Write Replace Warning Request is the NGAP warning message sent by the AMF to the NG-RAN node to start or replace public warning broadcast content such as ETWS or CMAS alerts.
Message Fact Sheet
Protocol
ngap
Network
5g
Spec
3GPP TS 38.413
Spec Section
Clause 9.2.8.1 and Warning Message Transmission Messages
AMF receives or determines public warning content that must be broadcast or must replace an active warning in NG-RAN.
Main purpose
Requests NG-RAN to broadcast warning content, replaces an existing warning message when matching identifiers are used, carries warning identification and broadcast-area information, carries warning content or delivery parameters, and allows NG-RAN to report handling result through Write Replace Warning Response.
Main specification
3GPP TS 38.413, Clause 9.2.8.1 and Warning Message Transmission Messages
What is Write Replace Warning Request in simple terms?
Write Replace Warning Request is the NGAP warning message sent by the AMF to the NG-RAN node to start or replace public warning broadcast content such as ETWS or CMAS alerts.
Requests NG-RAN to broadcast warning content, replaces an existing warning message when matching identifiers are used, carries warning identification and broadcast-area information, carries warning content or delivery parameters, and allows NG-RAN to report handling result through Write Replace Warning Response.
Why this message matters
Write Replace Warning Request is the AMF telling the gNB to start or replace a public warning broadcast. The most important fields identify the warning, target the area, and control repetition.
Where this message appears in the call flow
Write Replace Warning
AMF requests warning broadcast start or replacement, and NG-RAN reports the handling result.
Call flow position: AMF sends this initiatingMessage to request NG-RAN to start or replace warning broadcast content.
Typical state: NG-RAN validates identifiers, area targeting, repetition parameters, and warning content before responding.
Preconditions:
AMF has warning content or warning control information to broadcast.
NG-RAN is reachable over NGAP non-UE-associated signalling.
Message Identifier and Serial Number identify the warning instance.
Next likely message: Write Replace Warning Response
Warning area targeting
Targeting and repetition fields determine where and how often the warning is broadcast.
Call flow position: Warning Area List and Warning Area Coordinates can scope the broadcast area.
Typical state: NG-RAN restricts warning broadcast according to supported area identifiers or geographic information.
Preconditions:
Area targeting information is present when the warning should not apply to the whole served area.
Target cells, tracking areas, emergency areas, or coordinates are decodable and supported.
Next likely message: Targeted public warning broadcast
Broadcast repetition control
Write Replace starts or replaces content; cancel stops it; restart and failure indications report NG-RAN-side state.
Call flow position: Repetition Period and Number of Broadcasts Requested control how the warning is repeated.
Typical state: NG-RAN schedules warning broadcast repetitions according to request parameters.
Preconditions:
Repetition fields are present and valid.
Extended Repetition Period is decoded if used.
Next likely message: Consistent repeated warning transmission
Transport / encapsulation: NGAP over SCTP/IP between AMF and NG-RAN
Security context: The message may carry public warning content, warning security information, broadcast area targeting, and repetition parameters for emergency warning delivery.
Message Structure Overview
Write Replace Warning Request is an AMF-to-NG-RAN initiatingMessage for public warning broadcast.
Message Identifier and Serial Number identify the warning and warning instance.
Repetition Period and Number of Broadcasts Requested control repeated broadcast behavior.
Warning Area List and Warning Area Coordinates scope the target area when present.
Warning Type, Warning Security Information, Data Coding Scheme, and Warning Message Contents carry warning classification and content details.
ASN.1 for 5G NGAP - Write Replace Warning Request
WriteReplaceWarningRequest ::= SEQUENCE {
protocolIEs ProtocolIE-Container { {WriteReplaceWarningRequest-IEs} },
...
