Wireless Sensor Networks Nov 1, 2006 Jeon Bokgyun

2/26 Reference Tutorial, Wireless sensor networks, mobicom Introduction – Deborah 2.Sensor Node Platforms & Energy Issues – Mani 3.Time & Space Problems in Sensor Networks – Mani 4.Sensor Network Protocols – Deborah 5.Collaborative Signal Processing – Akbar 6.Discussion - All

3/26 Contents  Introduction  Features of sensor network  Sensor hardware platform  Energy issue  Time synchronization  Node localization  Sensor coverage  Conclusion

4/26 What is the sensor?  Sensor: a transducer that converts a physical, chemical, or biological parameter into an electrical signal  Actuator: a transducer that accepts an electrical signal and converts it into a physical, chemical, or biological action  Transducer: a device converting energy from one domain into another. The device may either be a sensor or an actuator.

5/26 Sensor Network Architecture  Tens of thousand nodes Densely deployed Internet, Satellite, etc Sink Task Manager

6/26 Sensor Network Applications  Seismic monitoring  Contaminant transport  Ecosystem monitoring  Transportation and urban monitoring  Infant monitoring  Personalized adv.  Etc.

7/26 Sensor Network Features  Densely deployed and prone to failure  The topology changes very frequently  May leverage broadcasting than point-to- point communications  May operate in aggregate fashion  Sensor nodes are limited in power, computational capacities, and memory  May not have global ID like IP address  Need tight integration with sensing tasks

8/26 Sensor Node HW Platform Capabilities Size, Power Consumption, Cost MICA Mote iBadge MK - II StarGate

9/26 Sensor Node HW Platform  COTS dust prototypes (Kris Pister et al.)  weC Mote (~30 produced, 1998)  Rene Mote (850+ produced, )  Dot (1000 produced, 2000)  Mica node ( produced, 2001)  Mica2 (2002)  MicaZ, Telos (2004)

10/26 Sensor Node SW Platform  TinyOS Programming concepts for resource constrained networked embedded devices  SOS Enabling dynamic embedded software.

11/26 Contents  Introduction  Features of sensor network  Sensor hardware platform  Energy issue  Time synchronization  Node localization  Sensor coverage  Conclusion

12/26 Where does the energy go? ADC : Analog to Digital Converter Power Unit Sensor ADCProcessorMemory Transceiver Location Finding System Mobilizer

13/26 Energy Observation  Communication >> computation (at short range)  Radio RX power May dominate (at short range)  Energy Spent in idle RX dominates lifetime energy consumption

14/26 Radio Energy Management  Short range links Shutdown based Turn off sender and receiver Topology management schemes exploit this  Long range links Scaling based Slow down transmissions Energy-aware packet schedulers exploit this

15/26 Motivation for Time Synchronization  Most applications require some synchronization accuracy Fire and flood tracking Animal movement Vehicle movement Gunshot detection

16/26 Synchronization in Sensor Network  Network time protocol (NTP) for Internet clock synchronization  Difference: for sensor networks Time synchronization requirements more stringent (µs instead of ms) Power limitations contain resources May not have easy access to synchronized global clocks NTP assumes that pairs of nodes are constantly connected and experience consistent communication delays Often, local synchronization sufficient

17/26 Network Time Protocol (NTP)  Primary servers (S1) synchronize to national time standards Satellite, radio, modem  Secondary servers (S2, …) synchronize to primary servers and other secondary servers Hierarchical subnet S3 S4 S2 S3 S1 S2 S1 S2 Primary Secondary

18/26 RBS (Reference Broadcast Sync.) NIC Sender Receiver Critical Path NIC Sender Receiver 1 Receiver 2 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

19/26 RBS (cont.)  Receiver to receiver synchronization  Two stage Transmitter broadcast clock time Receivers exchange observations  Assumptions Propagation delay is zero No clock skew  RBS outperforms NTP 11usec precision over 19.2K radios

20/26 Why is Localization Important?  Very fundamental component for many other services GPS does not work everywhere Smart Systems – devices need to know where they are Geographic routing & coverage problems People and asset tracking Need spatial reference when monitoring spatial phenomena

21/26 Techniques for Location Sensing  Measure proximity to “ landmarks ”  Dead reckoning: position relative to an initialization point  Measure direction of landmarks  Measure distance to landmarks  Measure difference in distances to two landmarks

22/26 Solving over multiple hops  Interative Multilateration Beacon node (known position) Unknown node (known position) Problems Error accumulation May get stuck!!!

23/26 Solving over multiple hops  Collaborative Multilateration

24/26 Sensor coverage  How well can the field be observed?  As the measure of QoS of a sensor network.  Example usage Commander Weakest path : what path is the enemy likely to take?

25/26 Worst-case Coverage  Voronoi Diagram Path of Maximal Breach of Surveillance in the sensor field lies on the Voronoi diagram lines. When adding node, the next node is deployed along the edge closest to the original nodes.

26/26 Conclusion  In the future, this wide range of application areas will make sensor networks an integral part of our lives.  Sensor network has various constraints.  Briefly, introduce energy issue, timing synchronization, node localization, and sensing coverage.