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“Link-level Measurements from an 802.11b Mesh

Network”Daniel Aguayo (MIT), John Bicket (MIT), Sanjit Biswas (MIT), Glenn Judd (CMU),

Robert Morris (MIT)

Summary presented by Gary Woo

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Outline

• Assumptions in “neighbor” abstraction

• What is Roofnet?

• Experiment/Results

• Hypotheses

• Cause

• Conclusion

• Additional experiments

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“Neighbor” abstraction

• Nodes are divided into pairs that can communicate and pairs that can not

• Examples of usage:– Wired networks

• Obvious physical connection

– MAC protocol in 802.11• Supported based on S/N and BER relationship

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Roofnet

• Located in Cambridge, MA next to MIT

• 38 nodes with roof top antennas

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Experiment setup

• Use 802.11b cards with Prism 2.5 chip set– Set to channel 3 (2.422Ghz)– Pseudo-IBSS (ad hoc) mode

• Run early Sunday morning June 6th, 2004• 1500 byte 802.11 broadcast for 90 seconds• Sequence Number, Arrival Time, Receive

Signal Strength Indication (RSSI), Silence values are saved

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Results

• Results show that nodes don’t adhere to “neighbor” abstraction

Red = expected

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Delivery probability burstyness

• Average loss rate are all about 50%

Sample (each point is 200 milliseconds)

Del

iver

y pr

obab

ilit

y

1 Mbit/s linksD

eliv

ery

prob

abil

itySample (each point is 200 milliseconds)

11 Mbit/s links

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Loss due to long interference?• Allan Deviation of loss rate measures fluctuations

(high means very bursty, low means more smooth)

Yellow box shows that around 80% of all pairs have low Allan Deviation of loss rate (around 0.06)

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Loss due to distance?

• Yellow circles are the same size (approximately centered around Sender)

• There is relation of distance/receiving signal, however not consistent

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Signal-to-noise ratio (dB)

Bro

adca

st p

acke

td

eliv

ery

pro

bab

ilit

y

Roofnet

Laboratory

Loss due to noise?• High signal and low noise should result in delivery• Roofnet doesn’t show strong correlation

Result for1 Mbit/s

High signal packets still lost

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Non-Roofnet packets observed per second (before the experiment)

Exp

erim

ent

pac

kets

lo

stp

er s

eco

nd

Loss due to 802.11 interference?

Few interference packets before experiment

• If cause is this interference, then the more interference packets the more packets would be drop

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So what is the cause?

• “when you have eliminated the impossible, whatever remains, however improbable, must be the truth” – Sherlock Holmes in The Adventure of the Beryl Coronet (1892)

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Reflected signals

• Buildings and other physical objects reflects signals

• Weaker duplicated signal are received by nodes after a delay

• RAKE receiver can suppress reflected copies with delays up to 250 nanoseconds

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Experiment for reflected signals

• Signal sent to the receiver, and combined with a duplicated version of the original signal (with added delay, and reduce strength)

Sender Receiver

delay attenuation

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Results for reflected signals experiment

• Yellow shows RAKE working• Green shows loss if reflected signal is almost as

powerful as original the receiver is confused• Blue shows intersection with the modulation’s

symbol boundaries

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Results for reflected signals experiment (continued)

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Link distance (feet or nanoseconds)

Cu

mu

lati

ve f

ract

ion

o

f li

nks

Distance between links

• Most links have greater than 500 nanosecond delays, RAKE can’t handle

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Conclusion

• No clear distinction of links that drop a lot of packets verses ones that receive most

• Link distance and Signal to Noise ratio has relation but not strong

• Most likely cause is multi-path fading (reflected signals)

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Varying signal strength

• The stronger the signal, the higher the probability of being delivered

1 Mbit/s

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Throughput compared based on bit rate

• Algorithm should wait until delivering half the packets before reducing bit rate

Shaded portion shows that X Mbit/s can successfully send more packets than other rates

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References and Acknowledgements

• Author's slides for SIGCOMM 2004 (figures in color were taken from these slides):– http://www.pdos.lcs.mit.edu/roofnet/sigcomm-talk.ppt

• Author’s electronic version of paper (other figures and tables were taken from this document):– http://www.pdos.lcs.mit.edu/~rtm/papers/p442-aguayo.pdf

• Roofnet’s group website:– http://www.pdos.lcs.mit.edu/roofnet/index.php