HLR/VLR

VLR - (Visitor Location Register)
A database of the subscribers who have roamed into the domain of another MNO (Mobile Network Operator/MSC (Mobile Switching Center).

Data stored in the VLR Database include:

• IMSI (the subscriber's identity number)
• Authentication data
• MSISDN (Mobile Subscriber's phone number)
• GSM services that the subscriber is allowed to access
• Access point (GPRS) subscribed
• The HLR address of the subscriber

Responsibilities of the VLR are:

• Inform the HLR that a subscriber has arrived in the particular area covered by the VLR
• Track where the subscriber is within the VLR area (location area) when no call is ongoing
• Authorize which services the subscriber is eligible
• Allocate roaming numbers during the processing of incoming calls
• Purge the subscriber record if a subscriber becomes inactive while in the area of a VLR. The VLR deletes the subscriber's data after a fixed time period of inactivity and informs the HLR
• Delete the subscriber record when a subscriber explicitly moves to another, as instructed by the HLR.

HLR - (Home Location Register)
Central database that contains details of each Mobile Subscriber that is authorized to use the GSM core network. HLRs stores the information of every SIM card issued by the mobile network operator. SIM cards have a unique identifier called an IMSI which is the primary key to each HLR record. MSISDN (Telephone Number) information is also kept within the SIM and is also primary key in the HLR database.

Data stored in the HLR using the IMSI record

• GSM services that the subscriber has requested
• GPRS settings authorizing the subscriber to the access packet services
• Current location of subscriber (VLR and serving GPRS support node/SGSN).
• Call divert settings applicable for each associated MSISDN.

The main responsibility of the HLR is to manage the movement of SIMs and MS.

Process used by the HLR to manage the movement of MS and SIMs

• Manage the mobility of subscribers by means of updating their position in administrative areas called location areas.
• Send the subscriber data to a VLR or SGSN when a subscriber first roams onto another MNO's network
• Broker between the G-MSC or SMSC and the subscriber's current VLR in order to allow incoming calls or text messages to be delivered
• Remove subscriber data from the previous VLR when a subscriber has roamed away from the visiting MNO's network