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5G User Plane Establishment Call Flow

5G User Plane Establishment explains how a session becomes a real data path instead of only a control-plane promise.

It combines PFCP forwarding setup, NGAP access resource creation, and first live packets into one operational procedure.

Introduction

This page is useful whenever a session appears to exist on paper but traffic still does not pass.

The strongest troubleshooting pattern is to compare what the gNB was asked to create, what the UPF actually installed, and what the first real packets did afterward.

What Is User Plane Establishment in Simple Terms?

What starts the procedure: A session needs an operational user-plane path.
What the UE and network want to achieve: Make packets flow reliably between UE, gNB, and UPF.
What success looks like: Both control and packet traces show the intended path is active.
What failure means: The path is missing, partial, stale, or mis-correlated.

Why this procedure matters

Many 5G incidents are not registration failures or session failures at all. They are user-plane-establishment failures where the session exists but the packets never got a healthy path.

Quick Fact Sheet

Procedure name	5G User Plane Establishment
Domain	5G NR + 5GC data-path creation between access and UPF
Main trigger	A PDU session or service needs an operational user-plane path
Start state	Control-plane context exists, but the final N3 path is not yet proven active
End state	Packets can flow between UE, gNB, and UPF over the intended user-plane path
Main nodes	UE, gNB, AMF, SMF, UPF
Main protocols	NGAP, PFCP, NAS support signaling, GTP-U
Main success outcome	The intended user-plane tunnel and access resources are active and carry traffic correctly
Main failure outcome	The session looks ready in control signaling but the packet path is missing, stale, or misrouted
Most important messages	PDU Session Resource Setup Request/Response, PFCP session establishment or update, first user-plane packets
Main specs	TS 23.502, TS 23.501, TS 29.244, TS 38.413

Preconditions

The UE has enough control-plane context to activate a session path.
The SMF can install or update UPF forwarding state.
The gNB can create the access-side user-plane resources.
Traffic or test packets are available to validate the path.

Nodes and Interfaces

Nodes involved

Node	Role in this procedure
UE	Generates the service packets that validate whether the path is truly usable.
gNB	Terminates the radio side and anchors the N3 tunnel toward the UPF.
AMF	Coordinates the control path that asks the gNB to set up user-plane resources.
SMF	Builds the tunnel and forwarding logic needed for session traffic.
UPF	Owns the user-plane endpoint, forwarding behavior, and packet handling for the session.

Interfaces used

Interface	Path	Role
NR-Uu	UE <-> gNB	Carries the radio-side packets for the session.
N2	gNB <-> AMF	Delivers access-side resource setup signaling.
N11	AMF <-> SMF	Connects session control to the access request.
N4	SMF <-> UPF	Creates or updates the user-plane forwarding state.
N3	gNB <-> UPF	Carries the live GTP-U traffic after setup.

End-to-End Call Flow

UE                gNB                AMF                SMF                UPF
|                  |                  |                  |                  |
|                  |<-- Resource Setup Request -----------|                  |
|                  |-- Resource Setup Response ----------->|                  |
|                  |                  |                  |-- PFCP install -->|
|==== first uplink and downlink packets over N3 ==================================>|

Major Phases

Phase	What happens
1. Session path need	A service or session event requires a usable user-plane path.
2. Core path preparation	SMF and UPF prepare the data path state.
3. Access resource setup	AMF instructs the gNB to establish access-side session resources.
4. First packet validation	Traffic checks that the N3 path is operational.
5. Stable data transfer	The path carries traffic normally and predictably.

Step-by-Step Breakdown

The network decides a user-plane path must become active

Sender -> receiver: Session logic -> SMF / AMF

Message(s): PDU session setup or resume context

Purpose: Trigger the work needed to make data packets flow, not just signaling state exist.

State or context change: The session transitions from planned connectivity to executable transport setup.

Note: This page focuses on the data path itself rather than the broader registration or session story around it.

SMF prepares forwarding state in the UPF

Sender -> receiver: SMF <-> UPF

Message(s): PFCP session establishment or update

Purpose: Create the rules that allow packets to be forwarded through the intended UPF path.

State or context change: The core-side user-plane anchor is ready or nearly ready.

Note: Without correct PFCP state, later access-side success cannot create a working packet path by itself.

The gNB receives resource setup instructions

Sender -> receiver: AMF -> gNB

Message(s): PDU Session Resource Setup Request

Purpose: Tell the RAN what transport and QoS context to activate for the session.

