HLR

The Home Location Register or HLR is the element where all of the subscribers' data is stored. It contains a database of all an operator's subscribers keyed on IMSI and MSISDN.

Data stored includes:

IMSI (the subscriber's identity number - this is the data key) MSISDN (the subscriber's phone number) GSM services that the subscriber has requested or been given Access Points (GPRS) allocated to the subscriber. Current Location of subscriber ( VLR and SGSN)

The HLR data is stored for as long as a subscriber remains with the operator, and therefore needs to be persistent, for example on disk drives.

The primary functions of the HLR are:

To send the subscriber data to a VLR when a subscriber first roams there. to send the subscriber data to an SGSN which requests

the information during a routing area update procedure To broker between the GMSC or SMSC and the subscriber's current VLR in

order to allow incoming calls or text messages to be delivered. To remove subscriber data from the old VLR when a subscriber

has roamed away from it.

VLR

The Visitors Location Register or VLR is a registry of which subscribers are currently in a particular area. The data stored in the VLR has been received from the HLR, or collected from the MS. In practice, for performance reasons, most vendors integrate the VLR directly to the MSC and, where this is not done, the VLR is very tightly linked with the MSC via a proprietary interface.

Data stored includes:

IMSI (the subscriber's identity number) authentication data MSISDN (the subscriber's phone number) GSM services that the subscriber is allowed to access Access Point (GPRS) subscribed the HLR address of the subscriber

The primary functions of the VLR are

to inform the HLR that a subscriber has arrived in the particular area covered by the VLR

to track where the subscriber is within the VLR area (location area) when no call is ongoing

to allow or disallow which services the subscriber may use to allocate roaming numbers during the processing of incoming calls

If a subscriber becomes inactive whilst in the area of a VLR then the VLR may delete the subscriber's data after some time and inform the HLR. This would happen when the phone has been switched off and left off or when the subscriber has moved to an area with no coverage for a long time. The case of a subscriber moving to another area is normally handled by the HLR telling the VLR to delete the subscriber's data explicitly.

What will be the effect if VLR is removing from the System?

VLR is said to be the largest coverage area for a particular MSC, and it stores all the visitors’ information. VLR is an entity that is integrated in MSC. Now if the VLR is removed from the network, first of all the existence of network is not possible. Even if a network is formed, where is the authentication, assigning a number, and several tasks which is whole responsibility of VLR are going to be performed. So a network existence without VLR is impossible.

MSC's VLR data base is failed and is no more operational. Now MSC doesn’t know the location of all MS coming under its domain. Now since all the MSC doesn’t have VLR so locating the MS is very difficult. So paging will happen to all the associated LACs with this particular MSC and in a result processing will exponentially increase and this will also become the cause of all MS power loss. Also BSC will page to its entire LAC as it is unable to sort it from BSIC. In a result the call will certainly be delayed.

I'd like to add HLR as a basic entity. AuC is also required while EIR is optional. I think with no VLR, there'd no MOC & possible as when the call reached MSC/MSS, it has to check from VLR if the subscriber can indeed make this call based on subscription profile that is copied from HLR into VLR. Paging and such problems would be there for MTC. However, if the mobility functions work in conjunction with VLR, then once again the call shall fail.

As normally, when a user want to call then VLR takes updates from HLR & then updates it to MSC so user is allowed to make call. Now consider the situation in which there is no VLR, so when user in one HLR area, want to call, MSC requests updates directly from HLR which on providing, user is allowed to call. It can be the case that network is not providing mobility as HLR is serving only its own area.

VLR is a database that acts as a bridge between MSC & HLR, So, it is necessary to communicate to VLR.