will instruct the originating SSP to put busy tone or reorder in the caller's ear. At this point, the calling party has several options, one of which is to invoke one of the many CLASS services such as automatic ringback. With automatic ringback, the network will monitor the called number for a period of time, waiting for the line to become available. As soon as it is, the call will be cut through, and the calling party will be notified of the incoming call via some kind of distinctive ringing. Thus, when a call is placed to a distant switch, the calling information is passed to SS7, which uses the caller's number, the called number, and SCP database information to choose a route for the call. It then deter- mines whether there are any special call handling requirements to be invoked such as CLASS services, and instructs the various switches along the way to process the call as appropriate. These features are made up of a set of services known as the Advanced Intelligent Network (AIN), a term coined by Telcordia (for- merly Bellcore, sold to Science Applications International Corporation, recently sold again to Providence Equity and Warburg Pincus). The SSPs (switches) are responsible for basic calling, while the SCPs manage the enhanced services that ride atop the calls. The SS7 network, then, is responsible for the signaling required to establish and tear down calls Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Telephony Chapter 3 and to invoke supplementary or enhanced services. It is critically impor- tant, even more so today as the network begins the complex process of migrating from a circuit-switched model to a VoIP packet-based model. More on that later in the book. Network Topology Before we enter the central office, we should pause to explain the topol- ogy of the typical network (please refer to Figure 3-9). The customer's telep