Gateway Mobile Services Switching Centre for the Answering party

Description

The GMSCA is a specific logical role of a Gateway Mobile Switching Centre (GMSC) defined within the context of Mobile Number Portability (MNP) and call routing procedures in 3GPP specifications. The 'A' stands for 'Answering party,' meaning the called subscriber. This term is used to precisely identify the GMSC that resides in the Interrogating Public Land Mobile Network (IPLMN). The IPLMN is the network that initially receives a call destined for a ported number and is responsible for interrogating the Number Portability Database (NPDB) or performing a Signaling Connection Control Part (SCCP) Global Title (GT) translation to determine the correct recipient network.

In a non-ported scenario, the GMSC in the subscriber's home network (HPLMN) is the GMSCA. However, with MNP, a subscriber's MSISDN (mobile number) is no longer tied to their original HPLMN. When a call is made to a ported number, it is first routed to a GMSC in the network associated with the number's original range (the IPLMN). This GMSC, now acting as the GMSCA, must first execute the MNP query process. It sends a signaling query (e.g., an SRI-for-SM or similar MAP message) that triggers an SCCP Global Title translation. This translation, based on the dialed MSISDN, points the query to the correct recipient network (the donor network's HLR or a central NPDB) to fetch routing information.

A key architectural nuance is that the GMSCA may also serve as the Visited MSC for the Answering party (VMSCA) if the called subscriber is physically located within the coverage area of the same network that is performing the GMSCA function. In this case, after completing the MNP interrogation and confirming the subscriber belongs to its own network (or is roaming within it), the node fulfills both gateway and serving switch roles. The specifications detail the signaling flows and decision logic to handle these different scenarios, ensuring that calls to ported numbers are correctly routed whether the subscriber is in their donor network, recipient network, or roaming elsewhere.

Purpose & Motivation

The GMSCA concept was formally defined to address the complexities introduced by Mobile Number Portability (MNP). Before MNP, call routing was simpler: a dialed MSISDN implicitly defined the home network (HPLMN), and the call was routed to a GMSC in that network. MNP broke this direct association, creating a scenario where the network that initially receives a call (based on the number's original range) is not necessarily the network that currently hosts the subscriber.

The purpose of defining the GMSCA role is to clarify the responsibilities and signaling behavior of the first GMSC that encounters a call to a potentially ported number. It standardizes how this node should behave: it must perform the additional step of number portability interrogation before, or as part of, the standard HLR interrogation procedure. This ensures interoperability between different operators' networks in a porting environment. The specification of scenarios where the GMSCA is also the VMSCA helps optimize call routing by avoiding unnecessary additional hops when the called party is within the same network that received the call, streamlining the connection and reducing signaling load.

Key Features

• Defined as the GMSC in the Interrogating PLMN (IPLMN) for the called (Answering) party
• Initiates the Mobile Number Portability (MNP) interrogation procedure for incoming calls
• May perform SCCP Global Title translation based on the called MSISDN to route queries to the correct network
• Can also assume the role of the Visited MSC for the Answering party (VMSCA) if the subscriber is located in its service area
• Integrates MNP logic with standard MAP-based HLR interrogation for routing information retrieval
• A critical functional role for ensuring correct call delivery in a number-portable environment

Evolution Across Releases

Defining Specifications