Downlink Data Notification User Plane Activation Call Flow for UE in CM-Idle State
This section describes the downlink data notification User Plane activation call flow when UE is in CM-Idle state.

Step | Description |
---|---|
1 | When 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, then based on the instruction from the SMF, the UPF buffers the downlink data. |
2 | The UPF sends data notification towards the SMF. This notification includes the N4 Session ID, the information to identify the QoS flow for the DL data packet, and the DSCP details. |
3 | The SMF sends the acknowledgement data notification to the UPF. |
4 | The SMF initiates the NAMF communication N1 and N2 message transfer toward 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, and N1 and N2 transfer failure notification target address. |
5 | As the UE is in CM-Connected state, the AMF initiates N1 and N2 transfer response. This response includes the “202 Accepted” status code and “ATTEMPTING_TO_REACH_UE” cause. |
6 | The AMF triggers the paging procedure towards the UE. |
7 | The UE receives the paging request and initiates the requested service to activate the session. |
8 | The AMF initiates the NSMF PDU Session Update SM Context Request towards SMF with connection state of user plane configured as activating. |
9 | The SMF responds back to the AMF with “200 OK” status code for the NSMF PDU Session Update SM Context Request. This request includes details, such as 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. |
10 | The AMF sends the NSMF PDU Session Update SM Context Request towards the SMF. This request contains the SM information of the N2 interface. The connection state of user plane is Activating. |
11 | The SMF initiates the N4 modification procedure towards 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 set forward action. |
12 | The UPF modifies the session and sends the acknowledgement of the modification to the SMF. |
13 | The SMF responds back to the AMF with “200 OK” status code for NSMF PDU Session Update SM Context Request with connection state of user plane as Activated. |