching fabric plan was similar to that of the earlier 5XB switch in being bidirectional and in using the call-back principle. The largest full access matrix switches in the system, however, were 8x8 rather than 10x10 or 20x16. Thus they required eight stages rather than four to achieve large enough junctor groups in a large office. Crosspoints being more expensive in the new system but switches cheaper, system cost was minimized with fewer crosspoints organized into more switches. The fabric was divided into ''Line Networks'' and ''Trunk Networks'' of four stages, and partially folded to allow connecting line-to-line or trunk-to-trunk without exceeding eight stages of switching.
For a switch with 1000 input customers and 1000 output customers, a full connection would require a matrix of 1000x1000, or 1 million, physical switches for full interconnection possibility. When one considers that a large telephone system can have many more than 1000 x 1000 customers, the hardware to establish a full interconnection can grow rapidly and exceed practical implementations. Agner Krarup Erlang first theorized a compromise which is based upon the concept that not all telephones lines are connected at the same time. From statistical theory, it is possible to design hardware that can connect most of the calls, in the sense of a high percentage, and block others as exceeding the design capacity. These are commonly referred to as ''blocking switches'' and are the most common in modern telephone exchanges. They are generally implemented as smaller switch fabrics in cascade. In many, a randomizer is used to select the start of a path through the multistage fabric so that the statistical properties predicted by the theory can be gained.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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