Achieving Fairness in a Distributed Ad-Hoc MAC

Summary

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.

This work was supported by the National Science Foundation under a NeTS grant.