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A Measurement Study of Vehicular Internet Access Using In Situ Wi-Fi Networks

A Measurement Study of Vehicular Internet Access Using In Situ Wi-Fi Networks. Vladimir Bychkovsky, Bret Hull, Allen Miu, Hari Balakrishnan, and Samuel Madden. MIT CSAIL. http://cartel.csail.mit.edu. Wi-Fi Is Everywhere.

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A Measurement Study of Vehicular Internet Access Using In Situ Wi-Fi Networks

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  1. A Measurement Study of Vehicular Internet Access Using In Situ Wi-Fi Networks Vladimir Bychkovsky, Bret Hull, Allen Miu, Hari Balakrishnan, and Samuel Madden MIT CSAIL http://cartel.csail.mit.edu

  2. Wi-Fi Is Everywhere What are the performance propertiesof organically grown Wi-Fi networks? Images from WiGLE.net and CarTel

  3. The Opportunity • Today: • Broadband connections are often idle • 65% of on-line households have Wi-Fi • What if … • … home users open up their APs … • … and share/sell the spare bandwidth? • Cellular alternative for mobile users: • Messaging (multimedia, e-mail, text) • Location-aware services • Mobile sensor networks (e.g. MIT project CarTel ) • Challenges • Legal, economic, security, policy issues • Performance

  4. Wi-Fi For Mobile Messaging: Will it work? • Wi-Fi cells are smaller than cellular cells • Is density sufficient? Are connections too short? • Organically grown, unplanned deployments • Uneven densities, AP churn, unpredictable • Back-of-the-envelope: • 55 km/hour: ~15 meters/s • ~150 meter AP coverage [Akella’05] • ~10 sec connectivity • What about connection overhead? • scan, associate, get IP, etc. • Current stacks too slow • How long does it take your laptop to get an IP here?

  5. Outline • Data and experimental method • Connectivity properties • Data transfer properties • Towards OpenWiFi networks

  6. Deployment and Data • 232 days of normal driving (07/05 – 07/06) • in Boston and Seattle • 290 hours of clean data • 260 distinct km of roads • 50% data from 15 km • 32,000 APs discovered • 2000 open • 75,000 AP join attempts • 9 cars: • Embedded PC • 200mW 802.11b @ 1MBps • GPS unit Area shown: ~21x15 km GPS unit Wi-Fi Antenna

  7. Experimental Method: Scanping get ip IP in cache? use cache try DHCP success No access points found scan open AP found associate success 3 seconds of lost pings get ip local AP ping ping success success success e2e ping tcp test upload

  8. Association Duration Definition scan associate get ip AP ping loss IP acquisition time

  9. IP Address Acquisition Cached IP Combined DHCP Fraction of successful attempts • Default DHCP timeout is too long • Simple fixes: • small DHCP timeout • caching leased IP IP acquisition delay (s)

  10. Outline • Data and experimental method • Connectivity properties • Data transfer properties • Towards OpenWiFi networks

  11. Association Duration Definition 1st AP ping received Last AP ping received scan associate get ip AP ping loss association duration time

  12. Association Duration • Associations last over tens of seconds even at vehicular speeds. • Median: 13 seconds • Mean: 24 seconds Fraction of associations Association duration (s)

  13. Connectivity vs. Speed Connections established at range of speeds. Little data at higher speeds (system is not optimized for subsecond connections yet) Fraction of associations Speed (km/h)

  14. Association Duration vs. Speed Association Duration (s) ~10 seconds at 55km/h Speed (km/h)

  15. Estimating AP Coverage 200 ft 100 m Procedure: • Note locations • Find bounding box • Report diagonal location at the time of connection

  16. Access Point Coverage • Open Wi-Fi access points have a significant coverage area even in urban setting. • Median: 100 m • Mean: 150 m Fraction of access points Diameter of AP coverage (meters)

  17. Urban Access Points Density Access points are highly clustered. Using multiple access points at the same time may further increase throughput. Fraction of successful scans Number of APs discovered per scan

  18. Time To Connectivity Definitions End-To-End connection Join Success (no e2e) Join failed (MAC filtering)

  19. Time Between Connectivity Join Attempts Join Successes E2E Success During normal driving we encounter a new access point every 23 seconds on average. Today we can only use one every 260 seconds on average. Fraction of events Time between events (s)

  20. Outline • Data and experimental method • Connectivity properties • Data transfer properties • Towards OpenWiFi networks

  21. Bytes Uploaded Per Connection Non-trivial amount of data: Median: 200 KBytes per connection Mean: 600 KBytes Fraction of connections Consistency check: 600 KBytes / 24 sec = 25 KBps Bytes received on server (KBytes)

  22. Impact of Mobility on Delivery Rate Packet delivery rate 80% delivery rate would cripple TCP Hypothesis: losses are non-uniform Speed (km/h)

  23. Related Work • Location and range of in situ Wi-Fi: • wardriving.com, wigle.com, wifimaps.com • Akella et al ’05, ‘06 • Vehicular Mobility of Wi-Fi client: • Ott and Kutscher ’04, ’05; Gass et al ’06; etc • Mobility in cellular networks: • Rodriguez ’04; Qureshi and Guttag ’05; etc This is the first end-to-end Wi-Fi performance study under normal driving conditions

  24. Outline • Data and experimental method • Connectivity properties • Data transfer properties • Towards OpenWiFi networks

  25. Towards Open Wi-Fi Networks • Today • Rampant, high-bandwidth use is a bad idea • “Unauthorized access” or “trespassing” • May violate ISP contract even if users “opt-in” • Solution: • Part I: provide economic incentives (Fon, etc) • Mobile user pay nominal fee • Home users “opt-in” • ISPs get a cut • Part II: provide technology • Tiered accounting, security, and QoS for home APs • Fast delay-tolerant stack for mobile users

  26. Conclusion • Today, during normal driving • New access point every 23 seconds (avg) • Associations last for 24 seconds (avg) • Median TCP upload: ~200 Kbytes • Connectivity is equi-probable at [0; 60] km/h • In situ APs are is highly clustered • Use multiple APs simultaneously • Simple techniques can improve DHCP latency OpenWiFi networks have tremendous potential. Will we tap into it? http://cartel.csail.mit.edu

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