Description
The Multicast/Broadcast Service Transport Function (MBSTF) is a core network function introduced in 5G System (5GS) architecture to support Multimedia Broadcast Multicast Service (MBMS) delivery. It operates within the 5G Core (5GC) and is responsible for managing the transport of multicast and broadcast user plane data. The MBSTF interfaces with other network functions such as the Session Management Function (SMF) and User Plane Function (UPF) to establish and maintain multicast/broadcast sessions. It handles key procedures including session announcement, modification, and termination, ensuring that data is efficiently distributed to the Radio Access Network (RAN) for transmission to user equipment (UE).
Architecturally, the MBSTF can be deployed as a standalone function or integrated with other user plane functions. It receives multicast/broadcast traffic from content providers or the UPF and forwards it using IP multicast protocols towards the RAN nodes (gNBs). The function supports both broadcast mode, where data is sent to all UEs in a service area, and multicast mode, where data is sent only to UEs that have joined a specific multicast group. It works in conjunction with the Broadcast/Multicast Service Center (BM-SC) for service provisioning and the Access and Mobility Management Function (AMF) for UE registration in multicast services.
The MBSTF plays a critical role in optimizing network resources by reducing duplicate transmissions of the same data over the core network. Instead of sending individual data streams to each UE, it aggregates traffic and uses point-to-multipoint delivery, which is especially beneficial for high-demand content like live sports or emergency broadcasts. It supports dynamic session management, allowing network operators to scale services based on demand and manage Quality of Service (QoS) parameters for multicast flows. The function also incorporates security mechanisms, integrating with authentication and key management functions to ensure that multicast content is delivered securely to authorized users only.
Purpose & Motivation
The MBSTF was created to address the growing demand for efficient group communication services in 5G networks. Previous generations like 4G LTE supported MBMS through the BM-SC and MCE (Multi-cell/multicast Coordination Entity), but the architecture was not fully integrated into the core network as a standardized function. This led to limitations in scalability, flexibility, and support for new 5G use cases such as massive IoT and ultra-reliable low-latency communications (URLLC). The MBSTF provides a native, cloud-native approach to multicast/broadcast within the 5GC, enabling seamless service delivery across diverse deployment scenarios.
The primary problem it solves is inefficient use of network bandwidth when delivering the same content to multiple users. Traditional unicast delivery would require separate data streams for each user, congesting the network and increasing latency. MBSTF enables network operators to offload popular content through multicast, reducing core network load and improving user experience. This is particularly important for bandwidth-intensive applications like 4K/8K video streaming, where unicast delivery becomes unsustainable during peak demand.
Furthermore, the MBSTF supports emerging 5G services such as Vehicle-to-Everything (V2X) communications, public safety networks, and software updates for IoT devices. By providing a standardized, scalable mechanism for group data delivery, it facilitates the commercialization of multicast/broadcast services and enables new revenue streams for operators. The function's integration with network slicing also allows dedicated multicast slices for specific services, ensuring performance isolation and tailored QoS.
Classification
Detected Changes Across Releases
from 3GPP Change RequestsSpecific changes extracted from the „Change history“ tables of 3GPP specifications (57 CRs across 5 releases). Complements the general historical overview above with the evidence-based evolution of this function.
In Release 15, the newly introduced MBSTF (Multicast/Broadcast Service Transport Function) provides the architectural enhancements required for 5G multicast-broadcast services, as defined in TS 23.247. The function establishes the necessary reference points within the 5G System to support these broadcast and multicast transport capabilities.
- Clarifications to: Protection at the network or transport layer, Authorization and authentication between network functions and the NRF TS 33.501CR0147
- Using TCP for reliable NAS transport between UE and N3IWF TS 23.501CR0692
- Transport Level Packet Marking TS 23.501CR0942
- Clarifications and editorials to clause 13.1 (Transport security for service based interfaces) TS 33.501CR0354
- Missing Feature Description on Transport Level Marking TS 29.244CR0075
- Transport Level Marking TS 29.244CR0179
In Release 16, the MBSTF enhancements specifically introduced support for forwarding broadcast and multicast packets for 5G-LAN and Ethernet-type PDU Sessions, including data forwarding procedures. The release also provided updates and clarifications for the 5G Virtual Network (5G VN) broadcast solution, detailing its usage of IP multicast mechanisms as defined in the relevant specifications.
