Different databases in mobile switching center are as follows:

• Home location register (HLR) database: The HLR stores information, both permanent and temporary, about each of the subscribers that "belongs" to it (i.e., for which the subscriber has its telephone number associated with the switching center).

• Visitor location register (VLR) database: One important, temporary piece of information is the location of the subscriber. The location is determined by the VLR into which the subscriber is entered. The visitor location register maintains information about subscribers that are currently physically in the region cov- ered by the switching center. It records whether or not the subscriber is active and other parameters associated with the subscriber. For a call coming to the subscriber, the system uses the telephone number associated with the subscriber to identify the home switching center ofthe subscriber. This switching center can find in its HLR the switching center in which the subscriber is currently physi- cally located. For a call coming from the subscriber, the VLR is used to initiate the call. Even if the subscriber is in the area covered by its home switching cen- ter, it is also represented in the switching center's VLR, for consistency.

• Authentication center database (AuC): This database is used for authentication activities of the system; for example, it holds the authentication and encryption keys for all the subscribers in both the home and visitor location registers. The center controls access to user data as well as being used for authentication when a subscriber joins a network. GSM transmission is encrypted, so it is private. A stream cipher, A5, is used to encrypt the transmis- sion from subscriber to base transceiver. However, the conversation is in the clear in the landline network. Another cipher, A3, is used for authentication.

• Equipment identity register database (EIR): The EIR keeps track of the type of equipment that exists at the mobile station. It also plays a role in security (e.g., blocking calls from stolen mobile stations and preventing use of the network by stations that have not been approved).