Visitor Location Register (VLR)

Visitor Location Register (VLR)

The VLR contains a copy of most of the data stored at the HLR. It is, however, temporary data which exists for only as long as the subscriber is “active” in the particular area covered by the VLR.

The VLR database will therefore contain some duplicate data as well as more precise data relevant to the subscriber remaining within the VLR coverage (here coverage means status of customer of last lac location area code updation )

The VLR provides a local database for the subscribers wherever they are physically located within a PLMN, this may or may not be the “home” system. This function eliminates the need for excessive and time-consuming references to the “home” HLR database.

The additional data stored in the VLR is listed below:

1. Location Area Identity (LAI).
2. Temporary Mobile Subscriber Identity (TMSI).
3. Mobile Station Roaming Number (MSRN).
4. Mobile status (busy/free/no answer etc.).

Location Area Identity

Cells within the Public Land Mobile Network (PLMN) are grouped together into geographical areas. Each area is assigned a Location Area Identity (LAI), a location area may typically contain 30 cells.

Each VLR controls several LAIs and as a subscriber moves from one LAI to another, the LAI is updated in the VLR. As the subscriber moves from one VLR to another, the VLR address is updated at the HLR.

Temporary Mobile Subscriber Identity (TMSI)

The VLR controls the allocation of new Temporary Mobile Subscriber Identity (TMSI) numbers and notifies them to the HLR. The TMSI will be updated frequently, this makes it very difficult for the call to be traced and therefore provides a high degree of security for the subscriber.

The TMSI may be updated in any of the following situations:

• Call setup.
• On entry to a new LAI.
• On entry to a new VLR.

Mobile Subscriber Roaming Number

As a subscriber may wish to operate outside its “home” system at some time, the VLR can also allocate a Mobile Station Roaming Number (MSRN). This number is assigned from a list of numbers held at the VLR (MSC). The MSRN is then used to route the call to the MSC which controls the base station in the MSs Current location.

The database in the VLR can be accessed by the IMSI, the TMSI or the MSRN. Typically there will be one VLR per MSC.