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5G SDAP Protocol: Release 18 Reference Guide

The Service Data Adaptation Protocol (SDAP) is the 5G user-plane sublayer responsible for QoS flow mapping, reflective QoS behavior, and SDAP header processing between upper layers and PDCP.

This page is the main SDAP reference hub for Release 18. It explains what SDAP is, where it sits in the protocol stack, what functions it covers, how QoS flow to DRB mapping works at a high level, how RRC configures SDAP behavior, and which adjacent protocol pages to open next.

Quick facts

Technology 5G NR
Area / Protocol SDAP (Service Data Adaptation Protocol)
Main spec 3GPP TS 37.324 Release 18
Stack position Between upper layers and PDCP in the NR user-plane stack
Main use QoS flow to DRB mapping, reflective QoS support, SDAP header handling, and control behavior such as End-Marker
Companion specs TS 38.300 for architecture and TS 38.331 for RRC SDAP configuration behavior

SDAP Topics

Overview, Scope, Terminology, and References | Architecture, Services, and Functions | Entity Handling and Data Transfer | QoS Flow to DRB Mapping and Default DRB | Reflective QoS | PDU Types and Data PDU Formats | SDAP Header Fields | End-Marker Control PDU and Flow Remapping | RRC SDAP-Config | MBS QoS Flow to MRB | Sidelink Overview and SL-DRB Mapping | SL-SDAP-Config and Cast Types | SL Data PDU Format and PQFI

Contents

  1. What is SDAP in 5G?
  2. SDAP Position in the 5G User Plane Stack
  3. Core SDAP Functions
  4. SDAP Quick Reference
  5. SDAP Reference Topics
  6. QoS Flow to DRB Mapping and Default DRB
  7. Reflective QoS, RQI, RDI, and RQ Timer
  8. SDAP PDU Types and Data PDU Formats
  9. SDAP Header Fields, QFI, RQI, RDI, and Bit Layout
  10. End-Marker Control PDU and Flow Remapping
  11. SDAP Procedures and Configuration Map
  12. RRC SDAP-Config, Default DRB, and QFI Mapping
  13. MBS QoS Flow to MRB
  14. Sidelink Overview and SL-DRB Mapping
  15. SL-SDAP-Config and Cast Types
  16. SL Data PDU Format and PQFI
  17. Related 5G Protocols
  18. References
  19. FAQ
  20. Related pages

What is SDAP in 5G?

SDAP is the user-plane protocol sublayer above PDCP that identifies or applies QoS flow treatment for packets and maps QoS flows to data radio bearers. In practical 5G reading, SDAP is the point where QoS intent from the core becomes bearer-facing user-plane behavior in the RAN.

That makes SDAP especially important when the question is not just “which bearer carries the data,” but “which QoS flow does this packet belong to, which DRB should carry it, and how does the UE learn or reflect that treatment?”

SDAP Position in the 5G User Plane Stack

SDAP sits above PDCP and below upper-layer packet handling. It is the layer that associates packets with QoS flow treatment before the packet moves deeper into the radio stack.

Applications / IP / Ethernet SDAP PDCP RLC / MAC / PHY QoS flow handling and bearer mapping Header compression, security, sequencing

Core SDAP Functions

  • transfer of user-plane data between upper layers and PDCP
  • mapping between QoS flows and DRBs in uplink and downlink
  • default DRB behavior when mapping rules require it
  • reflective QoS support through SDAP indications
  • SDAP header handling for selected packet flows
  • control support such as End-Marker behavior during remapping events

Those are the core anchors to keep in mind before diving into field-level SDAP details, because most SDAP debugging questions reduce to mapping, header presence, or configuration behavior.

SDAP Quick Reference

Topic What it covers
QFI QoS Flow Identifier used to associate packets with a QoS flow.
DRB mapping How SDAP maps QoS flows to one or more data radio bearers.
Default DRB Fallback bearer behavior used when explicit mapping is not present or when configuration relies on default treatment.
RQI Reflective QoS indication used by the UE to apply reflective treatment rules.
RDI Reflective mapping related indication visible in SDAP header behavior.
SDAP header Header carrying QoS-related fields such as QFI and reflective indicators for selected packet handling cases.
End-Marker Control PDU used in flow or bearer remapping situations.

SDAP Reference Topics

These cards organize the main SDAP reading paths on the hub and will map directly to the dedicated SDAP child pages published alongside this main reference page.

PDU types and data PDU formats

Open this for SDAP data and control PDU families, header-bearing formats, headerless formats, and transport applicability.

