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Wireless local area networks (WLANs) have become Increasingly popular due to the availability of affordable devices providing multirate capabilities. Under time- varying wireless channels, a device needs to tune its transmission rate dynamically for more efficient utilization of the physical link. Hence, rate adaptation mechanism, which is intentionally unspecified by the IEEE 802.11 standards, is critical to the system performance. In this paper, we propose a practical rate adaptation algorithm, Smart Sender, which utilizes both statistics and the received signal strength indicator (RSSI) of ACK packets to determine the transmission rate that maximizes the throughput. We implement our algorithm in commercial WLAN products and carry out extensive experiments for performance evaluation. The results demonstrate that using both statistics and RSSI of ACKs greatly improves system throughput and responsiveness under various wireless environments.