Description
The Home Location Register of the C subscriber (HLRC) is a logical network entity defined within 3GPP specifications, specifically for handling certain circuit-switched supplementary services. Unlike the HLRB, which is involved in standard mobile-terminated calls, the HLRC comes into play during more complex call scenarios involving three parties: the A (calling), B (originally called), and C (transferred-to or added) subscribers. The HLRC is the HLR that contains the subscription profile and service data for this third 'C subscriber'. Its primary role is to provide routing information and service authorization when a call is being forwarded to or involves this third party, as in Call Transfer (ECT) or Multi-Party (MPTY) services.
Architecturally, the HLRC is identical in form to any other HLR—it is a database node in the core network. Its distinction is purely functional and contextual within specific signaling flows. When a B subscriber initiates a Call Transfer to a C subscriber, the serving MSC of the B subscriber acts upon the request. To complete the transfer, it may need to route a new call leg to the C subscriber. This requires interrogating the C subscriber's HLR (the HLRC) to obtain routing information, following a similar procedure to a standard mobile-terminated call setup. The MSC (or a dedicated Service Control Point in some architectures) sends a MAP_SEND_ROUTING_INFORMATION request to the HLRC. The HLRC processes this request by verifying the C subscriber's service profile, checking for any applicable call barring or forwarding services, and interacting with the C subscriber's current VLR to fetch a Mobile Station Roaming Number (MSRN) for routing.
The HLRC's operation is tightly integrated with the service logic for supplementary services defined in 3GPP TS 23.079. It uses the same MAP interfaces (C and D) as the HLRB. The key difference lies in the trigger and the calling context. The interrogation of the HLRC is not initiated by a Gateway MSC for an inbound call, but typically by a visited MSC executing a service logic for an active call. This highlights the HLRC's role in enabling dynamic, mid-call feature interactions. Its proper function ensures that advanced services like Explicit Call Transfer, where a user can transfer an ongoing call to a third number, work seamlessly across network boundaries, including in roaming situations. The HLRC thus extends the foundational HLR-based routing paradigm to support richer telephony services.
Purpose & Motivation
The HLRC was defined to formalize the network behavior and signaling required for advanced supplementary services that involve a third party. Basic mobile telephony only required managing two parties (A and B). However, to increase the value and functionality of mobile networks, standards bodies introduced features like Call Transfer and Conference Calling. These features create scenarios where the network must interact with and route calls to a third subscriber (C) whose subscription is independent of the original two parties. The HLRC concept provides a clear reference point in the standards for this interaction.
It solves the problem of service ambiguity and ensures consistent implementation across different network vendors and operators. Without a defined concept for the C subscriber's HLR, the signaling procedures for services like Explicit Call Transfer would be poorly specified, leading to interoperability issues. The HLRC addresses the limitation of the simple A/B subscriber model by extending the proven HLR interrogation mechanism to a third party. This allowed operators to offer sophisticated business and personal communication features using the same reliable, centralized subscriber database infrastructure. Its creation was motivated by the desire to match and exceed the feature set of traditional business telephony systems (PBXs) on mobile networks.
Key Features
- Stores permanent subscriber data for the third party (C subscriber) in multi-party services
- Provides routing information (MSRN) for call forwarding and transfer scenarios
- Authorizes service invocation for the C subscriber (e.g., checking for barring)
- Integrates with supplementary service logic defined in 3GPP TS 23.079
- Utilizes standard MAP signaling interfaces (C and D) for queries and updates
- Enables advanced call features like Explicit Call Transfer and Multi-Party service
Evolution Across Releases
Introduced alongside the standardization of supplementary service procedures in the 3GPP core network. The initial definition established the HLRC's role in the signaling flows for services involving a C subscriber, particularly Call Transfer. It formalized how a serving MSC interacts with a third party's HLR using existing MAP protocols.
Defining Specifications
| Specification | Title |
|---|---|
| TS 23.079 | 3GPP TS 23.079 |