}
WriteReplaceWarningRequest-IEs NGAP-PROTOCOL-IES ::= {
{ ID id-MessageIdentifier CRITICALITY reject TYPE MessageIdentifier PRESENCE mandatory } |
{ ID id-SerialNumber CRITICALITY reject TYPE SerialNumber PRESENCE mandatory } |
{ ID id-WarningAreaList CRITICALITY ignore TYPE WarningAreaList PRESENCE optional } |
{ ID id-RepetitionPeriod CRITICALITY reject TYPE RepetitionPeriod PRESENCE mandatory } |
{ ID id-NumberOfBroadcastsRequested CRITICALITY reject TYPE NumberOfBroadcastsRequested PRESENCE mandatory } |
{ ID id-WarningType CRITICALITY ignore TYPE WarningType PRESENCE optional } |
{ ID id-WarningSecurityInformation CRITICALITY ignore TYPE WarningSecurityInformation PRESENCE optional } |
{ ID id-DataCodingScheme CRITICALITY ignore TYPE DataCodingScheme PRESENCE optional } |
{ ID id-WarningMessageContents CRITICALITY ignore TYPE WarningMessageContents PRESENCE optional } |
{ ID id-ConcurrentWarningMessageInd CRITICALITY ignore TYPE ConcurrentWarningMessageInd PRESENCE optional } |
{ ID id-ExtendedRepetitionPeriod CRITICALITY ignore TYPE ExtendedRepetitionPeriod PRESENCE optional } |
{ ID id-WarningAreaCoordinates CRITICALITY ignore TYPE WarningAreaCoordinates PRESENCE optional } |
{ ID id-SendWriteReplaceWarningIndication CRITICALITY ignore TYPE SendWriteReplaceWarningIndication PRESENCE optional } |
{ ID id-GlobalRANNodeID CRITICALITY ignore TYPE GlobalRANNodeID PRESENCE optional },
...
}
How to read this ASN.1
Decode Message Identifier and Serial Number first, then inspect the target area, repetition controls, content fields, and any security or concurrent-warning indicators. The paired response reports handling result.
5G NGAP - Write Replace Warning Request - Example Dump
Treat this as a teaching example based on the expected message structure, not as a captured network trace.
Message Identifier and Serial Number identify the warning instance.
Repetition Period and Number of Broadcasts Requested control repeated broadcast behavior.
Warning Area List and Warning Area Coordinates should be checked when targeting matters.
Important Information Elements
IE
Required
Description
Message Type
Yes
Identifies the NGAP PDU as WRITE-REPLACE WARNING REQUEST.
Message Identifier
Yes
Mandatory warning message type or category identifier.
Serial Number
Yes
Mandatory warning message instance identifier used for replacement, update, or repeated warning handling.
Warning Area List
Optional
Optional target area where the warning should be broadcast.
Repetition Period
Yes
Mandatory timing control for warning broadcast repetition.
Number of Broadcasts Requested
Yes
Mandatory count of requested warning broadcast repetitions.
Warning Type
Optional
Optional ETWS warning type information.
Warning Security Information
Optional
Optional security information for warning delivery.
Data Coding Scheme
Optional
Optional coding scheme for warning message contents.
Warning Message Contents
Optional
Optional warning text or payload to be broadcast.
Concurrent Warning Message Indication
Optional
Optional indicator for concurrent warning-message handling.
Extended Repetition Period
Optional
Optional extended timing control for longer repetition periods.
Warning Area Coordinates
Optional
Optional geographical area information for warning targeting.
Send Write Replace Warning Indication
Optional
Optional request for additional indication behavior.
Global RAN Node ID
Optional
Optional RAN node identity context.
Detailed field explanation
Message Type
Identifies the NGAP PDU as WRITE-REPLACE WARNING REQUEST.
Presence: Required
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.
Message Identifier
Mandatory warning message type or category identifier.
Presence: Required
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.
Serial Number
Mandatory warning message instance identifier used for replacement, update, or repeated warning handling.
Presence: Required
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.
Warning Area List
Optional target area where the warning should be broadcast.
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.
Repetition Period
Mandatory timing control for warning broadcast repetition.
Presence: Required
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.
Number of Broadcasts Requested
Mandatory count of requested warning broadcast repetitions.
Presence: Required
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.
Warning Type
Optional ETWS warning type information.
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.
Warning Security Information
Optional security information for warning 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.
Data Coding Scheme
Optional coding scheme for warning message contents.
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.
Warning Message Contents
Optional warning text or payload to be broadcast.
