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Standards

5G Multicast Broadcast Services (MBS)

5G MBS architecture across the user-service, 5G Core and NR/NG-RAN layers, and the 3GPP specifications behind it.

Overview​

5G Multicast Broadcast Services (MBS) is the 3GPP 5G System feature for delivering the same content to many devices at once over the 5G core and NR radio, used for live media, software updates and mission-critical group communication. Unlike LTE-based 5G Broadcast (see Standards: 5G Broadcast), MBS is native to the 5G core and New Radio (NR). Because MBS spans the whole stack, the specifications below are grouped by layer: user-service level, 5G core network, and NR / NG-RAN. 5G-MAG tracks and contributes to this work. For acronyms used here, see the Glossary.

Technology & Analysis

The implementer-facing analysis of the MBS architecture across its three layers.

Software Tools

5G-MAG's reference tools realising the MBS architecture.

Why MBS, and what changed from eMBMS​

The problem MBS solves is scale. Sending the same live stream to a large audience over conventional unicast means one copy per device, so radio and transport load grows linearly with the audience. Point-to-multipoint (PTM) delivery sends a single transmission that many devices decode, so the cost is decoupled from audience size. LTE already offered this through evolved Multimedia Broadcast Multicast Service (eMBMS), but eMBMS used a largely separate control and user plane and a distinct radio design, which made it costly to deploy alongside unicast.

5G MBS is designed to reuse the existing 5G System instead. It is integrated into the 5G Core service-based architecture and into the NR radio layer, reusing the Release 15/16 physical channels, reference signals, numerology and cyclic prefixes. The network can switch a multicast session between PTM and point-to-point (PTP) delivery per cell and per device, so a session behaves like unicast where that is more efficient (few receivers, poor channel) and like broadcast where PTM is more efficient (many receivers). This flexibility, and the shared control plane, are the main practical differences from eMBMS.

Architecture in three layers​

The specification list below is grouped to match the three layers a single MBS session passes through. Reading top down:

  • User-service layer. The MBS User Services architecture (TS 26.502) is an optional abstraction designed in SA4. A content provider (the MBS Application Provider, playing the AF/AS role) uses it to provision services, announce them to clients, ingest content and, optionally, repair lost objects over unicast. It is realised by two functions delegated from SA2: the Multicast/Broadcast Service Function (MBSF) on the control plane and the Multicast/Broadcast Service Transport Function (MBSTF) on the user plane. The bit formats and protocols are in TS 26.517.
  • 5G Core layer. The MBS system architecture (TS 23.247, Stage 2) defines the multicast and broadcast communication services, the MBS sessions that carry them, and the two ways the core moves MBS packets towards the radio: the 5GC shared method (one copy per MBS-capable RAN node over a shared GTP-U tunnel) and the 5GC individual method (a per-UE copy in a normal PDU session, used for MBS-incapable nodes). The MBS-specific core functions are the MB-SMF (session management) and MB-UPF (user plane); the AMF, PCF, NEF and NRF gain MBS extensions. Stage 3 procedures are in TS 29.532 (session management), TS 29.537 (policy control), TS 29.580 (MBSF services) and TS 29.581 (MBSTF transport services).
  • NR and NG-RAN layer. Once packets reach the gNB, the radio side (TS 38.300 family) chooses PTM or PTP and applies one of three Layer-2 delivery modes: delivery mode 1 (multicast, HARQ feedback and retransmissions, RRC_CONNECTED), delivery mode 2 (broadcast, no feedback, receivable in any RRC state) and the default unicast mode. Broadcast configuration is carried on the MCCH (pointed to by SIB20), traffic on the MTCH, and sessions are addressed by a Group RNTI (G-RNTI).

The developer-facing view of what the 5G-MAG reference tools implement across these layers is on the developer portal.

Specifications by release​

MBS was introduced in Release 17 as the "5MBS" work: the Stage 2 architecture (TS 23.247), the user-service layer (TS 26.502, TS 26.517), the core Stage 3 procedures (TS 29.532, TS 29.537, TS 29.580, TS 29.581) and the NR/NG-RAN support (TS 38.300 family, with SIB20, the MCCH/MTCH, G-RNTI and delivery modes 1 and 2). In Release 17, multicast reception (delivery mode 1) requires RRC_CONNECTED.

Release 18 ("5MBS Phase 2") extends the feature rather than replacing it. The RAN work adds MBS multicast reception in the RRC_INACTIVE state, introducing SIB24 and a dedicated multicast MCCH carrying the multicast configuration (see MBS Multicast Inactive RAN procedures). Architecture and security study work for Phase 2 was captured in study reports before being folded into the normative specifications. Later releases continue to maintain and extend these specifications; check the version of each specification you are targeting for the exact release content.

The tables in the sections that follow list each specification with its 3GPP number and title. Where a specification applies at a specific reference point or protocol layer, that mapping is given alongside.

This is a list of specifications in the scope of 5G Multicast Broadcast Services.

