Time Synchronization and Calibration in Sensor Networks

1. Time Synchronization and Calibration in Sensor Networks Prepared By: Muhammad Aslam Malik KAY RO¨ MER, PHILIPP BLUM, and LENNART MEIER Supervisor: Ivan Stojmenovic Date Presented: 18th March-2010

2. Outline • Introduction • Related Works • System Model • Communication Model • Proposed Systems • Synchronization Techniques • Multiple Nodes Synchronization Techniques • Measurement Techniques • Classes of Calibration • Project Milestone • Conclusion

3. Introduction • Sensor Networks are being used to monitor real world phenomena • Physical plays a pivotal role in sensor networks • Achievement of synchronization of physical time is complex task, due to many different challenging characteristics of sensor networks • Improvement in energy efficiency due to frequent switching of sensor nodes or components • Sensor networks also separate the sensor samples due to short time occurrence

4. Introduction (Cont) • Through multiple sensors it is possible to detect the proximity of an object, also further higher level of information (like speed, size, and shape) • Time synchronization for sensor networks is an active field of research • Calibration is very general and complex problem • The challenges for the future research is the development of methods and tools for the evaluation of time synchronization and calibration in large scale sensor networks

5. (Related Works) Physical Time Importance • The physical time is the essential requirement of many sensor network applications, on the other hand many traditional applications of time also depends upon sensor network • Rough classification of applications of physical time are a- Interaction between sensor network and external observer b- Among nodes of the sensor network c- Interaction between sensor network with real world

6. Physical Time Applications • Interface among observer, sensor network and environments

7. Time Synchronization and Calibration in Sensor Network Synchronization When a system operates with all parts in synchrony status is called as synchronization Sensor Network Sensor Network plays the role of an observer interfacing to external observer and environment Physical Time Time of occurrence of any physical event is referred as Physical Time Nodes Nodes are required to send, receive or to store the data Energy Efficiency Nodes helping in improving the energy efficiency due to frequent switching and components

8. Network Time Protocol (NTP) • NTP was designed for large-scale networks preferably static topology • Nodes are externally synchronized to a global reference time • These are injected into the network at many places through a set of master nodes • These master nodes are synchronized out of band for instance via GPS (which provides global time with a precision at a great extent below 1µ sec) • Nodes participating in NTP, leaf nodes are called clients, inner nodes are called Stratum L servers • L is considered as level of node in the hierarchy • Parents of each node must be specified in configuration files due to which nodes frequently exchange synchronization messages with their parents and then use achieved information to adjust their clocks by regularly incrementing them

9. Network Dynamics • Sensor nodes can be mobile, may be died due to the weakness of batteries or due to influence of environ- ments & new sensor nodes can be added at any point at any time • This operation happen in frequent manner and any unpredictable changes in the network topology can took place, even network partitions • Mobile nodes make the transportation of messages across partition boundaries by storing received messages and further transporting it as soon as a new partition is entered • End-to-end delay of such type of message path is very • unstable and hard to predict as well

10. System Model • All modelling are carried out in terms of discrete time and events • Any event represent the communication between nodes, a sensor measurement, the injection of time information at a node, and so on Clock Models • Digital clocks measure time intervals, they particularly having a counter “h” which can counts time steps of an ideally fixed length • We denote the reading of the counter at real time “t” as h(t), counter is incremented by an oscillator with a • rate(freq) “f”, so Rate f at time t is given as the first • derivation of h(t ):f(t)=dh(t)/dt

11. Clock Model • Any ideal clock having rate 1 at all times, but the rate of a real clock fluctuations over the time is due to changes in the supply voltages, temperature and so on • If fluctuation allowed to be arbitrary, the clocks reading obviously give no information at all • It is limited by known boundary, different types of boundary on the rate fluctuation lead to different types of clock models Constant-Rate Model • In this model rate is considered as constant • It is justified if required precision is small compared to the rate fluctuation

12. Software Clocks • A synchronization algorithms can either directly modify local clock “h” or otherwise Construction of a software clock ”c” • A software clock is a function which take a local clock value h(t) as input and transforming it to time c (h(t)) • This time is the final result of synchronization & therefore it is called as synchronized time • C(h(t))= +h(t)-h( ) • Software clock which starts with correct real time t0 then runs with the same speed as local clock ‘h’

13. Communication Models • Communication is required to achieve and maintain synchronization, different parameters, which may affect time synchronization are Unicast-verses Multicast • In this scenario, a message is sent by one network node and is received by at most one other network nodes, we referred it as unicast or point-to-point communication • On the other hand in Multicast Communication occurs when a message is sent by one network node and it is received by an arbitrary number of other network nodes Broad Cast The situation in which all nodes within transmission range are recipients are called as Broad Cast

