5G PDU Session Establishment Procedure Explained

Introduction

The PDU Session Establishment procedure in 5G networks allows a User Equipment (UE) to establish a data connection with an external data network such as the internet.

A PDU Session is the equivalent of a data bearer in LTE, enabling the UE to send and receive IP packets.

During this procedure:

• the UE requests a data session
• the network allocates IP connectivity
• QoS parameters are configured
• user-plane tunnels are created

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

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

What is a PDU Session?

A Protocol Data Unit (PDU) Session represents a logical connection between the UE and a Data Network (DN).

Examples of data networks include:

• the public internet
• private enterprise networks
• operator services

Each PDU session is associated with:

• a PDU Session ID
• a QoS flow
• a User Plane path

Network Functions Involved

The PDU Session Establishment procedure involves several core network components.

UE (User Equipment)

Initiates the PDU session request when an application requires data connectivity.

gNB

The gNB provides radio access and forwards control plane signaling.

AMF (Access and Mobility Management Function)

Handles signaling and forwards session requests to the SMF.

SMF (Session Management Function)

The SMF is responsible for:

• session creation
• IP address allocation
• QoS policy management

UPF (User Plane Function)

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

Interfaces Used

PDU Session Establishment Call Flow

Below is the simplified signaling sequence.

UE           gNB           AMF            SMF           UPF
 |            |             |              |             |
 |---PDU Session Establishment Request--->|
 |            |----N2--------------------->|
 |            |             |----N11----->|
 |            |             |             |----N4----->|
 |            |             |             |<---N4------|
 |            |             |<---N11------|
 |<--PDU Session Establishment Accept----|
 |            |                           |
 |===== User Plane Tunnel Setup ========>|

After this procedure completes, the UE can start sending IP traffic.

Step-by-Step Explanation

Step 1: PDU Session Establishment Request

The UE sends a PDU Session Establishment Request NAS message to the AMF.

The request contains:

• PDU Session ID
• requested data network (DNN)
• session type (IPv4 / IPv6 / Ethernet)
• requested slice (S-NSSAI)

Important parameters to check

Engineers should verify:

• PDU Session ID
• DNN (Data Network Name)
• requested S-NSSAI
• session type

Step 2: Session Management Request

The AMF forwards the request to the SMF via the N11 interface.

The SMF:

• validates subscription data
• selects a suitable UPF
• allocates an IP address for the UE

Important parameters to check

Check:

• SMF selection
• subscriber session limits
• IP address allocation

Step 3: UPF Configuration

The SMF configures the UPF using the N4 interface.

This includes:

• creating a user plane tunnel
• defining forwarding rules
• assigning QoS parameters

Important parameters to check

Verify:

• UPF selection
• forwarding rules
• QoS flow parameters

Step 4: PDU Session Establishment Accept

Once resources are allocated, the network sends PDU Session Establishment Accept to the UE.

The message contains:

• allocated IP address
• QoS parameters
• session configuration

Important parameters to check

Look for:

• assigned IP address
• QoS flow identifier
• session AMBR values

Step 5: User Plane Tunnel Activation

Finally, the N3 tunnel is established between the gNB and UPF.

This enables user data to flow through the network.

Data path after session setup

UE → gNB → UPF → Data Network

At this point, the UE can start sending application traffic.

Common PDU Session Types

Troubleshooting PDU Session Failures

Session Establishment Reject

Possible causes:

• invalid DNN
• slice not allowed
• subscription restrictions

No IP Address Assigned

Possible reasons:

• SMF allocation failure
• UPF misconfiguration
• address pool exhaustion

Data Traffic Not Flowing

Possible causes:

• N3 tunnel failure
• QoS rule mismatch
• UPF routing issue

Key NAS Messages

Relevant 3GPP Specifications

The PDU Session Establishment procedure is defined in:

• 3GPP TS 23.502 - 5G System Procedures
• 3GPP TS 24.501 - NAS Protocol
• 3GPP TS 38.300 - NR Overall Description

Published by the 3rd Generation Partnership Project.

Summary

The PDU Session Establishment procedure creates a data connection between the UE and a data network in 5G architecture.

It involves coordination between multiple network functions including:

• AMF
• SMF
• UPF

Once completed, the UE receives an IP address and can begin sending data through the 5G user plane.