Description
The Multicast/Broadcast Session Management Function (MB-SMF) is a specialized 5G Core Network (5GC) function defined in 3GPP Release 17 and beyond to support 5G Multicast-Broadcast Services (5G MBS). It is responsible for the session management aspects of point-to-multipoint data delivery. Unlike the standard Session Management Function (SMF) that manages unicast PDU sessions for individual UEs, the MB-SMF manages multicast/broadcast sessions that serve a group of UEs, such as those receiving live video streaming, software updates, or public warning messages.
Architecturally, the MB-SMF is a logical network function that can be deployed as a separate entity or co-located with a unicast SMF. It interfaces with other core network functions using the 5GC service-based architecture. Key interfaces include the Nmbmf interface to the Multicast/Broadcast Service Management Function (MB-UPF) for controlling the user plane, the Namf interface to the Access and Mobility Management Function (AMF) for coordinating with UEs joining/leaving a session, and the Npcf interface to the Policy Control Function (PCF) for applying multicast-specific policies. The MB-SMF is responsible for the lifecycle of a multicast session: it receives session establishment requests (e.g., from an Application Function), allocates a multicast IP address or tunnel endpoint identifier, selects and configures the appropriate MB-UPF(s) to create a multicast distribution tree, and manages the quality of service (QoS) for the multicast flow.
Its operation involves two primary modes: broadcast (where data is sent to all UEs in a service area) and multicast (where data is sent only to UEs that have explicitly joined a group). The MB-SMF works in tandem with the RAN, which has been enhanced with multicast capabilities (e.g., Single-Cell Point-To-Multipoint - SC-PTM, Multicast Broadcast Service - MBS). It provides the RAN with the necessary session context, including QoS profiles and multicast traffic routing information. For multicast mode, it also interacts with the AMF to manage the dynamic joining and leaving of UEs via IGMP/MLD proxy functionality or direct signaling, ensuring efficient use of radio and transport network resources by activating multicast bearers only when and where there are active receivers.
Purpose & Motivation
The MB-SMF was created to address the fundamental limitation of 5G networks prior to Release 17: the lack of native, efficient support for point-to-multipoint services within the 5G Core. While LTE had evolved with eMBMS (evolved Multimedia Broadcast Multicast Service), the initial 5G specifications focused on ultra-reliable low-latency and massive IoT use cases, leaving broadcast/multicast as an afterthought. The MB-SMF solves the problem of inefficient resource usage for group communication; using unicast transmissions to deliver the same content (like a live sports event) to thousands of users in the same cell wastes precious radio spectrum and core network bandwidth.
Its development was motivated by emerging use cases requiring efficient mass communication, such as live event streaming, in-vehicle infotainment, public safety communications, and over-the-air software updates for massive IoT device fleets. The MB-SMF, as part of the holistic 5G MBS architecture, provides a standardized, network-native mechanism to manage these services. It addresses the limitations of previous approaches (like eMBMS in LTE, which had complex integration and limited adoption) by being designed from the ground up within the flexible, service-based 5G Core. This allows for more dynamic session management, better integration with unicast services, and support for both broadcast and on-demand multicast, enabling new business models and services for operators.
Classification
Detected Changes Across Releases
from 3GPP Change RequestsSpecific changes extracted from the „Change history“ tables of 3GPP specifications (452 CRs across 6 releases). Complements the general historical overview above with the evidence-based evolution of this function.
In Release 15, the MB-SMF (Multicast/Broadcast Session Management Function) was newly introduced as a core network function within the 5G System architecture to enable multicast-broadcast services, as defined in TS 23.247. Its primary role is to manage multicast and broadcast PDU sessions, handling session establishment, modification, and release for point-to-multipoint data delivery. This introduction provides the foundational session management framework for 5G multicast-broadcast services.
- Addition of PDU Session type IPv4v6 TS 23.501CR0181
- Charging Session Definition TS 32.290CR0002
- Configuration information the UE may exchange with the SMF during the lifetime of a PDU Session TS 23.501CR0003
- Homogeneous support for IMS voice over PS Session supported indication TS 23.501CR0046
- UE support for Multi-homed IPv6 PDU Session TS 23.501CR0105
- User Plane management to support interworking with EPS TS 23.501CR0122
+ 50 more changes
In Release 16, the MB-SMF saw enhancements primarily related to the management of 5G LAN-type services and Time-Sensitive Networking (TSN) integration. This included new capabilities for 5G LAN group management and session binding for TSN networks, enabling the MB-SMF to support deterministic communication requirements. Furthermore, procedures were introduced for the MB-SMF to handle AF session binding to a PDU session specifically for TSN networks.
