HOME LOCATION REGISTER (HLR)

HLR FUNCTIONS

The HLR is a centralized network database that stores and manages all mobile subscriptions belonging to a specific operator. It acts as a permanent store for a person's subscription information until that subscription is cancelled. The information stored includes:

• Subscriber identity (i.e. IMSI, MSISDN)

• Subscriber supplementary services

• Subscriber location information (i.e. MSC service area)

• Subscriber authentication information

The primary functions of the HLR include:

• Subscription database management:

As a database, the HLR must be able to process data quickly in response to data retrieval and update requests from other network nodes. For this reason it acts as a database management system. Each subscriber record contains a substantial amount of parameters.

• Communication with MSC’s:

 When setting up calls to an MS, it is necessary for the HLR to contact the MSC serving the MS for routing information. By analyzing the MSISDN, MSC knows which HLR to contact worldwide for that MS’s subscription.

• Communication with GMSC’s:

During call set-up to an MS, the GMSC requests MS location information from the HLR, which then provides this in the form of routing information. Also, if the subscriber is detached the HLR will inform the GMSC that there is no need to perform further routing of the call.

• Communication with AUC’s:

Before any activity involving change or use of subscription information takes place, the HLR must retrieve new authentication parameters from an AUC.

• Communication with VLR’s/ILR’s:

 When an MS moves into a new MSC service area the VLR for that area requests information about the MS from the HLR of the subscriber. The HLR provides a copy of the subscription details, updates its MS location information and instructs the old VLR to delete the information it has about that MS. As the ILR acts as a VLR for AMPS subscribers, the HLR communicates with it in a similar way.

HLR IMPLEMENTATION

The HLR can be implemented in the same network node as the MSC/VLR (i.e. MSC/VLR/HLR) or as a stand-alone database. An MSC/VLR/HLR node is a suitable solution for a small startup GSM network as it saves hardware and signaling load on the links between MSC/VLR and HLR. A stand-alone HLR is a suitable solution for large networks. It has the following advantages:

• There are no traffic disturbances creating better reliability

• When the HLR is separate from the MSC/VLR, there is more capacity available for call handling in the MSC/VLR If the number of subscribers exceeds the capacity of a HLR, additional HLR's may be added.

HLR Redundancy

In order to provide additional network reliability, an additional “mated” HLR is used to mirror the data in a HLR and can automatically take over if required.