Home / Call Flows / 5G / Network Requested PDU Session

5G Network Requested PDU Session Call Flow

call-flow 5G NR | 5GC | Paging | NAS | SMF | UPF

5G Network Requested PDU Session is the network-triggered session setup flow used when incoming data or service needs to reach a UE that is not currently ready for delivery.

It combines reachability recovery, NAS service restoration, and session establishment into one end-to-end path.

Introduction

This procedure matters most when engineers need to explain how mobile-terminated services, push traffic, or operator-triggered services reach a UE that may be idle or inactive.

The most reliable troubleshooting path is to separate three checkpoints: paging success, Service Request success, and the correctness of the created session profile.

What Is Network Requested PDU Session in Simple Terms?

What starts the procedure: Incoming data or service requires the network to activate a session toward the UE.
What the UE and network want to achieve: Restore reachability and create the needed session quickly enough for the pending service.
What success looks like: The UE responds, the session is created, and the pending traffic is delivered.
What failure means: Reachability recovery, session creation, or actual service delivery breaks down.

Why this procedure matters

Many mobile-terminated data issues are misdiagnosed as pure paging trouble or pure session trouble. This page keeps the whole chain visible so engineers can see exactly where recovery stopped.

Quick Fact Sheet

Procedure name	5G Network Requested PDU Session Establishment
Domain	5G NR + 5GC network-triggered data-session setup
Main trigger	The network needs to deliver data or service toward a UE that does not currently have the needed active session
Start state	UE may be idle or inactive, and the target session is not active for the pending service
End state	The network-triggered session is created and the UE becomes reachable for the intended data or service delivery
Main nodes	UE, gNB, AMF, SMF, UPF, application or service trigger source
Main protocols	Paging, NAS Service Request, NAS session establishment, NGAP, PFCP
Main success outcome	The UE responds to paging, resumes connectivity, and the requested session becomes usable
Main failure outcome	Paging succeeds but session setup fails, or session setup succeeds without useful service delivery
Most important messages	Paging, Service Request, PDU Session Establishment Command or Accept, PDU Session Establishment Complete
Main specs	TS 23.502, TS 24.501, TS 23.501, TS 29.244

5G Network Requested PDU Session procedure flow

Preconditions

The network has a valid reason to deliver data or service toward the UE.
The target subscriber can be paged and restored into an active signaling context.
The requested DNN, slice, and policy treatment are allowed for the session.
SMF and UPF can create the session after the UE becomes reachable.

Nodes and Interfaces

Nodes involved

Node	Role in this procedure
UE	Responds to paging, restores signaling connectivity, and completes setup of the network-requested session.
gNB	Transmits paging, restores access context, and supports the data path once the session is ready.
AMF	Coordinates paging and signaling restoration toward the UE.
SMF	Owns the decision to activate the requested session for incoming data or service.
UPF	Anchors user-plane traffic for the session once it is created.
Service trigger source	Represents the application or network event that caused the session request.

Interfaces used

Interface	Path	Role
NR-Uu	UE <-> gNB	Carries paging response, restored signaling, and later user-plane traffic.
N1	UE <-> AMF	Carries Service Request and session setup results.
N2	gNB <-> AMF	Drives paging and access-side restoration.
N11	AMF <-> SMF	Coordinates the network-triggered session request.
N4 / N3	SMF <-> UPF and gNB <-> UPF	Create and carry the new data path.

End-to-End Call Flow

Service / SMF         AMF                gNB                UE                UPF
|                    |                  |                  |                  |
|-- trigger -------->|                  |                  |                  |
|                    |-- Paging ------->|----------------->|                  |
|                    |<-- Service Req --|<-----------------|                  |
|                    |-- setup session ------------------------------------->|
|<================ pending data delivered over new session ================>|

Major Phases

Phase	What happens
1. Incoming service need appears	An application or network trigger indicates data must be delivered toward the UE.
2. Reachability recovery	The network pages the UE and restores signaling connectivity if the UE is idle or inactive.
3. Session setup	The SMF creates the session needed for the requested service.
4. UE completion	The UE accepts the setup and the session becomes usable.
5. Service delivery	Real data or service traffic uses the new session.

Step-by-Step Breakdown

The network decides the UE needs a new or resumed session

Sender -> receiver: Application trigger -> SMF / AMF

Message(s): Service trigger and session decision

Purpose: Identify that incoming data cannot be delivered with the current UE state.

State or context change: The network prepares reachability recovery and session activation.

Note: Keep the trigger context clear so you can explain why paging and session setup happened together.

The UE is paged and reachability is restored

Sender -> receiver: AMF -> gNB -> UE

Message(s): Paging

Purpose: Wake the UE and recover a signaling path if it is not currently connected.

State or context change: The UE moves from idle or inactive reachability into active signaling exchange.

Note: A paging success does not prove the later session setup is healthy. Treat them as related but separate checkpoints.

