The IEEE 802.11 standard for wireless local area networks (WLANs) employs a medium access control (MAC), called distributed coordination function (DCF), which is based on carrier sense multiple access with collision avoidance (CSMA/CA). The collision avoidance mechanism utilizes the random backoff prior to each frame transmission attempt. The random nature of the backoff reduces the collision probability, but cannot completely eliminate collisions. It is known that the throughput performance of the 802.11 WLAN is significantly compromised as the number of stations increases. In this paper, we propose a novel distributed reservation-based MAC protocol, called early backoff announcement (EBA), which is backward compatible with the legacy DCF. Under EBA, a station announces its future backoff information in terms of the number of backoff slots via the MAC header of its frame being transmitted. All the stations receiving the information avoid collisions by excluding the same backoff duration when selecting their future backoff value. Through extensive simulations, EBA is found to achieve a significant increase in the throughput performance as well as a higher degree of fairness compared to the 802.11 DCF.