4 General

38.3233GPPNRPacket Data Convergence Protocol (PDCP) specificationRelease 17TS

4.1 Introduction

The present document describes the functionality of the PDCP.

4.2 Architecture

4.2.1 PDCP structure

Figure 4.2.1-1 represents one possible structure for the PDCP sublayer, and Figure 4.2.1-2 represents one possible structure for the PDCP sublayer used in L2 U2N relay case; they should not restrict implementation. The figures are based on the radio interface protocol architecture defined in TS 38.300 [2].

Figure 4.2.1-1: PDCP layer, structure view (normal)

Figure 4.2.1-2: PDCP layer, structure view (L2 U2N relay)

The PDCP sublayer is configured by upper layers TS 38.331 [3]. The PDCP sublayer is used for RBs mapped on DCCH, DTCH, MTCH, SCCH, and STCH type of logical channels. The PDCP sublayer is not used for any other type of logical channels.

Each RB (except for SRB0 for Uu interface) is associated with one PDCP entity. Each PDCP entity is associated with one, two, three, four, six, or eight RLC entities depending on the RB characteristic (e.g. uni-directional/bi-directional or split/non-split) or RLC mode:

– For split bearers, each PDCP entity is associated with two UM RLC entities (for same direction), four UM RLC entities (two for each direction), or two AM RLC entities;

– For RBs configured with PDCP duplication, each PDCP entity is associated with N UM RLC entities (for same direction), 2 × N UM RLC entities (N for each direction), or N AM RLC entities, where 2 <= N <= 4;

– For DAPS bearers, each PDCP entity is associated with two UM RLC entities (for same direction, one for source and one for target cell), four UM RLC entities (two for each direction on source cell and target cell), or two AM RLC entities (one for source cell and one for target cell);

– For UM MRBs, each PDCP entity is associated with one UM RLC entity (for MTCH or for downlink DTCH), two UM RLC entities (one for MTCH and one for downlink DTCH, or one for downlink DTCH and one for uplink DTCH), or three UM RLC entities (one for MTCH, one for downlink DTCH, and one for uplink DTCH);

– For AM MRBs, each PDCP entity is associated with one AM RLC entity (for downlink DTCH and uplink DTCH), or one UM RLC entity (for MTCH) and one AM RLC entity (for downlink DTCH and uplink DTCH);

– Otherwise, each PDCP entity is associated with one UM RLC entity, two UM RLC entities (one for each direction), or one AM RLC entity.

For the case of L2 U2N relay, all PDCP entities are associated with one SRAP entity.

4.2.2 PDCP entities

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.

Figure 4.2.2-1 represents the functional view of the PDCP entity for the PDCP sublayer; it should not restrict implementation. The figure is based on the radio interface protocol architecture defined in TS 38.300 [2].

For split bearers and DAPS bearers, routing is performed in the transmitting PDCP entity.

A PDCP entity associated with DRB can be configured by upper layers TS 38.331 [3] to use header compression or uplink data compression (UDC). A PDCP entity associated with MRB can be configured by upper layers TS 38.331 [3] to use header compression. In this version of the specification, the robust header compression protocol (ROHC), the Ethernet header compression protocol (EHC) and UDC are supported. Each header compression protocol is independently configured for a DRB/MRB.

Figure 4.2.2-1: PDCP layer, functional view

Figure 4.2.2-2 represents the functional view of the PDCP entity associated with the DAPS bearer for the PDCP sublayer; it should not restrict implementation. The figure is based on the radio interface protocol architecture defined in TS 38.300 [2].

For DAPS bearers, the PDCP entity is configured with two sets of security functions and keys and two sets of header compression protocols.

Figure 4.2.2-2: PDCP layer associated with DAPS bearer, functional view

4.3 Services

4.3.1 Services provided to upper layers

The PDCP layer provides its services to the RRC or SDAP layers. The following services are provided by PDCP to upper layers:

– transfer of user plane data;

– transfer of control plane data;

– header compression;

– uplink data compression;

– ciphering;

– integrity protection.

The maximum supported size of a PDCP SDU is 9000 bytes. The maximum supported size of a PDCP Control PDU is 9000 bytes.

4.3.2 Services expected from lower layers

A PDCP entity expects the following services from lower layers per RLC entity (for a detailed description see TS 38.322 [5]):

– acknowledged data transfer service, including indication of successful delivery of PDCP PDUs;

– unacknowledged data transfer service.

4.4 Functions

The PDCP layer supports the following functions:

– transfer of data (user plane or control plane);

– maintenance of PDCP SNs;

– header compression and decompression using the ROHC protocol;

– header compression and decompression using the EHC protocol;

– uplink data compression and decompression using the UDC protocol;

– ciphering and deciphering;

– integrity protection and integrity verification;

– timer based SDU discard;

– for split bearers and DAPS bearer, routing;

– duplication;

– reordering and in-order delivery;

– out-of-order delivery;

– duplicate discarding.