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

Packet loss in Wireless Packet loss in Wireless NetworksNetworks

A BC

A send RTS to B

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 lossAppropriate actions

◦For collision BEB

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: An OverviewCOLLIE: An OverviewClient ModuleAP ModuleServer Module (optional)

COLLIE: An OverviewCOLLIE: An Overview

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

PerformancePerformance

Successful almost 60%, false positive rate 2.4%

Metric voting scheme

Some ProblemsSome ProblemsRSS: universal cutoff impossibleCapture EffectPacket size

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

Questions?Questions?