Airtime fairness, or time-based fairness, has been well recognized as a method to solve WiFi performance anomalies and provide a balance between fairness and spectrum efficiency in multi-rate wireless networks. However, the definition of airtime is vague and simplistic. In this paper, it is demonstrated that current airtime fair scheduling results in unfairness in reality because overheads are neglected or unfairly counted. We introduce a notion of responsible airtime, which covers not only the data transmission time, but also all overheads, even a TCP ACK segment in TCP traffic. An approach based on responsible airtime can provide true time-based fairness, but responsible airtime is too complicated to directly handle. A practical method is thus introduced for evaluating responsible airtime fairness indirectly via throughput measurement. The key element, throughput fair share, of a node, is based on the baseline property in time-based fairness. For each node, an achieving ratio of actual throughput to the throughput fair share is determined, and a new fairness index considering deficiency as well as equity is applied. To validate the feasibility of responsible airtime fairness, we have developed a simple responsible airtime fair scheduler in access points for download traffic. Extensive simulation experiments are conducted in various network and traffic environments using the ns3 simulator. The results show that true time-based fairness is achievable in practice.
Keywords: WiFi sensor networks; airtime fairness; deficit round-robin; fairness index; multi-rate wireless networks; performance anomaly; responsible airtime; time-based fairness.