Various Aspects Of Wireless Sensor Networks

Published Date: 02 Nov 2017

Navina Krsna(Author)

Student (Undergraduate)

Multimedia University, MMU

Cheras, Malaysia

[email protected]

Danish(Author)

Student (Undergraduate)

Multimedia University, MMU

Serdang, Malaysia

Abstract—A wireless sensor network (WSN) has many real life applications. Basically, wireless sensor network consist of spatially distributed autonomous sensors which monitors physical or environmental conditions. Some of them are temperature, sound and pressure. What happens is it passes the data through the network to a main location. Nowadays, modern networks are said to be bi-directional, which means two way transmission and also there is control of the sensor activity. Besides, current sensors are smaller, cheaper and more intelligent. The development of wireless sensor networks was motivated by military applications. For instance, battlefield surveillance and in our modern world such networks are used in many industrial and consumer applications. The core of the wireless sensor networks are the nodes which can range from a few to several thousands. These nodes are usually connected to sensors. The sensors are equipped with wireless interfaces which can communicate with each other to form a network. The sensor network consist of several parts which include a radio transceiver with an antenna, a microcontroller, an electronic circuit for interfacing with sensors and a power source. The sensor node varies with size. Even the cost varies according to specifications. The topology of a wireless sensor network can range from a simple star network to an advanced multi-hop wireless mesh network. The propagation technique between the hops of the network can be routing or by flooding. The design process of wireless sensor networks depends vastly on the application of the sensors and other factors such as environmental, cost, hardware availability, system constraints. The reasoning towards this review research paper is to present a comprehensive review of the applications of wireless sensors networks currently in the modern world and present its features. Adhering to the top-down approach, we give an overview of several new applications and then give our suggestions on how to improve the application of various aspects of wireless sensor networks.

Keywords—component; formatting; style; styling; insert (key words)

Introduction

Since the era of globalization, wireless sensor networks (WSNs) generated an increasing interest from industrial and research perspectives [1–7]. A WSN can be generally described as a network

of nodes that work together to sense and might or might not control the environment enabling interaction between persons or computers and the surrounding environment [8]. On the other hand, WSNs enables new applications to be used and thus creates new markets, one thing to note is that, the design is affected by several constraints that call for new paradigms. In fact, the activity of sensing, processing, and communication under limited amount of energy, ignites a cross-layer design approach typically requiring the joint consideration of distributed signal/data processing, medium access control (MAC), and communication protocols [9]. This review research paper provides a review of WSNs features and its applications. Besides that, this review research paper also focuses on the aspects of features and standards of WSNs which fall under the technical aspects of WSN.

In Section III, the main application areas for WSNs are categorized according to the type of information measured or carried by the network. Applications, on top of the stack, set requirements that drive the selection of protocols and transmission techniques; at the other end, the wireless channel poses constraints to the communication capabilities and performance. Based on the requirements set by applications and the constraints posed by the wireless channel, the communication protocols and techniques are selected. The main features in WSNs design are described in Section II. Specifically, the design of energy efficient communication protocols is a very odd issue of WSNs, without significant precedent in wireless network history. Generally, when a node is in transmit mode, the transceiver drains much more current from the battery than the microprocessor in active state or the sensors and the memory chip. The ratio between the energy needed for transmitting and for processing a bit of information is usually assumed to be much larger than one. For this reason, the communication protocols need to be designed according to paradigms of energy efficiency, while this constraint is less restrictive for processing tasks. Then, the design of energy efficient communication protocols is a very peculiar issue of WSNs, without significant precedent in wireless network history. On the other

hand, data processing in WSNs may require consuming tasks to be performed at the microprocessor, much longer than the actual length of time a transceiver spends in transmit mode. This can cause a significant energy consumption by the microprocessor, even comparable to the energy consumed during transmission, or reception, by the transceiver. Thus, the general rule that the design of communication protocol design is much more important than that of the processing task scheduling is not always true.

