nal areas: 1. Supervisory 2. Address 3. Call progress audible-visual Another signaling breakdown is A. Subscriber signaling B. Interswitch (interregister) signaling 7.2 DEFINING THE FUNCTIONAL AREAS 7.2.1 Supervisory Signaling 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 off- hook) 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 SIGNALING 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. 7.2.2 Address Signaling Address signaling directs and routes a telephone call to the called subscriber. It originates as dialed digits or activated push-buttons from a calling subscriber. The local switch accepts these digits and, by using the information contained in the digits, directs the call to the called subscriber. If more than one switch is involved in the call setup, signaling is required between switches (both address and supervisory). Address signaling between switches is called interregist