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

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

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