Radio Link Control Explained

Radio Link Control (RLC) is a layer 2 Radio Link Protocol used in UMTS, LTE and 5G on the Air interface. This protocol is specified by 3GPP in TS 25.322^[1] for UMTS, TS 36.322^[2] for LTE and TS 38.322^[3] for 5G New Radio (NR).RLC is located on top of the 3GPP MAC-layer and below the PDCP-layer. The main tasks of the RLC protocol are:

• Transfer of upper layer Protocol Data Units (PDUs) in one of three modes: Acknowledged Mode (AM), Unacknowledged Mode (UM) and Transparent Mode (TM)
• Error correction through ARQ (only for AM data transfer)
• Concatenation, segmentation and reassembly of RLC SDUs (UM and AM)
• Re-segmentation of RLC data PDUs (AM)
• Reordering of RLC data PDUs (UM and AM);
• Duplicate detection (UM and AM);
• RLC SDU discard (UM and AM)
• RLC re-establishment
• Protocol error detection and recovery

RLC features specific to LTE only - 1) Re-segmentation.2) RLC SDU discard is notified by upper layer.

Notes and References

1. https://portal.3gpp.org/desktopmodules/Specifications/SpecificationDetails.aspx?specificationId=1176 3GPP TS 25.322 Radio Link Control (RLC) protocol specification
2. http://www.3gpp.org/ftp/Specs/html-info/36322.htm 3GPP TS 36.322 Evolved Universal Terrestrial Radio Access (E-UTRA); Radio Link Control (RLC) protocol specification
3. http://www.3gpp.org/ftp/Specs/html-info/38322.htm 3GPP TS 38.322 NR; Radio Link Control (RLC) protocol specification