sible for the areas of one or more MSCs. Mobile Stations are roaming freely and therefore, depending on their current location, they may be registered in one of the VLRs of their home network or in the VLR of a foreign network. Operation and Maintenance Subsystem (OMSS) The network operation is controlled and maintained by the Operation and Maintenance Subsystem (OMSS). Network control functions are monitored and initiated from an Operation and Maintenance Centre (OMC). The OMC has access to both the GMSC and BSC. Some of its functions are: administration and commercial operations (subscribers, end terminals, charging, statistics); security management; network configuration, operation, performance management; maintenance tasks. The OMC configures the BTS via the BSC and allows the operator to check the attached components of the system. User Authentication and Equipment Registration Two additional databases are responsible for the various aspects of system security. They are based primarily on the verification of the equipment and subscriber identity; therefore, the databases serve for user authentication, identification and registration. Confidential data and keys are stored or generated in the Authentication Centre (AUC). The Equipment Identity Register (EIR) stores the serial numbers (supplied by the manufacturer) of the terminals (IMEI), which makes it possible to block service access for mobile stations reported as stolen. Cellular Networks Addresses and Identifiers Mobile Station (MS) These are pieces of equipment used by mobile subscribers for accessing the services. They consist of two major components: the Mobile Equipment (ME) and the Subscriber Identity Module (SIM). In addition to the equipment identifier the International Mobile Station Equipment Identity (IMEI) the mobile station has subscriber identification (IMSI and MSISDN, or the Mobile Subscriber ISDN Number) as subscriber dependent data. Subscriber Identity Module (SIM) The Subscriber Identity Module (SIM) provides mobile equipment with an ide