5GSM Overview and Session State Model
5GSM is the session management part of NAS. It supports PDU session handling between the UE and the SMF, with session-side signaling carried through the AMF.
Use this page when the main question is how the 5GSM sublayer is organized, how its session states and transaction states work, and how establishment, modification, release, authentication, authorization, and status branches fit together before moving into deeper procedure or message pages.
Quick facts
| Technology | 5G |
|---|---|
| Area / Protocol | 5GSM in NAS |
| Main role | Session-side NAS control for PDU session handling between the UE and the SMF via the AMF |
| Main state models | UE-side session states, network-side session states, and separate procedure transaction states |
| Key identifiers | PDU Session ID and PTI (Procedure Transaction Identity) |
| Best next topics | PDU Session Establishment, PDU Session Modification, PDU Session Release, 5GSM States, 5GSM Cause Values |
Contents
- What 5GSM does in 5G NAS
- Types of 5GSM procedures
- UE-side 5GSM session state model
- UE procedure transaction states
- Network-side 5GSM session state model
- Network procedure transaction states
- General 5GSM concepts that support the state model
- Related 5GSM procedure pages
- Related 5GSM messages
- Specification map
- FAQ
- References
- Related pages
What 5GSM Does in 5G NAS
Main function of the 5GSM sublayer
The 5GSM sublayer is the session-side control part of NAS. It exists so the UE can ask for, maintain, change, or remove a PDU session and so the network can return the result with the right session context.
Relationship between UE, AMF, and SMF
In practical reading, 5GMM looks mainly like UE-to-AMF mobility-side control, while 5GSM is the UE-to-SMF session-side meaning carried through the AMF. That is why a session problem may be visible in NAS even though the session logic really belongs to the SMF side.
5GMM context and secure NAS as prerequisites
5GSM procedures are easier to understand after the UE already has usable mobility-side context and secure NAS exchange is in place. If the session branch never starts cleanly, the earlier checks are usually registration and authentication and security mode control.
When a PDU session can be handled
A PDU session can be handled when the session-side request has enough NAS context, security, and policy context behind it to be interpreted and applied. The exact session meaning then depends on the current 5GSM state, the chosen session type, and whether the branch is establishment, modification, release, or status handling.
Types of 5GSM Procedures
The 5GSM procedure map is easiest to read in three families: session-related procedures, transaction-related procedures, and the common status procedure. That grouping helps separate steady session meaning from transaction branches and abnormal handling.
Procedures related to PDU sessions
PDU Session Authentication and Authorization
Session-side authentication and authorization branch used when access to the data-network service needs its own control exchange.
Network-requested PDU Session Modification
Network-driven change of an already-established session, including QoS, policy, or parameter updates.
Network-requested PDU Session Release
Network-driven cleanup of an existing PDU session when service should be removed.
Service-level Authentication and Authorization
Service-level branch associated with data-network access control and related result transfer.
UE-requested PDU Session Establishment
Main entry point for creating a new session from the UE side.
Transaction-related procedures
Common procedure
UE-Side 5GSM Session State Model
The UE-side session model shows whether the UE has no active session, is waiting for establishment, already holds an active session, or is moving through modification or release handling.
Not every possible branch is visible in a simple state sketch, and some branches are narrower in scope for specific session variants. Use the state model as a practical reading map rather than as the only source of detailed procedure behavior.
PDU SESSION INACTIVE
Meaning: No active session exists for the selected session context.
When the UE enters this state: Before establishment begins or after release has completed.
Expected transition: The next meaningful branch is establishment or another new-session action.
Related procedure families: Session establishment, network-requested establishment, and fresh session creation.
PDU SESSION ACTIVE PENDING
Meaning: The UE has started establishment and is waiting for the network result.
When the UE enters this state: After a session establishment request is sent.
Expected transition: The branch should settle into active on acceptance or return to inactive on rejection.
Related procedure families: PDU Session Establishment Request, Accept, and Reject.
PDU SESSION ACTIVE
Meaning: The session is established and can support normal service.
When the UE enters this state: After successful establishment or after a successful modification branch returns to steady state.
Expected transition: The next meaningful branches are modification, transport of session-side signaling, or release.
