5G SDAP MBS QoS Flow to MRB Mapping
SDAP supports mapping between an MBS QoS flow and an MRB for downlink delivery. This is the multicast or broadcast branch of the SDAP mapping model, but it is intentionally simpler than DRB-based SDAP because MRB has no SDAP header and reflective QoS flow to MRB mapping is not supported.
Quick facts
| Technology | 5G NR |
|---|---|
| Area / Protocol | SDAP mapping between MBS QoS flow and MRB |
| Primary baseline | 3GPP TS 37.324 |
| Direction | Downlink-focused |
| Main limits | No SDAP header for MRB and no reflective QoS flow to MRB mapping |
| Receive-path rule | MRB reception uses the SDAP data PDU without header path |
What This Page Covers
This reference page covers the SDAP behavior specific to MBS QoS flow to MRB mapping. It focuses on the downlink mapping model, receive-path behavior, and the SDAP limits that make MRB different from ordinary DRB-based traffic.
What MBS QoS Flow to MRB Mapping Means
One of the supported SDAP functions is mapping between an MBS QoS flow and an MRB for downlink transport. This is the multicast or broadcast counterpart to the better-known QoS flow to DRB mapping used in ordinary Uu traffic.
The mapping exists so that MBS user-plane data associated with an MBS QoS flow can be carried over the appropriate multicast radio bearer.
Why MRB Is Different from DRB in SDAP
MRB-related SDAP behavior is intentionally simpler than DRB-based SDAP behavior. SDAP explicitly states that there is no SDAP header for MRB and that reflective QoS flow to MRB mapping is not supported.
That means MRB does not use the same header-carried signaling and reflective logic that appears on DRB-based downlink SDAP traffic.
Downlink Orientation of MRB Mapping
MBS QoS flow to MRB mapping is a downlink function. Unlike ordinary QoS flow to DRB mapping, there is no uplink bearer-selection model for MRB on this page because the SDAP function is specifically defined for downlink delivery.
This fits the delivery nature of multicast or broadcast data, where the user-plane path is focused on downlink distribution toward receiving UEs.
SDAP Receive-Path Behavior for MRB
In SDAP downlink handling, when the receiving SDAP entity receives an SDAP data PDU from lower layers and that PDU was received from an MRB, the UE retrieves the SDAP SDU using the data PDU without SDAP header.
This is the practical receive-path consequence of the rule that MRB has no SDAP header.
No SDAP Header for MRB
MRB does not use an SDAP header. Because of that, MRB-related SDAP handling does not use the
normal SDAP header field set such as QFI, RQI, and RDI
in the same way as header-bearing DRB traffic.
This should be kept explicit because it is easy to assume every SDAP path uses the same header model, but MRB is a deliberate exception in the Release 18 SDAP behavior set.
Reflective QoS Does Not Apply to MRB
Reflective QoS flow to MRB mapping is not supported. This is one of the biggest functional differences between MRB and DRB-based SDAP behavior.
As a result, MRB should not be described using RQI or RDI-driven reflective mapping updates.
MRB in the SDAP Function Model
The clean way to place MRB in the SDAP mapping model is:
- QoS flow to DRB mapping for ordinary Uu traffic
- MBS QoS flow to MRB mapping for downlink multicast or broadcast delivery
- PC5 QoS flow to SL-DRB mapping for sidelink
This makes it clear that MRB is a first-class SDAP branch, but with simpler behavior than DRB-based SDAP.
MRB vs DRB in SDAP
| Aspect | DRB | MRB |
|---|---|---|
| Main mapping role | QoS flow to DRB | MBS QoS flow to MRB |
| Direction | Uplink and downlink behavior | Downlink-focused |
| SDAP header | May be present depending on configuration | Not used |
| Reflective QoS | Supported where applicable | Not supported |
| Receive path | Headerless or header-bearing depending on DRB config | Headerless path only |
Relationship to Overall NR and NG-RAN MBS Architecture
This page gives only a narrow architecture bridge: MRB is part of the NR and NG-RAN support for MBS delivery, but the detailed MBS session setup, NG-RAN procedures, and bearer-context architecture belong on separate MBS-specific pages.
The main SDAP point is simple: SDAP provides the user-plane mapping between an MBS QoS flow and the multicast radio bearer used for downlink delivery.
Reference Examples
Example 1: MRB downlink reception
A UE receives MBS user-plane data on an MRB. The receiving SDAP entity retrieves the SDAP SDU using the headerless SDAP data PDU path.
Example 2: Why RQI and RDI do not apply
An engineer expects reflective QoS handling on an MRB receive path. SDAP says that reflective QoS flow to MRB mapping is not supported, so that expectation is incorrect.
Example 3: Why the header page does not apply
A reader looks for QFI, RQI, or RDI decoding on an MRB
path. That does not apply here because MRB does not use an SDAP header.
Recommended Diagram Blocks
- MBS QoS flow to MRB mapping diagram
- MRB receive path:
Lower layers -> SDAP data PDU without header -> SDAP SDU -> upper layers - Callout box:
No SDAP header for MRB - Callout box:
Reflective QoS flow to MRB mapping not supported
Key Specification References
- 3GPP TS 37.324 - primary SDAP specification for MBS QoS flow to MRB mapping and MRB-specific limits
- 3GPP TS 38.300 - NR overall architecture context for SDAP and MBS support
- 3GPP TS 38.401 - NG-RAN procedural context showing MBS QoS flow to MRB mapping information