Radio Bearer

Description

A Radio Bearer (RB) is the service provided by Layer 2 (the data link layer) for the transfer of user data between the User Equipment (UE) and the radio access network. It is a fundamental abstraction in 3GPP systems: it represents a logical channel of data flow over the air interface, with an associated set of quality-of-service (QoS) attributes that determine how the data is treated by the radio protocols. The RB is realized by the combination of Packet Data Convergence Protocol (PDCP), Radio Link Control (RLC), and Medium Access Control (MAC) entities, each of which contributes to the overall data transfer service.

In the protocol stack, a radio bearer maps to a specific configuration of PDCP, RLC, and MAC. The PDCP layer handles header compression, ciphering, and integrity protection; the RLC layer provides segmentation, reassembly, and reliable or unreliable delivery depending on the RLC mode (AM, UM, or TM); and the MAC layer multiplexes logical channels onto transport channels and handles HARQ and scheduling. The RB is identified by a radio bearer identity and is associated with a logical channel identity and a logical channel group for buffer status reporting. The QoS parameters of a radio bearer (e.g., priority, packet delay budget, packet error rate) are used by the scheduler to allocate radio resources.

Radio bearers come in two main types: Signaling Radio Bearers (SRBs) used for carrying RRC and NAS signaling messages, and Data Radio Bearers (DRBs) used for carrying user plane data. SRBs are typically configured with high priority and reliable delivery, while DRBs are configured according to the QoS requirements of the service (e.g., conversational voice, streaming video, best-effort data). In 5G, the concept of QoS flows was introduced, where a DRB can carry one or more QoS flows, each with its own QoS profile, and the mapping between QoS flows and DRBs is controlled by the network.

The radio bearer concept has evolved across generations. In UTRAN (3G), radio bearers were established per service with specific QoS attributes, and the RNC managed them. In E-UTRAN (LTE), the EPS bearer concept was introduced, with radio bearers being the radio part of an EPS bearer; the eNodeB manages the radio bearer configuration. In NG-RAN (5G), the QoS flow is the finest granularity of QoS treatment, and radio bearers are established to carry one or more QoS flows. Despite these architectural changes, the fundamental definition of a radio bearer as a Layer 2 service for user data transfer has remained stable since Rel-4, with the definition updated in Rel-19 to explicitly include GERAN, E-UTRAN, and NG-RAN alongside UTRAN.

Radio bearers are critical for ensuring that different services receive appropriate treatment over the radio interface. They enable the network to enforce QoS policies, prioritize traffic, and manage radio resources efficiently. The configuration of radio bearers is part of the radio resource control (RRC) protocol, which establishes, modifies, and releases them as the UE moves and as service requirements change.

Purpose & Motivation

The Radio Bearer concept exists to provide a standardized, layered abstraction for data transfer over the radio interface. Before its introduction, radio access protocols lacked a unified way to describe the service provided to upper layers, making it difficult to ensure consistent QoS treatment across different services and network architectures. The radio bearer solves this by defining a clear interface between Layer 2 and higher layers, allowing applications and core network functions to request specific QoS characteristics without needing to understand the details of the radio protocols.

Historically, as mobile networks evolved from voice-centric to data-centric, the need for differentiated treatment of various data flows became paramount. The radio bearer provides the mechanism to map service requirements (e.g., low latency, high reliability) to specific radio protocol configurations. It also enables mobility: when a UE moves between cells, the radio bearer can be re-established or reconfigured to maintain service continuity. The evolution of the definition to include multiple radio access technologies (GERAN, UTRAN, E-UTRAN, NG-RAN) reflects the multi-RAT nature of modern networks and the need for a common terminology.

Furthermore, the radio bearer concept supports the separation of control plane and user plane, with SRBs and DRBs respectively. This separation is essential for security, as signaling and data can be protected differently, and for QoS, as data bearers can be tailored to specific service needs. The radio bearer thus serves as a cornerstone of 3GPP's QoS architecture, enabling operators to offer differentiated services and manage network resources effectively.