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
0.4
0.5
0.6
0.7
0.8
0.9
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