UE-initiated Service Request Procedure
The UE in CM-IDLE state initiates the Service Request procedure to send uplink signaling messages, user data, or as a response to a network paging request. After receiving the Service Request message, the AMF performs authentication. After the establishment of the signaling connection to an AMF, the UE or network sends signaling messages, for example, PDU session establishment from UE to the SMF, through the AMF.
The Service Request procedure is used by a UE in CM-CONNECTED to request activatation of a User Plane connection for PDU sessions and to respond to a NAS Notification message from the AMF. When a User Plane connection for a PDU session is activated, the AS layer in the UE indicates it to the NAS layer.