Editing Network switching subsystem (section)

==Visitor location register (VLR)==<!-- inbound links -->

===Description===
The '''Visitor Location Register (VLR)''' is a database of the MSs ([[Mobile station]]s) that have roamed into the jurisdiction of the Mobile Switching Center (MSC) which it serves. Each main [[Base Transceiver Station|base transceiver station]] in the network is served by exactly one VLR (one [[Base transceiver station|BTS]] may be served by many MSCs in case of MSC in pool), hence a subscriber cannot be present in more than one VLR at a time.

The data stored in the VLR has either been received from the [[Network_switching_subsystem#Home_location_register_.28HLR.29|'''Home Location Register (HLR)''']], or collected from the MS. In practice, for performance reasons, most vendors integrate the VLR directly to the V-MSC and, where this is not done, the VLR is very tightly linked with the MSC via a proprietary interface. Whenever an MSC detects a new MS in its network, in addition to creating a new record in the VLR, it also updates the HLR of the mobile subscriber, apprising it of the new location of that MS. If VLR data is corrupted it can lead to serious issues with text messaging and call services.

Data stored include:  
* [[International mobile subscriber identity|IMSI]] (the subscriber's identity number).
* Authentication data.
* MSISDN (the subscriber's phone number).
* GSM services that the subscriber is allowed to access.
* [[GPRS Access Points|access point (GPRS)]] subscribed.
* The HLR address of the subscriber.
* SCP Address(For Prepaid Subscriber).

===Procedures implemented===
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.
* To purge the subscriber record if a subscriber becomes inactive whilst in the area of a VLR. The VLR deletes the subscriber's data after a fixed time period of inactivity and informs the HLR (e.g., 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).
* To delete the subscriber record when a subscriber explicitly moves to another, as instructed by the HLR.