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What information is stored in a Visitor Location Register (VLR)?

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The Visitor Location Register (VLR) is a key component in a mobile telecommunications network, specifically within the GSM (Global System for Mobile Communications) and related technologies such as UMTS (Universal Mobile Telecommunications System) and LTE (Long-Term Evolution). The VLR is responsible for storing temporary information about mobile subscribers who are currently within the coverage area served by a particular Mobile Switching Center (MSC). Here's a detailed technical explanation of the information stored in a Visitor Location Register:

  1. Subscriber Identity Information:
    • IMSI (International Mobile Subscriber Identity): The VLR stores the IMSI of each subscriber currently within its coverage area. The IMSI uniquely identifies a subscriber on a global scale and is composed of the Mobile Country Code (MCC), Mobile Network Code (MNC), and Mobile Subscriber Identification Number (MSIN).
  2. Location Area Information:
    • Location Area Identity (LAI): The LAI identifies the current location area of the subscriber. It consists of the Mobile Country Code (MCC), Mobile Network Code (MNC), and Location Area Code (LAC). The LAI allows the network to quickly locate the subscriber within a specific area.
  3. Subscriber Authentication Information:
    • Authentication Key (Ki): The VLR stores the subscriber's authentication key, Ki, which is used in the Authentication and Key Agreement (AKA) process during the initial connection setup to verify the identity of the subscriber and secure communication.
  4. Subscriber Service Profile:
    • Subscription Information: The VLR maintains information about the subscriber's subscribed services, including voice, data, and supplementary services. This includes details about the type of services the subscriber is allowed to use.
  5. Subscriber's Temporary Mobile Subscriber Identity (TMSI):
    • TMSI: The VLR assigns a Temporary Mobile Subscriber Identity (TMSI) to the subscriber while they are within the VLR's coverage area. The TMSI is used to temporarily identify the subscriber, providing a level of privacy by avoiding the use of the IMSI for routine communication.
  6. Mobile Station Roaming Number (MSRN):
    • MSRN: For subscribers who are currently roaming, the VLR assigns a Mobile Station Roaming Number (MSRN). The MSRN is used for routing calls to the subscriber while they are within the VLR's coverage area.
  7. Subscriber Location Update Status:
    • Update Status Flags: The VLR keeps track of the update status of the subscriber, indicating whether the subscriber's location information needs to be updated in the VLR or Home Location Register (HLR).
  8. Call Routing Information:
    • Call Forwarding and Routing Information: The VLR stores information related to call forwarding and routing for the subscriber, ensuring that calls are appropriately directed based on the subscriber's preferences and service configurations.
  9. Subscriber Mobility Management Information:
    • DRX Parameters: The VLR may store Discontinuous Reception (DRX) parameters related to the subscriber's mobility management. DRX is used to optimize power consumption in the mobile device by allowing it to periodically sleep and wake up to check for incoming messages.
  10. Subscriber Activity and Location History:
    • Location Update Timestamps: The VLR maintains timestamps for the subscriber's recent location updates and activity, providing a history of the subscriber's movements within the VLR's coverage area.
  11. Handover Information:
    • Handover-related Information: If the subscriber is involved in an ongoing call or data session, the VLR may store information related to handovers, ensuring a seamless transition of the subscriber's communication as they move between cells.

In summary, the Visitor Location Register (VLR) stores a range of technical information about mobile subscribers currently within its coverage area. This information is crucial for the proper functioning of the mobile network, enabling subscriber identification, authentication, and the efficient management of communication services while ensuring privacy and security.