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In traditional IEEE 802.11 Wireless LAN, since no traffic differentiation is provided, QoS control for real-time services can not be supported. In this paper, we propose a method to provide service differentiation in legacy IEEE 802.11 Wireless LAN, which uses Distribute Coordination Function (DCF) mode. For the low priority traffic, remaining backoff time is reset according to its pre-determined probability. The probability is given according to the application's category, e.g. real-time video/voice or best effort data. When it is needed to reset the remaining backoff time for low priority traffic, the terminal should recalculate the backoff time between the previous remaining time and the maximum contention window size. The recalculated backoff time tends to increase compared to the previous remaining backoff time. Accordingly, the whole backoff time for low traffic tends to increase. For the use of the proposed scheme, only simple modification at terminal, not at AP (Access Point), is required. So, it can be easily applied to normal IEEE 802.11 Wireless LAN environments.