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An Energy-Efficient MAC Protocol for Wireless Sensor Networks (S-MAC)
Wei Ye, John Heidemann, Deborah Estrin
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Sensor Network MAC Differences Energy Consumption
Difficult to recharge Request long lifetime of work
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Major source of energy waste Collision Overhearing Control Overhead Idle Listening
Listening to possible traffic that is not sent 50%-100% energy drain compared with receiving
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Main Contributions
Periodic listen and sleep Collision avoidance Overhearing avoidance Message passing
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What is the main idea this paper propose? Duty Cycles
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Listen for SYNC
td
Listen/Sleep Schedule AssignmentChoosing Schedule (1)
Sleep
Listen
Go to sleep after time t
Sleep
Listen
Broadcasts
A
B
Broadcasts
Go to sleep after time t- td
Synchronizer• Listen for a mount of time• If hear no SYNC, select its
own SYNC• Broadcasts its SYNC
immediately
Follower• Listen for a mount of time• Hear SYNC from A, follow
A’s SYNC• Rebroadcasts SYNC after
random delay td
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Listen for SYNC Go to sleep after time t2
td
Listen for SYNC
Listen/Sleep Schedule AssignmentChoosing Schedule (2)
Sleep
Listen
Go to sleep after time t1
Sleep
Listen
A
Broadcasts
Sleep
Listen
C
Broadcasts
B
Only need to broadcast once
1. B receive A’s schedule and rebroadcast it.
2. Hear a different SYNC from C.
3. Adapt both schedules
• Late sleep results to longer listening period
• Early sleep results to early wake up in the next schedule
Nodes only rarely adopt multiple schedules
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Listen/Sleep Schedule AssignmentMaintaining Schedule Reason: Clock drift Solution: Sent SYNC periodically
Listen interval is divided into two parts SYNC RTS/CTS/DATA/ACK No data transmission during synchronization
Node B listen
Node A listenNode A listen
Node B listen
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Collision and Overhearing Avoidance Collision Avoidance
Similar to 802.11 RTS/CTS Virtual carrier sense (NAV) Physical carrier sense
Overhearing Avoidance Problem: A node picks up packets that are
destined to other nodes Solution: Interfering nodes go to sleep when
overhear RTS, CTS or ACK.
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Example (To sleep, or not to sleep, this is a question)
A is talking to BD receives CTS from B -> sleep• D’s transmission will collide B’s
C receives RTS from A -> sleep• C cannot receive CTS/DATA from E
All immediate neighbours of transmitting node sleep
How long should they sleep?C and D update their NAVKeeping sleeping until NAV count down to zero
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Message Passing
How to transmit long message? One long packet Many small packets with RTS/CTS/DATA/ACK for
each
S-MAC: Divide into fragments, transmit all in burst RTS/CTS reserve medium for the entire sequence Fragment-errors recovery with ACK - no control
packets for fragments
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Acknowledgment to Pro. Jun Yang
Why use ACK?
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Hidden Terminal Example
CDATA
A B
CACK
A BACK
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Why utilize message passing?
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Message Passing
Advantages: Energy saving: immediate node go to sleep when sense
transmissions Reduces control overhead by sending multiple ACK
Disadvantage: Node-to-node fairness reduces
However, message-level latency reduces
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Experiment Listen time: 300msSleeping time: 1sSYNC: every 13s (10 listen/sleep period)A, B, C use the same schedule
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Heavy Traffic Light Traffic
Energy save due to avoiding overhearing by using message passing
Energy save due to periodic sleep 802.11
OA
SMAC
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OA: In light traffic status, sources nodes keep listening for quite a long time
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Heavy Traffic Light Traffic
SYNC overhead
Overhearing avoidance still benefit
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Discussion
What is the tradeoff by using duty cycles? How can we make the schedule more
intelligent?
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Acknowledgment to Jun Yang (CS, Duke) and Romit Roy Choudhury