Gateway MSC of the B subscriber

Description

The Gateway MSC of the B subscriber (GMSCB) is a functional role of a Mobile Switching Center (MSC) within the circuit-switched core network of 2G (GSM) and 3G (UMTS) systems. Its primary function is to act as the entry point into the home Public Land Mobile Network (HPLMN) for incoming calls destined for a mobile subscriber (the B-party). When a call is placed to a mobile number, the call is first routed to a GMSC. The GMSCB is specifically the GMSC that serves the called subscriber's home network. It performs the key procedure of Home Location Register (HLR) interrogation. Upon receiving the call, the GMSCB sends a Send Routing Information (SRI) message to the subscriber's HLR. The HLR, which stores the subscriber's current location (the Visitor Location Register or VLR address), responds with a Mobile Station Roaming Number (MSRN). The GMSCB then uses this MSRN to route the call to the MSC currently serving the subscriber (the Visited MSC or VMSC), where the final connection to the mobile device is established.

Architecturally, the GMSCB is not necessarily a separate physical node; it is a logical function that can be integrated into a standard MSC. Its operation is defined in the context of call control and mobility management procedures. The GMSCB interfaces with the HLR using the Mobile Application Part (MAP) protocol over the SS7 signaling network. It also handles the interworking between different network types, such as the Public Switched Telephone Network (PSTN) or other mobile networks, and the mobile core network, managing signaling conversion and call routing.

Its role is fundamental for the completion of Mobile-Terminated (MT) calls. Without the GMSCB's interrogation function, the network would have no efficient way to locate a roaming subscriber. It decouples the dialed Mobile Station International Subscriber Directory Number (MSISDN) from the physical routing number (MSRN), enabling subscriber mobility and number portability. The GMSCB is also involved in supplementary service handling for the terminating side, such as Call Forwarding Unconditional (CFU), where the HLR may provide alternative routing instructions.

Purpose & Motivation

The GMSCB was created to solve the fundamental problem of locating and routing calls to a mobile subscriber whose physical location can change dynamically. In fixed telephony, a phone number is tied to a specific physical line. In mobile networks, a subscriber's MSISDN is a logical identifier, not a location-specific one. The GMSCB, through its interrogation of the centralized HLR, provides the necessary translation from this logical identifier (MSISDN) to a temporary, routable address (MSRN) that points to the subscriber's current serving MSC.

This architecture addresses the limitations of static routing, enabling true user mobility. It allows calls to be correctly delivered whether the subscriber is in their home network or roaming in a visited network. The separation of the GMSC function for the calling (A) and called (B) parties provides clarity in call control procedures and billing. The GMSCB is a cornerstone of the GSM/UMTS core network design, establishing a clear anchor point in the home network for security, billing mediation, and service control for incoming traffic.

Key Features

• Acts as the entry point for Mobile-Terminated (MT) calls into the subscriber's HPLMN
• Interrogates the Home Location Register (HLR) using MAP signaling to obtain routing information
• Receives the Mobile Station Roaming Number (MSRN) from the HLR for call routing
• Routes the incoming call to the Visited MSC (VMSC) serving the subscriber
• Handles interworking between external networks (PSTN, other PLMNs) and the mobile core network
• Participates in supplementary service execution for the terminating call leg

Evolution Across Releases

Defining Specifications