Multicast/Broadcast Service

Description

Multicast/Broadcast Service (MBS) is a comprehensive framework within 3GPP standards designed for the efficient, simultaneous delivery of identical data streams from a single source to a potentially large group of users. Unlike unicast, where separate data streams are sent to each user, MBS utilizes point-to-multipoint (PTM) transmission at the radio and transport network levels, conserving valuable radio spectrum and core network bandwidth. The architecture supports both broadcast mode (content delivered to all users in a service area) and multicast mode (content delivered only to a subscribed group of users within an area).

In 5G System (5GS), MBS is integrated into the core network (5GC) and the NG-RAN. Key architectural components include the Multicast/Broadcast-Session Management Function (MB-SMF) for session management, the Multicast/Broadcast-User Plane Function (MB-UPF) for data forwarding, and the Multicast/Broadcast-Service Function (MBSF) for service announcement and membership management. In the RAN, a new logical entity, the MBS Gateway (MB-SGW), may be used to distribute MBS data to multiple gNBs. The radio interface employs MBS-specific bearers and scheduling. In 5G NR, this is realized through Physical Downlink Shared Channels (PDSCH) scheduled for multicast, using either Single-Cell Point-To-Multipoint (SC-PTM) or Multicast/Broadcast over a Single Frequency Network (MBSFN), where multiple synchronized cells transmit identical waveforms, allowing a UE to combine them as a single, stronger signal.

The service works by establishing an MBS session context across the network. The application server initiates a session, and the 5GC sets up the necessary multicast distribution tree for user plane data, identifying the target service area. UEs interested in receiving the content perform a service discovery and, for multicast, may join a specific multicast group via signaling with the core network. The RAN then schedules the MBS data on shared radio resources. UEs in RRC_IDLE, RRC_INACTIVE, or RRC_CONNECTED states can receive the broadcast/multicast traffic, enabling power-efficient reception for a massive number of devices. Key protocols involved include the MBS-specific adaptations of the Packet Data Convergence Protocol (PDCP), Radio Link Control (RLC), and Medium Access Control (MAC) layers to handle point-to-multipoint data duplication and scheduling.

MBS supports reliable delivery mechanisms, including application-level and radio-level error correction (e.g., RLC Acknowledged Mode for critical data), and seamless mobility where a UE moving between cells continues to receive the MBS session without interruption. The system also defines QoS flows for MBS, allowing different levels of service (e.g., high priority for emergency alerts, guaranteed bitrate for live video). This integrated approach allows MBS to support a wide range of data rates and service requirements, from low-rate IoT group messaging to high-definition video streaming.

Purpose & Motivation

MBS was created to solve the inherent inefficiency of using unicast transmission for popular content consumed by many users simultaneously in the same geographic area. Delivering live television, streaming major sports events, or distributing large software/firmware updates via individual unicast streams consumes excessive radio and transport network resources, leading to congestion, degraded quality for all users, and higher operational costs for network operators. MBS addresses this by sharing network resources for common content, dramatically improving spectral efficiency and network scalability for group communication services.

The evolution of MBS in 3GPP has historical roots in earlier technologies like Multimedia Broadcast Multicast Service (MBMS) introduced in 3G UMTS and evolved MBMS (eMBMS) in 4G LTE. These earlier versions faced challenges with complexity, limited device support, and business model adoption. The motivation for reinvigorating and enhancing MBS in 5G (starting in Rel-16/17) stems from new use cases demanding efficient mass communication: massive IoT software updates, Vehicle-to-Everything (V2X) group communications, public safety and mission-critical group messaging, and immersive media experiences like augmented reality (AR) live events. 5G's need to support diverse vertical industries provided a strong driver for a more flexible, integrated, and capable multicast/broadcast framework.

Furthermore, MBS solves the problem of network congestion during peak events and enables new revenue streams for operators through broadcast-like services. It also supports reliable low-latency group communications essential for industrial IoT and V2X applications, which were not fully addressed by previous MBMS implementations. By designing MBS as a native service within the 5G architecture, rather than a bolt-on feature, 3GPP ensures it can meet the stringent performance, reliability, and scalability requirements of future group-oriented applications across all 5G usage scenarios: enhanced Mobile Broadband (eMBB), massive Machine Type Communications (mMTC), and Ultra-Reliable Low Latency Communications (URLLC).