14. Symmetrical Verses Asymmetrical Links Symmetrical • The scenario in which a node “A” is able to receive the messages sent by node “B” if and only if node B can receive messages sent by node “A”, then this link between node A & B is called symmetrical • Asymmetrical Asymmetrical link is between a base station with high transmit power and a mobile device with low transmit power , beyond a certain distance between two, only communication in direction from the base station to the mobile device is possible

15. Delay Uncertainty • The delay has also the great importance, during message transfer between nodes gaining the time information • Send Time The time when message command is sent is termed as send time • The (medium) access time When node starts transmitting message is called access time • The Propagation Time Time duration between sender to receiver is called as propagation time Receive Time It is the time lasting from the reception of the signal to the arrival of the data at the application

16. Sources of Synchronization Errors • Clock synchronization algorithms has to face two problems, clock drift and message delay uncertainty • Influence of clock drift may dominate over that of message delays, the scenario in which communication is infrequent • Due to decrease in frequency of communication, the uncertainty due to clock drift increases • Uncertainty due to message delay remains constant

17. Classes of Synchronization • “Making clock show the same time” is termed as synchronization • Internal Vs External The time supplied from outside the network is termed as external synchronization, NTP performs external synchronization and sensor nodes synchronizing clocks to master nodes Internal Synchronization • Internal synchronization is the synchronization of all clocks in the network, without a predetermined master time, goal was consistency among network nodes

18. Life Time: Continuous Vs On-Demand • Life Time Life time of synchronization is period of time during which synchronization is required to hold Continuous • Synchronization is continuous, the network nodes exerts force to maintain synchronization at all times • On-Demand On-Demand synchronization can be as good as continuous synchronization with respect to synchronization quality but with much efficient way during that time between events, no synchronization and communication is required, and thus energy consumption can be kept at minimum level

19. Kinds of On-Demand Synchronization • Event Triggered In this scenario sensor nodes needs a synchronized clock only immediately after the occurrence of event, to compute time stamp for the moment in recent past when event took place (e.g. post facto synchronization) Time Triggered On-Demand Synchronization In this scenario data is collected during specific time from multiple sensor nodes For successful anticipated synchronization, it is sufficient to maintain a synchronization quality, which can guarantees that target time is not missed

20. Classes of Synchronization (cont) • Scope: All Nodes Vs Subsets • It defines which nodes in the network has to be synchronized • Depending on applications scope determines weather all nodes or only subsets of nodes has to be synchronized • Rate synchronization Vs Offset synchronization • Means that nodes measure all identical time-interval length in sensor networks, in this scenario sensor nodes measure time of appearance and disappearance of an object • Off-Set Synchronization • Nodes measure identical point in time, that is at some time “t” the software clocks of all nodes in the scope • show “t”

21. Scope and Life Time • Scope and life defines where and when synchronization is required • Scope • N1 • N3 • N4 • Scope • Time • N5 N1 N2 N3 N4 N5 N2 Scope

22. Time scale Transformation Vs Clock Synchronization • Two ways of time synchronization • In 1st method We can synchronize clocks, making all clocks displaying the same time at any given time • For achieving synchronization, we have to perform rate and off-set synchronization • 2nd Method is to transform timescales, meaning to transform local times of one node into local times of another node, both nodes are same in sense

23. Rate and Off-Set Synchronization

24. Time Instants Vs Time Intervals • Time Instant • It determines the specific time instant like “t=5” • Time-Interval • It determines the time with specific time intervals, like (“t ϵ [4.5, 5.5]”) • In both cases the time information can be refined by adding a statement about its quality • For example the time information may be guaranteed to be correct with a certain probability or even probability distribution for the time can be given • For sensor networks, guaranteed time interval is better

25. Synchronization Technique (Proposed Systems) Taking One Sample • The simple model showing the two nodes and which can exchange messages and synchronization between these nodes mean that they have established the relationship between their local clocks and and

26. Unidirectional synchronization • The unidirectional Scenario

27. Unidirectional Synchronization • The time synchronization contains or as estimating • If priori bounds are known for message delay , which is ≤ d ≤ , then estimation will be ≈ -1/2( + ) Alternatively minimizing in the worst case scenario is ≈ +1/2( + ) - and - - are lower and upper bounds on + and + are bounds on

