An Energy-Efficient MAC Protocol
for Wireless Sensor Networks
Wei Ye1, John Heidemann1, Deborah Estrin2
1USC Information Sciences Institute
2UCLA and USC/ISI
IEEE INFOCOM 2002
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Introduction
Wireless sensor network
•
•
•
•
•
Special ad hoc wireless network
Large number of nodes w/ sensors & actuators
Battery-powered nodes
energy efficiency
Unplanned deployment
self-organization
Node density & topology change
robustness
Sensor-net applications
• Nodes cooperate for a common task
• In-network data processing
IEEE INFOCOM 2002
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Medium Access Control in Sensor Nets
 Important attributes of MAC protocols
1.
2.
3.
4.
5.
6.
7.
Collision avoidance
Energy efficiency
Scalability in node density
Latency
Fairness
Throughput
Bandwidth utilization
IEEE INFOCOM 2002
Primary
Secondary
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Energy Efficiency in MAC
Major sources of energy waste
• Idle listening
0.14
0.12
0.1
Diffusion
Flooding
Omniscient Multicast
0.08
0.06
0.04
0.02
00
50
100
150
200
250
300
Network Size
Over always-listening MAC
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Average Dissipated Energy
(Joules/Node/Received Event)
(Joules/Node/Received Event)
Average Dissipated Energy
Energy consumption of typical 802.11 WLAN cards
idle:receive — 1:1.05 to 1:2 (Stemm 1997)
Example: directed diffusion (Intanagonwiwat 2000)
0.018
0.016
0.014
0.012
0.01
0.008
0.006
0.004
0.002
00
Flooding
Omniscient Multicast
Diffusion
50
100
150
200
250
Network Size
Over energy-aware MAC
4
300
Energy Efficiency in MAC
Major sources of energy waste (cont.)
• Idle listening
Dominant in sensor nets
Long idle time when no sensing event happens
• Collisions
• Control overhead
• Overhearing
Common to all
wireless networks
We try to reduce energy consumption from
all above sources
Combine benefits of TDMA + contention
protocols
IEEE INFOCOM 2002
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Sensor-MAC (S-MAC) Design
Tradeoffs
Latency
Fairness
Energy
Major components in S-MAC
•
•
•
•
Periodic listen and sleep
Collision avoidance
Overhearing avoidance
Massage passing
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Periodic Listen and Sleep
Problem: Idle listening consumes significant
energy
Solution: Periodic listen and sleep
listen
sleep
listen
sleep
• Turn off radio when sleeping
• Reduce duty cycle to ~ 10% (200ms on/2s off)
Latency
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Energy
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Periodic Listen and Sleep
Schedules can differ
Node 1
Node 2
listen
sleep
listen
listen
sleep
sleep
listen
sleep
• Prefer neighboring nodes have same schedule
— easy broadcast & low control overhead
Schedule 1
Schedule 2
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Border nodes:
two schedules
broadcast twice
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Periodic Listen and Sleep
Schedule Synchronization
• Remember neighbors’ schedules
— to know when to send to them
• Each node broadcasts its schedule every few
periods of sleeping and listening
• Re-sync when receiving a schedule update
• Schedule packets also serve as beacons for new
nodes to join a neighborhood
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Collision Avoidance
Problem: Multiple senders want to talk
Options: Contention vs. TDMA
Solution: Similar to IEEE 802.11 ad hoc
mode (DCF)
•
•
•
•
Physical and virtual carrier sense
Randomized backoff time
RTS/CTS for hidden terminal problem
RTS/CTS/DATA/ACK sequence
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Overhearing Avoidance
Problem: Receive packets destined to others
Solution: Sleep when neighbors talk
• Basic idea from PAMAS (Singh, Raghavendra 1998)
• But we only use in-channel signaling
Who should sleep?
• All immediate neighbors of sender and receiver
How long to sleep?
• The duration field in each packet informs other
nodes the sleep interval
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Message Passing
Problem: Sensor net in-network processing
requires entire message
Solution: Don’t interleave different messages
• Long message is fragmented & sent in burst
• RTS/CTS reserve medium for entire message
• Fragment-level error recovery — ACK
— extend Tx time and re-transmit immediately
Other nodes sleep for whole message time
Fairness
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Energy
Msg-level latency
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Msg Passing vs. 802.11 fragmentation
 S-MAC message passing
Data 19
RTS 21
ACK 18
CTS 20
...
Data 17
ACK 16
Data 1
...
ACK 0
 Fragmentation in IEEE 802.11
• No indication of entire time — other nodes keep listening
• If ACK is not received, give up Tx — fairness
Data 3
RTS 3
CTS 2
Data 3
ACK 2
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...
ACK 2
Data 1
...
13
ACK 0
Implementation on Testbed Nodes
Platform
Motes (UC Berkeley)
8-bit CPU at 4MHz,
8KB flash, 512B RAM
916MHz radio
TinyOS: event-driven
Compared MAC modules
1. IEEE 802.11-like protocol w/o sleeping
2. Message passing with overhearing avoidance
3. S-MAC (2 + periodic listen/sleep)
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Experiments
Topology and measured energy consumption
on source nodes
Average energy consumption in the source nodes
1800
Source 2
Sink 1
Sink 2
• Each source node sends
10 messages
— Each message has 400B
in 10 fragments
802.11-like protocol
Overhearing avoidance
S-MAC
1600
Energy consumption (mJ)
Source 1
• Measure total energy over
time to send all messages
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1400
1200
1000
800
600
400
200
0
2
4
6
8
Message inter-arrival period (second)
15
10
Conclusions
S-MAC offers significant energy efficiency
over always-listening MAC protocols
Future Plans
• Measurement of throughput and latency
Throughput reduces due to latency, contention,
control overhead and channel noise
• Experiments on large testbeds
~100 Motes, ~30 embedded PCs w/ MoteNIC
URL: http://www.isi.edu/scadds/
IEEE INFOCOM 2002
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