g network capacity, we must specify capacity re- quirements for these items as well, including the home location register (HLR), the visitor location register (VLR), short message service (SMS) equipment, and voice- messaging equipment. 26.2.1 HLR and VLR capacity The HLR has a record in its database for every subscriber based in its service area. The HLR database has to have enough storage capacity for all its customers. This is not necessarily a function of wireless system traffic load. If the service provider signs up lot of people who got cellular service mostly for emergency use, then there will be a lot 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. Capacity Calculations for Cellular Networks Capacity Calculations for Cellular Networks of subscribers who use their cell phones very little. The HLR must have records in its database for all these subscribers. The service provider has ample warning when this database is reaching its capacity as new subscribers are signed up. The VLR has database records for every subscriber roaming in its territory. This is a dynamic allocation that depends on the momentary concentration of nonresident cell phones in the area. Because of the dynamic nature of VLR traffic, this database can overload without warning. The registration workload for the HLR and VLR depends on the number of sub- scribers turning on their cell phones or moving into a new area. The HLR has some- what more work to do when its subscribers are roaming than when they are home. Every call start and stop generates HLR work, as well as VLR work for roamers. When the call starts, the HLR has to record that the user terminal is in use so that subse- quent calls can be handled correctly, and its available status has to be restored at the end of the call. The registration and call-processing loads are reflected by comparable workloads in th