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Wireless Sensor Networks(WSNs)

Wireless Sensor Networks(WSNs). Topics. Wireless Sensor Networks (WSNs) Research topics Networking sensors in WSNs Coverage of sensor networks Location service Sensor databases. Wireless Sensor Networks (WSNs). What is a sensor?

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Wireless Sensor Networks(WSNs)

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  1. Wireless SensorNetworks(WSNs)

  2. Topics • Wireless Sensor Networks (WSNs) • Research topics • Networking sensors in WSNs • Coverage of sensor networks • Location service • Sensor databases

  3. Wireless Sensor Networks (WSNs) • What is a sensor? • Light, noise, acoustic, temperature ,pressure, humidity sensors • Magnetometers, accelerometers • biosensors Berkeley Motes(WeC) light, temperature, 10 kbps @ 20m

  4. What is a Sensor Node? • Motes (UC Berkeley) : • 8-bit CPU at 4MHz, • 8KB flash, 512B RAM • Sensor is a tiny electronic devices with four major components • Sensing (外接sensor) • Communication • Processing • Power

  5. BASIC MOTE-KITS • 4 MICA2 Processor/Radio Boards • 4 MICA2DOT Quarter-Sized Processor Radio Boards • 3 MTS310 Sensor Boards (Acceleration, Magnetic, Light, Temperature, Acoustic, and Sounder) • 2 MTS510 Sensor Boards (Acceleration, Light, Microphone) • 2 MDA500, MICA2DOT Prototype and Data Acquisition Boards • 1 MIB510 Programming and Serial Interface Board . • MICAz • 802.15.4/ZigBee compliant Motes, sensor and data acquisition boards, and two different gateway/interface boards.

  6. What is a WSN? • WSN is a multi-hop wireless network consisting of a large number of small, low-cost, low-power sensor nodes to perform intended monitoring functions in the target area.

  7. Wireless Sensor Network Base station (Sink, Gateway) Internet Cooperation Extended coverage Fault tolerance Extended lifetime User Sensor Field Sensor Nodes

  8. (1324,1245) Data Wakeup Line of Bearing (LOB) Fusion center

  9. Limitations of a Sensor Node • Modest processing power(4MHs) • Little storage(512byte) • Short communication range (p  d4) • Limited power source (< 1.2v, < 0.5Ah)

  10. Sensor Networks v.s. MANETs • The number of sensor nodes in WSNs can be several orders of magnitude higher than the nodes in an ad hoc network • Sensor nodes are densely deployed • Sensor nodes are prone to failures • The topology of a sensor network changes very frequently.

  11. Sensor Networks v.s. MANETs • Sensor nodes mainly use broadcast communication paradigm whereas most ad hoc networks are based on point-to-point communications • Sensor modes are limited in power, computational capacities, and memory • Sensor nodes may not have global identification (ID)

  12. Applications • Military application • Monitoring friendly forces, equipment and ammunition • Battlefield surveillance • Targeting • Battle damage assessment • Nuclear, biological and chemical attack detection and reconnaissance(偵察) • C4ISRT systems: military command, control, communications, computing, intelligence, surveillance(監視), reconnaissance(偵察) and targeting

  13. Applications (cont.) • Environment application‧Forest fire detection‧Biocomplexity mapping of the environment‧Flood detection‧Precision agriculture • Health application‧Telemonitoring of human physiological(生理上的) data‧Tracking and monitoring doctors and patients inside a hospital‧Drug administration in hospital

  14. Applications (cont.) • Context-aware computing‧intelligent home, smart environment • Other commercial application • tracking chemical plumes(羽狀煙）: Ad Hoc, Just-in-time deployment for mitigating disasters (Berkeley team)

  15. A chemical gas leak has been detected • Need to get a real-time assessment of extent and movement of the gas and inform the evacuation • 3 UAVs (unmanned aerial vehicle) are immediately launched, each with 1000 chemical sensing nodes • Upon flying over the attack site, sensor nodes are released • Nodes self-organize into an ad hoc network, and relay tracking result back to emergency response command center.

