cooperative packet recovery in enterprise wireless - lans
DESCRIPTION
Cooperative Packet Recovery in Enterprise Wireless - LANs. Souvik Sen. HP Labs [email protected]. Duke University [email protected]. Mahanth Gowda. Sung- Ju Lee. Romit Roy Choudhury. Narus [email protected]. Duke University r [email protected]. Wireless LAN & EWLAN. Internet. - PowerPoint PPT PresentationTRANSCRIPT
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Cooperative Packet Recovery in Enterprise Wireless - LANs
Mahanth GowdaDuke University
Souvik SenHP Labs
Romit Roy ChoudhuryDuke University
Sung-Ju LeeNarus
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InternetInternet
Internet Internet
Internet
Internet
Wireless LAN & EWLAN
AP AP AP
AP AP AP
Controller
Internet
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Packet Combining
AP AP AP
AP AP AP
Controller
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Application Scenario• Upload traffic
• Usually multiple Access Points (APs) overhear client’s data• Increase bit rate, induce errors, recover via combining
4
Controller
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Do we Care for Upload Traffic ?• Upload traffic is increasing at a rapid pace because:
5
Cloud Computing
P2P File Access
Controller
Sensor Data Upload
Code Offloading
VoIP, Video Chat
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Packet combining is an old idea …
• Several creative works done in the past•MRD (Mobicom 2005)•Soft (Mobicom 2007)
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MAC Layer combining - MRD
Decoding
Corrected Packet
7
Signals Signals
Bits Bits
Low Bandwidth overhead,Less Gain
Bit Combining
Decoding
DemodulationDemodulation
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MAC Layer combining - SOFT
8
Signals Signals
Soft Bits Soft Bits
Soft DemodulationSoft Demodulation
Soft Combining
Soft Decoding Soft Decoding
Corrected Packet
Low Bandwidth overhead,Less Gain
Bits: 1 0 1 0Soft Values: 0.6 -0.3 0.2 -0.7
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PHY Layer combining - MRC
9
Signals Signals
Signals Signals
Symbol Combining
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• Weighted centroid of erroneously received symbols
Weights based on channel
quality
MRC (Maximal Ratio Combining)
Jointly decode possibly corrupt
symbols to infer the correct symbol
P1 P2 P3 P4
P5 P6 P7 P8
P9 P10 P11 P11
P13 P14 P15 P16
𝑆=∑ h 𝑖∗𝑅𝑖
∑ h𝑖
10
Received Symbol at
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PHY Layer combining (MRC)
11
Signals Signals
Signals Signals
Symbol Combining
Corrected Packet
Decoding
High gain, Prohibitive Bandwidth
Demodulation
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EPICENTERPractical Symbol Level Packet Combiner
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Bits
SymbolsIs there a
sweet spot?
Coarser Symbols?
Gai
n
Bandwidth Overhead
10101
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Contributions• Finding a sweet spot in the overhead-combining tradeoff• Aggressive rate estimation algorithm to leverage combining
Epicenter Client
Symbol Combining
Rate Estimation
Epicenter AP
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Bits
Symbols
Gai
n
Bandwidth Overhead
MRC Bits
I
Q
IQ
x 9x1.6x
Coarser Symbols
10101
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Gai
n IQ
I
Q
Epi 1x
IQ
Bandwidth Overheadx 9x1.6x 2x
Bits
Symbols
MRC Bits
Coarser Symbols
10101
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Gai
n IQ Epi 1x
IQ
Bandwidth Overheadx 9x1.6x 2x
Bits
Symbols
MRC Bits
I
QI
Q
Epi 2x
2.5x
Coarser Symbols
10101
Overhead of Soft = 3x
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Epicenter System Architecture
18
Signals Signals
Low FidelitySignals
Corrected Packet
High Gain,Low
Bandwidth
Low Fidelity Transformer
Coarse SymbolCombining
Low Fidelity Transformer
Low FidelitySignals
Demodulation
Decoding
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Rate Adaptation• Rate is a function of modulation and coding scheme• Higher modulations support higher rates, but tolerate few
errors
P1 P2
P3 P4
00
01
10
11
Error Vector = Received Symbol –
Transmitted Symbol
Wrong Demodulation with
16QAM
Correct Demodulation with
QPSK
v
P1 P2 P3 P4
P5 P6 P7 P8
P9 P10 P11 P12
P13 P14 P15 P16
0101000110011101
1100 1000 0000 0100
1110 1010 0110
1111 1011 0011 0111
0010
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‘Look up’ based Rate Estimation
P4
P8
P11
P13 P14 P15 P16
P1 P2 P3
P5 P7
P9 P10 P11
P6F1
F3
20
• Bit errors depend upon the distribution of error vectors
• The Expected Bit error rate in the fig is F1*0 + (F2)*1.5 + (F3)*3• Bit error rate helps in bit rate estimation• Empirical table of rates,
fractions, delivery ratio (DR) <Rate, F1, F2, F3, DR>
Key Value
F2
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P2 P3 P4
P5 P7 P8
P9 P10 P11 P12
P13 P14 P15 P16
P1
F2
F3
P6
Table Look up Example
P6
Testing 24 Mbps, 16 QAM
F1
Key<24, 10/20, 6/20, 4/20>
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P1 P2 P3 P4 P5 P6
P7 P9 P10 P11 P12
P14 P15 P16 P17 P18
P19 P21 P22 P23 P24
P25 P26 P27 P28 P29 P30
P31 P32 P33 P34 P35 P36
P8
P13
P20
F1 F2
F3
P8
Testing 54 Mbps, 64 QAM (Partial constellation shown)
Table Look up ExampleKey
<54, 5/20, 8/20, 7/20>
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Rate Prediction Algorithm• Estimate the distribution of Error Vectors using client AP
channels (details in paper)• Estimate Delivery Ratio for all 802.11 Data-rates/AP-
combination via look-up table
• Prescribe the Data-rate/AP- combination that maximizes throughput
Rate, Delivery Ratio
Key Value6,0.8,0.1,0.1 0.90
.. ..
Table Look-up (Details in paper)
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EVALUATIONEpicenter
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Methodology• Software Defined Radios – USRP + GnuRadio
•6 USRP-APs were co-located with real APs in our building •A client-USRP was mounted on a wheel chair•Client communicates with closest AP, others overhear
•Comparison of Following Schemes•Soft•Epi – {1x, 2x}•MRC – {Bits, Symbol}
• Accuracy of Rate Prediction
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Symbol Level – Reduced Errors
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Throughput Gain
~ 47% gain
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70-80% accurate
Rate Prediction
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More results in paper
• Bit Error Rates (Comparison with Soft)• Performance of AP selection• Downlink Throughput gain – Range amplifier
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Bits
SymbolsEpicenter,
Best of Both
Coarser Symbols
Gai
n
Bandwidth Overhead
Conclusion
10101
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Conclusion• Epicenter uses coarser representation of symbols for
combining•Preserves diversity among symbols necessary for combining•Enormously decreases bandwidth requirements
• Look up based rate prediction achieves 70-80% accuracy
• Epicenter achieves 25-90% throughput gain over 802.11 and 40% over Soft