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Energy–efficient Reliable Broadcast in Underwater Acoustic Networks

Energy–efficient Reliable Broadcast in Underwater Acoustic Networks. Paolo Casari and Albert F Harris III University of Padova, Italy University of Illinois at Urbana-Champaign. Standard network primitive Routing protocols Reprogramming of nodes Standard techniques Push method

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Energy–efficient Reliable Broadcast in Underwater Acoustic Networks

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  1. Energy–efficient Reliable Broadcast in Underwater Acoustic Networks Paolo Casari and Albert F Harris III University of Padova, Italy University of Illinois at Urbana-Champaign

  2. 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 Delay 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 Underwater Reliable Broadcast

  3. 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

  4. Underwater Attenuation-Noise • Attenuation is both distance and frequency dependent Absorption factor (frequency dependent as O(f2)) Spreading loss (k=2 for spherical) Absorption loss • Noise is frequency dependent • Four common components • Turbulence • Shipping • Wind • Thermal Dominant for low frequencies Dominant for high frequencies

  5. 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

  6. 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 < 3 km

  7. Underwater Acoustic Propagation Speed • 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 in water • 1,500 m/s Consider nodes 1 km apart Thermocline Varies by 20 ms over a depth of 4 km

  8. 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

  9. 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

  10. 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 Single-band Reliable Broadcast (SBRB)

  11. 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

  12. 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 Three protocols Single-band Reliable Broadcast SBRB, without FEC FSBRB, with FEC Dual-band Reliable Broadcast Evaluation

  13. Pull Method Saves Energy • For a large range of network densities, both energy and time to broadcast completion are minimized

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

  15. Future Directions • Enhancements • Add more intelligent FEC • Fountain-style codes • Reduce initial number of transmissions further • MAC and routing work • Implementation and deployments • Testbeds

  16. Thank You • Albert Harris III • aharris@cs.uiuc.edu • http://mobius.cs.uiuc.edu/~aharris/

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