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user. Even identifying, measuring, and recording a long-distance
toll call was accomplished through common control mechanical
devices.
With the advent of solid-state electronics, some or all of
the common control functions were accomplished by utilizing
integrated circuits on which processing (control) instructions
were encoded. Instead of electromechanical devices opening and
shutting in a predetermined (programmed) fashion to respond to
various alternative situations, electronic circuits would be
opened or closed in accordance with the embedded logic.
In the mid-1960s, new telephone switches began to use
specially designed processors called stored program control
(SPC), which execute programs encoded on magnetic tape or other
media, rather than wired-logic, to control certain switch
functions. At first, the tendency in digitalized switch design
was to centralize most switch control functions in one central
processing unit. By the end of 1985, it was deemed more
beneficial if certain control functions were performed by
decentralized processors controlled by the central processing
unit.
The advantages of SPC were that, because its program was
loaded by tape, rather than in electromechanical devices or
hard-wired integrated circuits, the program could be more easily
maintained (or changed), and the speed of electronics could be
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