In this paper, we present a generic plug-and-play controller that ensures fair and efficient operation of IEEE 802.11 infrastructure wireless local area networks with multiple co-channel access points, without any change to hardware firmware of the network devices. Our controller addresses performance issues of TCP transfers in multi-AP WLANs, by overlaying a coarse time-slicing scheduler on top of a cascaded fair queuing scheduler. The time slices and queue weights, used in our controller, are obtained from the solution of a constrained utility optimization formulation. A study of the impact of coarse time-slicing on TCP is also presented in this paper. We present an improved algorithm for adaptation of the service rate of the fair queuing scheduler and provide experimental results to illustrate its efficacy. We also present the changes that need to be incorporated to the proposed approach, to handle short-lived and interactive TCP flows. Finally, we report the results of experiments performed on a real testbed, demonstrating the efficacy of our controller.
CENTAUR @cite_6 is motivated by the observation that when the traffic pattern is dominated by downloads, the IEEE 802.11 distributed coordination function (DCF) leads to wasted airtime in networks with hidden and exposed terminals. To tackle this, the authors in @cite_6 propose a centralized controller for hidden and exposed terminal links; the objective is to ensure that transmissions to hidden nodes do not happen simultaneously, while transmissions to exposed nodes overlap, in time, as much as possible. Links that are not associated with hidden or exposed nodes use the IEEE 802.11 DCF to access the medium. Handling exposed terminals requires carrier sensing to be effectively disabled. To achieve this, the authors in @cite_6 use fixed backoffs at the access points (APs) --- this requires modifications to the AP firmware. Since most firmwares are not open-source, it is difficult to incorporate such modifications.
@cite_6
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The IEEE has standardized the 802.11 protocol for wireless local area networks. The primary medium access control (MAC) technique of 802.11 is called the distributed coordination function (DCF). The DCF is a carrier sense multiple access with collision avoidance (CSMA CA) scheme with binary slotted exponential backoff. This paper provides a simple, but nevertheless extremely accurate, analytical model to compute the 802.11 DCF throughput, in the assumption of finite number of terminals and ideal channel conditions. The proposed analysis applies to both the packet transmission schemes employed by DCF, namely, the basic access and the RTS CTS access mechanisms. In addition, it also applies to a combination of the two schemes, in which packets longer than a given threshold are transmitted according to the RTS CTS mechanism. By means of the proposed model, we provide an extensive throughput performance evaluation of both access mechanisms of the 802.11 protocol.