28. Round Trip Synchronization • Bidirectional Scenario

29. Round Trip Synchronization • In round trip scenario • If priori bounds about the message delays are known, which is • ≤ d ≤ • The node now knows that delay d is bounded by • Max(D- , ) and min( ,D- ) • The estimation ≈ -D/2, minimizing the worst case synchronization errors • -(D- ) and - are lower and upper bounds for • Round trip synchronization is better due to the reason as it provide the lower and upper bound synchronization error, it is called as probabilistic time synchronization, it continues till the synchronization error is below the specified threshold value • The only disadvantage is that number of messages are increased • than unidirectional

30. Round Trip Synchronization • Round Trip Scenario

31. Reference Broadcasting • Fig shows the reference broadcasting • In this technique a beacon is involved as well, delay d and are almost equal, Node sends time stamp to , and it measure D= - between arrival of two messages, then estimate to ≈ +D main advantage is broad cast message received concurrently, so better than all others

32. Synchronization of Multiple Nodes • Multiple nodes synchronization is desired, which help in adding of additional layer of complexity, due to which it can be avoided easily by using an overlay network providing virtual, single-hop communication from sensor node to a single master node • Synchronization error directly depends upon the message delay, and it is very difficult to control on logical link having many physical hops • Hence performance schemes have to be dealt with the multi-hop problem absolutely

33. Synchronization of Multiple Nodes • There are four approaches of multihop synchronization • Out-of-Band-Synchronization

34. Synchronization of Multiple Nodes • Clustering A B C

35. Synchronization of Multiple Nodes • Tree hierarchy scenario is most common solution of multihop synchronization problem

36. Multiple Nodes Synchronization • Unstructured Scenario • This type of synchronization, do not solve the problem perfectly and then perform pairwise synchronization • Symmetrical synchronization is carried out • It is having local approach can not use for global reference time

37. Measurements • Measurements Techniques

38. Measurements Techniques • Three fundamental different measurements strategies has been shown in figure • In fig a, single-hop RBS scheme is used to measure precision • The precision is achieved by the FTSP multi-hop algorithms • Through these techniques, having advantage that every node can evaluate and log its own precision • In fig b, sensor nodes generate some directly observable event Advantage of this scenario is that the precision of measurement is not limited by the resolution of node’s clocks • In fig c, it proposes to measures the precision achieved by one client node, a client node synchronizes over several hops to a master node • Master and client nodes are virtual nodes successfully implemented on a single physical node, and intermediate nodes are all separate • physical nodes

39. Classes Calibration • Internal vs. External: For internal calibration all software sensor ‘i’ should give the same output value ( ( (t)), if they are present in identical stimulus q(t) ( note that if for instance q(t)= C, then ( ( (t))= ( (t ))= C would mean that sensors 1 and 2 are inter-calib • For external calibration, the output of all software sensor must be presented to a specified scale( e.g. if q(t)= C, then • ( (t))= ( (t)= C is required) • Life-Time Continuous Vs on-demand: Some of the parameters which influence ‘h’ may change over time, calibration have to be repeated to be suitable to accept these parameters and calibration can be performed continuously or on-demand • Scope, All Nodes Vs Subsets: All nodes or subset might participate in calibration • For instance, only some nodes might be equipped with a specific • type of sensor or some sensor might be used by • some nodes

40. Project Milestone Background Studies Related Work Sudies Problem Identification Proposed Systems Results Analysis

41. Conclusions • Time Synchronization has declared special case of calibration, and many observations about time synchronization can be transferred to calibration • Time synchronization has been declared as great active field of research, while calibration still has not been responded very well by the researcher • Calibration is more complex than the synchronization • The future challenges in research is to develop the methods and tools for the evaluation of time synchronization and calibration for large sensor networks • Model-Based Calibration analysis will be presented in paper in detail, at present due to shortage of time has not been discussed in deatail

42. References • Kay Romer, Philipp Blum and Lennart Meier on Time Synchronization and Calibration in Sensor Network • Jeremy Eric Elson, on Time Synchronization in Wireless Sensor network, 2003 • J . Feng, S.Megerion, M. Potkonjak on Model-Based Calibration for Sensor Network

43. Question and Answer • Q1- • How energy efficiency can be achieved in sensor networks. • Ans- • Through frequent switching of sensor nodes and components (Sleep Nodes). • Q2-In which aspect the on-demand synchronization and continuous synchronization are equally good • Ans- • Both are equally good with respect to synchronization quality • Q-3 Why wired networks have less delay uncertainties as compared with wireless sensor networks • Ans-Due to lower link reliability and bandwidth