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We investigate the use of transmit power as a mechanism for service differentiation in contention based wireless local area networks (LAN). We use a dual transmit power scheme where the power levels are specially selected to guarantee capture whenever a collision occurs between a single high power frame and one or more low power frames. We employ the distributed coordination function (DCF) of the IEEE 802.11 wireless LAN standard for our simulation experiments. The performance metrics considered are throughput and medium access control (MAC) delay. We specifically investigate the level of differentiation achievable between high power and low power hosts given various ratios of high power to low power hosts. Our results show that differentiation is more distinct when the ratio of high power to low power hosts is small. It was also determined that in a cell with a fixed number of hosts capable of switching between the two power levels, there exists a threshold beyond which the MAC delay for the low power hosts actually decreases with increasing number of high power hosts.