WIRELESS SENSOR NETWORKS BY VENKAT KANCHERLA VIJAY CHAND

WIRELESS SENSOR NETWORKS BY VENKAT KANCHERLA VIJAY CHAND UYYURU

Introduction n n A sensor network consists of a large number densely populated sensors acting as nodes in the network. A sensor network is composed of a large number of sensor nodes, which are densely deployed either inside the phenomenon or very close to it. Sensor data is shared between the sensors and used as input to a distributed estimation system which aims to extract as much relevant information from the available sensor data The fundamental objectives for sensor networks are reliability, accuracy, flexibility, cost effectiveness and ease of deployment.

Introduction (contd…) n n A sensor network is made up of individual multifunctional sensor nodes The sensor node itself may be composed of various elements such as various multi-mode sensing hardware (acoustic, seismic, infrared, magnetic, chemical, imagers, microradars), embedded processor, memory, powersupply, communications device (wireless and/or wired) and location determination capabilities (through local or global techniques).

Wireless sensor network device designed to be the approximate size of a quarter.

Adhoc networks n n Ad hoc networks are a new paradigm of wireless communication for mobile hosts (which we call nodes). In an ad hoc network, there is no fixed infrastructure such as base stations or mobile switching centers. Mobile nodes that are within each other’s radio range communicate directly via wireless links, while those that are far apart rely on other nodes to relay messages as routers. Node mobility in an ad hoc network causes frequent changes of the network topology. Figure 1 shows such an example: initially, nodes A and D have a direct link between them. When D moves out of A’s radio range, the link is broken. However, the network is still connected, because A can reach D through C, E, and F.

Figure 1

Sensor networks VS ad hoc networks: n n n n The number of nodes in a sensor network 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 Sensor nodes mainly use broadcast, most ad hoc networks are based on p 2 p. Sensor nodes are limited in power, computational capacities and memory. Sensor nodes may not have global ID.

Wireless sensor have many application areas including Military, n Environmental, n Health, n Home, n Disaster relief, n Space exploration, n Chemical processing, n Other commercial n

Military Applications n n n n An integrated part of C 4 ISRT (command, control, communications, computer, surveillance, reconnaissance, targeting) systems: Enhanced logistics systems to monitor friendly forces, equipment and ammunition. Enhanced surveillance systems to detect intruders, chemical or biological attacks, underwater targets, firing guns and their locations. Enhanced reconnaissance systems that can run in inaccessible or contaminated terrains and beyond the enemy lines. Enhanced targeting and target tracking systems. Battle damage assessment systems. Enhanced guidance and navigation systems.

Civilian Applications: n n n n n Habitat monitoring. Environmental monitoring. Patient and elderly monitoring. Flood and forest fire detection. Disaster relief operations management. Traffic management. Smart office, home and car systems. Space exploration. Collaborative systems.

Health applications Telemonitoring of human physiological data n Tracking and monitoring patients and doctors inside a hospital n Drug administration in hospitals n

Factors influencing sensor network design Fault tolerance n Some sensor nodes may fail or be blocked due to lack of power, or have physical damage or environmental interference. The failure of sensor nodes should not affect the overall task of the sensor network. This is the reliability or fault tolerance issue. n Fault tolerance is the ability to sustain sensor network functionalities without any interruption due to sensor node failures. n The reliability Rk(t) or fault tolerance of a sensor node is modeled in using the Poisson distribution to capture the probability of not having a failure within the time interval (0, t): Rk(t) = e – λk t where λk is the failure rate of sensor node k and t is the time period.

Factors influencing sensor network design Production costs n The cost of a single node is very important to justify the overall cost of the networks. n The cost of a sensor node is a very challenging issue given the amount of functionalities with a price of much less than a dollar.

Factors influencing sensor network design Scalability n The number of sensor nodes deployed in studying a phenomenon may be on the order of hundreds or thousands. n Some times depending on the application we may increase the number of nodes, New schemes must be able to work with this number of nodes. They must also utilize the high density of the sensor networks. n Scalability measures the density of the sensor nodes. n Density = µ(R) =(N π R 2)/A where N is the number of scattered sensor nodes in region A, and R is the radio transmission range. Basically, µ(R) gives the number of nodes within the transmission radius of each node in region A.

