4 General

37.3243GPPEvolved Universal Terrestrial Radio Access (E-UTRA) and NRRelease 17Service Data Adaptation Protocol (SDAP) specificationTS

4.1 Introduction

The objective is to describe the SDAP architecture and the SDAP entity from a functional point of view. The specified functionality only applies to UE with connection to the 5G-CN and UE in NR sidelink communication.

4.2 SDAP architecture

4.2.1 SDAP structure

Figure 4.2.1-1 illustrates one possible structure for the SDAP sublayer; it should not restrict implementation. The figure is based on the radio interface protocol architecture defined in TS 38.300 [2].

Figure 4.2.1-1: SDAP sublayer, structure view

The SDAP sublayer is configured for DRBs by RRC (TS 38.331 [3]). The SDAP sublayer maps QoS flows to DRBs. One or more QoS flows may be mapped onto one DRB. One QoS flow is mapped onto only one DRB at a time in the UL.

The SDAP sublayer is configured for MRBs by RRC (TS 38.331 [3]). The SDAP sublayer maps MBS QoS flows to MRBs. One or more MBS QoS flows may be mapped onto one MRB.

In NR sidelink communication, the SDAP sublayer maps PC5 QoS flows to SL-DRBs. One or more PC5 QoS flows may be mapped onto one SL-DRB. One PC5 QoS flow is mapped onto only one SL-DRB at a time in the NR sidelink for transmission.

4.2.2 SDAP entities

The SDAP entities are located in the SDAP sublayer. Several SDAP entities may be defined for a UE. There is an SDAP entity configured for each individual PDU session or MBS session for NR Uu. For NR sidelink, SDAP entity is configured per Destination Layer-2 ID and cast type in the UE.

An SDAP entity receives/delivers SDAP SDUs from/to upper layers and submits/receives SDAP data PDUs to/from its peer SDAP entity via lower layers.

– At the transmitting side, when an SDAP entity receives an SDAP SDU from upper layers, it constructs the corresponding SDAP data PDU and submits it to lower layers;

– At the receiving side, when an SDAP entity receives an SDAP data PDU from lower layers, it retrieves the corresponding SDAP SDU and delivers it to upper layers.

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

Figure 4.2.2-1: SDAP layer, functional view

Reflective QoS flow to DRB mapping is performed at UE, as specified in the clause 5.3.2, if DL SDAP header is configured.

Reflective QoS flow to MRB mapping is not supported. There is no SDAP header for MRB.

For NR sidelink communication, reflective PC5 QoS flow to SL-DRB mapping is not supported.

4.3 Services

4.3.1 Services provided to upper layers

The SDAP sublayer provides its service to the user plane upper layers. The following services are provided by SDAP to upper layers:

– transfer of user plane data.

4.3.2 Services expected from lower layers

An SDAP entity expects the following services from lower layers:

– user plane data transfer service;

– in-order delivery except when out of order delivery is configured by RRC (TS 38.331 [3]).

4.4 Functions

The SDAP sublayer supports the following functions:

– transfer of user plane data;

– mapping between a QoS flow and a DRB for both DL and UL;

– mapping between an MBS QoS flow and an MRB for DL;

– mapping between a PC5 QoS flow and a SL-DRB for NR sidelink communication;

– marking QoS flow ID in both DL and UL packets;

– marking PC5 QoS flow ID in unicast of NR sidelink communication packets;

– reflective QoS flow to DRB mapping for the UL SDAP data PDUs.