Telephone exchange
Telecommunication system for establishing telephone calls
Telephone exchange
Digital telephony started with so called channel banks which converted analog trunk (inter-exchange) circuits into a digital stream of 24 circuits in North America. AT&T Bell Labs then developed an all digital trunk exchange manufactured by Western Electric, the AT&T manufacturing arm, call a #4ESS in the early 1970's. The technical barriers in developing an all digital class 5 end office (an office connecting directly to analogue subscribers telephone) were significant. Each subscribers line connects to a line circuit in the end office, line circuits thus constitute a large proportion of the hardware and cost is an important factor. An all electronic line circuit had to manage problems at the limits of electronics in the 1970's. These problems were summarized by the "death word" BORCHT. Battery feed - all telephone lines were powered by a 50v feed. Overload protection - equipment had to be resilient to lightning strikes, AC mains contact and other perils Ringdown - subscribers...
Read on Wikipedia · Image Joseph A. Carr, Attribution
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