ng is going wrong when it gets 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 Key Telephony Concepts two information requests from two separate systems for the same call. One of the sig- nals is from the intended serving MSC, and the other is from an unintended MSC. Part of the call-attempt message from a serving MSC to the HLR is a radio signal path measurement, at least for potential border cells. When it gets the two conflict- ing messages, the HLR uses the two radio signal strength measurements to deter- mine which of the two systems should be serving the call. It then tells the other sys- tem to abandon it. 15.2.4 User terminal state management The user terminal state is either active or inactive. An active mobile telephone is one that is available for call delivery. While an inactive mobile telephone cannot receive telephone calls, it may still be able to accept SMS deliveries. A user terminal is inactive under these circumstances: It has not registered anywhere. There is no valid location. It goes out of radio contact when it misses a registration, whether an autonomous regis- tration or one scheduled by the system. It is in sleep mode, a subscriber-designated setting where it can place calls but not receive them. The serving system has designated the user terminal inactive according to the server's own internal rules. An inactive user terminal remains inactive until a serving MSC sends the HLR a reg- istration notification message. 15.2.5 HLR and VLR fault recovery The ANSI-41 specification allows for the case where an HLR or VLR can lose its cur- rent status data for its subscribers. While we would hope that wireless service providers would do frequent backups on their HLR databases SO that the account records would be safe from mishap, the HLR and VLR keep a moment-by-moment record of the location and registration state of