Versatile low power media access for wireless sensor networks

Joseph Polastre Jason Hill David Culler

Computer Science Department University of California,Berkeley

JLH LabsCamino Capistrano Capistrano Beach

Computer Science Department University of California,Berkeley

Speaker: Yung-Lin Yu

ACM SenSys’04

Outline

• Introduction• Design and Implementation

– Clear Channel Assessment (CCA)– Low Power Listening (LPL)

• Evaluation• Experiment• Conclusion

Introduction• What is BMAC?

– A configurable MAC protocol for WSNs– Small core

• Factors out higher-level functionality

– Energy efficient• Goals

– Low Power operation– Effective collision avoidance– Simple and predictable– Small code size and RAM usage– Scalable to large numbers of nodes

Introduction (cont.)

• Reconfigure– Bidirectional interface for WSN application– Extend network lifetime by 50%

Design and Implementation• Traditional

– SMAC design• Users pre-configure duty cycle• Applications rely on S-MAC to adjust its operation as things c

hange

• BMAC– Small core functionality: media access control– RTS/CTS, ACKs, etc are considered higher layer functi

onality (services)• Applications can turn them on and off

– More flexible

Design and Implementation(cont.)

Design and Implementation(cont.)

• MAC must accurately determine if channel is clear– Need to tell what is noise and what is a signal– Ambient noise changes depending on the envir

onment• BMAC’s solution

– Use Clear Channel Assessment (CCA)• CCA is used to determine the state of the medium

Design and Implementation (cont.)

• 0=busy, 1=clear• Packet arrives between 22 and 54 ms• Single-sample thresholding produces several false ‘busy’ signals

Design and Implementation (cont.)• Low Power Listening

– Goal: minimize listen cost– Principles

• Node periodically wakes up, turns radio on and checks channel– Check interval variable

• If signal is detected, node powers up in order to receive the packet

• Node goes back to sleep– If a packet is received– After a timeout

• Preamble length matches channel checking period– No explicit synchronization required

• Noise floor estimation used to detect channel activity during LPL

Design and Implementation (cont.)

• LPL

125 ms 125 ms 125 ms 125 ms

ReceiverReceiver

Sender preamble

data

data

data

Evaluation

• LPL check interval vs Lifetime

Evaluation (cont.)

• LPL check interval vs neighborhood size

50ms25ms

Experiment• Wireless sensor node

– Mica2• Software

– TinyOS• Environment

– Unobstructed• Deployment

– Place the nodes with 1 meter spacing• Experiment Three subject

– Throughput– power consumption– Energy vs Latency

Experiment (cont.)

• Throughput (Channel Utilization)– 2.5 times than S-MAC

broadcast,4.5 time thanS-MAC unicast

• Because CCA andlower sync. overhead

– As the Nodes Increase• Channel contention

cause performance converge to S-MAC

0 5 10 15 200

2000

4000

6000

8000

10000

12000

14000

16000

0 5 10 15 200

0.1

0.2

0.3

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1

Throughput of a congested channel

Number of nodes

Perc

enta

ge o

f Cha

nnel

Cap

acity

B-MACB-MAC w/ ACKB-MAC w/ RTS-CTSS-MAC unicastS-MAC broadcastChannel Capacity

Thro

ughp

ut (b

ps)

Experiment (cont.)

• power consumption– Duty cycle increase

• In S-MAC, have moreSYNC overhead

• In B-MAC 1.no sync. requirements.2.reconfigure check interval to adept network bandwidth

Because SYNC overhead

Experiment (cont.)

• Energy vs Latency– 10-hop network– Source sends 100 byte

packet every 10 seconds

0 2000 4000 6000 8000 100000

50

100

150

200

250

300

350

400

450

500

550

Latency (ms)

Ener

gy (m

J)

Effect of latency on mean energy consumption

B-MACS-MACAlways On

S-MAC Default Configuration

B-MAC Default Configuration

11 10 9 3 2 111 10 9 3 2 1

Conclusions

• BMAC appears to be better than SMAC– Easier to tune– Has better channel assessment – Doesn’t use explicit sync packets– Doesn’t use RTS/CTS/ACK if it doesn’t have

to– Is smaller and less complex