Network-initiated Idle to Active Transition

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
StepDescription
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".

2The 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).

5The 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

8Upon receipt of successful response from UPF node, the SMF sets the upCnxState attribute to ACTIVATED for the PDU session.
9The 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).