5G NR RLC Layer | Functions, Modes, Data Structure, RRC Parameters

 

5G NR RLC Layer | Functions, Modes, Data Structure, RRC Parameters

 5G NR RLC layer including functions. It covers 5G NR RLC modes (TM mode, UM mode, AM mode), data structures (TMD, UMD, AMD), RLC PDUs (TMD PDU, UMD PDU, AMD PDU), data transfers (TM, UM and AM) and RRC parameters which defines RLC layer.

Introduction:
• RLC stands for Radio Link Control. 3GPP specifications TS 38.322 defines RLC protocol for UE and NR radio interface.
• As shown it lies between MAC on lower side and PDCP on higher side of the stack.
• Like previous cellular standards such as WCDMA and LTE, this standard (5G NR) also supports RLC modes viz. Transparent mode (TM mode), Unacknowledge Mode (UM mode) and Acknowledge mode (AM mode).

The figure-1 depicts 5G NR protocol stack showing position of RLC layer. As shown RLC layer provide services to the upper layers and it expects some services from the MAC Layer>> and PHY Layer>>.

The figure depicts data flow through various protocol layers of 5G NR stack.

RLC Modes | TM Mode, UM Mode, AM Mode

RLC configuration does not depend on 5G NR numerologies and it is associated with logical channels. TM mode is used for SRB0, paging and broadcast of system information. AM mode is used for SRBs. Either UM or AM mode is used for DRBs. ARQ procedure is supported within RLC sublayer.

Functions of RLC sublayer are as follows.
• Transfer of upper layer PDUs
• Sequence numbering independent of the one in PDCP (UM and AM)
• Error Correction through ARQ (AM only)
• Segmentation (AM and UM) and re-segmentation (AM only) of RLC SDUs
• Reassembly of SDU (AM and UM)
• Duplicate Detection (AM only)
• RLC SDU discard (AM and UM)
• RLC re-establishment
• Protocol error detection (AM only)

RLC layer expects following services from lower layer (i.e. MAC layer).
• Data transfer
• Notification of transmission opportunity.

TM Mode And TM Data Transfer Procedure

• A TM RLC entity uses logical channels viz. BCCH, DL CCCH, UL CCCH and PCCH to transmit or receive RLC PDUs.
• A TM RLC entity uses TMD PDU to transmit/receice data PDUs.
• During transmission, TMD PDUs are formed from RLC SDUs. It does not segment RLC SDUs and does not include any RLC headers in the TMD PDUs. During reception, TM RLC entity receives TMD PDUs and pass it to upper layers.

UM Mode And UM Data Transfer Procedure