  16. Design Issues of WSNs • Fault tolerance • Scalability • Production costs • Hardware constraints • Topology • Environment • Transmission media • Power consumption

  17. Fault Tolerance & Scalability • The failure of sensor nodes should not affect the overall task of the sensor network. • The system should be scalable enough to work with large number of nodes.

  18. Production Cost • Cost of individual node plays an important role. • Sensor nodes are densely deployed. Nodes must be cheap! • Use Bluetooth RF, US$10, should be less than US $1.

  19. Hardware Constraints Location Finding System Mobilizer Processing Unit Processor Transceiver ADC Sensor Storage Power Unit Power Generator

  20. The Mote Family Ref: from Levis & Culler, ASPLOS 2002

  21. Topology • Topology should be carefully maintained. • Three phases:‧Pre-deployment and deployment phase: Sensor nodes are either thrown in mass or placed one by one in the sensor field.‧Post-deployment phase: After deployment, topology changes due to the change in sensor nodes’ position or reach ability or failure.‧Re-deployment of additional nodes phase: Additional sensor nodes can be re-deployed at any time to replace the malfunctioning nodes or due to changes in task dynamics.

  22. Transmission Media • In a multi-hop sensor network, communicating nodes are linked by a wireless medium.‧Radio Frequency (FR) Do not need Line of Sight.WINS , PAMS ‧Infrared (IR) License-free and robust to interference form electrical device.‧Optical media Require line of sight (smart dust mote)

  23. Power Consumption • The wireless sensor node, being a micro-electronic device, can only be equipped with a limited power source (<0.5 Ah, 1.2V). • Two major power consumption‧Communication A sensor node expends maximum energy in data communication. ( transmit > receive)‧Data processing

  24. Identifying the Energy Consumers • Need to shutdown the radio From Tsiatis et al. 2002 TX RX IDLE CPU SLEEP SENSORS RADIO

  25. Research Issues • Localization and tracking • Time synchronization • Networking sensors (MAC, Network) • Topology control (network coverage) • WSN security • Sensor network databases

  26. .Data-centric paradigm: The operating paradigm of WSNs is centered around information retrieval from the underlying network, usually referred to as a data-centric paradigm. • Compared to the address-centric paradigm exhibited by traditional networks, the data-centric paradigm is unique in several ways. • New communication patterns resemble a reversed multicast tree. • In-network processing extracts information from raw data and removes redundancy among multiple source data. • Cooperative strategies among sensor nodes are used to replace the non-cooperative strategies for most Internet applications. • The development of appropriate routing strategies that take the above factors into consideration is challenging.

  27. 2). Collaborative information processing and routing: The data-centric paradigm involves two fundamental operations in WSNs: information processing and information routing. • Many research efforts are motivated by the fact that information processing and routing are mutually beneficial. While information processing helps reduce the data volume to be routed, information routing facilitates joint information compression (or data aggregation) by bringing together data from multiple sources. • It is often non-trivial to model and analyze the inter-relationship between information processing and routing. In many situations, the problem of finding a routing scheme in conjunction with joint compression for energy minimization turns out to be NP-hard.

  28. 3). Energy-efficient design: Once deployed, it is often infeasible or un-desirable to recharge sensor nodes or replace their batteries. • Energy conservation becomes crucial for sustaining a sufficiently long network lifetime. Among the various techniques proposed for improving energy-efficiency, cross-layer optimization has been realized as an effective approach. • Due to the nature of wireless communication, one performance metric of the network can be affected by various factors across layers. • Hence, a holistic approach that simultaneously considers the optimization at multiple layers enables a larger design space within which cross-layer tradeoffs can be effectively explored.

  29. 4). Network discovery and organization • Localization • Time synchronization • Deployment of sensor • Coverage

  30. 5). Security: • Since WSNs may operate in a hostile environment, security is crucial to ensure the integrity and confidentiality of sensitive information. To do so, the network needs to be well protected from intrusion and spoofing. • The constrained computation and communication capability of sensor nodes make it unsuitable to use conventional encryption techniques. Lightweight and application-specific architectures are preferred instead.

