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

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

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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 Akella05
• 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?

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Outline

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

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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 _at_ 1MBps
• GPS unit

Area shown 21x15 km
GPS unit
Wi-Fi Antenna
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Experimental Method Scanping
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
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Association Duration Definition
scan
associate
get ip
AP ping
loss
IP acquisition
time
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IP Address Acquisition
Fraction of successful attempts

• Default DHCP timeout is too long

• Simple fixes
• small DHCP timeout

• caching leased IP

IP acquisition delay (s)
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Outline

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

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

• Associations last over tens of seconds even at
  vehicular speeds.
• Median 13 seconds
• Mean 24 seconds

Fraction of associations
Association duration (s)
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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)
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Association Duration vs. Speed
Association Duration (s)
10 seconds at 55km/h
Speed (km/h)
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Estimating AP Coverage

• Procedure
• Note locations
• Find bounding box
• Report diagonal

location at the time of connection
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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)
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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
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Time To Connectivity Definitions
End-To-End connection Join Success (no e2e) Join
failed (MAC filtering)
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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.
Fraction of events
Time between events (s)
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Outline

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

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

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

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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

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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