A home location register (HLR) is a database containing pertinent data regarding subscribers authorized to use a global system for mobile communications (GSM) network. Some of the information stored in an HLR includes the international mobile subscriber identity (IMSI) and the mobile station international subscriber directory number (MSISDN) of each subscription. The IMSI uniquely identifies each Subscriber Identity Module (SIM) and serves as the primary key for each HLR record. The MSISDN (also called the mobile subscriber integrated services digital network) is a list of the telephone numbers for each subscription. Other information stored in the HLR includes services requested by or rendered to the corresponding subscriber, the general packet radio service settings of the subscriber, the current location of the subscriber and call divert settings.
Because cell phones, and their respective SIMs, are mostly mobile, the HLR serves as the main source of recent location information. The HLR is updated each time the SIM transfers into another location area. The HLR also plays a significant role in short message service (SMS) message sending. Before the SMS company forwards the message to the intended recipient, it scans through the HLR to find which mobile switching center (MSC) the recipient has recently used. If the target MSC reports that the recipient’s phone is not available, a message waiting flag is set in the HLR. If the recipient appears in another MSC (for example, when flying to another city), he still receives the message because the MSC will notify the HLR once the recipient is detected in its jurisdiction. Other cellular components actively working with the HLR include the gateway mobile switching center (G-MSC), visitor location register (VLR) and the authentication center (AUC).
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