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1 Chapter 6 Time synchronization

2 Outline 6.1. The Problems of Time Synchronization 6.2. Protocols Based on Sender/Receiver Synchronization Network Time Protocol (NTP) Timing-sync Protocol for Sensor Networks (TPSN) Flooding Time Synchronization Protocol (FTSP) Ratio-based time Synchronization Protocol (RSP) 6.3. Protocols Based on Receiver/Receiver Synchronization Reference Broadcast Synchronization (RBS) Hierarchy Referencing Time Synchronization (HRTS) 6.4. Summary

3 6.1. The problems of Time Synchronization

4 The Problems of Time Synchronization Why Need for Time Synchronization? Many of the applications of WSN needs the event with time stamp Ordering of the samples for reporting Events are reported by multiple nodes When WSN is energy save enabled, it need all nodes to be in sync in order to be in Idle or Active mode Medium Access Layer (MAC) Scheduling Order of messages may change while transmission

5 The Problems of Time Synchronization Affecting factors on Time Synchronization scheme Communication Overhead Available Bandwidth Accuracy Requirements Scalability Infrastructure Requirement

6 The Problems of Time Synchronization Source of Synchronization Error Clock Drift Major influence in WSN where communication is infrequent. Typical value 1 ~ 100 µs per second Communication Message Delay Send Time Access Time Propagation Time (Negligible for most of the cases) Receive Time

7 The Problems of Time Synchronization Types of Clock Digital Clock Changes in Supply Voltage, Temperature can drift the rate Constant, Bounded, or Bounded Variation Drifts Software Clock Value of counter converted in Time Offset time value can be added/subtracted to achieve time synchronization

8 6.2. Protocols Based on Sender/Receiver Synchronization

9 Protocols Based on Sender/Receiver Synchronization In this kind of protocols, a receiver synchronizes to the clock of a sender The classical Network Time Protocol (NTP) belongs to this class We have to consider two steps: Pair-wise synchronization: How does a single receiver synchronize to a single sender? Network wide synchronization: How to figure out who synchronizes with whom to keep the whole network / parts of it synchronized?

10 Network Time Protocol (NTP)

11 Network Time Protocol (NTP) Synchronizing Physical Clocks Computer Clocks in distributed system not in agreement Need to synchronize clocks External synchronization (ES) Synchronized with an external authoritative time source S S - C < D, where C is computer s clock Internal synchronization (IS) Synchronized with other computer in the distributed system C i - C j < D IS does not imply ES Clock C i and C j ES implies IS Within bound 2D may drift collectively

12 Network Time Protocol (NTP) Distributed System Type Synchronous distributed system Known upper bound on transmission delay Simplifies synchronization One process p1 sends its local time t to process p2 in a message m, p2 could set its clock to t + Ttrans where Ttrans is transmission delay from p1 to p2 Ttrans is unknown but min Ttrans max Set clock to t + (max - min)/2 then skew u/2 Asynchronous distributed system Internet is asynchronous system Ttrans = min + x where x 0

13 Network Time Protocol (NTP) Cristian s method (1989) for an asynchronous system A time server S receives signals from a UTC source Process p requests time in m r and receives t in m t from S p sets its clock to t + T round /2 Accuracy ± (T round /2 - min) : because the earliest time S puts t in message m t is min after p sent m r. the latest time was min before m t arrived at p the time by S s clock when m t arrives is in the range [t + min, t + T round - min] T round is observed RTT min is minimum delay between p and S m r p m t Time server,s

14 Network Time Protocol (NTP) Issues with Christian s Algorithms A single time server might fail, so they suggest the use of a group of synchronized servers It does not deal with faulty servers No authentication mechanism Inaccuracy increases if the delay between messages is nonnegligible

15 Network Time Protocol (NTP) A time service for the Internet - synchronizes clients to UTC Reliability Primary from servers redundant are connected paths, to scalable, UTC sources authenticates time sources Secondary servers are synchronized to primary servers Synchronization subnet - lowest level servers in users computers

16 Network Time Protocol (NTP) Synchronisation of servers The synchronization subnet can reconfigure if failures occur, e.g. a primary that loses its UTC source can become a secondary a secondary that loses its primary can use another primary Modes of synchronization: Multicast A server within a high speed LAN multicasts time to others which set clocks assuming some delay (not very accurate) Procedure call A server accepts requests from other computers (like Cristiain s algorithm). Higher accuracy. Useful if no hardware multicast. Symmetric Pairs of servers exchange messages containing time information Used where very high accuracies are needed (e.g. for higher levels)

