IP Adaptation Function

Description

The IP Adaptation Function (IPAF) is a core network element specified in the early phases of the IP Multimedia Subsystem (IMS) architecture. Its primary role is to facilitate seamless interworking between the packet-switched, SIP-based IMS domain and traditional circuit-switched telephony networks, such as the Public Switched Telephone Network (PSTN) or legacy Public Land Mobile Network (PLMN). Functionally, it acts as a signaling gateway and protocol translator. It resides at the boundary of the IMS, often co-located with or part of a Media Gateway Control Function (MGCF) or a dedicated Interworking Function (IWF).

Architecturally, IPAF interfaces with the CS network using protocols like ISDN User Part (ISUP) or Bearer Independent Call Control (BICC) over traditional SS7 or IP-based signaling transports. On the IMS side, it interfaces with Call Session Control Functions (CSCFs) using the Session Initiation Protocol (SIP). The IPAF performs mapping between the semantically different signaling messages and parameters of these two domains. For instance, it translates an ISUP Initial Address Message (IAM) into a SIP INVITE request, ensuring that called/calling party numbers, bearer capabilities, and call progress indicators are accurately conveyed. It also handles the translation in the reverse direction for responses and in-call signaling.

How it works involves stateful session management. The IPAF maintains the state of each interworking call, correlating the CS leg and the IMS leg. It modifies SIP headers and SDP bodies to align with CS bearer characteristics and vice-versa. Furthermore, it may interact with other IMS functions like the Breakout Gateway Control Function (BGCF) for routing decisions when a call originates in IMS and breaks out to the CS network. While later IMS architectures evolved and refined these interworking functions (with the MGCF taking a more prominent, integrated role), the IPAF represented the initial conceptual decomposition of the adaptation layer, clarifying the separation of signaling adaptation from media conversion (handled by a Media Gateway) and call control logic.

Purpose & Motivation

IPAF was created to solve a critical early challenge in the deployment of IMS: backward compatibility. When 3GPP defined IMS in Release 5 as the future all-IP service delivery platform, a vast installed base of circuit-switched networks and subscribers existed. For IMS to be viable, it had to interconnect with this legacy world to allow voice and other services to flow between IP and CS subscribers. The IPAF provided the dedicated functional entity to manage this complex signaling interworking.

The problem it addressed was the fundamental protocol mismatch. CS networks used stateful, connection-oriented signaling like ISUP, designed for dedicated timeslots. IMS used stateless, text-based SIP, designed for IP sessions. Without an adaptation function, calls between the domains would be impossible. IPAF abstracted this complexity, allowing IMS core elements (CSCFs) to communicate using SIP while the IPAF handled the translation to/from the CS world. This enabled phased network migration, allowing operators to introduce IMS islands that could still serve and interconnect with their entire subscriber base. Its specification helped formalize the mapping rules and behaviors necessary for reliable interworking, which were later absorbed and enhanced within the broader MGCF specification.

Key Features

• Protocol translation between SIP and ISUP/BICC
• Signaling interworking for call control
• Mapping of addressing and numbering information
• Adaptation of bearer and service characteristics
• State management for inter-domain call legs
• Interface towards both CS network and IMS core

Evolution Across Releases

Defining Specifications