Radio Link Control

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Radio link control (RLC) is a layer 2 protocol used in UMTS and LTE 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.

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