CS Adaptation Function

Description

The CS Adaptation Function (CSAF) is a core network element defined within the 3GPP architecture for IP Multimedia Subsystem (IMS)-based services. Its primary technical role is to provide the necessary adaptation layer between the legacy Circuit-Switched (CS) core network domain and the packet-based IMS. Architecturally, the CSAF is positioned as an intermediary function. It interfaces with the CS network on one side, typically connecting to Mobile Switching Centers (MSCs) using standard CS protocols like ISUP (ISDN User Part) or BICC (Bearer Independent Call Control). On the other side, it interfaces with the IMS core, specifically with the Media Gateway Control Function (MGCF) and the IMS Media Gateway (IMS-MGW), using SIP (Session Initiation Protocol) for call control and RTP/RTCP for media transport over IP.

From a functional perspective, the CSAF operates by performing protocol conversion and media adaptation. For signaling, it translates between CS-specific call control signaling (e.g., ISUP messages) and their SIP equivalents used within the IMS. This involves mapping call setup, teardown, and supplementary service information between the two domains. For the user plane (bearer), the CSAF manages the adaptation of the media stream. It handles the interworking between the 64 kbps PCM (Pulse-Code Modulation) voice channels used in the CS domain and the packetized voice codecs (like AMR) transported over RTP/IP in the IMS domain. This often involves the CSAF controlling an associated media gateway resource to perform the actual transcoding or pass-through of the media.

A key component of the CSAF's operation is its role in the IMS Service Centralization and Continuity (SCC) architecture, which was a major driver for its specification. In this context, the CSAF enables the anchoring of CS calls in the IMS. When a User Equipment (UE) initiates or receives a CS call, the CSAF allows the IMS to take control of that call session. This centralization in the IMS is crucial for enabling advanced service continuity features, such as Voice Call Continuity (VCC) and later Single Radio Voice Call Continuity (SRVCC), where an ongoing voice call can be seamlessly handed over between the CS domain and the IMS/PS domain.

The CSAF's implementation is critical for a phased network evolution. It allows mobile network operators to introduce and expand their IMS infrastructure for voice and multimedia services while their legacy CS network remains operational and serves subscribers who are not yet IMS-capable or are in areas without IMS coverage. By anchoring CS-originated sessions in the IMS, operators can begin to deploy new, IP-based services that are accessible even to users on the legacy network, creating a unified service layer. Technically, it decouples service logic (in the IMS) from the access technology, paving the way for the eventual retirement of the traditional CS core network.

Purpose & Motivation

The CSAF was created to solve a fundamental challenge in the transition from traditional circuit-switched mobile networks to all-IP networks based on the IMS. Prior to IMS, all voice and SMS services were delivered entirely through the Circuit-Switched (CS) core network (MSC/VLR). The introduction of IMS in Release 5 offered a new, IP-based platform for delivering multimedia services, but it created a parallel service silo. Without a bridge, subscribers on the CS network could not access IMS services, and IMS subscribers could not interoperate with the vast installed base of CS users. This bifurcation hindered the adoption of IMS and prevented operators from consolidating their service platforms.

The primary problem the CSAF addresses is service interoperability and network evolution. It enables the delivery of what appear to be traditional CS services (like voice calls) but are actually routed and controlled through the IMS. This allows network operators to start migrating service logic and intelligence to the modern, flexible IMS platform while continuing to utilize their existing CS network infrastructure for access and transport where needed. It was a key enabler for the 'Service Centralization' concept, where the IMS becomes the central point for service control regardless of the underlying access network (CS or PS).

Historically, its development in Release 7 was motivated by the need for concrete mechanisms to support IMS-based voice services and, critically, service continuity. Concepts like Voice Call Continuity (VCC) required a point of interconnection where a call session could be anchored and then transferred between domains. The CSAF provides that anchor point for CS-legacy calls. It solved the limitation of having two separate, incompatible service cores by acting as a gateway and adaptation layer, making the IMS the unifying service layer for both packet-switched and circuit-switched accesses. This was a vital step towards the long-term goal of a single, consolidated IP-based core network.

Key Features

• Protocol Interworking between CS (ISUP/BICC) and IMS (SIP) signaling
• Media Adaptation for user plane traffic between TDM/PCM and RTP/IP formats
• Call Anchoring in the IMS for sessions originating from the CS domain
• Support for IMS Service Centralization and Continuity (SCC) architecture
• Enabler for Voice Call Continuity (VCC) and Single Radio VCC (SRVCC) features
• Facilitates phased network migration from CS to all-IP IMS core

Evolution Across Releases

Defining Specifications