Time Synchronization Protocols in Wireless Sensor Networks

M. Maroti, B. Kusy, G. Simon, A. Ledeczi SenSys 2004 The Flooding Time Synchronization Protocol S. Ganeriwal, R. Kumar, M. B. Srivastava Sensys 2003 Timing-sync protocol for sensor networks Three Papers Jeremy Elson, Lewis Girod and Deborah Estrin OSDI 2002 Fine-Grained Network Time Synchronization using Reference Broadcasts

3 Time Synchronization. Getting all devices in a distributed system to the same time (clock Skew/offset) at exactly the same rate (clock drift).

4 Applications of Time Synchronization List a set of functions that need time synchronization Synchronized MAC Power Management Data TimeStamp Tracking Localization5.Real-time Services6. Profiling & Debug7. Coordinated Actuation 8.

5 Why Time Synchronization is Needed? Clock skew (offset): Difference between time on two clocks. Different start times Only local clock available initially Clock drift: Count at different rates. Different frequency of the oscillator. Accuracy : agreement between the oscillators expected and actual frequencies (freq. error) (PPM) Stability: drifting in frequency over time, because of short term factors (temperature Etc.) and long term factors (Aging)

6 The accuracy and drift Rubidium Cesium Quartz Drift rate Cost ~ s / day ~ s / day ~10 -6 s / day High Low~10 -6 s / s 1s /12 day ~10 -6 s / hr Desktop IPAQs Atomic Clocks embeded

7 Why Not Atomic clock CPU power drawn by motes ~ 25 mW Cost will be the deciding factor!

8 State-of-the-Art General solutions for time sync problem: GPS (Global Positioning System) NTP (Network Time Protocol)

9 GPS overview T1 = t1 + |X-s1|/c Gather four satellite signals and solve the non- linear system of equations. Figure source: Propagation delay

10 Global Positioning System (GPS) Single Hop Synchronization +’s Commercial receivers can achieve accuracy of better than 200ns. -’s Needs a clear sky view (not possible inside buildings, underwater etc.) Receivers need long settling time Receivers can be large, costly and high power consuming.

Synchronization Clients just adopt the timestamp in the beacon packet Send at T1 Base station Very simple, Provides ms accuracy. Neglects packet delay and delay jitters Same approach being used by NIST to synchronize electronic products such as wall clocks, clock radio, wrist watches etc. worldwide. WWVB signals are being transmitted from colarado.

12 NTP: Internet Synchronization A Send at T3 Recv at T4 T4 = T3 + DELAY- OFFSET Send at T1 Recv at T2 T2 = T1 + DELAY + OFFSET B OFFSET = {(T2-T1)-(T4-T3)}/2 DELAY = {(T2-T1)+(T4-T3)}/2 ClientPeer

13 Network Time Protocol (NTP) The standard C/S Internet Synchronization Protocol +’s Scalable Self configuration in multi hop networks (hierarchical) Robustness to failures and sabotage (multiple servers) -’s High Cost Pair-wise Not energy efficient S1 S2 ** S1 * S2 Clients (c)

14 Time Synchronization in Sensor Networks How is time synchronization in sensor networks different from the traditional networks? 1.Energy Utilization 2.Single hop vs. multi hop 3.Infrastructure-Supported vs. Ad-hoc 4.Static topology vs. Dynamic Topology 5.Connected vs. Disconnected 6.Dynamic time sync. requirements, depending on the application

M. Maroti, B. Kusy, G. Simon, A. Ledeczi SenSys 2004 The Flooding Time Synchronization Protocol First Papers

16 FTSP Basic Idea: Deal with Skew Receiver Time = Sender Time + Signaling Delay Sender Receiver Signaling Delay Challenging issue: estimate signaling Delay!!! Sender side delay Receiver side delay Propagation delay

17 Let’s look at delays in detail software MACpropagationTXRX software senderreceiver ALL DELAYS ARE VARIABLE ! Bottleneck Use low level time stamping

18 Delays in transmitting and receiving a message

19 FTSP Basic Idea II. Use periodic flooding to provide robustness

20 FTSP Idea III: Deal with Drift Real Time Ideal clock Node B clock Node A clock t1 Local node time 45 degree

21 Basic Idea III: Deal with Drift Receiver gets the multiple time stamps (Ta,Tb) Uses this to update the estimate on the clock drift. Time at Node A Time at Node B Offset } Slope = drift

22 Analysis of FTSP +’s  MAC layer delay are removed by low-level time- stamping  Robust  Clock Drift through linear regression  Support multi-hop time synchronization -’s  Relatively high cost in flooding  Special timestamp message needed

Second Papers Jeremy Elson, Lewis Girod and Deborah Estrin OSDI 2002 Fine-Grained Network Time Synchronization using Reference Broadcasts

24 RBS: Synchronize receivers Based on CesiumSpray system by Verissimo and Rodrigues Receiver-receiver synchronization Figure source: Courtesy of Jeremy Elson

25 Sender Receiver 1 Receiver 2 Critical Path Sender Receiver Critical Path Time Traditional critical path: From the time the sender reads its clock, to when the receiver reads its clock RBS: Only sensitive to the differences in receive time and propagation delay Magic behind RBS Figure source: Courtesy of Jeremy Elson

26 Analysis of RBS +’s  The biggest sources of non deterministic latency are removed.  Message does not need to have any time stamp neither the time when its sent is needed.  Multiple broadcasts allow tighter synchronization.  Outliers are handled gracefully because of the use of best fit line. -’s  Needs a network with physical broadcast channel (not possible in networks with point to point links)  They have not explored the scaling issues like automatic and dynamic election of set of nodes to send out beacons

S. Ganeriwal, R. Kumar, M. B. Srivastava Sensys 2003 Timing-sync protocol for sensor networks Third Papers

28 Tree-based Synchronization Pair-wise NTP over multi-hop

29 TPSN: Conventional sender-receiver synchronization A Send at T3 Recv at T4 T4 = T3 + DELAY- OFFSET Send at T1 Recv at T2 T2 = T1 + DELAY + OFFSET B OFFSET = {(T2-T1)-(T4-T3)}/2 DELAY = {(T2-T1)+(T4-T3)}/2 Based on NTP Enemy is non-determinism! Asymmetric delays and varying offset

30 Comparison of the time sync protocols. FTSPTPSNRBS Send/Access TimeTS at MAC Eliminated Prop. timeNot handledAcknowledgementsNot handled Byte AlignmentHandledAcknowledgementsNot handled Average error single hop1.48 us16.9 us29.1 us Traffic/loadLowHighLow Network hierarchyNot fixedNeededNot needed

31 Take Away Messages Need to deal with uncertainty in MAC access delay FTSP = Low Level Time-stamping + Flooding + Linear regression RBS = Receiver-Receiver Synchronization FTSP = Pairwise Multi-hop NTP