1、Definitions, symbols and abbreviations

QoS flow to DRB mapping rule: a mapping rule determining on which DRB packets of a QoS flow shall be carried.

Reflective QoS flow to DRB mapping: a QoS flow to DRB mapping scheme where a UE monitors the QoS flow to DRB mapping rule in the DL, and applies it to in the UL.

QFI    QoS Flow ID

RDI    Reflective QoS flow to DRB mapping Indication

RQI    Reflective QoS Indication

SDAP    Service Data Adaptation Protocol

2、SDAP General

2.1 SDAP 架构

SDAP结构：

下行 1个多个QOS流映射到一个DRB

上行 1个Qos流映射到一个DRB

SDAP实体：

可以为一个UE定义多个SDAP实体，一个PDU session配置一个SDAP实体；

2.2 SDAP服务

SDAP对用户面上层提供的服务：

-    transfer of user plane data.

SDAP期望的下层服务：

-    user plane data transfer service;

-    in-order delivery except when out of order delivery is configured by RRC .

2.3 SDAP功能

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;

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

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