A method and system for uplink-downlink transmission of message in a network. The method includes transmitting, by a user equipment (UE), to a radio access network (RAN) node, a random access channel (RACH) message including identifier information for identifying a serving gateway in a core network; receiving, by the UE, from the RAN node, a random access (RA) response message including uplink (UL) grant information; and transmitting, by the UE, to the RAN node, a data message including encrypted data in an idle mode. The data message is transmitted from the RAN node to the serving gateway based on the identifier information, and the encrypted data is generated by the UE, and the encrypted data is decrypted by the serving gateway.