1. Time synchronization for UWSN

2. Outline • Time synchronization knowledge • Typical time sync protocol • Time sync in UWSN • Discussion

3. Time Synchronization • Clock Offset • Clock skew T(t)= at + b

4. Skew and offset • offset causes constant error independent of time • skew causes increasing error as time progresses. • To avoid the need for frequent resynchronization, the synchronization algorithm should accurately estimate both the clock skew and offset.

5. Sources of error in estimating message latency

6. Synchronization Methods • Sender-Receiver Synchronization • Receiver-Receiver Synchronization

7. Sender-Receiver Synchronization

8. Receiver-Receiver Synchronization

9. Outline • Time synchronization knowledge • Typical time sync protocol • Time sync in UWSN • Discussion

10. Typical Time sync protocol • Network Time Protocol (NTP) • Reference Broadcast Synchronization (RBS) • Timing-sync Protocol for Sensor Networks (TPSN) • Flooding Time Synchronization Protocol (FTSP)

11. Not match with UWSN • It is impractical for a wireless device with a an atomic clock like NTP • RBS’ central algorithm is built on the simultaneous reception of reference broadcasts at all nearby nodes • TPSH does not model skew and not robust with topology change. • FTSP requires calibration on the hardware actually used in the deployment and suffers from the same problem as RBS, in that it assumes near instantaneous message propagation

12. Outline • Knowledge related to time synchronization • Typical time sync protocol for WSN • Time sync in UWSN • Discussion

13. Change for time sync in UWSN Acoustic Channel • long propagation delay • Synchronization overhead • Time-varying delay Time sync protocol • TSHL • MU-SYNC

14. TSHL and MU-Sync • Both TSHL and MU-SYNC try to minimize the synchronization error by estimating and compensating both the clock skew and offset, utilizing MAC-layer time stamping and bidirectional message exchange. • TSHL is designed for static underwater sensor networks, which assume long but constant propagation delay • MU-Sync has a higher overhead than the TSHL • TSHL perform linear regression only once to retrieve the estimated skew, while the MU-Sync performs it twice. • For MU-SYNC, cluster head broadcasts its neighboring nodes’ estimated skew and offset, every node learns the estimated skew and offset of all other nodes in the same cluster, instead of just the relative parameters between the cluster head and a particular node.

15. TSHL Two phases: • Skew synchronization • Offset synchronization

16. Skew Synchronization

17. Offset Synchronization

18. MU-Sync • Using a two-phase operation • Skew and offset acquisition phase • synchronization phase

19. Skew and offset acquisition phase: • The clock skew and offset is estimated by applying linear regression twice over a set of n reference beacons. • The first regression: • Obtain its first estimated skew • Estimate propagation delay • The second regression: • Extract the amount of propagation delay that each REF packet encounters • Calculate final estimated skew and offset

21. The first linear regression • Obtain first estimated skew , (Fig.3) • The value of first estimated skew then used to compute the amount of one-way propagation delay that each REF packet has encountered

23. Second linear regression • Subtract the estimated propagation delay corresponding to each of the data points to obtain a new set of data points. • The cluster head then runs the second linear regression to obtain the final estimated skew and offset of neighboring node y.

24. Synchronization phase • Cluster head broadcasts all neighbors’ clock skew and offset, so that every neighbor can keep track of these parameters. • When every node in the cluster knows the skew and offset of every other node in the cluster, cluster-wide synchronization has been achieved.

25. Synchronization phase

26. Error Analysis of Propagation Delay

27. Error Analysis of Propagation Delay • is an average of the propagation delay obtained from t4 −t3 and t2 −t1 • actual delay is t4 − t3 • the error of propagation delay estimation:

28. Error Analysis of Propagation Delay

33. Discussion • Fig. 8 (the worst-case) shows that the MU-Sync cannot cope when the duration of t3 − t2 is longer than approximately 25 s. • Reason: currently using half of the round trip time to estimate the one-way propagation delay, the estimation error depends on the value of

34. Discussion • MU-Sync takes into account both long and time-varying propagation delays. • The accuracy of the MU-Sync is highly dependent on the accuracy of the propagation delay estimation, as it is a major contributor to synchronization error UWSN.

35. Open issue • currently using half of the round-trip time as an estimation of the one-way propagation delay. • This may result in low accuracy if the propagation delay varies significantly within the round trip message exchange. • Future work will concentrate on how the varying propagation delay can be estimated more accurately, while maintaining low overhead.

36. Q&A