Energy Efficient MAC Protocol

1
Energy Efficient MAC Protocol for Wireless
Sensor Network
Haozhi Xiong Department of Electrical and
Computer Engineering The Ohio State University
2
Outline

• General view on MAC protocol in WSN
• Requirements
• Problems
• MAC protocols for WSN
• Conclusion

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General Purposes of MAC Protocol

• MAC protocol is to ensure that the channel can be
  accessed by multiple users, dealing with the
  situation of interference.
• Has a direct bearing on how reliably and
  efficiently data can be transmitted

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Requirement of WSN

• Application-level performance is the goal as
  opposed to per-node fairness
• Long battery life
• Delay
• Surveillance, Low traffic, Regular update
• Emergency, Quick response, Bursty heavy load

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Requirement of WSN contd

• Adaptive to changing topology
• Nodes die out
• Mobile nodes
• Applicable with limited computing capability

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MAC Protocol for WSN

• Energy-efficient in sense of achieved throughput
• Delay-optimized
• Robust
• As simple as possible

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Major problems in WSN MAC design

• Idle listening
• Listening when no traffic is sent
• Overhearing
• Receiving packets destined to other nodes
• Collision
• Retransmission
• Overhead
• Headers for signaling

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Overhearing

• Receiving packets that are not destined to the
  node
• Interception, waste of energy in receiving, error
  responding will cause potential collision

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Traffic Pattern

• Local broadcast
• Schedule exchange/update between neighbors
• Omni-directional transmission is desired
• Nodes to sink report
• Payload and signaling
• In favor of directional transmission

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Classification

• Basic idea
• Control the active time of radio
• Control the times of on-off switches
• Scheduling time-slotted system, wake-up by
  schedule, clock shift can be disastrous
• LPL (Low Power Listening) preamble sampling,
  wake-up tone

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S-MAC (Sensor-MAC)?

• Single frequency, schedule-based
• Active time interval, larger sleep interval
• Duty cycle listen interval / frame length

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S-MAC contd

• Total frame length is limited by latency
  requirement, buffer space, and active time
• Active time depends on message rate
  fragmentation is used while transmitting large
  messages

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S-MAC contd

• Synchronization period
• Nodes exchange schedules by sending SYNC packets
  to immediate neighbors at their scheduled listen
  time

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S-MAC contd

• Flat topology
• Virtual cluster comprises nodes with common
  schedule
• No coordination from cluster head

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S-MAC contd

• Coordinating sleeping
• listen for a fixed amount of time
• Following the existing schedule
• or
• Establishing new schedule if no schedule exists
  and announcing new schedule by SYNC

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S-MAC contd

• New schedule received by node A
• Discard current schedule if node A has no other
  neighbor
• or
• Adopt both schedules if node A has other
  neighbors

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S-MAC contd

• Periodical neighbor discovery
• reasons
• SYNC is corrupted by collision or interference
• SYNC is delayed due to busy medium
• The lesser neighbors a node has, the more
  frequent it search for neighbors.

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S-MAC contd

• Adaptive listening
• To resolve sleep latency
• Potential delay on every hop!
• Basic idea
• Let the nodes who overhear its neighbors
  transmission (ideally RTS or CTS) go to sleep and
  wake up for a short period at the end of
  transmission

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S-MAC contd

• Additional collision avoidance
• Record transmission time as NAV (Network
  Allocation Vector)?
• Check if NAV 0 before transmission attempt
• NAV 0 indicates ongoing transmission is over

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S-MAC contd

• Message passing
• Divide long messages into small fragments
• Transmit fragments in a burst through reserved
  channel

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S-MAC contd

• Features
• Loose synchronized due to large scale of
  intervals, no need for precise synchronization
• Virtual cluster
• Adaptive listen to reduce sleep delay
• Message passing

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S-MAC contd

• Cons
• Sleep latency
• Active time must be long enough to handle
  expected highest load, inefficient when load is
  lower.
• Essentially S-MAC trades energy with latency

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T-MAC (Timeout-MAC)?

• Minimize idle listening
• Using timeout to be adaptive to traffic during
  wakeup period
• Transmitting all messages in burst of variable
  length
• RTS/CTS provides both collision avoidance and
  reliable transmission

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T-MAC contd

• An active period ends when no activation event
  has occurred for a time TA

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T-MAC contd

• Definition of activation event
• The firing of a periodic frame timer (the
  beginning)
• The reception of any data on the radio
• The sensing of traffic (during collision)?
• The end-of-transmission of a nodes own data
  packet or acknowledgement
• End of neighbors transmission (knowledge from
  prior overhearing)?

