Combinational call

Description

A Combinational call is a service defined in 3GPP specifications that combines a circuit-switched (CS) speech teleservice with an IMS session to provide an enriched communication experience. Introduced in Release 7, this concept allows a user to establish a CS voice call and simultaneously add an IMS session (e.g., for video, file sharing, or messaging) between the same two UEs. The CS call and the IMS session are originated in one single UE and terminated in another single UE, creating a combined service that leverages the strengths of both domains.

The architecture for Combinational calls is specified in 3GPP TS 22.279 (Combined CS and IMS sessions) and TS 22.979 (Combined CS and IMS sessions; Stage 1). The service is designed to work in networks that support both CS and IMS, such as those with GERAN/UTRAN for CS and IMS core for packet-switched services. The UE must support simultaneous CS and PS (Packet Switched) connections, which is typically achieved through Dual Transfer Mode (DTM) in GERAN or multi-RAB in UTRAN. The IMS session is established over the PS domain, while the CS call uses the CS domain.

The key characteristic of a Combinational call is that the two services are correlated. The network must associate the CS call and the IMS session so that they are treated as a single logical session from the user's perspective. This correlation is achieved through the use of a Correlation Identifier (Correlation ID) that is exchanged between the UEs and the network. The Correlation ID is included in the signaling for both the CS call and the IMS session, allowing the network to link them. The SCC AS (Service Centralization and Continuity Application Server) may be involved in anchoring the IMS session and managing the correlation.

The service enables several use cases. For example, a user can make a voice call and then add a video stream to see the other party. Or they can share a file or a presentation during the call. The CS call provides reliable, high-quality voice, while the IMS session provides flexibility for multimedia. The two services can be controlled independently or together. The user can add or remove the IMS session without affecting the CS call, and vice versa.

Combinational calls were introduced in Release 7 as part of the effort to integrate CS and IMS services. They were particularly relevant during the transition period when CS networks were still dominant for voice, but IMS was being deployed for multimedia. The service allowed operators to offer enriched voice services without requiring a full migration to VoIP. Over time, as VoLTE and VoNR became prevalent, the need for Combinational calls diminished, but the concept remains defined in 3GPP specifications for backward compatibility and for networks that still rely on CS for voice. The definition has remained unchanged across all releases from Rel-7 to Rel-19, indicating its stability.

Purpose & Motivation

The Combinational call service was created to bridge the gap between circuit-switched and IMS domains, allowing users to enjoy enriched multimedia services while maintaining a reliable CS voice call. In the early days of IMS, CS networks provided ubiquitous voice coverage, but IMS offered advanced multimedia capabilities. Combinational calls allowed operators to leverage both, providing a stepping stone towards all-IP communication.

The primary problem it solves is the limitation of CS calls, which are typically voice-only. By adding an IMS session, users can share video, images, or other data during a voice call without interrupting the voice service. This is similar to the concept of Rich Communication Services (RCS) but at a lower level, integrating CS and IMS at the session layer. It also addresses the need for simultaneous services in networks that do not support VoLTE, enabling multimedia enrichment for CS calls.

The motivation for introducing Combinational calls in Release 7 was to enable operators to offer differentiated services and compete with over-the-top (OTT) providers. It provided a standardized way to combine CS and IMS, ensuring interoperability between devices and networks. Although the industry has largely moved to VoLTE and VoNR, the Combinational call concept remains a part of 3GPP history and is still specified for legacy support. It represents an important step in the evolution of mobile communication services.