Write Replace Warning Response is the F1AP non-UE-associated message the gNB-DU sends to the gNB-CU to report handling of a Write Replace Warning Request.
Message Fact Sheet
Protocol
f1ap
Network
5g
Spec
3GPP TS 38.473
Spec Section
Sections 8.5.1 and 9.2.4.2
Direction
gNB-DU -> gNB-CU successfulOutcome
Message Type
Public warning broadcast response
Full message name
Write Replace Warning Response
Protocol
F1AP
Technology
5G
Direction
gNB-DU -> gNB-CU successfulOutcome
Interface
F1-C
Signaling bearer / channel
Non-UE-associated F1AP signaling for public warning response / SCTP carried F1AP successfulOutcome after Write Replace Warning Request
Typical trigger
The gNB-DU has received and processed a Write Replace Warning Request from the gNB-CU.
Main purpose
Acknowledges DU processing of a warning broadcast request, correlates the result through Transaction ID, reports cells where broadcast handling completed, identifies UEs that may need dedicated SI delivery, and synchronizes public-warning state between CU and DU.
What is Write Replace Warning Response in simple terms?
Write Replace Warning Response is the F1AP non-UE-associated message the gNB-DU sends to the gNB-CU to report handling of a Write Replace Warning Request.
Acknowledges DU processing of a warning broadcast request, correlates the result through Transaction ID, reports cells where broadcast handling completed, identifies UEs that may need dedicated SI delivery, and synchronizes public-warning state between CU and DU.
Why this message matters
Write Replace Warning Response is the DU's answer to the CU warning request. Match Transaction ID first, then check completed cells and any UEs that need dedicated SI delivery.
Where this message appears in the call flow
Write-Replace Warning
Procedure branch: the DU acknowledges warning request handling and broadcast preparation.
Call flow position: The gNB-DU sends Write Replace Warning Response after handling a Write Replace Warning Request from the gNB-CU.
Typical state: The DU has validated warning information, prepared broadcast handling, and reports the transaction result to the CU.
Preconditions:
The DU received a matching Write Replace Warning Request.
Transaction ID is available for correlation.
The DU has evaluated target cells and UEs that may not receive SI from broadcast.
Next likely message: Warning SI broadcast continues in target cells
Broadcast completion reporting
Decode branch: match the transaction, then inspect completed cells and UEs needing dedicated SI delivery.
Call flow position: The response may include Cell Broadcast Completed List to report cells where warning broadcast handling completed.
Typical state: The CU uses completed-cell reporting to track DU-side public warning coverage.
Preconditions:
The request targeted one or more DU cells.
The DU can map warning handling result to NR CGI values.
Next likely message: CU monitors warning state or sends follow-up PWS control
Dedicated SI delivery support
Scope branch: distinguish warning activation response from UE paging and future warning cancellation.
Call flow position: The response may include Dedicated SI Delivery Needed UE List for UEs unable to receive system information from broadcast.
Typical state: The CU can inform those UEs through dedicated RRC message delivery.
Preconditions:
The DU has active UE context knowledge for affected UEs.
Some UEs cannot rely on broadcast SI reception for the warning update.
Next likely message: Dedicated RRC delivery toward listed UEs
Transport / encapsulation: F1AP over SCTP/IP between gNB-DU and gNB-CU
Security context: Write Replace Warning Response does not establish UE security. It reports public warning broadcast handling and optional dedicated SI delivery needs.
Message Structure Overview
Write Replace Warning Response is a non-UE-associated successfulOutcome from gNB-DU to gNB-CU.
Transaction ID is the mandatory top-level correlation field.
Cell Broadcast Completed List is optional and reports completed DU cell broadcast handling.
Dedicated SI Delivery Needed UE List is optional and identifies UEs that need dedicated RRC delivery of updated SI.
Message Identifier and Serial Number are not top-level F1AP IEs in this response; they are part of the warning/PWS information in the related request.
This page follows TS 38.473 Release 18 terminology. Simplified warning-procedure summaries sometimes list Message Identifier and Serial Number for the response, but in F1AP those identifiers are carried in the related warning/PWS information rather than as top-level response IEs.
Treat this as a teaching example based on the expected message structure, not as a captured network trace.
Match Transaction ID with the preceding Write Replace Warning Request.
Use Cell Broadcast Completed List to verify DU cell coverage.
If Dedicated SI Delivery Needed UE List appears, check that the CU sends dedicated RRC information to the listed UEs.
Important Information Elements
IE
Presence
Description
Message Type
Mandatory
Identifies the F1AP PDU as WRITE-REPLACE WARNING RESPONSE.
Transaction ID
Mandatory
Correlates the response with the original Write Replace Warning Request.
Cell Broadcast Completed List
Optional
Optional list of DU cells where warning broadcast handling completed. Each item includes NR CGI.
Criticality Diagnostics
Optional
Optional protocol diagnostics for non-fatal IE or procedure handling issues.
