Energy

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Energyefficient Reliable Broadcast in Underwater
Acoustic Networks

• Paolo Casari and Albert F Harris III
• University of Padova, Italy
• University of Illinois at Urbana-Champaign

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Scenario Programming a underwater sensor network
You want to reprogram a entire sensor network
wirelessly !!!!
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Problem

• You want to implement reliable broadcast in an
  underwater sensor network
• Constraints
• Energy ?
• Error prone channel

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Easy solution

• Use standard techniques from RF broadcast ?
• Pros
• No redesign required
• Easily write a paper !
• Cons
• Underwater channel characteristics are different
  leading to poor performance
• Paper gets rejected !!

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Our (the authors) solution

• First try and understand the how the underwater
  channel characteristics differ from RF
• Leverage underwater channel characteristics to
  design energy efficient reliable broadcast

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Outline

• Underwater reliable broadcast basics
• Underwater channel characteristics
• Broadcast protocols designed
• Evaluation
• Conclusions

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Underwater Reliable Broadcast

• Standard network primitive
• Routing protocols
• Reprogramming of nodes
• Standard techniques
• Push method
• Each node sends broadcast out upon receiving
• Optimization techniques
• Reduce number of sending nodes
• Challenge
• Very expensive
• Energy consumption
• Time
• Underwater channel
• Bandwidth challenged
• Energy challenged

• Techniques
• Forward error correction (FEC)
• Mitigate error rate
• Combined short link / long link communication
• Minimize energy consumption/delay
• Metrics
• Energy consumption
• Broadcast completion time

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Three Important Underwater Channel Characteristics

• Bandwidth
• Distance dependent
• AN factor
• Attenuation
• Noise
• Transmission power
• Signal-to-noise requirement
• AN factor
• Delay
• Location in water
• Salinity and temperature of water

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Underwater Attenuation-Noise

• Attenuation is both distance and frequency
  dependent

Absorption factor (frequency dependent as O(f2))
Spreading loss (k2 for spherical)
Absorption loss

• Noise is frequency dependent
• Four common components
• Turbulence
• Shipping
• Wind
• Thermal

Dominant for low frequencies
Dominant for high frequencies
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Bandwidth-Distance Relationship

• Find frequency center
• Frequency with minimal attenuation given the
  distance
• Find bandwidth
• 3 dB definition for example

• Both the frequency center AND the bandwidth vary
  with distance between nodes

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Transmit Power

• Signal-to-noise ratio (SNR)
• Related to
• Bandwidth (B(l))
• Attenuation (A(l,f))
• Noise (N(f))
• Calculate needed transmit power (W)
• Distance between nodes
• SNR threshold

Knee in curve appears at lt 3 km
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Underwater Acoustic Propagation Speed
Consider nodes 1 km apart

• Speed
• c O(T3)O(T2S)O(z2)
• Temperature (T)
• Salinity (S)
• Depth in water (z)
• T is dependent on z
• Value
• Rate of change
• Average speed
• 1,500 m/s

Varies by 20 ms over a depth of 4 km
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Towards Broadcasting

• Leverage underwater properties
• Turn challenges into benefits
• Bandwidth-distance relationship
• Use new pull model
• Reduce the number of redundant transmissions
• Use FEC
• Reduce the need for retransmissions

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Simple Reliable Broadcast (SRB)

• Standard push method protocol
• Node begins broadcast
• Upon receiving broadcast
• Re-broadcast message
• If broadcast is received incomplete
• Wait for timeout
• Potential for some other neighbor to transmit
  needed packet
• Send retransmission request to neighbors

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Single-band Reliable Broadcast (SBRB)

• Problem
• Short links
• Reduced coverage
• Nodes fail to overhear broadcast
• Long links
• Expensive
• Increase contention in the network
• Solution Pull method
• Using high-power, long links for notifications
• Using low-power short links for data

• Upon receiving a complete broadcast message
• Transmit notification on long link
• Wait for transmission requests
• Upon receiving a broadcast request message
• Nodes with complete broadcast contend for channel
• Winning node broadcasts, other go back to listen
  mode

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Dual-band Reliable Broadcast

• Idea
• Instead of sending wasted data for notification
  on long link, make use of the bits
• Works like SBRB, except
• FEC data is sent over long link instead of
  notification

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Evaluation

• Three protocols
• Single-band Reliable Broadcast
• SBRB, without FEC
• FSBRB, with FEC
• Dual-band Reliable Broadcast

• Baseline Simple Reliable Broadcast
• Each node re-broadcasts using low-power short
  links
• SRB, without FEC
• FSRB, with FEC
• Generate random topologies
• 5 km x 5 km x 5 km network
• Control maximum closest neighbor distance (varied
  between 100 m and 2 km)
• Vary number of nodes between 40 and 700

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Pull Method Saves Energy

• For a large range of network densities, both
  energy and time to broadcast completion are
  minimized

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Conclusions

• Reliable broadcast
• Standard network primitive required by protocols
  and applications
• Leverage channel properties
• Reduce redundant transmissions
• Leverage FEC
• Reduce retransmissions

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Future Directions

• Enhancements
• Add more intelligent FEC
• Fountain-style codes
• Reduce initial number of transmissions further
• Implementation and deployments
• Testbeds

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Discussions

• Can a acoustic modem transmit at two different
  frequencies without causing collision
• How is this use of multiple channels to transmit
  data and signaling information applicable to
  terrestrial networks

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Similar idea in terrestrial networks