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 Computer Science and Artificial Intelligence Laboratory

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A measurement study of vehicular internet access
using in situ Wi-Fi networksVladimir
Bychkovsky, Bret Hull, Allen Miu, Hari
Balakrishnan, and Samuel MaddenMIT Computer
Science and Artificial Intelligence
Laboratory
By Anup Jaya
Prakash
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Outline of the presentation

• Introduction
• Questions
• Answers The Measurement Study
• The Experiment
• Connectivity Results
• Packet Loss and Data Transfers
• Discussions
• Conclusion

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Introduction

• 14.3 million homes in the US have Wi-Fi access
  points.
• Most of the links are often idle.
• Can be used by others by providing controlled
  access.
• What would be the performance?

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Questions

• What is the distribution of the duration of
  connectivity per AP?
• What is the distribution of the duration of
  disconnectivity
• How long does it take for a client to scan,
  associate, and obtain an IP address?
• What is the distribution of the coverage region
  of an AP?
• What is the distribution of packet loss and data
  transfer rates?
• What is the effect of a cars speed on these
  metrics?

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Answers The Measurement Study

• A set of in situ open APs deployed in and around
  Boston.
• 9 Distinct cars fitted with embedded computers.
• They try to associate with an AP and if
  successful, try to obtain an IP address.
• Next they do an end-to-end ping to a well known
  IP.
• If this is successful, they start periodic local
  pings to next hop IP router and initiates a
  transfer to the internet site.

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Answers

• The Measurement study is based on uploads. Two
  reasons are provided for this
• New Applications treat vehicles as data source in
  wireless sensor networks ( CarTel Project )
• Download performance will be at least as good as
  the upload performance
• The Results are Divided into 2 categories
• Connectivity
• Data transfer performance

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The Experiment

• CarTel Embedded Computer Has a 802.11b Wi-Fi
  card, GPS unit, 128 MB RAM, 1 GB Flash memory,
  running Linux 2.4.31.
• Scanping Application used for the
  experimentation purpose
• GPS used to find position and speed of the car.
• Computer draws power from the car. Boots up when
  ignition is turned

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The Experiment - Processes

• Scan Scan for APs in the Area
• Association Try to Associate with one of the
  APs
• Address Configuration Acquire an IP
• Single end-to-end ping Try for an end-to-end
  ping
• Connectivity and uploads Measure bandwidth and
  connectivity

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Scan

• Discovery of the APs in an area
• For each discovered AP, the ESSID, BSSID,
  Frequency, Signal strength and privacy bits (if
  any) are logged.
• Done till at least one AP is discovered, then
  proceeds to next step

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Association

• Scanping issues a command to Wi-Fi interface to
  associate with the AP.
• For multiple APs, highest signal strength is
  taken into account
• The result of association along with start time
  and duration of operation is logged
• Jumps to first step if failed else runs tcpdump
  to monitor furthur networking activity.

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Address Configuration

• Uses dhcpcd to obtain an IP address.
• Checks local cache for information on current AP.
• If exists then uses the information.
• Otherwise tries to obtain an IP address
• If it fails, the client ties out after 5 secs and
  proceeds to step 1.

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Single End-to-End Ping

• Once it obtains an IP, it starts a end-to-end
  ping every 200ms until the first successful ping
  or until 2 sec elapse.
• This is to estimate end-to-end connectivity
  duration

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Connectivity and Uploads

• 2 processes in parallel
• AP Pings Ping the first hop router every 100ms
  and log the time and the result of the ping
• TCP uploads Establish a TCP connection to
  central sever and deliver data.

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Timeline of the Operations
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Data Summary
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Connectivity Results

• The Connectivity results are organized into 4
  parts namely
• Wi-Fi Association and IP Address Acquisition
  first 3 graphs
• Connectivity Duration 4th, 5th and 6th graphs
• Periods without connectivity 7th graph
• AP coverage 8th, 9th and 10th graphs

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Connectivity Results
CDF for distribution of time for various phases
of activities after a successful association
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Connectivity Results
Distribution of scan and association times
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Connectivity Results
Distribution of time for different types of IP
acquisition
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Connectivity Results
CDF of Association Durations
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Connectivity Results
CDF of Average speeds for Associations
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Connectivity Results
Plot of Connection duration Vs Speed
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Connectivity Results
CDF of time between connectivity events between
4 types of events
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Connectivity Results
CDF of no. of APs discovered in successful scan
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Connectivity Results
Fraction of associations to any give AP
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Connectivity Results
CDF of Connection Coverage
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Packet Loss and Data Transfers

• These Results are divided into two categories
• Wi-Fi packet loss rates
• TCP throughput

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Wi-Fi Packet Loss Rates
CDF of fraction of AP pings that succeeded per
connection
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Wi-Fi Packet Loss Rates
Plot of Wi-Fi packet delivery rate vs the car
speed
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TCP Throughput
CDF of duration between association and first TCP
data ACK
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TCP Throughput
CDF of per connection end-to-end throughput
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TCP Throughput
CDF of per connection bytes received at server
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Discussion

• The idea of the paper holds much promise
• What are the other issues in implementation of
  such networks
• Lets see them one by one

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Open Wi-Fi Networks

• Incentives to users and ISPs
• Tiered security level with different levels of
  access controll and data rates at different tiers

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Connectivity and Transport

• Continuous connectivity to mobile users
• If not continuous then some modeling change would
  be needed

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Fast and Friendly Connection Establishment

• Fast connection establishment and fair
  utilization of bandwidth necessary
• Three possible optimizations
• Connection initiation timing optimize transport
  protocol to address this issue
• Fairness Rate limiting at APs or use
  cooperative TCP protocols

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Conclusion

• With just 3.2 of total APs participating in the
  system, the clients remain connected for 24 sec
  on an average.
• The mean period of disconnectivity was 260
  seconds which would reduce if the participation
  increased
• Several commercial, legal and policy issues need
  to be addressed in order to make this vision a
  reality

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Questions ?