NAS 5G AMF to UE 3GPP TS 24.501

5G NAS - Registration Reject

Registration Reject is the NAS message the AMF sends when a 5G registration attempt is refused and the UE must stop the current registration path.

Message Fact Sheet

Protocol	nas	Network	5g
Spec	3GPP TS 24.501	Spec Section	8.2.9
Direction	AMF to UE	Message Type	5GMM signaling

Full message name	5G NAS - Registration Reject
Protocol	NAS
Technology	5G
Direction	AMF to UE
Interface	N1
Signaling bearer / channel	NAS signaling / Dedicated NAS message, commonly transported via DL Information Transfer
Typical trigger	Sent when the AMF cannot accept the registration request because of subscription, roaming, identity, slice, or policy reasons.
Main purpose	Stops the current registration procedure and returns the 5GMM cause and any backoff or policy context that explains why registration was not accepted.
Main specification	3GPP TS 24.501, 8.2.9
Release added	Release 15
Procedures where used	5G Initial Registration, Mobility Registration Update
Related timers	T3346 when provided
Related cause values	5GMM cause, T3346 value

What is Registration Reject in simple terms?

Registration Reject is the NAS message the AMF sends when a 5G registration attempt is refused and the UE must stop the current registration path.

Stops the current registration procedure and returns the 5GMM cause and any backoff or policy context that explains why registration was not accepted.

Why this message matters

Registration Reject is the network telling the UE that the current registration attempt is not allowed, along with the reason why.

Where this message appears in the call flow

5G Initial Registration

Call flow position: Negative AMF response when the UE's registration attempt cannot be accepted.

Typical state: UE is still trying to enter a registered mobility-management state.

Preconditions:

The AMF has evaluated the Registration Request and related checks.

Next likely message: Retry, wait, or remain deregistered depending on cause

Call flow position

Previous message(s): Registration Request, Authentication Failure, Identity Response

Next message(s): Backoff or retry behavior, Fresh Registration Request, Deregistered state handling

Message direction and transport

Sender and receiver: AMF to UE

Interface: N1

Domain: Core-side mobility management rejection with radio-side transport dependency

Signaling bearer: NAS signaling

Logical channel: Dedicated NAS message, commonly transported via DL Information Transfer

Transport / encapsulation: NAS 5GS message carried end-to-end between AMF and UE

Security context: May be sent plain or protected depending on the exact stage and available NAS context.

Message Structure Overview

Registration Reject is operationally centered on the returned cause.
For troubleshooting, engineers usually prioritize the 5GMM cause, any T3346 backoff, and the stage at which the AMF decided to reject.

Structure Syntax

This message is not typically analyzed as ASN.1 on the wire. Engineers usually inspect it as a NAS or protocol field decode rather than an ASN.1 tree.

Registration Reject is a NAS 24.501 message, not an ASN.1 structure.

Decoded Message Dump

Registration Reject
  Extended Protocol Discriminator: 5G Mobility Management
  Security Header Type: Plain NAS
  Message Type: Registration Reject
  5GMM Cause: PLMN not allowed
  T3346 Value: 12 minutes

How to read this dump

Start with the 5GMM cause because it determines nearly all next-step troubleshooting.
T3346 is critical because it changes retry timing and can make a network look unavailable when the UE is actually respecting backoff.

Important Information Elements

IE	Required	Description
`5GMM cause`	Yes	Primary failure reason explaining why registration was refused.
`T3346 value`	Optional	Backoff timer that tells the UE how long it should wait before trying again.

Detailed field explanation

`5GMM cause`

Primary failure reason explaining why registration was refused.

Presence: Required

`T3346 value`

Backoff timer that tells the UE how long it should wait before trying again.

Presence: Optional

What to check in logs and traces

Read the 5GMM cause first and map it to the actual scenario.
Check whether T3346 is present and whether the UE behavior matches that backoff.
Correlate the reject with earlier identity, authentication, slice, or roaming checks.
Confirm whether the message was sent plain or protected and whether that matches the procedure stage.

Common Issues and Troubleshooting

UE receives Registration Reject repeatedly.

Likely cause: The underlying cause is persistent, such as subscription mismatch, roaming restriction, wrong identity context, or barred service.

What to inspect: Check the exact 5GMM cause, slice context, PLMN/TAI alignment, and AMF policy assumptions.

Next step: Troubleshoot by cause rather than retrying blindly, especially if T3346 is present.

The UE does not retry for a long time after reject.

Likely cause: The AMF provided T3346 and the UE is correctly backing off.

What to inspect: Check the timer value and compare it with later UE access behavior.

Next step: Validate whether the retry pause is expected instead of treating it as a separate access failure.

Detailed explanation

5G NAS - Registration Reject Explained

Registration Reject is the NAS failure message used when the network refuses a 5G registration attempt. It is one of the most important troubleshooting messages in NAS because it gives the formal reason why the AMF did not accept the UE.

What is Registration Reject in simple terms?

The UE asks to register. The network checks the request and decides that registration cannot continue. It then sends Registration Reject back to the UE with a cause value and, in some cases, a retry timer.

Why this message matters

This message matters because it turns a generic “registration failed” symptom into a cause-driven investigation. It tells you whether the problem is likely to be:

roaming or PLMN related
subscription or slice related
identity related
temporary backoff or policy related

Where this message appears in the call flow

UE                              gNB / AMF
|--- Registration Request ------>|
|<-- Registration Reject --------|
|   UE stops this path or waits  |

It appears at the negative decision point of the registration procedure.

Transport characteristics

sender: AMF
receiver: UE
interface: N1
access-side transport: commonly DL Information Transfer
security: may be plain or protected depending on the stage

What Registration Reject means operationally

Operationally, Registration Reject is the AMF’s explicit decision that the current registration attempt cannot continue.

This is the point where engineers should stop treating the issue as a generic access problem and start troubleshooting by cause. The most useful first checks are:

the returned 5GMM Cause
any T3346 retry backoff
the earlier identity, slice, roaming, or security context that led to the reject

Important Information Elements

IE	Why it matters
`5GMM cause`	Main reason the registration attempt was refused.
`T3346 value`	Controls retry backoff when present.

Example message dump

Registration Reject
  Extended Protocol Discriminator: 5G Mobility Management
  Security Header Type: Plain NAS
  Message Type: Registration Reject
  5GMM Cause: PLMN not allowed
  T3346 Value: 12 minutes

How to read this dump

Read the 5GMM Cause first.
Check whether T3346 explains delayed retry behavior.
Correlate the reject with earlier identity, authentication, and requested registration context.

What to check in logs

inspect the 5GMM Cause before looking elsewhere
verify whether T3346 is present and whether the UE respects that backoff
correlate the reject with the preceding Registration Request, identity exchange, and authentication path
avoid misreading timer-driven silence after reject as a separate radio problem

FAQ

What is Registration Reject in 5G?

It is the AMF's failure response when a registration attempt cannot be accepted.

What should engineers inspect first in Registration Reject?

The 5GMM cause and any T3346 backoff timer.

Does Registration Reject always mean radio failure?

No. It usually points to mobility-management, subscription, identity, slice, or policy problems rather than radio decode failure.

Summary

Registration Reject is the NAS message the AMF sends when a 5G registration attempt is refused and the UE must stop the current registration path.

Advanced Engineer Notes

The reject cause is often only the start of the analysis. Engineers still need to inspect earlier request content and AMF decision context.
A reject with T3346 changes retry behavior and can mislead troubleshooting if timer-driven silence is mistaken for radio failure.

Related message pages

Related Procedures

5G Initial Registration

Related Tools

Use This Reference in Practice

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.