Wireless sensor networks dcs
- 1. 1 Wireless Sensor Networks Dhara Shah Outline Overview – Definition Factors influencing WSN design Features of WSN – Distinguished features from other MAN WSN Protocol Stack Data Handling Media access control schemes Architecture for a Wireless Sensor Network Applications Overview A Wireless Sensor Network (WSN) consists of base stations and a number of wireless sensors (nodes). sensor – A transducer – Converts physical phenomenon e.g. heat, light, motion, vibration, and sound into electrical signals sensor node – Basic unit in sensor network – Contains on-board sensors, processor, memory, transceiver, and power supply sensor network – Consists of a large number of sensor nodes – Nodes deployed either inside or very close to the sensed phenomenon Definition “A wireless sensor network (WSN) is a wireless network consisting of spatially distributed autonomous devices using sensors to cooperatively monitor physical or environmental conditions, such as temperature, sound, vibration, pressure, motion or pollutants, at different locations.” - Wikipedia 4
- 2. 2 Sensor Node Components 5 Sensor Node Components Sensing Unit Processing Unit Transceiver Unit Power Unit Location Finding System (optional) Power Generator (optional) Mobilizer (optional) 6 Sensor Nodes 7 Worldsens Inc. Sensor Node Crossbow Sensor Node WSN Communication Architecture 8
- 3. 3 Factors Influencing WSN Design Fault tolerance Scalability Production costs Hardware constraints Sensor network topology Environment Transmission media Power Consumption – Sensing – Communication – Data processing9 Features of WSN Requirements – Nodes with small size, large number and low cost – Small size implies small battery – Constrained by energy consumption and communication – Low cost & energy implies low power CPU – Radio with minimum bandwidth and range – Ad-hoc deployment implies no maintenance or battery replacement To increase network lifetime, no raw data is transmitted Features of WSN Ad Hoc Wireless Networks – Large number of self-organizing static or mobile nodes that are possibly randomly deployed Near(est)-neighbor communication Wireless connections – Links are fragile, possibly asymmetric – Connectivity depends on power levels and fading – Interference is high for omnidirectional antennas Features of WSN Energy is a major constraint – Nodes are battery-powered – Nobody is going to change the batteries. So, each operation brings the node closer to death. To save energy: – Sleep as much as possible. – Acquire data only if indispensable. – Use data fusion and compression. – Transmit and receive only if necessary. Receiving is just as costly as sending.
- 4. 4 Features of WSN Scalability and Reliability – Self-configure and robust to topology changes (e.g., death of a node) – Maintain connectivity: Base Station should reach all nodes – Ensure coverage Maintenance – Reprogramming is the only practical kind of maintenance. – It is highly desirable to reprogram wirelessly. Features of WSN Data Collection – Centralized data collection puts extra burden on nodes close to the base station. Clever routing can alleviate that problem – Clustering: data from groups of nodes are fused before being transmitted, so that fewer transmissions are needed – Often getting measurements from a particular area is more important than getting data from each node – Security and authenticity should be guaranteed. However, the CPUs on the sensing nodes cannot handle fancy encryption schemes. Features of WSN Power Supply – AA batteries power the vast majority of existing platforms. They dominate the node size. – Alkaline batteries offer a high energy density at a cheap price. The discharge curve is far from flat, though. – Lithium coin cells are more compact and boast a flat discharge curve. Rechargeable batteries: Who does the recharging? – Solar cells are an option for some applications. – Fuel cells may be an alternative in the future. – Energy scavenging techniques are a hot research topic Features of WSN Radio – Commercially-available chips – Available bands: 433 and 916MHz, 2.4GHz ISM bands – Typical transmit power: 0dBm. – Narrowband (FSK) or Spread Spectrum communication. DS-SS (e.g., ZigBee) or FH-SS (e.g., Bluetooth) – Relatively low rates (<100 kbps) save power – Sensitivity: as low as -110dBm
- 5. 5 Features of WSN CPU – The Microcontroller Unit (MCU) is the primary choice for in-node processing. – Power consumption is the key metric in MCU selection. – The MCU should be able to sleep whenever possible, like the radio. – Memory requirements depend on the application – ATmega128L and MSP430 are popular choices Sensors – The power efficiency of the sensors is also crucial, as well as their duty cycle. – MEMS techniques allow miniaturization. Distinguishing features of WSN WSNs are ad hoc networks – wireless nodes that self-organize into an infrastructure less network BUT, in contrast to other ad hoc networks: Sensing and data processing are essential WSNs have many more nodes and are more densely deployed Hardware must be cheap; nodes are more prone to failures WSNs operate under very strict energy constraints WSN nodes are typically static The communication scheme is many-to-one (data collected at a base station) rather than peer-to-peer WSN Protocol Stack 19 WSN Protocol Stack Sensor management protocol – Provides software operations needed to perform administrative tasks e.g. moving sensor nodes, turning them on an off Sensor query and data dissemination protocol – Provides user applications with interfaces to issue queries and respond to queries – Sensor query and tasking language (SQTL) Directed diffusion Sensor MAC (S-MAC) IEEE 802.15.4 20
