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UW-FLASHR: Achieving High Channel Utilization in a Time-Based Acoustic MAC Protocol

UW-FLASHR: Achieving High Channel Utilization in a Time-Based Acoustic MAC Protocol. Goals. Time-based MACs (TDMA, CSMA, etc.) have advantages over FDMA & CDMA in some UWAN scenarios Can Time-based MAC be practical for UWANs? How to overcome challenges of UWANs?.

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UW-FLASHR: Achieving High Channel Utilization in a Time-Based Acoustic MAC Protocol

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  1. UW-FLASHR: Achieving High Channel Utilization in aTime-Based Acoustic MAC Protocol

  2. Goals • Time-based MACs (TDMA, CSMA, etc.) have advantages over FDMA & CDMA in some UWAN scenarios • Can Time-based MAC be practical for UWANs? • How to overcome challenges of UWANs?

  3. UnderWater Acoustic Networks (UWANs) Acoustic (sound) waves instead of RF because salt water is conductive Due to noise and VERY high absorption at high frequencies, data rate in IDEAL scenario is 50,000 bits/sec Sound travels through water at 1,500 meters/sec Example, sending 100 bytes 250 meters: RF: 0.000080s transmit, 0.0000008s propagation UW: 0.016000s transmit, 0.1666667s propagation 200x slower 200,000x slower

  4. Why we shouldn’t use Time-based MAC • High ratio of propagation delay to transmission delay (3-4 orders of magnitude higher than RF) + exclusive access constraint = low utilization • Carrier sensing and time synchronization difficult • Determining propagation delays difficult • Centralized control undesirable

  5. Why we should use Time-based MAC Simpler hardware than CDMA & FDMA Multiple slow channels can be less desirable than a single fast channel More time spent sending/receiving Additional delay and delay is already high More opportunities to sleep

  6. Time-based is not inherently bad If data transmissions are 1 time unit and propagation between nodes are as shown, could send/receive 10 packets in 8 time units

  7. For Time-based to be practical No exclusive access constraint No precise clock synchronization No knowledge of propagation delays No large guard times between transmissions Decentralized

  8. For Time-based to be practical No exclusive access constraint No precise clock synchronization No knowledge of propagation delays No large guard times between transmission Decentralized

  9. Scheduling Transmissions Loose clock synchronization Send control messages during experimental portion to build schedule for established portion

  10. Scheduling Transmissions (2) Nodes can not directly exchange precise time information

  11. Scheduling Transmissions (3) Relative times allow for nodes to exchange precise time information.

  12. Scheduling Transmissions (4) Process repeats until all transmissions have been scheduled If a conflict occurs, nodes use similar “delta” value exchanges (described in paper) to narrow the search for a suitable time slot ACK frames are scheduled in the same way as DATA frames

  13. Evaluation QualNet modified to simulate acoustic channel Spherical path loss, thorp attenuation Nodes randomly placed in square single-hop terrain, max delay between 10ms and 1000ms Node clocks randomly initialized Generate 50 or 1000 byte (76ms or 582ms) CBR traffic between random nodes Utilization is maximum total duration of all DATA and ACK frames received in a single cycle divided by the cycle size (including wasted experimental portion and boundaries)

  14. Example of Schedule Being Built Animation

  15. Example of Constructed Schedule

  16. Example of Constructed Schedule In 693ms, 467ms of data sent and received (67%) In 560ms, 311ms of data sent and received (55%) 76ms data duration, Max propagation delay 500ms Approximately 25% utilization versus at most 15% with exclusive access Delay from node 5 to 8 is 364ms

  17. Utilization vs Propagation Delay

  18. Utilization vs Data Payload Size

  19. Utilization vs Short-Term Variation

  20. Room for Improvement?

  21. From First Real Example Eight 582ms transmissions in 10s (46%) Max propagation delay is ~700ms

  22. Optimal Scheduling Optimal scheduling is ten 582ms xmits in ~5.3s (109%)

  23. Another Random Topology Ten 582ms xmits in ~4.6s (125%)

  24. Different Nodes Transmitting Ten 582ms xmits in ~5.4s (105%)

  25. Scale Delays 3x Longer Ten 582ms xmits in ~4.7s (125%) Max propagation delay is ~2s

  26. Conclusions • Time-based MAC protocol can be practical and efficient (both in terms of utilization and energy) • For small data payloads, we have shown ~2X increase over exclusive access protocols • A lot of room to improve scheduling, time wasted in experimental portion and boundaries, etc.

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