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. Wi-Fi Is Everywhere. - PowerPoint PPT PresentationTRANSCRIPT
A Measurement Study of Vehicular Internet Access
Using In Situ Wi-Fi Networks
Vladimir Bychkovsky, Bret Hull, Allen Miu,
Hari Balakrishnan, and Samuel Madden
http://cartel.csail.mit.edu
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, 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)– 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
GPSunit
Wi-FiAntenna
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
Association Duration Definition
scan
associate
get ip AP ping loss
IP acquisitio
ntime
Fract
ion
of
succ
ess
ful att
em
pts
IP acquisition delay (s)
Cached IPCombinedDHCP
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
scan
associate
get ip AP ping loss
association
durationtime
Last AP ping received
1st AP ping received
Association Duration
Association duration (s)
Fract
ion
of
ass
oci
ati
on
s
Associations last over tens of seconds even at vehicular speeds.• Median: 13 seconds• Mean: 24 seconds
Connectivity vs. SpeedFr
act
ion
of
ass
oci
ati
on
s
Speed (km/h)
Connections established at range of speeds. Little data at higher speeds (system is not optimized for subsecond connections yet)
Ass
oci
ati
on
Dura
tion
(s)
Speed (km/h)
Association Duration vs. Speed
~10 seconds at 55km/h
Estimating AP CoverageProcedure:
1. Note locations2. Find bounding
box3. Report diagonal
200 ft
100 m
location at the time of connection
Fract
ion
of
acc
ess
poin
ts
Diameter of AP coverage (meters)
Access Point CoverageOpen 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.
Fract
ion
of
succ
ess
ful sc
an
s
Number of APs discovered per scan
Time To Connectivity Definitions
End-To-End connectionJoin 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 SuccessesE2E Success
Time between events (s)
Fract
ion
of
even
ts
Outline• Data and experimental method
• Connectivity properties
•Data transfer properties
• Towards OpenWiFi networks
Bytes Uploaded Per ConnectionNon-trivial amount of data:Median: 200 KBytes per connectionMean: 600 KBytes
Fract
ion
of
con
nect
ion
s
Bytes received on server (KBytes)
Consistency check:600 KBytes / 24 sec = 25 KBps
Impact of Mobility on Delivery Rate
80% delivery rate would cripple TCPHypothesis: losses are non-uniformP
ack
et
deliv
ery
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;
etcThis 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