20-1©2008 Raj JainCSE574sWashington University in St. LouisWireless Sensor Wireless Sensor NetworksNetworksRaj Jain Washington University in Saint LouisSaint Louis, MO [email protected]/Video recordings of this lecture are available at:http://www.cse.wustl.edu/~jain/cse574-08/20-2©2008 Raj JainCSE574sWashington University in St. LouisOverviewOverview! 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 Communication20-3©2008 Raj JainCSE574sWashington University in St. LouisSensor ApplicationsSensor ApplicationsBattlefield Surveillance Chemical, Biological WeaponsHabitat exploration of animals Patient heart rate, blood pressureCrops and AgricultureForest Fires and Flood Detection20-4©2008 Raj JainCSE574sWashington University in St. LouisSensor (vs. AdSensor (vs. Ad--Hoc)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 aggregation20-5©2008 Raj JainCSE574sWashington University in St. LouisSensor Network ArchitectureSensor Network Architecture1. Layered: Base station, one-hop layer, 2-hop layer, ...2. Clustered: Nodes elect and communicate through cluster heads20-6©2008 Raj JainCSE574sWashington University in St. LouisData DisseminationData 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 neighborSinksSources20-7©2008 Raj JainCSE574sWashington University in St. LouisDirected DiffusionDirected 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 pathACDFEBEventSinkACDFEBEventSinkSourceACDFEBSinkSourceACDFEBSinkSource20-8©2008 Raj JainCSE574sWashington University in St. LouisGeographic Hash tableGeographic 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 locationSinkSourceHomeLocation20-9©2008 Raj JainCSE574sWashington University in St. LouisData GatheringData 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 leader20-10©2008 Raj JainCSE574sWashington University in St. LouisMAC Protocols for Sensor NetworksMAC Protocols for Sensor NetworksThree types:1. Fixed Allocation: Predetermined assignment2. Demand Based: Based on need3. 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 practice20-11©2008 Raj JainCSE574sWashington University in St. LouisLocation DiscoveryLocation 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 equations20-12©2008 Raj JainCSE574sWashington University in St. LouisGlobal Positioning System (GPS)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.html20-13©2008 Raj JainCSE574sWashington University in St. LouisQuality of a Sensor NetworkQuality 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 sensorsFind the least covered path that could be followed by enemy! Voronoi Diagram: Cost = Distance from nearest sensorFind the maximum cost path. ! Opposite Problem: Find the best covered path20-14©2008 Raj JainCSE574sWashington University in St. LouisSensor StandardsSensor 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 ZigBee20-15©2008 Raj JainCSE574sWashington University in St. LouisTime SynchronizationTime 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 clock20-16©2008 Raj JainCSE574sWashington University in St. LouisTransport Layer IssuesTransport Layer Issues! Reliable transmission of data from sources to sinksPSFQ:! Ask previous hop to
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