An Energy-Efficient MAC Protocol for Wireless Sensor Networks

1
An Energy-Efficient MAC Protocol for Wireless
Sensor Networks

• Wei Ye1, John Heidemann1, Deborah Estrin2
• 1USC Information Sciences Institute
• 2UCLA and USC/ISI

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

3
Medium Access Control in Sensor Nets

• Important attributes of MAC protocols
• Collision avoidance
• Energy efficiency
• Scalability in node density
• Latency
• Fairness
• Throughput
• Bandwidth utilization

4
Energy Efficiency in MAC

• Major sources of energy waste
• Idle listening
• Energy consumption of typical 802.11 WLAN cards
• idlereceive 11.05 to 12 (Stemm 1997)
• Example directed diffusion (Intanagonwiwat 2000)

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Energy Efficiency in MAC

• 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

6
Sensor-MAC (S-MAC) Design

• Tradeoffs

• Major components in S-MAC
• Periodic listen and sleep
• Collision avoidance
• Overhearing avoidance
• Message passing

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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)

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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
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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 Dont 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

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Msg Passing vs. 802.11 fragmentation

• S-MAC message passing

• 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

• Compared MAC modules
• IEEE 802.11-like protocol w/o sleeping
• Message passing with overhearing avoidance
• S-MAC (2 periodic listen/sleep)

15
Experiments

• Topology and measured energy consumption on
  source nodes

• Each source node sends 10 messages
• Each message has 400B in 10 fragments
• Measure total energy over time to send all
  messages

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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/

Thank You!