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Abstract:
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This paper describes the design, implementation, and evaluation of Cabernet, a system to deliver data to and from moving vehicles using open 802.11 (WiFi) access points encountered opportunistically during travel. Network connectivity in Cabernet is both fleeting (access points are typicallywithin range for a few seconds) and intermittent (because the access points don't provide continuous coverage), and suffers from high packet loss rates over the wireless channel. On the positive side, in the absence of losses, achievable data rates over WiFi can reach many megabits per second. Unfortunately, current protocols don't establish end-to-end connectivity fast enough, don't cope well with intermittent connectivity, and don't handle high packet loss rates well enough to achieve this potential throughput. Cabernet incorporates two new techniques to improve data delivery throughput: QuickWifi, a streamlined client-side process to establish end-to-end connectivity quickly, reducing the mean time to establish connectivity from 12.9 seconds to less than 366 ms and CTP, a transport protocol that distinguishes congestion on the wired portion of the path from losses over the wireless link to reliably and efficiently deliver data to nodes in cars. We have deployed the system on a fleet of 10 taxis, each running several hours per day in the Boston area. Our experiments show that CTP improves throughput by a factor of 2x over TCP and that QuickWifi increases the number of connectionsby a factor of 4x over unoptimized approaches. Thus, Cabernet is perhaps the first practical system capable of delivering data to moving vehicles over existing short-range WiFi radios, with a mean transfer capacity of approximately 38 megabytes/hour per car, or a mean rate of 87 kbit/s. |