- 5GS Logical TSN bridge management TS 23.501CR1002
- QoS for Multi-Access PDU Session TS 23.501CR0770
- Clarification on MA PDU session TS 23.501CR1033
- Further detailing of 5G LAN group management TS 23.501CR1052
- Subscription Information Influence on PDU Session Rate Control TS 23.501CR1251
- Handling of Stored Small Data Rate Control Status at Subsequent PDU Session Establishment TS 23.501CR1252
+ 124 more changes
In Release 17, key enhancements for the MB-SMF focused on improving service resilience and network integration. Specifically, new procedures were introduced for MBS session restoration upon MB-UPF or NG-RAN failure, both with and without restart. Furthermore, the release defined mechanisms for MB-SMF registration and discovery, as well as support for selecting collocated MB-SMF/SMF entities.
- MA PDU sessions with connectivity over E-UTRAN/EPC and non-3GPP access to 5GC TS 23.501CR2527
- Informative guideline on supporting session/service continuity between SNPN and PLMN when using N3IWF TS 23.501CR2563
- Adding the usage of Session Management Congestion Control Experience analytics TS 23.501CR2708
- Enabling restricted PDU Session for remote provisioning of UE via User Plane TS 23.501CR2709
- Clarification on UE provides PDU Session Pair ID based on URSP rules TS 23.501CR2736
- KI#3B, Resolving EN for Future PDU Session TS 23.501CR2791
+ 102 more changes
In Release 18, key enhancements for the MB-SMF included support for the graceful or gradual termination of PDU sessions during network slice decommissioning and optimizations for managing the temporal validity of network slices. Furthermore, the release introduced the ability for multicast MBS sessions to be activated, deactivated, or updated following an AMF failure, improving service resilience. These updates were part of broader improvements in group management and session handling for multicast-broadcast services.
- UPF event exposure service for TSC management TS 23.501CR3720
- The support of Home Routed PDU Session supporting Session Breakout in VPLMN (HR-SBO) TS 23.501CR3830
- Change of Network Slice instance for PDU sessions TS 23.501CR3867
- Service area provisioning and LADN aspects for enhanced group management TS 23.501CR3914
- Considering ML model management capability during ADRF discovery and selection TS 23.501CR3929
- Support non-3GPP access leg of MA-PDU Session with PDN connection in EPC TS 23.501CR3937
+ 97 more changes
In Release 19, key enhancements for the MB-SMF included the introduction of a specific multicast MBS session restoration procedure for N3mb path failures and support for in-session unicast repair of MBS object distribution. The release also clarified architectural aspects for Local Offloading Management, including mechanisms for PCF awareness of I-SMF insertion and the discovery of an I-SMF for this purpose. Furthermore, updates were made to UPF event exposure during N4 session release and to QoS monitoring during UPF relocation.
- AMF event exposure for supporting on-demand broadcast of LCS assistant data TS 23.501CR5580
- HSBO_roaming traffic offloading via session breakout in HPLMN TS 23.501CR5748
- MWAB BH PDU session clarification TS 23.501CR5828
- NAT functionality in the UPF of BH PDU Session TS 23.501CR5650
- KI#1 Architecture for Local Offloading Management TS 23.501CR5752
- UPF event exposure at N4 Session release TS 23.501CR5782
+ 47 more changes
In Release 20, the MB-SMF gained support for an Ethernet type of Backhaul PDU Session for Multicast and Broadcast services (MWAB), expanding the supported PDU Session types beyond IP. Additionally, corrections were made to the procedures for Energy Consumption calculation to properly account for scenarios involving redundant transmission or a PDU Session with multiple PDU Session Anchors.
Explore further
Broader topics and technologies where MB-SMF plays a role.
Defining Specifications
3GPP specifications that define or reference MB-SMF, with the latest known release. Sourced from the 3GPP document catalog — see methodology.
| Specification | Title | Release |
|---|---|---|
| TS 23.501 vk00 | 5G System Architecture Stage 2 | Rel-20 |
| TS 23.527 vj50 | 5G System Restoration Procedures | Rel-19 |
| TS 29.244 vj40 | PFCP Specification for Control/User Plane Separation | Rel-19 |
| TS 29.510 vj50 | NRF Service Based Interface Protocol | Rel-19 |
| TS 29.513 vj40 | 5G PCC Signalling Flows & QoS Mapping | Rel-19 |
| TS 29.518 vj50 | AMF Service Based Interface Protocol | Rel-19 |
| TS 29.522 vj40 | 5G NEF Northbound APIs Stage 3 | Rel-19 |
| TS 29.532 vj30 | MB-SMF Service Based Interface Protocol | Rel-19 |
| TS 29.537 vj10 | 5G Multicast/Broadcast Policy Control Services | Rel-19 |
| TS 29.581 vj20 | MBSTF Service Based Interface Protocol Specification | Rel-19 |
| TS 32.240 vj40 | Charging Management Architecture & Principles | Rel-19 |
| TS 32.290 vj50 | 5G Charging for Service Based Interface | Rel-19 |
| TS 32.291 vj40 | Charging Management: Service-Based Interface Protocol | Rel-19 |