Multicast/Broadcast Service Associated Signalling

Description

MBS-associated signalling is a concept introduced in 3GPP Release 19 to describe the mechanism by which E1AP messages related to a specific Multicast/Broadcast Service (MBS) session are associated with that session over the E1 interface. The E1 interface connects the gNB-CU-CP (Central Unit Control Plane) and the gNB-CU-UP (Central Unit User Plane) within a gNB in the 5G RAN architecture. When E1AP messages are exchanged for an MBS session, they need to be correctly associated with that session in both the gNB-CU-CP and gNB-CU-UP. This is achieved through the MBS-associated logical E1-connection, which serves as a dedicated logical connection for the MBS session. The definition is specified in 3GPP TS 37.483 (E1AP specification) and TS 38.473 (F1AP specification), indicating its relevance to both E1 and F1 interfaces.

The E1 interface is a critical component of the 5G RAN disaggregated architecture, where the gNB is split into a Central Unit (CU) and one or more Distributed Units (DU). The CU is further split into Control Plane (CU-CP) and User Plane (CU-UP) functions. The E1 interface carries signalling between CU-CP and CU-UP, including bearer context management, UE context management, and MBS session management. For MBS, the CU-CP is responsible for controlling the MBS sessions, while the CU-UP handles the user plane data. To ensure that E1AP messages (e.g., Bearer Context Setup, Bearer Context Modification) for an MBS session are correctly routed and processed, an MBS-associated logical E1-connection is established. This logical connection is identified by an MBS session identifier and is used to associate all E1AP messages pertaining to that session.

The MBS-associated signalling mechanism works as follows: When an MBS session is established, the gNB-CU-CP and gNB-CU-UP exchange E1AP messages to set up the necessary user plane resources. These messages include an MBS session identifier that uniquely identifies the session within the gNB. The MBS-associated logical E1-connection is then used for all subsequent E1AP messages related to that session, such as modifications or releases. This ensures that both the CU-CP and CU-UP can maintain the correct context for the MBS session and apply the appropriate configurations. The logical connection is not a physical connection but a logical association that is maintained by the E1AP protocol.

The introduction of MBS-associated signalling in Rel-19 addresses the need for efficient and reliable management of MBS sessions in the disaggregated RAN architecture. In earlier releases, MBS support was defined but the E1AP procedures for MBS were not fully specified. Rel-19 fills this gap by defining how E1AP messages are associated with MBS sessions, enabling seamless operation of MBS in a CU/DU split deployment. This is particularly important for network operators who deploy 5G in a disaggregated manner, as it ensures that MBS can be supported with the same flexibility and performance as other services.

From a protocol perspective, the MBS-associated signalling uses the existing E1AP message structure but adds the MBS session identifier to the relevant messages. The gNB-CU-CP and gNB-CU-UP maintain a mapping between the MBS session identifier and the logical E1-connection. When an E1AP message is received, the receiving node uses the MBS session identifier to determine which MBS session the message pertains to and processes it accordingly. This allows multiple MBS sessions to be managed simultaneously over the same E1 interface, each with its own logical connection. The mechanism is similar to how UE-associated signalling works on E1, where each UE has a logical E1 connection identified by a UE ID.

The MBS-associated signalling is a key enabler for MBS in 5G-Advanced networks. It ensures that MBS sessions can be set up, modified, and released efficiently in a CU/DU split architecture. Without this mechanism, MBS would be limited to monolithic gNB deployments, which are less common in modern 5G networks. By standardizing the association of E1AP messages with MBS sessions, 3GPP ensures interoperability between different vendors' CU-CP and CU-UP implementations, facilitating multi-vendor deployments. This is crucial for the widespread adoption of MBS and for delivering services like live streaming, public safety communications, and IoT group messaging.

Purpose & Motivation

MBS-associated signalling was created to solve the problem of associating E1AP messages with specific MBS sessions in the 5G disaggregated RAN architecture. In a gNB split into CU-CP and CU-UP, the E1 interface carries control plane signalling for various functions, including MBS. Without a standardized association mechanism, it would be ambiguous which MBS session an E1AP message refers to, leading to potential misconfigurations and service disruptions. The MBS-associated logical E1-connection provides a clear and unambiguous way to link E1AP messages to their corresponding MBS sessions.

The motivation for this concept stems from the growing importance of MBS in 5G-Advanced. As operators deploy more MBS services, the need for efficient and reliable session management becomes critical. The disaggregated RAN architecture, while offering flexibility and scalability, introduces complexity in signalling. MBS-associated signalling addresses this complexity by defining a logical connection that simplifies the association process. It allows the CU-CP and CU-UP to maintain separate contexts for each MBS session and to process E1AP messages correctly, even when multiple sessions are active simultaneously.

Prior to Rel-19, MBS support in the RAN was defined primarily for monolithic gNBs or with limited E1AP procedures. The lack of a standardized association mechanism meant that vendors had to implement proprietary solutions, which hindered interoperability. Rel-19 standardizes this aspect, ensuring that MBS can be deployed in multi-vendor disaggregated networks. This is essential for the widespread adoption of MBS and for enabling new business models such as venue casting, group communications, and mission-critical services. The MBS-associated signalling thus represents a crucial step in the evolution of MBS towards full commercial deployment.