Review Applications of Networks: An Up-to-Date Survey

Dionisis Kandris 1,* , Christos Nakas 1, Dimitrios Vomvas 1 and Grigorios Koulouras 2,3

1 microSENSES Research Laboratory, Department of Electrical and Electronic Engineering, Faculty of Engineering, University of West Attica, GR-12241 Athens, Greece; [email protected] (.N.); [email protected] (D.V.) 2 TelSiP Research Laboratory, Department of Electrical and Electronic Engineering, School of Engineering, University of West Attica, GR-12241 Athens, Greece; [email protected] 3 Hellenic and Post Commission, GR-15125 Athens, Greece * Correspondence: [email protected]

 Received: 31 December 2019; Accepted: 20 February 2020; Published: 25 February 2020 

Abstract: Wireless Sensor Networks are considered to be among the most rapidly evolving technological domains thanks to the numerous benefits that their usage provides. As a result, from their first appearance until the present day, Wireless Sensor Networks have had a continuously growing range of applications. The purpose of this article is to provide an up-to-date presentation of both traditional and most recent applications of Wireless Sensor Networks and hopefully not only enable the comprehension of this scientific area but also facilitate the perception of novel applications. In order to achieve this goal, the main categories of applications of Wireless Sensor Networks are identified, and characteristic examples of them are studied. Their particular characteristics are explained, while their pros and cons are denoted. Next, a discussion on certain considerations that are related with each one of these specific categories takes place. Finally, concluding remarks are drawn.

Keywords: wireless ; wireless sensor networks

1. Introduction A Wireless Sensor Network (WSN) is a group of spatially dispersed sensor nodes, which are interconnected by using wireless communication [1]. As seen in Figure1, a , also called mote, is an electronic device which consists of a processor along with a storage unit, a transceiver module, a single sensor or multiple sensors, along with an analog-to-digital converter (ADC), and a power source, which normally is a battery. It may optionally include a positioning unit and/or a mobilization unit. A sensor node uses its sensor(s) in order to measure the fluctuation of current conditions in its adjacent environment. These measurements are converted, via the ADC unit, into relative electric signals which are processed via the node’s processor. Via its transceiver, the node can wirelessly transmit the data produced by its processor to other nodes or/and to a selected sink point, referred to as the Base Station. As illustrated in Figure2, the Base Station, by using the data transmitted to itself, is able to both perform supervisory control over the WSN it belongs to and transmit the related information to human users or/and other networks [2].

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A sensor node uses its sensor(s) in order to measure the fluctuation of current conditions in its adjacent environment. These measurements are converted, via the ADC unit, into relative electric signals which are processed via the node’s processor. Via its transceiver, the node can wirelessly transmit the data produced by its processor to other nodes or/and to a selected sink point, referred to as the Base Station. As illustrated in Figure 2, the Base Station, by using the data transmitted to itself, is able to both perform supervisory control over the WSN it belongs to and transmit the related information to Figure 1. The typical architecture of a sensor node used in Wireless Sensor Networks (WSNs). humanFigure users 1. or/andThe typical other architecture networks [2]. of a sensor node used in Wireless Sensor Networks (WSNs). A sensor node uses its sensor(s) in order to measure the fluctuation of current conditions in its adjacent environment. These measurements are converted, via the ADC unit, into relative electric signals which are processed via the node’s processor. Via its transceiver, the node can wirelessly transmit the data produced by its processor to other nodes or/and to a selected sink point, referred to as the Base Station. As illustrated in Figure 2, the Base Station, by using the data transmitted to itself, is able to both perform supervisory control over the WSN it belongs to and transmit the related information to human users or/and other networks [2].

