e difference in message count between a handoff forward and handoff back is only one message, so the message count from subsequent handoffs on a call mostly depends on whether there are many tandem-MSC handoffs and whether there are many path minimizations. And any call that ends with its serving MSC not the same as its anchor MSC generates two mes- sages. The same analysis that estimates the ANSI-41 workload also can be used to es- timate the demand on the MSC-to-MSC links dedicated to handoff traffic. 25.2 Roaming We want to know how much signaling activity is associated with roaming subscribers. Handoffs only involve ANSI-41 signaling when a call moves from one MSC region to an- other, but roaming involves significant signaling for any subscriber activity in an MSC region other than the user's home MSC region. 1 When a roaming user terminal is detected in a wireless telephone system, the serv- ing MSC sends the home location register (HLR) a message through its visitor loca- tion register (VLR) requesting service qualification and expects a reply message. This message exchange takes place when the visiting user terminal is detected initially. If the original service qualification covers a particular coverage area and the user ter- minal is detected outside that area, then the serving MSC requests service qualifica- tion again. This does not have to be a new MSC region because service qualification may be limited to just a part of the entire MSC service area. Also, a service qualifica- tion can have a time limit, and a new service qualification is required when that time limit expires. 1In this chapter we are using the ANSI-41 definition of roaming, where a user terminal is roam- ing whenever it is being served by an MSC other than its home MSC. The subscriber may not be notified of or charged for roaming in this technical sense of the term. Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is su