Overview#Visitor Location Register (VLR) is a database of the subscribers who have roamed into the jurisdiction of the MSC (Mobile Switching Center) which it serves.
Each main base station in the network is served by exactly one VLR (one BTS may be served by many MSCs in case of MSC in pool), hence a subscriber cannot be present in more than one Visitor Location Register at a time.
The data stored in the Visitor Location Register has either been received from the Home Location Register, or collected from the Mobile Station. In practice, for performance reasons, most vendors integrate the Visitor Location Register directly to the V-MSC and, where this is not done, the Visitor Location Register is very tightly linked with the Mobile Switching Center via a proprietary interface. Whenever an Mobile Switching Center detects a new Mobile Station in its network, in addition to creating a new record in the Visitor Location Register, it also updates the Home Location Register of the mobile subscriber, apprising it of the new location of that Mobile Station. If Visitor Location Register data is corrupted it can lead to serious issues with text messaging and call services.
Data stored include:
- 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.
- Home Location Register address of the subscriber.
- SCP Address(For Prepaid Subscriber).