Mitigating power law delays: The use of polynomial backoff in IEEE 802.11 DCF

The IEEE 802.11 wireless local area network (WLAN) standard was originally designed for best-effort services, targeting at providing high throughput and throughput fairness. However, high system throughput does not necessarily translate to good delay performance. Recent studies show that exponential backoff, the key collision avoidance mechanism in distributed coordination function (DCF) of 802.11, is fundamentally defected in the sense that it induces divergent moments of medium access delay. Essentially, the medium access delay follows a power law distribution, a subclass of heavy tailed distribution. With practical system configurations, the delay variance can easily approach infinity, which translates to service starvation of some users and eventually leads to severe unfairness among users. In this paper, we show that the power law delay distribution can be mitigated if exponential backoff is replaced by a polynomial backoff mechanism. Through rigorous analysis, we find that all delay moments are finite with polynomial backoff, thus fundamentally solving the problem of starvation and unfairness. In addition, polynomial backoff yields a similarly high throughput as exponential backoff with a practical network size when the order of the polynomial backoff function is set reasonably. In this sense, we argue that polynomial backoff is a better alternative than exponential backoff in IEEE 802.11 DCF, especially when there are increasingly more broadband multimedia traffics with stringent delay requirements in the network.