Associated Protocol Data Unit Session

Description

The Associated PDU Session is a concept introduced in 3GPP Release 19 within the context of Multicast and Broadcast Services (MBS) in the 5G System (5GS). It refers to a Protocol Data Unit (PDU) Session that is associated with a multicast MBS session. This association serves two primary purposes: first, it is used for the 5GC Individual MBS traffic delivery method, where the 5G Core Network (5GC) delivers MBS data to a UE individually via a PDU Session rather than via a shared multicast radio bearer; second, it is used for signalling related to a user's participation in a multicast MBS session, such as join and leave requests.

The architecture of the Associated PDU Session involves several key network functions. The Session Management Function (SMF) manages the PDU Session and its association with the multicast MBS session. The Multicast/Broadcast Service Function (MBSF) and the Multicast/Broadcast Service Transport Function (MBSTF) are responsible for the overall management of MBS sessions. The Access and Mobility Management Function (AMF) handles the UE's mobility and access, while the User Plane Function (UPF) is responsible for the user plane data forwarding. The UE interacts with the network through the PDU Session to join or leave a multicast MBS session and to receive MBS data individually if the 5GC Individual MBS traffic delivery method is used.

The Associated PDU Session is established when a UE requests to join a multicast MBS session. The UE sends a PDU Session Establishment Request or a PDU Session Modification Request to the SMF, including the MBS session identifier. The SMF then associates the PDU Session with the multicast MBS session and informs the MBSF. The SMF also configures the UPF to handle the MBS traffic for that UE. If the UE is in a location where multicast MBS is not available via shared radio resources, the 5GC may use the Associated PDU Session to deliver MBS data individually to the UE. This ensures service continuity and allows the UE to receive MBS content regardless of its location or radio conditions.

The Associated PDU Session also plays a crucial role in signalling. When a UE wants to join a multicast MBS session, it sends a join request via the Associated PDU Session. Similarly, when it wants to leave, it sends a leave request. These signalling messages are transported over the PDU Session's control plane, typically via NAS (Non-Access Stratum) messages between the UE and the SMF. The SMF then coordinates with the MBSF to update the multicast session membership. This signalling is essential for managing group communications and ensuring that only authorized UEs receive the multicast content.

The concept of the Associated PDU Session is defined in 3GPP TS 23.247, which specifies the architectural enhancements for 5G multicast-broadcast services, and in 3GPP TS 29.244, which defines the Packet Forwarding Control Protocol (PFCP) used between the SMF and UPF. The Associated PDU Session enables flexible and efficient delivery of multicast services, combining the benefits of multicast transmission over shared radio resources with the reliability of individual delivery when needed. It represents a significant step forward in the support of group communication services in 5G and beyond.

Purpose & Motivation

The Associated PDU Session was created to address the need for a flexible and efficient mechanism to manage user participation in multicast MBS sessions and to support individual delivery of multicast traffic when shared delivery is not feasible. In earlier generations, multicast and broadcast services were delivered using dedicated bearers or shared channels without a direct association with a user's PDU Session. This limited the ability to manage group membership dynamically and to provide seamless service continuity across different radio conditions.

The motivation for introducing the Associated PDU Session in Release 19 stems from the growing demand for multicast services in 5G, such as live streaming, group communications, and public safety applications. These services require efficient use of network resources, especially in dense areas where many users consume the same content. Multicast transmission over shared radio resources (e.g., using MBS radio bearers) is highly efficient, but it may not be available everywhere or for all users. The Associated PDU Session provides a fallback mechanism: if a UE cannot receive multicast via shared resources, the 5GC can deliver the MBS data individually via the Associated PDU Session, ensuring service continuity.

Furthermore, the Associated PDU Session simplifies the signalling for joining and leaving multicast sessions. By using the existing PDU Session framework, the network can leverage established procedures for session management, QoS, and mobility. This reduces the complexity of introducing multicast services and ensures interoperability with existing 5G core network functions. The Associated PDU Session also enables the network to apply differentiated QoS to MBS traffic on a per-user basis, if needed, and to collect charging and analytics data for multicast services.