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connect each subscriber directly to every other subscriber or to
have a centralized switch within a network with the same number
of direct lines as there are combinations of customers. Rather,
to satisfy the economic considerations of a telephone system, the
switch function in a given area is centralized at an office (the
central office), which receives calls placed by customers and
then routes those calls through one of a number of outlets to
other subscribers within the local network or to other local
networks via long-distance trunks.
The earliest automated switching systems used direct
progressive control to operate the switch, whereby relay switches
along the path connecting the calling and the called parties
would be closed as each digit in a telephone number was dialed
until a complete connection was made.
To address certain disadvantages of progressive control
systems, telephone designers in the 1940s began incorporating
registers, devices which store and release dialed telephone
numbers into telephone switches. Using registers, systems could
be devised for looking ahead to ascertain the best possible
routing. Moreover, with such use, common control, or control of
various telephone functions by a centralized mechanism shared by
separate lines, was possible. By the 1950s, switches used
electromechanical switches and relays to accomplish the key
control functions of a switch on a common basis, including
determining routing, seizing trunk lines, and ringing the call
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Last modified: May 25, 2011