Wireless sensor networks

A WSN can be defined as a network of devices, denoted as nodes, which can sense the environment and communicate the information gathered from the monitored field through

wireless links [1–9]. The data is forwarded, possibly via multiple hops, to a sink (sometimes denoted as controller or monitor) that can use it locally or is connected to other networks (e.g., the Internet) through a gateway. The nodes can be stationary or moving. They can be aware of their location or not. They can be homogeneous or not.

This is a traditional single-sink WSN (see Figure 1, left part). This single-sink scenario suffers from the lack of scalability: by increasing the number of nodes, the amount of data gathered by the sink increases and once its capacity is reached, the network size cannot be augmented. Moreover, for reasons related to MAC and routing aspects, network performance cannot be considered independent from the network size. A more general scenario includes multiple sinks in the network (see Figure 1, right part) [13]. Given a level of node density, a larger number of sinks will decrease the probability of isolated clusters of nodes that cannot deliver their data owing to unfortunate signal propagation conditions. In principle, a multiple-sink WSN can be scalable (i.e., the same performance can be achieved even by increasing the number of nodes), while this is clearly not true for a single-sink network. However, a multi-sink WSN does not represent a minor extension of a single-sink case for the network engineer. In many cases nodes send the data collected to one of the sinks, selected among many, which forward the data to the gateway, toward the final user (see Figure 1, right part). From the protocol viewpoint, this means that a selection can be done, based on a suitable criteria that could be, for example, minimum delay, maximum throughput, minimum number of hops, etc. Therefore, the presence of multiple sinks ensures better network performance with respect to the single-sink case (assuming the same number of nodes is

deployed over the same area), but the communication protocols must be more complex and should be designed according to suitable criteria.

Applications of wireless sensor networks

The variety of possible applications of WSNs in the real world is practically unlimited, from environmental monitoring [14], health care [15], positioning and tracking [16], to logistic, localization, and so on. A possible classification for applications is provided in this section. It is important to underline that the application strongly affects the choice of the wireless technology to be used. Once application requirements are set, in fact, the designer has to select the technology which allows to satisfy these requirements. To this aim the knowledge of the features, advantages and disadvantages of the different technologies is fundamental. Owing to the importance of the relationship between application requirements and technologies, we

report in this Section some example requirements and we devoted Sections 5 and 6 to an overview of the main features of the most promising technologies provided for WSNs.

Applications of wireless sensor networks:

Area monitoring is a common usage of WSNs. For area monitoring the WSNs is usually deployed over a region where some activity is to be monitored. It normally happens to be for military use such as the use of sensors to detect enemy intrusion or invasion. For example, the intrusion of enemy fighter pilots or submarines. This is a major factor in the safety of a particular nation as it consists of the security of a nation. Besides, other security monitoring is for oil refineries and highly secured research laboratories.

Environmental monitoring consists of wireless sensors which are deployed to monitor the environment. The core usage is in the earth science research field whereby the sensors monitor volcanoes for scientific research and eruption signs. The ocean is also monitored for weather patterns and sea levels. Besides that, the glaciers and the forest are some of the examples where environmental monitoring applies. One of the major area is the air quality monitoring whereby sensors are used to protect the environment, animals and humans from being affected by air pollution. The sensors perform their job by constantly measuring the levels of pollutants in the air real-time. The results will then be termed as air pollutant index (API). It’s an interesting application of sensors since the conditions can change dramatically easily in hazardous areas which could result in serious consequences. Therefore, the wireless sensors act as a potential safety barrier to protect humans for such hazardous environments. Then, there are the environmental magnitudes such as the temperature, humidity and light. These magnitudes can be easily obtained by using wireless sensors to monitor them. For instance, the temperature at the mountain or under the deep sea and the humidity of the particular area. Furthermore, there are also gas and particle concentration sensors. They basically perform the task of monitoring gaseous and particles which can range from hazardous to non-hazardous. This simplifies the task of humans as they need not be exposed to such harsh environment which would be harmful to health. For example, monitoring the concentration of Carbon Monoxide gas (CO) which is a toxic gas which can lead to intoxification. Then there is ambient monitoring which monitors rainfall, wind speed, wind direction , UV levels and Atmospheric pressure. It is vital to use wireless sensors to measure these as it is easier to do so and the accuracy is higher. This aids human work and is more practical. The ambient sensors are normally used by weather forecasters to aid their forecasting. Next, comes the interior and exterior monitoring sensors. Interior monitoring measures the gas levels at hazardous environments which requires the use of robust and trustworthy equipment that meet the industrial regulations. Since, hazardous environments need accurate measurements to ensure the safety of humans or living things in that particular environment. This is followed by exterior monitoring. Whereby the outdoor air quality requires the use of accurate wireless sensors but, it has to be durable enough to withstand rain, wind and probably other harsh conditions. Besides that, the sensor also has to be self-sufficient in term that it needs the use of energy harvesting techniques that would ensure the sensors extended autonomy to equipment which most probably will be difficult to access. For example, sensors high in the Swiss Alps, considering the strong wind and harsh conditions up there, it’s obvious that the power supply is going to be a tough task to supply to altitude that high.

