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PHD THESIS ON
WIRELESS SENSOR NETWORKS
Sensor Network (Snet) is a distributed system (Dsis) that makes a field sensors of a different type of interconnected communication network. Sensor output data are divisible and are input to the Dsis input for their estimation. The Dsis task is to extract the most likely information on the phenomenon being monitored based on available sensor data. The main operational and economic characteristics of Snet are: • high reliability in operation; • relatively high accuracy; • flexibility; • low price; • easily allocate sensors in space. Snet is formed from individual multifunctional sensor nodes (Sensor NodSnod). In most cases, Snods connect wirelessly to communication thus forming a wireless sensor network (WSN). The WSN consists of batterypowered modules that are essentially Snods. The building blocks of these modules are: • sensor: data generator • radio primo-transmitter: handing over or transmitting through the network data received from its neighbors (routing data) • one or more processors: control the operation of sensors and primers, process data, and implement network and routing protocols. In most cases, Sets are implemented as data-centric, not as address centric systems. This means that the queries are directed to the region made by a topologically arranged group of clusters rather than a specific sensor address. Within a single cluster, there is an aggregator node that collects data from Snod's associated cluster, analyzes, aggregates, and after the surrender. Basically, the aggregated analysis of local data is performed by a cluster aggregator within a cluster. This greatly reduces requirements for communication bandwidth. Data aggregation increases the level of accuracy, and at the same time incorporates redundancy-data, which compensates for failures in nodes. Bearing in mind that the sensor modules are battery-powered devices, and that the available battery power is limited, the energy efficiency of the module has a direct impact on the life of the sensor. When the module stops working, its data collection not only stops, but the network loses the availability of the module to further forward (rout) data. Because of the aforementioned, energy efficiency has a direct impact on how long, not only individual sensors, but also the entire network will function successfully. It is therefore of great importance to look at the problem of energy efficiency from the point of view of all the details regarding the design of the module and the whole network. Analyzing the work of Snet, we note that a number of information processing techniques are used to: Manipulate and analyze sensor data; Extraction of important traits; the ambient that is being monitored; Efficient storage and transfer of important information The protocols and algorithms proposed for traditional wireless ad-hoc networks do not meet all the requirements set by Snet's. The specifics of Snet's characteristics are as follows: The number of Snodes is much higher than the number of nodes in an ad-hoc network; Snods are densely distributed; Snodes use a broadcast communication paradigm in relation to ad-hoc nodes using point-to-point communications; Snodes do not have global identification because of the large number of sensors. There are two types of Snods that vary widely in the type of Snets that are
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PHD THESIS ON WIRELESS SENSOR NETWORKS installed: 1. Proactive Networks: Nodes in the network periodically include sensors, measures the size of the environment, and deliver data of interest. 2. Reactive Networks: Are networks where nodes are awake all the time react to sudden changes in the network. These types of nodes are suitable for real-time system applications. Snods are resolutely distributed in the sensor field. Every Snod is able to Gathers data and directs them to the destination. The smart sensory node structure (smart sensor node) consists of the following four basic building blocks: sensor unit (sensing unit); transceiver unit (transceiver unit); process unit; power unit. Snod's optional components are: sensor location sensor unit - such as a GPS receiver; a mobilizer-block to launch Snod, is used when the sensor needs to be mobile; an energy generator - a power conversion unit, for example, a solar battery. The sensor accepts the measured size at the input and converts it into an electrical signal. After conditioning, the signal is input to the ADC input, so it is accepted by the processor after the conversion is performed. The processor, over data, performs some type of processing signal, and, depending on how it is programmed, transmits the resulting information to the network with the help of the transceiver. A power supply unit may be a battery unit. PS combines: awareness of power and routing awareness, integrity of data with network protocol, and energy efficiency of communication via wireless media. The physical level is in charge of implementing modulation techniques, as well as transmitting and receiving signals. The link level is responsible for reducing collisions when data is being transmitted. Network-level is in charge of routing. The transport level takes care of the delivery of data from one end user to another. The application level takes into account the provision of services. The levels of energy, mobility and tasks control the consumption, movement and tasks between the sensor nodes. These levels help the sensor node to coordinate the sensor task and