An Adaptive Energy-Efficient MAC Protocol for Wireless Sensor Network

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An Adaptive Energy-Efficient MAC Protocol for
Wireless Sensor Network

• Tijs van Dam, Koen Langendoen
• SenSys03
• Ku Dara
• Network Security LAB at KAIST
• 2006.09.14

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Contents

• Introduction
• S- MAC drawbacks
• T- MAC
• Experiments
• Conclusions

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Introduction

• Traditional MAC Protocols
• Design to maximize packet throughput, minimize
  latency and provide fairness
• Protocol design for wireless sensor networks
• Focuses on minimizing energy consumption
• What was the most wasted energy in traditional
  MAC protocol?
• Idle listening
• A node does not know when it will be the receiver
  of a message from one of its neighbors
• It must keep its radio in receive mode at all
  times
• Ex) sensor application 1/sec, messages fairly
  short, transmit(5ms) ,
• receive(5ms),
  990ms on listening while nothing happens(99)

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S-MAC

• Idle listening problem solution
• Duty cycle is involved, each node sleep
  periodically
• S-MAC in sensor network
• Single-frequency contention-based protocol
• Time is divided into fairly large-frame (frame
  1sec)
• Every frame has two parts active part (200ms)
  /sleep part (800ms)
• duty cycle listen interval / frame length (20)
• All messages are packed into the active part
• Tradeoff
• Energy efficiency ?, throughput ? , latency?

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Drawbacks of S-MAC

• Active (Listen) interval long enough to handle
  to the highest expected load
• If message rate is less energy is still wasted
  in idle-listening
• S-MAC fixed duty cycle is NOT OPTIMAL

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T-MAC Preliminaries(1)

• Basic idea
• To utilize an active and a sleep cycle, similar
  to S-MAC
• To introduce an adaptive duty cycle by
  dynamically ending the active part
• An active period ends when no activation event
  has occurred for a time TA
• Activation event
• The reception of any data on the radio (RTS, CTS,
  DATA, ACK)
• The sensing of communication on the radio
  (overhearing)
• Difference in the duty cycle
• S-MAC - fixed duty cycle
• T-MAC Dynamic duty cycle

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T-MAC Preliminaries(2)

• Normal MAC protocols messages are spread out
  over the whole time frame
• S-MAC active time is fixed
• T-MAC the active time is dynamically adjusted
  (i.e., be shorten) by timing out on hearing
  nothing during some time period (TA)

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T-MAC RTS Operation (1)

• Contention Interval Fixed contention interval
• In contention-based protocols, like IEEE 802.11
• a back-off scheme is used contention interval
  increases when traffic is higher
• Reduce the probability of collision when load is
  high
• In the T-MAC protocol
• Every node transmits its queued messages in a
  burst at the start of the frame
• In burst, the traffic is mostly high
• Waiting and listening for random time within a
  Fixed contention interval
• Tuned for maximum load.

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T-MAC RTS Operation (2)

• RTS Retries
• No CTS reply for RTS?
• The receiving node has not heard the RTS due to
  collision
• The receiving node is prohibited from replying
  due to an overheard RTS or CTS
• Receiving node is asleep
• Solutions
• Retransmit RTS if no answer
• If there is still no reply after two retries, it
  should give up and go to sleep

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T-MAC Choosing TA

• Determining TA
• The interval TA must be long enough to receive at
  least the start of the CTS packet
• TA gt CRT
• C contention interval length
• R RTS packet length
• T turn around time, time between RTS end CTS
  start
• Larger TA increases the energy used
• In experiments, used TA 1.5 x (C R T)

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T-MAC Overhearing Avoidance

• S-MAC
• But implemented as an option in T-MAC
• Node goes to sleep after overhearing RTS/CTS of
  other nodes communication
• Although overhearing avoidance saves energy, it
  must not be used when maximum throughput is
  required

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T-MAC Asymmetric Communication (1)

• Early-Sleeping Problem unidirectional (A to D)
• Node goes to sleep when a neighbor still has
  messages for it

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T-MAC Asymmetric Communication (2)

• Future request-to-send (FRTS)
• Let others know that we still have a message for
  it, but cannot access the medium
• C sends FRTS to future target of an RTS packet
• FRTS has duration field
• FRTS might affect data so, DATA postponed until
  FRTS is over To prevent others from taking
  medium, A send DS(Data Send) packet

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T-MAC Asymmetric Communication (3)

• Taking priority on full buffers
• When a nodes transmit/routing buffers are almost
  full, it may prefer sending than receiving
• Receive RTS, send its own RTS to others instead
  of CTS
• Advantage in a nodeto-sink communication pattern

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Experiments

• S-MAC Vs. T-MAC

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Simulation setup and parameters

• Simulator OMNeT
• Built a network of 100 nodes in a 10 by 10 grid
  (8 neighbor)
• Energy consumption
• S-MAC protocol
• A frame length of one second, and with several
  lengths of the active time, varying from 75 ms to
  915 ms.
• T-MAC protocol
• Always used a frame length of 610ms and an
  interval TA with a length of 15 ms
• Can optionally be deployed with overhearing
  avoidance, full-buffer priority, and FRTS

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Homogeneous local unicast

• Nodes send packets to one of their neighbors at
  random
• T-MAC Used overhearing avoidance, but no FRTS or
  full-buffer priority mechanisms

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Nodes-to-sink communication

• Nodes send messages to a single sink node Send
  message to corner node
• Shortest path routing, no data aggregation
• T-MAC Used overhearing avoidance, FRTS
  full-buffer priority mechanisms

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Early-sleeping Problem

• Nodes send messages to a single sink node Send
  message to corner node
• Shortest path routing, no data aggregation

T-MAC FRTS Vs. Priority Vs. FRTS
Priority Vs. No measures
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Conclusions And Future Work

• T-MAC dynamically adapts a listen/sleep duty
  cycle
• Early sleeping problem
• Proposed FRTS full-buffer priority
• Trade-off throughput vs. energy efficiency
• Future work
• Experiment with mobile network
• Apply virtual clustering in the S-MAC