Multicast Multicast-Broadcast Services (MBS) session

Description

A multicast MBS session is a service delivery construct in 5G Multicast-Broadcast Services (MBS) that carries multicast communication service to a group of authorized or subscribed UEs. It is defined in 3GPP TS 23.247 (architectural enhancements for 5G multicast-broadcast services) and supported by codec and media handling specifications such as TS 26.502 and TS 26.802. The session is characterized by three elements: the content to be sent, the list of UEs that may receive the service, and optionally a geographical area within which the content is distributed. This distinguishes it from a broadcast MBS session, which is intended for all UEs in a broadcast service area without a specific receiver list.

Architecturally, the multicast MBS session is established and managed through the 5G core network. The MBSF (Multicast/Broadcast Service Function) and MBSU (Multicast/Broadcast Service User plane) interact with the SMF, UPF, and AMF to set up the session, manage membership, and deliver the user plane data. The AF (Application Function) or an MBS application server provides the content and service parameters. UEs join the session either by explicit signaling or by being pre-configured, and the network tracks which UEs are eligible to receive the multicast data. The RAN then delivers the content using either point-to-point (PTP) or point-to-multipoint (PTM) transmission, depending on the number and distribution of receivers in a cell.

A key operational aspect is the dynamic choice between PTM and PTP delivery in the radio access network. When many UEs in a cell are interested, the gNB can use a single PTM transmission over shared radio resources, greatly improving spectral efficiency. When only a few UEs are present, PTP delivery may be more efficient. The multicast MBS session therefore provides a unified service context that the RAN can realize with different transmission modes. The optional geographic area allows the network to restrict distribution to specific cells or tracking areas, which is useful for regional content, local advertising, or public safety scenarios.

Multicast MBS sessions support a range of applications, including live streaming, group communications, software distribution, and mission-critical services. They can be delivered in both connected and idle modes, with mechanisms for session announcement, join/leave procedures, and service continuity during mobility. QoS handling for MBS sessions uses dedicated QoS flows and 5QI values appropriate for streaming or conversational media, and the session can be secured through existing 5G security mechanisms.

Compared with unicast delivery, multicast MBS sessions reduce core and RAN resource consumption when the same content is sent to many users. Compared with broadcast, they offer finer control over the receiver set and location, enabling personalized or regionalized group services. This makes the multicast MBS session a central building block of 5G MBS, complementing broadcast MBS sessions and legacy eMBMS approaches.

Purpose & Motivation

The multicast MBS session exists to deliver the same content efficiently to a defined group of users without duplicating unicast resources for each recipient. In mobile networks, popular live events, software updates, and group communications can involve thousands of users consuming identical data. Unicast delivery wastes spectrum and core capacity in such cases, while pure broadcast lacks the ability to target a specific subscriber set or geographic zone. The multicast MBS session fills this gap by combining group targeting with efficient point-to-multipoint transmission.

Historically, 3GPP addressed group delivery with eMBMS in LTE, but that framework was broadcast-centric and had limited flexibility for dynamic membership and hybrid unicast/multicast operation. With 5G, the industry sought a more integrated approach that could leverage the service-based architecture, support mobility and QoS, and coexist smoothly with unicast. Rel-17 and Rel-18 laid the groundwork for 5G MBS, and Rel-19 refined the multicast session concept, including its characterization by content, receiver list, and optional geographic area.

The multicast MBS session also supports new business models such as localized content delivery, enterprise group services, and public safety communications, where controlling who receives the content and where it is distributed matters. By making the receiver list and geographic scope first-class attributes of the session, the network can enforce service policies, optimize resource usage, and meet regulatory or commercial constraints.