Editing Network switching subsystem (section)

==Home location register (HLR) {{anchor|HLR}}==
The '''home location register''' (HLR) is a central database that contains details of each mobile phone subscriber that is authorized to use the GSM core network. There can be several logical, and physical, HLRs per [[Public Land Mobile Network|public land mobile network]] (PLMN), though one [[international mobile subscriber identity]] (IMSI)/MSISDN pair can be associated with only one logical HLR (which can span several physical nodes) at a time.

The HLRs store details of every [[SIM card]] issued by the mobile phone operator. Each SIM has a unique identifier called an IMSI which is the [[primary key]] to each HLR record.

Another important item of data associated with the SIM are the MSISDNs, which are the [[telephone number]]s used by mobile phones to make and receive calls.  The primary 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 [[unique key]] to the HLR record. The HLR data is stored for as long as a subscriber remains with the mobile phone operator.

Examples of other data stored in the HLR against an IMSI record is:
* GSM services that the subscriber has requested or been given.
* [[General Packet Radio Service]] (GPRS) settings to allow the subscriber to access packet services.
* Current location of subscriber (VLR and [[SGSN|serving GPRS support node]]/SGSN).
* [[Call forwarding|Call divert]] settings applicable for each associated MSISDN.

The HLR is a system which directly receives and processes [[Mobile Application Part|MAP]] transactions and messages from elements in the GSM network, for example, the location update messages received as mobile phones roam around.

===Other GSM core network elements connected to the HLR===
The HLR connects to the following elements:
* The G-MSC for handling incoming calls
* The VLR for handling requests from mobile phones to attach to the network
* The SMSC for handling incoming SMSs
* The <span lang="id">voice mail</span> system for delivering notifications to the mobile phone that a message is waiting
* The AuC for authentication and ciphering and exchange of data (triplets)

===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 procedure.
* Send the subscriber data to a VLR or SGSN when a subscriber first roams there.
* Broker between the G-MSC or SMSC and the subscriber's current VLR in order to allow [[GSM services#Call set up process for incoming calls|incoming calls or text messages to be delivered]]{{Broken anchor|date=2024-07-22|bot=User:Cewbot/log/20201008/configuration|target_link=GSM services#Call set up process for incoming calls|reason= The anchor (Call set up process for incoming calls) [[Special:Diff/8922898|has been deleted]].}}.
* Remove subscriber data from the previous VLR when a subscriber has roamed away from it.
* Responsible for all SRI related queries (i.e. for invoke SRI, HLR should give sack SRI or SRI reply).