5G to 4G Handover Call Flow

This section describes the 5G to 4G handover call flow.

5G to 4G Handover Call Flow Description

1

The SRC GNB (source gNB) sends a Handover Required message to the SRC AMF (source AMF).

2, 3

The source AMF finds MME based on the received target ID. AMF finds the MME IP address through the AMF configuration or NRF discovery for the matching target ID.

4

When source AMF finds the IP address, it sends a Forward Relocation Request to the TGT-MME (target MME).

5

The target MME sets up resources at the enodeB, updates the S-GW, and responds to the source AMF with Forward Relocation Reponse.

6, 7

AMF requests SMF to setup Indirect Forwarding Tunnels.

The SMF responds to the request from the AMF.

8

The source AMF sends a Handover Command to the source gNB.

9

As the handover completes on the MME, the target MME notifies the source AMF by sending a Forward Relocation Complete Notification.

10

The source AMF responds with a Forward Relocation Complete ACK to target MME.

11

The AMF starts a timer to clean up all the local resources and indirect tunnels.

12, 13

The AMF requests the SMF to clean up the IDFT tunnels created.

The SMF responds to the AMF request.

14

The source AMF sends a UE Context Release Command to the source gNB to clean up all the resources at source gNB.

15

The source gNB responds with a UE Context Release Complete to the source AMF.