 a large number of mobile stations. The HLR and VLR have procedures to follow if these data are lost. The HLR is required to keep a nonvolatile storage area with a list of all possible serv- ing systems and another list of all active serving systems, systems actually serving one or more of its subscribers at any particular time. (A disk file is nonvolatile enough for these purposes. The ANSI-41 requirement for nonvolatile is simply that the file is not lost when the power goes off or when a computer is shut down and restarted.) In case of data failure at the HLR, when the HLR recovers, it sends an unreliable roamer data directive message to every serving system on its list so that the serving MSCs can send messages updating the HLR data files. In this way, the HLR can start fresh with a clean and correct representation of its own subscribers. The VLR has similar protection against data failure. It keeps a nonvolatile storage area with a list of all possible HLRs and a list of all HLRs with active visiting sub- 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 ANSI-41 scribers. Should the VLR have a failure, when it recovers, it sends a bulk deregistra- tion message requesting an update to all the HLRs on its list. 15.2.6 Roamer call delivery When a roamer makes a call, the serving MSC and VLR coordinate with the HLR to authenticate the subscriber's validity and level of service. This should be done when the user terminal registers. From this point, the subscriber's call looks like an ordinary mobile telephone call. Incoming calls require more coordination between home and serving systems be- cause the call comes into the originating system rather than the serving system. ANSI- 41 allows that the originating system where the call comes in may not be the same as the subscriber's home system. The originating system is called the gateway s