Controller User Equipment

Description

The Controller UE is the central device in a 3GPP Collaborative Session, responsible for managing the session and determining the services available on the remote leg. Introduced in Release 9 as part of the IMS enhancements for multi-device support, the Controller UE acts as the anchor point for session control. In a typical scenario, a user might have multiple devices—such as a smartphone and a tablet—participating in a single session. The Controller UE is the device that holds the session control, while other devices (Controllee UEs) are subordinate to it. The Controller UE's service profile, stored in the network, dictates which services (e.g., voice, video, messaging) are allowed on the remote leg, ensuring that the session adheres to the user's subscription and preferences.

Architecturally, the Controller UE interacts with the IMS core network via the Gm interface, using SIP for session signaling. It also communicates with the SCC AS, which is the network entity that coordinates the Collaborative Session. The SCC AS uses the Controller UE's service profile to authorize session modifications and to manage the remote leg. The Controller UE may also support media flows itself, meaning it can be both the session controller and a media endpoint. Additionally, it can request IUT Media Control Related Procedures, such as transferring a media flow to another device or adding a new flow. These requests are processed by the SCC AS, which ensures they are consistent with the service profile.

The Controller UE's role is defined by its authority over the session. Unlike Controllee UEs, which can only request changes, the Controller UE can authorize or reject requests from other devices. This hierarchical structure prevents conflicts and maintains a consistent session state. The Controller UE is also responsible for managing the remote leg, which is the network-side portion of the session that connects to the remote party. The service profile of the Controller UE determines the services on this remote leg, meaning that if the Controller UE's profile does not include a particular service, it cannot be used in the session, even if a Controllee UE supports it.

Technically, the Controller UE must support a range of capabilities, including SIP session management, IUT procedures, and service profile handling. It may also need to support media negotiation and control protocols. The 3GPP specifications 23.237 and 23.831 define the architecture and procedures for Collaborative Sessions, including the roles of Controller and Controllee UEs. The Controller UE is typically the device that initiates the Collaborative Session, but it can also be assigned the role through procedures like Controller UE transfer, where a Controller capable UE takes over control.

The definition of the Controller UE has evolved slightly since Release 9. The initial definition focused on session control and media flow provision. In Release 10, the wording was refined to explicitly include the ability to request IUT Media Control Related Procedures, aligning with the Controllee UE definition and reflecting the maturation of IUT. Subsequent releases have maintained this definition, indicating stability. The Controller UE remains a key concept in IMS multi-device services, enabling flexible and user-centric session management.

Purpose & Motivation

The Controller UE concept was created to enable a single user to manage a session across multiple devices while maintaining a consistent service experience. Before Release 9, IMS sessions were device-centric, meaning each device had its own session and there was no unified control. This led to fragmented user experiences, especially when users wanted to use multiple devices simultaneously. The Controller UE solves this by designating one device as the session controller, whose service profile governs the entire session. This ensures that the session adheres to the user's subscription and that all devices operate under a unified policy.

The primary problem addressed by the Controller UE is the need for centralized control in multi-device scenarios. Without it, devices might independently request session modifications, leading to conflicts and inconsistent service levels. The Controller UE acts as the authoritative entity for session control, authorizing requests from Controllee UEs and managing the remote leg. This simplifies network operations and enhances security, as the network only needs to trust the Controller UE for session control decisions.

Historically, the introduction of the Controller UE in Release 9 was part of the broader IUT framework, which aimed to support seamless session continuity and media sharing across devices. The specifications 23.237 and 23.831 were developed to define the architecture and procedures. The evolution of the definition in Release 10 to include IUT Media Control Related Procedures reflects the practical need for the Controller UE to actively manage media flows, not just session control. This has ensured the concept's relevance across releases up to Rel-19, as multi-device usage continues to grow. The Controller UE remains a foundational element for advanced IMS services, enabling features like session transfer, media sharing, and unified service profiles.