Achieving Fairness in a Distributed Ad-Hoc MAC

Many distributed multiple access (MAC) protocols use an exponential backoff mechanism. In that mechanism, a node picks a random backoff time uniformly in an intervals that doubles in size after a collision. When used in an Ad-Hoc network, this backoff mechanism is unfair towards nodes in the middle of the network. Indeed, such nodes tend to experience more collisions than nodes with fewer neighbors; consequently, they often choose larger delays than those other nodes. We propose a different backoff mechanism that achieves a fairer allocation of the available bandwidth by decreasing the backoff delay upon collision or failure to send a packet. That is, a node becomes more aggressive after each failure. Accordingly, we call the mechanism the Impatient Backoff Algorithm (IBA). The nodes maintain the stability of the algorithm by resetting, in a distributed way, the average backoff delays when they become too small. We perform a Markov analysis of the system to prove stability and fairness in simple topologies. We also use simulations to study the performance of IBA in random Ad-Hoc networks and compare with an exponential backoff scheme. Results show that IBA achieves comparable mean throughput, while delivering significantly better fairness.