Abstract-CSMA/CA, the contention mechanism of the IEEE 802.11 DCF medium access protocol, has recently been found vulnerable to selfish backoff attacks consisting in nonstandard configuration of the constituent backoff scheme. Such attacks can greatly increase a selfish station's bandwidth share at the expense of honest stations applying a standard configuration. The paper investigates the distribution of bandwidth among anonymous network stations, some of which are selfish. A station's obtained bandwidth share is regarded as a payoff in a noncooperative CSMA/CA game. Regardless of the IEEE 802.11 parameter setting, the payoff function is found similar to a multiplayer Prisoners' Dilemma; moreover, the number (though not the identities) of selfish stations can be inferred by observation of successful transmission attempts. Further, a repeated CSMA/CA game is defined, where a station can toggle between standard and nonstandard backoff configurations with a view of maximizing a long-term utility. It is argued that a desirable station strategy should yield a fair, Pareto efficient, and subgame perfect Nash equilibrium. One such strategy, called CRISP, is described and evaluated.Index Terms-Ad hoc LAN, game theory, MAC protocol, selfish behavior.

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A class of contention-type MAC protocols (e.g., CSMA/CA) relies on random deferment of packet transmission, and subsumes a deferment selection strategy and a scheduling policy that determines the winner of each contention cycle. This paper examines contention-type protocols in a noncooperative an ad-hoc wireless LAN setting, where a number of stations self-optimise their strategies to obtain a more-than-fair bandwidth share. Two scheduling policies, called RT/ECD and RT/ECD-1s, are evaluated via simulation It is concluded that a well-designed scheduling policy should invoke a noncooperative game whose outcome, in terms of the resulting bandwidth distribution, is fair to non-self-optimising stations.

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