EPS Fallback Guard Timer Call Flow
This section describes the 5G to EPS fallback guard timer call flow.

Step | Description |
---|---|
1 | gNB sends the dedicated bearer creation failure information with the fallback cause through AMF. |
2 | EPS fallback timer starts. |
In the successful EPS fallback with 5G to 4G handover scenario, Steps 3 –12 happen. | |
3 | EPS fallback timer stops and triggers pending dedicated bearer creation. |
4 | SMF(+S5-C) sends the PFCP session modification request to UPF(+S5-U). |
5 | PDR and FAR are created. |
6 | UPF(+S5-U) sends the PFCP session modification response to SMF(+S5-C). |
7 | The information on the created PDR with the GTP-U TEID is available. |
8 | SMF(+S5-C) sends the Create Bearer Request to SGW. |
9 | SGW sends the Create Bearer Response to SMF(+S5-C). |
10 | SMF(+S5-C) sends the PFCP Session Modification Request to UPF(+S5-U). |
11 | UPF(+S5-U) sends the notification of the successful dedicated bearer creation to PCF. |
12 | EPS fallback guard timer stops. |
13 | PCF sends the “200 OK” acknowledgment to SMF(+S5-C) |
In the EPS fallback timer expiry before handover completion scenario, Steps 14–16 happen. | |
14 | SMF(+S5-C) sends the failure notification of the dedicated bearer creation to PCF. |
15 | PCF sends the “200 OK” acknowledgment to SMF(+S5-C). |
16 | The 5G to 4G handover procedure continues. |