17 Network Time Protocol (NTP) Messages exchanged between a pair of NTP peers All modes use UDP Each message bears timestamps of recent events: Local times of Send and Receive of previous message Local times of Send of current message Recipient notes the time of receipt T i ( we have T i-3, T i-2, T i-1, T i ) In symmetric mode there can be a non-negligible delay between messages Server B T i-2 T i-1 Time m m' Server A T i- 3 T i Time

18 Network Time Protocol (NTP) Accuracy of NTP For each pair of messages between two servers, NTP estimates an offset o i between the two clocks and a delay d i (total time for the two messages, which take t and t ) T i-2 = T i-3 + t + o and T i = T i-1 + t - o This gives us (by adding the equations) : = + = d i = t + t = T i-2 - T i-3 + T i - T i-1 Also (by subtracting the equations) o = o i + (t - t )/2 where o i = (T i-2 - T i-3 + T i-1 - T i )/2 Using the fact that t, t >0 it can be shown that o i - d i /2 o o i + d i /2. Thus o i is an estimate of the offset and d i is a measure of the accuracy

19 Network Time Protocol (NTP) Techniques to Improve Accuracy NTP servers filter pairs <o i, d i >, estimating reliability from variation, allowing them to select peers High variability in successive pairs implies unreliable data Accuracy depends on the delay between the NTP servers Accuracy of 10s of millisecs over Internet paths (1 on LANs) Accuracy of 10s of millisecs over Internet paths (1 on LANs) Peer selection Lower stratum peer favoured over higher stratum server Peer with lower synchronization dispersion preferred synchronization dispersion is the sum of variability in data from the server to the root

20 Timing-sync Protocol for Sensor Networks (TPSN)

21 Timing-sync Protocol for Sensor Networks (TPSN) Introduction We present a Timing-sync Protocol for Sensor Networks (TPSN) that works on the conventional approach of sender-receiver synchronization Pairwise-protocol similar to LTS, but timestamping at node i happens immediately before first bit appears on the medium, and timestamping at node j happens in interrupt routine Comparison with Reference Broadcast Synchronization (RBS)

22 Timing-sync Protocol for Sensor Networks (TPSN) Network Model The network is always-on Every node maintains 16-bit register as clock Sensor has unique ID Build hierarchical topology for the network Node at level i can connect with at least one node at level i-1

23 Timing-sync Protocol for Sensor Networks (TPSN) Level discovery Phase Trivial Synchronization Phase Pair-wise sync is performed along the edge of hierarchical Pair-wise sync is performed along the edge of hierarchical structure

24 Timing-sync Protocol for Sensor Networks (TPSN) Level discovery Phase The root node is assigned a level 0 and it initiates this phase by broadcasting a level_discovery packet level_discovery packet contains the identity and the level of the sender The immediate neighbors of the root node receive this packet and assign themselves a level (level = level +1) This process is continued and eventually every node in the network is assigned a level. On being assigned a level, a node neglects any such future packets. This makes sure that no flooding congestion takes place in this phase

25 Timing-sync Protocol for Sensor Networks (TPSN) Synchronization Phase T1: A is sender, starting sync by sending synchronization_pulse packet to B T2 = T1 + + d where is the clock drift d is propagation delay T3: B replies acknowledgement containing T1, T2, T3 T4: A receive Ack and T4 = T3 - + d. So: = [(T2 - T1) - (T4 - T3)] / 2 d = [(T2 - T1) + (T4 - T3)] / 2

26 Timing-sync Protocol for Sensor Networks (TPSN) Synchronization Phase T2: BA T1: replies receive B A is receive sender, acknowledgement an Ack the starting synchronization and get timestamp containing by sending _pulse T4 packet and synchronization_pulse timestamping T1,T2,T3 immediately packet to B with timestamp T1 T2 T1,T2,T3 B T1 A T4 At time t3 t1 t4 t2

27 Timing-sync Protocol for Sensor Networks (TPSN) Simulation and Comparison

28 Timing-sync Protocol for Sensor Networks (TPSN) Simulation and Comparison

29 Flooding Time Synchronization Protocol (FTSP)

30 Flooding Time Synchronization Protocol (FTSP) Introduction The FTSP synchronizes the time to possibly multiple receivers utilizing a single radio message Linear regression is used in FTSP to compensate for clock drift

