1. Jayant Gupchup Phoenix, EWSN 2010 Phoenix: An Epidemic Approach to Time Reconstruction Jayant Gupchup †, Douglas Carlson †, Răzvan Musăloiu-E. †,*, Alex Szalay ±, Andreas Terzis † Department of Computer Science, Johns Hopkins University † Department of Physics and Astronomy, Johns Hopkins University ± Google *

2. Jayant Gupchup Phoenix, EWSN 2010 where: Environmental Monitoring

3. Jayant Gupchup Phoenix, EWSN 2010 Design Goals and Targets  Target Lifetime : 1 year o Duty-cycle (~ 5%)  Accuracy Requirements o Milliseconds (ms) - Seconds (s) o Online Synchronization not needed  Delay-tolerant networks o Basestation collects data opportunistically o NOT “sample-and-send”  All measurements require timestamps o Not just events

4. Jayant Gupchup Phoenix, EWSN 2010 Naïve Time Reconstruction  Measurements are timestamped using motes local clock  Basestation collects data  Time reconstruction algorithm: Assigns measurements a global timestamp

5. Jayant Gupchup Phoenix, EWSN 2010 Reconstruction is NOT Synchronization  Asynchronous operation o Each mote has its own operation schedule o No attempt to match schedules  Motes o Agnostic of network time / global time o Do not process time information o Do not have an onboard Real-Time Clock (RTC) (E.g. Telos, Mica2, MicaZ, IRIS)

6. Jayant Gupchup Phoenix, EWSN 2010 Phoenix Performance  Accuracy o Order of seconds, ~ 6 PPM (ignoring temperature effects)  Yield : Fraction of measurements assigned timestamps o ≥ 99%  Overheads: o Duty-Cycle: 0.2% o Space: 4%  Yield performance maintained: o Presence of random, frequent mote reboots o Absence of global clock source for months

7. Jayant Gupchup Phoenix, EWSN 2010 Background and Related Work

8. Jayant Gupchup Phoenix, EWSN 2010 Reboots and Basestation ? ? ?

9. Jayant Gupchup Phoenix, EWSN 2010 Cub Hill – Year long deployment

10. Jayant Gupchup Phoenix, EWSN 2010 Cub Hill : Time Reconstruction Nodes Stuck (Data Loss) Watchdog Fix Basestation Down Reboot Problems

11. Jayant Gupchup Phoenix, EWSN 2010 Rate of Reboots

12. Jayant Gupchup Phoenix, EWSN 2010 Reconstruction Challenges  Motes reboot at random o Downtime is non-deterministic  Dependence on basestation  Temporary network partitions  Mote clock o Varies per mote o Skew changes over time

13. Jayant Gupchup Phoenix, EWSN 2010 Related Work  Linear Regression for Time Rectification o Fidelity and Yield in a Volcano Monitoring Sensor Network, Werner-Allen et al., OSDI 2006  Reboot Problems o Lessons from the Hogthrob Deployments, Chang et al., WiDeploy 2008 o Trio: Enabling sustainable and scalable outdoor wireless sensor network deployments, Dutta et al., SPOTS 2006  State preservation after reboots o Surviving sensor network software faults, Chen et al., SIGOPS 2009  Data-driven Temporal Integrity o Recovering temporal integrity with data driven time synchronization, Lukac et al., IPSN 2009 o Sundial: Using sunlight to reconstruct global timestamps, Gupchup et al., EWSN 2009

14. Jayant Gupchup Phoenix, EWSN 2010 Phoenix

15. Jayant Gupchup Phoenix, EWSN 2010 Big Picture 1 2 3 Base Station

16. Jayant Gupchup Phoenix, EWSN 2010 Terminology  Segment: S tate defined by a monotonically increasing local clock (LC) o Comprises  Anchor: o : Time-references between 2 segments o : Time-references between a segment and global time  Fit : Mapping between one time frame to another o Defined over : Neighbor Fit o Defined over : Global fit  Fit Parameters o Alpha (α) : Skew o Beta (β) : Offset  Goodness of Fit : Metric that estimates the quality of the fit o E.g. : Variance of the residuals

17. Jayant Gupchup Phoenix, EWSN 2010 2-Phase  Phase-I : Data Collection (In-network)  Phase-II : Timestamp Assignment (Database)

