The gNB receives the UE's first uplink NAS message and needs to forward it to the AMF before a UE-associated NG context is fully established on both sides.
Main purpose
Carries the initial UE NAS message toward the core network and provides the AMF with radio-side identity, location, and access context needed to start UE-specific processing.
Main specification
3GPP TS 38.413, 8.6.1, 9.2.5.1
Release added
See specification history
Procedures where used
5G Initial Registration, 5G Mobility Registration Update, 5G Periodic Registration Update, UE Triggered Service Request from CM-IDLE
What is Initial UE Message in simple terms?
NGAP initiating message used by the NG-RAN to transfer the UE's first uplink NAS message to the AMF.
Carries the initial UE NAS message toward the core network and provides the AMF with radio-side identity, location, and access context needed to start UE-specific processing.
Why this message matters
Initial UE Message is the first NGAP wrapper that carries the UE's NAS request from the gNB to the AMF. It is where radio-side access turns into core-network signaling.
Where this message appears in the call flow
5G Initial Registration
Call flow position: First N2 message carrying the UE's Registration Request from NG-RAN to AMF.
Typical state: UE has completed early RRC setup and the AMF has not yet processed UE-specific NAS context for this access attempt.
Preconditions:
RRCSetupComplete has carried the dedicated NAS message from the UE.
NG-RAN has assigned a RAN UE NGAP ID.
User location and establishment cause are known on the access side.
Next likely message: Downlink NAS Transport or Initial Context Setup Request
UE Triggered Service Request from CM-IDLE
Call flow position: Initial N2 transfer of the first uplink NAS service request toward the AMF.
Typical state: UE is transitioning from idle or connection management idle state toward active core interaction.
Preconditions:
RAN has recovered enough UE context to carry the initial NAS message.
NG-RAN can provide location and cause information.
Transport / encapsulation: NGAP over SCTP/IP between NG-RAN and AMF
Security context: Protected by the transport and N2 association, but the embedded NAS-PDU may still be plain NAS during initial registration.
Message Structure Overview
Initial UE Message is an NGAP initiating message with a fixed procedure code and a set of protocolIEs.
Four IEs are the real operational core: RAN UE NGAP ID, NAS-PDU, User Location Information, and RRC Establishment Cause.
Optional identity and slice-related IEs become important when AMF selection, reroute, or advanced access handling is involved.
ASN.1 for 5G NR - Initial UE Message
InitialUEMessage ::= SEQUENCE {
protocolIEs ProtocolIE-Container { {InitialUEMessageIEs} },
...
}
InitialUEMessageIEs NGAP-PROTOCOL-IES ::= {
{ ID id-RAN-UE-NGAP-ID CRITICALITY reject TYPE RAN-UE-NGAP-ID PRESENCE mandatory } |
{ ID id-NAS-PDU CRITICALITY reject TYPE NAS-PDU PRESENCE mandatory } |
{ ID id-UserLocationInformation CRITICALITY reject TYPE UserLocationInformation PRESENCE mandatory } |
{ ID id-RRCEstablishmentCause CRITICALITY ignore TYPE RRCEstablishmentCause PRESENCE mandatory } |
{ ID id-FiveG-S-TMSI CRITICALITY reject TYPE FiveG-S-TMSI PRESENCE optional } |
{ ID id-UEContextRequest CRITICALITY ignore TYPE UEContextRequest PRESENCE optional },
...
}
How to read this ASN.1
Read this message as an NGAP envelope around the first uplink NAS payload. In practice, engineers should focus on whether the mandatory IEs are present and whether the NAS-PDU, location, and identity context are internally consistent.
The critical debugging line is usually the NAS-PDU because it tells you what procedure the AMF will start.
RAN UE NGAP ID is the anchor for later NGAP correlation before AMF UE NGAP ID appears.
User Location Information must align with the cell and TAI where the UE actually accessed.
Important Information Elements
IE
Required
Description
RAN UE NGAP ID
Yes
Unique UE identity allocated by the NG-RAN node for the UE-associated NGAP context.
NAS-PDU
Yes
The UE to AMF NAS message carried transparently by the NG-RAN, such as Registration Request or Service Request.
User Location Information
Yes
Provides access-side location details such as NR CGI and TAI so the AMF knows where the UE initiated access.
