Call Continuity Control Function

Description

The Call Continuity Control Function (CCCF) is a core component within the IP Multimedia Subsystem (IMS) architecture, specifically defined to support the Voice Call Continuity (VCC) feature. It acts as an Application Server (AS) that anchors voice calls to enable domain transfer between the Circuit-Switched (CS) domain (e.g., traditional GSM/UMTS voice) and the IMS/PS (Packet-Switched) domain (e.g., VoIP over LTE or Wi-Fi). When a VCC-capable user initiates or receives a call, the CCCF is invoked to control the call's routing and manage the necessary signaling for potential handovers. It interfaces with the Serving-Call Session Control Function (S-CSCF) via the ISC (IMS Service Control) interface and can communicate with the CS domain through the MGCF (Media Gateway Control Function) or directly with the MSC (Mobile Switching Centre) using CAMEL (Customised Applications for Mobile network Enhanced Logic) or other signaling protocols, depending on the network deployment.

Architecturally, the CCCF contains logic to execute domain selection at call origination and termination, determining whether the call should be established in the CS or IMS domain based on user registration, capabilities, and network conditions. During an active call, it monitors for triggers—such as changes in radio access technology (e.g., moving from Wi-Fi to cellular coverage)—that necessitate a domain transfer. Upon receiving a transfer request, the CCCF coordinates with other network functions to re-route the call leg, update the media path, and ensure signaling continuity. This involves procedures like executing a third-party call control, where the CCCF acts as a back-to-back user agent (B2BUA) to bridge the two domains, maintaining the call session while swapping the underlying transport.

Key internal components of the CCCF include a domain transfer function, which handles the logic and signaling for switching domains; a call state machine, which tracks the active call's status across domains; and interfaces to subscriber data (e.g., for VCC subscription information). Its role extends beyond basic handover: it also manages features like dual radio aspects (where a device might use separate radios for CS and PS simultaneously) and ensures that supplementary services (e.g., call hold, forwarding) are preserved during transfers. The CCCF works in tandem with other VCC entities, such as the Network Domain Selection (NeDS) function for initial domain choice and the CAMEL service environment for CS interactions, forming a comprehensive solution for call continuity in early IMS deployments.

Purpose & Motivation

The CCCF was created to address the challenge of maintaining voice call continuity as mobile networks evolved toward all-IP architectures with the introduction of IMS in 3GPP Release 5. Prior to VCC and the CCCF, voice calls were predominantly circuit-switched, and emerging VoIP services over IMS or Wi-Fi operated in isolated domains, leading to dropped calls when users moved between these domains. For example, a user on a Wi-Fi VoIP call would lose connectivity upon leaving Wi-Fi coverage unless the network could handover the call to the cellular CS network. The CCCF solves this by providing a centralized control point to manage seamless transitions, enabling operators to offer converged voice services and encouraging adoption of IMS-based voice without sacrificing reliability.

Historically, the motivation stemmed from the need to support Fixed-Mobile Convergence (FMC) and early Voice over LTE (VoLTE) precursors, where ensuring service continuity was critical for user acceptance. Without the CCCF, dual-mode devices (supporting both CS and PS voice) would face service interruptions, limiting the practicality of IMS deployments. The CCCF, as part of the VCC framework, allowed operators to leverage existing CS infrastructure while migrating to IMS, providing a smooth transition path. It addressed limitations of earlier approaches, such as proprietary solutions or simple network switching, by standardizing the handover procedures within 3GPP, ensuring interoperability and consistent user experience across multi-vendor networks.

Key Features

• Anchors voice calls for domain transfer between CS and IMS domains
• Acts as an IMS Application Server (AS) for Voice Call Continuity (VCC) service logic
• Performs domain selection at call origination and termination based on subscriber and network conditions
• Coordinates third-party call control to re-route media and signaling during handovers
• Interfaces with S-CSCF via ISC and with CS network via CAMEL or MGCF protocols
• Maintains call state and supplementary services across domain transitions

Evolution Across Releases

Defining Specifications