  31. Networking WSNs • Power limitation of a sensor node plays an important role. • The sensor MAC protocol---S-MAC IEEE/ACM transactions on networking, Vol. 12, No. 3, June 2004

  32. Common to all wireless networks Energy Efficiency in MAC • Major sources of energy waste • Idle listening • Long idle time when no sensing event happens • Collisions • Control overhead • Overhearing • Try to reduce energy consumption from all above sources • TDMA requires slot allocation and time synchronization • Combine benefits of TDMA + contention protocols

  33. Latency Fairness Energy Sensor-MAC (S-MAC) Design(Wei et al. 2002) • Tradeoffs • Major components of S-MAC • Periodic listen and sleep • Collision avoidance • Overhearing avoidance • Message passing

  34. sleep listen listen sleep Periodic Listen and Sleep • Problem: Idle listening consumes significant energy • Nodes do not sleep in IEEE 802.11 ad hoc mode • Solution: Periodic listen and sleep • Turn off radio when sleeping • Reduce duty cycle to ~10% (200 ms on/2s off) • Increased latency for reduced energy

  35. Node 1 sleep sleep listen listen Node 2 sleep sleep listen listen Schedule 1 Schedule 2 Periodic Listen and Sleep • Schedules can differ • Preferable if neighboring nodes have same schedule • — easy broadcast & low control overhead Border nodes: two schedules broadcast twice

  36. Periodic Listen and Sleep • Schedule maintenance • Remember neighbors’ schedules — to know when to send to them • Each node broadcasts its schedule every few periods (Sync packet, saying when it will enter sleep, relative to the Sync) • Refresh on neighbor’s schedule when receiving an update • Schedule packets also serve as beacons for new nodes to join a neighborhood

  37. Collision Avoidance • Problem: Multiple senders want to talk • Options: Contention vs. TDMA • Solution: Similar to IEEE 802.11 ad hoc mode (DCF)實際上只用RTS, CTS • Physical and virtual carrier sense • Randomized backoff time • RTS/CTS for hidden terminal problem • RTS/CTS/DATA/ACK sequence

  38. Overhearing Avoidance • Problem: Receive packets destined to others • Solutions: see the paper

  39. Message Passing • Problem: In-network processing requires entire message • Solution: Don’t interleave different messages • Long message is fragmented & sent in burst • RTS/CTS reserve medium for entire message • If a fragment lost, re-transmit it.(802.11 will abort the whole message) • Other nodes sleep for whole message time

  40. Routing • Given a topology, how to route data? • MANET: Reactive[DSR], proactive[AODV], TORA, GPSR[KarpKung00] • Address Centric • Distinct paths from each source to sink. • Usually has address concept • Data Centric Routing

  41. Advantages • Communication overhead for binding is minimized • In-network processing is enabled because the content moving through the network is identifiable by intermediate nodes. This allows further energy saving through data aggregation and compression.

  42. Data Centric Routing • Basic idea • name data (not nodes) with externally relevant attributes • Data type, time, location of node, SNR, etc • diffuse requests and responses across network using application driven routing • Data sources publish data, Data clients subscribe to data

  43. Routing of WSNs • Most of previous work focuses data centric routing

  44. Data gathering/Routing schemes

  45. Flooding • Each node receiving a data or management packet repeats it by broadcasting, unless a maximum number of hops for the packet is reached or the destination of the packet is the node itself. • It does not require costly topology maintenance and complex route discovery algorithms.

  46. Disadvantages • Implosion: duplicated messages are sent to the same node • Overlap: two nodes share the same observation region, both send the sensed results, neighbor nodes receives duplicated messages • Resource blindness: does not take into account the available energy resource.

  47. Gossiping • Derived from flooding by randomly selects another sensor node to send the data. • Disadvantages: it takes a long time to propagate the message to all sensor nodes.

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