31 Flooding Time Synchronization Protocol (FTSP) Network Model Every node in the network has a unique ID Each synchronization message contains three fields: timestamp rootid seqnum The node with the smallest ID will be only one root in the whole network

32 Flooding Time Synchronization Protocol (FTSP) The root election phase FTSP utilizes a simple election process based on unique node IDs Synchronization phase

33 Flooding Time Synchronization Protocol (FTSP) The root election phase When a node does not receive new time synchronization messages for a number of message broadcast periods The node declares itself to be the root Whenever a node receives a message, the node with higher IDs give up being root Eventually there will be only one root

34 Flooding Time Synchronization Protocol (FTSP) Synchronization phase Root and synchronized node broadcast synchronization message Nodes receive synchronization message from root or synchronized node When a node collects enough synchronization message, it estimates the offset and becomes synchronized

35 Flooding Time Synchronization Protocol (FTSP) Timestamp rootid seqnum Root Timestamp rootid seqnum A B Synchronized Node Unsynchronized node C

36 Flooding Time Synchronization Protocol (FTSP) Simulation and Conclusion

37 Ratio-based time Synchronization Protocol (RSP)

38 Ratio-based time Synchronization Protocol (RSP) The RSP use two synchronization messages to synchronize the clock of the receiver with that of sender The RSP also can extend to multi-hop synchronization The nodes in the wireless sensor network construct a tree structure and the root of this tree is the synchronization root The global time of the root is flooding out to the nodes through the tree structure

39 Ratio-based time Synchronization Protocol (RSP) The local clock time of a sensor device is provided by the quartz oscillator inside itself transformation formula between t and Ci (t): (1) : the local clock time of a sensor node i. t : the Coordinated Universal Time (UTC). : the drift ratio. : the offset of node i s clock at time t. By (1), the local clock times of two sensor nodes i and j have the following relationship: (2) : relative drift ratio : offset between the clocks of nodes i and j

40 Ratio-based time Synchronization Protocol (RSP) Reference node Sensor node calculate the clock drift ratio θ = (T3 T1)/(T4 T2).

41 Ratio-based time Synchronization Protocol (RSP) Reference node Sensor node According to the ratio, each node can estimate the local time of reference node in the following way. (3) : the local time of sensor node :the corresponding local time of the reference node. : the initial offset between reference node and sensor node.

42 Ratio-based time Synchronization Protocol (RSP) It can be calculated using linear interpolation with the four timestamps the can be derived as follows (4)

43 Ratio-based time Synchronization Protocol (RSP) Therefore, we can derive (5) from (3) and (4): (5) each sensor node can estimate the local time of reference node, that is, the global time of the network

44 Ratio-based time Synchronization Protocol (RSP) Reference node Sensor node R S (T 1 ) (T 3 )

45 Ratio-based time Synchronization Protocol (RSP) But the clock drift (θ) is unstable and changed with time As using a large time interval, the relative drift ratio of two sensor nodes will become unreliable Therefore, the time-stamps of (5) are needed to be refreshed if the time interval is larger than a threshold α

46 Ratio-based time Synchronization Protocol (RSP) Assume the synchronization message is sent by the reference node per 3 minutes, and α is set to 10 minutes 3 minutes When time of t5 : T9 T1 = 12 > 10 mins The reference point must be changed, but the new reference point must Satisfy the condition of T9 Tnew larger than a threshold β Assume threshold β is set to 3 minutes

47 Ratio-based time Synchronization Protocol (RSP) T9 Tnew 3 minutes 3 minutes So, the new point is changed into time of t4 and do the same operations, and calculate again. : relative drift ratio. : offset between the clocks of nodes i and j

48 Ratio-based time Synchronization Protocol (RSP) multi-hop : R node and A node are already synchronized above method. But B node and R node are not neighborhood, so: (B node calculated clock drift and offset ) R A B Msg.1 Msg.2 (Using local time of A node can estimate global time)

