Controllee User Equipment

Description

The Controllee UE is a fundamental component of the 3GPP Collaborative Session architecture, introduced in Release 9 as part of the IP Multimedia Subsystem (IMS) enhancements for multi-device scenarios. In a Collaborative Session, a single session is established between the network and a user who may be using multiple devices simultaneously. The Controllee UE represents one of these devices, typically the one that is not controlling the session but is actively participating in media exchange. For example, a user might have a smartphone acting as the Controller UE and a tablet or laptop as the Controllee UE, with the tablet receiving video streams while the smartphone manages the session.

Architecturally, the Controllee UE interacts with the IMS core network through the Gm interface, using SIP for session signaling. It may also use the IUT (Inter-UE Transfer) interface, specifically the IUT Media Control Related Procedures, to request modifications to the media flows it handles. However, these requests are not executed autonomously; they are sent to the Controller UE or to the Service Centralization and Continuity Application Server (SCC AS) for authorization. The SCC AS is the key network entity that coordinates the Collaborative Session, ensuring that the Controller UE's service profile governs the overall session and that any changes requested by Controllee UEs are consistent with that profile.

The Controllee UE's role is defined by its subordination to the Controller UE. While it can initiate requests for media control—such as adding or removing a media flow, or transferring a flow to another device—it cannot unilaterally enforce these changes. This hierarchical relationship ensures that the user's primary device (the Controller UE) retains ultimate control over the session, preventing conflicts and maintaining a consistent user experience. The Controllee UE must support the necessary protocols and procedures to participate in the Collaborative Session, including SIP registration, session establishment, and media negotiation.

In terms of capabilities, the Controllee UE may support a subset of the features available to the Controller UE. It is not required to be capable of becoming a Controller UE, although some devices may support both roles. The distinction is important for network operations: the SCC AS uses the Controller UE's service profile to determine which services are allowed on the remote leg (the network side of the session), and the Controllee UE's requests are evaluated against this profile. This design allows for flexible device usage while maintaining centralized control.

The Controllee UE concept has evolved since its introduction in Release 9. The initial definition focused on providing media flows and being subordinate for session control. In Release 10, the wording was refined to explicitly mention the ability to request IUT Media Control Related Procedures, reflecting the maturation of the IUT procedures and the need to clarify the Controllee UE's interaction capabilities. Subsequent releases have maintained this definition, indicating stability in the architectural role. The Controllee UE remains relevant in modern networks, especially with the proliferation of multi-device users and the increasing demand for seamless session continuity across devices.

Purpose & Motivation

The Controllee UE concept was created to address the growing need for multi-device collaboration in IMS-based services. Before Release 9, IMS sessions were typically tied to a single device, and users could not easily share or transfer media flows between multiple devices within the same session. This limitation became increasingly problematic as users began owning multiple devices (e.g., smartphones, tablets, laptops) and expected seamless service continuity. The Controllee UE, along with the Controller UE, enables a Collaborative Session where one user can use multiple devices simultaneously, with one device controlling the session and others participating as media endpoints.

The primary problem solved by the Controllee UE is the coordination of media flows across devices while maintaining a single session context. Without this concept, each device would need to establish separate sessions, leading to duplicated signaling, inconsistent service profiles, and a fragmented user experience. The Controllee UE allows the network to treat multiple devices as part of one logical session, with the Controller UE acting as the anchor. This simplifies session management, reduces signaling overhead, and enables advanced features like media transfer between devices.

Historically, the introduction of the Controllee UE in Release 9 was part of a broader effort to enhance IMS with Inter-UE Transfer (IUT) capabilities. The 3GPP specifications 23.237 and 23.831 defined the architecture and procedures for IUT, including the roles of Controller and Controllee UEs. The evolution of the definition in Release 10 to explicitly include the ability to request IUT Media Control Related Procedures reflects the practical need for Controllee UEs to initiate certain actions, such as requesting to add a media flow, while still being subject to authorization. This balance between flexibility and control is central to the design and has ensured the concept's longevity across releases up to Rel-19.