Radio Link Control

Description

The Radio Link Control (RLC) sublayer is a key component of the Layer 2 protocol stack in 3GPP radio access networks, including UMTS, LTE, and NR. Positioned between the Medium Access Control (MAC) layer below and the Packet Data Convergence Protocol (PDCP) layer above, RLC is responsible for reliable data transfer over the radio link. It operates in three modes: Transparent Mode (TM), Unacknowledged Mode (UM), and Acknowledged Mode (AM), each tailored to different service requirements. In TM, RLC passes data without adding headers, used for delay-sensitive services like voice. In UM, it provides segmentation and reassembly with sequence numbering but no retransmissions, suitable for streaming or broadcast. In AM, it adds error correction through Automatic Repeat Request (ARQ), ensuring reliable delivery for data services. RLC works by receiving service data units (SDUs) from PDCP, segmenting or concatenating them into protocol data units (PDUs) for transmission via MAC. It manages buffers, handles retransmissions in AM, and ensures in-sequence delivery to upper layers. Key components include the RLC entity, which maintains state variables and timers, and the RLC bearer, which corresponds to a logical channel. RLC interacts with MAC for scheduling and HARQ, adapting to radio conditions to optimize throughput and latency. Its role is fundamental in mitigating errors from the physical layer, supporting QoS differentiation, and enabling efficient use of radio resources across evolving 3GPP technologies.

Purpose & Motivation

The RLC protocol was created to address the inherent unreliability and variability of wireless radio links, which are prone to errors, delays, and packet loss due to fading, interference, and mobility. In early cellular systems, simple data transfer mechanisms were insufficient for supporting diverse services like voice, video, and internet access with varying reliability and latency needs. RLC solves this by providing a flexible, mode-based framework that ensures data integrity and order, adapting to service requirements. Historically, before 3GPP standardization, proprietary solutions led to interoperability issues. RLC, introduced in R99, established a unified approach for UMTS, evolving through releases to handle increased data rates and new use cases in LTE and NR. It addresses limitations of lower-layer protocols like PHY and MAC, which lack end-to-end reliability mechanisms, by offering ARQ and segmentation capabilities. The motivation was to enable efficient, reliable communication over the air interface, supporting the growth from circuit-switched voice to packet-switched multimedia, and later to 5G's ultra-reliable low-latency communications. RLC remains essential for maintaining link quality and enabling advanced features like carrier aggregation and dual connectivity.