From Saturation to Non-Saturation: A Study on 802.11 Networks

There have been extensive studies on the performance and behavior of 802.11 networks. These studies primarily focus on the so-called saturation regime, where all clients/user queues always have packets to send. A prominent study is the work by Bianchi [1], as well as numerous subsequent studies. This saturation operating regime may be viewed as the limiting case when all the arrival rates in the system approach infinity. This paper departs from this class of study in that we focus on a nonsaturation regime, where user queues are finite with arrival rates below the saturation level. We demonstrate that the widely studied saturation throughput is inherently a pessimistic notion and deserves better understanding. We provide examples of systems of users with finite arrival rates where the total throughput of the system is significantly greater than the saturation throughput. Furthermore, we study the throughput and delay performance of an 802.11 network in a non-saturation scenario, and show that for arrival rates very close to the saturation throughput, the system behavior is qualitatively very different from that in the saturation case. We attempt to explain these observations by constructing and analyzing a number of both realistic and idealized MAC schemes. We also argue that under realistic channel models and user asymmetry, the conventional measures of fairness (defined in terms of throughput) are distinct from fairness in terms of delay.