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

Outline of the presentation Introduction Questions Answers – The Measurement Study The Experiment Connectivity Results Packet Loss and Data Transfers Discussions Conclusion

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?

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?

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.

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

The Experiment CarTel Embedded Computer – Has a b Wi-Fi card, GPS unit, 128 MB RAM, 1 GB Flash memory, running Linux 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

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

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

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.

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.

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

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.

Timeline of the Operations

Data Summary

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

Connectivity Results … CDF for distribution of time for various phases of activities after a successful association

Connectivity Results … Distribution of scan and association times

Connectivity Results … Distribution of time for different types of IP acquisition

Connectivity Results … CDF of Association Durations

Connectivity Results … CDF of Average speeds for Associations

Connectivity Results … Plot of Connection duration Vs Speed

Connectivity Results … CDF of time between connectivity events between 4 types of events

Connectivity Results … CDF of no. of AP ’ s discovered in successful scan

Connectivity Results … Fraction of associations to any give AP

Connectivity Results … CDF of Connection Coverage

Packet Loss and Data Transfers These Results are divided into two categories  Wi-Fi packet loss rates  TCP throughput

Wi-Fi Packet Loss Rates CDF of fraction of AP pings that succeeded per connection

Wi-Fi Packet Loss Rates … Plot of Wi-Fi packet delivery rate vs the car speed

TCP Throughput CDF of duration between association and first TCP data ACK

TCP Throughput … CDF of per connection end-to- end throughput

TCP Throughput … CDF of per connection bytes received at server

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

Open Wi-Fi Networks Incentives to users and ISPs Tiered security level with different levels of access controll and data rates at different tiers

Connectivity and Transport Continuous connectivity to mobile users If not continuous then some modeling change would be needed

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

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

Questions ?