Communications Technology Handbook, 2nd Edition

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 ...