HLR (home location register)

A database residing in a local wireless network that contains service profiles and checks the identity of a local subscriber. (Cingular)

The Home Location Register or HLR is a central database that contains details of each mobile phone subscriber that is authorized to use the GSM core network.

More precisely, the HLR stores details of every SIM card issued by the mobile phone operator. Each SIM has a unique identifier called an IMSI which is one of the primary keys to each HLR record.

The next important items of data associated with the SIM are the telephone numbers used to make and receive calls to the mobile phone, known as MSISDNs. The main MSISDN is the number used for making and receiving voice calls and SMS, but it is possible for a SIM to have other secondary MSISDNs associated with it for fax and data calls. Each MSISDN is also a primary key to the HLR record.

Examples of other data stored in the HLR in a SIM record is:

* GSM services that the subscriber has requested or been given
* GPRS settings to allow the subscriber to access packet services
* Current Location of subscriber (VLR and SGSN)
* Call divert settings applicable for each associated MSISDN.

The HLR data is stored for as long as a subscriber remains with the mobile phone operator.

At first glance, the HLR seems to be just a database which is merely accessed by other network elements which do the actual processing for mobile phone services. In fact the HLR is a system which directly receives and processes MAP transactions and messages. If the HLR fails, then the mobile network is effectively disabled as it is the HLR which manages the Location Updates as mobile phones roam around.

As the number of mobile subscribers has grown in mobile phone operators the HLR has become a more powerful computer server rather than the traditional telephone exchange hardware in the early days of GSM.

Other GSM Core Network Elements connected to the HLR

The HLR connects to the following elements:

* the Gateway MSC (G-MSC) for handling incoming calls
* The VLR for handling requests from mobile phones to attach to the network
* The SMSC for handling incoming SMS
* The voice mail system for delivering notifications to the mobile phone that a message is waiting

Procedures implemented

The main function of the HLR is to manage the fact that SIMs and phones move around a lot. The following procedures are implemented to deal with this:

* Manage the mobility of subscribers by means of updating their position in administrative areas called 'location areas', which are identified with a LAC. The action of a user of moving from one LA to another is followed by the HLR with a Location area update while retrieving information from BSS as BSIC (cell identifier).
* Send the subscriber data to a VLR or SGSN when a subscriber first roams there.
* Broker between the GMSC 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 it.

(Wikipedia)