State or context change: The access side begins creating the path that meets the core-side UPF state.

Note: This is the most important checkpoint for proving the gNB got the intended session context.

The gNB confirms resource creation

Sender -> receiver: gNB -> AMF

Message(s): PDU Session Resource Setup Response

Purpose: Acknowledge that the access-side transport path was created.

State or context change: The network now claims the path should be live end to end.

Note: Treat this as a strong clue, not final proof. The first packets still matter most.

Live traffic validates the user plane

Sender -> receiver: UE <-> gNB <-> UPF

Message(s): First uplink and downlink packets

Purpose: Prove that the N3 path, gNB context, and UPF rules work together in reality.

State or context change: The session reaches operational data transfer.

Note: If the first packets fail, the user plane was never truly established no matter how good the control plane looked.

Important Messages in This Flow

Message	Protocol	Direction	Purpose in this procedure	What to inspect briefly
PDU Session Resource Setup Request	NGAP	AMF -> gNB	Requests access-side setup for the session user plane.	Best view of what the gNB was asked to create.
PDU Session Resource Setup Response	NGAP	gNB -> AMF	Confirms the gNB finished the requested setup.	Use it to compare requested vs accepted resources.
PFCP Session Establishment or Update	PFCP	SMF <-> UPF	Creates the forwarding state in the UPF.	Critical for proving the user plane exists in the core.
First user-plane packets	GTP-U	UE <-> UPF via gNB	Reveal whether the path actually carries traffic.	The most important operational checkpoint.

Important Parameters to Inspect

Parameter	What it is	Where it appears	Why it matters	Common issues
PDU Session ID	The session whose user plane is being created.	NGAP, PFCP, NAS context	Anchor for every later correlation step.	Wrong session ID is a common source of confusion.
Tunnel endpoint context	Transport identifiers used on the active path.	NGAP and GTP-U state	Shows whether the gNB and UPF agree on the path.	Mismatch causes blackholing immediately.
QoS and DRB mapping	Access-side treatment tied to the data path.	Resource setup signaling	Ensures the data path is not only alive but correctly treated.	Missing mapping often looks like path failure.
PFCP forwarding rules	UPF-side logic for the session packets.	N4 trace	Best proof of core-side readiness.	Stale or partial rules create silent failure.
First packet timing	Delay between setup and real traffic.	Live traces	Useful for proving whether setup converged quickly or stalled.	Long gaps can expose hidden setup issues.

Success Criteria

The correct session is targeted for user-plane setup.
PFCP creates the expected forwarding state in the UPF.
The gNB accepts and creates the requested access-side resources.
First packets pass correctly over the intended path.

Common Failures and Troubleshooting

Symptom	Likely cause	Where to inspect	Relevant message(s)	Relevant interface(s)	Likely next step
Setup responses are present but packets never flow	Control-plane setup succeeded, but the real user-plane path is broken.	PFCP state, tunnel IDs, and first packets.	Resource Setup Response	N3, N4	This is the defining user-plane-establishment failure.
Uplink works but downlink fails	The path is only partially correct or forwarding state is asymmetric.	First uplink vs downlink packets and UPF state.	Live packet traces	N3, GTP-U	Always inspect directionality explicitly.
The wrong session path is activated	Resource setup correlated to the wrong session context.	PDU Session ID across NGAP and PFCP traces.	Setup Request / Response	N2, N4	This is especially easy to miss on busy UEs.
The path comes up with the wrong QoS behavior	The transport exists, but QoS or DRB mapping is inconsistent.	QoS and DRB mapping plus packet KPIs.	Resource setup signaling	NR-Uu, N3	Treat this as a user-plane establishment defect, not only a QoS issue.

Related sub-procedures

Related message reference pages

Related troubleshooting pages

FAQ

What is User Plane Establishment?

It is the process of making the real packet path active between the UE, gNB, and UPF.

How is it different from PDU Session Establishment?

PDU Session Establishment is the broader session procedure; user-plane establishment focuses specifically on the operational data path.

What proves success?

The intended N3 path comes up and first packets pass correctly in both directions.

What should I inspect first?

Start with PDU Session ID, PFCP state, PDU Session Resource Setup messages, and first packets.

Why are first packets more important than control-plane success?

Because the packet path is the actual service outcome; signaling only predicts that outcome.