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.
Protocol Stack
Release Timeline
Detected Changes Across Releases
from 3GPP Change RequestsSpecific changes extracted from the „Change history“ tables of 3GPP specifications (45 CRs across 5 releases). Complements the general historical overview above with the evidence-based evolution of this function.
In Release 15, the RLC function was updated to support new dual-connectivity architectures like EN-DC, NGEN-DC, and NE-DC, requiring corrections and clarifications for RLC bearer setup and establishment for LTE bearers within these configurations. New procedures were introduced for reporting RLC failures, and enhancements were made for RLC operation with PDCP duplication, specifying the PDCP association with RLC for configured radio bearers. Additional corrections and clarifications addressed state variables, SN sizes for both AM and UM (including for NB-IoT), polling, retransmission selection, and security for UM bearers during termination point changes.
- EN-DC impacts to LTE RLC TS 36.322CR0132
- Correction on SN size for RLC AM TS 36.322CR0141
- Clarification on RLC UM SN size for NB-IoT TS 36.322CR0145
- Corrections regarding RLC failure reporting TS 36.331CR3629
- Clarification of primary and secondary RLC entity TS 36.331CR3752
- Correction for the establishment of LTE RLC bearers for (NG)EN-DC and NE-DC TS 36.331CR4185
+ 7 more changes
In Release 16, the RLC function was significantly enhanced to support Integrated Access and Backhaul (IAB) networks, introducing the Backhaul (BH) RLC channel and specifying its management, including transmission suspension upon IAB-MT failure. The release also added new capabilities for NR Sidelink (SL), such as total L2 buffer size and RLC RTT reporting, and introduced configuration for RLC out-of-order delivery. Furthermore, it included clarifications and security improvements for procedures involving RLC AM and UM bearers during termination point changes and RRC resume/reestablishment.
- Incorrect restriction for RLC UM radio bearers TS 36.331CR4385
- Addition of total L2 buffer size and RLC RTT for NR SL TS 38.306CR0547
- Correction on RLC spec to support the BAP as upper layer TS 38.322CR0036
- CR to 38.322 on Backhaul RLC Channel TS 38.322CR0037
- PDCP entity associated with AM RLC entity TS 38.323CR0052
- Corrections on default BH RLC channel TS 38.331CR1954
+ 10 more changes
In Release 17, the RLC updates primarily involved corrections and clarifications for Sidelink (SL) relay operation. These included specific fixes for the RLC entity establishment procedures for SL-SRB0 and SL-SRB1, and a clarification on RLC bearer re-association. The release also contained a correction for the RLC-Config information element code.
In Release 18, the RLC function saw targeted corrections and clarifications for specific operational scenarios. These included handling for multi-path relay with N3C, data volume calculations for bearers associated with multiple RLC entities, and the processing of the Delay Critical Indication from PDCP. The release also provided corrections for PC5 sidelink RLC channel handling and release procedures, as well as for the handling of bearers during LTM cell switch execution in NR-DC and for sidelink RLC mode indication.
- RLC correction for multi-path relay with N3C TS 38.322CR0063
- Data volume calculation for DSR when associated with at least two RLC entities TS 38.323CR0133
- Correction for Delay Critical Indication from PDCP to RLC TS 38.323CR0144
- Correction to PC5 RLC channel handling TS 38.331CR5270
- Correction to PC5 RLC Channel Release TS 38.331CR5292
- Handling of radio and RLC bearers at LTM cell switch execution in NR-DC TS 38.331CR5383
+ 1 more changes
In Release 19, the RLC function was updated to introduce specific enhancements for Extended Reality (XR) traffic, as detailed in a new specification. The release also included miscellaneous corrections to these R19 XR-related RLC procedures. Furthermore, a correction was made to the RLC channel configuration for PC5 Relay communication.
Explore further
Broader topics and technologies where RLC plays a role.
Defining Specifications
3GPP specifications that define or reference RLC, with the latest known release. Sourced from the 3GPP document catalog — see methodology.
