Solvability of a Markovian model of an IEEE 802.11 LAN under a backoff attack

Ad-hoc IEEE 802.11 LANs employ CSMA/CA at the MAC layer. CSMA/CA selects a random backoff time from a contention window that grows from w_min to w_max upon consecutive frame collisions. A selfish backoff attack by configuring small w_min and/or w_max (i.e., systematic selection of short backoff times) brings the attacking station an unfairly large long-term bandwidth share. To evaluate the impact of backoff attacks we use Bianchi´s model of a saturated CSMA/CA network. So far, its solvability has been rigorously established only for identical w_min and w_max at all stations. We ask whether the model remains solvable for a general backoff attack scenario, where w_min and w_max differ from station to station. Although we find the answer negative, we formulate a sufficient condition for solvability that covers realistic backoff attacks. Based on the obtained bandwidth distribution and power efficiency we point to several features of CSMA/CA under a backoff attack.