1. Fine-Grained Network Time Synchronization using Reference Broadcasts Jeremy Elson, Lewis Girod, and Deborah Estrin U.C.L.APresenter: Todd Fielder

2. Time Synchronization  Applications to Sensor Networks SecuritySecurity  Cryptography Coordinate Future ActionCoordinate Future Action  Domain Specific: Integrate proximity detections into a velocity estimateIntegrate proximity detections into a velocity estimate Suppress redundant messages by recognizing duplicated sensed events.Suppress redundant messages by recognizing duplicated sensed events.

3. Sources of Time Synchronization Errors  Send Time Message construction time varies among sendersMessage construction time varies among senders Non-determinant: Dependant on instantaneous load on CPU.Non-determinant: Dependant on instantaneous load on CPU.  Access Time Channel access time is indeterminateChannel access time is indeterminate Non-determinant: Dependant on instantaneous load on Network.Non-determinant: Dependant on instantaneous load on Network.  Propagation Time Distance to receivers variesDistance to receivers varies Effectively zero:Effectively zero:  RF close to speed of light  RBS only takes into account differences in distance of receivers However: Increases if receivers are at different sides of sender.However: Increases if receivers are at different sides of sender.  Receive Time Time to deliver to application variesTime to deliver to application varies  messages in queue can cause delay. Mitigated by reading timestamp at interrupt time.Mitigated by reading timestamp at interrupt time.

4. Reference Broadcast Synchronization  Synchronizes a set of receivers with one another Traditional method synchronizes senders with receivers.Traditional method synchronizes senders with receivers.  Receivers use the arrival time of a ‘reference broadcasts’ as a point of reference for comparing their clocks. Avoids errors due to Send Time and Access Time.Avoids errors due to Send Time and Access Time.

5. Synchronization Algorithm  Phase Offset Estimation Transmitter broadcasts a reference packet to two receivers.Transmitter broadcasts a reference packet to two receivers. Each receiver records the time according to its local clock.Each receiver records the time according to its local clock. Receivers Exchange their observations and synchronize with the average.Receivers Exchange their observations and synchronize with the average.  Synchronizes relative timescales. Global time is not important in Sensor Networks.Global time is not important in Sensor Networks.  Or is it?

6. Synchronization Algorithm (cont.)  Estimation of Clock Skew AccuracyAccuracy   The agreement between an oscillator’s expected and actual frequencies. StabilityStability   An oscillator’s tendency to stay at the same frequency over time.   least-squares linear regression Finds the best fit line through the phase error observations over time.

7. Additional Comments  Requires additional overhead between all communicating nodes. If two nodes cannot communicate directly, multi-hop communication is required or substantial message passing.If two nodes cannot communicate directly, multi-hop communication is required or substantial message passing.  No energy usage graphs presented.  How does a node know if reference broadcasts is meant for it? May cause nodes to unnecessarily expend energy to synchronize.May cause nodes to unnecessarily expend energy to synchronize.  However, more efficient than traditional protocols because receivers do not need to maintain synchronization. Can synchronize after hours or days of sleep.Can synchronize after hours or days of sleep.  Do receivers need to maintain synchronization in NTP?

8. Experiment  Traffic scenarios: Heavy trafficHeavy traffic Light trafficLight traffic  Three synchronization techniques tested: RBS-synchronizes receivers.RBS-synchronizes receivers. NTP-uses GPS and a NTP server to synchronize to an external timescale.NTP-uses GPS and a NTP server to synchronize to an external timescale. NTP offset-NTP limits rate at which to correct phase error this variation does not, in order to allow for more fair testing of synchronization precision.NTP offset-NTP limits rate at which to correct phase error this variation does not, in order to allow for more fair testing of synchronization precision.

9. Experimental Results (application Level)  Light Traffic RBS performed 8 times better than NTPRBS performed 8 times better than NTP  RBS: 6.29 +/- 6.45 usec. Causes of jitter:Causes of jitter:  Code path through network stack  Time required to schedule daemon  System calls to get current time  NTP: 51.18 +/- 53.30 usec.  Heavy Traffic RBS performance remained almost consistent while NTP performance degraded 30 foldRBS performance remained almost consistent while NTP performance degraded 30 fold  RBS: 8.44 usec  NTP: 1,542 usec *For reasons unknown, performance of NTP-offset was much worse than NTP.

10. Experimental Results (Kernal Level)  Timestamps acquired at the network interrupt handler. RBS performance improved considerablyRBS performance improved considerably  1.85 +/- 1.26usec. Result limited by 1usec clock resolutionResult limited by 1usec clock resolution

11. Multi-Hop Synchronization  Nodes 1,2,3,4 synchronize to each other based on A’s broadcast.  Nodes 4,5,6,7 synchronize to each other based on B’s broadcast.  Node 4 is now common among all nodes and can synchronize A and B based on a best fit line.  At next reference broadcast, all nodes will be synchronized.

12. Time Routing  Not necessary to have a common node actively synchronize two distinct regions.  Can dynamically determine the “time- route” for packets to be routed and converted on the fly. Route packets throughRoute packets through “gateway” node, which can determine conversion from one time base to another.

13. Performance  Each conversion step can introduce error. How is that error amplified as the hop- length increases?How is that error amplified as the hop- length increases?  Average per-hop error is e  For n hops, estimated error is e (n^1/2)  Experimental Results For a 4 hop network, average error is 3.68 +/- 2.57usec.For a 4 hop network, average error is 3.68 +/- 2.57usec.

14. Additional Comments  Causes Gateway nodes to perform additional computation.  Is there a way to verify gateway node is not compromised? Can C be a “watchdog” overCan C be a “watchdog” over 4,7,8,9 to ensure that all times reported are in a specified range? Is there a way for C toIs there a way for C to Know that 8 and 9 should be reporting the same value, i.e. that they are both in the same region?

15. External Timescales  Treat GPS as a Broadcasts Beacon.  To Work effectively, GPS must be attached to every Broadcast beacon in network. Need to distribute additional computation load.Need to distribute additional computation load. Time-routing causes some conversion between unsynchronized regions.Time-routing causes some conversion between unsynchronized regions.