An Energyefficient MAC protocol for wireless Sensor Networks

1
An Energy-efficient MAC protocol for wireless
Sensor Networks
(Wei Ye, John Heidemann, Deborah Estrin)

• Lim Aeri
• 2002. 10 14

2
Outline

• Introduction
• Design considerations
• S-MAC
• Implementation
• Result discussion
• Conclusion

3
Introduction

• Wireless Sensor Networks
• monitoring, medical systems and robotic
  exploration etc.
• Large number of distributed nodes and self
  organizing
• Normally battery operated - power limited
• active only when event occurs
• Assumption
• Composed of many small nodes deployed in an ad
  hoc fashion
• Nodes must self-configure
• Dedicated to a single application or a few
  collaborative applications
• In-network processing store-and-forward fashion
• Applications will have long idle periods and can
  tolerate some latency

4
Design considerations

• Energy efficiency
• Collision
• Overhearing
• Control packet overhead
• Idle listening
• Scalability
• New nodes join
• Old nodes die
• secondary
• Fairness
• Latency
• Throughput
• Bandwidth Utilization

gt Reduce energy consumption !!
gt Support good scalability !!
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S-MAC proposed scheme

• S-MAC ( Sensor MAC )
• MAC protocol especially for sensor networks
• Collision by using RTS and CTS
• Overhearing by switching the radio off not for
  the my packets
• Control overhead by message passing
• Idle listening by periodic listen and sleep
• cost
• some reduction in both per-hop fairness and
  latency
• This does not necessarily result in lower
  end-to-end fairness and latency

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S-MAC main feature

• Periodic listen and sleep
• Collision and Overhearing avoidance
• Message passing

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S-MAC Periodic listen and sleep(1)

• In sleep mode
• radio off and sets a timer to awake later
• Sleep schedule
• Each node has a sleep schedule and sleep schedule
  table for neighbors
• Broadcast periodically
• Duration of sleep and listen time can be selected
  based on the application scenario
• Synchronization
• Same sleep schedule neighboring nodes are
  synchronized
• Not all neighboring nodes can synchronize
  together

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S-MAC Periodic listen and sleep(2)

• Choose sleep schedule
• first listen for a certain amount of time.
• SYNCHRONIZER
• If it does not hear, it randomly chooses a
  schedule and broadcast its schedule immediately.
• FOLLOWER
• it just follows this neighbors schedule and
  waits for a random delay and broadcasts its
  schedule.
• Adopts more than two schedules
• If a node receives a neighbors schedule after it
  selects its own schedule, it adopts to both
  schedules and broadcasts its own schedule before
  going to sleep.

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S-MAC Periodic listen and sleep(3)

• Maintain synchronization (loosely)
• Done by periodic updating using a SYNC packet.
• SYNC packets
• Sender Address Time of next sleep
• Listen interval is divided into two parts
• SYNC / RTS

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S-MAC Collision and Overhearing avoidance

• Collision avoidance
• Similar to IEEE802.11 using RTS/CTS mechanism
• Perform carrier sense(virtual/physical) before
  initiating a transmission
• If a node fails to get the medium, it goes to
  sleep until the receiver is free.
• Overhearing avoidance
• Duration field in each packet
• Time for the remaining transmission
• NAV (network allocation vector)
• For virtual carrier sense
• Node can know how long he has to stay in sleep
  mode

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S-MAC Message passing

• Long message transmission
• A message a collection of meaningful,
  interrelated units of data
• disadvantageous
• re-transmission cost is high
• Fragmentation
• into small packets
• high control overhead
• Fragmentation and burst transmission
• Reduces latency of the message
• Reduces control overhead and switching cost
• Node-to-node fairness is reduced
• nodes with small packets to send has to wait till
  the message burst is transmitted

12
implementation

• Simplified IEEE 802.11 DCF
• physical and virtual carrier sense
• Back-off and retry
• RTS/CTS/DATA/ACK packet exchange and
  fragmentation support
• Simple S-MAC
• Message passing with overhearing avoidance
• Complete S-MAC
• all the features are implemented
• Periodic sleep and wake , overhearing avoidance,
  message passing

13
implementation

• Topology
• Two-hop network with two sources and two sinks
• Sources generate message which is divided into
  fragments
• Traffic load is changed by varying the
  inter-arrival period of the message

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Simulation results(1)

• Energy consumption in the source nodes

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Simulation results(2)

• Percentage of time that the source nodes are in
  the sleep mode

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Simulation results(3)

• Energy consumption in the intermediate node

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Conclusion

• Novel technique
• Period listen/sleep
• Virtual cluster auto-synchronization on sleep
  schedule
• In-channel signaling
• Message passing
• For Heavy traffic load
• Overhearing avoidance
• Efficiently transmitting a long message
• For Light traffic load
• Adjust the sleep time according to traffic
  patterns