Air pollution Monitoring is currently being utilized at several major cities around the world like London and Brisbane in order to keep track of the concentration of dangerous gaseous for citizens. Air pollutants like Carbon Monoxide(CO) and haze are very harmful to citizens well being. The sensors are deployed by taking advantage of ad-hoc wireless links rather than wired installation which greatly improve the speed and the easy of usage. Since, the sensors will be more mobile for testing readings in different areas. Simply put there are various architectures that can be used for air monitoring as well as alternate data analysis methods to further improve the outcome of the results.

Forest Fire detection basically works using the nodes which carry sensors which can be installed in forest to detect whenever a fire has started. They sensors which can be equipped can range from measurements of temperature, humidity and types of gases produced by forest fires. These sensors play an important role as they are crucial for early detection in order for firefighters to be successful in their actions of saving properties, humans , animals and crops. Provided the wireless sensor network is deployed efficiently firefighters will be able to know when a fire is started and how fast it is spreading so they can curb the losses to the minimum.

A Landslide detection system also uses the aid of wireless sensor networks to detect slight movement on or in the earth crust. And variations of different aspect which may occur during or before a landslide. Given the data provided by the sensors it is easier to identify landslide prone areas and take early precaution. This can avoid lives being lost and property damage in the long term.

Water quality monitoring system involves analyzing water properties in streams, lakes, oceans, as well as underground reservoirs. The system functions with the use of many wireless sensors distributed which enables the formation of a precise map of the water status. This also acts as a permanent deployment for monitoring stations which have tough access without the need for manual retrieval. Therefore, it is easier to identify whether the water is contaminated or the presence of harmful elements which would make the water unsuitable for consumption. And it eases the burden of detecting water levels in the reservoirs underground.

The application of wireless sensor networks can effectively prevent natural disasters like flash floods, earthquakes and volcanic eruptions.

The sensors function with the usage of wireless nodes which have been successfully deployed in rivers and on the earth crust. So that the sensor can monitor the changes in water level and movement under the earth crust in real-time.

Then, there is Industrial monitoring. Where the machine health monitoring system uses wireless sensor networks to check machinery condition based maintenance (CBM) as they offer significant cost savings and enable new functionalities. Besides, in wired systems the usage of sensors is often limited since the cost for wiring is very high. As such previously inaccessible locations like hazardous, restricted areas with rotating machinery can be reached using the current high technology wireless sensors.

Moreover, sensor networks are also used for data logging. As the term states it collects data for monitoring environmental information. For instance, monitoring the temperature of the fridge or the level of water in the coolants of nuclear powered stations. The statistic can then be used to explain how certain systems function. The major asset using wireless sensor networks is the real-time or live data feed. Since, the data will be always up-to-date.

This is followed by industrial sense and control applications. Currently, there are plenty of wireless sensor network communication protocols which have been developed. Those day the expectance was more towards saving energy but nowadays it’s more towards wider aspects such as wireless link reliability, real time capabilities and quality of service. These new aspects are considered the future of for all applications in industrial and related sense and control applications. Therefore, by replacing the old wire based networks with the wireless networks more productivity can be obtained.

