A method of informing a selected CN operator from a UE to a RAN in an NS system where a plurality of CN operators shares the RAN is provided. Upon request of transmission of an IDT, the UE acquires a CN operator ID selected from a multi-ID list broadcasted from a cell in which an RRC connection has been established, and transmits an IDT message with the selected CN operator ID to the RAN. Upon receipt of the IDT message, the RAN determines whether the IDT message includes a selected CN operator ID. In the presence of the selected CN operator ID, the RAN stores the selected CN operator ID in a UE context. In the absence of the selected CN operator ID, the RAN reads a stored selected CN operator ID from the UE context. The RAN then selects a CN operator using the selected CN operator ID.