5G to 4G Handover with Collapsed UPF
The following illustration describes the 5G to 4G handover call flow with collapsed UPF.

Step | Description |
---|---|
1 |
As part of UE initial attach, N4 session is already established with SMF and UPF. The MME sends Create Session Request (CSReq) to SGW-C/cnSGW. If it’s a handover request, the SGW-C/cnSGW selects the same UPF that is selected by the SMF. |
2 |
The SGW-C/cnSGW sends Sx Establishment Request (Sxa) to the UPF. At UPF:
|
3 | The UPF sends Sx Establishment Response (Sxa) back to SGW-C/cnSGW. |
4 | The SGW-C/cnSGW sends Create Session Response to the MME. |
5 | The MME sends Modify Bearer Request to the SGW-C/cnSGW. |
6 |
The SGW-C/cnSGW sends Sx Modify Request (Sxa) to the UPF. The UPF updates eNodeB F-TEID for downlink data. |
7 | The UPF sends Sx Modify Response (Sxa) to the SGW-C/cnSGW. |
8 | The SGW-C/cnSGW sends Modify Bearer Request to the SMF/IWF. |
9 |
The SMF/IWF sends Sx Modify Bearer Request (N4) to the UPF. The UPF:
|
10 | The UPF sends Sx Modify Response (N4) to the SMF/IWF. |
11 | The SMF/IWF sends Sx Modify Response to the SGW-C/cnSGW. |
12 | The SGW-C/cnSGW sends Modify Bearer Response to the MME. |