Initial Attach on 4G for 5G Capable Device Call Flow
This section describes the Initial Attach on 4G for 5G Capable Device call flow.

Step |
Description |
---|---|
1 |
Established UPF association with control planes. cnSGW-C sends Sx Association Req/Rep to UPF. |
2 |
SMF sends Sx Association Req/Rep to UPF. |
3 |
MME sends Create Session Request to cnSGW-C after selecting co-located cnSGW-C and SMF. |
4 |
cnSGW-C sends Sx Establishment Req (SXA) to UPF after selecting UPF using DNN profile, Cell ID, and local factors. |
5 |
cnSGW-C receives Sx Establishment Res from UPF. |
6 |
cnSGW-C sends Create Session Request to SMF including SGW-U FQDN. |
7 |
SMF must find cnSGW-C selected UPF as per received SGW-U FQDN and select the same UPF to achieve data throughput. SMF tries to select same UPF when IP pools are available. Same UPF supports the PDN type. SMF may not select same UPF in few in-compatible scenarios. SMF send Sx Establishment Req N4 to UPF. |
8 |
SMF receives Sx Establishment Res-N4 from UP. |
9 |
cnSGW-C receives Create Session Response from SMF. |
10 |
MME sends Modify Bearer Request to cnSGW-C. |
11 |
cnSGW-C sends Sx Modify Req (SXA) to SMF. |
12 |
cnSGW-C receives Sx Modify Res (SXA) to SMF. |
13 |
cnSGW-C sends Modify Bearer Response to MME. |