Sift: A MAC Protocol for Event-Driven
Wireless Sensor Networks
Kyle Jamieson, Hari Balakrishnan, Y.C. Tay
LNCS Springer 2006
Suho Yang
(CS710: November 4, 2008)
KAIST
Contents
Introduction
Motivation
Our proposal: Sift
Performance Evaluation
Conclusion & Discussion
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Introduction
Event-driven WSN
Report an event information when an phenomenon occurs
↔ periodic traffic
Latency-sensitive applications
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Introduction
Sift
Focus
Designing MAC protocol to handle event-driven traffic
Challenges
Low latency
Good throughput
Good fairness
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Motivation: Problems of Traditional CSMA
Problems of traditional contention-based MAC in WSN
Spatial correlated contention
Not suitable for bursty traffic
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Motivation: Problems of Traditional CSMA
Problems of traditional non-persistent CSMA
When the channel is idle,
•1-persistent CSMA: transmit immediately with
100% probability
Busy Medium
•p-persistent CSMA : transmit immediately with
p*100% probability
•Non-persistent CSMA: transmit after waiting
for a random amount of time and checks again
Timeslot: opportunity for a node to begin transmitting
Pick a timeslot chosen uniformly in [0, CW]
Listen up to chosen slot
Transmit if nobody else started transmitting
Wait more if somebody else started transmitting
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Motivation: Problems of Traditional CSMA
Problems of traditional non-persistent CSMA
Successful transmission case
Slot choice (slot #4)
Node A:
Slot choice (slot #8)
Node B:
Collision case
Slot choice (slot #4)
Node A:
Slot choice (slot #4)
Node B:
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Motivation: Problems of Traditional CSMA
Problems of traditional non-persistent CSMA
High contention causes collisions in CSMA
Due to uniform distribution!
Unacceptable collision rate above
~15 transmitting sensors
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Motivation: Problems of Traditional CSMA
Solution for this problem
Create more slots
Conventional approach
Called “binary exponential backoff” (BEB)
Acknowledgement?
Yes
Reduce CW
No
Double CW
and resend
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Motivation: Problems of Traditional CSMA
Problem of BEB
Takes time for every node to increase CW
Especially if traffic is spatially-correlated and bursty
Waste backoff slots if collisions cause CW to increase
We are interested in the collision-free transmission of only the first R of N
potential reports of some event
sink
sink
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Our proposal: Sift
Sift is a MAC protocol for ...
Event-driven traffic
Low-latency requirements
Sift’s properties
Extremely simple
Offers up to 7-fold lower latency
Goal
Design a MAC protocol that minimizes the latency taken to send R of
without collisions
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Our proposal: Sift
Main Idea
Change the way we pick slots
Instead of uniform distribution
Use small and fixed-size contention window
No BEB
Not all sensing nodes need to report an event
It is enough for a subset of the event reports to reach the data sink
Out of N nodes, only the first R nodes report (the remainder are suppressed)
Changing the Distribution for picking transmission slot
Use an geometrically-increasing probability distribution
→ Reduce the chance of collisions
→ Reduce wastage of backoff slots
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Our proposal: Sift
Sift’s slot selection distribution
Increasing exponential distribution
(1   ) CW r
pr 

CW
1
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Our proposal: Sift
Nodes choosing each slot →
Why use this pdf?
A
Bins represent backoff slots →
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Our proposal: Sift
Optimal non-persistent CSMA performance
With knowledge of number of nodes (IEEE JSAC ’04)
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Our proposal: Sift
Sift approaches Optimal
Sift needs no knowledge of the number of nodes
Sift keeps success rate above
this unacceptable range
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Performance Evaluation
Simulation-based
NS-2
Comparisons with
802.11 (BEB), 802.11/copy (=copy overheard CW+countdown timer)
Experiment Setup (Event-driven traffic pattern)
Topology: Single-hop to one base station
N nodes sense and report an event
R (≤ N ) reports are required
If a node hears ≥ R reports then it suppresses its own event report
BS
E.g. N=4, R=3
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Performance Evaluation
Simulation_1-1: Latency
Sift outperforms when N is large
R=16
R=1
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Performance Evaluation
Simulation_1-2: Latency
Sift outperforms as R Increases
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Performance Evaluation
Simulation_2: Fairness
Sift outperforms 802.11 in terms of fairness
Eight nodes
64 nodes
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Performance Evaluation
Simulation_3: Hidden terminal experiment setup
Separate 128 sensors into mutually-hidden clusters
Nodes in one cluster cannot hear nodes in another
All nodes send to the base station
Result: hidden terminal collisions at the base station
Base Station
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Performance Evaluation
Simulation_3: Hidden terminal experiment setup
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Conclusion
Sift is ideal for sensor networks, where...
it is often sufficient that any R of N sensors that observe an event report it
spatially-correlated contention occurs
sudden changes in the number of sensors that are trying to send data
Key idea
Use a geometrically-increasing probability distribution for picking a
transmission slot within a fixed-size contention window
Sift is a latency-enhancing MAC for event-driven sensor networks
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Discussion
The lack of mentions about ...
Energy consumption
No attention about ...
How to determine R?
Only manual setting
Too strong assumption
Single-hop communication to one base station
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Thank you
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Appendix A: Optimal Non-Persistent CSMA
Existing MAC protocols in WSN
Let s be a slot number, assume N ≥ 2 sensors transmitting. Define:
0,
s 1

N 1

f s ( N )  
N 1 

 , s  2
 N  f (N )
s 1


1  f K r ( N )
*
*
*
*
pr 
1  p1  p2    pr 1
N  f K r ( N )


“Collision Minimizing CSMA and its Applications to Wireless Sensor Networks.” IEEE JSAC, 2004
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