Dedicated SI Delivery Needed UE List
Optional
Optional list of UEs unable to receive system information from broadcast and needing dedicated SI delivery.
Detailed field explanation
Message Type
Identifies the F1AP PDU as WRITE-REPLACE WARNING RESPONSE.
Presence: Mandatory
In practice: In practice, compare this field with the original request and with any later release-dependent optional fields so you can see whether the network accepted the same service model the UE asked for.
Transaction ID
Correlates the response with the original Write Replace Warning Request.
Presence: Mandatory
In practice: In practice, compare this field with the original request and with any later release-dependent optional fields so you can see whether the network accepted the same service model the UE asked for.
Cell Broadcast Completed List
Optional list of DU cells where warning broadcast handling completed. Each item includes NR CGI.
Presence: Optional
In practice: In practice, compare this field with the original request and with any later release-dependent optional fields so you can see whether the network accepted the same service model the UE asked for.
Criticality Diagnostics
Optional protocol diagnostics for non-fatal IE or procedure handling issues.
Presence: Optional
In practice: In practice, compare this field with the original request and with any later release-dependent optional fields so you can see whether the network accepted the same service model the UE asked for.
Dedicated SI Delivery Needed UE List
Optional list of UEs unable to receive system information from broadcast and needing dedicated SI delivery.
Presence: Optional
In practice: In practice, compare this field with the original request and with any later release-dependent optional fields so you can see whether the network accepted the same service model the UE asked for.
What to check in logs and traces
Confirm message direction is gNB-DU to gNB-CU.
Verify it follows the matching Write Replace Warning Request.
Match Transaction ID with the request.
Check Cell Broadcast Completed List and NR CGI values when present.
Check Dedicated SI Delivery Needed UE List when present.
Inspect Criticality Diagnostics if present.
Confirm warning SI is active in expected cells.
Confirm follow-up dedicated RRC delivery for UEs that cannot receive broadcast SI.
Common Issues and Troubleshooting
Response is missing after Write Replace Warning Request.
Likely cause: The DU may have rejected the request at protocol level, lost the F1AP transaction, or failed before producing the successful outcome.
What to inspect: Check Transaction ID, Error Indication, DU logs, SCTP/F1-C health, and whether the request was valid.
Next step: Fix request content or F1-C transport issues, then resend the warning request if the warning remains active.
Completed-cell reporting is incomplete.
Likely cause: Some target NR CGI values may not be served by the DU, cells may be unavailable, or broadcast setup failed in selected cells.
What to inspect: Compare requested Cells To Be Broadcast List with Cell Broadcast Completed List and DU served-cell inventory.
Next step: Correct cell-area mapping and investigate DU cell state for missing cells.
Some connected UEs do not receive updated warning SI.
Likely cause: The UEs may be unable to receive SI from broadcast and require dedicated SI delivery.
What to inspect: Check Dedicated SI Delivery Needed UE List and follow-up RRC delivery from the CU.
Next step: Trigger or verify dedicated RRC message delivery for the listed UE contexts.
LTE / 5G / Variant Comparison
ETWS handling
ETWS response handling confirms DU acceptance of rapid disaster warning broadcast configuration. Timing, repetition, and cell coverage are critical.
CMAS and public warning handling
CMAS or public warning handling may cover wider areas and richer warning payloads. The response helps the CU track DU-side cell completion and dedicated delivery needs.
Compared with Paging
Paging targets selected UE identities for reachability. Write Replace Warning Response acknowledges mass broadcast warning handling.
Compared with PWS Cancel Response
Write Replace Warning Response reports start or replacement handling. PWS Cancel Response reports cancellation or removal handling.
FAQ
What is Write Replace Warning Response in F1AP?
It is the F1AP successful outcome the gNB-DU sends to the gNB-CU after handling a Write Replace Warning Request.
Who sends this message?
The gNB-DU sends Write Replace Warning Response to the gNB-CU over F1-C.
What does Transaction ID do?
Transaction ID correlates the response with the original Write Replace Warning Request.
What is Cell Broadcast Completed List?
It is an optional list of DU cells, identified by NR CGI, where warning broadcast handling completed.
Does this response confirm all UEs received the warning?
No. It confirms DU-side request handling and optional completed-cell reporting, not per-UE warning reception.
What is Dedicated SI Delivery Needed UE List?
It identifies UEs that are unable to receive system information from broadcast and may need the CU to deliver SI through dedicated RRC signalling.
How is this different from Paging?
Paging is UE-specific reachability signalling. Write Replace Warning Response is an infrastructure acknowledgement for public warning broadcast handling.
How do you troubleshoot warning response failures?
Check the matching request, Transaction ID, completed-cell list, dedicated-SI-needed UE list, DU cell state, broadcast logs, and any Criticality Diagnostics.
Decode this message with the 3GPP Decoder, inspect the related message database, or open the matching call flow to see where this signaling step fits in the full procedure.
