5GSM States in 5G NAS
5GSM states describe where a PDU session sits in session-side NAS control. They help explain whether a session is not yet established, already active, or moving through a temporary establishment, modification, or release branch.
Use this page when a 5GSM message or cause does not make sense until you first place the session in the correct session-side state.
Quick facts
| Technology | 5G |
|---|---|
| Area / Protocol | 5GSM states in NAS |
| Main use | Show where a PDU session sits in session-side NAS progression and which messages make sense next |
| UE-side states | PDU SESSION INACTIVE, PDU SESSION ACTIVE PENDING, PDU SESSION ACTIVE, PDU SESSION INACTIVE PENDING, PDU SESSION MODIFICATION PENDING |
| Network-side states | PDU SESSION INACTIVE, PDU SESSION ACTIVE, PDU SESSION MODIFICATION PENDING, PDU SESSION INACTIVE PENDING |
| Related topics | PDU session establishment, modification, release, PTI, PDU Session ID, 5GSM cause values |
Contents
Overview
The 5GSM state model helps explain session-side meaning in traces. It shows whether the UE is asking for a new PDU session, working inside an already-established session, or cleaning up a session that is leaving use.
This matters because the same session message can be valid in one session position and clearly abnormal in another. The most useful next checks are usually session establishment, session modification, session release, or 5GSM cause values.
UE state flow
The UE-side 5GSM state flow moves from inactive session context into active-pending, active, modification pending, and inactive-pending branches depending on whether the session is being created, changed, or released.
Network state flow
The network-side 5GSM view is a little simpler. It moves between inactive, active, modification-pending, and inactive-pending depending on whether the network is accepting, modifying, or releasing a session.
Quick lookup
Use the quick lookup table for fast state matching, then move into the UE or network state sections when you need the more detailed meaning behind a state transition or a stalled session-side branch.
| State | Side | Typical meaning | Typical procedures |
|---|---|---|---|
| PDU SESSION INACTIVE | UE | No active PDU session exists for the target session context. | PDU Session Establishment, network-requested establishment, and fresh session creation |
| PDU SESSION ACTIVE PENDING | UE | Session establishment has started and the UE is waiting for the network result. | PDU Session Establishment accept or reject handling |
| PDU SESSION ACTIVE | UE | The PDU session is established and active. | modification, release, session transport, and status handling |
| PDU SESSION INACTIVE PENDING | UE | The session is being moved toward inactive state. | PDU Session Release command or release-reject handling |
| PDU SESSION MODIFICATION PENDING | UE | Session modification is in progress. | PDU Session Modification command, reject handling, and return to active or inactive outcome |
| PDU SESSION INACTIVE | Network | The network does not currently keep the session as active. | establishment accept or reject handling and fresh setup decisions |
| PDU SESSION ACTIVE | Network | The network holds an active PDU session context. | PDU Session Modification, PDU Session Release, and active service handling |
| PDU SESSION MODIFICATION PENDING | Network | The network is processing modification of an active session. | PDU Session Modification complete, reject handling, and transition back to active or inactive |
| PDU SESSION INACTIVE PENDING | Network | The network is moving an active session toward inactive state. | release completion and final move into inactive session state |
UE states
The UE-side 5GSM states describe whether the session is inactive, being created, active, being modified, or being released.
PDU SESSION INACTIVE
No active PDU session exists for the target session context.
Entered when: Before establishment or after release has completed.
What it means: There is no active session for data transfer in this state.
Typical procedures: PDU Session Establishment, network-requested establishment, and fresh session creation
Trace note: This is the baseline state when a session does not yet exist or has already been torn down.
PDU SESSION ACTIVE PENDING
Session establishment has started and the UE is waiting for the network result.
Entered when: After PDU Session Establishment Request.
What it means: The session is being created but is not active yet.
Typical procedures: PDU Session Establishment accept or reject handling
Trace note: This state explains why the request exists in the trace but the session is not usable yet.
PDU SESSION ACTIVE
The PDU session is established and active.
Entered when: After PDU Session Establishment Accept or successful return from modification handling.
What it means: The session can carry user-plane service and can be modified or released.
Typical procedures: modification, release, session transport, and status handling
Trace note: This is the main steady state for an active PDU session.
PDU SESSION INACTIVE PENDING
The session is being moved toward inactive state.
Entered when: After PDU Session Release Request from the UE side.
What it means: Release has started but the final inactive state is not reached yet.
Typical procedures: PDU Session Release command or release-reject handling
Trace note: This is the temporary release branch before the session becomes fully inactive.
PDU SESSION MODIFICATION PENDING
Session modification is in progress.
Entered when: After PDU Session Modification Request when a live session is being changed.
What it means: The session exists, but the requested change has not yet settled.
Typical procedures: PDU Session Modification command, reject handling, and return to active or inactive outcome
Trace note: This is the key temporary state when a live PDU session is being updated.
Network states
The network-side 5GSM states describe whether the network currently treats the session as inactive, active, under modification, or under release cleanup.
PDU SESSION INACTIVE
The network does not currently keep the session as active.
Entered when: Before establishment or after release completion.
What it means: No active session data path is maintained for the target session.
Typical procedures: establishment accept or reject handling and fresh setup decisions
Trace note: This is the network-side baseline when the session is not active.
PDU SESSION ACTIVE
The network holds an active PDU session context.
Entered when: After PDU Session Establishment Accept.
What it means: The session exists and can be maintained, modified, or released.
Typical procedures: PDU Session Modification, PDU Session Release, and active service handling
Trace note: This is the main network-side steady state for an established session.
PDU SESSION MODIFICATION PENDING
The network is processing modification of an active session.
Entered when: After PDU Session Modification Command.
What it means: The session remains under change processing until completion or rejection settles it.
Typical procedures: PDU Session Modification complete, reject handling, and transition back to active or inactive
Trace note: This is the network-side branch for parameter or policy changes to an established session.
PDU SESSION INACTIVE PENDING
The network is moving an active session toward inactive state.
Entered when: After PDU Session Release Command.
What it means: Release is in progress but final inactive settlement is still pending.
Typical procedures: release completion and final move into inactive session state
Trace note: This temporary network-side state explains why release is visible but cleanup is not finished yet.
Why states matter
Before deciding what a 5GSM message means, ask three questions:
1. Does this PDU session already exist?
2. Is another session-side procedure already in progress?
3. Does the next message fit that session position?
This is often the fastest way to spot a session-side state mismatch or a procedure collision. If the state is already clear but the failure reason is not, the next page is usually 5GSM cause values.
Example
A simple example is a UE sending PDU Session Establishment Request.
Start with no active session context for the target session.
PDU Session Establishment Request starts the move toward an established session.
PDU Session Establishment Accept completes the branch and settles the session into active context.
A reject means the session never reached the established position and should be read with a 5GSM cause.
FAQ
What are 5GSM states?
They are the session-side NAS state positions that explain where a PDU session sits in establishment, maintenance, or release progression.
Why are 5GSM states useful?
They help explain whether a visible PDU session message fits the current session context or whether the trace shows a state mismatch.
Do 5GSM states replace cause values?
No. States explain session position, while causes explain why the session procedure could not continue or was rejected.
References
- 3GPP TS 24.501, Non-Access-Stratum (NAS) protocol for the 5G System
- 3GPP TS 23.502, Procedures for the 5G System