5G Network Requested PDU Session Establishment Explained

Introduction

In 5G networks, a PDU Session is typically initiated by the User Equipment (UE) when an application requires connectivity to a data network.

However, there are scenarios where the network itself initiates the PDU session. This is known as Network Requested PDU Session Establishment.

This mechanism is useful for services such as:

• push notifications
• IoT device activation
• operator service triggers
• mobile terminated data sessions

The procedure is defined by the 3rd Generation Partnership Project in 3GPP TS 23.502.

Why Network Requested Sessions Are Needed

Normally, the UE initiates a session when it needs connectivity.

But sometimes the network must deliver data to the UE, even when the UE is idle.

Examples include:

• incoming application data
• push messages
• IoT device commands
• network-triggered services

To handle this scenario, the network triggers a Network Requested PDU Session Establishment.

Network Functions Involved

The procedure involves several 5G Core Network Functions.

UE (User Equipment)

The UE responds to paging and completes session establishment.

gNB

The gNB provides radio access and forwards signaling messages.

AMF (Access and Mobility Management Function)

The AMF handles mobility management and initiates paging.

SMF (Session Management Function)

The SMF triggers the PDU session establishment when the network needs to deliver data.

UPF (User Plane Function)

The UPF forwards user data packets between the UE and the data network.

Interfaces Used

Network Requested PDU Session Call Flow

Below is the simplified signaling sequence.

SMF            AMF            gNB            UE
 |              |              |              |
 |--Session Trigger----------->|
 |              |--Paging----->|
 |              |              |----Paging-->|
 |              |              |              |
 |              |<--Service Request-----------|
 |              |              |              |
 |<--N11--------|              |              |
 |              |              |              |
 |---PDU Session Establishment Command------>|
 |              |              |              |
 |<--PDU Session Establishment Complete------|

This flow shows how the network wakes up the UE and triggers session establishment.

Step-by-Step Explanation

Step 1: Session Trigger in SMF

The SMF receives a trigger indicating that data must be delivered to the UE.

Possible triggers include:

• application server request
• network policy trigger
• incoming packet detection at UPF

The SMF then initiates the session establishment procedure.

Important parameters to check

Engineers should verify:

• SMF trigger source
• PDU session ID
• data network name (DNN)

Step 2: UE Paging

If the UE is in Idle Mode, the AMF initiates paging.

Paging is sent through the gNB to locate the UE.

Important parameters to check

Check:

• paging identity (5G-S-TMSI)
• tracking area list
• paging response timer

Step 3: Service Request from UE

Once paged, the UE responds with a Service Request.

This message establishes a signaling connection with the network.

Important parameters to check

Verify:

• service type
• UE identity
• security context

Step 4: PDU Session Establishment Command

The network sends PDU Session Establishment Command to the UE.

This message instructs the UE to establish a data session.

It contains:

• PDU session ID
• QoS parameters
• session configuration

Important parameters to check

Look for:

• QoS Flow Identifier (QFI)
• session AMBR
• IP address allocation

Step 5: Session Completion

The UE confirms the session setup by sending PDU Session Establishment Complete.

After this step:

• user plane tunnels are activated
• data can flow between the UE and the network.

Data Path After Session Setup

Once the procedure completes, the data path is established.

UE -> gNB -> UPF -> Data Network

Common Use Cases

Troubleshooting Network Requested Sessions

UE Not Responding to Paging

Possible causes:

• UE out of coverage
• paging configuration error
• incorrect tracking area list

Session Establishment Fails

Possible reasons:

• SMF configuration issue
• incorrect subscription data
• slice restrictions

Data Not Delivered

Possible causes:

• UPF routing failure
• tunnel setup issue
• QoS mismatch

Key Messages

Relevant 3GPP Specifications

The procedure is defined by the 3rd Generation Partnership Project in:

• 3GPP TS 23.502 - 5G System Procedures
• 3GPP TS 24.501 - NAS Protocol

Summary

The Network Requested PDU Session Establishment procedure allows the 5G network to initiate data sessions toward a UE when incoming data needs to be delivered.

The procedure includes:

1. session trigger at the SMF
2. UE paging by the AMF
3. service request from the UE
4. session establishment signaling

This mechanism ensures that mobile devices can receive network-triggered data even when idle.