Multicast/Broadcast Service Session

Description

An MBS (Multicast/Broadcast Service) session is a fundamental concept in 3GPP Release 17 and later, defined in TS 23.247, that represents either a multicast session or a broadcast session. It provides a mechanism for delivering the same content to multiple users simultaneously, optimizing network resource usage compared to unicast delivery. The MBS session is established and managed by the 5G Core Network (5GC) and can be delivered over both 5G NR and LTE radio access technologies.

Architecturally, an MBS session involves several key network functions. The MBSF (Multicast/Broadcast Service Function) and MBSU (Multicast/Broadcast Service User plane) are core network entities that handle session management and user plane data distribution. The session can be in different states: idle, active, or inactive. For multicast sessions, users must join the session (via signaling) to receive data, while broadcast sessions are available to all users in a broadcast service area without explicit joining. The session context includes information such as the MBS session ID, service area, QoS parameters, and the list of participating UEs.

How it works: When a content provider or application server wants to deliver content to multiple users, it requests the creation of an MBS session from the 5GC. The 5GC allocates resources and establishes the necessary bearers. For multicast, UEs that wish to receive the content perform a join procedure, which may involve signaling to the AMF and SMF. The SMF then manages the user plane paths, potentially using local multicast (e.g., in the RAN) to optimize delivery. The RAN (gNB) can use either point-to-point (PTP) or point-to-multipoint (PTM) transmission depending on the number of users and radio conditions. For broadcast, the session is announced and UEs can receive it without joining, using pre-configured parameters.

Key components include the MBS session ID, which uniquely identifies the session within a PLMN; the MBS service area, which defines the geographic region where the service is available; and the QoS flow parameters, which ensure the required quality of service. The MBS session can be delivered using either the individual MBS traffic delivery method (where each UE has a separate user plane connection) or the shared MBS traffic delivery method (where a single user plane connection is shared among multiple UEs). The choice depends on factors like the number of UEs, their distribution, and the service requirements.

The MBS session plays a crucial role in enabling efficient group communication, live streaming, and IoT software updates. It is a key enabler for services like Mission Critical Push-to-Talk (MCPTT), V2X (Vehicle-to-Everything) communication, and public safety applications. By leveraging multicast/broadcast, the network can significantly reduce the load on the core network and radio access network, especially when many users are interested in the same content. The MBS session concept is also being enhanced in later releases to support new features like MBS for IoT devices and integration with network slicing.

Purpose & Motivation

The MBS session was introduced to address the growing demand for efficient delivery of popular content to many users simultaneously. Traditional unicast delivery, where each user receives a separate data stream, consumes significant network resources, especially in scenarios like live sports events, software updates, or group communications. Multicast and broadcast technologies allow the network to transmit data once and have multiple users receive it, dramatically improving spectral efficiency and reducing core network load.

Before MBS, 3GPP networks had limited support for multicast/broadcast, primarily through eMBMS (Evolved Multimedia Broadcast Multicast Service) in LTE. However, eMBMS had limitations in terms of flexibility, integration with 5G core, and support for new use cases. The MBS session concept in 5G was designed to be more flexible, scalable, and integrated with the service-based architecture of the 5GC. It supports both multicast and broadcast modes, dynamic session management, and seamless mobility.

The motivation for MBS sessions includes enabling new revenue-generating services such as live video streaming, immersive media, and mission-critical communications. It also addresses the need for efficient network resource usage in dense environments like stadiums, airports, and smart factories. By providing a standardized framework for multicast/broadcast, 3GPP ensures interoperability and allows operators to deploy these services using multi-vendor equipment.