- Support of forwarding of broadcast and multicast packets TS 23.501CR1659
- Clarification on PDU Session management for 5G-LAN multicast TS 23.501CR1202
- Data forwarding for 5G-LAN multicast TS 23.501CR1328
- Correction of NAS transport for LCS TS 23.501CR1578
- Multicast forwarding for Ethernet type PDU Session TS 23.501CR1722
- Updates to the 5G VN broadcast solution TS 23.501CR1857
+ 8 more changes
In Release 17, the MBSTF function was enhanced with new event reporting capabilities, including a Broadcast Delivery Status event and a Multicast Transport Address Change Context Status event. It also introduced support for transporting PMF information via the N4 interface and clarified the protocol usage for unicast transport over the Nmb9 reference point. Furthermore, essential corrections and clarifications were made to the MBSTF APIs and to multicast security context handling in the session creation procedure.
- Multimedia Priority Service (MPS) Phase 2 support for Data Transport Service TS 23.501CR2536
- Additional authorization functionality in support of MPS for Data Transport Service TS 23.501CR2971
- Transport Level Marking information for PFCP sessions over N4mb TS 29.244CR0622
- Broadcast Delivery Status event TS 29.532CR0012
- Multicast Transport Address Change Context Status event TS 29.532CR0014
- KI#3, clarification on the TSCTSF functionality and configuration for transport protocols TS 23.501CR3160
+ 9 more changes
In Release 18, the MBSTF saw new capabilities for interworking with a Time Sensitive Networking (TSN) enabled transport network, including support for a TL-Container and the fully centralized configuration model. It also introduced explicit support for Reduced Capability (RedCap) UEs within MBS broadcast sessions and enabled location-dependent broadcast sessions managed by multiple MB-SMFs. Furthermore, security procedures were streamlined by removing the requirement for security key exchange between the MBSF and MBSTF.
- Interworking with TSN network deployed in the transport network TS 23.501CR3811
- SNPN broadcast system information and manual network selection for localized service TS 23.501CR4095
- TL-Container for the support of TSN enabled Transport Network TS 29.244CR0734
- RedCap UEs Information for a broadcast MBS session TS 29.532CR0085
- Correction on Support of Time Sensitive Networking (TSN) enabled Transport Network (TN) TS 23.501CR4955
- Clarifications of Event ID broadcast in SIB9 TS 23.501CR5039
+ 6 more changes
In Release 19, key enhancements for the MBSTF included the support of the Media over QUIC (MoQ) transport protocol on the N4 interface and new mechanisms for transport level packet marking. Specifically, these mechanisms leverage PDU Set QoS information, including PDU Set Importance, to enable DSCP marking over N3/N9 interfaces for HR PDU sessions. Furthermore, the scope of the MBSTF was refined by removing time services endpoints.
- AMF event exposure for supporting on-demand broadcast of LCS assistant data TS 23.501CR5580
- Leveraging PDU Set QoS information for DSCP marking over N3/N9 in the transport network TS 23.501CR5407
- XRM_Ph2_KI3 Leveraging PDU Set QoS information for DSCP marking over N3/N9 in the transport network TS 23.501CR6050
- Support of MoQ Transport protocol on N4 TS 29.244CR0897
- Leveraging PDU Set QoS information for DSCP marking over N3/N9 in the transport network TS 29.244CR0912
- Transport Level Marking Indication TS 29.244CR0987
+ 4 more changes
Explore further
Broader topics and technologies where MBSTF plays a role.
Defining Specifications
3GPP specifications that define or reference MBSTF, 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 26.502 vj30 | 5G Multicast-Broadcast User Services Architecture | Rel-19 |
| TS 26.512 vj10 | 5G Media Streaming Protocols & APIs | Rel-19 |
| TS 26.517 vj10 | 5G MBS User Service Protocols and Formats | Rel-19 |
| TS 26.802 vj20 | Multicast Enhancements for 5G Media Streaming | Rel-19 |
| TS 26.804 vj10 | 5G Media Streaming Extensions Study | Rel-19 |
| TS 29.244 vj40 | PFCP Specification for Control/User Plane Separation | Rel-19 |
| TS 29.532 vj30 | MB-SMF Service Based Interface Protocol | Rel-19 |
| TS 29.580 vj30 | 5G MBSF Service Interface Stage 3 Specification | Rel-19 |
| TS 29.581 vj20 | MBSTF Service Based Interface Protocol Specification | Rel-19 |
| TS 33.501 vk00 | 5G Security Architecture and Procedures | Rel-20 |
| TR 33.850 vh00 | 5G MBS Security Study | Rel-17 |