Diagnosing Wireless Packet Losses in 802.11: Separating Collision from Weak Signal

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Diagnosing Wireless Packet Losses in 802.11
Separating Collision from Weak Signal

• Presented By
• Jacob H. Cox Jr
• For
• ECE 256 Wireless Networking and Mobile
  Computing
• February 10, 2009

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Acknowledgments

• Authors Shravan Rayanchu, Arunesh Mishra,
  Dheeraj Agrawal, Sharad Saha, Suman Banerjee
• Kuo-Chung Wang (Slide Presentation)
• http//lion.cs.uiuc.edu/group_seminar_past/fall06/
  group_seminar_slides/kim-rateadaptation06.pptRRAA

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Presentation Outline

• Packet Loss Problem
• Current Rate Adaption Schemes
• COLLIE Overview
• COLLIE Metrics
• COLLIE Analysis
• Conclusion

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Motivation

• Packet Loss
• 2 Causes Weak Signal and Collision
• 802.11 Solution Inadequate
• defaults to BEB for a substantial number of
  packet losses
• Question
• Does the type of packet loss matter?
• What if we could determine its cause?

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Problem Defined

• Collision or Weak Signal, why does knowing
  matter?

Beamforming?
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Fixing packet loss

• Appropriate actions
• For collision
• BEB

CW Max
Retries
REF http//pages.cs.wisc.edu/shravan/coll-infoco
m.pdf
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Rate Adaptation

• 802.11 a/b/g standards allow for the use of
  multiple transmission rates
• 802.11a, 8 rate options (6,9,12,18,24,36,48,54
  Mbps)
• 802.11b, 4 rate options (1,2,5.5,11Mbps)
• 802.11g, 12 rate options (11a set 11b set)
• Some papers report that rate adaptation is
  important yet unspecified in 802.11 standards

Reference Robust Rate Adaptation in 802.11
Networks Presentation by Kuo-Chung Wang
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Rate Adaptation Example

• Rate adaptation affects throughput performance
  and should be adjusted by channel condition

12Mbps
Sender
Rate Too High Rate Too Low
Increases Loss Ratio Capacity Under-Utilized
Decreased Throughput Decreased Throughput
Reference Robust Rate Adaptation in 802.11
Networks Presentation by Kuo-Chung Wang
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Related Work
Rate Adaptation Algorithms Differentiate
between loss behaviors Adapt to realistic
scenarios Handle hidden stations ARF
Auto-rate Fallback CARA Collision-Aware Rate
Adaptation MRD Multi-Radio Diversity RBAR
Receiver Based Auto Rate RRAA Robust Rate
Adaptation Algorithm
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RAA Problem
Receiver
With hidden terminals, reducing the rate prolongs
transmission time for each packet and results in
more collisions
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Introduction to COLLIE

• 802.11, CARA, and RRAA use multiple attempts to
  deduce cause of packet loss
• COLLIE uses a direct approach
• Error packet kickback
• Client analysis

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COLLIE

• Collision Inferencing Engine
• Utilizes receiver feedback
• Analyzes
• Bit and symbol level error patterns
• Received signal strength
• Design
• Signal analysis algorithms
• Link layer protocol which adjusts link layer
  parameters

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Link Adaptation Mechanism Enhancements

• Auto Rate Fallback (ARF)
• Used in conjunction w/COLLIE for this paper
• Rate adaption mechanism enhanced with inferencing
  component
• Using COLLIE, observed throughput gains of 20-60

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COLLIE Continued
?
Client
AP
Data
X
Feedback
Received Signal Strength
Adjust Data Rate/Power Or Contention Window
Collision Inference Algorithm
Bit error distribution and patterns
Symbol error patterns
Note assumes Feedback is successfully received
and senders MAC address is decoded correctly by
the AP
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Metrics for Analysis

• Received Signal Strength (RSS) S I
• S Signal Strength
• I Interference
• Bit Error Rate (BER) total incorrect bits
• Symbol level errors errors within transmission
  frame
• Multiple tools used to analyze symbol-level
  errors

http//pages.cs.wisc.edu/shravan/coll-infocom.pdf
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Symbol-level Errors

• Symbol Error Rate (SER)- symbols received in
  error
• Errors Per Symbol (EPS)- average errors within
  each symbol
• Symbol Error Score (S-score) , where Bi is a
  burst of n bits

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S-Score
Collision

• 0011 0011 0011 ? 0111 1011 0010

Channel Fluctuation

• 0011 0011 0011 ? 0011 1101 0011

http//pages.cs.wisc.edu/shravan/coll-infocom.pdf
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Experimental Design

• Three possibilities at R
• Packet received without error
• Packet received in error
• No packet received

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Experimental Design

• Two transmitters, T1 and T2
• Two receivers, R1 and R2
• Receiver R hears all signals

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Analysis of Results
Metric Collision Weak Signal
RSS Higher (90 gt -73dBm) Lower (98 lt -73dBm)
BER Higher (24 lt 12 BER) Lower (98 lt 12 BER)
SER Unremarkable Unremarkable
EPS Higher (45 lt 28 EPS) Lower (98 lt 28 EPS)
S_Score Higher (28 lt 500) Lower (98 lt 500)
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Analysis of Results
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Begs the Question

• Is it worth it? Successful almost 60, false
  positive rate of 2.4

Check out this accuracy?
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Design Components
Client Module
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Multi-AP COLLIE

• Error packet sent to a central COLLIE server
• Most important where the capture effect is
  dominant

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Multi-AP Results

• Static situation averaged 30 gains in throughput
• For multiple collision sources and high mobility,
  throughput gains reached 15-60

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Collision Analysis
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Some Problems

• Capture Effect
• Packet size
• Packet Kickback

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Conclusions

• COLLIE implementation achieves increased
  throughput (20-60) while optimizing channel use
• 40 reduction in retransmission costs
• Implementation can be done over standard 802.11,
  resulting in much lower startup costs than other
  protocols

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Questions?