  In 802.11 WLANs, adapting the contention parameters to network conditions
results in substantial performance improvements. Even though the ability to
change these parameters has been available in standard devices for years, so
far no adaptive mechanism using this functionality has been validated in a
realistic deployment. In this paper we report our experiences with implementing
and evaluating two adaptive algorithms based on control theory, one centralized
and one distributed, in a large-scale testbed consisting of 18 commercial
off-the-shelf devices. We conduct extensive measurements, considering different
network conditions in terms of number of active nodes, link qualities and
traffic generated. We show that both algorithms significantly outperform the
standard configuration in terms of total throughput. We also identify the
limitations inherent in distributed schemes, and demonstrate that the
centralized approach substantially improves performance under a large variety
of scenarios, which confirms its suitability for real deployments.
