1. HIERARCHY REFERENCING TIME SYNCHRONIZATION PROTOCOL Prepared by : Sunny Kr. Lohani, Roll – 16 Sem – 7, Dept. of Comp. Sc. & Engg.

2. OVERVIEW  Introduction  Time synchronization  Hierarchy referencing time synchronization  Summary

3. INTRODUCTION  Wireless Sensor Networks (WSNs) consist of numerous small sensors that are wirelessly connected to each other. In these networks, time synchronization is an important issue. Correct time-stamping of events is crucial for data processing.  Time synchronization protocols that are used for synchronization in traditional wired networks are not applicable in WSNS.  The different aspects of WSNs related to synchronization are : i) Sensors ii) Dynamic operations iii) Network topology iv) Operating System

4. TIME SYNCHRONIZATION IN TRADITIONAL NETWORKS Network Time Protocol (NTP) : A client using NTP connects to a server to synchronize time. The time given by the server is corrected with half the measured round trip time. NTP is a two-way time synchronization method, in which a timestamp is sent in two directions. Simple Network Time Protocol (SNTP) : It allows operation in a stateless remote- procedure call (RPC) mode. It is less complex then the full NTP, which makes it easier to implement, but decreases accuracy. Classless Time Protocol (CTP) : It uses a peer-to-peer approach. Using this protocol, a node sends and receives packets only to and from its direct neighbors. The CTP uses the same format and number of network messages as the NTP. Its performance is better than the performance of the NTP without an increase of complexity.

5. TIME SYNCHRONIZATION METHODS FOR WSNS Reference Broadcast Synchronization(RBS): It is a time synchronization protocol in which nodes send reference beacons to their neighbors, by making use of the broadcast possibility of the network. These beacons do not contain a timestamp. Receivers use the arrival time of these beacons as points of reference for comparing their clocks. A beacon can be sent by any node in the network, thus no special nodes are needed. Probabilistic extension to RBS: This extension is based on the possibility of RBS to send multiple beacon messages to improve accuracy. For this protocol we need to specify the maximum synchronization error and the confidence probability. The minimum number of messages and the synchronization interval are then derived from this specification. This is possible, because the error among receivers has a normal distribution.

6. TIME SYNCHRONIZATION METHODS FOR WSNS (CONT..) Timing-sync Protocol for Sensor Networks (TPSN): It synchronizes time in a sensor network by first creating a hierarchical structure and then synchronizing nodes along this structure. In this way, a global timescale is established throughout the network. A hierarchical topology is created by assigning a level to every node in the network. Only one node is assigned level 0, which will be the master node. Every other node at level i can communicate with at least one other node at level i - 1. Time-Diffusion Synchronization Protocol: It is a protocol that allows synchronization of a whole sensor network. Initially, an election protocol is run to select a set of master nodes. These masters then send a broadcast message, containing their local time. All receivers send back a message, allowing the masters to calculate the round-trip times. The average of these round-trip times is used to estimate the one-way delay between the master and its neighboring nodes.

7. TIME SYNCHRONIZATION METHODS FOR WSNS (..CONT) Tsync: It is a bidirectional time synchronization service that offers both a push mechanism for synchronizing the whole network as well as a pull mechanism for on-demand synchronization of a single node. This protocol uses the ability of node radios to communicate over multiple channels for frequency diversity. By doing this, it is possible to reduce packet collisions and prevent jamming. All time-critical packets are sent through a dedicated channel, called the clock channel. Lightweight Tree-based Synchronization (LTS): It consists of a push mechanism and a pull mechanism. In push mechanism, nodes synchronize to their parents by means of pair wise synchronization. In pull mechanism, When a node n determines that it needs to be synchronized, it sends a synchronization request to a master node. In order for n to synchronize with a master node, all nodes on the path must be synchronized using pair wise synchronization.

8. TSYNC  TSync combines: HRTS (Hierarchy Referencing Time Synchronization) : A protocol to synchronize a broadcast domain to one of its members ITR (Individual-based Time Request): A sender-/receiver protocol similar to LTS/TPSN A network wide synchronization protocol  HRTS provides a technique to synchronize a group of nodes to a reference node with only three packets.

9. 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.

10. HIERARCHY REFERENCING TIME SYNCHRONIZATION (HRTS) [..CONT]  LTS (Light weight 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.

11. HIERARCHY REFERENCING TIME SYNCHRONIZATION (HRTS) [..CONT]  LTS: Pairwise synchronization

12. HIERARCHY REFERENCING TIME SYNCHRONIZATION (HRTS) [..CONT]

13. LTS : Pairwise Synchronization  Offset : O = Δ(t5 )=L i (t5) - L j (t5)= [L i (t8)+ L i (t1)- L j (t6)- L j (t5)] / 2  Because L i (t5) = [L i (t8) + L i (t1)- (L j (t6)- L j (t5))] / 2  Benefit : Only two packet transmissions with each pair. Benefit of RBS  Idea : Ignore transmission delay.  By this idea, one packet can synchronize every node in one hop. Combining the two protocol’s benefit, the HRTS finds good solution to synchronize nodes in hierarchical way.

14. HIERARCHY REFERENCING TIME SYNCHRONIZATION (HRTS) [..CONT]

15.  Root node triggers time synchronization at t1 with timestamp LR(t1).  Node i timestamps packet at time t2 with L i (t2) and node j timestamps it at t2’ with L j (t2’).  Node i formats a packet and timestamps it at time t3 with L i (t3) – the packet includes the values L i (t2) and L i (t3).  Root node R timestamps the answer packet at time t4 with L R (t4) and computes its offset O R,i with node i’s clock.  O R,i =L i (t2) - L R (t2) = L i (t2) – (L R (t1) + L R (t4) – (L i (t3)- L i (t2))/2 =[ (L i (t2) - L R (t1)) – (L R (t4)- L i (t3))]/2  Root node R broadcasts the values O R,i and L i (t2)

16. HIERARCHY REFERENCING TIME SYNCHRONIZATION (HRTS) [..CONT]  The root node R can estimate the offset O R,i between its own clock and the local clock i in a similar fashion as the protocol LTS.  Root R broadcast the values O and L i (t2) to all nodes.  Node i simply subtract the offset O R,i from its local clock.  Node j can compute O j,i directly as O j,i = L i (t2) – L j (t’2) and O R,j = O R,i – O j,i

17. HIERARCHY REFERENCING TIME SYNCHRONIZATION (HRTS) [..CONT]  Node j is not involved in any packet exchange.  By this scheme is possible to synchronize an arbitrary number of nodes to R’s clock with only three packets.  The synchronization uncertainty comes from: The error introduced by R when estimating O R,i The error introduced by setting t 2 = t 2 ’.  This makes HRTS only feasible for geographically small broadcast domains.

18. HIERARCHY REFERENCING TIME SYNCHRONIZATION (HRTS) [..CONT]  Both kinds of uncertainty can again be reduced by: time stamping outgoing packets as lately as possible (relevant for t 1 and t 3 ). time stamping incoming packets as early as possible (relevant for t 2, t 2 ’, t 4 ).  The authors propose to use extra channels for synchronization traffic when late time stamping of outgoing packets is not an option. Rationale: keep MAC delay small

19. HIERARCHY REFERENCING TIME SYNCHRONIZATION (HRTS) [..CONT]

20. 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.