Figure 2. The typical architecture of a WSN. Figure 2. The typical architecture of a WSN. The collaborative use of a sufficient quantity of such sensor nodes, enables a WSN to perform The collaborative use of a sufficient quantity of such sensor nodes, enables a WSN to perform simultaneoussimultaneous data data acquirement acquirement of of ambient ambient informationinformation at at several several points points of of interest interest positioned positioned over over widewide areas. areas. The The inexpensive inexpensive production production ofof sensorsensor nodes of of this this kind, kind, which which despite despite their their relatively relatively smallsmall size, size, have have exceptionally exceptionally advanced advanced sensing, sensing, processing, processing, and communicationand communication abilities abilities has become has feasiblebecome due feasible to continuous due to continuous technological technological advances. advances. For this For reason, this reason, although although WSNs WSNs were initiallywere usedinitially mainly used for militarymainly for purposes, military nowadays purposes, they nowadays support they an ever-growingsupport an ever range‐growing of applications range of of Figure 2. The typical architecture of a WSN. diffapplicationserent types [of3]. different types [3]. TheTheThe aim aim collaborative of of this this paper paper use is ofis to toa studysufficient study characteristic characteristic quantity of suchexamples sensor of ofnodes, different diff erentenables existing existing a WSN applications applicationsto perform of of WSNs,WSNs,simultaneous both both widely widely data used usedacquirement and and novel novel of ones, ambientones, andand information examine various variousat several issues issues points of of interestof interest interest related relatedpositioned to this to thisover topic. topic. Specifically,Specifically,wide areas. in Section inThe Section inexpensive2 applications 2 applications production of WSNs of aresensorWSNs classified no aredes classifiedof according this kind, according to which their naturedespite to their intotheir corresponding naturerelatively into categoriescorrespondingsmall andsize, theirhave categories specificexceptionally and features their advanced are specific examined, sensing, features throughprocessing, are examined, the and presentation communication through of the indicative presentation abilities examples. has of Specificindicativebecome concerns feasibleexamples. arising due Specific to from continuous thisconcerns study technological arising are discussed from advances. this in study Section For arethis3. discussed Finally,reason, although concluding in Section WSNs remarks3. Finally,were are drawnconcludinginitially in Section used remarks4 .mainly are fordrawn military in Section purposes, 4. nowadays they support an ever-growing range of applications of different types [3]. 2. Main2. Main WSNThe WSN aim Applications Applications of this paper is to study characteristic examples of different existing applications of WSNs, both widely used and novel ones, and examine various issues of interest related to this topic. Various applications of WSNs are currently either already in mature use or still in infant stages VariousSpecifically, applications in Section of 2 WSNs applications are currently of WSNs either are already classified in matureaccording use orto stilltheir in nature infant into stages of of development. In this paper, WSN applications are classified according to the nature of their use development.corresponding In this categories paper, WSNand thei applicationsr specific features are classified are examined, according through to the the nature presentation of their useof into into six main categories which, as illustrated in Figure 3, namely are: military, health, environmental, six mainindicative categories examples. which, Specific as illustrated concernsin arising Figure from3, namely this study are: are military, discussed health, in Section environmental, 3. Finally, flora flora and fauna, industrial, and urban. and fauna,concluding industrial, remarks and are urban.drawn in Section 4. In each category, various subcategories are considered. In what follows in this section, the nature In each category, various subcategories are considered. In what follows in this section, the nature of 2.each Main one WSN of these Applications categories and subcategories is explained. Additionally, through the indicative of eachexamination one of these of characteristic categories andexamples subcategories of them, their is explained. particular Additionally, features are explained, through thewhile indicative their examinationbenefits and of problems characteristic are denoted. examples of them, their particular features are explained, while their benefitsMoreover, and problems various are methodologies denoted. and technical means that are used in these applications either for sensing or for processing purposes are discussed, while similarities and dissimilarities existing among them are identified.

