Description
VLRA is not a separate physical network element but a logical role assigned to a standard VLR within the context of a specific call or session setup. In 3GPP specifications, particularly those dealing with call control (e.g., TS 23.018) and Camel service logic, the terms 'VLRA' and 'VLRB' are used to unambiguously identify which VLR is associated with the originating subscriber (A-party) and which is associated with the terminating subscriber (B-party). When a mobile subscriber initiates a call (becomes the A-subscriber), the VLR that currently holds their temporary service profile assumes the role of VLRA for the duration of that call establishment and control process.
During a mobile-originated call setup, the VLRA performs several critical functions. It validates that the A-subscriber is allowed to originate calls and has the necessary subscription rights. It interacts with the serving MSC (which becomes the MSC-A) to initiate the call setup signaling. The VLRA provides the A-subscriber's profile, including any originating call screening or barring services, to the MSC. If Camel (Customized Applications for Mobile network Enhanced Logic) services are invoked for the A-party, the VLRA plays a key role in triggering the appropriate gsmSCF (Camel service node) by detecting the configured trigger points in the subscriber's profile.
Furthermore, the VLRA is central to charging processes. It generates or forwards charging data records (CDRs) related to the originating leg of the call. These records, identified with the role of VLRA, include details such as the calling party number, the serving MSC, start time, and, for later correlation, the call identifier. This distinction between VLRA and VLRB is essential in complex call scenarios, such as calls between two roaming subscribers in different countries, where the originating and terminating networks need to clearly identify the responsible nodes for service control, lawful interception, and billing settlement.
Purpose & Motivation
The concept of VLRA (and VLRB) was introduced to provide precise functional addressing within 3GPP signaling and procedures. In a distributed network with millions of roaming scenarios, a simple 'VLR' identifier is insufficient to specify *which* VLR—the caller's or the callee's—is being referenced in a given message or procedure. This distinction solves ambiguity in call control, service logic execution, and charging.
Its specification was motivated by the need for clear and unambiguous protocol design, especially for advanced telephony services like Camel. Camel allows network operators to define custom call control logic (e.g., prepaid, virtual private network). For a Camel-controlled call, the network must trigger service logic in the A-subscriber's home network (using VLRA) and potentially different logic in the B-subscriber's home network (using VLRB). Without the A/B designation, signaling messages could be misinterpreted, leading to incorrect service execution or charging.
This functional split also addresses the requirements of inter-operator accounting and lawful interception. When two different operators are involved in a call (one for the caller, one for the callee), billing records must clearly attribute charges to the correct network element. Similarly, interception orders must be directed to the specific VLR (VLRA or VLRB) serving the target subscriber. Thus, VLRA is a conceptual tool that brings precision and reliability to multi-operator, service-rich mobile communications.
Key Features
- Logical role assigned to a physical VLR when its served subscriber is the calling party (A-subscriber)
- Key node for executing originating call control procedures and subscription checks
- Triggers Camel service logic for the A-party based on detected trigger points
- Generates charging data for the originating leg of a call or session
- Identified in signaling protocols (e.g., MAP, CAP) to route messages correctly
- Essential for unambiguous node identification in multi-operator roaming scenarios
Evolution Across Releases
Formally introduced in the specifications to provide clear distinction between the VLRs serving the calling and called parties. This was particularly important for the enhanced Camel Phase 2 and Phase 3 specifications, enabling precise control and charging for both legs of a call in separate networks.
Defining Specifications
| Specification | Title |
|---|---|
| TS 23.018 | 3GPP TS 23.018 |
| TS 23.079 | 3GPP TS 23.079 |