From a theoretical standpoint, backpressure-based techniques present elegant cross-layer rate control solutions that use only local queue information. It is only recently that attempts are being made to design real world wireless protocols using these techniques. To aid this effort, we undertake a com-prehensive experimental evaluation of a backpressure-based network stack over the USC tutornet wireless testbed, a 100 node wireless sensor network (WSN) testbed spanning two floors. To the best of our knowledge this is the first such study in the context of a WSN. Our evaluation yields three key insights. First, we show that in a WSN, contrary to pre-vious proposals, the gains in implementing queue prioritiza-tion over a CSMA MAC are negligible. Hence backpressure-based protocols can be implemented for a WSN without mod-ifying the underlying CSMA MAC. Second, we show that the performance of backpressure-based protocols is highly sensitive to a parameter setting that depends upon current traffic conditions. Therefore, practical backpressure proto-cols must provide for automatic parameter adaptation. We conjecture that this insight, though presented here in the con-text of a WSN, holds for any general multi-hop wireless net-work. Third, our comparative evaluation with existing rate control protocols shows that with optimal parameter settings, backpressure-based stack can give upto a factor of two im-provement in throughput performance, albeit at the cost of increased queue sizes. 1
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