EPS Fallback Guard Timer Call Flow

This section describes the 5G to EPS fallback guard timer call flow.

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