49 6.3. Protocols Based on Receiver/Receiver Synchronization

50 Protocols Based on Receiver/Receiver Synchronization In this class of schemes The receivers of packets synchronize among each other, not with the transmitter of the packet Reference Broadcast Synchronization (RBS) Synchronize receivers within a single broadcast domain RBS does not modify the local clocks of nodes, but computes a table of conversion parameters for each peer in a broadcast domain

51 Reference Broadcast Synchronization (RBS)

52 Reference Broadcast Synchronization (RBS) Introduction Reference broadcasts do not have an explicit timestamp Receivers use reference broadcast s arrival time as a point of reference for comparing nodes clocks Receivers synchronizes with one another using the message s timestamp (which is different from one receiver to another)

53 Reference Broadcast Synchronization (RBS) Types of errors in Traditional Synchronization protocol Send Time Latency time spent at the sender to construct the message Access Time Latency time spent at the sender to wait for access to transmit the message time spent at the sender to wait for access to transmit the message Prorogation Time Latency time spent by the message in traveling from the sender to the receiver Receive Time Latency time spent at the receiver to receive the message from the channel and to notify the host

54 Reference Broadcast Synchronization (RBS) Types of errors in RBS Phase error due to nodes clock that contains different times Clock skew due to nodes clock that run at different rate

55 Reference Broadcast Synchronization (RBS) Difference between RBS & Traditional Synchronization protocol RBS Synchronizes a set of receivers with one another Supports both single hop and multi hop networks Traditional Senders synchronizes with receivers mostly supports only single hop networks

56 Reference Broadcast Synchronization (RBS) The phase offset with the clock skew is estimated by: Least-squares linear regression graph From the best-fit line of the graph, following can be inferred: Slope of the line : Clock skew of the nodes clock Intercept of the line : Phase of the nodes clock

57 Reference Broadcast Synchronization (RBS) Basic idea to estimate phase offset and clock skew for non-deterministic receivers: Transmitter broadcasts m reference packets Each of the n receivers records the time that the reference was received, according to its local clock The receivers exchange their observation Each Receiver i can compute its phase offset to any other receiver j

58 Reference Broadcast Synchronization (RBS) Formula for calculating the phase offset and clock skew of receiver r 1 with other receiver r 2 : T r,b : r s clock when it received broadcast b, for each pulse k that was received by receivers r 1 and r 2, we plot a graph : x = T r1, k y = T r2,k T r1,k Diagonal line drawn through the points represents the best linear fit to the data

59 Reference Broadcast Synchronization (RBS) Diagonal line minimizes the residual error (RMS) Therefore, we go for calculating the slope and intercept of the diagonal line Time value of r 1 is converted to time value of r 2 by combining the slope and intercept data obtained

60 Reference Broadcast Synchronization (RBS) Reference Packet Step1: Transmitter Step2: Receiver broadcasts records its local Step3:Use clock, Least-squares and exchange linear regression observation to estimate phase Finish offset RBS A:Local time A B:Local time B Transmitter Receiver

61 Reference Broadcast Synchronization (RBS) Conclusion Advantages of RBS Can be used without external timescales Does not require tight coupling between sender and its network interface Largest resources of latency (that exists in Traditional Time Synchronization Protocol) is removed from critical path Limitations of RBS Does not support point to point communication

62 6.3.2 Hierarchy Referencing Time Synchronization (HRTS)

63 Hierarchy Referencing Time Synchronization (HRTS) Goal : Synchronize the vast majority of a WSN in a lightweight manner Idea Combine the benefits of LTS and RBS

64 Hierarchy Referencing Time Synchronization (HRTS) LTS : Lightweight Time Synchronization Goal Synchronize the clocks of all sensor nodes of a subset of nodes to one reference clock It considers only phase shifts and does not try to correct different drift rates

65 Hierarchy Referencing Time Synchronization (HRTS) LTS : Pairwise Synchronization At time t1 t2 t3 t4 Record t1 t4 n1 n2 Record t2 t3 Sync packet Reply packet In this packet contains t2 and t3

66 Hierarchy Referencing Time Synchronization (HRTS)

67 Hierarchy Referencing Time Synchronization (HRTS) LTS : Pairwise Synchronization Offset : [L i (t 8 )+ L i (t 1 )- L i (t 6 )- L i (t 5 )] / 2 Benefit : only two packet transmission with each pair

68 Hierarchy Referencing Time Synchronization (HRTS) Benefit of RBS Idea : ignore transmission delay By this idea, one packet can synchronize every node in one hop

69 Hierarchy Referencing Time Synchronization (HRTS) Combining the two protocol s benefit, the HRTS finds good solution to synchronize nodes in hierarchical way

70 Hierarchy Referencing Time Synchronization (HRTS)

71 Hierarchy Referencing Time Synchronization (HRTS) First step : R broadcast packet Second step : i reply packet ( For R and i, the two step is like LTS.) Third step : R calculate offset and broadcast packet Forth step : i and j calculate the offset ( For j, it s like RBS.)

