An EnergyEfficient MAC Protocol for Wireless Sensor Networks

1
An Energy-Efficient MAC Protocol for Wireless
Sensor Networks

• Tijs van Dam

2
Quick recap

• MAC protocol determines next node to use the
  medium
• Evaluate MAC protocols based on energy usage
• Use realistic communication patterns for
  evaluation

3
Classic CSMA

• If air is clear, send if receiving, wait
• How do we know when to receive?
• Listen always!
• Listening costs 200x more energy than sleeping
• Not energy-efficient

4
S-MAC protocol

• Fixed active/sleep duty cycle
• Synchronization of active times
• Not optimal for fluctuating message load
• time (events)
• location (important nodes)

5
T-MAC protocol

• Make duty cycle dynamic
• End active time with time-out
• All messages at start of frame

6
T-MAC Protocol (2)

• RTS/CTS/DATA/ACK exchange
• Synchronization not critical
• No increasing backoff fixed contention interval
• Overhearing avoidance

7
Early sleeping problem
A
B
C
D
Ready-To-Send
8
Early sleeping problem
A
B
C
D
!
Clear-To-Send

• B overhears CTS

9
Early sleeping problem
A
B
C
D
?
Data

• B must remain silent
• A times out and goes to sleep

10
Early sleeping problem
A
B
C
D
Ready-To-Send

• When B can send, A is sleeping
• B must wait until next frame (problem!)

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Early sleeping problem

• A node goes to sleep while another node still has
  a message for it
• Occurs mostly in asymmetric communication
• uni-directional flow (nodes-to-sink)
• edge of active area
• Throughput decreases dramatically

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Solutions

• After overhearing a CTS, quickly send a
  Future-Request-To-Send
• Target node will be awake
• Short pause between CTS and DATA
• When receiving an RTS, ignore and send own RTS
• only when running out of buffers
• only after two tries
• Solutions increase throughput with 100

13
Simulation setup

• 10 x 10 grid network
• Patterns local unicast, nodes-to-sink
• Active areas simulate real-world events
• Realistic nodes
• energy
• radio properties
• DAS2 cluster
• 32 64 cpus
• 2 mins / simulation, gt 40,000 msgs

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Simulation results
Nodes-to-sink pattern, msglength 20
4.5
CSMA
4
3.5
3
2.5
energy mA / node
2
1.5
1
0.5
0
0
0.05
0.1
0.15
0.2
0.25
0.3
load msg / node / s
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Simulation results (2)
Event-based local unicast, msglength20
4.5
CSMA
4
3.5
S-MAC
3
2.5
energy used avg. mA / node
2
1.5
1
0.5
T-MAC
0
0
1
2
3
4
5
6
7
8
peak load msg / node / s
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Real implementation

• Implemented T-MAC on EYES nodes
• FRTS and priority not implemented (yet)
• Seems to work well, no data loss yet
• When idle, radio is off 97.5 of the time

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Conclusions

• T-MAC reduces energy. A lot.
• Simple implementation
• Simple idea can lead to dramatic improvements
• Paper submitted for SENSYS03
• Infrared demo