 used to control a few transceivers. They communicate over a specified interface known as the Abis interface. By having a standard specification different suppliers can be used for each. One link further up the chain from the base station controller is the Mobile Switching Centre (MSC). This is defined in the GSM specifications Base station network subsystem Figure 10.7 Basic base station configuration for a GSM system Cellular telecommunications 265 Figure 10.8 A typical cellular telecommunications base station located on a tall building covering an area of a town to act like a normal ISDN, or PSTN switching node. In addition to this it provides all the functionality to provide subscriber registration, authenti- cation, call routing and handovers. The MSC also has access to the Authentication Centre, a protected database that stores a copy of the secret key kept in each subscriber's SIM card. It also checks the IMEI to ensure that the phone has not been reported stolen and that it is type approved. Only when the checks have been accomplished is the phone allowed onto the network. The Home Location Register (HLR) is an important register. It is a large database in the system that stores the administrative information for each subscriber in a particular network. It details the location, i.e. the base station with which the mobile will communicate, whether the mobile is 266 Newnes Guide to Radio and Communications Technology turned on and the services to which the user has access. To enable roaming, which is a key element of GSM, a further register called the Visitor Location Register (VLR) is provided to enable visitors to register on a network, for example when they are abroad. At registration the VLR obtains information from the user's HLR SO that while the user is within the visited network he can be treated like a local user. The GSM standard also defines the frequencies to be used: 890-915 MHz is used for the uplink and 935-960 MHz is used for the downlink. In some countries the full allocation is not availab