On another note, WSNs are also applied for water/wastewater monitoring systems. They involve aspects such as the quality of surface or underground water , to the monitoring of the country’s water infrastructure. The 3 major systems are water quality magnitudes which monitor temperature, pH value, specific electrical conductance (EC) and dissolved Oxygen(O2). These aspects are very important for human water consumption and all living things. Followed by the water distribution network monitoring which monitors the flow and pressure levels of water, leakage detection, water levels and remote metering. These aspects would be very helpful for underground reservoirs and water supply systems. It aids the delivery process of water supply to the common household till industrial sites. Finally there is the natural disaster prevention system which monitors for flood and drought and give an early warning so necessary precautions can be taken.

The deployment of wireless sensor networks in agriculture based industry is a common thing nowadays. Since, it frees the farmers of the need to maintain wirings in difficult environments. The deployment of gravity feed water systems can be monitored by using pressure transmitters to monitor water tank levels and water pumps can be operated using input/output devices and the water usage is measured and wirelessly transmitted back to a control centre for billing. This enables efficient water usage and prevents wastage.

Another vital usage of WSNs is for Greenhouse monitoring. Where sensors can control the temperature and humidity levels inside man made greenhouses. When the temperature and humidity drops below certain levels the sensor notifies the system which triggers other systems like misting systems, open vents and turning on fans which automatically controls the temperature of the greenhouses.

Finally, there is structural monitoring which involves the aspects of engineering and architecture. It is mainly used to monitor the movements of infrastructures such as buildings, highway flyovers, tunnels underground and bridges. These structures need to be monitored real-time as it involves infrastructures which need constant monitoring to ensure the safety of the general public. Besides that, monitoring assets remotely enables engineers to monitor infrastructures without the need for costly on site inspections as well as having the advantage of daily data updates. In contrast, according to traditional method data is usually collected on a weekly or monthly basis using physical site visits, which involves site closure in some cases. On another note, the data collected is far more accurate using wireless sensor networks compared to manual on the site inspections. Also some of the useful uses of WSNs are for structural health monitoring like the amount of load the structure can handle, the vibration level, the formation of cracks and the fatigue of structures. Besides that, wireless sensor network also has the capability to monitor the wind and weather condition. For example, the humidity level outside and the wind direction affecting the infrastructure. Some sensor networks also monitor the traffic situation to warn motorist of traffic congestions and also sensors which can monitor the pollution levels to warn the general public on the potential air pollution. The major structural monitoring involves the monitoring of bridges. It’s important that for bridges measurements of loads and the effects the load has on the bridge. This can help to identify possible fatigues of the structure and engineers can take early precautions to avoid any mishaps. Another use of wireless sensors is for passive localization and tracking purposes. For example, tracking people with tags and ID cards. So these, things can be monitored wirelessly. They are normally connected by wireless links in a mesh wireless sensor infrastructure. Finally, there is smart home monitoring whereby wireless sensors are used to monitor the security of the compound and the presence of human in unauthorized areas. This gives real-time security. Some sensors are even embedded in objects to form a WSN which enables network activity support services.

3.1. Applications Classification

One of the possible classifications distinguishes applications according to the type of data that must be gathered in the network. Almost any application, in fact, could be classified into two categories: event detection (ED) and spatial process estimation (SPE). In the first case sensors are deployed to detect an event, for example a fire in a forest, a quake, etc. [17–19]. Signal processing within devices is very simple, owing to the fact that each device has

