Downlink Data Notification User Plane Activation Call Flow for UE in CM-Connected State

This section describes the downlink data notification User Plane activation call flow when UE is in the CM-Connected state.

Downlink Data Notification User Plane Activation Call Flow for UE in CM-Connected State
Downlink Data Notification User Plane Activation Call Flow Description for UE in CM-Connected State
StepDescription
1When the UPF receives the downlink data for a PDU session and if no AN tunnel information is saved in the UPF for the PDU session, the UPF buffers the downlink data. The buffering is done based on the instruction from the SMF.
2The UPF sends data notification towards SMF. This notification includes the N4 session ID, the information to identify the QoS flow for the DL data packet, and the DSCP details.
3The SMF sends the acknowledgement data notification to the UPF.
4The SMF initiates the NAMF communication N1 and N2 message transfer towards the AMF. This message transfer includes details, such as PDU session ID, N2 SM information (QFIs, QoS profiles), CN N3 tunnel information, S-NSSAI, ARP, Paging Policy Indicator, 5QI, N1 and N2 transfer failure notification target address, and the PDU session resource setup request IE.
5As the UE is in CM-Connected state, the AMF initiates N1 and N2 transfer response. This response includes the “200 OK” status code and the “N1_N2_TRANSFER_INITIATED” cause.
6The User Plane Reactivation procedures begin. The reactivation procedures set up the radio resources and activate the user plane to establish the N3 tunnel.
7The AMF sends the NSMF PDU Session Update SM Context Request toward SMF. This request contains the SM information of the N2 interface. The connection state of user plane is activated.
8The SMF sends the N4 modification procedure toward the UPF to activate the session and to update the AN tunnel information, which is the IP and TEID. The session is activated by performing the remove buffer action and the set forward action.
9The UPF modifies the session and sends the acknowledgement of the modification to the SMF.
10The SMF responds back to the AMF with “200 OK” status code for NSMF PDU Session Update SM Context Request with the connection state of user plane as activated.
Note

The following N1 and N2 response error cases are handled:

  • For 404 Context Not Found status, a PDU session is released.

  • For 504 or 403 status with the "UE_IN_NON_ALLOWED_AREA" and "NOT_REACHABLE" cause, an N4 modification request is sent to drop the buffered packets and to not send the CP notification for the downlink data.

  • For the N1and N2 transfer notification failure, the N4 modification request is sent to drop the buffered packets and to not send the CP notification for downlink data.

  • For 409 status code with the Retry After timer value, the N1 and N2 transfer is re-initiated after the timeout value.

  • For the 409 status code with "HIGHER_PRIORITY_REQUEST_ONGOING" cause, the lower priority N1 and N2 transfers are not allowed. Only the higher priority transfers are communicated to the AMF.