A frequency offset estimation method for a multi-carrier communication system is provided. The lower end of the enclosure is weighted and contains a second plurality of smaller holes for rapid drainage of water without loss of the trapped leeches. According to the present invention so configured, since transmission delay time information of a frame is considered when scheduling a sequence of frame transmission, there is an advantage in that waste of wireless resources due to retransmission delay of frames can be prevented. The priority arbitration of queuing elements is made according to a two level hierarchy to increase the speed of priority queue management and therefore the switching throughput.