reduce energy consumption. The physical level is in charge of: frequency selection; signal sign (signal detection); data protection; propagation effects; modulation scheme; power efficiency. In Snet, the most frequently used frequency for transmitting signals belongs to the RF 915 MHz ISM band. Signal detection is based on the following principle: Based on the set of values {xj} that are monitored, the {hypothesis} is true. The target broadcasting frequency that may be present or not depends on the parameter estimation {fk} of {xj}, i.e. selected Fourier transform coefficients related to the wavelength. When it comes to the effects of signal propagation, the identification of the spatial separation of Snods must be achieved by the thick distribution of nodes. Snoods have built-in small antennas, and are arranged on an uneven surface. The higher the density, the nodes are closer to one another, and the greater the probability of achieving a good connection at a small distance, as well as the elimination of other negative effects of signal propagation. Using the simplest Path Loss Model, signal losses can be expressed as (1 / d) n where n is near 4. The choice of the modulation scheme is critical from the aspect of energy efficiency. M-array modulations (within a single symbol, a larger number of bits are transmitted) reduces the amount of power required to deliver the information, but then the electronic delivery / reception circuitry (Tx / Rx) is too complex and consumes a lot of energy. In binary modulation schemes such as BPSK, FSK and ASK (transmitting is ON / OFF), there is a better power balance on the transfer side and the power consumed by the Tx / Rx circuit. Start-up time has a major impact on average energy per bit (Eb). When short-packed packets are used during transmission, then the total impact is greater than the start-up energy (includes header transmission), and not the one consumed for transfer useful information. In other words, Eb is very dependent on the scope of the package. The link level is responsible for: multiplexing data streams; framing; physical addressing; flow control; error control; access control; the medium access control layer is a very important aspect of the transmission because the wireless medium is a shared medium. Namely, having in mind that radios from a large number of transmitters are performed on the same frequency, it is inevitable that their interference will occur. This is, in the end, the cause of the collision. The role of the MAC is to identify when and how each node can be transmitted over a wireless communication channel. The primary MAC attributes to be identified are: collision avoidance; energy efficiency; scalability and adaptability; efficient bandwidth utilization; latency (latency); throughput. Existing MAC protocols cannot be used with WSN because
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PHD THESIS ON WIRELESS SENSOR NETWORKS located at a distance from one skip relative to the nearest base station, and the role of the protocol is primarily subordinate to achieving high QoS and more efficient use of the bandwidth. Since the sensor nodes do not have a central controlling agent, this means that the existing methods cannot be used. MAC protocols for sensor networks can be classified into: contention-based protocols (CBPs) and scheduling protocols (scheduled protocols). For crash-based protocols, instead of specifying the time when the node will hand over, the nodes compete for the channel, resulting in a co-ordinated based acceleration. The classic examples of the CB MAC protocol are ALOHA and CSMA / CD. At ALOHA, the node hands the packet at the time of generation (pure ALOHA) or in a reusable slotted slot (ALOHA). Packages that come to the collision are ejected and later surrendered. In CSMA, before the surrender, the node is listening to the channel. If it detects that the channel is busy, it delays access and attempts to access the channel later. In the CSMA / CA mechanism, before the transmitter starts transmitting data, a handshake procedure is established between the transmitter and the receiver. The purpose of the RTS-CTS handshake procedure is to inform the neighbors of both the transmitter and the receiver that they will receive the surrender. For example, with MACA based on CSMA / CA both the RTS and CTS adds a field indicating the amount of data to be transmitted. This is given to the knowledge of other nodes for how long they will not be able to perform the surrender. With an enhanced MACA, called MACW, after the transfer of each data packet, an ACK packet is introduced which enables the fast recovery of the link level from transmission errors. Transmission between transmitter and receiver is performed according to the RTS-CTS-DATA-ACK sequences. In the 802.11, when it comes to the distributed function (DCF), all the proposals introduced by CSMA / CA, MACA and MACW have been adopted, and in addition numerous improvements have been made, such as virtual carrier sense, binary exponential back-off, as well as supporting the fragmentation process.
REFERENCES Umar, I. A., Mohd, H., Z., Sali, A., Zulkarnain, Z. A. (2016). A Resource Bound Secure Forwarding Protocol for Wireless Sensor Networks. Sensors, 16. Estévez, F. J., Glösekötter, P.; González, J. (2016). A Dynamic and Adaptive Routing Algorithm for Wireless Sensor Networks. Sensors, 16. Welsh, M. & Mainland, G. (2004) Programming sensor networks using abstract regions. Proc. of the 1st Symposium on Networked Systems Design and Implementation, San Francisco, CA.