Idle/Active DDN Handling with Collapsed UPF

The following illustration describes the Idle/Active DDN handling with collapsed UPF.

Idle/Active DDN Handling with Collapsed UPF Call Flow Description
Step Description
1

As part of UE initial attach, N4 and Sxa session is already established with SMF and UPF. At UPF, N4+Sxa session exists and are interconnected. At MME, the UE goes from Active to Ide state.

The MME sends Release Access Bearer Request to the SGW-C/cnSGW. The SGW-C/cnSGW informs UPF to buffer packets.

2 The SGW-C/cnSGW sends Sx Modify Request (Sxa) to the UPF. The UPF updates FAR action towards eNodeB to buffer state. 6 tuple flows are onloaded for buffering.
3 The UPF sends Sx Modify Response (Sxa) back to SGW-C/cnSGW.
4 The MME sends Release Access Bearer Response to the SGW-C/cnSGW.
5 The ISP/Gi sends the received downlink packet to the UPF. The packet received by UPF at N4 session is passed to interconnect Sxa session for buffering.
6 The UPF sends Usage Report Request (Sxa) to the SGW-C/cnSGW.
7 The SGW-C/cnSGW sends Usage Report Response (Sxa) to the UPF.
8 The SGW-C/cnSGW sends DDN notification to the MME.
9 The MME sends DDN Acknowledgment back to the SGW-C/cnSGW.
10

At MME, the UE moves from Idle to Active. The MME sends Modify Bearer Request to the SGW-C/cnSGW. The SGW-C/cnSGW:

  • Updates eNodeB F-TEID in FAR

  • Updates FAR action to Forward

11

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

  • Updates eNodeB F-TEID for downlink data

  • Updates TEP entries at VPP with new Remove TEID

  • Releases buffered packets by finding respective 6 tuple streams

12 The UPF sends Sx Modify Response (Sxa) to the SGW-C/cnSGW.
13 The SGW-C/cnSGW sends Modify Bearer Response to the MME.