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

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

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, , text) –Location-aware services –Mobile sensor networks (e.g. MIT project CarTel ) Challenges –Legal, economic, security, policy issues –Performance

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?

Outline Data and experimental method Connectivity properties Data transfer properties Towards OpenWiFi networks

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 1MBps –GPS unit GPS unit Wi-Fi Antenna Area shown: ~21x15 km

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

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

Outline Data and experimental method Connectivity properties Data transfer properties Towards OpenWiFi networks

Association Duration Definition scanassociateget ipAP pingloss association duration time Last AP ping received 1 st AP ping received

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

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

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

Estimating AP Coverage Procedure: 1.Note locations 2.Find bounding box 3.Report diagonal 200 ft 100 m location at the time of connection

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

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

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

Time Between Connectivity 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. Join Attempts Join Successes E2E Success Time between events (s) Fraction of events

Outline Data and experimental method Connectivity properties Data transfer properties Towards OpenWiFi networks

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

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

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

Outline Data and experimental method Connectivity properties Data transfer properties Towards OpenWiFi networks

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

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?