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.
Concurrent Warning Message Indication
Optional indicator for concurrent warning-message handling.
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.
Extended Repetition Period
Optional extended timing control for longer repetition periods.
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.
Warning Area Coordinates
Optional geographical area information for warning targeting.
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.
Send Write Replace Warning Indication
Optional request for additional indication behavior.
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.
Global RAN Node ID
Optional RAN node identity context.
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 direction is AMF to NG-RAN node.
Decode Message Identifier and Serial Number correctly.
Check Warning Area List against the intended broadcast scope.
Validate Repetition Period and Number of Broadcasts Requested.
Check Warning Message Contents and Data Coding Scheme together.
Find the matching Write Replace Warning Response.
Check later PWS Failure Indication if broadcast fails.
Common Issues and Troubleshooting
Warning is broadcast in the wrong area.
Likely cause: Warning Area List or Warning Area Coordinates may be wrong, unsupported, or missing.
What to inspect: Decode area identifiers and compare them with intended target cells, tracking areas, emergency areas, or coordinates.
Next step: Correct area targeting and confirm NG-RAN supports the provided area form.
Warning replacement behaves unexpectedly.
Likely cause: Message Identifier and Serial Number may not match the intended active warning instance.
What to inspect: Compare Message Identifier and Serial Number with previous Write Replace Warning Request messages.
Next step: Use the correct warning identifiers for replacement or send PWS Cancel when stopping a warning is intended.
Warning content is missing or unreadable.
Likely cause: Warning Message Contents may be absent or Data Coding Scheme may not match the encoded payload.
What to inspect: Decode Data Coding Scheme and warning content together.
Next step: Fix content encoding or provide the expected warning payload.
Warning repeats too often or too rarely.
Likely cause: Repetition Period, Extended Repetition Period, or Number of Broadcasts Requested may be misconfigured.
What to inspect: Check repetition timing and count fields.
Next step: Adjust repetition parameters and confirm NG-RAN applies them as expected.
No response from NG-RAN.
Likely cause: NG-RAN may reject or fail to process the request, or the response may be filtered from the trace.
What to inspect: Search for Write Replace Warning Response, Error Indication, or PWS Failure Indication.
Next step: Correlate by warning identifiers and procedure timing.
LTE / 5G / Variant Comparison
Compared with Write Replace Warning Response
Request asks NG-RAN to start or replace warning broadcast. Response reports the handling result.
Compared with PWS Cancel Request
Write Replace Warning Request starts or replaces warning content. PWS Cancel Request stops an active warning.
Compared with PWS Restart and Failure Indications
Restart and Failure Indications are NG-RAN reports. Write Replace Warning Request is AMF-initiated broadcast control.
FAQ
What is Write Replace Warning Request in NGAP?
It is the AMF-to-NG-RAN warning message used to start or replace public warning broadcast content such as ETWS or CMAS alerts.
Who sends Write Replace Warning Request?
The AMF sends Write Replace Warning Request to the NG-RAN node.
What is the response to this message?
NG-RAN responds with Write Replace Warning Response.
What is Message Identifier used for?
Message Identifier identifies the type or category of warning message being broadcast.
What is Serial Number in warning messages?
Serial Number identifies the warning-message instance and helps distinguish replacement, update, and repeated warning handling.
What is the difference between Write Replace Warning Request and PWS Cancel Request?
Write Replace Warning Request starts or replaces warning broadcast content. PWS Cancel Request stops or cancels an active warning broadcast.
How does Warning Area List control broadcast scope?
Warning Area List tells NG-RAN the area where warning broadcast should apply, such as supported cells, tracking areas, emergency areas, or other area identifiers.
What fields control warning repetition?
Repetition Period and Number of Broadcasts Requested are the main fields. Extended Repetition Period may provide longer timing control.
Can this message replace an existing warning?
Yes. It can replace an existing warning when the warning identifiers and serial number behavior indicate replacement of active warning content.
How do you troubleshoot failed public warning broadcast?
Check identifiers, serial number, target area, repetition settings, content encoding, the matching Write Replace Warning Response, and any later PWS Failure Indication.
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.
