KAIST Sift: A MAC Protocol for Event-Driven Wireless Sensor Networks Suho Yang (CS710: November 4,...

KAIST

Sift: Sift: A MAC Protocol A MAC Protocol for Event-for Event-Driven Wireless Sensor NetworksDriven Wireless Sensor Networks

Suho Yang(CS710: November 4, 2008)

Kyle Jamieson, Hari Balakrishnan, Y.C. Tay

LNCS Springer 2006

Sift: A MAC Protocol for Event-Driven Wireless Sensor Networks / 24/ 24

ContentsContents

Introduction

Motivation

Our proposal: Sift

Performance Evaluation

Conclusion & Discussion

22


Event-driven WSNReport an event information when an phenomenon occurs

↔ periodic traffic

Latency-sensitive applications

33

IntroductionIntroduction


SiftFocus

Designing MAC protocol to handle event-driven traffic

ChallengesLow latency

Good throughput

Good fairness

IntroductionIntroduction


Problems of traditional contention-based MAC in WSNSpatial correlated contention

Not suitable for bursty traffic

Motivation: Problems of Traditional CSMAMotivation: Problems of Traditional CSMA


Problems of traditional non-persistent CSMA

Timeslot: opportunity for a node to begin transmitting

Pick a timeslot chosen uniformly in [0, CW]

Listen up to chosen slotTransmit if nobody else started transmitting

Wait more if somebody else started transmitting


Busy Medium

When the channel is idle,

•1-persistent CSMA: transmit immediately with 100% probability

•p-persistent CSMA : transmit immediately with p*100% probability

•Non-persistent CSMA: transmit after waiting for a random amount of time and checks again


Problems of traditional non-persistent CSMASuccessful transmission case

Collision case


Node A:

Node B:

Slot choice (slot #4)


Node A:

Node B:




Problems of traditional non-persistent CSMAHigh contention causes collisions in CSMA


Unacceptable collision rate above ~15 transmitting sensors

Due to uniform distribution!


Solution for this problemCreate more slots

Conventional approach

Called “binary exponential backoff” (BEB)


Acknowledgement?

Reduce CWDouble CWand resend

Yes No


Problem of BEBTakes 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


Sift is a MAC protocol for ...Event-driven traffic

Low-latency requirements

Sift’s propertiesExtremely simple

Offers up to 7-fold lower latency

GoalDesign a MAC protocol that minimizes the latency taken to send R of without collisions

Our proposal: SiftOur proposal: Sift


Main IdeaChange the way we pick slots

Instead of uniform distribution

Use small and fixed-size contention windowNo BEB

Not all sensing nodes need to report an eventIt 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 slotUse an geometrically-increasing probability distribution

→ Reduce the chance of collisions

→ Reduce wastage of backoff slots



Sift’s slot selection distributionIncreasing exponential distribution


rCW

CW

rp

1

)1(


Why use this pdf?


A

Bins represent backoff slots →

Nod

es c

hoos

ing

each

slo

t →


Optimal non-persistent CSMA performanceWith knowledge of number of nodes (IEEE JSAC ’04)



Sift keeps success rate above this unacceptable range

1616

Sift approaches OptimalSift needs no knowledge of the number of nodes



Performance EvaluationPerformance Evaluation

Simulation-basedNS-2

Comparisons with802.11 (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 requiredIf a node hears ≥ R reports then it suppresses its own event report

1717

BS

E.g. N=4, R=3



Simulation_1-1: LatencySift outperforms when N is large

1818

R=16

R=1



Simulation_1-2: LatencySift outperforms as R Increases

1919



Simulation_2: FairnessSift outperforms 802.11 in terms of fairness

2020

Eight nodes 64 nodes



Simulation_3: Hidden terminal experiment setupSeparate 128 sensors into mutually-hidden clusters

Nodes in one cluster cannot hear nodes in another

All nodes send to the base stationResult: hidden terminal collisions at the base station

2121

Base Station



Simulation_3: Hidden terminal experiment setup

2222


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 ideaUse 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

ConclusionConclusion


The lack of mentions about ...Energy consumption

No attention about ...How to determine R?

Only manual setting

Too strong assumptionSingle-hop communication to one base station

DiscussionDiscussion


Thank youThank you

2525


Existing MAC protocols in WSNLet s be a slot number, assume N ≥ 2 sensors transmitting. Define:

Appendix A: Optimal Non-Persistent CSMAAppendix A: Optimal Non-Persistent CSMA

*1

*2

*1

*

1

1

1)(

)(1

2,)(

1

1,0

)(

rrK

rKr

N

s

s

pppNfN

Nfp

sNfN

N

s

Nf

“Collision Minimizing CSMA and its Applications to Wireless Sensor Networks.” IEEE JSAC, 2004

KAIST Sift: A MAC Protocol for Event-Driven Wireless Sensor Networks Suho Yang (CS710: November 4,...

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