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In Wi-Fi networks, mobile nodes compete for accessing the shared channel by means of a random access protocol called distributed coordination function (DCF), which is long term fair. But recent drivers allow users to configure protocol parameters differently from their standard values in order to break the protocol fairness and obtain a larger share of the available bandwidth at the expense of other users. This motivates a game theoretical analysis of DCF. Previous studies have already modeled access to a shared wireless channel in terms of non-cooperative games among the nodes, but they have mainly considered ad hoc mode operation. In this paper we consider the role of the access point (AP) in infrastructure mode operation, for mitigating or discouraging such selfish behaviors. Solving a mechanism design problem, we use the AP as a network coordinator, for encouraging node strategies which maximize a global utility function. We analyze both unidirectional and bidirectional (uplink and downlink) traffic scenarios. It is well known that nodes selfishness jeopardize performance, but we show that simple changes to AP operation can let the system achieve optimal performance in spite of nodes selfishness. Instead for the bidirectional scenario the legacy behaviour of an AP is sufficient to guarantee quasi optimal performance.