Related procedure families: Modification, release, status handling, and live session maintenance.
PDU SESSION INACTIVE PENDING
Meaning: The UE is moving the session toward inactive state.
When the UE enters this state: After release has started but before final cleanup is complete.
Expected transition: The branch should return to inactive once release processing finishes.
Related procedure families: PDU Session Release Request, Command, Complete, and reject branches.
PDU SESSION MODIFICATION PENDING
Meaning: The session exists, but a change is still being processed.
When the UE enters this state: After a session modification branch starts on an already-active session.
Expected transition: The branch should settle back into active or move toward another state if the session can no longer continue normally.
Related procedure families: PDU Session Modification Request, Command, Complete, and rejection handling.
UE Procedure Transaction States
The transaction-state view is separate from the session-state view. It answers a different question: is there an open PTI-based transaction right now, or not?
A session can already exist and still have an open transaction branch. That is why a state mismatch often comes from confusing session state with transaction state.
| Transaction state | Meaning | Reference reading purpose |
|---|---|---|
| PROCEDURE TRANSACTION INACTIVE | No transaction is currently open for that PTI. | This is the steady baseline when no session-side transaction is in progress for the selected procedure transaction identity. |
| PROCEDURE TRANSACTION PENDING | A transaction is currently open and waiting to settle. | This is the temporary branch that explains why a later session-side message is still part of the same transaction rather than a new independent request. |
Network-Side 5GSM Session State Model
The network-side model is slightly smaller, but it answers the same practical question from the network view: is the session inactive, active, under modification, or being cleaned up toward inactive state?
PDU SESSION INACTIVE
Meaning in the network: The network does not currently treat the session as active.
What triggered entry: Before establishment or after release cleanup is finished.
Expected transition: The next meaningful branch is establishment or another new-session decision.
Related procedure families: Establishment acceptance or rejection handling.
PDU SESSION ACTIVE
Meaning in the network: The network holds an active session context.
What triggered entry: After successful establishment has been accepted and applied.
Expected transition: The next meaningful branches are modification, maintenance, or release.
Related procedure families: Modification command, release command, and active service handling.
PDU SESSION INACTIVE PENDING
Meaning in the network: The network is processing session removal.
What triggered entry: After release begins but before final cleanup completes.
Expected transition: The branch should settle into inactive once release cleanup finishes.
Related procedure families: Release command, release completion, and cleanup toward inactive state.
PDU SESSION MODIFICATION PENDING
Meaning in the network: The network is processing a change to an existing active session.
What triggered entry: After a modification branch starts on an already-active session.
Expected transition: The branch should settle back into active or move toward inactive if the session cannot remain established.
Related procedure families: Modification command, modification complete, and reject handling.
Network Procedure Transaction States
The network also keeps a separate transaction-state view. It is the same practical idea as the UE-side model: either no PTI-based transaction is open, or a transaction is still pending.
| Transaction state | Meaning | Reference reading purpose |
|---|---|---|
| PROCEDURE TRANSACTION INACTIVE | No transaction is currently open for that PTI. | This is the steady baseline when no session-side transaction is in progress for the selected procedure transaction identity. |
| PROCEDURE TRANSACTION PENDING | A transaction is currently open and waiting to settle. | This is the temporary branch that explains why a later session-side message is still part of the same transaction rather than a new independent request. |
General 5GSM Concepts That Support the State Model
The state model becomes easier to read once PTI, PDU Session ID, session types, QoS-side context, and general session-control concepts are placed around it.
| Concept | Reference reading purpose |
|---|---|
| PTI in 5GSM procedures | PTI, or Procedure Transaction Identity, separates one session-side transaction from another. It matters whenever the trace needs you to distinguish a new transaction from a response inside an already-open branch. |
| PDU session types | Common session types include IPv4, IPv6, IPv4v6, Ethernet, and Unstructured. The selected type affects how the requested data service is understood and which session context details matter later. |
| PDU session management model | The 5GSM model groups establishment, modification, release, authentication, and authorization around one session context. A UE can hold multiple sessions, each with its own session-side control meaning. |
| IP address allocation overview | Session-side NAS carries the meaning that a data session should be created and parameterized, including address-related context. Use the dedicated QoS and session-context page for the deeper IP view. |
| Quality of service overview | 5GSM also carries QoS-side meaning such as QoS rules, QoS flow descriptions, Session-AMBR, and reflective QoS. These often appear during establishment and modification rather than in the overview model itself. |
Other 5GSM concepts
Other session-side topics often appear as supporting context rather than as the first question on the page. They still matter because they explain why one session branch behaves differently from another.
