Feature Description
The EPS fallback failures occurred when the SMF received temporary errors with the N1N2 Transfer Request message during N2HO or XnHO collision.
The SMF sent N1N2TransferReq for PCF-Init-Modify procedure to enforce voice flows. The AMF detected collision with ongoing N2HO or XnHO and responded back with N1N2TransferResp — "409-Conflict" HTTP status code, "Handover-In-Progress" cause, or "Temporary-Reject-Registration-Ongoing" cause. The SMF was unable to retransmit the N1N2TransferReq message after N2HO or XnHO so that gNB could send N2Resp with EPS fallback. Since the SMF was unable to send the N1N2TransferReq message, it led to EPSFB failures.
To handle the EPSFB failures, the SMF performs retransmissions for the N1N2TransferReq message based on N11-failure-profile configured under access-Profile. Once all the retries are exhausted, the SMF increments with failure reason "N1N2XferMaxRetryExhausted" under PDU Modify procedure.