Initial Attach with SGW-C/cnSGW and SMF/IWF
The following illustration describes the initial attach call flow with collapsed UPF.

Step | Description |
---|---|
1 | Create Session Request (CSReq) is received by SGW-C/cnSGW and it selects the UPF. |
2 |
The SGW-C/cnSGW sends Sx Establishment Request (Sxa) to the UPF. The UPF:
|
3 | The UPF sends Sx Establishment Response (Sxa) back to SGW-C/cnSGW. |
4 |
The SGW-C/cnSGW sends CSReq to SMF/IWF. The SMF/IWF selects the same UPF that is selected by SGW-C/cnSGW. |
5 | The SMF/IWF sends Sx Establishment Request (N4) to the UPF. |
6 | The UPF sends Sx Establishment Response (N4) to the SMF/IWF. |
7 | The SMF/IWF sends Create Session Response to the SGW-C/cnSGW. |
8 |
The SGW-C/cnSGW sends Sx Modify Request (Sxa) to the UPF. The UPF updates P-GW data F-TEID as part of Egress FAR. The UPF also interconnects Sxa and N4 session using internal logic and removes already-created Bearer Stream (3 tuple). |
9 | The UPF sends Sx Modify Response (Sxa) to the SGW-C/cnSGW. |
10 | The SGW-C/cnSGW sends Create Session Response to the MME. |
11 | The MME sends Modify Bearer Request to the SGW-C/cnSGW. |
12 |
The SGW-C/cnSGW sends Sx Modify Request (Sxa) to the UPF. The UPF updates eNodeB F-TEID as part of Ingress FAR. |
13 | The UPF sends Sx Modify Response to the SGW-C/cnSGW. |
14 | The SGW-C/cnSGW sends Modify Bearer Response to the MME. |