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C-MAC: Model-driven Concurrent Medium Access Control for Wireless

Sensor Networks1

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Mo Sha ; Guoliang Xing ; Gang Zhou ; Shucheng Liu ; Xiaorui Wang

City University of Hong Kong; Michigan State UniversityCollege of William and Mary; University of Tennessee Knoxville

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1 4

3

1 23 4

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Outline

• Motivation

• Power control and interference models

• Design of C-MAC

• Performance evaluation

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• Habitat monitoring, structural monitoring etc.– Sample the environment at high rates– Ex: sample @100 Hz for finding structural defect

• Limited storage capacity

• High network throughput

Data-intensive Sensing Applications

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MACs for Wireless Sensor Nets• CSMA-based MAC protocols

– S-MAC, T-MAC, B-MAC, X-MAC…– Conservative, low throughput

• TDMA-based MAC protocols– TRAMA, DCQS, DRAND…– High maintenance overhead

• Hybrid MAC protocols– SCP, Funneling-MAC and Z-MAC – Not designed for high throughput

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s1

r1

s2

Collision

Background of CSMA

r2

• S1: Sender 1• S2: Sender 2• R1: Receiver 1• R2: Receiver 2

Packet may be corrupted

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s1

r1

s2

r2

Traffic Demand

Sense Channel (CCA Check)

If Channel is Clear,Transmit

Traffic Demand

Sense Channel (CCA Check)

If Channel is not Clear,Random Delay

(1)(2)

• S1: Sender 1• S2: Sender 2• R1: Receiver 1• R2: Receiver 2

Background of CSMA

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s1

r1

s2

Collision

Is Packet Corrupted?

r2

• S1: Sender 1• S2: Sender 2• R1: Receiver 1• R2: Receiver 2

Is each packet corrupted?

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A Case of Concurrency

s1

r1s2

r2

Power is fixed to be 15.

power increases from level 1 to 31

(1) Run1: CSMA disabled

(2) Run2: CSMA enabled

Chipcon 2420 radio;

31 tunable power levels;

256 kbps transceiver;

TinyOS-1.X.

Link 1Link 2

Tmote Sky mote

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Experimental Result

Golden Zone:Power of Sender 1 is 15.Power of Sender 2 is between 9 and 16.

Golden Zone

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Observations• Concurrent TXs are possible despite contention• CSMA tries avoids interference by disabling

concurrency– Back-off and channel reservation

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Key Questions

• How to enable concurrency?– Carefully control TX power for each sender

• How to control TX power?– Empirical power control and interference models

• Power decay model• Signal-to-Interference-Noise ratio (SINR) model

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Outline

• Application and Related Work

• Motivation

• Models• Design of MAC protocol

• Evaluation

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• Classical exponential decay model:

Power Decay Model

RSS = P / distα

log(RSS) = log(P)- α log(dist)

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• Experimental setup– One sender, multiple receivers at different positions– Experiments in 4 different environments

• Classical exponential decay model:

Power Decay Model

RSS = P / distα

log(RSS) = log(P)- α log(dist)

Office corridor grass field parking lot

not accurate!

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Power Decay Model

• Near-linear RSSdBm vs. transmission power level– Overhead can be reduced

Re

ce

ive

d S

ign

al S

tren

gth

(dB

m)

Transmission Power Level

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Pair-wise Power Decay Model

• Received signal strength (RSS) at r when s transmits with power Ps is given by

RSSr(s) = a x Ps + b

• a and b are interpolated using multiple measurements – a is estimated once– b is updated periodically

s

r

Ps

RSSr(s)

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PRR vs. Signal-to-Interference-Noise Ratio (SINR)

• Classical model doesn't capture the gray region

office, no interfererparking lot, no interferer office, one interferer

Noise + Interference

Received Signal Strength (RSS)

0~3 dB is

"gray region"

Packet R

eception Ratio (%

)

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Probabilistic SINR Model

• PRR(SINRi ) (1≤ i ≤ m)

PRR(0.5) PRR(1) PRR(1.5) PRR(2) …..

