Distributed WiFi Analysis

1
Distributed Wi-Fi Analysis

• By Arun Chhetri

2
Index

• 802.11 Introduction
• Problem Description
• Previous Work Motivation
• Proposed Solution
• State Diagram
• Contention Finding Algorithm
• Block Diagram
• Results
• Acknowledgements
• References

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802.11 Introduction

• Extension of IEEE 802.3 protocol (ethernet)
• Defines media access control and physical layer
  for wireless network
• Uses CSMA/CA as compared to CSMA/CD in Ethernet
• Basic Service Set - BSS (Infrastructure BSS and
  Independent BSS)
• Extended Service Set ESS
• Data is transferred as frames with maximum size
  of 2304 bytes
• Bigger frames are fragmented
• Three types of frame
• Control Frame
• Data Frame
• Management Frame

802.11 Architecture
4
A familiar question

• How to give best Wi-Fi experience to users ???????

5
A familiar situation

• Wi-Fi is not working !! call support

• Problems
• Contention with nearby wireless devices
• Bad AP channel assignment
• Microwave Owen
• Congestion
• Poor choice of AP
• 802.11 DOS attack
• .

AP
N1
N2
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Problem Description
AP7
AP6
6
3
S2
5
AP1
AP2
1
4
6
3
3
3
S5
2
4
S1
5
5
1
4
6
4
6
AP3
2
2
2
3
AP4
3
1
1
S6
5
S3
5
S4
4
6
4
6
5
Sniffers
2
2
1
1

• What will happen when all the access points
  are transmitting at same channel
• How one node For e.g. N1 in AP1 will behave in
  time with node N3 in AP3

7
Problem Description

• To analyze the relationship between different
  wireless nodes present in distributed Wi-Fi
  system
• Contention Graph
• Probability of Successful transmission
• Further these reports can be used find problems
  in a Wi-Fi setting
• To check whether random back-off algorithm of a
  wireless node is in compliance with IEEE
  specification
• Efficiency of nodes, upstream and downstream
  data, network usage etc.

8
Previous Work and Motivation

• Yeo An Accurate Technique for Measuring the
  Wireless Side of Wireless Networks
• Packet capturing and merging technique
• Rodrig and Mahajan Measurement-based
  Characterization of 802.11 in a Hotspot Setting
• Packet capturing, overheads, retransmission as a
  factor of signal strength and how many active
  nodes are present in network
• Jigsaw Solving the Puzzle of Enterprise 802.11
  Analysis
• Large scale deployment of sniffers
• Reconstruct a global view of network activity of
  802.11 network

In future work, we intend to develop a
comprehensive portfolio of analyses and use them
to drive more complex operational questions
(e.g., what are the root causes of transient
outages and performance degradations) and
research questions (e.g., how to better optimize
wireless protocols and their interactions with
other protocol layers)
We infer that the transmission from s failed to
reach r when we do not observe an ACK from r.
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State Diagram
DCF State Diagram
NAV State Diagram
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Contention Finding algorithm
Atomic Sequences
RTS
CTS
Frame
ACk
So, all those frames which breaks any atomic
sequence, can cause contentionIf there is
retransmission of two or more than two different
data frames which were part of two or more
different atomic sequences which overlaps in
time, then there are fair chances that they will
collided.
Two stations will exponentially back-off, due to
the loss of ack
Two Stations S1 and S2 where they both cannot
sense each other
S2 Frame z
S2 Frame z
S2
SIFS
SIFS
RTS
CTS
S1 Frame x
S1 Frame x
S1
NAV (RTS CTS)
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Block Diagram

• Layer 4
• Presenting information to user
• Inference System
• Contention Finder
• Network Statistics
• Random Back-off compliance test

• Layer 3
• Create Indexes
• Find contention in traces
• Find relations between nodes
• Check random back-off compliance with IEEE
• specification

• Layer 2
• Parsing pcap format data.

• Layer 1
• Synchronizing all the sniffers
• Scheduling sniffer to sniff channels
• Kismet

12
Results
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Results
14
Results
15
Acknowledgements

• Dr. Rong Zheng for her advice
• Mr. Amit Rane, Suman and Shweta which also
  contributed in the development of this tool.

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References

• IEEE 802.11 specification
• 802.11 Wireless network The Definitive guide
• IEEE 802.11 Handbook- A Designers companion
• Jigsaw Solving the Puzzle of Enterprise 802.11
  Analysis
• IEEE 802.11 MAC Simulator with DCF and
  Power-Saving Mode by Charles Song
• http//djw.cs.washington.edu//papers/trace-ewind.p
  df
• www.tcpdump.org/pcap