gnal) if the callED party's mobile telephone is not available. A registration in a system other than the home system creates a record for the sub- scriber in the new system's visitor location register (VLR) and updates the location and status information in its HLR record. At this time, the serving system receives verifi- cation of the user terminal's identity and the service capabilities the subscriber should receive. Authorization can be denied to mobile identification numbers known to be fraudulent and to subscribers not paying their cellular telephone bills. 12.5.6 Small numbers of calls Much of the engineering design of the PSTN is oriented toward supporting many sub- scribers and many calls through a relatively small number of local exchange offices and other facilities. This model reduces cost by achieving economies of scale. In contrast, wireless telephone systems are limited by the capacity of the air inter- face, and they grow by adding base stations. Cellular networks grow by adding new lo- cations and facilities rather than by expanding the capacity of existing facilities. Wire- less network providers add mobile switching centers as well, but the primary element of growth is new cells. Since a typical cell handles 10 to 100 simultaneous calls, the base-station-to-MSC transport in a wireless telephone system never reaches the econ- omy possible in systems where one transport pipe has hundreds of lines. In landline telephony, small trunk groups usually grow into large trunk groups as systems expand to serve more subscribers. In wireless telephony, small pipes grow into more small pipes as more cells are added to meet increasing subscriber demand. 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. Telephony Engineering Concepts Telephony Engineering Concepts The number of user terminals is dictated by the number of subscribers. After u