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.