to compare the measured quantity with a given threshold and to send the binary information to the sink(s). The density of nodes must ensure that the event is detected and forwarded to the sink(s) with a suitable probability of success while maintaining a low probability of false alarm. The detection of the phenomenon of interest (POI) could be performed in adecentralized (or distributed) way, meaning that sensors, together with the sink, cooperatively undertake the task of identifying the POI. However, unlike in classical decentralized detection problems, greater challenges exist in a WSN setting. There are stringent power constraints for each node, communication channels between nodes and the fusion center are severely bandwidth-constrained and are no longer lossless (e.g, fading, noise and, possibly, external sources of interference are present), and the observation at each sensor node is spatially varying. In the context of decentralized detection, cooperation allows exchange of information among sensor nodes to continuously update their local decisions until consensus is reached across the nodes. In SPE the WSN aims at estimating a given physical phenomenon (e.g., the atmospheric pressure in a wide area, or the ground temperature variations in a small volcanic site), which can be modelled as a bi-dimensional random process (generally non-stationary). In this case the main issue is to obtain the estimation of the entire behavior of the spatial process based on the samples taken by sensors that are typically placed in random positions [20–23]. The measurements will then subject to proper processing which might be performed either in a distributed manner by the nodes, or centrally at the supervisor. The estimation error is strictly related to nodes density as well as on the spatial variability of the process. Higher nodes density lead to a more accurate scalar field reconstruction at the expense of a larger network throughput and cost. In the recent literature, different works addressed the estimation of a scalar field using random WSNs. As an example, [20] presents a distributed algorithm able to estimate the gradient of a generic smooth physical process (energy constraints and nodes failure are not considered there); in [21] the relationship

between the random topology of a sensor network and the quality of the reconstructed field is investigated and some guidelines on how sensors should be deployed over a spatial area for efficient data acquisition and reconstruction are derived. Distributed source coding techniques can be successfully exploited to reduce the amount of data to be transmitted and hence to improve the network energy efficiency [24]. There exist also applications that belong to both categories. As an example, environmental monitoring applications could be ED- or SPE-based. To the first category belong, for example, the location of a fire in a forest, or the detection of a quake, etc. (see Figure 2). Alternatively, the estimation of the temperature of a given area belongs to the second category. In general, these applications aim at monitoring indoor or outdoor environments, where the supervised area may be few hundreds of square meters or thousands of square kilometers, and the duration of the supervision may last for years. Natural disasters such as floods, forest fires, earthquakes may be perceived earlier by installing networked embedded systems closer to

places where these phenomena may occur. Such systems cannot rely on a fixed infrastructure and have to be very robust, because of the inevitable impairments encountered in open environments. The system should respond to environment changes as quick as possible. The environment to be observed will mostly be inaccessible by the human all the time. Hence, robustness plays an important role. Also security and surveillance applications have some demanding and challenging requirements such as real-time monitoring and high security. Another application that could belong to both the above defined categories is devoted to the realization of energy efficient buildings. In this application, in fact, sensor nodes could aim at estimating a process (SPE), but also events (ED). In this case the WSN is distributed in buildings (residential or not) to manage efficiently the energy consumption of all the electric appliances. Consequently, nodes have to continuously monitor the energy consumed by all appliances connected to the electrical grid. Therefore, sensors have to estimate a process, that is the energy consumption which varies with time, but in some cases, they could be used to detect some events. As an example, sensors could detect the arrival of a person in a room to switch on some electrical appliances.

The European Joint Undertaking ARTEMISIA has funded the project eDIANA (Embedded Systems for Energy Efficient Buildings), focused on the above described application scenario. The project,

in fact, aims at achieving energy efficient buildings through innovative solutions based on networked embedded systems. The eDIANA approach is to achieve greater efficiency in the use of resources, prioritizing energy as scarce resource, more flexibility in the provision of resources and better situation awareness for the citizen and for service and infrastructure owners. This will be achieved through the deployment and inter-operation of embedded systems throughout the eDIANA environment of buildings and intra-building units.

