5G SDAP PDU Types and Data PDU Formats

Quick facts

What This Page Covers

This reference page covers the SDAP wire-format layer: PDU categories, data PDU variants, format applicability by transport case, and the control PDU used for End-Marker signaling.

SDAP PDU Types

SDAP defines two broad PDU classes:

• SDAP Data PDU
• End-Marker Control PDU

The SDAP Data PDU itself appears in multiple structural variants depending on whether SDAP header presence is configured and whether the context is downlink, uplink, or unicast sidelink.

SDAP Data PDU Without SDAP Header

In the headerless format, the SDAP PDU consists only of a data field and does not include an SDAP header. This is the simplest SDAP wire format.

This format is used when header presence is not configured, and it is also the receive-side path used for MRB traffic because MRB does not use an SDAP header.

DL SDAP Data PDU With SDAP Header

The DL SDAP data PDU with header is the downlink format used when the DRB is configured with SDAP header presence. This format carries the data field together with the SDAP header fields needed for downlink QoS-aware handling.

This is the format associated with downlink reflective QoS-related handling, because header-bearing downlink SDAP data allows the UE to process QFI and reflective indications carried in the SDAP header.

UL SDAP Data PDU With SDAP Header

The UL SDAP data PDU with header is the uplink format used when the selected DRB is configured with SDAP header presence. It carries the SDAP header and the SDAP SDU data field.

This format is especially important for default DRB behavior because the SDAP procedures state that the default DRB is always configured with UL SDAP header.

SL SDAP Data PDU With Header for Unicast Sidelink

SDAP defines a dedicated data PDU format with header for unicast NR sidelink communication. This is the sidelink header-bearing counterpart to the Uu header-bearing formats.

The spec limits this to unicast: for groupcast and broadcast NR sidelink communication, only SDAP data PDU without SDAP header is supported.

End-Marker Control PDU

In addition to data PDUs, SDAP defines an End-Marker Control PDU. This is the control-format part of the wire-format layer and should be treated separately from ordinary data PDU formats.

On this page, the End-Marker is introduced at format level only. Its procedure-level role during flow remapping and bearer transition belongs on a later dedicated End-Marker page.

SDAP Format Comparison

Which Transport Cases Use Which SDAP Format

DRB over Uu

DRB-based Uu transport can use either headerless or header-bearing SDAP data PDU formats depending on RRC configuration of SDAP header presence.

MRB

MRB does not use an SDAP header, so the no-header SDAP data PDU path applies for MRB reception behavior.

SL-DRB over PC5

For NR sidelink, unicast may use the SL SDAP data PDU with header, while groupcast and broadcast support only the headerless data PDU format.

Field Families Used in SDAP Formats

This format page should mention the field families that appear in SDAP PDUs, but the exact semantics should be delegated to the dedicated header-fields page when that page is live.

• Data field
• D/C field
• QFI field
• RQI field
• RDI field
• PQFI-related sidelink field set

Reference Examples

Example 1: DRB without SDAP header

If a DRB is configured without SDAP header presence, SDAP sends and receives the SDAP SDU using the data PDU without SDAP header.

Example 2: DL DRB with SDAP header

If a downlink DRB has SDAP header configured, the UE receives a DL SDAP data PDU with header and can process QFI and reflective indications from that header.

Example 3: Sidelink groupcast

For NR sidelink groupcast, the header-bearing SL SDAP PDU format is not used. Only the headerless SDAP data PDU format is supported.

Key Specification References

Next SDAP Reference Pages