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Moreover, various methodologies and technical means that are used in these applications either for sensing or for processing purposes are discussed, while similarities and dissimilarities existing amongAppl.Appl. themSyst. Syst. Innov. Innov. are 2020 identified. 2020, 3, ,3 14, 14 3 3of of 24 24

Figure 3. Overview of the most popular categories of applications of WSNs. FigureFigure 3. 3. Overview Overview of of the the most most popular popular categories categories of of applications applications of of WSNs. WSNs. 2.1. Military Applications 2.1.2.1.The Military Military military Applications Applications domain is not only the first field of human activity that used WSNs but it is also consideredTheThe tomilitary military have motivateddomain domain is is not thenot only initiationonly the the first first of sensorfield field of of networkhuman human activity activity research. that that Smartused used WSNs WSNs Dust [but 4but] is it it ais is typical also also exampleconsideredconsidered of these to to have initialhave motivated motivated research the etheff orts,initiation initiation which of of weresensor sensor performed network network research. research. in the late Smart Smart 90 sDust Dust in order [ 4[4] ]is is toa a typical developtypical sensorexampleexample nodes of of which these these despiteinitial initial research research their very efforts, efforts, small which which size wouldwere were performed performed be capable in in of the the accomplishing late late 90 90 s s in in order order spying to to develop develop activities. sensorsensorThe technological nodes nodes which which despite despite advances their their achieved very very small small since size size would would then madebe be capable capable WSNs of of accomplishing accomplishing capable of supporting spying spying activities. activities. various operationsTheThe [ 5 technological].technological In Figure4, advances theadvances main subcategories achievedachieved since since of thenthen the military mademade WSNsWSNs applications capablecapable of of WSNs supportingsupporting which variousvarious namely areoperations battlefieldoperations [5]. surveillance, [5]. In In Figure Figure 4, 4, combat the the main main monitoring, subcategories subcategories and of of intruder the the military military detection, applications applications are illustrated of of WSNs WSNs which which along namely namely with the typesareare of battlefield battlefield sensors that surveillance, surveillance, are most combat commonlycombat monitoring, monitoring, used in and them.and intruder intruder detection, detection, are are illustrated illustrated along along with with the the typestypes of of sensors sensors that that are are most most commonly commonly used used in in them. them.

FigureFigureFigure 4. The 4. 4. The The subcategories subcategories subcategories of of theof the the military military military applications applications applications of of WSNs WSNs and and and the thethe types types of of sensors sensors sensors they they they use. use. use.

Specifically,Specifically,Specifically, Chemical, Chemical,Chemical, Biological, Biological,Biological, Radiological, Radiological,Radiological, Nuclear Nuclear and andand Explosive ExplosiveExplosive (CBRNE),(CBRNE), (CBRNE), andand and ToxicToxic Toxic IndustrialIndustrialIndustrial Material Material Material (TIM) (TIM) (TIM) sensors sensors sensors may may may be be be used used used to to to detectdetect detect thethe the presence presence of ofof such suchsuch substances. substances. To To To detect detect detect intrusion,intrusion,intrusion, WSNs WSNs WSNs may may may use use use infrared, infrared, infrared, photoelectric, photoelectric, photoelectric, laser,laser, laser, acoustic,acoustic, acoustic, and andand vibration vibrationvibration sensors. sensors. sensors. Similarly, Similarly, Similarly, RAdioRAdioRAdio Detection Detection Detection And And And Ranging Ranging Ranging (RADAR), (RADAR), (RADAR), LIght LIght LIght Detection Detection Detection And And Ranging Ranging (LIDAR), (LIDAR), LAser LAser LAser Detection Detection Detection AndAndAnd Ranging Ranging Ranging (LADAR) (LADAR) (LADAR) and and and ultrasonic ultrasonic ultrasonic sensors sensors sensors areare are usedused used by by nodes nodes in inin WSNs WSNs in in in order order order to to to detect detect detect the the the distancedistancedistance from fromfrom objects objectsobjects of interest. ofof interest.interest. Likewise, Likewise,Likewise, LADAR LADARLADAR and infrared andand infraredinfrared sensors sensorssensors are used areare for usedused imaging forfor imagingimaging purposes. purposes.purposes. Additionally, Additionally, the the flexibility flexibility that that WSNs WSNs have have in in their their structure structure enables enables them them to to adapt adapt to to variousvarious requirements. requirements. For For instance, instance, in in battlefield battlefield operations operations large large‐scale‐scale WSNs WSNs consisting consisting of of many many thousandsthousands ofof nodes,nodes, whichwhich areare nonnon‐manually‐manually deployed,deployed, areare used.used. InIn urbanurban warfarewarfare andand forceforce

