The Home Location Register (HLR) is the main database of permanent subscriber information for a mobile network. The HLR is an integral component of CDMA (code division multiple access), TDMA (time division multiple access), and GSM (Global System for Mobile communications) networks. Maintained by the subscriber's home carrier (or the network operator where the user initiated the call), the HLR contains pertinent user information, including address, account status, and preferences. The HLR interacts with the Mobile Switching Center (MSC), which is a switch used for call control and processing. The MSC also serves as a point-of-access to the Public Switched Telephone Network (PSTN - the fixed network). The third integral element is the Visiting Location Register (VLR), which maintains temporary user information (such as current location) to manage requests from subscribers who are out of the area covered by their home system.
When a user initiates a call, the switching equipment determines whether or not the call is coming from the device's home area. If the user is out of the home area, the area VLR sends out a request for information required to process the call. An MSC queries the HLR identified by the call for information, which it relays to the appropriate MSC, which in turn relays it to the VLR. The VLR sends routing information back to the MSC which allows it to find the station where the call originated, and, finally, the mobile device to connect. Communications between the elements are based on Signaling System (SS7) protocols and signaling.
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