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be prevented. The economy requirement refers to the need to
provide service at a competitive price. To that end, it is
necessary to avoid overbuilding a system despite the desire for
availability, reliability, and privacy. Availability and
privacy, which generally are presumed requirements, voice
quality, price, and reliability are the bases by which systems
compete, with reliability being the key differentiating basis.
The reliability standards for telephone switches are far more
stringent than for most modern computer systems.
An automated telephone switch comprises (1) the switching
network, (2) various interfaces, and (3) control mechanisms. The
heart of the switch is the network, which consists of individual
devices designed to connect (and disconnect) communication paths.
Before integrated circuit switching, automated switching was
accomplished through a series of electromagnetic relays. When a
call was placed, a physical connection path would be formed by
closing the appropriate relays. In its most simplified form,
i.e., a network system consisting of only two telephone
customers, the operation of the switch would involve nothing more
than closing and opening the relay switch on the line running
between the two customers. When the relay was closed, the lines
of the two customers would be connected and the switch would have
functioned, enabling conversation.
A local telephone system may consist of hundreds of
thousands of customers. It would be cost prohibitive either to
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Last modified: May 25, 2011