5G NR PDCP layer | functions, architecture, procedures, PDU formats

This page describes overview of 5G NR PDCP layer including functions. It covers PDCP architecture (structure, entities), PDCP procedures for data transfer during transmit/receive operation, Data PDU and Control PDU formats of PDCP layer etc.

Introduction:
• PDCP stands for Packet Data Convergence Protocol. 3GPP specifications TS 38.323 defines PDCP protocol.
• As shown it lies between RLC on lower side and RRC on higher side of the control protocol stack.
• In the data user plane it lies on the top as shown.

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

The figure depicts data flow through various protocol layers of 5G NR stack. PDCP provides following services to the upper layers.
• Transfer of user plane data and control plane data
• Header compression/decompression using ROHC
• Ciphering/Deciphering
• Integrity protection

PDCP expects following services supported from lower layers.
• Acknowledged data transfer service
• Unacknowledges data transfer service

5G NR PDCP layer functions

Functions of PDCP layer are as follows.
• transfer of data (user plane or control plane)
• maintenance of PDCP SNs
• header compression and decompression using ROHC protocol
• ciphering and deciphering
• integrity protection and integrity verification
• timer based SDU discard
• for split bearers, routing is performed
• Activation/Deactivation of PDCP duplication
• reordering and in-order delivery
• out-of-order delivery
• duplicate discarding

PDCP architecture (structure, entities)

• The architecture is based on radio interface protocol.
• PDCP sublayer is configured by RRC.
• It is used for RBs mapped on logical channels which include DCCH and DTCH.
• Each RB is associated with 1 PDCP entity. Each PDCP entity is associated with 1/2/4 RLC entities which depends on RB characteristics or RLC modes. RB characteristics are uni-directional / bi-directional or split/non-split.
• For non-split bearers , each PDCP entity is associated with 1 UM RLC entity/2 UM RLC entities/1 AM RLC entity.
• For split bearers, each PDCP entity is associated with 2 UM RLC entities/4 UM RLC entities/2 AM RLC entities(same direction).

PDCP entity:
• The PDCP entities are located in the PDCP sublayer. Several PDCP entities may be defined for a UE. Each PDCP entity is carrying the data of one radio bearer.
• A PDCP entity is associated either to the control plane or the user plane depending on which radio bearer it is carrying data for.

• The figure depicts functional view of PDCP entity used for the PDCP sublayer.
• The data can be either uncompressed PDCP SDU or compressed PDCP SDU. Uncompressed data is associated with user plane or control plane. Compressed data is associated with user plane only.
• As per Plane, PDU can be of two types viz. control PDU or data PDU.
• Control PDU types include either PDCP status report or interspersed ROHC feedback.

PDCP Procedures for data transfer

• There are three main PDCP entity handling procedures viz. PDCP entity establishment, re-establishment and release.
• After establishment, PDCP procedures are associated with either transmitting operation or receiving operation.
• As shown during transmit operation, PDCP entity receives SDU from upper layer. On this received SDU various operations are performed before it is given to lower layers. It is later passed to radio interface (Uu).
• When UE transmits NG-RAN receives and when NG-RAN transmits UE receives.
• Similar functionalities are performed when data PDU is received from lower layers.
• PDCP SDU size and PDCP control PDU size are both 9000 bytes (maximum).
• The length of PDCP SN is either 12 bits or 18 bits. It is configured by upper layers.

PDCP layer data PDU and control PDU formats

• A PDCP PDU is a bit string that is byte aligned (i.e. multiple of 8 bits) in length.
• PDCP SDUs are bit strings that are byte aligned (i.e. multiple of 8 bits) in length.
• A compressed or uncompressed SDU is included into a PDCP Data PDU from the first bit onward.

Following figure depicts PDCP Data PDU format with 12 bits PDCP SN. This format is applicable for SRBs.

Following figure depicts PDCP Data PDU format with 12 bits PDCP SN. This format is applicable for UM DRBs and AM DRBs.

Following figure depicts PDCP Data PDU format with 18 bits PDCP SN. This format is applicable for UM DRBs and AM DRBs.

Following figure depicts PDCP Control PDU format which carries one PDCP status report. This format is applicable for AM DRBs.

PDCP Control PDU format carrying one interspersed ROHC feedback is applicable for UM DRBs and AM DRBs.

References

• 3GPP TS 38.323, Packet Data Convergence Protocol (PDCP) specification (Release 15)

5G NR Numerology | 5G NR Terminology

5G NR Control channels | 5G NR Traffic Channels | 5G NR Reference Signals and sequences

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