5G Sidelink SDAP Data PDU Format and PQFI

Quick facts

What This Page Covers

This reference page covers the sidelink SDAP wire-format layer: the SL SDAP data PDU with header for unicast, the role of PQFI, and the restriction that groupcast and broadcast use only the headerless SDAP data PDU format.

Where the Sidelink SDAP Format Fits

SDAP has a dedicated data PDU format with header for unicast NR sidelink communication. This is separate from the downlink and uplink Uu SDAP formats and should be treated as its own sidelink branch in the SDAP format tree.

The sidelink format exists because PC5 QoS flow handling needs a sidelink flow identifier context and a PDU structure tied to SL-DRB operation rather than DRB operation.

The most direct signaling bridge is RRCReconfiguration, because that is where SL-SDAP-Config-r16 typically turns header presence and cast-type rules into the format path later used by the sidelink bearer.

SL SDAP Data PDU With Header for Unicast

The header-bearing sidelink SDAP format is defined specifically for unicast NR sidelink communication. This is the only sidelink case where SDAP defines an SDAP data PDU with header.

The cleanest visual model is a simple split between a compact sidelink SDAP header and the payload data field, with the header carrying the PC5 QoS flow identification context.

Groupcast and Broadcast Use Only Headerless SDAP

For groupcast and broadcast NR sidelink communication, only SDAP Data PDU without SDAP header is supported.

This is one of the most important sidelink format rules on the page because it prevents the unicast format from being over-applied across all sidelink cast types.

The broader sidelink context is easiest to anchor through NR system information for broadcast support and 5G Sidelink UE Information NR for UE-side sidelink context around those cast-specific behaviors.

What PQFI Means

PQFI is the PC5 QoS Flow Identifier. It is the sidelink flow identifier used in the SDAP context for PC5 QoS flow handling, parallel to the role that QFI plays in Uu SDAP.

On this page, PQFI is best understood as the field that anchors the sidelink header to a specific PC5 QoS flow whenever the header-bearing unicast format is used.

PQFI vs QFI

The identifiers play parallel roles, but they belong to different SDAP transport contexts, so a sidelink trace should not be interpreted using the Uu identifier model by default.

Relationship to SL-SDAP-Config-r16

The wire format used on sidelink depends on the RRC configuration in SL-SDAP-Config-r16, especially sl-SDAP-Header-r16 and sl-CastType-r16. Header presence decides whether the header-bearing sidelink PDU is used, while cast type decides whether that header-bearing option is applicable at all.

In practice, this means RRCReconfiguration is the key message to line up against the wire format you later see on the bearer, because it is the point where those config fields are delivered to the UE.

Sidelink Header Structure Guidance

The unicast sidelink SDAP header should be shown as its own mini-diagram rather than merged with the downlink or uplink Uu SDAP header diagrams. The main visual split is:

• sidelink SDAP header
• PQFI-oriented flow identification
• payload data field

That separation makes it much easier to connect the wire format back to SL-SDAP-Config-r16 without importing Uu-specific header behavior that does not belong in the PC5 branch.

Sidelink Format Applicability Matrix

The key format rule is simple: groupcast and broadcast do not use the header-bearing sidelink SDAP data PDU.

How the Sidelink SDAP PDU Is Used

On the transmitting side, if the selected SL-DRB is configured with SDAP header presence, the UE constructs the SL SDAP Data PDU with header; otherwise it constructs the SDAP data PDU without header and submits it to lower layers.

On the receiving side, the UE retrieves the SDAP SDU according to whether the received sidelink PDU uses the header-bearing or headerless format. The clearest way to understand why one format was chosen is to place the PDU side by side with the relevant RRCReconfiguration and the matching SL-SDAP-Config-r16 values.

Recommended Diagram Blocks

• SL SDAP Data PDU with header for unicast: [ SL SDAP Header | Data ]
• Headerless sidelink SDAP data PDU: [ Data ]
• PQFI callout inside the sidelink header
• Applicability callout showing that groupcast and broadcast use the headerless format only

Reference Examples

Example 1: Unicast sidelink with header

A unicast sidelink DRB is configured with sl-SDAP-Header-r16 present. The UE constructs an SL SDAP Data PDU with header, and that header carries the PC5 QoS flow identification context.

Example 2: Groupcast sidelink

A groupcast sidelink transmission does not use the SL SDAP Data PDU with header. It is limited to the SDAP data PDU without SDAP header.

Example 3: PQFI-oriented interpretation

A sidelink trace shows a PC5 QoS flow identifier in the sidelink SDAP context. That should be interpreted as PQFI-oriented sidelink flow identification, not as the normal Uu QFI field model.

Key Specification References

• 3GPP TS 37.324 - primary SDAP specification for sidelink data PDU format, PQFI-related context, and cast-type format restrictions
• 3GPP TS 38.331 - RRC configuration context for sidelink SDAP header presence and cast type through SL-SDAP-Config-r16

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