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Fast rate adaptation has long been recognized as an effective way to improve the PHY-layer data rate of wireless networks. However, in random access wireless networks such as IEEE 802.11 wireless LANs, MAC-layer throughput is dominated by stations with the lowest transmission rates, resulting in an underutilization of spectrum bandwidth. In this paper, we propose a novel two-level medium access framework, referred to as two-level MAC, to solve the aforementioned problem and to significantly improve system spectrum efficiency through the exploitation of multiuser diversity. The key idea of two-Level MAC is to introduce a second level of deterministic channel access on top of the traditional IEEE 802.11 DCF protocol. By doing so, higher priority is granted to high-rate stations in a fully distributed manner. Meanwhile, collisions among potential contending stations are drastically reduced. Through analysis, we show how such two-Level MAC can be optimized to achieve the maximum system throughput. The superiority of the proposed protocol is verified through analyses and extensive simulations.