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
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_se
minar_slides/kim-rateadaptation06.ppt+RRAA
Presentation Outline
•
•
•
•
•
•
Packet Loss Problem
Current Rate Adaption Schemes
COLLIE Overview
COLLIE Metrics
COLLIE Analysis
Conclusion
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?
Problem Defined
• Collision or Weak Signal, why does knowing
matter?
Fixing packet loss
• Appropriate actions
– For collision
CW Max
• BEB
Retries
REF: http://pages.cs.wisc.edu/~shravan/coll-infocom.pdf
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
Rate Adaptation Example
• Rate adaptation affects throughput performance and
should be adjusted by channel condition
54Mbps
12Mbps
Signal is good
Signal becomes weaker
Sender
Receiver
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
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
RAA Problem
12Mbps Sender
Signal is still good
54Mbps
12 Mbps
Signal is good
Sender
Receiver
12Mbps
Signal is still good
Sender
54Mbps
Signal is good
Sender
With hidden terminals, reducing the rate
prolongs transmission time for each packet and
54Mbpsresults
Sender in more collisions
Signal is good
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
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
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%
COLLIE Continued
Client
AP
Data
Feedback
Adjust
Data Rate/Power
Or
Contention Window
Received Signal
Strength
Collision Inference
Algorithm
Symbol error
patterns
Bit error distribution
and patterns
Note: assumes Feedback is successfully received and sender’s MAC address is
decoded correctly by the AP
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
Symbol-level Errors
• Symbol Error Rate (SER)- % symbols received
in error
• Errors Per Symbol (EPS)- average # errors
within each symbol
n
2
• Symbol Error Score (S-score):  Bi , where Bi
i 1
is a burst of n bits
S-Score
Collision

0011 0011 0011 
n
S-Score =
i 1
0111 1011 0010
Bi  12  12  12  3
2
Channel Fluctuation
• 0011 0011 0011
 0011 1101 0011
n
S-Score =
i 1
Bi  02  32  02  9
2
http://pages.cs.wisc.edu/~shravan/coll-infocom.pdf
Experimental Design
• Three possibilities at R:
1. Packet received without error
2. Packet received in error
3. No packet received
Experimental Design
• Two transmitters, T1 and T2
• Two receivers, R1 and R2
• Receiver R hears all signals
Analysis of Results
Metric
Collision
Weak Signal
RSS
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)
Analysis of Results
Begs the Question
• Is it worth it? Successful almost 60%, false positive rate of 2.4%
Check out this accuracy?
Design Components
Client Module
Multi-AP COLLIE
• Error packet sent to a central COLLIE server
• Most important where the capture effect is dominant
Multi-AP Results
• Static situation averaged 30% gains in
throughput
• For multiple collision sources and high
mobility, throughput gains reached 1560%
Collision Analysis
Some Problems
• Capture Effect
• Packet size
• Packet Kickback
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
Questions?