Home Location Register of the B subscriber

Description

The Home Location Register of the B subscriber (HLRB) is a fundamental component within the circuit-switched core network domain of 2G (GSM) and 3G (UMTS) systems as defined by 3GPP. It is not a separate physical node but a logical designation for the Home Location Register (HLR) that serves the called party, or terminating subscriber, in a telephony session. The HLR is a central database that contains permanent subscriber data, including the International Mobile Subscriber Identity (IMSI), Mobile Station International Subscriber Directory Number (MSISDN), subscribed supplementary services (like call forwarding or barring), and authentication parameters. For a mobile-terminated call, the Gateway MSC (GMSC) queries the HLRB to obtain routing information, specifically the address of the MSC/VLR currently serving the B subscriber. This process, known as the HLR interrogation, is critical for establishing the call path.

Architecturally, the HLRB interfaces with several other core network nodes via standardized signaling protocols, primarily the Mobile Application Part (MAP). Key interfaces include the C and D interfaces. The C interface connects the GMSC to the HLRB for routing information retrieval. The D interface connects the HLRB to the Visitor Location Register (VLR) associated with the serving MSC; this interface is used for updating location information, managing subscriber data in the VLR, and handling authentication procedures. The HLRB's role is purely in the control plane; it handles signaling messages to manage mobility and service logic but does not process user voice or data traffic.

During a call setup to a mobile subscriber, the originating network's GMSC sends a MAP_SEND_ROUTING_INFORMATION message to the HLRB. The HLRB checks the subscriber's profile, including any active call forwarding or barring services. If the call is allowed, the HLRB queries the current VLR (via the D interface if necessary) to obtain a temporary routing number called the Mobile Station Roaming Number (MSRN). The HLRB then returns this MSRN to the requesting GMSC, which uses it to route the call to the serving MSC. Beyond call routing, the HLRB is involved in procedures like location updating (when the subscriber moves), subscriber data management, and the restoration of subscriber data in a VLR after a failure. Its centralized database nature makes it a critical point for subscriber management and a potential single point of failure, which led to the development of more distributed architectures in later systems.

Purpose & Motivation

The HLRB concept exists to provide a clear logical and functional distinction within call routing and service control procedures in circuit-switched mobile networks. In any telephony exchange, there is a calling party (A subscriber) and a called party (B subscriber). The network needs to locate and route the call to the B subscriber, whose subscription data is stored in their designated HLR. The HLRB designation explicitly identifies which HLR—the one belonging to the B subscriber—must be interrogated to retrieve the necessary routing information. This solves the fundamental problem of locating a mobile subscriber who can be anywhere within the network's coverage area or even roaming in a foreign network.

Historically, before cellular networks, landline telephony used fixed physical lines, making routing trivial. The introduction of mobility required a dynamic lookup mechanism. The HLR, as a centralized subscriber database, was the solution. The differentiation between the HLR of the A subscriber (which might be queried for originating services) and the HLR of the B subscriber (for terminating services) provides clarity in signaling standards and network implementations. It addresses the limitation of not having a fixed terminal point by decoupling subscriber identity from physical location. The procedures defined around the HLRB ensure that mobile-terminated calls can be completed efficiently, supporting national and international roaming, which was a key commercial driver for GSM's success.

Key Features

• Stores permanent subscriber data for the called party (B subscriber)
• Provides routing information (MSRN) to the GMSC for mobile-terminated call setup
• Manages mobility through location update procedures with the VLR
• Handles authentication and security information for the subscriber
• Controls and provisions supplementary telephony services (e.g., call forwarding, barring)
• Interfaces with GMSC (C interface) and VLR (D interface) using MAP signaling

Evolution Across Releases

Defining Specifications