April 2013
Beginner
480 pages
23h 17m
English
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36 Telephony and associated systems
Historically, telephony refers to the transmission of analogue voice signals over open wire or twisted pair unshielded copper cables. The bandwidth of such a system is restricted to 300 to 3400 Hz, which is just sufficient to provide readily recognisable speech of adequate quality.
The interconnections between subscribers is provided via exchanges that select routes or circuits through the system for each telephone call, this circuit switching being achieved by the use of relays and solenoids in the early Strowger type exchanges. Later in the 1960s, this gave way to the use of cross-bar and reed relay switched exchanges to achieve a more flexible means of control. Since that period, there have been significant ...
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