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

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

Non-roaming 5GS to EPS Handover with Alternate RAT Tunnel Creation Call Flow Description
Step	Description
1	A 5G session is established among UE, E-UTRAN, NGRAN, MME, AMF, S-GW, and SMF+PGW-C. NG-RAN initiates handover towards AMF.
2	AMF sends the Nsmf PDU Session Context Request to SMF+PGW-C.
3	SMF+PGW-C sends the Nsmf PDU Session CreateSMContext Response to AMF. This response includes details on the CP and UP tunnels for both the default and dedicated bearers.
4	AMF sends the Relocation Request to MME.
5	MME sends the Create Session Request to S-GW.
6	S-GW sends the Create Session Response to MME.
7	MME sends the Handover Request to E-UTRAN.
8	E-UTRAN sends the acknowledgment of the Handover Request to MME.
9	MME sends the Relocation Response to AMF.
10	AMF sends the handover command to NG-RAN.
11	NG-RAN sends the handover command to UE.
12	UE sends the handover completion message to NG-RAN.
13	The UL data path is set up among UE, E-UTRAN, NG-RAN, MME, AMF, S-GW, SMF+PGW-C, and UPF. E-UTRAN sends the Handover Notify message to MME.
14	MME sends the notification on the relocation completion to AMF.
15	AMF sends the acknowledgment message of relocation completion to MME.
16	MME sends the Modify Bearer Request to S-GW.
17	S-GW sends the Modify Bearer Request to SMF+PGW-C.
18	SMF+PGW-C sends the N4 Session Modification Request to UPF. This request includes details for updating DL FAR with S-GW tunnel for both the default and dedicated bearers.
19	The 4G Tunnels are updated with DL FAR information (S-GW tunnel information). SMF+PGW-C sends the Modify Bearer Response to S-GW.
20	The DL data path is set up among UE, E-UTRAN, NG-RAN, MME, AMF, S-GW, SMF+PGW-C, and UPF. S-GW sends the Modify Bearer Response to MME.