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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 Computer Science and Artificial Intelligence Laboratory By – Anup Jaya Prakash. - PowerPoint PPT PresentationTRANSCRIPT
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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 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 car’s speed on these metrics?
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Answers – The Measurement Study A set of in situ open AP’s
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 AP’s in the Area Association – Try to Associate with one
of the AP’s 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 AP’s 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 AP’s 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 AP’s or use cooperative TCP protocols
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Conclusion With just 3.2% of total AP’s 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 ?