Distributed Coordinate Functions (DCF) is the mandatory medium access method for IEEE 802.11 Wireless Local Area Networks (WLANs), which is designed for infrastructure WLANs where all end users connect with Access Point (AP) with single-hop links. DCF doubles its Contention Window (CW) before each retransmission. However, in multi-hops WLANs, a node may encounter different kinds of transmission failures from different transmission directions. In this paper, we will point out that, in some situations doubling CW before retransmitting makes DCF exploit the medium inefficiently. We also propose a method to resolve this problem by employing different backoff mechanisms for different transmission directions. NS-2 simulations demonstrate a significant throughput improvement of our enhanced MAC over the native MAC.