5G to EPS Handover with N26 Interface
5G to EPS handover with N26 interface is defined in 3GPP TS 23.502, Section 4.11.1.2.1. The following diagram shows the detailed call flow for N26 interface.

Step | Description |
---|---|
1 |
In Step 2b, the SMF+PGW-C sends the 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 is already present with SMF. The SMF also contains the bearer IDs obtained from the Bearer ID Allocation procedure. SMF+PGW-C creates new PDRs for the N4 session and gets TEID allocated for each bearer as required by the 4G system. |
2 |
In Step 10b, SMF+PGW-C sends N4 Modification Request to UPF to create additional PDRs and FARs to receive the redirected DL data over the indirect tunnel from NG RAN and forward them to eNodeB. The uplink PDRs in this case has the QFI to match forwarded DL data from NG RAN and the associated QER does not have the QFI as data needs to be forwarded to eNodeB. Also, the FAR redirects the received data to eNodeB over appropriate tunnel based on the QFI. |
3 |
At Step 11, for the QoS flows indicated in QoS Flows for Data Forwarding, NG-RAN initiates data forwarding through the UPF based on the CN Tunnel Info for Data Forwarding per PDU Session. Then the UPF maps data received from the data forwarding tunnel(s) in the 5GS to the data forwarding tunnel(s) in EPS and sends the data to the target eNodeB through the Serving GW. |
4 |
In Step 15, the SMF sends N4 Modification Request to 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 eNodeB. |
5 |
At Step 21, the SMF sends N4 Modification Request to the UPF to delete the indirect forwarding tunnel. |
Other call flows related to EPS to 5G and 5G to EPS handover with N26 interface, or without N26 interface are defined in 3GPP 23.502, Section 4.11.1.2.1 and Section 4.11.2.