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

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. |