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This procedure is used when the network needs signalling connection with a UE.
Step 1:
-> When a UPF receives downlink data for a PDU session and there is no AN tunnel info stored in UPF for the PDU session, the data may buffer the downlink data or forward the downlink data to the SMF.
Step 2a: UPF to SMF: Data Notification
-> UPF will send Data NOtification to SMF, that has N4 Session ID, Information to identify the QoS Flow for the DL data packet, DSCP
Step 2b: SMF to UPF: Data Notification Ack.
Step 2c.
-> The UPF forwards the downlink data packets towards the SMF
Step 3a: [Conditional] SMF to AMF: Namf_Communication_N1N2MessageTransfer
-> Namf_Communication_N1N2MessageTransfer includes
SUPI,
PDU Session ID,
N1 SM container (SM message),
N2 SM information (QFI(s),
QoS profile(s),
CN N3 Tunnel Info,
S-NSSAI),
Area of validity for N2 SM information,
ARP,
Paging Policy Indicator,
5QI,
N1N2TransferFailure Notification Target Address,
Extended Buffering support
Step 3b: [conditional] The AMF responds to the SMF.
-> If the UE is in CM-IDLE state at the AMF, and the AMF is able to page the UE the AMF sends a Namf_Communication_N1N2MessageTransfer response to the SMF immediately to indicate to the SMF that AMF is attempting to reach UE and the N2 SM information provided in step 3a
Step 3c: [Conditional] SMF responds to the UPF
-> SMF may notify the UPF about the User Plane setup failure.
Step 4a.
-> If the SMF receives an indication from the AMF that the UE is unreachable or reachable only for regulatory prioritized service and the SMF determines that Extended Buffering does not apply
Step 4b.
-> [Conditional] If the UE is in CM-IDLE state in 3GPP access and the PDU Session ID received from the SMF in step 3a has been associated with 3GPP access and based on local policy the AMF decides to notify the UE through 3GPP access even when UE is in CM-CONNECTED state for non-3GPP access, the AMF may send a Paging message to NG-RAN node(s) via 3GPP access.
Step 4c.
-> [Conditional] If the UE is simultaneously registered over 3GPP and non-3GPP accesses in the same PLMN, and the UE is in CM-CONNECTED state in 3GPP access and the PDU Session ID in step 3a is associated with non-3GPP access, the AMF sends a NAS Notification message containing the non-3GPP Access Type to the UE over 3GPP access and sets a Notification timer. Step 5 is omitted.
Step 5: [Conditional] AMF to SMF: Namf_Communication_N1N2Transfer Failure Notification.
Step 6.
-> If the UE is in CM-IDLE state in 3GPP access, upon reception of paging request for a PDU Session associated to 3GPP access, the UE shall initiate the UE Triggered Service Request procedure
Step 6a.
-> After receiving the Reject Paging Indication, the AMF notifies the SMF using the Namf_Communication_N1N2MessageTransfer Failure Notification
Step 7.
-> If the AMF has paged the UE to trigger the Service Request Procedure, the AMF shall initiate the UE configuration update procedure
Step 8.
-> The UPF transmits the buffered downlink data toward UE via (R)AN node which performed the Service Request procedure.
Source: TS 38 502