RRC Establishment Cause
Yes
Indicates why the UE requested the RRC connection, for example mo-Signalling, mo-Data, emergency, or mt-Access.
5G-S-TMSI
Optional
Temporary UE identity used by the NG-RAN and AMF for UE identification and AMF selection when available from the radio interface.
UE Context Request
Optional
Requests the AMF to trigger Initial Context Setup toward the NG-RAN.
Allowed NSSAI
Optional
Carries allowed slice information that may be relevant for routing, AMF handling, or reroute scenarios.
AMF Set ID
Optional
Indicates rerouting information when the message has been redirected toward another AMF.
Source to Target AMF Information Reroute
Optional
Carries AMF reroute-related context in AMF reallocation scenarios.
Selected PLMN Identity
Optional
Identifies the selected PLMN in applicable access and sharing scenarios.
Detailed field explanation
RAN UE NGAP ID
Unique UE identity allocated by the NG-RAN node for the UE-associated NGAP context.
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.
NAS-PDU
The UE to AMF NAS message carried transparently by the NG-RAN, such as Registration Request or Service 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.
User Location Information
Provides access-side location details such as NR CGI and TAI so the AMF knows where the UE initiated access.
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.
RRC Establishment Cause
Indicates why the UE requested the RRC connection, for example mo-Signalling, mo-Data, emergency, or mt-Access.
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.
5G-S-TMSI
Temporary UE identity used by the NG-RAN and AMF for UE identification and AMF selection when available from the radio interface.
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.
UE Context Request
Requests the AMF to trigger Initial Context Setup toward the NG-RAN.
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.
Allowed NSSAI
Carries allowed slice information that may be relevant for routing, AMF handling, or reroute scenarios.
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.
AMF Set ID
Indicates rerouting information when the message has been redirected toward another AMF.
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.
Source to Target AMF Information Reroute
Carries AMF reroute-related context in AMF reallocation scenarios.
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.
Selected PLMN Identity
Identifies the selected PLMN in applicable access and sharing scenarios.
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 the message is InitialUEMessage and not a later Uplink NAS Transport.
Check the NAS-PDU payload type and decode it fully.
Validate RAN UE NGAP ID continuity across later NGAP messages.
Inspect NR CGI, TAI, and PLMN values in User Location Information.
Check whether 5G-S-TMSI presence or absence matches the UE identity situation.
Compare the RRC Establishment Cause against the preceding radio procedure.
Common Issues and Troubleshooting
AMF does not continue the procedure after Initial UE Message.
Likely cause: The NAS-PDU, identity, or location information may be malformed or inconsistent.
What to inspect: Decode NAS-PDU, User Location Information, 5G-S-TMSI, and establishment cause together.
Next step: Compare the full NGAP envelope with a known-good registration or service request trace.
AMF reroutes or selects an unexpected AMF.
Likely cause: Identity, slice, or location information may be driving different AMF selection logic.
What to inspect: Check 5G-S-TMSI, Allowed NSSAI, PLMN, TAI, and any reroute-related IEs.
Next step: Validate AMF selection assumptions and routing policies.
Core-side procedure fails even though radio access looks normal.
Likely cause: The problem may be inside the embedded NAS-PDU rather than the NGAP wrapper.
What to inspect: Correlate the NGAP message with the decoded Registration Request or Service Request.
Next step: Troubleshoot the NAS payload and its surrounding authentication or registration handling.
LTE / 5G / Variant Comparison
Compared with Uplink NAS Transport
Initial UE Message is used for the first uplink NAS transfer before a mature UE-associated NG context exists on both sides. Uplink NAS Transport is typically used later once the UE context is already established.
FAQ
What is Initial UE Message in 5G?
It is the NGAP message sent by the NG-RAN to the AMF to forward the UE's first uplink NAS message and establish the initial UE-associated NG signaling context.
Is Initial UE Message a NAS message?
No. It is an NGAP message that transports a NAS-PDU inside it.
What NAS message is usually carried in Initial UE Message?
Common examples are Registration Request and CM-IDLE Service Request related NAS messages.
What is the difference between Initial UE Message and Uplink NAS Transport?
Initial UE Message is used for the first uplink NAS transfer when no UE-associated NG context has been set up yet. Uplink NAS Transport is used later once the UE-associated NG context already exists.
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.