home location register and visitor location register

The Home Location Register (HLR) and Visitor Location Register (VLR) are two essential components in a mobile telecommunications network, particularly in the context of GSM (Global System for Mobile Communications) and other cellular networks. These registers play a crucial role in managing the location and mobility of mobile subscribers.

1. Home Location Register (HLR):
   • Definition: The HLR is a central database that stores subscriber information for each mobile user within a specific mobile network. It serves as the home database for subscribers, containing essential information about their subscription and current status.
   • Key Functions:
     • Subscriber Data Storage: The HLR stores subscriber information such as the International Mobile Subscriber Identity (IMSI), subscriber services, current location area, and other relevant details.
     • Authentication and Authorization: When a subscriber initiates a call or data session, the HLR is queried to authenticate the user and authorize the requested services.
     • Call Routing: The HLR is responsible for providing information to route incoming calls to the current location of the subscriber.
   • Subscriber Data: The information stored in the HLR includes:
     • IMSI (International Mobile Subscriber Identity)
     • MSISDN (Mobile Subscriber ISDN Number)
     • Subscriber services and features
     • Authentication and encryption keys
     • Subscriber status (active, suspended, etc.)
   • HLR Interaction:
     • When a subscriber turns on their mobile device, the HLR is queried to retrieve the subscriber's current location information.
     • During call setup, the HLR is consulted to authenticate the subscriber and obtain necessary information for call routing.
2. Visitor Location Register (VLR):
   • Definition: The VLR is a temporary database that stores information about subscribers currently within a specific geographic area or location area covered by a Mobile Switching Center (MSC). The VLR is associated with a particular MSC.
   • Key Functions:
     • Temporary Subscriber Information: The VLR contains temporary information about subscribers currently in the coverage area, including their current location, status, and services.
     • Call Processing: The VLR is involved in call setup and processing, including subscriber authentication, call routing, and handover procedures.
     • Interactions with HLR: The VLR communicates with the HLR to obtain subscriber information and authentication during call setup.
   • Subscriber Data in VLR:
     • MSISDN
     • IMSI
     • Location area and cell information
     • Subscriber status in the visited network
     • Authentication and encryption information
   • VLR Interaction:
     • When a subscriber enters a new location area, the VLR associated with the serving MSC is updated with the subscriber's information.
     • The VLR communicates with the HLR to obtain additional subscriber information and to perform necessary authentication.

Interaction Between HLR and VLR:

• When a subscriber roams into a new location area, the VLR queries the HLR for information about the subscriber.
• The HLR provides the necessary data to the VLR for temporary storage.
• During call setup or other transactions, the VLR may communicate with the HLR to update subscriber information or request additional services.