18. Jayant Gupchup Phoenix, EWSN 2010 Architecture Summary  Motes  Global Clock Source  Basestation

19. Jayant Gupchup Phoenix, EWSN 2010 Anchor Collection – I : Beaconing Each Mote: Beacons time-state periodically Beacon interval~ 30s Duty-cycle overhead: 0.075% 435102400 9773600 2839600000 43 97 28

20. Jayant Gupchup Phoenix, EWSN 2010 Anchor Collection – II : Storage 435102800 9773800 Each Mote: Stays up (30s) after reboot Listens for announcements Wakes up periodically (~ 6 hrs) Stays up (30s) Listens for announcements Stores anchors Duty-Cycle : 0.14% 43 28 97

21. Jayant Gupchup Phoenix, EWSN 2010 Anchor Collection – III : Global References 9774000 G-Mote: Connected to a global clock source Beacon its time-state (30s) Store Global References (6 hrs) Global clock source (GPS, Basestation etc) 284102435 284102455, 1217351879 97 43

22. Jayant Gupchup Phoenix, EWSN 2010 43-5 (B) 43-5 (B) 97-7 (A) 97-7 (A) 28-4 (G) 28-4 (G) 97-7 (A) 97-7 (A) 43-5 (B) 43-5 (B) 28-4 (G) 28-4 (G) Time Reconstruction (outside the network) χ = 2 χ = 2.5 χ = 7 Segment Graph

23. Jayant Gupchup Phoenix, EWSN 2010 Evaluation: Simulation & Experiments

24. Jayant Gupchup Phoenix, EWSN 2010 Evaluation Metrics  Yield: Fraction of samples assigned timestamps (%)  Average PPM Error: PPM Error per measurement:  Duty Cycle Overhead: Fraction of time radio was on (%)  Space Overhead: Fraction of space used to store anchors (%)

25. Jayant Gupchup Phoenix, EWSN 2010 Simulation: Missing Global Clock Source Simulation Period : 1 Year

26. Jayant Gupchup Phoenix, EWSN 2010 Simulation: Wake Up Interval Anchor collection rate should be significantly faster than the rate of reboots

27. Jayant Gupchup Phoenix, EWSN 2010 Simulation: Segments to anchor with

28. Jayant Gupchup Phoenix, EWSN 2010 Olin Deployment - 19 Motes - 21 Day Deployment - 62 segments - One Global clock mote

29. Jayant Gupchup Phoenix, EWSN 2010 Deployment Accuracy

30. Jayant Gupchup Phoenix, EWSN 2010 Naïve Yield Vs Phoenix Yield Phoenix Yield: 99.5%

31. Jayant Gupchup Phoenix, EWSN 2010 Conclusion  Phoenix timestamps: o > 99% of the collected measurements o With accuracy in order of seconds  Phoenix is Robust to: o Basestation failures for days-months o Random mote reboots  Paying a price of: o 0.2% increase in duty cycle o 4% space overhead

32. Jayant Gupchup Phoenix, EWSN 2010 Questions ?

33. Jayant Gupchup Phoenix, EWSN 2010 Extra:

34. Jayant Gupchup Phoenix, EWSN 2010 Discussion / Future Work  Choosing the right link metric o Factor number of anchor points o Temporal separation of anchors o Combining the metrics along a “fit” path  Adaptive anchor collection o If rate of reboots is unknown  Compare with online timestamping (FTSP)

35. Jayant Gupchup Phoenix, EWSN 2010 Simulation Parameters ParameterTypeDefault Value Clock SkewUniform Distribution~ U (40 70) [ppm] Segment ModelNon-Parametric (Cub Hill)median : 4 days TopologyCub Hill (53 nodes) Communication Delay (end-to-end) Uniform Distribution~ U (5 15) [ms] Packet Reception RatioLog-Normal Path LossPr(2.0) = -59.28 η = 2.04 σ = 6.28 Constant Constant NUM_SEGMENTSConstant4 Sampling Frequency (measurements) Constant10 mi

36. Jayant Gupchup Phoenix, EWSN 2010 Reboots: Long downtimes

37. Jayant Gupchup Phoenix, EWSN 2010 Clock Skews

38. Jayant Gupchup Phoenix, EWSN 2010 Temperature dependence Source: http://focus.ti.com/lit/an/slaa322b/slaa322b.pdf