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 架構
SDAP結構：
下行 1個多個QOS流映射到一個DRB
上行 1個Qos流映射到一個DRB
SDAP實體：
可以為一個UE定義多個SDAP實體，一個PDU session配置一個SDAP實體；
2.2 SDAP服務
SDAP對用戶面上層提供的服務：
- transfer of user plane data.
SDAP期望的下層服務：
- 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.