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

Step |
Description |
---|---|
1 |
As the UE transitions to 5GC, the SRC-ENodeB (source eNB) sends a Handover Required message to the SRC MME (source MME). |
2 |
The MME does target-ID analysis, and chooses the TGT AMF (target AMF). The source MME sends a Forward Relocation Request to the TGT AMF (target AMF). |
3 |
The target AMF creates associations with the SMF for the PDU sessions that are acceptable on the target AMF. |
4 |
The SMF responds to the Create Request from the target AMF. |
5 |
The target AMF selects the TGT GNB (taget gNB) that serves the UE, and sends a Handover Request to the target GNB. |
6 |
After the GNB has allocated resources to the UE, it responds with a Handover Request ACK to the target AMF. |
7, 8 |
The target AMF updates the SMF with information from the target GNB. |
8 |
The SMF responds to the target AMF. |
9 |
The target AMF responds to the request from the source MME using a Forward Relocation Response message. |
10 |
The source MME sends a Handover Command to the SRC ENodeB (source eNB) to complete the handover. |
11 |
Target gNB sends a Handover Notify to the target AMF when the UE handover gets completed. |
12 |
The target AMF notifies the source MME using a Forward Relocation Complete notification. |
13 |
The source MME acknowledges the message with a Forward Relocation Complete acknowledge. |
14, 15 |
The target AMF notifies the SMF on the completion of the handover. The SMF acknowledges the notification. |
Note | When AMF receives the Registration Request message, it performs the following:
|