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
What are the performance properties
of organically grown Wi-Fi networks?
Images from WiGLE.net and CarTel
• Today:
The Opportunity
– 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
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)
Area shown:
~21x15 km
– 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
GPS
unit
Wi-Fi
Antenna
Experimental Method: Scanping
No access
points
found
scan
open AP found
get ip
IP in cache?
use cache
try DHCP
success
associate
success
3 seconds of lost pings
get ip
local AP ping
success
e2e ping
success
tcp test upload
ping
success
Association Duration Definition
scan associate
get ip
IP
acquisition
time
AP ping
loss
Fraction of successful attempts
IP Address Acquisition
Cached IP
Combined
DHCP
• Default DHCP timeout is too long
• Simple fixes:
• small DHCP timeout
• caching leased IP
IP acquisition delay (s)
Outline
• Data and experimental method
• Connectivity properties
• Data transfer properties
• Towards OpenWiFi networks
Association Duration Definition
1st AP ping Last AP ping
received
received
scan associate
get ip
AP ping
association
duration
time
loss
Fraction of associations
Association Duration
Associations last over tens of
seconds even at vehicular speeds.
• Median: 13 seconds
• Mean: 24 seconds
Association duration (s)
Fraction of associations
Connectivity vs. Speed
Connections established at
range of speeds. Little data
at higher speeds (system is
not optimized for subsecond
connections yet)
Speed (km/h)
Association Duration (s)
Association Duration vs. Speed
~10 seconds at 55km/h
Speed (km/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
Access Point Coverage
Open Wi-Fi access points have
a significant coverage area
even in urban setting.
• Median: 100 m
• Mean: 150 m
Diameter of AP coverage (meters)
Fraction of successful scans
Urban Access Points Density
Access points are highly
clustered. Using multiple access
points at the same time may
further increase throughput.
Number of APs discovered per scan
Time To Connectivity Definitions
End-To-End connection
Join Success (no e2e)
Join failed (MAC filtering)
Fraction of events
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)
Outline
• Data and experimental method
• Connectivity properties
• Data transfer properties
• Towards OpenWiFi networks
Fraction of connections
Bytes Uploaded Per Connection
Non-trivial amount of data:
Median: 200 KBytes per
connection
Mean: 600 KBytes
Consistency check:
600 KBytes / 24 sec = 25 KBps
Bytes received on server (KBytes)
Packet delivery rate
Impact of Mobility on Delivery Rate
80% delivery rate would cripple TCP
Hypothesis: losses are non-uniform
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?
http://cartel.csail.mit.edu