A Study of Network Performance and Customer Behavior During Direct-Distance-Dialing Call Attempts in The U.S.A.

A complex sequence of interactions and reactions is initiated each time a person or machine attempts to call another person or machine via the switched public telephone network. In the case of local calls, the setup process involves station equipment, subscriber loops, at least one local switching office (end office) with its multitude of equipment, and perhaps interoffice trunks and local tandem offices with possible local alternate routing capabilities. Several local switching arrangements are illustrated in Fig. la and b. In the case of long distance (toll) calls, the switching arrangements are more complex because of a five-level switching hierarchy, and they are more flexible because of the extensive use of alternate routing. 1 A standard toll switching arrangement is illustrated in Fig. lc. At one extreme of this switching arrangement, a toll call may encounter ten switching offices interconnected by seven final intertoll trunks and two toll connecting trunks; at the other extreme, a toll call may encounter two end offices connected by a single direct intertoll trunk. In between these extremes, a toll call may be established through several switching offices interconnected by a combination of toll connecting, final intertoll, and high-usage intertoll trunks. In general, a toll call may be established in several different ways between the same two originating and terminating stations. In Fig. lc, there are four separate routes which may be traversed. The toll switching algorithm establishes calls on a "link-by-link" basis.