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Ryad Ben-El-Kezadri, Giovanni Pau Network Research Lab, UCLA

TimeRemap : Stable & Accurate Time in Vehicular Networks. Ryad Ben-El-Kezadri, Giovanni Pau Network Research Lab, UCLA. REVE Workshop - 2010. Outline. Problem & Application Available clocks on nodes TimeRemap algorithm Testbed & Results. Outline. Problem & Application Problem

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Ryad Ben-El-Kezadri, Giovanni Pau Network Research Lab, UCLA

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  1. TimeRemap: Stable & Accurate Time in Vehicular Networks Ryad Ben-El-Kezadri, Giovanni Pau Network Research Lab, UCLA REVE Workshop - 2010

  2. Outline • Problem & Application • Available clocks on nodes • TimeRemap algorithm • Testbed & Results

  3. Outline • Problem & Application • Problem • Application & constraints • Scenarios • Available clocks on nodes • TimeRemap algorithm • Testbed & Results

  4. Problem & Application > Problem • General Problem Synch. clocks • Vehicular nets • Comm. : Off-the-shelf HW • 802.11 NIC • Sync. : No extra HW • GPS receiver Environment & constraints

  5. Time constraints > Comm. constraints > Problem & Application > Appli. Synchronization Contextualization/ Distributed Sensing : Evt ↔Time & Place Network functioning Network monitoring Pollution monitoring

  6. Problem & Application > Constraints • Time constraints • Inter-packet/2 : ~100 s • Slot/2 : ~5 s Nodes to track Pkt Tracker • Comm. constraints • No contact between trackers Sync signaling impossible

  7. Problem & Application > Scenarios The largest RT system to attack Google car

  8. Outline • Problem & Application • Available clocks on nodes • Packet timestamping • OS & NIC characterization • Key idea • TimeRemap algorithm • Testbed & Results

  9. Wallclock time modified Probabilistic error Ex. Time to read the clock No good No good Available clocks > Timestamping • A packet event e, a timestamping system S • When did e come at the antenna twalle / fS(twalle) ? • Clock model • fS(twalle) = a twalle + b + error • Available systems : OS, NIC GPS • - fOS(twalle) = twalle + errorant-OS • - fNIC(twalle) = at twalle + bt OS NIC e e

  10. Stable but,… Bad precision Available clocks > OS characterization 0) Generate periodic e 1)Difference between OS time and wallclock for each e (phase)

  11. σ Available clocks > OS characterization 0) Generate periodic e 1)Difference between OS time and wallclock for each e (phase) 2)Compute phase variations over interval (freq.)& observe freq variations

  12. σ Available clocks > OS characterization 0) Generate periodic e 1)Difference between OS time and wallclock for each e (phase) 2)Compute phase variations over interval(freq.)& observe freq variations Allan variation plot σ

  13. 2σ Available clocks > OS characterization 0) Generate periodic e 1)Difference between OS time and wallclock for each e (phase) 2)Compute phase variations over interval(freq.)& observe freq variations

  14. 2σ Available clocks > OS characterization 0) Generate periodic e 1)Difference between OS time and wallclock for each e (phase) 2)Compute phase variations over interval(freq.)& observe freq variations Allan variation plot 2σ σ

  15. 3σ Available clocks > OS characterization 0) Generate periodic e 1)Difference between OS time and wallclock for each e (phase) 2)Compute phase variations over interval(freq.)& observe freq variations

  16. 3σ Available clocks > OS characterization 0) Generate periodic e 1)Difference between OS time and wallclock for each e (phase) 2)Compute phase variations over interval (freq.)& observe freq variations Allan variation plot 2σ 3σ σ

  17. Good precision but,… Drift Available clocks > NIC characterization 1)Difference between NIC time and wallclock for each e (phase)

  18. σ Available clocks > NIC characterization 1)Difference between NIC time and wallclock for each e (phase) 2)Compute phase variations over interval(freq.)& observe freq variations

