nd the HLR and two messages between the serving MSC and the HLR. Roamer call delivery is a very important and fairly common cellular event. And roamer call delivery can have many outcomes. The user terminal may be available to answer the call, the user terminal may be out of the service area, or the user terminal may be busy on another call. The subscriber may or may not answer the call. The sub- scriber may have the bad timing to initiate a call after the assignment of a temporary local directory number (TLDN) but before the call setup to the TLDN arrives through the telephone network. In all these cases, however, there is a location request message from the originating MSC to the HLR and some kind of response back from the HLR. Also, unless the user terminal has powered down and sent a message to the base station saying so, there is a message from the HLR to the serving MSC requesting a TLDN and an acknowledg- ment, favorable or otherwise. This means that we should estimate the volume of roamer terminated calls for each MSC. We should do this for each MSC in the role of home MSC and in the role of serv- ing MSC. The traffic patterns in a major metropolitan area may have significant asym- metries between the two because commuters buy service in their suburban neighbor- hoods and use their cell phones in their city-center workplaces. Call Processing Virtually every time a subscriber does something with his or her user terminal, the serving MSC contacts the HLR, and the HLR sends some response. Whenever a call is made or received, the user terminal registers, and the HLR verifies its validity. When the subscriber picks up voice messages, the serving MSC signals the HLR, which, in 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. ANSI-41 Signaling Capacity Capacity and Quality Principles turn, signals the voice-message system. When the subscribe