The following call flow depicts the network-initiated idle to active transition.
Idle to Active Transition (Network-initiated) Call Flow
Idle to Active Transition (Network-initiated) Call Flow Description
Step
Description
1
The UPF sends "PFCP session report request" to the SMF.
Report Type as DLDR (Downlink Data Report).
The "Downlink Data Report" IE contains corresponding "PDR ID".
2
The SMF sends the PFCP session report response.
3
The SMF sends "N1N2MessageTransfer" to AMF with the following attributes:
SUPI, PDU Session ID,
N2SMInformation as "ngapIeType":77 (id-PDUSessionResourceSetupListSUReq), "ngapMessageType":27 (id-PDUSessionResourceSetup).
PDUSessionResourceSetupListSUReq has the PDU session id, QFI, QoS profile, UPF's GTP-U F-TEID for uplink traffic, QFI, QoS profile, S-NSSAI, User Plane Security Enforcement, UE Integrity Protection Maximum Data Rate, and Cause.
Area of validity for N2 SM information, ARP, Paging Policy Indication, 5QI, N1N2TransferFailure Notification Target Address (n1n2FailureTxfNotifURI).
4
The SMF receives the “N1N2TransferResponse” with the following status codes:
200/202 OK and cause as "N1_N2_TRANSFER_INITIATED" (proceed to Step 6).
409/504 and Cause “UE_IN_NON_ALLOWED_AREA” (proceed to Step 7).
5
The AMF sends the N1N2 Transfer failure response. If the UE is not reachable, move to Step 7.
6
Subsequently, the AMF requests the SMF by sending POST request with the following information:
N2 SM information received from the 5G-AN, including the transport layer address and tunnel endpoint of the downlink termination point for the user data for the current PDU session (i.e. 5G-AN's GTP-U F-TEID for downlink traffic), if the 5G-AN succeeded in establishing resources for the PDU sessions.
7
The SMF initiates PFCP Session Modification Procedure towards UPF with down link FAR updated with following options:
If N2 Transfer is successful, Forwarding Action is enabled along with remote node “forwarding parameters” details like IP address and GTP-U F-TEID.
If the cause of transfer failure is ATTEMPTING_TO_REACH_UE or UE_IN_NON_ALLOWED_AREA:
Update FAR > Apply Action > NOCP: 1
Update FAR > Apply Action > DROP:1
PFCPSMReq-Flags > DROBU:1
If the cause of transfer failure is UE_NOT_REACHABLE:
Update FAR > Apply Action > NOCP: 0
Update FAR > Apply Action > DROP:1
PFCPSMReq-Flags > DROBU:1
8
Upon receipt of successful response from UPF node, the SMF sets the upCnxState attribute to ACTIVATED for the PDU session.
9
The SMF then initiates 200 OK response including the upCnxState attribute set to ACTIVATED towards AMF (Only if Step 6 is completed and response is received from Step 8).