Collaborative Session

Description

A Collaborative Session is a fundamental concept in the IP Multimedia Subsystem (IMS) that allows a single logical session to be composed of multiple access legs originating from different user equipment (UEs) that share the same IMS subscription. This concept, introduced in 3GPP Release 9, enables a user to use multiple devices simultaneously within a single communication session, with the network presenting the session as a single, unified session to the remote party. The architecture is defined in 3GPP TS 23.237 (IP Multimedia Subsystem (IMS) Service Continuity) and TS 23.838 (IMS Service Continuity Management Object (MO)).

The core architectural component enabling Collaborative Sessions is the Service Centralization and Continuity Application Server (SCC AS). The SCC AS acts as a Back-to-Back User Agent (B2BUA) that anchors all access legs of the collaborative session. When a user initiates or receives a session, the SCC AS ensures that the remote leg—the session as seen by the far-end party—is a single IMS session. The SCC AS manages the correlation between the multiple access legs and the single remote leg, handling media negotiation, session control, and media transfer between devices.

Each device participating in a Collaborative Session has its own access leg to the SCC AS. These access legs can be established over different access networks (e.g., LTE, Wi-Fi, 5G) and can involve different media types. The SCC AS maintains the session state and coordinates the media flows. For example, a user might start a video call on a smartphone and then transfer the video component to a tablet while keeping the audio on the smartphone. The remote party experiences this as a single video call, unaware of the underlying multi-device composition.

Collaborative Sessions rely on the IMS Service Continuity procedures, which include session transfer, media transfer, and access leg management. The Controller UE is the device that has the authority to perform control operations on the Collaborative Session, such as releasing the session, invoking supplementary services, or authorizing requests from other UEs. The Controller role can be transferred between devices. The SCC AS enforces these control rules and ensures that only authorized operations are performed.

The concept of Collaborative Sessions is closely tied to the broader IMS Service Continuity framework, which aims to provide a seamless user experience across multiple devices and access networks. It enables use cases such as media sharing, session mobility, and multi-device conferencing. By abstracting the multi-device nature of the session from the remote party, it simplifies interoperability and ensures that legacy devices can participate without modification. Collaborative Sessions are a key enabler for converged communication services in both consumer and enterprise environments.

Purpose & Motivation

The primary purpose of Collaborative Sessions is to allow a single user with multiple devices to participate in a communication session as if they were using a single device. This addresses the growing trend of users owning multiple devices (smartphones, tablets, laptops, wearables) and wanting to use them together seamlessly. Before this concept, IMS sessions were tied to a single device, making it difficult to share media or transfer sessions between devices without interrupting the communication.

Collaborative Sessions solve the problem of session fragmentation across devices. They provide a standardized way for the network to coordinate multiple access legs and present a unified session to the remote party. This enables service continuity, where a user can move a session from one device to another, or add a device to an ongoing session, without the remote party experiencing any disruption. It also allows for media sharing, where different media streams (audio, video, text) can be distributed across devices according to user preference or device capabilities.

The motivation for introducing Collaborative Sessions in Release 9 was to enhance the IMS Service Continuity framework, which initially focused on session transfer between access networks for a single device. As users began to use multiple devices simultaneously, the need arose for a more flexible architecture that could handle multiple devices within a single session. This concept was later extended and refined in subsequent releases, with the definition evolving to emphasize the set of access legs and the role of the SCC AS. It laid the foundation for advanced multi-device services and remains a key feature of IMS.