EPS to 5G Handover with N26 Interface – Execution Call Flow
This section describes the call flow of the execution of the EPS to 5G Handover with the N26 interface.

Step | Description | ||
---|---|---|---|
1 |
Call handover initiation starts from the UE and E-UTRAN toward each other, proceeds from E-UTRAN to S-GW. Then for roaming calls, call handover initiation proceeds from S-GW to the UPF+P-GW-U. The MME sends the handover command to E-UTRAN. | ||
2 | The E-UTRAN sends the handover command to the UE. | ||
3 | The UE sends the confirmation message to NG-RAN for the received handover to 5G-RAN. | ||
4 | The NG-RAN sends the Handover Notification message to the AMF. | ||
5 | The AMF sends the Forward Relocation Complete Notification to the MME. | ||
6 | The MME sends the Acknowledgment Response for the received Forward Relocation Complete Notification. | ||
7 | The AMF sends NsmfPDUSessionUpdateSMContext Request to SMF +PGW-C. This request includes Handover Complete Indication for PDU Session ID details. For indirect forwarding, a timer in SMF+PGW-C starts to check when resources in UPF are to be released. | ||
8 | The SMF performs N4 Modification Request with UPF+PGW-U to update the DL tunnel information for the FARs that are associated with DL PDRs of the 5G session. The DL data path is activated. At this point, the indirect tunnel also exists. | ||
9 |
The SMF informs PCF of the RAT type change.
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10 | The SMF sends NsmfPDUSessionUpdateSMContext Response, with PDU Session ID, to AMF. The SMF confirms the reception of Handover Complete. | ||
11 | After the timer that started in Step 7 expires, the SMF sends N4 Modification Request to UPF. This request is to remove the PDRs and FARs that are associated with the indirect data tunnel. | ||
12 | The UE starts the EPS to 5GS mobility registration procedure and sends it to H-PCF. | ||
13 | The E-UTRAN performs the resource cleanup in EPC by MME. |