Browsing by Author "Camp, Joseph D."

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    Experimental and analytical evalution of embedded link performance with small-scale channel fluctuations
    (2009) Camp, Joseph D.; Knightly, Edward W.
    We have deployed a first-of-its-kind, urban-scale wireless mesh network which provides Internet access to 1000's of users spanning multiple square kilometers in an underserved area in Houston, TX. However, in this and other urban environments, IEEE 802.11 link performance is both misunderstood and poor-performing due to complex node interactions which are affected by a vast array of factors including topology, channel conditions, modulation rate, packet sizes, and physical layer capture. In this thesis, I draw from 100's of thousands of urban measurements and develop an analytical model to understand the performance of links embedded in the aforementioned complex scenarios. My focus is on two fundamental concepts involving embedded links. First, choosing the modulation rate which maximizes the throughput is imperative since each bit of the (overly-)shared medium is critical. Yet, all existing rate adaptation mechanisrns fail to track the ideal rate even in a simple, non-mobile urban scenario. Using a custom cross-layer framework, I implement multiple and previously un-implemented rate adaptation mechanisms to reveal the reasons for the failure and design rate adaptation mechanisms which are able to track urban and downtown vehicular and non-mobile environments. Second, I pose a basic, yet unsolved problem: given a time-varying channel and traffic matrix in the aforementioned complex scenario, predict the throughput of an embedded link and understand the complex interactions of factors that lead to its performance. By performing thousands of measurements of embedded links on an urban mesh network and developing an analytical model, this work is the first to show that even a 1 dB change in channel state can yield a bi-modal shift in throughput that emulates a change in node connectivity. Finally, I apply our model and experimentation to modulation rate selection and the interaction of control and data traffic to show that understanding these complex interdependencies leads to operation in improved performance regimes. My work has implications for this and other urban communities which have unequal access to Internet resources, enabling a high-speed access infrastructure at extremely low cost.
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    Measurement driven deployment of a two-tier urban mesh access network
    (2006) Camp, Joseph D.; Knightly, Edward W.
    Multihop wireless mesh networks can provide Internet access over a wide area with minimal infrastructure expenditure. In this work, we investigate key issues involved in deploying such networks including individual link characteristics, multihop application layer performance, and network-wide reliability and throughput. We perform extensive measurements in a two-tier urban scenario to characterize the propagation environment and correlate received signal strength with application layer throughput. Further, we measure competing, multihop flow traffic matrices to empirically define achievable throughputs of fully backlogged, rate limited, and web-emulated traffic. We find that while fully backlogged flows produce starving nodes, rate-controlling flows to a fixed value yields fairness and high aggregate throughput. Likewise, transmission gaps occurring in statistically multiplexed web traffic, even under high offered load, remove starvation and yield high performance.