3.2. Examples of Application Requirements

Due to the wide variety of possible applications of WSNs, system requirements could change significantly. For instance, in case of environmental monitoring applications, the following requirements

are typically dominant: energy efficiency, nodes are battery powered or have a limited power supply; low data rate, typically the amount of data to be sensed is limited; one-way communication, nodes act only as sensors and hence the data flow is from nodes to sink(s); wireless backbone, usually in environmental monitoring no wired connections are available to connect sink(s) to the fixed network. Significantly different are the requirements of a typical industrial application where wireless nodes are used for cable replacement: reliability, communication must be robust to failure and interference; security, communication must be robust to intentional attacks; inter-operability, standards are required; high data rate, the process to be monitored usually carries a large amount of data; two-way communication, in industrial applications nodes typically act also as actuators and hence the communication between sink(s) and nodes must be guaranteed; wired backbone, sinks can be connected directly to the fixed network using wired connections. Even if requirements are strongly application dependent, one of the most important issues in the design of WSNs, especially in such scenarios where power supply availability is limited, is energy efficiency. High energy efficiency means long network lifetime and limited network deployment and maintenance costs. Energy efficiency can be achieved at different levels starting from the technology level (e.g., by adopting low consumption hardware components), physical layer, MAC, routing protocols up to the application level. For example, at physical and MAC layers, nodes could operate with low duty cycle by spending most of their time in sleeping mode to save energy. This poses new problems such as that nodes may not wake up at the same time, due to the drifts of their local clocks, thus making the communication impossible. Suitable network synchronization schemes are mandatory in this case [8, 25].

Main Features in Wireless Sensor Networks Design

The main features of WSNs, as could be deduced by the general description given in the previous sections, are: scalability with respect to the number of nodes in the network, self-organization, self-healing, energy efficiency, a sufficient degree of connectivity among nodes, low-complexity, low cost and size of nodes. Those protocol architectures and technical solutions providing such features can be considered as a potential framework for the creation of these networks, but, unfortunately, the definition of such a protocol architecture and technical solution is not simple, and the research still needs to work on it [5]. The massive research on WSNs started after the year 2000. However, it took advantage of the outcome of the research on wireless networks performed since the second half of the previous century. In particular, the study of ad hoc networks attracted a lot of attention for several decades, and some researchers tried to report their skills acquired in the field of ad hoc networks, to the study of WSNs. According to some general definitions, wireless ad hoc networks are formed dynamically by an autonomous system of nodes connected via wireless links without using an existing network

infrastructure or centralized administration. Nodes are connected through "ad hoc" topologies, set up and cleared according to user needs and temporary conditions [11]. Apparently, this definition can include WSNs. However, this is not true. This is the list of main features for wireless ad hoc networks: unplanned and highly dynamical; nodes are "smart" terminals (laptops, etc.); typical applications include realtime or non-realtime data, multimedia, voice; every node can be either source or destination of information; every node can be a router toward other nodes; energy is not the most relevant matter; capacity is the most relevant matter [11].Apart from the very first item, which is common to WSNs, in all other cases there is a clear distinction between WSNs and wireless ad hoc networks. In WSNs, nodes are simple and low-complexity devices; the typical applications require few bytes sent periodically or upon request or according to some external event; every node can be either source or destination of information, not both; some nodes do not play the role of routers; energy efficiency is a very relevant matter, while capacity is not for most applications. Therefore, WSNs are not a special case of wireless ad hoc networks. Thus, a lot of care must be used when considering protocols and algorithms which are good for ad hoc networks, and using them in the context of WSNs.

8. Conclusions

The aim of this paper is to discuss some of the most relevant issues of WSNs, from the application, and technical features. The first part aims to explain in detail what wireless sensor networks are all about. The second part mainly presses on the applications of wireless sensor network in the current world we live in today. Finally, the paper provides a vision on future trends of the short- and long-term research on WSNs.

9. Acknowledgment

The main purpose of this review research paper is to have a further understanding on WSNs and its applications. We would like to take this opportunity to thank Mr. Ayman bin Salleh for his initiation in assigning us this assignment of writing a review research paper in order to improve and support our understand of the course data communication and networking. By writing this review research paper a lot of research had to be done on the internet and also lots of reading and referring to Journal papers and topics related to WSNs. As such, we have gained a lot of knowledge in this topic related to data communication and networking. Finally, we have also got some experience in writing a review research paper which will surely aid us in our future undertakings.

10. References and Notes