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It is well known that IEEE 802.11 provides a physical layer multi-rate capability, and hence MAC layer mechanisms are needed to exploit this capability. Several solutions have been proposed to achieve this goal. However, these solutions only consider how to exploit good channel quality for the direct link between the sender and the receiver. Since IEEE 802.11 supports multiple transmission rates in response to different channel conditions, data packets may be delivered faster through a relay node than through the direct link if the direct link has low quality and low rate. In this paper, we propose a novel MAC layer relay-enabled distributed coordination function (DCF) protocol, called rDCF, to further exploit the physical layer multi-rate capability. We design a protocol to assist the sender, the relay node and the receiver to reach an agreement on which data rate to use and whether to transmit the data through a relay node. Considering various issues such as bandwidth utilization and channel errors, we propose techniques to further improve the performance of rDCF. Simulation results show that rDCF can significantly improve the system performance when the channel quality of the direct link is poor.