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5GSM Cause Values in 5G NAS

5GSM cause values explain why a session-management procedure did not continue normally. They are most useful in PDU session establishment, modification, release, and session-status handling when the visible question is why a requested session action was rejected or could not proceed.

Use this page when the failure belongs to session-side NAS meaning rather than mobility-side meaning and you need to place the cause in the correct PDU session context before moving into message-level detail.

Quick facts

Technology 5G
Area / Protocol 5GSM cause values in NAS
Main use Explain why a session-management procedure was rejected, failed, or moved into status handling
Common procedures PDU session establishment, modification, release, authentication, status handling
Reader goal Separate session-policy, state, resource, and payload issues before moving into detailed session troubleshooting
Related topics PDU Session Establishment Reject, 5GSM Status, PTI, PDU Session ID, 5GSM states

Contents

  1. Overview
  2. Where they appear
  3. Cause families
  4. How to read them
  5. Example
  6. FAQ
  7. References
  8. Related pages

Overview

A 5GSM cause should be read as part of a session procedure, not as an isolated code. The important context is which PDU session procedure failed, whether the session already existed, what the earlier session state looked like, and which PTI and PDU Session ID were active.

In practice, 5GSM causes help answer questions such as: was the session rejected because it was not allowed, because the request did not fit the current session state, because the requested resources were not accepted, or because the message sequence moved into abnormal handling?

Where they appear

Procedure area Typical message view What the cause helps explain
Session establishment PDU Session Establishment Reject Why a requested new session was not accepted.
Session modification PDU Session Modification Reject Why a change to an existing session could not continue.
Session release or cleanup Release branch or abnormal handling message Whether the session could be cleaned up normally or had to move into failure handling.
Status handling 5GSM Status Why the session message sequence moved into abnormal or unexpected-protocol handling.

PDU Session Establishment

Open the session setup flow when the cause appears during first session creation.

PDU Session Management

Use the wider session flow view when the cause belongs to modification, release, or restoration context.

Cause families

Use the cause-family table for fast grouping, then move into the related session procedure, message, or state page when you need more context around policy rejection, state mismatch, resource limitations, or protocol errors.

Cause family Main meaning Typical next check
Session not allowed or not accepted The network rejects the requested session action on policy, subscription, slice, or service grounds. Check DNN, S-NSSAI, subscription, and session request intent.
Session state mismatch The message does not fit the current session-side NAS state. Check whether the session already exists, was already released, or is waiting on another pending branch.
QoS or resource-related handling The requested session outcome cannot be granted in the requested form. Inspect requested QoS, session context, and later network-selected parameters.
Protocol or abnormal handling The session sequence moves into status handling rather than normal completion. Check PTI, PDU Session ID, message ordering, and missing prerequisites.

How to read them

Trace reading order
1. Identify the failed 5GSM procedure.
2. Confirm the PDU Session ID and PTI.
3. Find the exact message carrying the cause.
4. Place the cause against the current 5GSM state.
5. Decide whether the issue is policy, state, resource, or protocol handling.

This matters because a session reject can look similar across procedures even when the real reasons are very different.

Example

A common example is a UE sending PDU Session Establishment Request and receiving PDU Session Establishment Reject with a 5GSM cause.

Example read path
Confirm the message belongs to 5GSM rather than 5GMM.
Read the reject together with PDU Session ID, PTI, DNN, slice context, and earlier session state.
Then decide whether the failure is policy-related, session-state-related, or resource-related.

FAQ

What are 5GSM cause values?

They are session-management reject or abnormal-condition indicators that explain why a PDU session procedure did not continue normally.

Are 5GSM causes only for session establishment?

No. They also matter in session modification, release, authentication, and status handling branches.

Should a 5GSM cause be read by itself?

No. It should be read with the session procedure, PDU Session ID, PTI, and the earlier 5GSM state.

References