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Additionally, the flexibility that WSNs have in their structure enables them to adapt to various requirements. For instance, in battlefield operations large-scale WSNs consisting of many thousands of nodes, which are non-manually deployed, are used. In urban warfare and force protection operations, WSNsAppl. Syst. used Innov. consist 2020, 3 of, 14 hundreds of manually deployed nodes. In other-than-war operations4 all of scales24 of WSNs and deployment methods are used [5]. protection operations, WSNs used consist of hundreds of manually deployed nodes. In other‐than‐ WSNs that have been developed for battlefield surveillance, combat monitoring, and intruder war operations all scales of WSNs and deployment methods are used [5]. detection are examined in the following subsection. WSNs that have been developed for battlefield surveillance, combat monitoring, and intruder detection are examined in the following subsection. 2.1.1. Battlefield Surveillance 2.1.1.In Battlefield applications Surveillance of this type, the sensor nodes of the WSN may be deployed on a battlefield nearby of theIn paths applications that enemy of this forces type, maythe sensor use. nodes The main of the advantage WSN may be provided deployed is on that a battlefield the WSN nearby not only canof bethe spontaneously paths that enemy positioned forces may but use. also The can main function, advantage without provided need is for that continuous the WSN not attendance only can and maintenance.be spontaneously The terrain positioned of the but battlefield also can in function, most cases without is absolutely need for variable. continuous This attendance plays an important and rolemaintenance. for both the The coverage terrain and of thethe energy battlefield consumption in most ofcases the is sensor absolutely nodes. variable. Some of theseThis plays applications an areimportant presented role below. for both the coverage and the energy consumption of the sensor nodes. Some of these applicationsIn [6], the are use presented of WSN below. technology for ground surveillance is studied. Specifically, the authors proposeIn a [6], system the use that of consists WSN technology of low-cost for common ground surveillance nodes which is are studied. capable Specifically, of sensing the magnetic authors and acousticpropose signals a system produced that consists by various of low‐ movingcost common target nodes objects. which The are system capable aims of sensing to detect magnetic and categorize and variousacoustic targets, signals such produced as vehicles by various and troopmoving movements, target objects. based The system on the spatialaims to didetectfferences and categorize of the signal strengthvarious detected targets, such by the as vehicles sensors. and Figure troop5 illustrates movements, the based architecture on the spatial of this differences system. of the signal strength detected by the sensors. Figure 5 illustrates the architecture of this system.

FigureFigure 5. 5.Illustration Illustration of the the system system architecture architecture proposed proposed in in[6] [ 6for] for target target tracking. tracking.

InIn [7 [7]] a a submarine submarine detection detection system for for Anti Anti-Submarine‐Submarine Warfare Warfare (ASW) (ASW) is presented. is presented. The The system system consistsconsists of of inexpensive inexpensive multi-sensing multi‐sensing units that that combine combine both both active active and and passive passive sonars. sonars. The The system system cancan be be scaled scaled to ato large a large number number of sensors,of sensors, which which are are deployed deployed in littoral in littoral waters waters according according to a to specific a patternspecific and pattern ocean and depth. ocean These depth. sensing These sensing units utilize units utilize their passivetheir passive sonar sonar to detect to detect a diesel-electric a diesel‐ submarineelectric submarine and their and active their sonar active to sonar confirm to confirm their target. their target. The unit The that unit has that confirmed has confirmed a target, a target, notifies itsnotifies neighboring its neighboring sensing units sensing by units using by an using alarm an signal alarm that signal contains that contains its ID code. its ID Whenevercode. Whenever the units sendthe multipleunits send alarm multiple signals alarm within signals a predefined within a predefined period of time,period an of alert time, is triggered.an alert is Furthermore,triggered. theFurthermore, system can acquirethe system low can False acquire Alarm low Rate False (FAR) Alarm due Rate to the (FAR) very due low to range the very of the low active range sonar of the (50 m) thatactive solves sonar the (50 acoustic m) that multipath solves the problemacoustic multipath of the conventional problem of sonobuoys. the conventional sonobuoys.

