X2 and S1 based Handover for EPS Session Connected to SMF
This section describes the X2 and S1-Based Handover for EPS Session Connected to SMF.

Step | Description |
---|---|
1 | The SGW sends the Modify Bearer Request to the SMF. This request includes the user location information IE, UE time zone IE, and the serving network IE per PDN connection to the associated PDN GWs information that is received from the MME. |
2 | In case of change in S-GW, SMF or P-GW sends the PFCP Session Modification Request to the UPF. |
3 | If Step 2 occurs, the UPF sends the PFCP Session Modification Response to SMF or PDN GW. |
4 | After receiving the response from the UPF, the SMF or P-GW sends the Modify Bearer Response to S-GW. |
5 | If PCF has armed notification for QoS modification, the SMF or P-GW sends a notification to the PCF. |
6 | If Step 5 occurs, the PCF sends the “200 OK” acknowledgment to the SMF or P-GW. |
7 | If PCF has armed notification for ULI or RAT modifications, SMF or PDN GW sends a notification to PCF. |
8 | If Step 7 occurs, PCF sends the “200 OK” acknowledgment to SMF or PDN GW. |
9 | If CHF has armed notification for QoS, ULI, or RAT modifications, SMF or PDN GW sends a notification to PCF. |
10 | If Step 9 occurs, PCF sends the “200 OK” acknowledgment to SMF or PDN GW. |
The following call flow shows the X2-based handover with S-GW relocation:

For call flow description, see the section 5.5.1.1.3 "X2-based handover with Serving GW relocation" from 3GPP TS 23.401.
The following call flow shows the S1-based handover:

For call flow description, see section 5.5.1.2.2 "S1-based handover, normal" from 3GPP TS 23.401.