Interworking Selection Function

Description

The Interworking Selection Function (ISF) is a critical component within the IP Multimedia Subsystem (IMS) architecture, specifically designed to manage interoperability between IMS-based networks and legacy circuit-switched (CS) networks, such as those using Signaling System No. 7 (SS7). It operates as a logical function, typically implemented within network entities like the Media Gateway Control Function (MGCF) or Session Border Controller (SBC). The ISF's primary role is to analyze an incoming session request destined for a CS network and decide on the optimal interworking protocol to use.

When a session originates from an IMS user (using SIP) and targets a user in a CS network, the ISF evaluates several parameters. These include the capabilities of the terminating network (supported interworking methods), the service type requested (e.g., voice call, video call), and any operator policies. Based on this analysis, it selects either SIP-I (SIP with encapsulated ISUP) or SIP-T (SIP for Telephones). SIP-I is used when detailed ISUP signaling information needs to be preserved end-to-end for advanced telephony services. SIP-T is often used for basic voice call interoperability where full ISUP transparency is not required.

The function works by intercepting the SIP INVITE request. It examines the SIP headers and the Session Description Protocol (SDP) body. It may also query external databases or policy functions. Once the selection is made, the ISF instructs the associated media gateway controller to apply the chosen protocol. The MGCF then performs the necessary signaling conversion, translating SIP messages into the appropriate ISUP messages for the CS network, and manages the corresponding media gateway to handle voice path interworking.

Its architectural role is pivotal in the IMS service layer. It enables the IMS network to act as a modern overlay that can still communicate with the vast installed base of legacy telephone networks. This allows for a gradual migration to all-IP networks without breaking existing services. The ISF ensures that interworking decisions are made dynamically and per-session, allowing for flexible service deployment and efficient use of network resources.

Purpose & Motivation

The ISF was created to solve the complex problem of interoperability between the new, all-IP IMS networks and the existing, global circuit-switched telephony infrastructure. Prior to IMS, telecommunications were predominantly based on CS technology with standardized SS7/ISUP signaling. The introduction of IMS, based on SIP, created a protocol gap. Simple gateways could translate calls, but they often failed to preserve rich signaling information necessary for advanced services like call forwarding, caller ID, or call waiting across the network boundary.

Early interworking solutions were static and service-limited. A gateway might be configured to use only SIP-T, which simplifies ISUP messages, potentially losing service information. This motivated the need for a intelligent selection function. The ISF allows network operators to deploy a single IMS edge that can dynamically choose the best interworking method based on the specific call and the capabilities of the destination network. This preserves service continuity and feature transparency as users migrate from legacy to IMS services.

Historically, its development in Release 8 was part of the broader IMS standardization effort to make IMS a viable replacement for, and interoperable with, traditional telephony networks. It addressed the limitation of 'one-size-fits-all' gateway approaches, enabling more sophisticated and service-rich interworking. This was crucial for the commercial success of IMS, as operators needed to guarantee that new IMS subscribers could still communicate with everyone on the old network without service degradation.