Factors influencing sensor network design Hardware constraints

Hardware components n n A sensor node is made up of four basic components, a sensing unit, a processing unit, a transceiver unit, and a power unit. additional application-dependent components such as a location finding system, power generator, and mobilizer. sensors and analog-to-digital converters (ADCs): The analog signals produced by the sensors based on the observed phenomenon are converted to digital signals by the ADC, and then fed into the processing unit. The processing unit, which is generally associated with a small storage unit, manages the procedures that make the sensor node collaborate with the other nodes to carry out the assigned sensing tasks.

Hardware components n n A transceiver unit connects the node to the network. Power units may be supported by power scavenging units such as solar cells. Most of the sensor network routing techniques and sensing tasks require knowledge of location with high accuracy. Thus, it is common that a sensor node has a location finding system. A mobilizer may sometimes be needed to move sensor nodes when it is required to carry out the assigned tasks.

Hardware constraints n All of these subunits may need to fit into a matchbox-sized module. n These nodes must consume extremely low power. n Operate in high volumetric densities have low production cost, be dispensable and autonomous, operate unattended, and be adaptive to the environment.

Factors influencing sensor network design Sensor network topology : n Deploying a high number of nodes densely requires careful handling of topology maintenance n Pre-deployment and deployment phase : Sensor nodes can be either thrown in as a mass or placed one by one in the sensor field. They can be deployed by dropping from a plane, delivered in an artillery shell, rocket, or missile, and placed one by either a human or a robot. n Post-deployment phase : After deployment, topology changes are due to change in sensor nodes position, reachability (due to jamming, noise, moving obstacles, etc. ), available energy, malfunctioning, and task details. n Re-deployment of additional nodes phase : Additional sensor nodes can be redeployed at any time to replace malfunctioning nodes or due to changes in task dynamics.

Factors influencing sensor network design Environment n Sensor nodes are densely deployed either very close or directly inside the phenomenon to be observed. They usually work unattended in remote geographic areas : n Interior of a large machinery n Bottom of an ocean n Inside a twister n Surface of an ocean during a tornado n Biologically or chemically contaminated field n Battlefield beyond the enemy lines n Home or a large building n Large warehouse n Fast moving vehicles n Drain or river moving with current.

Factors influencing sensor network design Transmission media: In a multihop sensor network, communicating nodes are linked by a wireless medium. To enable global operation, the chosen transmission medium must be available worldwide. n Radio : The Wireless Integrated Network Sensors (WINS) architecture uses radio links for communication. n infrared : Infrared communication is license-free and robust to interference from electrical devices. Infrared-based transceivers are cheaper and easier to build n optical media : Another interesting development is that of the Smart Dust mote, which is an autonomous sensing, computing, and communication system that uses the optical medium for transmission.

Factors influencing sensor network design Power consumption: n The wireless sensor node, being a microelectronic device, can only be equipped with a limited power source (< 0. 5 Ah, 1. 2 V) n In a multihop ad hoc sensor network, each node plays the dual role of data originator and data router. n The main task of a sensor node in a sensor field is to detect events, perform quick local data processing, and then transmit the data. Power consumption can hence be divided into three domains: n Sensing n Communication n Data processing

Environmental monitoring n n n Redwood trees are so large that entire ecosystems exist within their physical envelope. Climatic factors determine the rate of photosynthesis, water and nutrient transport, and growth patterns. microclimatic structure varies over regions of the forest. water transport rates and the scale of respiration may influence the microclimate around a tree, effectively creating its own weather All these factors influence the habitat dynamics of species existing in and on the tree.

Wireless sensor node for environment monitoring

Wireless sensor node for environment monitoring n On top, two incident-light sensors measure total solar radiation, specifically light and photosynthetically active radiation, the bands at which chlorophyll are sensitive. n An identical pair of sensors on the bottom, to monitor relative humidity, barometric pressure, and temperature n Contains a small computer, data storage, battery, and low-power radio to collect data, process it, and route information among the nodes and to the outside world

WSN climate data

Results n The measurements show that within the expected daily cycle, the top of the tree experiences much wider climatic variation than the forest floor. n These weather fronts create powerful temperature and moisture gradients that could be instrumental in understanding growth dynamics, water intake, and nutrient transport over such large structures, yet they cannot be observed with sparse instrumentation.

Communication architecture of sensor networks n The sensor nodes are usually scattered in a sensor field as shown below:

Sensor communication n n Each of these scattered sensor nodes has the capabilities to collect data and route data back to the sink. Data are routed back to the sink by a multi-hop infrastructure less architecture through the sink. The sink may communicate with the task manager node via Internet or satellite. The design of the sensor network is influenced by many factors, including fault tolerance, scalability, production costs, operating environment, sensor network topology, hardware constraints, transmission media, and power consumption.