0 1 2 3 4 SINR (dB)

Packet R

eception Ratio (%

)

20

40

60

80

100

Classical deterministic SINR ModelOur probabilistic SINR model

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SINR Models in Different Settings

different signal strength

different # of interferers

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Outline

• Application and Related Work

• Motivation

• Models

• Design of MAC protocol• Evaluation

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Concurrency Check

InterferenceAssessment

Throughput Prediction

Data Transmission

Random Delay

Dro

pp

ed

pass

fail

max count reached

no improvement

fail

Traffic Snooping

Received Data from App

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Concurrency Check

1. Overhear m packets (say, belonging to K links)2. For each of link uv, predict the PRR if s

transmits with min power PRRv(SNRv)

3. If the PRR of any link would drop below α (i.e., 20%), fails

RSSv(Pu)

RSSv(Psmin) + Ir+Nr

SNRv =

stored in data packet

RSSv(Psmin) = av Psmin + bv

compute from v's RSS model

compute PRRv(SNRv) from v's interference

model

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Throughput Prediction• s tries to transmit to r• s overhears m packets (belonging to K links) • s finds power P that maximizes

• If negative, abort, otherwise transit a block of B packets

}{rKv Σ PRRv( SNRv ) – |K|

assuming 100% PRR for all active links

RSSr(P)

Interferencer+Noiser

SNRr =

obtained from handshaking

RSS model

PRR model

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Outline

• Application and Related Work

• Motivation

• Models

• Design of MAC protocol

• Evaluation

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Performance Evaluation• Implemented in Tmote testbed with TinyOS-1.x• 16 Tmotes deployed in a 25x24 ft office• 8 senders and 8 receivers

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System Throughput

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System Delay

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Energy Consumption

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Thanks!

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• RTS/CTS– Support data-intensive sensing applications.

• habitat monitoring [1], structural monitoring [2] and etc.

– Not for low-load applications.

Block Transmission

[1] R. Szewczyk, A. Mainwaring, J. Polastre, J. Anderson, and D. Culler. An analysis of a large scale habitat monitoring application. In SenSys, 2004.[2] N. Xu, S. Rangwala, K. K. Chintalapudi, D. Ganesan, A. Broad, R. Govindan, and D. Estrin. A wireless sensor network for structural monitoring. In SenSys, 2004.

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Components

ConcurrentTransmission

Engine

Power Decay Model

SINRModel

InterferenceAssessment

On

line

Mo

del E

stimatio

n

Concurrency Check

Throughput Prediction

Traffic Snooping

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• Conflict– Senders can not concurrent transmit whatever

the sending power is, when

s1

r1

s2

r2

Multi-Channel

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Recent studies on multi-channel

Figures in this slide are from

[1] Yafeng Wu et al, Realistic and Efficient Multi-Channel Communications in Wireless Sensor Networks, INFOCOM 2008.

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Signal-to-Interference-Noise Ratio (SINR) model

• Classical deterministic SINR model:

Noise + Interference

Received Signal Strength (RSS)

0 1 2 3 4 SINR (dB)

Packet R

eception Ratio (%

)

20

40

60

80

100

PRR = 1 If

0 Otherwise

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Time Sequence

Sender

Receiver

Jammer 1

Jammer n

…

syn packet

data packet

Time

RSS MeasurementsNoise Level Measurement

jam packet

Send event Receive/measure event

data packet

jam packet jam packet

jam packet

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System ExperimentPerformance with different block size

Throughput

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System ExperimentPerformance with different block size

Delay

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System ExperimentPerformance with different block size

Energy Consumption

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Related Work

• CSMA-based MAC protocols– S-MAC, T-MAC, B-MAC and X-MAC….

• TDMA-based MAC protocols– TRAMA, DCQS and DRAND…

• Hybrid MAC protocols– SCP, Funneling-MAC and Z-MAC

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Motivation Experiment

Golden Zone:Power of Sender 1 is 15.Power of Sender 2 is between 9 and 16.

Golden Zone