ECE 256
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.ppt+RRAA
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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 Loss2 Causes: Weak Signal and Collision
• 802.11 Solution Inadequatedefaults 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
RetriesREF: http://pages.cs.wisc.edu/~shravan/coll-infocom.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
Sender Receiver
54MbpsSignal is goodSignal becomes weaker
12Mbps
Reference: Robust Rate Adaptation in 802.11 Networks Presentation by Kuo-Chung Wang
Rate Too High Rate Too LowIncreases Loss Ratio Capacity Under-Utilized
Decreased Throughput Decreased Throughput
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Related WorkRate Adaptation Algorithms
–Differentiate between loss behaviors –Adapt to realistic scenarios–Handle hidden stations
ARF ~ Auto-rate FallbackCARA ~ Collision-Aware Rate AdaptationMRD ~ Multi-Radio DiversityRBAR ~ Receiver Based Auto RateRRAA ~ Robust Rate Adaptation Algorithm
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RAA Problem
ReceiverSender
54MbpsSignal is good
12 Mbps
Sender54MbpsSignal is good
Sender
54MbpsSignal is good
Sender12MbpsSignal is still good
Sender12Mbps
Signal is still good
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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?Data
Feedback
Collision Inference Algorithm
Received Signal Strength
Bit error distribution and patterns
Symbol error patterns
X
Note: assumes Feedback is successfully received and sender’s MAC address is decoded correctly by the AP
Adjust Data Rate/Power
Or Contention Window
Client AP
COLLIE Continued
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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
2
1
n
ii
B
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S-Score
• 0011 0011 0011 0011 1101 0011
Collision
Channel Fluctuation
0011 0011 0011 0111 1011 0010
S-Score =
2 2 2 2
1
1 1 1 3n
ii
B
S-Score = 2 2 2 2
1
0 3 0 9n
ii
B
http://pages.cs.wisc.edu/~shravan/coll-infocom.pdf
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Experimental Design
• Three possibilities at R:1. Packet received without error2. Packet received in error3. 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 SignalRSS Higher (90% > -73dBm) Lower (98% < -73dBm)
BER Higher (24% =< 12% BER) Lower (98% =< 12% BER)
SER Unremarkable Unremarkable
EPS Higher (45% =< 28% EPS) Lower (98% =< 28% EPS)
S_Score Higher (28% =< 500) Lower (98% =< 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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Client Module
Design Components
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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?