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T-MAC contd

• Node successfully transmitting 3 packets

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T-MAC contd

• Determining TA
• RTS starts transmission, the TA should be at
  least long enough to hear the CTS
• TA gt RTS CTS Turn_around_Time

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T-MAC contd

• Early sleeping problem
• Reason asymmetric communication
• Node to sink
• Border of highly active part of network

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T-MAC contd

• Solution1
• Future request-to-send

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T-MAC contd

• Use of FRTS
• Send FRTS when overhearing CTS destined to
  another nodes
• Do not send FRTS if communication is sensed right
  after CTS
• Do not send FRTS if the node has already been
  prohibited by prior RTS/CTS
• Receiver of FRTS wakes up as dictated by FRTS
  (FRTS contains transmission time from CTS)

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T-MAC contd

• DS (data-send) packet is used to occupy the
  channel while giving time for FRTS to be sent

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T-MAC contd

• Solution 2
• Full buffer priority

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T-MAC contd

• Nodes with buffer almost full may prefer sending
  to receiving
• After a node receiving RTS, it sends its own RTS
  without responding CTS.
• A node may take advantage of full buffer priority
  only after several failed contentions (for
  example 2)?

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T-MAC contd

• Pros
• Adaptive active time
• Cons
• Early sleeping problem

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P-MAC (Pattern-MAC)?

• A nodes sleep-wakeup schedule is determined by
  its own traffic and the traffic of its neighbors.

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P-MAC contd

• Pattern
• Pattern is a string of bits indicating the
  tentative sleep-wake plan and is subject to
  change.
• 1 wake at a slot
• 0 sleep at a slot
• Pattern is NOT equal to schedule.
• Schedule is derived from a nodes own pattern and
  its neighbors pattern.

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P-MAC contd

• Pattern generation
• Let Pj be the pattern of node j
• A sequence of N slots is called a period
• Pj repeats over N slots if the length of Pj is
  lesser than N

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P-MAC contd

• Example of a pattern
• N 5, Pj 01, then the tentative plan will be
  01010
• 01010 is denoted as 031
• 0m1, m 0,1,...,N-1

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P-MAC contd

• Pattern growing
• If the node has no data to send, the number of
  0 doubles in current pattern until threshold
  th.
• Beyond threshold, the number of 0 increases by
  1.
• 1, 01, 021, 041, , 0th1, 0th01, 0th021, 0th031,
  , 0N-11
• If the node has data to send, the pattern is
  restored to default 1

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P-MAC contd

• Pattern exchange
• STF (Super Time Frame)
• PRTF (Pattern Repeat Time Frame)

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P-MAC contd

• PETF (Pattern Exchange Time Frame)
• The last generated pattern during PRTF becomes
  the pattern for next PRTF, and will be advertised
  to neighbors during PETF.

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P-MAC contd

• w a slot in PRTF for all nodes to stay awake,
  downstream nodes can update pattern to 1 and
  wake up quickly

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P-MAC contd

• TR is chosen long enough to handle a complete
  data transmission (CWRTSCTSDATAACK)
• TE is chosen long enough to broadcast a pattern.

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P-MAC contd

• Schedule generation

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X-MAC

• Asynchronous protocols like B-MAC and Wise-MAC,
  rely on LPL (low power listening) also called
  preamble sampling.

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X-MAC (contd)

• LPL drawbacks
• Extended waiting time even if the receiver has
  already wake up
• Cannot tell whos the intended receiver until the
  end of preamble

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X-MAC (contd)

• Strobed preamble
• Allowing interruption and wake up faster
• Short preamble
• embedded with address information of the target

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X-MAC (contd)

• Energy-efficient
• Low latency (reduced preamble length)
• No need for synchronization
• Low overheads
• Less complex

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Conclusion

• The special requirement for MAC design in WSN
• Energy-efficient
• Delay-optimized
• Robust
• Simple
• Synchronous MAC based on schedule
• SMAC, TMAC, PMAC
• Asynchronous MAC based on preamble
• XMAC (BMAC, WiseMAC)

Big Mac ?