  19. σ Available clocks > NIC characterization 1)Difference between NIC time and wallclock for each e (phase) 2)Compute phase variations over interval(freq.)& observe freq variations Allan variation plot 2σ 3σ σ

  20. Available clocks > NIC characterization 1)Difference between NIC time and wallclock for each e (phase) 2)Compute phase variations over interval(freq.)& observe freq variations

  21. 2σ Available clocks > NIC characterization 1)Difference between NIC time and wallclock for each e (phase) 2)Compute phase variations over interval(freq.)& observe freq variations Allan variation plot 2σ 3σ σ

  22. Available clocks > NIC characterization 1)Difference between NIC time and wallclock for each e (phase) 2)Compute phase variations over interval(freq.)& observe freq variations

  23. 3σ Available clocks > NIC characterization 1)Difference between NIC time and wallclock for each e (phase) 2)Compute phase variations over interval(freq.)& observe freq variations Allan variation plot 2σ 3σ σ

  24. OS -stable -not precise NIC -not stable -precise Construct a 3rd clock which leverages : -the stability of the OS -the precision of the NIC Available clocks > Summary & Idea

  25. Outline • Problem & Application • Available clocks on nodes • TimeRemap algorithm • Steps • Zoom on regression • Detection of group of outliers • The TimeRemap “clock” • Testbed & Results

  26. TimeRemap Algo > Steps Capture OS & NIC timestamps Data segmentation 1 2 NIC tstamps NIC tstamps OS tstamps OS tstamps e’s e’s

  27. TimeRemap Algo > Steps Extract NIC & OS supports (regression) for each segment Remap NIC tstamps to OS support for each segment 4 3 NIC tstamps OS tstamps NIC tstamps OS tstamps supports e’s e’s

  28. Timestamping delay (probablistic) TimeRemap Algo > Regression Outlier suppression A two step regression over each segment Compute support conversion parameters (a,b) 3.a 3.b OS tstamps OS tstamps  1st Regression line (for selection)  2nd Regression line y=ax+b (for remapping)  ☻ ☻  ☻ ☻ ☻ ☻ NIC tstamps NIC tstamps

  29. GPS GPS e’s OS NIC OS NIC TimeRemap Algo >Group of outliers Group of outliers (GPS fix loss,…) Error between OS tstamps : Error b/w remapped NIC tstamps : Error b/w remapped NIC tstamps with detection of group of outliers: Reuse the support conversion parameters of previous segment

  30. TimeRemap Algo >Timeremap clock TimeRemap service Global time (GT) TimeRemap “clock” NIC clock Local time (LT) What time is it? • The TimeRemap “clock” can be used by • the OS to convert the NIC tsamps to GT • the NICs to convert a LT to GT & share it with other NICs

  31. Outline • Problem & Application • Available clocks on nodes • TimeRemap algorithm • Testbed & Results

  32. Testbed & Results > Testbed OLSR ‘VANET’+ Madwifi NIC Comm. C C C C C C C C C C Chrony+ shmpps Sync. Internet NTP time GPS/PPS electric pulse See http://sites.google.com/site/gpssync/ for our GPS/PPS multiplexer specs GPS receiver Garmin 18 LVC

  33. Testbed & Results > Results • Compare timestamps b/w cars (nodes) • OS vs TimeRemap tstamps • Performance metrics • Mean sync error • Std dev • Outlier ratio (Outlier if error>30 s) • Some cars are stressed with the stressed linux command • 3 scenarios • No car stressed • All cars stressed • Half cars stressed, Half cars not stressed

  34. Testbed & Results > Results Timeremap does not produce any outlier

  35. Conclusion • TimeRemap leverages • the stability of the OS • the precision of the NIC • Performance • Mean sync error reduced to 3sec • No outlier

  36. Perspective • Deployment on the road • Better stability

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