Data in sensor networks n n n Sensor networks are predominantly data-centric rather than address-centric. Given the similarity in the data obtained by sensors in a dense cluster, aggregation of the data is performed locally. Summary or analysis is done by aggregator node to reduce the bandwidth requirement. A network hierarchy and clustering of sensor nodes allows for network scalability, robustness, efficient resource utilization and lower power consumption.

Data in sensor networks (contd. . ) n n n Dissemination of sensor data in an efficient manner requires the dedicated routing protocols to identify shortest paths. Redundancy must be accounted for to avoid congestion resulting from different nodes sending and receiving the same information. We need redundancy to ensure reliability. Data dissemination may be either query driven or based on continuous updates. The operation of a sensor network includes a variety of information processing techniques for the manipulation and analysis of sensor data, extraction of significant features, along with the efficient storage and transmission of the important information.

Communication architecture of sensor networks n The protocol stack used by sink and all sensor nodes is as shown below:

Communication architecture of sensor networks Application layer An application layer management protocol makes the hardware and software of the lower layers transparent to the sensor network management applications. n Sensor management protocol (SMP) n Task assignment and data advertisement protocol (TADAP) n Sensor query and data dissemination protocol (SQDDP)

Communication architecture of sensor networks Transport layer n This layer is especially needed when the system is planned to be accessed through Internet or other external networks. n No attempt thus far to propose a scheme or to discuss the issues related to the transport layer of a sensor network in literature. n The communication between user and sink is TCP/UDP via the Internet or Satellite. n The communication between the sink and sensor nodes may be purely by UDP protocols.

Communication architecture of sensor networks Network layer n Power efficiency is always an important consideration. n Sensor networks are mostly data centric. n Data aggregation is useful only when it does not hinder the collaborative effort of the sensor nodes. n An ideal sensor network has attribute-based addressing and location awareness.

Communication architecture of sensor networks • Maximum available power (PA) route • Minimum energy (ME) route • Minimum hop (MH) route • Maximum minimum PA node route

Communication architecture of sensor networks n n Maximum available power (PA) route: The route that has maximum total available power is preferred. (route 4) Minimum energy (ME) route: The route that consumes ME to transmit the data packets between the sink and the sensor node is the ME route. (route 1) Minimum hop (MH) route: The route that makes the MH to reach the sink is preferred. (route 3) Maximum minimum PA node route: The route along which the minimum PA of the other routes is preferred. (route 3)

Communication architecture of sensor networks n n n Data Aggregation: It can be perceived as a set of automated methods of combining the data that comes from many sensor nodes into a set of meaningful information. It is also known as Data Fusion. It is a technique used to solve the implosion and overlap problems in data-centric routing. In this technique, a sensor network is usually perceived as a reverse multicast tree as shown in the fig. where the sink asks the sensor nodes to report the ambient condition of the phenomena.

Communication architecture of sensor networks Data aggregation

Communication architecture of sensor networks Data link layer The data link layer is responsible for the medium access and error control. It ensures reliable point -to-point and point-to-multipoint connections in a communication network.

Communication architecture of sensor networks Medium access control n Creation of the network infrastructure n Fairly and efficiently share communication resources between sensor nodes

Communication architecture of sensor networks Power saving modes of operation Operation in a power saving mode is energy efficient only if the time spent in that mode is greater than a certain threshold.

Communication architecture of sensor networks Error control n Forward Error Correction (FEC) n Automatic Repeat Request (ARQ). Simple error control codes with low-complexity encoding and decoding might present the best solutions for sensor networks.

Communication architecture of sensor networks Physical layer The physical layer is responsible for frequency selection, frequency generation, signal detection, modulation and data encryption.

Communication architecture of sensor networks n n n Power management: This plane manages how a sensor node uses its power. Mobility management: This plane detects and registers the movement of sensor nodes, so a route back to the user is always maintained and the sensor nodes can keep track of who are their neighbor sensor nodes. Task management: This plane is needed, so that sensor network nodes can work together in a power efficient way , route data in a mobile sensor network, and share resources between sensor nodes.

Conclusion The flexibility, fault tolerance, high sensing fidelity, low-cost and rapid deployment characteristics of sensor networks create many new and exciting application areas for remote sensing. n In the future, this wide range of application areas will make sensor networks an integral part of our lives. n

Questions? 1. 2. 3. What are factors influencing sensor network design? What are different components of a sensor node? What is data aggregation?

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