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An Energy-Efficient MAC Protocol for Wireless Sensor Networks (S-MAC)

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Title: An Energy-Efficient MAC Protocol for Wireless Sensor Networks (S-MAC)


1
An Energy-Efficient MAC Protocol for Wireless
Sensor Networks (S-MAC)
  • Wei Ye, John Heidemann, Deborah Estrin

2
Sensor Network MAC Differences
  • Energy Consumption
  • Difficult to recharge
  • Request long lifetime of work

3
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

4
Main Contributions
  • Periodic listen and sleep
  • Collision avoidance
  • Overhearing avoidance
  • Message passing

5
  • What is the main idea this paper propose?
  • Duty Cycles

6
Listen/Sleep Schedule Assignment
  • Choosing Schedule (1)
  • Synchronizer
  • Listen for a mount of time
  • If hear no SYNC, select its own SYNC
  • Broadcasts its SYNC immediately

Listen
A
Sleep
Go to sleep after time t
Listen for SYNC
Broadcasts
  • Follower
  • Listen for a mount of time
  • Hear SYNC from A, follow As SYNC
  • Rebroadcasts SYNC after random delay td

Listen
Sleep
Go to sleep after time t- td
td
Broadcasts
7
Listen/Sleep Schedule Assignment
  • Choosing Schedule (2)
  • B receive As schedule and rebroadcast it.
  • Hear a different SYNC from C.
  • Adapt both schedules
  • Late sleep results to longer listening period
  • Early sleep results to early wake up in the next
    schedule

Listen
Sleep
Go to sleep after time t1
Listen for SYNC
Broadcasts
Listen
B
Sleep
td
Broadcasts
Only need to broadcast once
Listen
C
Nodes only rarely adopt multiple schedules
Sleep
Go to sleep after time t2
Listen for SYNC
8
Listen/Sleep Schedule Assignment
  • Maintaining 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

9
(No Transcript)
10
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.

11
Example (To sleep, or not to sleep, this is a
question)
  • A is talking to B
  • D receives CTS from B -gt sleep
  • Ds transmission will collide Bs
  • C receives RTS from A -gt 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 NAV
  • Keeping sleeping until NAV count down to zero

12
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

13
Acknowledgment to Pro. Jun Yang
Why use ACK?
14
Hidden Terminal Example
15
Why utilize message passing?
16
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

17
Experiment
Listen time 300ms Sleeping time 1s SYNC every
13s (10 listen/sleep period) A, B, C use the same
schedule
18
Energy save due to periodic sleep
802.11
Energy save due to avoiding overhearing by using
message passing
OA
SMAC
Heavy Traffic
Light Traffic
19
OA In light traffic status, sources nodes keep
listening for quite a long time
20
SYNC overhead
Overhearing avoidance still benefit
Heavy Traffic
Light Traffic
21
Discussion
  • What is the tradeoff by using duty cycles?
  • How can we make the schedule more intelligent?

22
  • Acknowledgment to Jun Yang (CS, Duke) and Romit
    Roy Choudhury
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