- 6. 6 Data Handling : Data-Centric Routing Interest distribution is performed to assign sensing tasks to sensor nodes – Sinks broadcast the interest – Sensor nodes broadcast an advertisement for available data Requires attribute-based naming – Users are more interested in querying the attribute of the phenomenon, rather than querying an individual node – E.g. the sensor nodes in the area where temperature is greater than 75 °F 21 Data handling: Data Aggregation Data coming from multiple sensor nodes are aggregated if they are about the same attribute of the phenomenon when they reach the same routing node on the way back to the sink – Solves implosion and overlap problem – Energy efficient 22 Data Handling: Data-dissemination Conventional Methods Direct communication with the base station – Sensor nodes communicate with the base station directly. – Energy consuming. Multi-hop Scheme – Transmit through some other intermediate nodes. – Energy consuming. Low Energy Adaptive Clustering Hierarchy (LEACH) Designed for sensor networks where an end-user wants to remotely monitor the environment. Where the data from the individual nodes must be sent to a central base station, often located far from the sensor network. Desirable properties for protocols on these networks: – Use 100’s – 1000’s of nodes – Maximize system lifetime – Maximize network coverage – Use uniform, battery operated nodes The use of distributed cluster formation and local processing – Reduce global communication along with randomized rotation – Cluster-heads allows LEACH to achieve the desired properties while being energy-efficient. Data-dissemination Schemes (Cont.)
- 7. 7 Clustering Hierarchy Data-dissemination Schemes (Cont.) Media Access Control (MAC) Scheme Two categories of MAC schemes for wireless networks Contention-based schemes – Designed for minimum delay and maximum throughput. – Require transceivers to monitor the channel at all times. Reservation-based or schedule-based schemes – Detect the neighboring radios before allocating collision-free channel to a link. – TDMA —a natural choice for sensor networks. MAC Scheme (Cont.) TDMA-based solutions The self-organizing “super frame” algorithm – Super frame = TDMA period + BOOTUP period + unused bandwidth – Performs well only under the specific conditions. Power Aware Clustered TDMA (PACT) – Divide the TDMA structure into control slot and data slot. – Hard to maintain the cluster when there are mobile nodes. MAC Scheme (Cont.) Other solutions Sensor-MAC (SMAC) – Nodes periodically sleep. – A node sleeps during transmission period of other nodes. – Not suitable for time-critical applications.
- 8. 8 Architecture for a WSN Special addressing requirement Local unique addresses Data-centric Example: Each node has an unique number. Attribute-based naming architecture Data is named by one or more attributes. Example: Each node is distinguished by an attribute – GPS sensors are practical for this. Architecture for a WSN (cont.) An address-free architecture (AFA) Advantage – Randomly select probabilistically unique identifier for each transaction. – Spatial locality. – Temporal locality. Drawback – Not applicable when static addressing of nodes is needed. – Identifier conflict. WSN Operating Systems TinyOS Contiki MANTIS BTnut SOS Nano-RK 31 Overview (Cont.) Wireless Sensor Node: Components
- 9. 9 Applications Applications of Wireless Sensor Networks Military and national security application Environment monitoring (examples coming) Medical application (example coming) Nearly anything you can imagine Environment monitoring (1) Great Duck Island • 150 sensing nodes deployed throughout the island relay data temperature, pressure, and humidity to a central device. • Data was made available on the Internet through a satellite link. Environment monitoring (2) Zebranet: a WSN to study the behavior of zebras • Special GPS-equipped collars were attached to zebras • Data exchanged with peer-to-peer info swaps • Coming across a few zebras gives access to the data Medical application Intel deployed a 130-node network to monitor the activity of residents in an elder care facility. Patient data is acquired with wearable sensing nodes (the “watch”) • Vital sign monitoring • Accident recognition • Monitoring the elderly
- 10. 10 Final Remarks Can you think of any applications where a wireless sensor network would be the best solution? Do you foresee wireless sensor networks becoming ubiquitous within the next ten years? During your lifetime?