Time Synchronization for High Latency Acoustic Networks

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Time Synchronization for High Latency Acoustic
Networks

• Affan A. Syed and John Heidemann
• Presented by Peng Xie

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Acoustic Channel

• High propagation delay
• 1500 m/s
• Low data rate
• 5 kbps
• Short-range
• 50-500m

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Time Synchronization

• Causality order
• Lamport clock
• Relative clock
• relative draft
• Global clock
• Synchronized to reference clock

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Time Synchronization

• Clock Offset
• Clock skew

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Sources of Errors in Time Synchronization
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Synchronization Methods

• Sender-Receiver Synchronization
• Receiver-Receiver Synchronization

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Sender-Receiver Synchronization
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Receiver-Receiver Synchronization
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The Challenges of High Latency
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The Challenges of High Latency (Contd)

• Protocols based on one-way exchanges

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The Challenges of High Latency (Contd)

• Protocols based on two-way exchange
• T1fs(t1), T2fB(t1d)
• T3fB(t3), T4fs(t3d)

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The Challenges of High Latency (Contd)

• No clock skew

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The Challenges of High Latency (Contd)

• Effect of clock skew
• Error

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Time Synchronization for High Latency Channel
(TSHL)

• Skew synchronization
• Offset synchronization

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Skew Synchronization
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Skew Synchronization (Contd)

• Linear regression the data points

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Offset Synchronization
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Offset Synchronization (Contd)
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Evaluations

• Simulation setup
• Skew40ppm
• Offset10µs
• Number of Beacon Messages25
• Granularity1µs
• Receive Jitter15 µs

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Effect of Distance Variation
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Effect of Time Since Synchronization
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Effect of Clock Skew
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Effect of Number of Sync Beacons
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Effect of Granularity
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Conclusions

• Protocols for RF perform poorly
• Skew has to been considered even during the
  synchronization exchanges

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Discussions

• Burden of beacon node?
• Broadcasting beacon messages
• Synchronizing all the nodes
• Effect of contention in MAC layer