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A novel channel access control for the IEEE 802.11 wireless standard is proposed to provide differentiated service in a wireless LAN. It relies on controlling the length of the inter-frame space (IFS) interval, during which a station has to detect a quiet channel before decrementing its backoff time. We derive a simple model to mathematically analyze the service differentiation achievable with arbitration IFS (AIFS), such as weight- or priorityproportional throughputs, and then verify its effectiveness via simulation. The simulation results confirmed the correctness of the proposed control scheme, and demonstrated its effectiveness. A small difference between stations' IFS values is shown to make substantial service differentiation between them. This, in turn, reduces the channel idle time which could be very long if the control relies on the commonly-used contention window size, rather than IFS. The proposed scheme, therefore, achieves higher channel utilization, especially when the total number of stations is not large.