| Specification | Title | Release |
|---|---|---|
| TS 03.071 v7b0 | Location Services (LCS) Stage 2 Description | Rel-7 |
| TR 21.905 vj00 | 3GPP Technical Terms and Definitions | Rel-19 |
| TS 23.050 v1100 | UMTS Network Principles and Architecture | R99 |
| TS 23.060 vj00 | GPRS Service Description Stage 2 | Rel-19 |
| TS 23.107 vj00 | UMTS QoS Framework | Rel-19 |
| TS 23.146 vj00 | 3G Facsimile Group 3 Technical Realization | Rel-19 |
| TS 23.207 vj00 | End-to-End QoS Framework for GPRS | Rel-19 |
| TS 23.221 vj00 | 3GPP System Architectural Requirements | Rel-19 |
| TR 23.910 v1400 | UMTS Circuit Switched Bearer Services Overview | Rel-5 |
| TR 23.979 vj00 | PoC over 3GPP Systems Architectural Requirements | Rel-19 |
| TS 25.201 vj00 | UTRA Physical Layer General Description | Rel-19 |
| TS 25.221 vj00 | UTRA TDD Physical Layer Specification | Rel-19 |
| TS 25.222 vj00 | UTRA TDD Multiplexing & Channel Coding | Rel-19 |
| TS 25.301 vj00 | UE-UTRAN Radio Interface Protocol Architecture | Rel-19 |
| TS 25.302 vj00 | UTRA Physical Layer Services | Rel-19 |
| TS 25.321 vj00 | MAC Protocol Specification for UTRAN | Rel-19 |
| TS 25.322 vj00 | RLC Protocol Specification | Rel-19 |
| TS 25.323 vj00 | Packet Data Convergence Protocol (PDCP) Specification | Rel-19 |
| TS 25.324 vj00 | Broadcast/Multicast Control Protocol | Rel-19 |
| TS 25.331 vj00 | UTRAN RRC Protocol Specification | Rel-19 |
| TS 25.423 vj00 | UTRAN RNSAP Specification | Rel-19 |
| TR 25.912 vj00 | Evolved UTRA and UTRAN Technical Report | Rel-19 |
| TR 25.931 vj00 | UTRAN Signalling Procedures Examples | Rel-19 |
| TS 26.804 vj10 | 5G Media Streaming Extensions Study | Rel-19 |
| TS 26.822 vj20 | 5G RTP Configurations Study Phase 2 | Rel-19 |
| TR 26.902 vj00 | Video Codec Performance for 3GPP Packet Services | Rel-19 |
| TR 26.926 vj00 | Traffic Models & Quality Evaluation for Media/XR in 5G | Rel-19 |
| TR 26.935 vj00 | Speech Codec Performance for Packet Switched Multimedia | Rel-19 |
| TR 26.937 vj00 | 3GPP PSS Characterization | Rel-19 |
| TS 29.163 vj00 | Interworking between 3GPP IM CN and CS networks | Rel-19 |
| TS 29.235 vj00 | SIP-I CS Core Network Interworking | Rel-19 |
| TS 33.105 vj00 | 3G Security: Cryptographic Algorithm Requirements | Rel-19 |
| TS 36.201 vj00 | LTE Physical Layer General Description | Rel-19 |
| TS 36.300 vj00 | E-UTRAN Radio Interface Protocol Architecture Overview | Rel-19 |
| TS 36.302 vj00 | E-UTRA Physical Layer Services | Rel-19 |
| TS 36.306 vj00 | E-UTRA UE Radio Access Capability Parameters | Rel-19 |
| TS 36.322 vj00 | E-UTRA Radio Link Control Protocol Specification | Rel-19 |
| TS 36.323 vj00 | PDCP Protocol Specification | Rel-19 |
| TS 36.331 vj00 | LTE RRC Protocol Specification | Rel-19 |
| TS 36.938 v900 | E-UTRAN to 3GPP2/Mobile WiMAX Mobility | Rel-9 |
| TS 37.320 vj00 | Minimization of Drive Tests (MDT) Overview | Rel-19 |
| TR 37.901 vf10 | UE Application Layer Data Throughput Performance | Rel-15 |
| TS 38.201 vj00 | NR Physical Layer General Description | Rel-19 |
| TS 38.306 vj00 | NR UE Radio Access Capability Parameters | Rel-19 |
| TS 38.322 vj00 | NR Radio Link Control (RLC) Protocol | Rel-19 |
| TS 38.323 vj00 | Packet Data Convergence Protocol (PDCP) | Rel-19 |
| TS 38.331 vj00 | NR Radio Resource Control (RRC) Protocol Specification | Rel-19 |
| TS 38.470 vj10 | F1 Interface Introduction | Rel-19 |
| TS 43.051 vj00 | GERAN Stage 2 Service Description | Rel-19 |
| TS 43.064 vj00 | GPRS Radio Interface Lower-Layer Functions | Rel-19 |
| TS 43.129 vj00 | PS Handover in GERAN A/Gb and GAN Modes | Rel-19 |
| TS 43.318 vj00 | Generic Access Network (GAN) Stage 2 | Rel-19 |
| TR 43.902 vj00 | GAN Enhancements Feasibility Study | Rel-19 |
| TS 44.060 vj00 | GERAN RLC/MAC Protocol Specification | Rel-19 |
| TS 44.160 vg00 | GERAN Iu Mode RLC/MAC Protocol Specification | Rel-16 |
| TS 44.318 vj00 | Generic Access Network (GAN) Interface Procedures | Rel-19 |
| TR 45.902 vj00 | Flexible Layer One (FLO) for GERAN | Rel-19 |
| TS 48.016 vj00 | Gb Interface Network Service Specification | Rel-19 |
