Domain Transfer Function

Description

The Domain Transfer Function (DTF) is a critical component within the IP Multimedia Subsystem (IMS) architecture specifically defined for the Voice Call Continuity (VCC) feature introduced in 3GPP Release 7. The DTF resides in the user's home IMS network. Its primary operational role is to serve as the anchor point for a VCC-enabled voice call. When a subscriber initiates or receives a call with VCC, the session is always routed through the DTF in the home network, regardless of whether the user is initially connected via the CS domain (like traditional GSM/UMTS voice) or the IMS/PS domain (like VoIP over LTE).

How it works is integral to the domain transfer procedure. The DTF maintains the ongoing call leg toward the remote party (e.g., another mobile user or a PSTN number). Simultaneously, it manages the call leg toward the user equipment (UE) via the current access domain. When a handover between domains is required—for instance, moving from an IMS VoIP call over WLAN to a CS call over GERAN due to deteriorating WLAN coverage—the UE or network triggers a transfer procedure. The DTF is instructed to switch the UE-facing call leg from the old domain to the new one. It coordinates with other VCC entities like the CS Adaptation Function (CSAF) for CS legs and potentially the Service Centralization and Continuity Application Server (SCC AS) in later architectures. The DTF executes the switch, ensuring the media path is re-anchored with minimal interruption to the user.

Key components interacting with the DTF include the VCC Application (which may be co-located), the CSAF for interfacing with the MSC, and the Serving-Call Session Control Function (S-CSCF) which routes initial SIP messages to it. The DTF's role is pivotal for transparency; the remote party is unaware of the domain transfer, as the DTF maintains the session on their side. This architecture centralizes control and state management for the VCC service, enabling robust continuity logic and subscriber service execution from the home network.

Purpose & Motivation

The DTF was created to solve the problem of seamless voice call continuity between different technological domains, specifically between the legacy Circuit-Switched (CS) domain and the emerging IP Multimedia Subsystem (IMS) packet-switched domain. Prior to VCC and the DTF, a voice call established in one domain (e.g., CS) would have to be dropped if the user moved to an area only supported by the other domain (e.g., IMS over WLAN), leading to a poor user experience. The motivation was to enable operators to deploy IMS-based voice services (like VoIP over WLAN) while leveraging the ubiquitous coverage of the existing CS network for reliability, creating a combined service offering.

The historical context is Release 7, where IMS was being positioned for multimedia services, but radio coverage for pure IP access (like early LTE or WLAN) was spotty. VCC was a key enabler for fixed-mobile convergence and early VoLTE precursors. The DTF addressed the limitation of previous approaches where the two domains were completely separate silos with no coordinated handover mechanism. By introducing a centralized anchor function in the home IMS, the DTF provided a standardized, network-controlled method for transferring the active session, preserving the call and ensuring service continuity, which was a fundamental requirement for user acceptance of new VoIP services.

Key Features

• Acts as the session anchor point in the home IMS for all VCC calls.
• Manages two call legs: one toward the remote party and one toward the UE via its current domain.
• Executes the domain transfer by swapping the UE-facing call leg from CS to IMS or vice versa.
• Interworks with the CS Adaptation Function (CSAF) to handle CS domain signaling and media.
• Ensures service continuity is transparent to the remote calling/called party.
• Centralizes VCC service logic and subscriber state in the home network.

Evolution Across Releases

Defining Specifications