INTRODUCTION TO WIRELESS SENSOR NETWORKS. CHAPTER 7: TIME SYNCHRONIZATION Anna Förster

Transcription

1 INTRODUCTION TO WIRELESS SENSOR NETWORKS CHAPTER 7: TIME SYNCHRONIZATION Anna Förster

2 OVERVIEW 1. Clocks and Delay Sources 2. Requirements and Challenges 3. Time Synchronization Protocols 1. Lightweight Tree Synchronization 2. Reference Broadcast Synchronization 3. NoTime

3 Clocks and Delay Sources Clock resolution or rate is the step at which the clock progresses and can be read Clock skew or drift is the difference between the progressing speeds of two clocks Clock offset is the difference between the time shown by two different clocks Clock jitter is when the individual ticks of a clock are not perfectly the same clock time offset jitter resolution this clock is too fast this clock is correct clock skew this clock is too slow real time

4 Requirements and Challenges (1) Two clocks drift apart even after synchronization The maximum drift rate of a clock dictates the synchronization period sync = 2 where ρ is the maximum drift rate in ppm (parts per million), δ is the maximum allowed drift in seconds and τsync is your needed synchronization period.

5 Requirements and Challenges (2) The time synchronization protocol cannot jump back and forth in time. Time synchronization accuracy is the maximum offset between the synchronized clock and a reference clock (for example global time). Time synchronization precision is the maximum offset between any two synchronized clocks. Time synchronization precision is important to understand the sensor data.

6 Understanding Sensor Data t1 < t2 < t3 < t4 < t5 < t6 t6 < t1 < t2 < t5 < t4 = t3 sensor node 1 sensor node 2 sensor node 3 event at time t1 event at time t2 event at time t3 event at time t4 event at time t5 event at time t6 t1 t2 t3 t4 t5 t6 t1 t2 t3 t4 t5 t6 two people passing from left to right???

7 Time Synchronization Protocols Externally Synchronized: there is an external source of correct time data. Internally Synchronized: consistent view from all nodes, but not globally correct time.

8 Sender-Receiver Synchronization init message send sender A receiver B Long critical path access critical path propagation receive time process message time

9 Receiver-Receiver Synchronization init message send access sender A time propagation process message receive receiver B time process message receive receiver C time critical path Shorter critical path RBS critical path

10 Lightweight Tree Synchronization (LTS) Build a spanning tree from root (sink) to all nodes Sink has globally correct time Sink sends a message to its children to sync them, then traverse the tree Use sender-receiver synchronization Precision depends on depth of tree and does not scale

11 Reference Broadcast Synchronization (RBS) Uses receiver-receiver synchronization Broadcast correct time to all neighbors Total precision increases with number of broadcasts Scales well Not very efficient in terms of communication overhead

12 NoTime Protocol Do not try to time sync the nodes directly Provide enough information so that the sink can reconstruct the real time Save delays on the way Precision depends on: Propagation Delay Hardware Delay Local time resolution delay_total = 14 sec real_time = '2014,7,23,10,19,43' event_time = '2014,7,23,10,19,29' sink 1 5 delay += 2 sec delay += 7 sec delay = 5 sec event

13 SUMMARY Clock skew causes two clocks to progress at different rates, while clock jitter causes a clock to tick irregularly. Provide efficiency and simplicity: Regular synchronization is needed. Available time cannot jump back and forth in. Lightweight tree synchronization protocol (LTS) uses the sender-receiver synchronization message exchange and is not scalable. Broadcast reference protocol (RBS) uses the receiverreceiver synchronization message exchange, has greater precision than LTS and is scalable. NoTime does not synchronize the nodes, but computes the innetwork delay for each data packet, which enables the sink to compute the original event s real time.