UE Triggered Service Request

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 state to request activation of User Plane connection for PDU Sessions and to respond to a NAS Notification message from the AMF.

For any Service Request, the AMF responds with a Service Accept message to synchronize PDU Session status between UE and network, if necessary. If the Service Request cannot be accepted by the network, the AMF responds with a Service Reject message to UE. The Service Reject message includes an indication or cause-code requesting the UE to perform Registration Update procedure. The Service Reject message is sent for unknown subscriber or if the TAC in Service Request does not match the last known user location.