Disclosed is a bitmap structure which enables the size of a bitmap field containing reception result information to be significantly reduced while fully performing its acknowledgment function. To this end, a message region for recording indicators, which enables reception success or failure for the maximum allowable SN level packets treatable by block ACK to be confirmed, is assigned. A message region for recording only the reception results for unsuccessfully received packets is also assigned. A receiving party confirms the unsuccessfully received packets through the indicators, and retransmits the unsuccessfully received packets. Also, a transmitting party provides the number of SN level packets and the maximum number of fragmentation packets to the receiving party. The receiving party determines an optimized bitmap configuration scheme, and transmits the reception results for the respective fragmentation packets to the transmitting party based on the determined bitmap configuration scheme.