Abstract

Media access protocols in wireless networks require each contending node to wait for a backoff time chosen randomly from a given range, before attempting to transmit on a shared channel. However, selfish stations might try to acquire an unfair portion of the channel resources, at the expense of the cooperating nodes, by not following the protocol specifications. For example, they might choose smaller backoff values more often than would be dictated by pure chance. In this thesis, we study how to detect such misbehavior as well as how nodes might be induced to adhere to the protocol.

We first introduce a game-theoretic framework that models an abstracted version of the medium access protocol as a strategic static game. We are interested in designing a game which exhibits a unique Nash equilibrium corresponding to a pre-specified full-support distribution profile. In the cooperation inducement context, the Nash equilibrium for the game would correspond to protocol compliance on behalf of the participating nodes. We identify an exact condition on the number of players and the number of their strategies that must be met to guarantee the existence of such a game.

Further, we propose a new protocol called XVBEB in order to determine based on the stations' backoff values choices whether they are behaving accordingly or selfishly. We describe how to deduce the backoff values in XVBEB based on observations of transmissions by nodes in the network and the collision timeline, which is rarely feasible with the IEEE 802.11 backoff procedure. Given a set of backoff values used by an XVBEB node, we describe how to conclude with a specified level of certainty whether the node is indeed adhering to the protocol.

Finally, we evaluate the performance of a network of XVBEB nodes and compare it against a standard IEEE 802.11 network. Simulation results show that the throughput of XVBEB is better than that of 802.11 for saturated CBR traffic. Furthermore, XVBEB also exhibits lower packet loss, delay and delay variation than 802.11 for both VBR and VoIP traffic for a variety of load conditions.