ieee infocom 2002 1 an energy-efficient mac protocol for wireless sensor networks wei ye 1, john...
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IEEE INFOCOM 2002 1
An Energy-Efficient MAC Protocol for Wireless Sensor Networks
Wei Ye1, John Heidemann1, Deborah Estrin2
1USC Information Sciences Institute2UCLA and USC/ISI
IEEE INFOCOM 2002 2
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
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Medium Access Control in Sensor Nets
Important attributes of MAC protocols1. Collision avoidance2. Energy efficiency3. Scalability in node density4. Latency5. Fairness6. Throughput7. Bandwidth utilization
Primary
Secondary
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Major sources of energy waste• Idle listening
Energy consumption of typical 802.11 WLAN cardsidle:receive — 1:1.05 to 1:2 (Stemm 1997)
Example: directed diffusion (Intanagonwiwat 2000)
Energy Efficiency in MAC
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Network Size
Diffusion
Omniscient MulticastFlooding
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Network Size
Diffusion
Omniscient Multicast
Flooding
Over always-listening MAC Over energy-aware MAC
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Major sources of energy waste (cont.)• Idle listening
Long idle time when no sensing event happens
• Collisions• Control overhead• Overhearing
We try to reduce energy consumption from all above sources
Combine benefits of TDMA + contention protocols
Energy Efficiency in MAC
Common to all wireless
networks
Dominant in sensor nets
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Sensor-MAC (S-MAC) Design
Tradeoffs
Major components in S-MAC• Periodic listen and sleep• Collision avoidance• Overhearing avoidance• Message passing
Latency
Fairnes
s
Energy
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Periodic Listen and Sleep
Problem: Idle listening consumes significant energy
Solution: Periodic listen and sleep
• Turn off radio when sleeping• Reduce duty cycle to ~ 10% (200ms
on/2s off)
sleeplisten listen sleep
Latency Energy
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Periodic Listen and Sleep
Schedules can differ
• Prefer neighboring nodes have same schedule— easy broadcast & low control overhead Border nodes:
two schedules
broadcast twice
Node 1
Node 2
sleeplisten listen sleep
sleeplisten listen sleep
Schedule 2
Schedule 1
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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. TDMASolution: 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 immediatelyOther nodes sleep for whole message time
Fairnes
s
Energy
Msg-level
latency
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Msg Passing vs. 802.11 fragmentation
S-MAC message passing
RTS 21 ......
Data 19ACK 18CTS 20
Data 17ACK 16
Data 1ACK 0
RTS 3 ......
Data 3ACK 2CTS 2
Data 3ACK 2
Data 1ACK 0
Fragmentation in IEEE 802.11 • No indication of entire time — other nodes
keep listening• If ACK is not received, give up Tx —
fairness
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Implementation on Testbed Nodes
PlatformMotes (UC Berkeley)
8-bit CPU at 4MHz,8KB flash, 512B RAM916MHz radio
TinyOS: event-drivenCompared 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
Source 1
Source 2
Sink 1
Sink 2
• Each source node sends 10 messages — Each message has
400B in 10 fragments
• Measure total energy over time to send all messages
0 2 4 6 8 10
200
400
600
800
1000
1200
1400
1600
1800Average energy consumption in the source nodes
Message inter-arrival period (second)
Energy consumption (mJ)
802.11-like protocol Overhearing avoidanceS-MAC
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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/