Supervisory signaling provides information on line or circuit condition. It informs a switch whether a circuit (internal to the switch) or a trunk (external to the switch) is busy or idle, when a called party is off-hook or on-hook, and when a calling party is on-hook or off-hook.
Supervisory information (status) must be maintained end-to-end on a telephone call, whether voice data or facsimile is being transported. It is necessary to know when a calling subscriber lifts her/his telephone off-hook, thereby requesting service. It is equally important that we know when the called subscriber answers (i.e., lifts her telephone offhook) because that is when we may start metering the call to establish charges. It is also
Fundamentals of Telecommunications, Second Edition, by Roger L. Freeman ISBN 0-471-71045-8 Copyright © 2005 by Roger L. Freeman important to know when the calling and called subscribers return their telephones to the on-hook condition. Now is when charges stop, and the intervening trunks comprising the talk path as well as the switching points are then rendered idle for use by another pair of subscribers. During the period of occupancy of a speech path end-to-end, we must know that this particular path is busy (i.e., it is occupied) so no other call attempt can seize it.
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