InterWorking Mobile Switching Center

Description

The InterWorking Mobile Switching Center (IWMSC) is a core network element defined in 3GPP specifications, primarily for GSM and UMTS networks. Its primary function is to act as a gateway and mediator between the mobile network and other external networks, such as the Public Switched Telephone Network (PSTN), Integrated Services Digital Network (ISDN), or other mobile networks using different technologies (e.g., CDMA). Architecturally, it is a specialized type of MSC that contains additional interworking functions (IWF). It sits at the boundary of the mobile network, interfacing with external networks via standardized signaling protocols like SS7 (Signaling System No. 7).

Operationally, the IWMSC performs critical protocol conversion and adaptation. When a call originates from a mobile subscriber and is destined for a subscriber on an external network, the call signaling and user traffic pass through the IWMSC. It translates mobile-specific signaling (e.g., MAP, BICC) into protocols understood by the external network (e.g., ISUP for PSTN). This includes converting call control messages, addressing information (like translating MSISDN to E.164 numbers), and adapting bearer characteristics. For user plane traffic, it may handle codec transcoding if the voice codecs used in the mobile network (e.g., AMR) are not supported by the external network.

Its role is pivotal for ensuring seamless service interoperability. The IWMSC is not just a passive gateway; it actively participates in call setup, management, and teardown procedures. It works in conjunction with the Gateway MSC (GMSC) for call routing. While a GMSC is responsible for querying the HLR to find the serving MSC of a called subscriber, the IWMSC handles the physical and logical interconnection to the external world. In many implementations, the functions of the GMSC and IWMSC can be combined in a single network node. The IWMSC is a key component for enabling basic telephony service roaming and interconnection in early 2G and 3G networks before the full adoption of IP-based core networks.

Purpose & Motivation

The IWMSC was created to solve the fundamental problem of network interconnection in the early days of digital mobile telecommunications. As GSM networks were deployed globally, they needed to interconnect with the vast existing infrastructure of fixed-line PSTN networks to allow mobile users to call landline phones and vice-versa. Furthermore, as mobile operators using different radio technologies (like GSM and CDMA) emerged, there was a need for these networks to interoperate. The purpose of the IWMSC is to bridge these technological divides.

Historically, before standardized interworking functions, proprietary gateways were used, leading to compatibility issues and increased costs. The 3GPP standardization of the IWMSC provided a uniform architecture and set of procedures for interconnection. It addressed limitations such as incompatible signaling systems, differing numbering plans, and mismatched bearer capabilities (e.g., voice coding). By defining a dedicated network element, 3GPP ensured that mobile networks could be built as standalone systems while still being able to connect to the global telecommunications ecosystem, which was crucial for the commercial success and ubiquity of mobile services.

Key Features

• Protocol conversion between mobile network signaling (e.g., MAP, BICC) and external network signaling (e.g., ISUP, TUP)
• Bearer adaptation and codec transcoding for voice traffic interoperability
• Address translation between mobile subscriber identifiers (MSISDN) and PSTN/ISDN numbering formats
• Call control and mediation for setup, management, and release of inter-network calls
• Interfaces with both the core mobile network (via MSC interfaces) and external networks (via SS7 links)
• Support for supplementary services interworking, like call forwarding, across network boundaries

Evolution Across Releases

Defining Specifications