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.

Execution Call Flow for the EPS to 5G Handover with the N26 Interface
Execution Call Flow Description for the EPS to 5G Handover with the N26 Interface
StepDescription
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.

2The E-UTRAN sends the handover command to the UE.
3The UE sends the confirmation message to NG-RAN for the received handover to 5G-RAN.
4The NG-RAN sends the Handover Notification message to the AMF.
5The AMF sends the Forward Relocation Complete Notification to the MME.
6The MME sends the Acknowledgment Response for the received Forward Relocation Complete Notification.
7The 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.
8The 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.

Important
Cisco SMF does not support this step in this release.
10The SMF sends NsmfPDUSessionUpdateSMContext Response, with PDU Session ID, to AMF. The SMF confirms the reception of Handover Complete.
11After 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.
12The UE starts the EPS to 5GS mobility registration procedure and sends it to H-PCF.
13The E-UTRAN performs the resource cleanup in EPC by MME.