Wireless Sensor Networks Raj Jain Washington University in Saint Louis Saint Louis MO 63130 Jain cse wustl edu These slides are available on line at http www cse wustl edu jain cse574 06 Washington University in St Louis CSE574s 17 1 2006 Raj Jain Overview Sensor Applications Sensor Network Architecture Data Dissemination MAC Protocols for Sensor Networks Location Discovery Quality of a Sensor Network Time Synchronization Transport Layer Issues Sensor Network Security Real Time Communication Washington University in St Louis CSE574s 17 2 2006 Raj Jain Sensor Applications Battlefield Surveillance Chemical Biological Weapons Crops and Agriculture Forest Fires and Flood Detection Habitat exploration of animals Patient heart rate blood pressure Washington University in St Louis CSE574s 17 3 2006 Raj Jain Sensor vs Ad Hoc Large scale Batteries may not be replaceable May not have global identifiers Queries may be data centric rather than address centric Who s temperature is more than 95 degree vs What is your temperature Geographical routing Data fusion Data aggregation Washington University in St Louis CSE574s 17 4 2006 Raj Jain Sensor Network Architecture 1 Layered Base station one hop layer 2 hop layer 2 Clustered Nodes elect and communicate through cluster heads Washington University in St Louis CSE574s 17 5 2006 Raj Jain Data Dissemination Sources Sinks and Events Data Gathering Sources send periodically to central collection points base station Data Diffusion Sinks propagate their interests type of data or event Nodes cache interests and report events when detected Flooding Implosion duplicate messages overlap multiple sources blind no consideration of energy or resources Gossiping Randomly select a neighbor Sinks Sources Washington University in St Louis CSE574s 17 6 2006 Raj Jain Directed Diffusion Sensor nodes generate queries Flooded to entire network Intermediate node cache the queries and the previous neighbor A gradient rate is applied at each hop to the query Data is propagated along the reverse path proportional to the gradient Sink can reinforce a path by requesting higher rates along that path B A Sink B A Sink Washington University in St Louis C D C D Event F E F Source E CSE574s 17 7 B A Sink B A Sink Event F Source E C D C D F Source E 2006 Raj Jain Geographic Hash table Query key is hashed in a x y coordinate and is sent to a node nearest to that coordinate Home Location k The data is hashed and sent to its home location from where it is propagated to the sinks Uniform Storage load Redundancy can be used for home location Home Location Source Sink Washington University in St Louis CSE574s 17 8 2006 Raj Jain Data Gathering Gathering From all sensor nodes to the BS Minimize delay energy PEGASIS Power Efficient Gathering for Sensor Information Systems Each nodes combines its data in the message and sends to its nearest neighbor not visited before Starting from the farthest node Ending at the leader which passes it to the base A Token is passed backwards from the leader Washington University in St Louis CSE574s 17 9 2006 Raj Jain MAC Protocols for Sensor Networks Three types 1 Fixed Allocation Predetermined assignment 2 Demand Based Based on need 3 Contention based No delay guarantee Self Organizing MAC for Sensors SMACS Capacity Data rate Neighbors synchronize and agree on times for transmission Only neighbors synchronize Synch energy saved Sleep when not transmitting Further energy savings TDMA FDMA TDMA FDMA CSMA are also possible Bluetooth 802 11 and ZigBee are MACs used in practice Washington University in St Louis CSE574s 17 10 2006 Raj Jain Location Discovery Location Stamp on data Indoor Localization Reference nodes in each location Atomic Multi Lateration Need 3 references Iterative Multi Lateration Nodes with known location become references for others Collaborative Multi Lateration Use quadratic equations Washington University in St Louis CSE574s 17 11 2006 Raj Jain Global Positioning System GPS US Department of Defense 12B Man made stars 24 Satellites and their ground stations Triangulation Measures travel time of radio signal Distance Satellites broadcast current time and their location using a Direct Sequence Code 1023 chips per bit 3 satellites give x y z 4 satellites give x y z t Correct for any delays experienced through the atmosphere http www edu observatory org gps tutorials html Washington University in St Louis CSE574s 17 12 2006 Raj Jain Quality of a Sensor Network Quality Coverage Exposure Exposure Ability to observe a target Ability decreases with the distance from the target Coverage How well is the region covered with sensors Find the least covered path that could be followed by enemy Voronoi Diagram Cost Distance from nearest sensor Find the maximum cost path Opposite Problem Find the best covered path Washington University in St Louis CSE574s 17 13 2006 Raj Jain Sensor Standards 802 11 Bluetooth ZigBee IEEE 1451 Smart Transducer Interface for Sensors and Actuators Seven parts 1451 0 through 1451 6 dealing with different issues 1451 5 is wireless interface specifies 802 11 bluetooth and ZigBee Washington University in St Louis CSE574s 17 14 2006 Raj Jain Time Synchronization GPS not accessible inside buildings under water Send a time stamp to neighbor One way Delay Send Time Preparing the message Access Time media access propagation time receive time processing at receiver Best to timestamp the message at the PHY layer of the receiver Post Facto Synchronization Announce time along with the event Everyone else synchronizes to it Leader periodically sends sync messages which are flooded Distributed election of the leader based on a random number Resynchronization Upon merger of partitions Better to advance the clock Washington University in St Louis CSE574s 17 15 2006 Raj Jain Transport Layer Issues Reliable transmission of data from sources to sinks PSFQ Ask previous hop to retransmit if error Fetch Forward to next hops Pump Pump slowly and fetch quickly PSFQ Minimize storage maximize reliability Farthest node sends a report of delivery status to the source Intermediate nodes append their status to the same message Washington University in St Louis CSE574s 17 16 2006 Raj Jain Sensor Network Security Public key too compute intensive Localized Encryption and Authentication Protocol LEAP All nodes have an individual key shared with BS All nodes also have a group key Group key and Sensor ID Master key of the sensor Hello to neighbor using group
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