EPS to 5G Handover with N26 Interface – Preparation Call Flow

This section describes the call flow of the preparation of the EPS to 5G Handover with the N26 interface.

Preparation Call Flow for the EPS to 5G Handover with the N26 Interface
Preparation Call Flow Description for the EPS to 5G Handover with the N26 Interface
StepDescription
1Call handover initiation starts from UE and E-UTRAN toward each other, proceeds from E-UTRAN to the S-GW. Then for roaming calls, call handover initiation proceeds from S-GW to the UPF+P-GW-U.
2The E-UTRAN sends the Handover Call Request to the MME.
3The MME forwards the Relocation Request to the AMF.
4The AMF invokes the NsmfPDUSessionCreateSMContext service operation on SMF. The PGW-C+SMF address identifies this service operation. The service operations can be UE EPS PDN Connection, AMF ID, or Direct Forwarding Flag. The AMF then indicates the handover preparation to avoid switching the UP path. The SMF searches for the corresponding PDU session that is based on EPS Bearer Contexts. The AMF includes Direct Forwarding Flag to inform the SMF of the applicability of indirect data forwarding.
5

If you have deployed the dynamic PCC, the SMF+PGW-C initiates the SMF-initiated SM Policy Modification toward the PCF.

Important

Cisco SMF does not support this step in this release.

6The PGW-C+SMF sends the N4 Session Modification to PGW-U+UPF to establish the CN tunnel for a PDU Session. The PGW-U+UPF receives the uplink packets from NG-RAN. This step involves creating uplink PDRs and FARs for the 5G session along with the QFIs that are mapped from the existing 4G bearers.
7

The PGW-C+SMF sends a NsmfPDUSessionCreateSMContext Response to the AMF. This response includes PDU Session ID, S-NSSAI, and N2 SM Information.

The N2 SM Information includes PDU Session ID, S-NSSAI, QFIs, QoS Profiles, EPS Bearer Setup List, mapping between EBIs and QFIs, CN Tunnel information, and cause code details.

The SMF includes mapping between EBIs and QFIs as the N2 SM Information container. If the P-GW-C+SMF determines that session continuity from EPS to 5GS is not supported for the PDU session, then the P-GW-C+SMF does not provide the Session Manager information for the corresponding PDU session. However, the P-GW-C+SMF includes the cause code details for rejecting the PDU session transfer in the N2 SM information.

8The V-SMF and V-UPF establish an N4 session with each other.
9The AMF sends the Handover Request to NG-RAN.
10The NG-RAN sends an acknowledgment for the received Handover Request to the AMF.
11

The AMF sends a NsmfPDUSessionUpdateSMContext Request, T-RAN SM N3 forwarding information list message to the SMF for updating the N3 tunnel information.

The NsmfPDUSessionUpdateSMContext request includes a PDU Session ID, S-NSSAI, and N2 SM Information. The tunnel information exists in the NGAP IE DL Forwarding UP TNL Information of the Handoff Request Acknowledgment that is received from NG-RAN.

12The SMF+PGW-C performs the N4 session modification toward UPF+PGW-U to create the indirect tunnel to forward the DL data from eNodeB to NG-RAN. This step includes creating UL PDRs for the redirected DL data and associating FARs with them to forward the FARs to NG-RAN. The mapping of these PDRs and FARs is based on QFI and the corresponding bearer ID.
13The PGW-C+SMF sends the NsmfPDUSessionUpdateSMContext Response to the AMF. This response includes PDU Session ID, EPS Bearer Setup List, and CN tunnel information for data forwarding. At this point, the indirect tunnels are established for DL data forwarding.
14The AMF sends the Forward Relocation Response to the MME.
15The MME sends the creation request for the indirect data forwarding tunnel to the S-GW. The S-GW sends the response for the indirect data forwarding tunnel to the MME.