5G to 4G Handover with Collapsed UPF

The following illustration describes the 5G to 4G handover call flow with collapsed UPF.

5G to 4G Handover with Collapsed UPF Call Flow Description
Step Description
1

As part of UE initial attach, N4 session is already established with SMF and UPF.

The MME sends Create Session Request (CSReq) to SGW-C/cnSGW. If it’s a handover request, the SGW-C/cnSGW selects the same UPF that is selected by the SMF.
2

The SGW-C/cnSGW sends Sx Establishment Request (Sxa) to the UPF. At UPF:

  • SxDemux selects the same SessMgr instance extracted from the P-GW F-TEID that is received in FAR. Both Sxa and N4 session are on the same SessMgr.

  • Allocates Sxa session

  • Allocates S-GW Ingress and Egress local F-TEID

  • Interconnects Sxa and N4 session using internal logic and doesn’t install Bearer Stream (3 tuple)
3 The UPF sends Sx Establishment Response (Sxa) back to SGW-C/cnSGW.
4 The SGW-C/cnSGW sends Create Session Response to the MME.
5 The MME sends Modify Bearer Request to the SGW-C/cnSGW.
6

The SGW-C/cnSGW sends Sx Modify Request (Sxa) to the UPF.

The UPF updates eNodeB F-TEID for downlink data.
7 The UPF sends Sx Modify Response (Sxa) to the SGW-C/cnSGW.
8 The SGW-C/cnSGW sends Modify Bearer Request to the SMF/IWF.
9

The SMF/IWF sends Sx Modify Bearer Request (N4) to the UPF. The UPF:

  • Updates N4 session FAR towards S-GW with F-TEID

  • Updates TEP entries at VPP with new F-TEID
10 The UPF sends Sx Modify Response (N4) to the SMF/IWF.
11 The SMF/IWF sends Sx Modify Response to the SGW-C/cnSGW.
12 The SGW-C/cnSGW sends Modify Bearer Response to the MME.