MBS QoS flow to MRB

Use this for multicast or broadcast SDAP behavior, MRB mapping, and how it differs from DRB-based transport.

QoS Flow to DRB Mapping and Default DRB

The most important operational job of SDAP is mapping QoS flows to the correct data radio bearers. That mapping is what turns QoS policy and session context from the core into bearer-level handling on the radio side.

  • one DRB can carry one or more QoS flows depending on configuration
  • a default DRB can be used when packets need fallback bearer treatment
  • mapping behavior matters in both uplink and downlink reading
  • if the mapping looks wrong, the real issue may be QoS context, RRC configuration, or bearer setup rather than pure SDAP header parsing

When the bearer side is unclear, pair this section with Radio Bearers in 5G and 5G PDU Session Establishment.

Reflective QoS, RQI, and RDI

Reflective QoS is one of the reasons SDAP exists as a distinct user-plane layer. The SDAP header can carry indications that allow the UE to apply reflective treatment logic without treating every flow as a static bearer-only decision.

  • RQI is the reflective QoS indication used by the UE in reflective behavior
  • RDI is a related SDAP header indicator used in mapping-related handling
  • reflective behavior needs to be read together with configuration and flow context, not as a standalone bit field

In practice, if the question is “why did the UE apply this QoS treatment,” reflective QoS is one of the first branches to check before assuming PDCP or RLC is the source of the behavior.

SDAP PDU Types and Header Fields

At hub level, the important distinction is between data transfer behavior and control behavior. For data transfer, the main reading questions are whether the SDAP header is present, which QFI is signaled, and whether reflective indicators such as RQI or RDI are relevant.

  • data PDU handling is where QFI and SDAP header presence matter most
  • header-present versus header-absent behavior should be read together with bearer configuration
  • control PDU behavior matters mainly around remapping and transition handling

End-Marker Control PDU and Flow Remapping

End-Marker handling belongs to the control side of SDAP rather than the steady-state user-plane data path. It becomes relevant when flows are being remapped or when bearer transitions must be handled cleanly.

That means an End-Marker question is usually not just “what is this PDU,” but “what mapping or bearer change just happened, and which part of the configuration or transition logic caused it?”

SDAP Procedures and Configuration Map

SDAP behavior is split across packet handling, mapping logic, header behavior, and RRC configuration. That is why the protocol makes the most sense when read together with companion references instead of in isolation.

Specification or layer Why it matters for SDAP
TS 37.324 Main SDAP specification covering services, procedures, and PDU behavior.
TS 38.300 NR architecture context around bearer model and QoS handling.
TS 38.331 RRC signaling and SDAP-Config details, including mapped QoS flow context.

RRC Configuration Overview for SDAP

In NR, SDAP behavior is configured through RRC, especially through SDAP-related configuration associated with PDU session, DRB, and mapped QoS flow context. This is where default DRB behavior, SDAP header expectations, and flow mapping assumptions become operationally real.

  • SDAP-Config ties bearer setup to QoS flow treatment
  • default DRB behavior must be read together with the RRC bearer view
  • sidelink uses separate SDAP-related configuration logic

When the field-level SDAP behavior looks wrong, check whether the real issue is in RRC configuration or bearer context before blaming the SDAP layer itself.

Sidelink, MBS, and Advanced SDAP Topics

Release 18 SDAP coverage is not limited to baseline Uu unicast behavior. Advanced SDAP reading also includes sidelink SDAP handling, cast-specific configuration, and MBS-related flow-to-bearer mapping.

  • sidelink SDAP introduces separate mapping and configuration questions around SL-DRB treatment
  • MBS-related behavior adds multicast and broadcast flow-to-bearer context beyond ordinary unicast DRB mapping
  • these advanced branches matter when the standard user-plane picture no longer explains the trace

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

FAQ

What is the difference between SDAP and PDCP?

SDAP handles QoS flow mapping and SDAP header behavior, while PDCP handles sequencing, security, compression, and several other user-plane functions below SDAP.

Why does SDAP matter for QoS troubleshooting?

Because SDAP is the layer where QoS flow treatment is associated with bearer handling. If packets are being carried on the wrong bearer or reflective treatment looks wrong, SDAP is one of the first protocol layers to inspect.

Is SDAP configured by RRC?

Yes. RRC signaling provides the configuration context that makes SDAP mapping and header behavior operational in the UE and RAN.

Does SDAP matter only for unicast traffic?

No. Release 18 coverage also extends into advanced areas such as sidelink and MBS-related SDAP behavior.