ferences. A mobile switching center (MSC) spends much of its time managing radio link assignment and handoff. An MSC also must convert the compressed voice channel optimized for the radio link into a pulse code modulation (PCM) signal accept- able to the PSTN. We will now look at several of the management functions specific to mobile switches. 11.4.1 Registration Each wireless user terminal registers with the system to announce its presence. The system therefore has a complete picture of all the wireless telephones in its service area. This picture is not perfect because wireless telephones can leave the service area or lose power. In Sec. 12.5.2 we describe technologies designed to identify user termi- nals that were registered but which are no longer accessible on the system. When a user terminal recognizes a new system, it registers again. Each MSC keeps a list of the user terminals that are active in its coverage area, and it pages only those terminals for incoming calls. 11.4.2 Paging When a call comes in for a registered user, the wireless system pages the telephone. Only in the earliest days of AMPS were there few enough cellular telephones that we could afford the luxury of paging the entire system for every incoming call. In the huge wireless market of today, the relationship between registration and paging is an im- portant capacity design issue. While unanswered pages are wasteful of system resources, not paging a subscriber expecting an incoming call is far worse. The system design has to be aggressive in tracking and hunting down the intended recipient of any incoming call attempt. Many subscribers are familiar with a typical result of a network's failure to find a subscriber's 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. The PSTN and Telephone Switching The PSTN and Telephone Switching phone. The subscriber never hears the c