Collaborative Session Control

Description

Collaborative Session Control refers to the specific control operations that can be performed on a Collaborative Session, as defined in 3GPP TS 23.237 and TS 23.831. These operations are restricted to the Controller UE, which is the device designated as having the authority to manage the session. The concept was introduced in Release 10 as part of the ongoing evolution of IMS Service Continuity, providing a clear framework for managing multi-device sessions.

The Controller UE is one of the UEs participating in the Collaborative Session. It is responsible for making decisions that affect the entire session, such as releasing the session, invoking supplementary services (e.g., call hold, call forwarding), and authorizing requests for Inter-UE Transfer (IUT) Media Control Related Procedures from other UEs. The SCC AS enforces these control rules, ensuring that only the Controller UE can perform these operations. Other UEs in the session, known as Controllee UEs, may request certain actions, but they require authorization from the Controller UE.

The control operations encompass several key areas. First, session release: only the Controller UE can terminate the entire Collaborative Session, which results in the release of all access legs and the remote leg. Second, supplementary services: the Controller UE can invoke services like call hold, call waiting, or call forwarding for the session. Third, authorization of IUT Media Control Related Procedures: when a Controllee UE wants to perform an action such as adding or removing media, or transferring media to another device, it must send a request to the Controller UE, which then authorizes or rejects it. The SCC AS mediates these requests and enforces the decisions.

The Controller role is not fixed; it can be transferred from one UE to another through a Controller Transfer procedure. This allows the user to switch control to a different device, for example, when moving from a smartphone to a tablet. The transfer is initiated by the current Controller UE and executed via the SCC AS. The new Controller UE then assumes all control responsibilities. This flexibility is essential for a seamless user experience.

Collaborative Session Control is a critical component of the IMS Service Continuity architecture. It ensures that despite the distributed nature of the session across multiple devices, there is a single point of control, preventing conflicts and ensuring consistent session management. It also provides a security mechanism, as only the authorized Controller UE can perform sensitive operations. The procedures are defined in detail in 3GPP specifications, including the signaling flows and the role of the SCC AS. This concept has been maintained across releases, with the definition remaining stable since its introduction in Release 10.

Purpose & Motivation

Collaborative Session Control exists to provide a clear and authoritative mechanism for managing multi-device IMS sessions. Without it, multiple devices in a Collaborative Session could issue conflicting commands, leading to session instability or unauthorized actions. The concept solves the problem of distributed control by designating a single Controller UE that has the final say on session-affecting operations.

The motivation for introducing this concept in Release 10 was the need to formalize the control aspects of Collaborative Sessions, which were introduced in Release 9. While Release 9 defined what a Collaborative Session is, it did not fully specify how control operations should be managed. Release 10 filled this gap by defining Collaborative Session Control, ensuring that the session can be managed reliably and securely. It also enabled the authorization framework for Inter-UE Transfer procedures, which allow media to be moved between devices.

By centralizing control in the Controller UE, the network can enforce policies and prevent unauthorized changes. This is particularly important in scenarios where multiple users might share devices or where devices have different trust levels. The Controller UE concept also simplifies the implementation of supplementary services, as the network only needs to interact with one device for service invocation. Overall, Collaborative Session Control is essential for the practical deployment of multi-device services in IMS.