5G NR RLC layer | functions, modes, data structure, RRC parameters

This page describes overview of 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

It uses logical channels viz. DL DTCH or UL DTCH. It uses UMD PDU which can carry one complete RLC SDU or one RLC SDU segment. Complete transmission and reception process is defined in 3GPP TS 38.322 document which is shown in the figure.

AM mode and AM data transfer procedure

AM RLC entity uses DL/UL DCCH or DL/UL DTCH logical channels. It transmits and receiver AMD PDUs which can carry either one complete RLC SDU or one RLC SDU segment. AM RLC entity transmits and receives STATUS PDU as control PDU which is mentioned below. Complete transmitting side and receiving side procedure is shown in the figure. The same has been described in detail in 38.322 document.

data structures | TMD, UMD, AMD

RLC PDU is a bit string. RLC SDUs are bit strings which are byte aligned in length. Following are structures of TMD, UMD and AMD.

TMD structure

UMD structure

AMD structure

5G NR RLC Status PDU SN 12 bit and SN 18 bit

Here SI (Segmentation Info) field is of 2 bits in length. It can be interpreted as follows.
00 : Data field contains all bytes of an RLC SDU
01 : Data field contains the first segment of an RLC SDU.
10 : Data field contains the last segment of an RLC SDU.
11 : Data field contains neither the first nor last segment of an RLC SDU.

➤SN refers to Sequence number field. It can be 12 bits or 18 bits for AMD PDU. It can be 6 bits or 12 bits for UMD PDU.
➤SO refers to Segment Offset which is about 16 bits in length.
➤D/C field indicates Data/Control Field. Value of "0" indicates it is control PDU where as value of "1" indicates it is data PDU.
➤P field indicates polling bit of length equals 1 bit. Value of "0" indicates "status report not requested" where as value of "1" indicates "status report is requested".
➤CPT field is 3 bits in size. value of "000" indicates it is "STATUS PDU" and value of "001" is reserved.

RRC parameters for RLC

Following RRC parameters are used to define RLC layer. The IE (Information Elements) are RLC-Bearerconfig IE and RLC config IE.

References

• 3GPP TS 38.322, V15.2.0 (2018-06), Radio Link Control (RLC) protocol specification (Release 15)
• 3GPP TS 38.331, V15.2.0 (2018-06), Radio Resource Control (RRC) protocol specification (Release 15)