Idle/Active DDN Handling with Collapsed UPF
The following illustration describes the Idle/Active DDN handling with collapsed UPF.

| Step | Description |
|---|---|
| 1 |
As part of UE initial attach, N4 and Sxa session is already established with SMF and UPF. At UPF, N4+Sxa session exists and are interconnected. At MME, the UE goes from Active to Ide state. The MME sends Release Access Bearer Request to the SGW-C/cnSGW. The SGW-C/cnSGW informs UPF to buffer packets. |
| 2 | The SGW-C/cnSGW sends Sx Modify Request (Sxa) to the UPF. The UPF updates FAR action towards eNodeB to buffer state. 6 tuple flows are onloaded for buffering. |
| 3 | The UPF sends Sx Modify Response (Sxa) back to SGW-C/cnSGW. |
| 4 | The MME sends Release Access Bearer Response to the SGW-C/cnSGW. |
| 5 | The ISP/Gi sends the received downlink packet to the UPF. The packet received by UPF at N4 session is passed to interconnect Sxa session for buffering. |
| 6 | The UPF sends Usage Report Request (Sxa) to the SGW-C/cnSGW. |
| 7 | The SGW-C/cnSGW sends Usage Report Response (Sxa) to the UPF. |
| 8 | The SGW-C/cnSGW sends DDN notification to the MME. |
| 9 | The MME sends DDN Acknowledgment back to the SGW-C/cnSGW. |
| 10 |
At MME, the UE moves from Idle to Active. The MME sends Modify Bearer Request to the SGW-C/cnSGW. The SGW-C/cnSGW:
|
| 11 |
The SGW-C/cnSGW sends Sx Modify Request (Sxa) to the UPF. The UPF:
|
| 12 | The UPF sends Sx Modify Response (Sxa) to the SGW-C/cnSGW. |
| 13 | The SGW-C/cnSGW sends Modify Bearer Response to the MME. |