5G to EPS Handover Call Flow

This section describes the 5G to EPS handover call flow with N26 interface.

The 5G to EPS Handover procedure for the EPS session is compliant with 3GPP 23.502, section 4.11.1.2.1.

  1. The AMF requests the SMF to provide the SM Context using Nsmf_PDUSession_ContextRequest.

  2. The SMF sends N4 Session Modification to the UPF to establish the CN tunnel for each EPS bearer. The bearer mapping to the 5G QoS and PCC rules received from PCC must already be present with the SMF. The SMF must also have the bearer IDs obtained from the Bearer ID Allocation procedure. The SMF creates new PDRs for the N4 session and gets TEID allocated for each bearer as required by the 4G system.

  3. The SMF provides EPS bearer contexts to the AMF. The SMF also provides the CN tunnel information to AMF for all bearers for the uplink traffic from E-UTRAN.

  4. If indirect data forwarding applies, the AMF sends the Nsmf_PDUSession_UpdateSMContext Request (S-GW address(es) and S-GW DL TEID(s) for data forwarding) to the SMF, for creating the indirect data forwarding tunnel.

  5. The SMF sends N4 Modification Request to the UPF to create additional PDRs and FARs to receive the redirected DL data over the indirect tunnel from NG RAN and forwards them to eNodeB. The uplink PDRs must have QFI to match the forwarded DL data from NG-RAN and the associated QER will not have QFI as data needs to be forwarded to the eNodeB. The FAR redirects the received data to the eNodeB over appropriate tunnel based on the QFI.

  6. The S-GW sends Modify Bearer Request to the SMF with DL TEIDs on the SMF for the bearers.

  7. The SMF sends N4 Modification Request to the UPF to activate the DL data path to E-UTRAN. At this time, both the indirect tunnel and the direct DL path are activated towards the eNodeB.

  8. The SMF sends the Modify Bearer Response to S-GW.

  9. The AMF initiates Nsmf_PDUSession_UpdateSMContext Request service operation with an indication to release the forwarding tunnels.

  10. The SMF sends N4 Modification Request to the UPF to remove the PDRs and FARs for the indirect tunnels. The PDRs and FARs for the 5G session which are not required are also removed.