Combinational Service

Description

A Combinational Service is a communication service that integrates a Circuit Switched (CS) call with one or more IP Multimedia Subsystem (IMS) components, such as video, file sharing, or text, between the same set of participants. This concept, defined in 3GPP specifications 22.279 and 22.979, allows users to enhance a traditional voice call with additional multimedia services without losing the voice connection. The CS call provides the voice component, while the IMS session delivers the IP-based multimedia components. The service is established such that both the CS and IMS components are between the same participants, ensuring a synchronized and coherent user experience.

The architecture of a Combinational Service involves the coordination of both the CS domain and the IMS domain. The CS call is set up through the traditional circuit-switched network (e.g., GSM/UMTS), while the IMS components are established via the IP-based IMS core network. The User Equipment (UE) must support both CS and IMS capabilities simultaneously, often requiring dual-radio or dual-transfer mode operation. The service logic resides in the UE and the network, with the IMS application server managing the multimedia components. Key network elements include the Mobile Switching Center (MSC) for CS call control, the Call Session Control Function (CSCF) for IMS session control, and the Application Server (AS) for service logic. The UE uses the CS call as the anchor and adds IMS components on demand, such as during a voice call, the user can initiate a video share or file transfer.

The operation of a Combinational Service begins with the establishment of a CS call between two or more users. Once the CS call is active, the UE can initiate an IMS session to add multimedia components. The IMS session setup involves SIP signaling between the UE and the IMS core, with the Application Server coordinating the addition of the multimedia component. The CS and IMS components are correlated using identifiers such as the Call-ID or a dedicated correlation identifier, ensuring that both components are associated with the same communication session. The service can be initiated by either party, and the multimedia component can be added or removed dynamically during the call. The quality of service (QoS) for the IMS component is managed through the IMS QoS framework, while the CS call retains its own QoS characteristics.

Key technical aspects include the need for simultaneous CS and PS (Packet Switched) connections, which may require Dual Transfer Mode (DTM) in GERAN or multi-radio access in other systems. The UE must support the Combinational Service capability, and the network must be configured to allow the service. The IMS components can include real-time video, file sharing, text messaging, or other IP-based services. The service is standardized to ensure interoperability across different vendors and networks. Security is handled through the existing CS and IMS security mechanisms, with additional correlation security to prevent unauthorized access.

Combinational Services were introduced in Release 7 as part of the evolution towards richer communication services. They represent an early attempt to merge CS and IMS domains, paving the way for fully IP-based services like VoLTE and ViLTE. Although the concept has been largely superseded by all-IP solutions, it remains a significant milestone in the convergence of circuit-switched and packet-switched networks. The specifications 22.279 and 22.979 provide the detailed requirements and procedures for implementing Combinational Services, ensuring a standardized approach to service delivery.

Purpose & Motivation

Combinational Services were created to address the need for enriched voice communication beyond traditional circuit-switched voice calls. Before their introduction, users could only make voice calls over CS networks or use separate IP-based services like video calls over IMS, but not simultaneously in an integrated manner. The service allows users to add multimedia components to an ongoing voice call, enhancing the communication experience without interrupting the voice session. This was particularly important during the transition period when CS networks were still dominant, and IMS was being deployed for new services.

The motivation stemmed from the desire to leverage the strengths of both CS and IMS domains: the reliability and ubiquity of CS voice and the flexibility of IMS for multimedia. By combining them, operators could offer innovative services like video sharing during a voice call, which was not possible with standalone CS or IMS services. This also provided a stepping stone towards all-IP communication, allowing operators to gradually introduce IMS services while maintaining CS voice. The limitations of previous approaches included the inability to have simultaneous voice and multimedia sessions, and the lack of standardized mechanisms to correlate CS and IMS sessions. Combinational Services solved these by defining the architecture and procedures for integrating the two domains, ensuring a seamless user experience.