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Adaptive transmit power control in 802.11 wireless LANs (WLANs) on a per-link basis helps increase network capacity and improves battery life of WiFi-enabled mobile devices. However, it faces the following challenges: 1) it can exacerbate receiver-side interference and asymmetric channel access; 2) it can incorrectly lead to lowering the data rate of a link; 3) mobility-induced channel variations at short timescales make detecting and avoiding these problems more complex. Despite substantial prior research, state-of-the-art solutions lack comprehensive techniques to address the above problems. In this paper, we design and implement Symphony, a synchronous two-phase rate and power control system whose agility in adaptation enables us to systematically address the three problems while maximizing the benefits of power control on a per-link basis. We implement Symphony in the Linux MadWifi driver and show that it can be realized on hardware that supports transmit power control with no modifications to the 802.11 MAC, thereby fostering immediate deployability. Our extensive experimental evaluation on a real testbed in an office environment demonstrates that Symphony : 1) enables up to 80% of the clients in three different cells to settle at 50%-94% lower transmit power than a per-cell power control solution; 2) increases network throughput by up to 50% across four realistic deployment scenarios; 3) improves the throughput of asymmetry-affected links by 300%; and 4) opportunistically reduces the transmit power of mobile clients running VOIP calls by up to 97% while only causing a negligible degradation of voice quality.