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

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

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

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

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

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

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

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

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

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

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

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

13. 13 Available clocks > OS characterization 1) Difference between OS time and wallclock for each e (phase) 2) Compute phase variations over interval (freq.) & observe freq variations 0) Generate periodic e 2σ2σ

14. 14 Available clocks > OS characterization 1) Difference between OS time and wallclock for each e (phase) 2) Compute phase variations over interval (freq.) & observe freq variations 0) Generate periodic e 2σ2σ σ Allan variation plot 2σ2σ

15. 15 Available clocks > OS characterization 1) Difference between OS time and wallclock for each e (phase) 2) Compute phase variations over interval (freq.) & observe freq variations 0) Generate periodic e 3σ3σ

16. 16 Available clocks > OS characterization 1) Difference between OS time and wallclock for each e (phase) 2) Compute phase variations over interval (freq.) & observe freq variations 0) Generate periodic e 3σ3σ σ Allan variation plot 2σ2σ 3σ3σ

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

18. 18 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 σ

19. 19 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 σ Allan variation plot 2σ2σ 3σ3σ σ

20. 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. 21 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 σ Allan variation plot 2σ2σ 3σ3σ 2σ2σ

22. 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. 23 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 σ Allan variation plot 2σ2σ 3σ3σ 3σ3σ

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

25. 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. 26 TimeRemap Algo > Steps Capture OS & NIC timestamps e’s NIC tstamps OS tstamps e’s NIC tstamps OS tstamps Data segmentation 1 2

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

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

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

30. 30 TimeRemap Algo > Timeremap clock TimeRemap service NIC clock TimeRemap “clock” What time is it? Global time (GT) Local time (LT) 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. 31 Outline Problem & Application Available clocks on nodes TimeRemap algorithm Testbed & Results

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

33. 33 Testbed & Results > Results 3 scenarios No car stressed All cars stressed Half cars stressed, Half cars not stressed 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

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

35. 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. 36 Perspective Deployment on the road Better stability