diagnosing wireless packet losses in 802.11: separating collision from weak signal shravan rayanchu,...
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Diagnosing Wireless Packet Diagnosing Wireless Packet Losses in 802.11:Losses in 802.11:Separating Collision from Separating Collision from Weak SignalWeak Signal
Shravan Rayanchu, Arunesh Mishra, Dheeraj Agrawal, Sharad Saha, Suman Banerjee
MotivationMotivationPacket Loss
2 Causes
Solution Inadequate802.11
Can we determine cause of packet loss?
Packet loss in Wireless Packet loss in Wireless NetworksNetworks
A BC
While A is transmitting,
C initiates RTS to B
Packet loss in Wireless Packet loss in Wireless NetworksNetworks
A BC
Since neither A nor B knows the other is transmitting, both RTS’s
are sent and collide at B, resulting in packet loss
Packet loss in Wireless Packet loss in Wireless NetworksNetworks
A BC
Since neither A nor B knows the other is transmitting, both RTS’s
are sent and collide at B, resulting in packet loss
Packet loss in Wireless Packet loss in Wireless NetworksNetworks
A
B
C
Here A and C are in just barely in range of each other, but both are
in range of B
Packet loss in Wireless Packet loss in Wireless NetworksNetworks
A
B
C
A send its RTS to C, which is received and B is silenced
Packet loss in Wireless Packet loss in Wireless NetworksNetworks
A
B
C
C send its CTS to A, but the packet is not heard due to weak signal caused by interference by
noise
Detecting Packet LossDetecting Packet LossRecap: 2 causes of packet loss802.11 Solution
◦BEBDifferent causes lead to different
solutions
Fixing Packet LossFixing Packet Loss◦For low signal
Increase power Decrease data rate How to differentiate?
C EA
D
B
Rate = 20
Rate = 10
Introduction to COLLIEIntroduction to COLLIE802.11, CARA, and RRAA use
multiple attempts to deduce cause of packet loss
COLLIE direct approach Error packet kickbackClient analysis
COLLIE: Single APCOLLIE: Single APAP error packet kickbackClient-side analysisProblem: how can the AP
successfully re-transmit packet?
Experimental DesignExperimental Design
Two transmitters, T1 and T2Two receivers, R1 and R2Receiver R hears all signals
Experimental DesignExperimental DesignThree possibilities at R:1. Packet received without error2. Packet received in error3. No packet received
Error MetricsError MetricsThree error metrics:Bit Error Rates (BER)Symbol Error Rates (SER)Error Per Symbol (EPS)
Metrics for AnalysisMetrics for AnalysisReceived Signal Strength (RSS) =
S + IHigh RSS collisionLow RSS channel fluctuations
Bit Error Rate (BER) = total # incorrect bits
BER is higher for collisions, lower for low signal
RSS: The DetailsRSS: The Details
Of all packets lost due to low signal, 95% had an RSS less than -73dB, compared to only 10% for collisions
Metrics for AnalysisMetrics for AnalysisSymbol level errors: errors
within transmission frameMultiple tools used to analyze
symbol-level errors
FramingFraming
0011 0011 0011 0011 1101 0011
Collision
Channel Fluctuation
0011 0011 0011 0111 1011 0010
Symbol-level ErrorsSymbol-level ErrorsSymbol Error Rate (SER)- # symbols
received in errorErrors Per Symbol (EPS)- average # errors
within each symbolSymbol Error Score (S-score): calculated
as , where Bi is a burst of n bits2
1
n
ii
B
74% accuracy
S-ScoreS-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
Multi-AP COLLIEMulti-AP COLLIEError packet sent to a central
COLLIE server
Most important where the capture effect is dominant
ResultsResultsStatic situation average of
30% gains in throughputFor multiple collision sources and
high mobility, throughput gains of 15-60%
ConclusionsConclusionsCOLLIE implementation achieves
increased throughput (20-60%) while optimizing channel use
Implementation can be done over standard 802.11, resulting in much lower startup costs than other protocols