Interworking MSC

Description

The Interworking MSC (IW-MSC) is a specialized Mobile Switching Centre function defined by 3GPP, primarily in TS 29.235, to handle signaling interworking at the control plane between the legacy circuit-switched (CS) telephony network and IP-based networks. Unlike the IW-MGW which handles media, the IW-MSC focuses on call control signaling translation. It acts as a signaling gateway and protocol converter, sitting at the boundary between the CS domain (using signaling protocols like ISDN User Part - ISUP or Bearer Independent Call Control - BICC) and an IP-based domain like the IMS or a corporate SIP network (using the Session Initiation Protocol - SIP).

Architecturally, the IW-MSC can be implemented as a standalone node or integrated with other functions like a Media Gateway Control Function (MGCF). Its core operation involves receiving an incoming call setup request in one protocol, interpreting the call parameters (called/calling numbers, service indicators), and mapping them to construct an equivalent outgoing call request in the target protocol. For example, when interworking from CS to IMS, it receives an ISUP Initial Address Message (IAM), maps the dialed number to a SIP URI, and generates a SIP INVITE request towards the IMS core. It manages the entire signaling dialog, translating subsequent messages like ACM (Address Complete), ANM (Answer), and REL (Release) to their SIP equivalents (180 Ringing, 200 OK, BYE).

The IW-MSC also handles critical supplementary service interworking, translating between CS-based supplementary service signaling (using Facility messages) and SIP-based methods. It is responsible for number format adaptation, screening, and routing decisions based on the interworking scenario. In a typical deployment for an operator migrating to IMS, the IW-MSC (often coupled with an MGCF for full MGW control) allows legacy MSCs in the network to route calls destined for IMS subscribers transparently, treating the IW-MSC as just another MSC in the CS network. This enables a phased migration where the IMS appears as a virtual CS exchange to the legacy infrastructure.

Purpose & Motivation

The IW-MSC was developed to solve the signaling interoperability challenge that accompanies the user plane challenge addressed by the IW-MGW. As operators began deploying IMS and other SIP-based networks, they needed a way for the existing, vast signaling infrastructure of their CS networks (based on SS7 ISUP) to communicate with the new IP-based signaling world (SIP). Without this, call control could not be established between the domains, even if a media path could be built. The IW-MSC provides the necessary protocol translation layer to allow call setup, teardown, and feature invocation across the network boundary.

Its creation was motivated by the same gradual migration strategy as the IW-MGW. It allowed network planners to introduce IMS islands into their network without having to immediately replace all legacy MSCs and switches. The IW-MSC acts as a gateway, making the IMS network look like a traditional CS exchange from the perspective of the legacy network, and vice-versa. This addressed the significant limitation and cost of a "forklift" upgrade. It specifically solved problems like routing calls to subscribers who had been migrated to IMS-based VoIP services, enabling business trunking connections between a corporate SIP PBX and the mobile network, and facilitating interconnection with other operators who were at different stages of their network modernization.