MBS User Services​

  • 3GPP TS 26.502 - 5G Multicast-Broadcast Services; User service architecture
  • 3GPP TS 26.517 - 5G Multicast-Broadcast User Services; Protocols and Formats
  • 3GPP TS 29.580 - 5G System; Multicast/Broadcast Service Function services; Stage 3
  • 3GPP TS 29.581 - 5G System; Multicast/Broadcast Service Transport Services; Stage 3

5G Core Network specifications​

MBS architecture​

  • 3GPP TS 23.247 - Architectural enhancements for 5G multicast-broadcast services

Network Function services​

  • 3GPP TS 29.532 - 5G System; 5G Multicast-Broadcast Session Management Services; Stage 3
  • 3GPP TS 29.537 - 5G System; Multicast/Broadcast Policy Control services; Stage 3

Protocols​

  • 3GPP TS 29.244 - Interface between the Control Plane and the User Plane nodes
  • 3GPP TS 38.413 - NG-RAN; NG Application Protocol (NGAP)
  • 3GPP TS 24.501 - Non-Access-Stratum (NAS) protocol for 5G System (5GS); Stage 3
  • 3GPP TS 29.281 - General Packet Radio System (GPRS) Tunnelling Protocol User Plane (GTPv1-U)

The following table maps each core-network plane to its protocol and the specification that defines it.

Control plane​
ProtocolDefining spec
Packet Forwarding Control Protocol (PFCP)TS 29.244
NG Application Protocol (NGAP)TS 38.413
Non-Access Stratum (NAS)TS 24.501
User plane​
ProtocolDefining spec
GPRS Tunnelling Protocol User Plane (GTP-U)TS 29.281

NR and NG-RAN specifications​

NR​

The two tables below list the NR radio protocol layers per plane and the specification that defines each. Channel annotations follow each table as a note; the abbreviations are: PSS/SSS (Primary/Secondary Synchronisation Signal), PBCH (Physical Broadcast Channel) carrying the MIB (Master Information Block), PDCCH (Physical Downlink Control Channel), PDSCH (Physical Downlink Shared Channel) carrying SIB20/SIB21 (System Information Blocks) and MCCH (Multicast Control Channel), and MTCH (Multicast Traffic Channel).

Control plane​
LayerProtocolDefining spec
RRCRadio Resource ControlTS 38.331
PDCPPacket Data Convergence ProtocolTS 38.323
RLCRadio Link ControlTS 38.322
MACMedium Access ControlTS 38.321
PHYPhysical layerTS 38.211, 38.212, 38.213, 38.214

Channels: PSS/SSS/PBCH(MIB), PDCCH, PDSCH(SIB20/SIB21/MCCH).

User plane​
LayerProtocolDefining spec
SDAPService Data Adaptation ProtocolTS 37.324
PDCPPacket Data Convergence ProtocolTS 38.323
RLCRadio Link ControlTS 38.322
MACMedium Access ControlTS 38.321
PHYPhysical layerTS 38.211, 38.212, 38.213, 38.214

Channels: PDSCH(MTCH).

NG-RAN​

Interfaces​

NG-RAN can be split into a Central Unit (CU) and Distributed Unit (DU), with the CU further divided into control-plane (CU-CP) and user-plane (CU-UP) parts. The reference points below connect these elements and the 5G core; the gNB is the 5G base station, the AMF is the Access and Mobility Management Function, and the UPF is the User Plane Function.

  • Xn (38.420, 38.423): connects two gNBs (5G base stations)
  • NG (38.410, 38.413): NG-c = N2 (to the AMF, Access and Mobility Management Function); NG-u = N3 (to the UPF, User Plane Function)
  • F1 (38.470, 38.473): F1-c (PDCP-c to RLC); F1-u (PDCP-u to RLC)
  • E1 (38.460, 38.463): CU-CP (RRC + PDCP-c) to CU-UP (SDAP + PDCP-u)
References to verify

The release placement statements in "Specifications by release" were not all confirmed against a primary source: the "5MBS" / "5MBS Phase 2" work-item naming, the Release 17 introduction of the listed specifications, and the Release 18 placement of MBS multicast reception in RRC_INACTIVE (SIB24 and the dedicated multicast MCCH). Verify against the specific 3GPP release and specification versions you are targeting.

5G-MAG tracking and contribution focus​

5G-MAG tracks and contributes to 5G MBS standardisation, and maintains reference tools that implement the architecture across all three layers described above (user-service, 5G Core, and NR/NG-RAN). The current focus areas are:

  • Release 18 "5MBS Phase 2": following the RAN extensions that add MBS multicast reception in the RRC_INACTIVE state, introducing SIB24 and a dedicated multicast MCCH (see MBS Multicast Inactive RAN procedures).
  • Cross-layer reference implementation: the developer portal documents how the reference tools realise the user-service (TS 26.502 / TS 26.517), 5G Core (TS 23.247 and the related Stage 3 specifications), and NR/NG-RAN (TS 38.300 family) layers described above.

For the implementer-facing analysis of this architecture, see Technical Documentation: Multicast & Broadcast in 5G.

note

Refer to the Standards repository to contribute to this documentation.