| Concept | Reference reading purpose |
|---|---|
| LADN | Local area data-network behavior that matters when session availability depends on the UE location context. |
| DNN-based congestion control | Session handling influenced by congestion policies tied to the selected data network name. |
| S-NSSAI-based congestion control | Session behavior influenced by slice-specific congestion policy. |
| URSP and upper-layer interaction | Upper-layer steering and policy interaction that can shape how session requests are selected or interpreted. |
| ProSeP-related upper-layer interaction | Upper-layer interaction relevant to proximity-service-related session behavior. |
| 3GPP PS data off | Session-side behavior when packet-switched data usage is restricted or switched off under policy control. |
| Multi-homed IPv6 PDU session | IPv6 session behavior where more than one home or address-related context matters. |
| Network rejection not due to congestion control | Session rejection behavior that should not be confused with congestion-based refusal. |
| Small data rate control | Rate-control behavior relevant to low-volume or controlled session traffic patterns. |
| Serving PLMN rate control | Rate control tied to the current serving PLMN context. |
| Reliable data service | Session-side support for reliable delivery behavior where that service model applies. |
| Header compression for control-plane CIoT optimizations | Special optimization context for compact control-plane data handling. |
| Edge computing enhancements | Session-side handling that may depend on edge-related service or routing context. |
| Redundant PDU sessions | Session behavior for redundancy-oriented service continuity models. |
| Maximum group data rate limitation control | Rate-limitation behavior applied to grouped traffic or grouped service context. |
| UL PDU set handling | Handling related to uplink PDU set behavior in session-side control. |
QoS Rules, IP Addressing, and Session Context
Open this when the overview needs to turn into QoS rules, PDU address context, Session-AMBR, or other reusable session parameters.
5GSM States
Open the dedicated state page when the next question is about exact state meaning rather than the wider 5GSM map.
Specification Map
| Area | What it covers |
|---|---|
| 5GSM overview and state model | Overview, session-state model, and transaction-state model. |
| General on elementary 5GSM procedures | PTI, session types, QoS-side context, address-related context, and other general session-side concepts. |
| Network-requested 5GSM procedures | Authentication, authorization, network-side modification, and network-side release branches. |
| UE-requested 5GSM procedures | UE-driven establishment, modification, and release branches. |
| 5GSM status procedure | Status handling for abnormal or unsupported session-side signaling. |
| 5GS session management messages | The main 5GSM message families used across establishment, modification, release, authentication, and status. |
FAQ
What is 5GSM in 5G NAS?
It is the 5GS session management part of NAS and is used for PDU session handling between the UE and the SMF via the AMF.
What is the main function of the 5GSM sublayer?
Its main function is to control session-side establishment, modification, release, related authentication or authorization, and status handling.
What are the UE-side 5GSM session states?
The UE-side session states are PDU SESSION INACTIVE, PDU SESSION ACTIVE PENDING, PDU SESSION ACTIVE, PDU SESSION INACTIVE PENDING, and PDU SESSION MODIFICATION PENDING.
What are the network-side 5GSM session states?
The network-side session states are PDU SESSION INACTIVE, PDU SESSION ACTIVE, PDU SESSION INACTIVE PENDING, and PDU SESSION MODIFICATION PENDING.
What is the difference between a PDU session state and a procedure transaction state?
A session state describes the position of the session itself, while a transaction state describes whether a PTI-based transaction is currently inactive or pending.
What types of procedures are defined in 5GSM?
The overview groups them into PDU-session-related procedures, transaction-related procedures, and the common 5GSM status procedure.
References
- 3GPP TS 24.501, Non-Access-Stratum (NAS) protocol for the 5G System
- 3GPP TS 23.501, System architecture for the 5G System
- 3GPP TS 23.502, Procedures for the 5G System