Broadcast MBS session

Description

The Broadcast MBS session is a fundamental concept in 5G Multicast-Broadcast Services (MBS), introduced in 3GPP Release 19. It is an MBS session type specifically designed for broadcast communication services, where the same content is delivered to all users in a specific geographical area. Unlike multicast sessions, which require users to join and may involve authorization, broadcast sessions are open to any UE within the broadcast area without prior subscription. The broadcast MBS session is characterized by two main attributes: the content to be sent (e.g., a live video stream, emergency alerts) and the geographical area where the content is distributed. The network establishes the broadcast session and allocates resources for transmission. The broadcast MBS session can be delivered using either multicast broadcast (point-to-multipoint) or unicast (point-to-point) methods, depending on network configuration and UE capabilities. In the 5G core, the broadcast MBS session is managed by the MBSF (Multicast/Broadcast Service Function) and the MB-SMF (Multicast/Broadcast Session Management Function). The session is established via signaling between the MBSF and the MB-SMF, and the user plane data is distributed via the MB-UPF (Multicast/Broadcast User Plane Function) to the RAN nodes, which then broadcast to UEs. The broadcast MBS session supports various QoS requirements, and the network ensures that the broadcast content is delivered with the required quality. The concept is specified in 3GPP TS 23.247 (Architectural enhancements for 5G multicast-broadcast services), TS 26.502 (5G MBS user service architecture) and TS 26.802 (5G MBS codecs). Broadcast MBS sessions are essential for services like live TV, emergency alerts, and public safety communications, where efficient distribution to many users is critical.

Purpose & Motivation

The Broadcast MBS session exists to provide an efficient and scalable way to deliver broadcast content over 5G networks. Traditional cellular networks were designed for unicast traffic, and delivering the same content to many users individually would consume excessive network resources. Broadcast MBS sessions solve this by enabling point-to-multipoint transmission, where a single stream is broadcast to all users in a geographic area, significantly improving spectral efficiency. This is particularly important for services like live sports, news, and emergency alerts, where many users demand the same content simultaneously. The concept was motivated by the need to support broadcast services in 5G, similar to LTE's eMBMS (evolved Multimedia Broadcast Multicast Service), but with enhanced flexibility and integration with 5G core. The broadcast MBS session also addresses the limitations of unicast delivery for popular content, reducing network congestion and improving user experience. It enables new business models for content providers and mobile network operators, such as broadcast TV over 5G.