ls to address the increasing capacity (remember that the first tech- nique for spectrum reuse is to break down the coverage areas into smaller chunks), there are more base transceiver stations. Therefore, we need an intermediate device, the base station controller, to control a group of base station transceivers. The base station controllers feed into the mobile switching center, which interfaces into the group of databases that enable roaming, billing, and interconnection, as well as interfacing to a gateway mobile switching center, which passes on the relevant bill- ing information SO that the home service provider can produce invoices. A number of databases are critical to the process of roaming: Home location register-The home location register provides information about the subscribers in an area controlled by a particular mobile switching center. Visitor location register-The visitor location register stores information about the calls being made by roaming subscribers and periodically for- wards information to subscribers' home service providers for billing and 2G: Digital Cellular Radio Gateway MSC (GMSC) = Base Transceiver Station (BTS) VLR = Visitor Location Register BSC = Base Station Controller HLR = Home Location Register MSC = Mobile Switching Center AUC = Authentication Center EIR = Equipment Identity Register Figure 14.4 The digital cellular architecture other purposes. Each mobile service switching center must have a visitor location register. Authentication center-The authentication center protects the subscriber, and therefore the carrier, from unauthorized access, providing security fea- tures such as encryption and customer identification. It is associated with the home location register. Equipment identity register-This database registers the mobile equip- ment types and maintains a database of equipment that has been stolen or blacklisted for some reason. As mentioned earlier in this chapter, one of the problems in the wireless realm is that a plethora of standards have been developed and a