The UE sends Service Request

Sender -> receiver: UE -> gNB -> AMF

Message(s): Service Request

Purpose: Restore service connectivity so the network can finish delivering the requested session.

State or context change: The access and NAS path are re-established for further signaling.

Note: If Service Request fails, the session problem is really a reachability-recovery problem first.

The network creates the requested session

Sender -> receiver: AMF <-> SMF <-> UPF

Message(s): Network-triggered session establishment and PFCP setup

Purpose: Create the session that matches the incoming service need.

State or context change: The core network binds session, policy, and forwarding state to the restored UE context.

Note: Validate DNN, slice, and service intent carefully because these sessions are often created under time pressure from incoming traffic.

The UE completes session activation and data arrives

Sender -> receiver: SMF -> AMF -> UE and user plane

Message(s): Session completion and first service packets

Purpose: Move from signaling success into real delivery of the pending data or service.

State or context change: The network-requested session becomes operational.

Note: The strongest proof of success is pending traffic actually reaching the UE on the new session.

Important Messages in This Flow

Message	Protocol	Direction	Purpose in this procedure	What to inspect briefly
Paging	RRC / NGAP	AMF -> gNB -> UE	Recovers reachability toward the UE.	Check identity, paging area, and response timing.
Service Request	NAS	UE -> AMF	Restores active service connectivity.	Use it to confirm the UE responded to paging correctly.
PDU Session Establishment Command or Accept	NAS	Network -> UE	Creates the requested session after reachability is restored.	Inspect session ID, QoS, and service profile.
PDU Session Establishment Complete	NAS	UE -> Network	Confirms the UE activated the session locally.	Useful for separating session-activation failure from later traffic failure.

Important Parameters to Inspect

Parameter	What it is	Where it appears	Why it matters	Common issues
Trigger source	Why the network initiated the session.	Before paging and session setup	Explains the operational reason for the whole sequence.	Without it, the flow looks arbitrary.
Paging identity and area	How the network tries to reach the UE.	Paging phase	Important for reachability troubleshooting.	Wrong area or stale identity breaks recovery before session setup starts.
PDU Session ID	Identifier of the requested session.	Establishment phase	Lets you correlate the incoming service need with the created session.	Wrong ID causes confusing post-recovery behavior.
DNN and slice	Requested service environment for the session.	Session creation and acceptance	Critical for proving the network created the intended session.	Wrong profile creates a usable but incorrect session.
First downlink packets	Real service traffic delivered after setup.	Post-establishment validation	Best final proof that the network-requested path worked.	Without real traffic, success is only partial.

Success Criteria

The network has a clear trigger for the session and pages the correct UE successfully.
The UE restores signaling using Service Request or equivalent recovery steps.
The created session has the right DNN, slice, QoS, and session identity.
Pending traffic or service actually reaches the UE after setup.

Common Failures and Troubleshooting

Symptom	Likely cause	Where to inspect	Relevant message(s)	Relevant interface(s)	Likely next step
UE never responds to paging	Reachability is not restored, so session setup cannot complete meaningfully.	Paging identity, area, and UE availability.	Paging	NR-Uu, N2	Treat this as a reachability failure first.
Paging succeeds but Service Request fails	The UE wakes up but cannot restore the signaling path needed for session setup.	Service Request and access restoration logs.	Service Request	N1, N2	Separate paging recovery from NAS recovery.
Session is created but the wrong service profile is used	The network-triggered setup selected the wrong DNN, slice, or QoS profile.	Session parameters and first traffic behavior.	Establishment Accept or Command	N1, N11	This is a correctness issue, not just a reachability issue.
Session completes but pending data still does not arrive	The user plane or application path is still wrong after setup.	PFCP state, N3 path, and first downlink packets.	Complete and post-session traffic	N3, N4, service layer	The real test is delivery of the originally pending traffic.

What to Check in Logs and Traces

Trace the incoming service trigger before looking at paging.
Validate paging identity and area separately from later session outcomes.
Use Service Request as the main checkpoint for restored UE reachability.
Inspect the created session for DNN, slice, QoS, and PDU Session ID correctness.
Prove success with the first pending downlink packets, not just completion signaling.

Related sub-procedures

Related message reference pages

Related troubleshooting pages

Notes

Network-requested sessions are a chain procedure. Treat paging, reachability recovery, and session correctness as separate checkpoints even though they happen close together.

FAQ

What is Network Requested PDU Session Establishment?

It is a 5G procedure where the network triggers creation of a session because incoming data or service needs to reach the UE.

Why is paging often part of it?

Because the UE may be idle or inactive and must first become reachable again.

What proves success?

The UE responds, the session is created with the right profile, and the pending traffic actually reaches the UE.

How is it different from normal session establishment?

The trigger comes from the network or incoming service, not from the UE starting a new application flow.

What should I inspect first?

Start with the incoming trigger, paging, Service Request, and then the created session profile.