A method and system for uplink-downlink transmission of message in a network. The method includes establishing, by a user equipment (UE), a security context for data transmission between the UE and a serving gateway in the network; generating, by the UE, in an idle mode, a data message including an encrypted data packet, the encrypted data packet being generated by encrypting a data packet to transmit to the serving gateway based on the security context; transmitting, by the UE, in the idle mode, to a base station (BS), a random access channel (RACH) message including a UE identifier (ID); receiving, by the UE, in the idle mode, from the BS, a contention resolution message including grant information for an uplink (UL) transmission; and transmitting, by the UE, in the idle mode, to the BS, the data message including the encrypted data packet.