5G SDAP Overview: Scope, Terminology, and References
This reference page introduces the Service Data Adaptation Protocol (SDAP) by covering its scope, companion references, abbreviations, and key terminology. It is the starting point for understanding the SDAP vocabulary and specification context before moving into procedures, headers, QoS flow mapping, RRC configuration, and sidelink-specific handling.
Quick facts
| Technology | 5G NR |
|---|---|
| Area / Protocol | SDAP overview and terminology |
| Primary baseline | 3GPP TS 37.324 Release 18 |
| Main focus | Scope, references, abbreviations, and definitions |
| Key terms | QFI, DRB, default DRB, reflective QoS, RQI, RDI, PQFI, SL-DRB, and MBS |
| Best companion context | SDAP hub, PDCP, RRC, and radio bearers |
What This Page Covers
This reference page focuses on the front section of the SDAP specification: the scope of the protocol, the main companion specifications, the core abbreviations, and the definitions needed to read the rest of the SDAP specification correctly.
This page is intentionally limited to the overview and terminology layer of SDAP. Detailed behavior such as QoS flow mapping, SDAP headers, control PDUs, RRC configuration, MBS handling, and sidelink data formats is better handled on dedicated SDAP child pages.
SDAP Scope in the Specification
The SDAP specification defines the Service Data Adaptation Protocol for E-UTRA and NR, including the protocol services, functions, procedures, and PDU formats used for SDAP operation. In 5G usage, SDAP is the user-plane layer that supports QoS-aware packet treatment between upper layers and PDCP.
At a high level, SDAP covers user-plane data transfer, QoS flow mapping, marking and interpretation of QoS-related identifiers, reflective QoS support, and associated behavior for standard radio bearer transport as well as newer areas such as sidelink and advanced bearer cases.
Where SDAP Fits in the 5G Protocol Stack
In the NR user plane, SDAP sits above PDCP and below upper-layer packet handling. Its role is to associate packets with QoS treatment and bearer mapping before lower-layer transport processing continues through PDCP, RLC, MAC, and PHY.
Key SDAP References
SDAP should not be read in isolation. TS 37.324 is the main SDAP definition, but practical SDAP interpretation also depends on overall NR architecture and RRC configuration specifications.
| Specification | Why it matters |
|---|---|
| 3GPP TS 37.324 | Primary SDAP specification covering services, procedures, and PDU formats. |
| 3GPP TS 38.300 | NR overall architecture and protocol context. |
| 3GPP TS 38.331 | NR RRC signaling and SDAP-Config or SL-SDAP-Config structures. |
Additional companion specifications become more relevant on deeper pages, especially when reflective QoS, bearer setup, or 5G Core QoS context is part of the question.
Common SDAP Abbreviations
| Abbreviation | Meaning |
|---|---|
| SDAP | Service Data Adaptation Protocol |
| QFI | QoS Flow Identifier |
| DRB | Data Radio Bearer |
| RQI | Reflective QoS Indication |
| RDI | Reflective QoS Flow to DRB Mapping Indication |
| PQFI | PC5 QoS Flow Identifier |
| SL-DRB | Sidelink Data Radio Bearer |
| MRB | Multicast Radio Bearer |
| MBS | Multicast/Broadcast Service |
Key SDAP Terms and Definitions
QoS Flow to DRB Mapping Rule
A QoS flow to DRB mapping rule determines how packets belonging to a given QoS flow are associated with a specific data radio bearer for transport.
Reflective QoS Flow to DRB Mapping
Reflective QoS flow to DRB mapping is the UE-side behavior where mapping information is derived from downlink treatment indications rather than only from explicit configuration.
QoS Flow Identifier (QFI)
The QFI identifies the QoS flow associated with a packet or SDAP PDU and is one of the central
fields in SDAP header interpretation and mapping behavior.
PC5 QoS Flow Identifier (PQFI)
The PQFI is the sidelink counterpart used for applicable NR sidelink SDAP handling, especially
where SDAP headers are present for supported cast modes.
Default DRB
The default DRB is the bearer used for SDAP handling when a packet does not yet have an explicit stored QoS flow to DRB mapping rule and default bearer treatment is configured.
Reflective QoS Indication (RQI)
RQI is an SDAP-related indication used to support reflective QoS behavior at the receiving UE.
Reflective QoS Flow to DRB Mapping Indication (RDI)
RDI is used in SDAP header-related behavior to indicate reflective mapping updates at the UE side.
How the SDAP Terminology Fits Together
In practical SDAP operation, packets are associated with QoS flows, those flows are mapped to radio bearers,
and SDAP headers may carry identifiers or indications such as QFI, RQI, and
RDI. For sidelink, similar concepts extend to PC5 flows and SL-DRB handling,
including PQFI where applicable.
- QoS flow identifies the service-treatment context
QFIidentifies that QoS flow in SDAP-related handling- mapping rules connect the QoS flow to a
DRB - default
DRBprovides fallback transport behavior RQIandRDIsupport reflective QoS behaviorPQFIextends related concepts into supported sidelink cases
Think of one downlink packet for a voice service. The packet belongs to a QoS flow, the SDAP header can carry
the QFI for that flow, the SDAP mapping rule decides which DRB should carry it, and
reflective information such as RQI or RDI helps the UE keep the right treatment in
sync with what it sees from the network.
Next SDAP Reference Pages
Related Content
Specification References
- 3GPP TS 37.324: Service Data Adaptation Protocol (SDAP)
- 3GPP TS 38.300: NR overall description and architecture
- 3GPP TS 38.331: NR Radio Resource Control (RRC) protocol specification