Critique #2 M. Sha, G. Hackmann and C. Lu, Real-world Empirical - - PowerPoint PPT Presentation

Critique #2

Ø M. Sha, G. Hackmann and C. Lu, Real-world Empirical Studies on Multi- Channel Reliability and Spectrum Usage for Home-Area Sensor Networks, IEEE Transactions on Network and Service Management, 10(1): 56-69, March 2013. Ø Due on 2/13 (Tuesday)

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Characterizing Wireless in Homes

Chenyang Lu

Cyber-Physical Systems Laboratory

Home Area Network

Ø Smart energy: meters, thermostats, home appliances… Ø Smart health: collect vital signs, measure sleep, detecting falls… Ø Need reliable and power-efficient Home Area Networks (HANs)

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Wireless Home Area Networks

Ø Advantages

q Do not require wired infrastructure. q Easily and inexpensively retrofit existing homes. q Energy efficiency

Ø Reliability challenges

q Unpredictable environment q Crowded 2.4 GHz ISM band

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Channels in the 2.4 GHz ISM Band

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Empirical Studies in Homes

Ø 2.4Hz spectrum study of existing wireless signals Ø IEEE 802.15.4 link reliability in all 16 channels

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Spectrum Usage Traces

Ø Collected from the 2.4 GHz spectrum in six apartments and an office

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Methodology

Ø Spectrum usage between 2.400 GHz and 2.495 GHz Ø Wi-Spy 2.4x spectrum analyzer

q Sweep across the 2.4 GHz spectrum q Sampling period: 40 ms q Signal strength reading on each of 254 discrete frequencies

Ø Traces over 7 days in 6 apartments and Bryan Hall

q Normal daily activities q 15,120,000 readings for each of the 254 frequencies q 2.5 GB of data per location

Ø Signal strength readings à binary values based on a threshold

q 0: idle channel; 1: busy channel.

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Questions

• 1. Is there a channel free in all apartments?
• 2. Does spectrum usage change over time?
• 3. Do homes have similar spectrum usage patterns as offices?
• 4. Is 802.11 the dominant interferer in homes?

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Is there a channel free in all homes?

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Ø No, due to diverse spectrum usage patterns in homes Channel Occupancy Rate

Does spectrum usage change?

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Spectrum occupancy exhibits considerable variability over time.

Daily standard deviation Hourly standard deviation 5-minute standard deviation

Is Wi-Fi the dominant interferer?

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Is Wi-Fi dominating the spectrum?

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Is Wi-Fi dominating the spectrum?

Ø While Wi-Fi is a major source of interference, others can be non-negligible contributors to spectrum occupancy.

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Empirical Studies in Homes

Ø 2.4GHz spectrum study of existing wireless signals Ø IEEE 802.15.4 link reliability in all 16 channels

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Methodology

Ø Platform

q Tmote Sky and TelosB motes q IEEE 802.15.4 compliant Chipcon CC2420 radio q 16 channels (11-26) in 5 MHz steps q TinyOS 2.1 using default CSMA/CA MAC layer

Ø Packet Reception Rate (PRR) of all 802.15.4 channels

q 10 apartments, 24 hours per apartment q A node broadcast 100 packets per channel to multiple receivers,

cycling through all 16 channels in 5 minutes

q Receivers recorded the PRRs in onboard Flash

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Questions

• 1. Is there a persistently reliable channel?
• 2. If a good channel cannot be found, are retransmissions

sufficient to deal with packet loss?

• 3. If no single channel is reliable, can we exploit channel diversity

to achieve reliability?

• 4. Do channel conditions exhibit cyclic behavior over time?
• 5. Is reliability strongly correlated among different channels?

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Is there a persistently reliable channel?

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Different links within a same apartment

Ø Link reliability varies among apartments and links.

Different apartments

Is there a persistently reliable channel?

Ø No, because of temporal variability.

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Lowest PRR observed on each link’s most reliable channel

Is retransmission sufficient?

Ø No, due to burstiness of transmission failures.

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Is channel hopping effective?

Ø Yes!

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How often needs a link switch channel?

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Number of channel hops required under an optimal schedule (one link selected randomly per apartment)

Ø Only a small number of channel hops per day.

Can hopping be scheduled statically?

Ø Channel conditions are not cyclic à channel-hopping decisions must be made dynamically.

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Pearson product-moment correlation coefficient

How should new channels be selected?

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Conditional probability of channel failure

Ø Reliability is correlated among adjacent channels. Ø A link should hop far away from a failing channel.

Correlation of channel reliability

Spectrum Analysis: Observations

• 1. There is no common idle channel across different homes.
• 2. Spectrum occupancy in homes exhibits variability over time,

whether looking at timescales of days, hours, or minutes..

• 3. While Wi-Fi is a major source of interference in homes, other

interferers cannot be ignored.

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802.15.4 Link Study: Observations

• 1. Link reliability varies greatly from channel to channel.
• 2. There is not always a single persistently reliable channel.
• 3. Retransmissions are insufficient due to bursty transmission failures.
• 4. Channel hopping can improve long-term link reliability.

q Only a few channel hops per day can maintain reliable links. q Channel conditions are not cyclic à channel-hopping decisions must be

made dynamically.

q Reliability is correlated across adjacent channels à a link should move

far away from a failing channel.

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ARCH: Adaptive and Robust Channel Hopping

Ø Receiver-oriented protocol

q Insight: links have different channel conditions within a home. q Different receivers may have different channels.

Ø Monitor channel condition

q Maintain a sliding window of ETX values of incoming links q Mark channel unreliable if ETX values exceed threshold

Ø Select a new channel

q Insight: strong correlation of link failures in adjacent channels. q Probabilistically chooses a new channel q The further away a channel is from the current channel, the more likely it

will be chosen.

Ø Upon selecting a new channel, nodes notify neighbors of this change. Neighbors update their neighbor tables.

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Paper

Ø M. Sha, G. Hackmann and C. Lu, Real-world Empirical Studies on Multi-Channel Reliability and Spectrum Usage for Home-Area Sensor Networks, IEEE Transactions on Network and Service Management, 10(1): 56-69, March 2013. Ø M. Sha, G. Hackmann and C. Lu, ARCH: Practical Channel Hopping for Reliable Home-Area Sensor Networks, IEEE Real-Time and Embedded Technology and Applications Symposium (RTAS'11), April 2011.

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