Radio Link Control bearer configuration

Description

The RLC bearer configuration is a concept introduced in 3GPP Release 15 for 5G New Radio (NR). It represents the lower-layer portion of a radio bearer configuration, specifically encompassing the Radio Link Control (RLC) entity configuration and the logical channel configuration. A radio bearer in NR is associated with a Service Data Adaptation Protocol (SDAP) entity, a Packet Data Convergence Protocol (PDCP) entity, and one or more RLC bearers. The RLC bearer configuration is therefore a subset of the full radio bearer configuration, focusing on the RLC and MAC layers.

In the NR protocol stack, the RLC layer sits between PDCP and MAC. It provides services such as segmentation and reassembly of RLC SDUs, error correction through ARQ, and duplicate detection. The RLC bearer configuration specifies parameters for the RLC entity, including the RLC mode (TM, UM, or AM), sequence number size, and various timers and counters. It also includes the logical channel configuration, which defines the logical channel identity, priority, prioritised bit rate, and bucket size duration. These parameters are used by the MAC scheduler to allocate radio resources.

The RLC bearer configuration is signaled to the UE via Radio Resource Control (RRC) messages. In 5G NR, the RRC protocol is specified in TS 38.331. The configuration is part of the RadioBearerConfig information element, which contains a list of RLC bearers. Each RLC bearer is associated with a logical channel and an RLC entity. The UE applies the configuration to establish the necessary protocol entities and to configure the MAC layer for scheduling.

The concept of an RLC bearer configuration allows for flexible mapping of radio bearers to logical channels. In LTE, the radio bearer configuration was more monolithic, with PDCP, RLC, and logical channel parameters often configured together. In NR, the separation into RLC bearer configuration enables more dynamic and modular configuration, which is beneficial for features like dual connectivity, carrier aggregation, and multi-connectivity. For example, in dual connectivity, a radio bearer may be split across a master node and a secondary node, with different RLC bearers configured for each node.

The RLC bearer configuration has remained stable across Releases 15 through 19, with only minor formatting changes in the definition text. This stability reflects the maturity of the NR RLC and MAC specifications. The term is primarily used in RRC specification TS 38.331 and in the LTE RRC specification TS 36.331 for the LTE side of EN-DC (E-UTRA-NR Dual Connectivity). In EN-DC, the LTE RRC configures the LTE RLC bearer, while NR RRC configures the NR RLC bearer.

For engineers and students, understanding the RLC bearer configuration is essential for comprehending how radio bearers are established and how data flows through the NR protocol stack. It bridges the higher-layer bearer concepts (PDCP, SDAP) with the lower-layer scheduling and transmission functions (MAC, PHY). Proper configuration of RLC bearers is critical for achieving desired QoS, reliability, and latency.

Purpose & Motivation

The RLC bearer configuration was introduced in 5G NR to provide a modular and flexible way to configure the lower layers of a radio bearer. In previous generations like LTE, the radio bearer configuration was more tightly integrated, making it less adaptable to advanced features such as dual connectivity and network slicing. The separation of the RLC bearer configuration allows the network to independently configure RLC and logical channel parameters for each leg of a split bearer, enabling efficient multi-connectivity.

Another motivation is the need to support diverse service requirements in 5G, such as enhanced mobile broadband (eMBB), ultra-reliable low-latency communication (URLLC), and massive machine-type communication (mMTC). Each service may require different RLC modes, logical channel priorities, and scheduling parameters. By defining a separate RLC bearer configuration, the network can tailor these parameters per bearer without affecting other layers.

The concept also simplifies the specification by clearly delineating the responsibilities of the RLC and MAC layers versus the PDCP and SDAP layers. This modularity aids in interoperability and testing, as each layer's configuration can be validated independently. Overall, the RLC bearer configuration addresses the limitations of monolithic bearer configurations by providing a granular, extensible framework for radio resource management in NR.