Visitor Location Register for the B-subscriber

Description

VLRB is the logical counterpart to VLRA, representing the functional role of a standard VLR when its served mobile subscriber is the recipient (B-subscriber) of a call or session. This distinction is critical within the call termination process. When a call is routed towards a mobile subscriber, the Gateway MSC (GMSC) queries the subscriber's HLR for routing information. The HLR, in turn, sends a MAP Provide Roaming Number request to the VLR where the subscriber is currently registered. This VLR, now acting as the VLRB, allocates a temporary Mobile Station Roaming Number (MSRN) and responds to the HLR, enabling the GMSC to route the call to the serving MSC (which becomes the MSC-B).

The VLRB's responsibilities are focused on termination. It validates that the B-subscriber is allowed to receive calls (e.g., not barred) and is reachable (not detached). It manages the paging procedure within its Location Area to locate the specific mobile device. For the called party, the VLRB also handles the invocation of any terminating Camel services, such as customized call forwarding or incoming call screening, based on trigger points in the B-subscriber's profile fetched from the HLR. The VLRB interacts with its co-located MSC to complete the call setup to the terminating device.

In charging, the VLRB generates charging data records for the terminating leg of the call. These records are separate from those created by the VLRA and are crucial for inter-operator settlements, especially in international roaming cases where the calling and called networks are different. The clear separation of VLRA and VLRB roles ensures that service logic, billing, and lawful interception mandates are applied accurately to each party involved in the communication, providing a clean architectural separation of control functions for the two ends of a connection.

Purpose & Motivation

The definition of VLRB, alongside VLRA, was driven by the necessity for symmetric and unambiguous handling of both parties in a mobile call within network protocols and procedures. As mobile networks evolved to support complex roaming and advanced value-added services, it became inadequate to refer simply to 'the VLR' in signaling specifications. The network needed to know precisely whether an operation (like fetching a roaming number, applying call barring, or triggering a Camel service) pertained to the caller's side or the callee's side.

This distinction solves critical problems in inter-network communication and service provisioning. For instance, in a call between a subscriber roaming in Network X and a subscriber roaming in Network Y, the home networks of both subscribers need to interact with the correct visiting network. The VLRB identifier allows the HLR of the B-subscriber to correctly address its query for an MSRN. It also enables the correct application of terminating services (e.g., 'Call Forward on Busy') which are controlled by the B-subscriber's service profile and executed in the VLRB/MSC-B, independent of the originating network's procedures.

Furthermore, VLRB is essential for accurate charging and legal compliance. Terminating network operators charge for the use of their radio and core network resources to deliver a call to a roaming subscriber. The charging records from the VLRB form the basis for these charges. Similarly, lawful interception orders for a target subscriber must be executed by the VLR currently serving them, which is the VLRB for incoming communications. Thus, the VLRB concept ensures clarity, fairness, and compliance in multi-operator mobile ecosystems.

Key Features

• Logical role assigned to a physical VLR when its served subscriber is the called party (B-subscriber)
• Allocates Mobile Station Roaming Numbers (MSRN) for inbound call routing upon HLR request
• Manages terminating call control procedures, including paging and subscriber reachability checks
• Triggers Camel service logic for the B-party (e.g., call forwarding, screening)
• Generates charging data records for the terminating leg of a call
• Critical node for executing mobile-terminated call delivery in roaming scenarios

Evolution Across Releases

Defining Specifications