No data / PDU session failure: 5GSM cause codes, N4 PFCP failures

PDU Session Reject Causes

When a UE registers successfully but cannot establish a data session, the SMF sends a PDU Session Establishment Reject with a 5GSM cause code. Cause #26 "Insufficient resources" means the SMF or UPF cannot allocate session resources — typically IP address pool exhaustion, UPF resource limits reached, or the N4 PFCP session establishment between SMF and UPF failed. Cause #27 "Missing or unknown DNN" occurs when the DNN (Data Network Name) requested by the UE is not configured in the network, is misspelled in UE settings, or is absent from the subscriber's allowed DNN list in UDM. Cause #28…