Non-roaming EPS to 5GS Handover with Alternate RAT Tunnel Creation Call Flow

This section describes the call flow of non-roaming EPS to 5GS handover with an alternate RAT tunnel creation.

Non-roaming EPS to 5GS Handover with Alternate RAT Tunnel Creation Call Flow Description
Step	Description
1	A 5G session is established among UE, E-UTRAN, NG-RAN, MME, AMF, S-GW, and SMF+PGW-C. E-UTRAN initiates handover towards MME.
2	MME sends the Forward Relocation Request to AMF.
3	AMF sends the Nsmf PDU Session CreateSMContext request to SMF+PGW-C. This request includes details on the UE PDN connection and handover state.
4	SMF+PGW-C checks for the following procedures: N10 subscription or you can subscribe, if required. N7 Policy Update, if trigger is enabled. N10 Registration, if required. SMF+PGW-C sends the Nsmf PDUSession CreateSMContext Response to AMF. This response includes the N2 Setup Request with the UPF UL tunnel information.
5	AMF sends the handover request to NG-RAN.
6	NG-RAN sends the acknowledgment of the handover request to AMF.
7	AMF sends the Nsmf PDU Session Update SMF Context Request to SMF+PGW-C. This request includes the handover request with the gNB DL tunnel information.
8	SMF+PGW-C sends the Nsmf PDU Session Update SMF Context Response to AMF.
9	AMF sends the Forward Relocation Response to MME.
10	MME sends the Handover Command to E-UTRAN.
11	E-UTRAN sends the Handover Command to UE.
12	UE sends the message on handover completion to NG-RAN.
13	NG-RAN sends the Handover Notify message to AMF.
14	AMF sends the Nsmf PDU Session Update SMF Context Request to SMF+PGW-C after the completion of the handover.
15	SMF+PGW-C sends the N4 Session Modification Request to UPF. This request is for updating the DL FAR with the gNB tunnel information.
16	The 5G Tunnel is updated with gNB tunnel information for the DL data. UPF sends the N4 Modification Response to SMF+PGW-C.
17	SMF+PGW-C sends the Nsmf PDU Session Update SMContext Response to AMF.