5G Secondary PDU Session Setup Explained

Introduction

In 5G networks, a Secondary PDU Session is an additional data session established between the User Equipment (UE) and a Data Network (DN) while a primary PDU session already exists.

This allows the UE to connect to multiple data networks simultaneously.

For example, a UE may have:

• one PDU session for internet access
• another session for enterprise services
• another session for operator services

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

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

Why Secondary PDU Sessions Are Needed

Modern applications often require connectivity to multiple networks at the same time.

Using secondary PDU sessions, the UE can maintain multiple simultaneous data connections.

Network Functions Involved

Several 5G Core Network Functions participate in this procedure.

UE (User Equipment)

Initiates the secondary session request when a new network connection is required.

gNB

Provides radio connectivity and forwards signaling messages.

AMF (Access and Mobility Management Function)

Handles control-plane signaling and forwards session requests.

SMF (Session Management Function)

The SMF manages the creation of the secondary PDU session.

UPF (User Plane Function)

The UPF handles user plane traffic between the UE and the data network.

Interfaces Used

Secondary PDU Session Setup 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----|

Although the signaling looks similar to the primary PDU session establishment, the difference is that another session already exists.

Step-by-Step Explanation

Step 1: Secondary Session Request

The UE sends a PDU Session Establishment Request to create an additional session.

The message contains:

• new PDU Session ID
• requested Data Network Name (DNN)
• requested slice (S-NSSAI)

Important parameters to check

Engineers should verify:

• PDU session ID
• DNN
• S-NSSAI
• session type

Step 2: Session Management Processing

The AMF forwards the request to the SMF.

The SMF:

• verifies subscriber authorization
• selects a UPF
• allocates an IP address

Important parameters to check

Check:

• SMF selection
• subscription data
• slice authorization

Step 3: UPF Configuration

The SMF configures the UPF using the N4 interface.

The UPF creates:

• forwarding rules
• packet detection rules
• QoS enforcement rules

Important parameters to check

Verify:

• UPF selection
• QoS flow parameters
• forwarding rules

Step 4: Session Establishment Accept

The network sends PDU Session Establishment Accept to the UE.

The message contains:

• assigned IP address
• QoS parameters
• session configuration

Important parameters to check

Look for:

• PDU session ID
• QoS Flow Identifier (QFI)
• session AMBR

Step 5: User Plane Activation

Once configuration is complete, the user plane tunnel is activated.

Data can now flow through the secondary session.

Data Path After Setup

After the session is established, the data path becomes:

UE -> gNB -> UPF -> Data Network

Multiple sessions may exist simultaneously.

Differences Between Primary and Secondary Sessions

Troubleshooting Secondary PDU Sessions

Session Establishment Rejected

Possible causes:

• subscription restrictions
• slice mismatch
• DNN not allowed

No IP Address Assigned

Possible reasons:

• SMF configuration error
• address pool exhaustion
• UPF selection failure

Data Traffic Not Flowing

Possible causes:

• N3 tunnel failure
• QoS mismatch
• routing configuration issue

Key NAS Messages

Relevant 3GPP Specifications

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

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

Summary

The Secondary PDU Session Setup procedure allows a 5G device to connect to multiple data networks simultaneously.

This enables:

• enterprise connectivity
• network slicing
• advanced multi-service applications

Secondary PDU sessions are an important feature of flexible service delivery in 5G architecture.