Multimedia Broadcast Multicast Service E-UTRAN Radio Access Bearer

Description

The MBMS E-RAB (E-UTRAN Radio Access Bearer) is a term used in 3GPP specifications to denote the combination of two distinct bearers: the data bearer established between the eNodeB (eNB) and the User Equipment (UE(s)) to transport MBMS data, and the MBMS M1 data bearer. This concept is defined in 3GPP TS 36.443 and TS 36.444, which specify the M2 and M3 interfaces for MBMS over LTE. The MBMS E-RAB is a critical component of the MBMS architecture in LTE, enabling the efficient delivery of broadcast and multicast services over the radio access network. It represents the end-to-end bearer from the core network to the UE for MBMS data, ensuring that the necessary quality of service (QoS) and transport characteristics are maintained.

Architecturally, the MBMS E-RAB spans multiple network entities. The M1 data bearer is the interface between the MBMS Gateway (MBMS GW) and the eNB, responsible for transporting MBMS data packets from the core network to the radio access network. The data bearer between the eNB and the UE(s) is the radio bearer that carries the MBMS data over the air interface. Together, these two bearers form the MBMS E-RAB, which provides a complete path for MBMS data from the BM-SC to the UE. The MBMS E-RAB is established and managed by the eNB, which coordinates with the MBMS GW and the Mobility Management Entity (MME) to set up the necessary resources. The M2 interface is used for control plane signaling between the MME and the eNB for MBMS session management, while the M3 interface is used for control plane signaling between the MME and the MBMS GW.

The MBMS E-RAB supports both broadcast and multicast modes of operation. In broadcast mode, the E-RAB is established for all UEs in a specific service area, without requiring individual UE authorization. In multicast mode, the E-RAB is established only for UEs that have joined the multicast group, and the network may optimize resource usage based on group membership. The E-RAB also supports features such as counting, which allows the network to determine the number of UEs interested in a service, and admission control, which ensures that sufficient resources are available before establishing the bearer. These features help operators manage network resources efficiently and ensure a good user experience.

Key components of the MBMS E-RAB include the M1 data bearer, which uses the GTP-U protocol over the S1-U interface between the MBMS GW and the eNB; the radio bearer, which uses the LTE-Uu interface between the eNB and the UE; and the control plane interfaces M2 and M3, which use the SCTP protocol for signaling. The MBMS E-RAB is identified by an E-RAB ID, which is used to correlate the bearer across different interfaces. The eNB is responsible for mapping the M1 data bearer to the appropriate radio bearer and for managing the radio resources. The MBMS GW is responsible for forwarding MBMS data from the BM-SC to the eNB over the M1 interface.

In operation, the establishment of an MBMS E-RAB begins with the BM-SC initiating an MBMS session and sending a session start request to the MBMS GW. The MBMS GW then sends a session start request to the MME, which forwards it to the eNB over the M2 interface. The eNB sets up the necessary radio resources and responds with a session start response. The MBMS GW then establishes the M1 data bearer and begins forwarding MBMS data to the eNB, which transmits it over the radio bearer to the UEs. The E-RAB remains active for the duration of the MBMS session and is released when the session ends. Throughout the session, the eNB may perform counting to determine if there are enough UEs to justify the use of MBMS, and may switch to unicast delivery if the number of interested UEs is low.

The MBMS E-RAB has evolved over multiple 3GPP releases, with enhancements in areas such as improved resource efficiency, support for new services, and integration with 5G. It remains a fundamental concept for MBMS delivery in LTE and is specified in the relevant 3GPP technical specifications.

Purpose & Motivation

The MBMS E-RAB was created to provide a standardized and efficient way to transport MBMS data from the core network to the UE over the LTE radio access network. Before its introduction, there was no unified bearer concept that combined the core network transport and the radio bearer for MBMS. The MBMS E-RAB solves this by defining a single logical bearer that encompasses both the M1 data bearer and the radio bearer, simplifying the management and coordination of MBMS sessions. It enables the network to treat the entire path from the MBMS GW to the UE as a single entity, ensuring consistent QoS and transport characteristics.

Historically, the need for MBMS E-RAB arose with the introduction of MBMS in LTE (Release 9). While MBMS was initially defined for UMTS in Release 6, the LTE architecture required a new set of interfaces and protocols to support MBMS efficiently. The MBMS E-RAB was introduced to address the specific requirements of LTE, such as the need for a flat architecture and the use of IP-based transport. It provides a clear separation between the control plane and user plane, with the M2 and M3 interfaces handling control signaling and the M1 interface handling user data. This separation allows for flexible deployment and efficient resource management.

The MBMS E-RAB also addresses the challenge of delivering the same content to multiple users without wasting radio resources. By using a shared bearer, the network can transmit MBMS data once and have all interested UEs receive it, rather than establishing individual unicast bearers for each UE. This is particularly important for services like mobile TV and live events, where many users may be interested in the same content. The E-RAB supports counting and admission control, which help operators decide whether to use MBMS or unicast based on the number of interested users, further optimizing resource usage. Overall, the MBMS E-RAB is a key enabler for efficient and scalable MBMS delivery in LTE networks.