A core network node and method for use in a wireless system are provided. The method includes transmitting, by a core network node, one or more service data unit (SDUs) to a base station; and transmitting, by the core network node, a control message including respective length information of the one or more SDUs to the base station. The length information included in the control message is used by the base station for processing the one or more SDUs, and the control message is transmitted after the one or more SDUs are transmitted.