72 Hierarchy Referencing Time Synchronization (HRTS)

73 6.4. Summary

74 Summary Time synchronization is important for both WSN applications and protocols Using hardware like GPS receivers is typically not an option, so extra protocols are needed Post-facto synchronization allows for time synchronization on demand, otherwise clock drifts would require frequent resynchronization and thus a constant energy drain

75 Summary Some of the presented protocols take significant advantage of WSN peculiarity like: small propagation delays the ability to influence the node firmware to timestamp outgoing packets late, incoming packets early

76 References [1] Ed. Ivan Stojmenovic, Handbook of Sensor Networks Algorithms and Architectures, [2] F. Sivrikaya,and B.Yener, Time Synchronization in Sensor Networks: A Survey,2004. ( [2] J. Elson, L. Girod, and D. Estrin,Fine-Grained Network Time Synchronization using Reference Broadcasts. (In Proceedings of the Fifth Symposium on OSDI 2002) [3] S. Ganeriwal, R. Kumar, and M. Srivastava, Timing-Sync Protocol for Sensor Networks. (SenSys 03) [5] D. L. Mills. Network Time Protocol (Version 3) Specification, Implementation and Analysis. RFC 1305, [6] D. L. Mills. Improved Algorithms for Synchronizing Computer Network Clocks. IEEE/ACM Transactions on Networking, 3(3): , [7] D. L. Mills. Adaptive Hybrid Clock Discipline Algorithm for the Network Time Protocol. IEEE/ACM Transactions on Networking, 6(5): , 1998.

77 References [8] S. Ganeriwal, R. Kumar, S. Adlakha, and M. Srivastava. Network-Wide Time Synchronization in Sensor Networks. Technical Report NESL , Networked and Embedded Systems Lab (NESL), University of California, Los Angeles (UCLA), [9] S. Ganeriwal, R. Kumar, and M. B. Srivastava. Timing-Sync Protocol for Sensor Networks. In Proceedings of the 1st ACM International Conference on Embedded Networked Sensor Systems (SenSys), pages , Los Angeles, CA, November [10] Miklós Maróti, Branislav Kusy, Gyula Simon, Ákos Lédeczi, The Flooding Time Synchronization Protocol, In Proceedings of the 2ed ACM International Conference on Embedded Networked Sensor Systems (SenSys), pages 39 49, Baltimore, MD, USA, [11] J.-P. Sheu, W.-K. Hu, and J.-C. Lin, Ratio-Based Time Synchronization Protocol in Wireless Sensor Networks, Telecommunication Systems, Vol. 39, No. 1, pp , Sep [12] J. Elson, L. Girod, and D. Estrin. Fine-Grained Network Time Synchronization using Reference Broadcasts. In Proceedings of the Fifth Symposium on Operating Systems Design and Implementation (OSDI 2002), Boston, MA, December 2002.

78 References [13] H. Dai and R. Han. TSync: A Lightweight Bidirectional Time Synchronization Service for Wireless Sensor Networks. ACM SIGMOBILE Mobile Computing and Communications Review, 8(1): , 2004.

79 Recommend Reading Particular Challenges and Constraints for Time Synchronization Algorithms in WSN J. Elson and K. R omer. Wireless Sensor Networks: A New Regime for Time Synchronization. In Proceedings of the First Workshop on Hot Topics In Networks (HotNets-I), Princeton, NJ, October J. E. Elson. Time Synchronization in Wireless Sensor Networks. PhD dissertation, University of California, Los Angeles, CA, Department of Computer Science, Other Time Synchronization Protocol Lightweight time synchronization protocol (LTS) J. V. Greunen and J. Rabaey. Lightweight Time Synchronization for Sensor Networks. In Proceedings of the 2nd ACM International Workshop on Wireless Sensor Networks and Applications (WSNA), San Diego, CA, September 2003.

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