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

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Sift A MAC Protocol for Event-Driven Wireless
Sensor Networks
Kyle Jamieson, Hari Balakrishnan, Y.C. Tay
MIT Computer Science and Artificial
Intelligence Laboratory Dept. of Computer
Science, National University of Singapore
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Types of Traffic in Sensor Networks

• Periodic traffic
• Animal habitat monitoring
• Indoor environment
• Temperature
• Room occupancy
• Medical monitoring
• Patient vital signs

• Event-driven traffic
• Failure of mechanical structures
• Water pipes
• Airplane wings
• Medical emergencies
• Vehicle tracking

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Airplane Wing Example
For critical systems, low latency is important!
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Sift

• Focus of our work
• Designing MAC protocol to handle event-driven
  workload
• Challenges
• Low-latency
• Good throughput
• Good fairness

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Problems for Traditional MAC

• Spatially-correlated contention correlation
  between geographical neighbors traffic.
• Bursty traffic the number of senders can quickly
  change.
• Suppression (counter-intuitively)

Suppression often, not all sensing nodes need to
report an event.
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The Status Quo CSMA

• Basis of existing sensornet MAC layers
• B-MAC, S-MAC
• Timeslot opportunity for a node to begin
  transmitting
• Process repeats after each packet

Busy Medium
Time
MAC Goal only one node transmit at a time
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The Status Quo CSMA
Time

• Pick a timeslot chosen uniformly in 0, CW
• Listen up to chosen slot
• Transmit if nobody else started transmitting
• Wait if somebody else started transmitting

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Example A Successful Transmission

• A and B happened to choose different slots
• Node A chooses slot 4, hears nothing, transmits
• Node B chooses slot 8, hears Node A, waits

Node A
Node B
Time
Success exactly one node in first non-vacant slot
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Example A Collision

• A and B happened to choose slot 4
• Both listen and hear nothing
• Both transmit simultaneously

Node A
Node B
Time
Collision 2 nodes in first non-vacant slot
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High Contention Causes Collisions in CSMA
Numerical simulation
Unacceptable collision rate above 15
transmitting sensors
Uniform distribution fills up, quickly
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Solving the Problem of Collisions in CSMA

• Create more slots
• Conventional approach
• Called binary exponential backoff (BEB)
• Change the way we pick slots
• Sift takes this approach

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Create More SlotsBinary Exponential Backoff
(BEB)

• The basis for Ethernet, B-MAC, S-MAC, 802.11,
  MACAW, many other MAC layers

Acknowledgement?
Yes
No
Reduce CW
Double CW and resend
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Problems with 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
• Especially with suppression

BEB causes performance to suffer
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Our Proposal Sift

• Sift is a MAC protocol for sensor networks
• Event-driven traffic
• Low-latency requirements
• Sifts Properties
• Extremely simple
• Offers up to 7-fold lower latency
• Maintains good channel utilization (throughput)

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Sift Changing the Distribution

• Keep number of slots the same (simple)
• Use an increasing non-uniform slot selection
  probability distribution
• Make collisions unlikely for large range of N
• Reduce the chance of collisions
• Penalty one packet- or RTS-time (ms)
• Reduce wastage of backoff slots
• Penalty one slot time (µs)

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Balls and Bins Analogy

• Bin represents a backoff slot in the contention
  window
• Bin height represents probability of picking that
  slot
• Ball represents a single nodes slot choice

A
Bins represent backoff slots ?
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Why an Increasing Slot-Selection Function?
Nodes choosing each slot ?
Bins represent backoff slots ?
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Sifts Slot Selection Distribution
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Optimal Non-Persistent CSMA Performance
Numerical simulation
With knowledge of number of nodes (IEEE J-SAC 04)
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Sift Approaches Optimal
Numerical simulation
Sift keeps success rate above this unacceptable
range
Sift needs no knowledge of the number of nodes
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Experimental Setup

• Simulation-based results (ns-2)
• Compare 802.11 (BEB), Sift, and 802.11/copy
• 802.11/copy send CW in each packet, copy
  overheard CW

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Event-driven Traffic Pattern

• Event-based traffic pattern
• 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

E.g. N4, R3
Base Station
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Sift Outperforms When N is Large
Experimental evaluation R1,16
R16
R1
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Sift Outperforms as R Increases
Experimental evaluation N128
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Exploring Sifts Performance Space
Experimental evaluation
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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

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Sift Performs Well with Hidden Terminals
Experimental evaluation N128, R1
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Sift Resilient to Jitter in Event Time
Experimental evaluation N128, R64
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Sift Improves Fairness
Experimental evaluation
64 nodes
Eight nodes
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Trace-Driven Experimental Setup

• Simulated vehicle tracking
• Captured live video from a street scene
• Extract motion events from image analysis
• Event trace drives ns-2 simulation
• 128 sensors laid out in a grid over the scene
• Sensors nearby each event send traffic in
  response to movement

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Sift Outperforms When R is Large
Trace-driven experimental evaluation
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Related Work

• TDMA suffers in terms of latency
• PTD (Mowafi et al.), TSMA (Chlamtac et al.)
• BEB-based protocols waste time in backoff
• MACAW (Bharghavan et al.), S-MAC (Ye et al.),
  FAMA (Garcia-Luna-Aceves et al.)
• The HIPERLAN standard for wireless LANs uses
  noise bursts of exponentially-distributed length
• Periodic-sleeping and other MAC protocols can
  work with Sift
• S-MAC (Ye et al.), B-MAC (Polastre)

Sift is a composable MAC primitive
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Conclusion

• Sift is a latency- (and sometimes throughput-)
  enhancing MAC for event-driven sensor networks
• Sift can be used as a building block in many MAC
  protocols

http//nms.csail.mit.edu/projects/sift
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Detailed Experimental Parameters

• Average of five runs with different random number
  seeds for each run
• ARQ with 5 retransmit limit
• Control packets sent at 1 MBps data at 2 MBps
• 20 µs slot time 192 bit preamble 30 byte packet
• 802.11 CWmin31, CWmax1023

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Sift Provides Good Throughput
32 nodes
Two nodes
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Optimal Non-Persistent CSMA

• Let s be a slot number, assume N 2 sensors
  transmitting. Define

Collision Minimizing CSMA and its Applications
to Wireless Sensor Networks. IEEE J. Selected
Areas in Comm. 226 (2004) pp. 1048-1058