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This letter considers an unfairness problem between uplink and downlink in 802.11 WLANs. One of existing solutions for this problem is giving a larger transmission opportunity (TXOP) limit to an AP than stations. This TXOP differentiation scheme, however, does not work well in error-prone environments since a packet bursting during a TXOP duration is terminated when a data transmission fails due to channel errors. To overcome this, we propose a new scheme in which an AP controls its minimum contention window size and TXOP limit dynamically according to the packet error rate and the number of stations. Our simulation results show that the proposed scheme provides fair channel accesses between uplink and downlink in error-prone 802.11 WLANs.