5G Inter-AMF Mobility Procedure Explained

Introduction

In 5G networks, Inter-AMF Mobility occurs when a User Equipment (UE) moves to an area served by a different Access and Mobility Management Function (AMF).

In such cases, the UE context must be transferred from the source AMF to the target AMF while maintaining the UE’s connectivity and mobility management.

This procedure ensures:

• continuous network registration
• seamless mobility across AMF regions
• uninterrupted data sessions

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

• 3GPP TS 23.502 - 5G System Procedures
• 3GPP TS 23.501 - System Architecture
• 3GPP TS 24.501 - NAS Protocol

When Inter-AMF Mobility Happens

Inter-AMF mobility typically occurs when:

• the UE moves into a different AMF service area
• the UE performs mobility registration update
• the network reassigns the UE to another AMF

Typical scenarios include:

• inter-region mobility
• load balancing between AMFs
• roaming scenarios

Network Functions Involved

UE (User Equipment)

Initiates the mobility update when entering a new registration area.

Source gNB

The base station currently serving the UE.

Target gNB

The base station serving the UE after mobility.

Source AMF

The current mobility management entity responsible for the UE.

Target AMF

The new AMF that takes over mobility management for the UE.

Interfaces Used

Inter-AMF Mobility Call Flow

Below is the simplified signaling sequence.

UE        Source gNB      Target gNB      Source AMF      Target AMF
 |            |               |              |               |
 |--Measurement Report------>|              |               |
 |            |--Handover Request---------->|               |
 |            |               |              |--Context----->|
 |            |               |              |               |
 |<--RRC Handover Command----|              |               |
 |====== UE moves to target gNB ======|
 |            |               |              |               |
 |----Registration Update------------------------------>|
 |                              |              |<--Context|
 |                              |              | Transfer |
 |                              |              |----------|
 |<--Registration Accept--------------------------------|

During this procedure, the UE moves to a new AMF while maintaining its session context.

Step-by-Step Explanation

Step 1: Measurement Reporting

The UE measures neighboring cells and reports measurements to the source gNB.

These measurements include:

• RSRP
• RSRQ
• SINR

Important parameters to check

Engineers should verify:

• measurement event triggers
• neighbor cell configuration
• signal quality thresholds

Step 2: Handover to Target gNB

The source gNB initiates a handover procedure to the target gNB.

If the target gNB belongs to a different AMF region, inter-AMF mobility will be triggered.

Important parameters to check

Check:

• target gNB configuration
• AMF region mapping
• neighbor relationships

Step 3: AMF Context Transfer

The source AMF transfers the UE context to the target AMF.

The context includes:

• UE identity
• security parameters
• mobility context
• session information

Important parameters to check

Verify:

• UE context transfer success
• security key continuity
• session management context

Step 4: Registration Update

The UE performs a Mobility Registration Update with the target AMF.

This step ensures that the UE is now managed by the new AMF.

Important parameters to check

Check:

• registration type
• UE identity (5G-GUTI / SUCI)
• tracking area list

Step 5: Registration Accept

The target AMF sends a Registration Accept message to the UE.

After this step:

• the UE is successfully registered with the new AMF
• mobility management continues normally

Data Path After Mobility

After inter-AMF mobility completes, the data path becomes:

UE -> Target gNB -> UPF -> Data Network

User sessions remain active during the mobility procedure.

Differences Between Inter-AMF and Inter-gNB Mobility

Troubleshooting Inter-AMF Mobility

Registration Update Failure

Possible causes:

• incorrect AMF configuration
• missing registration area configuration
• UE identity mismatch

Context Transfer Failure

Possible reasons:

• N8 interface connectivity issue
• AMF software mismatch
• security context transfer failure

Session Interruption

Possible causes:

• SMF session context mismatch
• UPF routing issues
• mobility configuration errors

Key Messages in Inter-AMF Mobility

Relevant 3GPP Specifications

The Inter-AMF Mobility procedure is defined by the 3rd Generation Partnership Project:

• 3GPP TS 23.502 - 5G System Procedures
• 3GPP TS 23.501 - System Architecture
• 3GPP TS 24.501 - NAS Protocol

Summary

The Inter-AMF Mobility procedure enables a UE to move between different AMF service areas while maintaining connectivity.

The process includes:

1. measurement reporting by the UE
2. handover to the target gNB
3. context transfer between AMFs
4. mobility registration update

This ensures seamless mobility across different core network regions in 5G networks.