energy-efficient low power listening in noisy environmentslu/cse521s/slides/aedp.pdf · m. sha, g....
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Energy-Efficient Low Power Listening in Noisy Environments
Chenyang Lu Cyber-‐Physical Systems Laboratory Department of Computer Science and Engineering
Wireless Sensors in Noisy Environments
Source: AT&T Labs
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Noise in Residen4al Environments Ø Spectrum usage of 2.4 GHz band in six apartments and Bryan Hall.
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CCA
Low Power Listening Ø Effective energy efficient MAC approach in clean environmentsØ BoX-MAC-2: Default MAC protocol of TinyOS
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Sender
Recipient
Packet
RX
ACK
False Wakeup Caused by Noise
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Recipient RX
CCA
Time out
Sender
Noise
False Wakeup Rates in Homes
Ø Measurement in an apartmentq Ideal: duty cycle < 1%, lifetime > 1 month.
q Real: duty cycle > 4%, lifetime < 1 week.
Ø This is a wide spread problem!
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Wakeup Check
K readings ≥ wakeup threshold à wakeup
Poll CCA pin N Bmes
Increase wakeup threshold
0 1 1 1 1 1 1 0 0 0
RSSI
wakeup threshold
0 0 0 1 0 0 0 0 0 0
wakeup threshold
RSSI
No wakeup
Idea: Adapt Wakeup Threshold
wakeup threshold
RSSI min packet
max noise
min packet RSS > wakeup threshold > max noise
RSSI min packet max noise
No wakeup threshold will work, but it will be a bad link anyway
Impact on False Wakeup Rate
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False wakeup rate under interference of an 802.11n router
Objec4ves
1. Maintain link reliability (ETX threshold) 2. Reduce false wakeup rate (wakeup rate threshold)
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AEDP: Adap4ve Energy Detec4on Protocol
Ø Controlled variablesq ETX: recent expected transmission count in a sliding window.
q WR: recent wakeup rate in the sliding window
q WRL: long-term average wakeup rate à battery life
Ø Adapt wakeup threshold T online q if ETX > threshold à T=Tmin
q if ETX ≤ threshold• WR > threshold à T=T+△T
• WR ≤ threshold AND WRL > threshold à no change.• WR ≤ threshold AND WRL ≤ threshold à T=T-△T
q Reset T periodically to react to network changes
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Cri4cal Flaw of BoX-‐MAC-‐2 in TinyOS
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• Sender: gap between transmissions is 8ms• >> time needed to receive a (soft) ACK (≤2.4ms)
• Receiver: wakeup check lasts 8.8ms (poll CCA pin 400 times)
• >> time needed to receive an entire packet (<4.3ms)!
• Contradict the intended short wakeup check à energy waste!
Sender
Recipient
ACK
RX
8ms
CCA
8.8ms
Fix for BoX-‐MAC-‐2 in TinyOS • Sender: reduce gap between transmissions from 8msà2.8ms
• > time needed to receive a (soft) ACK (≤2.4ms)• Receiver: shorten wakeup check 8.8msà2.9ms (poll CCA pin 115 times)
• < time it takes to receive an entire packet (<4.3ms)
• Shorter wakeup check à lower duty cycle
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Sender
Recipient (fixed)
RX
CCA ACK
Duty Cycle in an Apartment
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AEDP reduces mean duty cycle by• 45.5% over fixed BoX-MAC-2• 65.1% over the default BoX-MAC-2
Effects of Links’ Signal Strength
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• Stronger links benefit more• Does no harm to weak links
Comparison with A-‐MAC
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• AEDP wins at low data rates• AEDP wakeup check: 2.9ms • A-MAC polling cost: 6.2ms
• A-MAC wins at high data rates• Avoids sending duplicated packets to wake up receiver
Related Works
Ø Optimize CCA to avoid collision [Brodsky SigComm’09, Boano EWSN’10]
Ø AEDP focuses on CCA threshold’s role in wakeup
Ø Enhance reliability under interference [Srinivasan TOSN’10, Liang SenSys’10]
Ø AEDP focuses on energy efficiency in noisy environments
Ø Receiver-initiated MAC [Sun SenSys’08, Dutta SenSys’10]
Ø AEDP addresses sender-initiated MACØ Complementary to each other for different data rates
Ø ContikiMAC performs two CCA checks spaced slightly apart [Dunkels ’11]
Ø AEDP performs only one CCA check à more efficient
Ø SoNIC classifies specific types of interference sources [Frederik IPSN’13]
Ø AEDP is a generic approach for all noise
Conclusion Ø False wakeups compromise energy efficiency of Low Power
Listening in noisy environments.
Ø AEDP: Adaptive Energy Detection Protocol for LPL q Self-tunes wakeup threshold online.
q Implemented in TinyOS 2.1.1 on TelosB.
q Fixes critical flaw of BoX-MAC-2 implementation.
Ø AEDP mitigates the impact of noise on radio duty cycles. q Significantly reduces false wakeup rate under LPL.
q Adapts to links with different signal strength.q Outperforms A-MAC for low data rate applications.
M. Sha, G. Hackmann and C. Lu, Energy-Efficient Low Power Listening for Wireless Sensor Networks in Noisy Environments, ACM/IEEE International Conference on Information Processing in Sensor Networks (IPSN'13), April 2013.