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Measurements of In-Motion 802.11 Networking

Measurements of In-Motion 802.11 Networking. Richard Gass and James Scott Intel Research Cambridge Christophe Diot Thomson. Outline. Motivation Experimental Setup Results Conclusions. Motivation. 802.11 Access Points are everywhere Predominant usage model is stationary

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Measurements of In-Motion 802.11 Networking

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  1. Measurements of In-Motion 802.11 Networking Richard Gass and James Scott Intel Research Cambridge Christophe Diot Thomson

  2. Outline • Motivation • Experimental Setup • Results • Conclusions WMCSA-06 April 6-7, 2006Semiahmoo Resort, Washington, USA

  3. Motivation • 802.11 Access Points are everywhere • Predominant usage model is stationary • Periods of use measured in minutes or hours • User stationary or slow-moving during use • Device in standby between uses • New and future devices are “always on” and have 802.11 • PDAs or push-email devices • Smart phones with WiFi e.g. Imate SP5 • Low-power long-life trend in notebook computers WMCSA-06 April 6-7, 2006Semiahmoo Resort, Washington, USA

  4. Our research: Study in-motion use of 802.11 APs • How does off-the-shelf equipment perform • Without using additional antennae or other hardware • Without tweaking the link layer parameters • Is the wireless channel well behaved? • Is it resilient to relative speed of device and AP? • How does a channel perform over the course of a drive-by? • What should in-motion users expect? • Study backhaul network effects as well as WiFi • Study performance of current applications in this scenario WMCSA-06 April 6-7, 2006Semiahmoo Resort, Washington, USA

  5. Experiment location WMCSA-06 April 6-7, 2006Semiahmoo Resort, Washington, USA

  6. Straight • Flat • Nobody around • Can test a range of speeds WMCSA-06 April 6-7, 2006Semiahmoo Resort, Washington, USA

  7. Experimental setup WMCSA-06 April 6-7, 2006Semiahmoo Resort, Washington, USA

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  10. AP Linksys WAP55AG Out of the box configuration Laptops – IBM Thinkpads T30 : Backhaul Simulator T41 : Mobile Client T42 : Stationary Server Operating system Linux Fedora Core 3 Wireless Cards Intel 2915ABG (In-Motion) Netgear WAG511 (monitoring) DHCP not used Traffic Generation TCP - iperf HTTP - wget and apache UDP – bash /dev/udp interface Traffic Collection Tcpdump (layer 3) Kismet (layer 2) Hardware/Software used WMCSA-06 April 6-7, 2006Semiahmoo Resort, Washington, USA

  11. Experimental procedure Start scripts Get to target speed Hit “enter” as first marker passed Deassociate Out of range Hit “enter” as last marker passed Association happens and data transfer starts Set experimental parameters Confirm experimental parameters WMCSA-06 April 6-7, 2006Semiahmoo Resort, Washington, USA

  12. Experimental parameters • Various speeds from city speeds to highway speeds • 5, 15, 25, 35, 55, 75 mph target speeds • Actual speed measured by noting the time over the 1000m range • Various traffic types • TCP – 1500 byte packets • UDP – Cycling packet sizes 50, 100, 200, 400, 800, 1500 • HTTP – Downloaded local caches of 6 popular webpages • Backhaul network • Direct link (100Mbit/s Ethernet) • 1 Mbit/s bandwidth limit • 100 ms added latency each way • Both bandwidth limit and added latency • 2 repetitions of each run • Total number of runs : 108 • Four 12-hour days for 4 people WMCSA-06 April 6-7, 2006Semiahmoo Resort, Washington, USA

  13. Distance in range • Speed reduces usable range • But not to zero! 392 Meters WMCSA-06 April 6-7, 2006Semiahmoo Resort, Washington, USA

  14. Association time • Transfer opportunity decreases with speed • Association time effectively levels out while time associated continues to fall with speed • Significant transfer opportunities still exist 13.7 sec WMCSA-06 April 6-7, 2006Semiahmoo Resort, Washington, USA

  15. Total data transferred • Data transferred follows expected behaviour (1/x) • In-motion channel not crippling transfers • Useful amounts of data transfer possible • 25mph : 27 MB • 55mph: 8 MB WMCSA-06 April 6-7, 2006Semiahmoo Resort, Washington, USA

  16. Instantaneous throughput • We saw the same channel “phases” as Ott/Kutscher • Entry phase where bandwidth ramps up • Production phase with full, constant bandwidth • Exit phase with bandwidth ramping down Entry Production Exit WMCSA-06 April 6-7, 2006Semiahmoo Resort, Washington, USA

  17. 5 mph 35 mph 75 mph 5.5Mbit/s TCP 3.5Mbit/s UDP HTTP does not utilize channel 1.8Mbit/s HTTP WMCSA-06 April 6-7, 2006Semiahmoo Resort, Washington, USA

  18. Packet loss (UDP) Does the size of the packet have an effect? • < 1% loss (for all traces) • Packet size has little effect at network layer • Link-layer examination shows: • Virtually no loss in production phase • Majority of losses at link layer occur at edge of wireless range WMCSA-06 April 6-7, 2006Semiahmoo Resort, Washington, USA

  19. Backhaul effects on TCP • Bandwidth limit has obvious effect • Latency is coped with by TCP • Transfer sizes drop by ~4x Delay BW limit WMCSA-06 April 6-7, 2006Semiahmoo Resort, Washington, USA

  20. Backhaul effects on HTTP • Opposite effects to TCP: • Bandwidth limit has little effect • Latency is deadly • Due to high number of round-trips to download webpages • Get index, here is index, get image 1, here is image 1, etc • Each using slow start (pipelining does not help across servers) • Transfer size drops ~15x • With backhaul, http wastes >90% of bandwidth BW Limit Delay WMCSA-06 April 6-7, 2006Semiahmoo Resort, Washington, USA

  21. Implications • 802.11b provides useful transfer sizes while in-motion • Even using off-the-shelf equipment without modifying link layer • Without adding external antennae • Periodic in-motion opportunities could support user needs • Must make transfers asynchronous with human activity • No time for manual authentications e.g. for AP, VPN or email • Start-of-day protocols (DHCP, VPN, etc) execute during entry phase • Must cope with varying channel conditions which might cause loss etc • Cannot afford many round trips, or long timeouts to retransmit • Application layer protocols (HTTP, IMAP, SMTP, etc) need to reduce round-trip reliances to make better use of short connections • E.g. by adding a “bulk” mode to the protocol WMCSA-06 April 6-7, 2006Semiahmoo Resort, Washington, USA

  22. Our future work • Haggle – networking framework for mobile devices allowing applications to make use of all connectivity opportunities, including neighbourhood, in-motion, infrastructure, device mobility, etc • YAAP (Yet Another Access Point) • Personal, always-on device acting as network proxy for notebook, etc • Can opportunistically utilize in-motion connections and later provide the data to laptop when it comes out of standby • We are running a demo now: try it out! • Connect to access point “yaap-01” with 802.11 WMCSA-06 April 6-7, 2006Semiahmoo Resort, Washington, USA

  23. Thanks Questions/Comments More info: http://www.cambridge.intel-research.net/haggle/research.php Traces available online WMCSA-06 April 6-7, 2006Semiahmoo Resort, Washington, USA

  24. BACKUP WMCSA-06 April 6-7, 2006Semiahmoo Resort, Washington, USA

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