Shared Inter Working Function

Description

The Shared Inter Working Function (SIWF) was a core network element introduced in 3GPP Release 4 as part of the evolution towards an all-IP core. Architecturally, it resided within the circuit-switched (CS) domain and acted as a shared gateway or mediation point. Its primary role was to handle interworking between the mobile CS domain and various external networks, most notably the Public Switched Telephone Network (PSTN) and other legacy telephony systems. Unlike dedicated interworking units per MSC, the SIWF was designed as a pooled resource, serving multiple Mobile Switching Centers (MSCs) or Media Gateways (MGWs) within a network. This shared model was a key architectural shift.

Operationally, the SIWF managed two critical aspects: signaling and user-plane traffic conversion. For signaling, it translated between the mobile-specific signaling protocols (like BICC or SIP-I used in the core) and the legacy ISUP signaling prevalent in the PSTN. For the user plane, it handled the conversion of voice traffic between the packet-based transport (e.g., over IP or ATM) used in the 3GPP core and the traditional TDM (Time-Division Multiplexing) circuits of the PSTN. This involved codec transcoding and adaptation of bearer protocols.

The SIWF's implementation was closely tied to the separation of control and user plane advocated in Release 4. It could be deployed as a standalone node or its functions integrated into a Media Gateway. By centralizing interworking capabilities, it allowed network operators to simplify their core network topology, reduce the number of network interfaces requiring management, and achieve economies of scale. Its shared nature meant capital and operational expenses were lower compared to deploying interworking functionality at every MSC site, facilitating a more cost-effective migration path from legacy TDM cores to packet-based Next Generation Networks (NGN).

Purpose & Motivation

The SIWF was created to address the challenges and costs associated with the transition from traditional circuit-switched mobile networks to packet-switched architectures in the early 2000s. Prior to Release 4, interworking with the PSTN was typically handled by dedicated functionality within each Mobile Switching Centre (MSC), leading to duplicated equipment and inefficient resource utilization across the network. As operators began deploying packet-based backbones (using IP or ATM), a need arose for a more efficient gateway to bridge the old and new worlds.

The primary problem it solved was enabling seamless voice and signaling communication between the emerging 3GPP packet core and the ubiquitous, entrenched PSTN without requiring a massive, simultaneous overhaul of the entire network infrastructure. By creating a shared, centralized interworking resource, 3GPP provided a pragmatic migration tool. It reduced the cost and complexity of introducing packet-switched Media Gateways (MGWs) and Call State Control Functions (CSCFs) by allowing them to connect to a common SIWF for all legacy PSTN traffic. This approach lowered the barrier for adopting new network architectures and supported the industry's strategic move towards all-IP networks.

Key Features

• Shared resource architecture serving multiple MSCs or Media Gateways
• Signaling interworking between BICC/SIP-I and legacy ISUP protocols
• User-plane media conversion between packet (IP/ATM) and TDM circuits
• Centralized point for codec transcoding and bearer adaptation
• Supports the Release 4 control/user plane separation concept
• Reduces network complexity and cost for PSTN interconnection

Evolution Across Releases

Defining Specifications