rolled by the MSC corresponds to that controlled by the VLR, simplifying the signaling required. Note that the MSC does not contain information about particular mobile stations-this information is stored in the location registers. The other two registers are used for authentication and security pur- poses. The Equipment Identity Register (EIR) is a database that contains a list of all valid mobile equipment on the network, where its International Mobile Equipment Identity (IMEI) identifies each mobile station. An IMEI is marked as invalid if it has been reported stolen or is not type approved. The Authentication Center (AuC) is a protected database that stores a copy of the secret key stored in each subscriber's SIM card, which is used for authentication and ciphering of the radio channel. Figure 1-17 "B" Interface The VLR. 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. Introduction to GSM Chapter 1 The Cell As it has already been explained, a cell, identified by its Cell Global Iden- tity (CGI) number, corresponds to the radio coverage of a base transceiver station. In a macrocell environment, the radius distance is between 3 to 35 km. The distances are calculated on the basis of a round-trip between the BTS and the mobile to provide sufficient bit error rate (BER) and power to satisfy quality speech. Location Area A location area (LA), identified by its location area identity (LAI) number, is a group of cells served by a single MSC/VLR. One MSC/VLR combination has several location areas. The LA is part of the MSC/VLR service area in which a mobile station may move freely without any updating of location messaging to the MSC/VLR controlling the location area. MSC/VLR Service Area A group of location areas under the control of the same MSC/VLR defines the MSC/VLR area. A single PLMN can have several MSC/VLR service areas. MSC/VLR is a sole 