  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.
