NAS 5G AMF to UE 3GPP TS 24.501

5G NAS - Service Reject

Service Reject is the NAS message the network sends when a service request is refused even though the UE already has registration context.

Message Fact Sheet

Protocol	nas	Network	5g
Spec	3GPP TS 24.501	Spec Section	8.2.16
Direction	AMF to UE	Message Type	5GMM signaling

Full message name	5G NAS - Service Reject
Protocol	NAS
Technology	5G
Direction	AMF to UE
Interface	N1
Signaling bearer / channel	NAS signaling / Dedicated NAS message, commonly via DL Information Transfer
Typical trigger	Sent when the network cannot accept the requested service continuation.
Main purpose	Stops the current service-request path and tells the UE that active service cannot be restored under the current conditions.
Main specification	3GPP TS 24.501, 8.2.16
Release added	Release 15
Procedures where used	Service Request
Related timers	T3346 when applicable
Related cause values	5GMM cause

What is Service Reject in simple terms?

Service Reject is the NAS message the network sends when a service request is refused even though the UE already has registration context.

Stops the current service-request path and tells the UE that active service cannot be restored under the current conditions.

Why this message matters

Service Reject means the network refused to restore active service even though the UE already had registration context.

Where this message appears in the call flow

Service Request

Call flow position: Negative AMF response after Service Request.

Typical state: UE is registered but service resumption was not accepted.

Preconditions:

The UE sent Service Request.

Next likely message: Retry or alternative recovery depending on cause

Call flow position

Previous message(s): Service Request

Next message(s): Backoff, retry, or alternative signaling behavior

Message direction and transport

Sender and receiver: AMF to UE

Interface: N1

Domain: Core-side mobility management signaling

Signaling bearer: NAS signaling

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

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

Security context: Usually protected because it follows a protected Service Request path.

Message Structure Overview

Service Reject is cause-driven and should be analyzed with the earlier Service Request.

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.

Service Reject is a NAS message defined in 24.501.

Decoded Message Dump

Service Reject
  Extended Protocol Discriminator: 5G Mobility Management
  Security Header Type: Integrity protected and ciphered
  Message Type: Service Reject
  5GMM Cause: UE not reachable for requested service

How to read this dump

As with other reject-style NAS messages, the cause is the first field to inspect.

Important Information Elements

IE	Required	Description
`5GMM cause`	Yes	Explains why the requested service continuation was refused.

Detailed field explanation

`5GMM cause`

Explains why the requested service continuation was refused.

Presence: Required

What to check in logs and traces

Check the 5GMM cause.
Correlate with the preceding Service Request and access-side resume state.
Inspect whether backoff or later retry behavior matches the returned context.

Common Issues and Troubleshooting

Paging or resume succeeds but active service still fails.

Likely cause: The AMF refused service continuation after evaluating the Service Request.

What to inspect: Check the Service Reject cause and the service type requested earlier.

Next step: Troubleshoot by cause instead of treating the issue as only radio-side.

Detailed explanation

5G NAS - Service Reject Explained

Service Reject is the AMF’s negative response when the UE requests active service continuation using existing registration context and the network refuses that request.

For beginners, the simple meaning is: the UE is already registered, but the network did not allow service to continue right now.
For engineers, the message is important because it separates service-continuation failure from fresh registration failure.

What is Service Reject in simple terms?

The UE asks for active service using Service Request. The network evaluates that request and sends Service Reject when service continuation cannot be allowed.

Why this message matters

This message matters because it tells engineers the problem is not simply “the UE is not registered.” The UE already had valid context, but the network still refused the requested service continuation.

That often shifts debugging toward:

the specific 5GMM cause
the requested service type
AMF service-state decisions
later retry or backoff behavior

Where this message appears in the call flow

UE                              gNB / AMF
|<-- Paging or trigger ----------|
|--- Resume / access ----------->|
|--- Service Request ----------->|
|<-- Service Reject -------------|

Transport characteristics

Direction: AMF to UE
Interface: N1
Transport on access side: commonly via DL Information Transfer
Security expectation: usually protected because it follows a protected service path

What Service Reject means operationally

Operationally, Service Reject means the UE reached the service-request decision point but the core network did not allow service to continue. That makes it a key message for separating:

access recovery that worked
from service restoration that did not

If the radio path looks healthy but the user still has no active service, this message is often the turning point in the trace.

Important Information Elements

IE	Why it matters
`5GMM cause`	Explains why service continuation was refused.

Example message dump

Service Reject
  Extended Protocol Discriminator: 5G Mobility Management
  Security Header Type: Integrity protected and ciphered
  Message Type: Service Reject
  5GMM Cause: UE not reachable for requested service

How to read this dump

Read the 5GMM cause first.
Correlate it with the preceding Service Request and access-side state.
Then check whether the UE retries, backs off, or falls into another recovery path.

What to check in logs

inspect the exact 5GMM cause
verify whether radio resume or access signaling succeeded earlier
compare the reject with the service type requested by the UE
check later retry behavior instead of assuming the issue is purely radio-side

FAQ

What does Service Reject mean in 5G?

It means the UE was registered, but the network did not accept the requested service continuation.

Summary

Service Reject is the NAS message the network sends when a service request is refused even though the UE already has registration context.

Advanced Engineer Notes

This message is useful for separating service-continuation failure from registration failure.

Related message pages

Related Procedures

5G RRC Resume

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.