2.1.2.2.1.2. Combat Combat Monitoring Monitoring WithinWithin a a battlefield battlefield the the firingfiring of guns, guns, mortars mortars artillery, artillery, and and other other weaponry weaponry creates creates sound, sound, heat, heat, andand vibrations. vibrations. This This information information can can be be recordedrecorded withwith the use of WSNs WSNs and and provides provides an an expectation expectation of theof location the location of the of the enemy. enemy. This This type type of of application application is is described described below. below. InIn [8 [8]] the the use use of of acoustic acoustic sensorsensor arrays arrays suspended suspended below below tethered tethered aerostats aerostats to detect to detect and and localize localize moving vehicles, transient signals from mortars, artillery, small arms fire, and locate their source is moving vehicles, transient signals from mortars, artillery, small arms fire, and locate their source presented. is presented. The specific detection system can be used in conjunction with an acoustic vector sensor, to The specific detection system can be used in conjunction with an acoustic vector sensor, to amplify amplify the possibility of locating the threat using the shockwave created by the supersonic bullet theand possibility the muzzle of blast locating created the by threat the gun using [9]. the shockwave created by the supersonic bullet and the muzzleThe blast authors created of by [10] the describe gun [9]. a system consisting of interconnected Body Sensor Networks (BSNs), a sub‐family of WSNs, for real‐time health monitoring of soldiers. A key component of this system is a BSN that integrates various physiological and biomedical sensors. These sensors are an accelerometer, an EEG simulator, and a SpO2 sensor, which can be embedded within an advanced

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The authors of [10] describe a system consisting of interconnected Body Sensor Networks (BSNs), a sub-family of WSNs, for real-time health monitoring of soldiers. A key component of this system is a BSN that integrates various physiological and biomedical sensors. These sensors are an accelerometer, an EEG simulator, and a SpO2 sensor, which can be embedded within an advanced combat helmet wornAppl. by Syst. each Innov. soldier. 2020, 3, 14 They monitor the various information of health status in real time,5 such of 24 as blood pressure, oxygen saturation, and heart rate. By utilizing the data collected, various methods combat helmet worn by each soldier. They monitor the various information of health status in real can be applied in order to train soldiers more efficiently and prepare them more adequately for time, such as blood pressure, oxygen saturation, and heart rate. By utilizing the data collected, future engagements. various methods can be applied in order to train soldiers more efficiently and prepare them more 2.1.3.adequately Intruder for Detection future engagements. 2.1.3.The Intruder knowledge Detection of the location of the enemy is considered to be one of the most critical pieces of informationThe knowledge during military of the location operations. of the Whichever enemy is considered side in a conflict to be one has of thisthe most knowledge critical pieces is one of step aheadinformation and closer during to victory. military With operations. the utilization Whichever of WSNs side within in a conflict a battlefield, has this intruders knowledge can is beone detected step in goodahead time and to closer prevent to victory. loss of suppliesWith the or utilization territory. of WSNs within a battlefield, intruders can be detectedAn intruder in good can time be to detected prevent in loss various of supplies ways. or Theterritory. authors of [11] developed a system that can recognizeAn intruders intruder can by utilizingbe detected Unattended in various Acousticways. The and authors Seismic of [11] Sensors developed to record a system vibrations that can and soundsrecognize of typical intruders soldier by actions.utilizing ToUnattended detect an Acoustic intruder, and with Seismic the aid Sensors of these to sensors,record vibrations typical military and activities,sounds such of typical as equipment soldier actions. handling, To detect walking, an intruder, rifle loading, with the etc., aid have of these been sensors, measured typical and military recorded in aactivities, controlled such environment as equipment and handling, in the walking, field. Although rifle loading, the distance etc., have of been detection measured may and be recorded small, the informationin a controlled provided environment by the network and in the can field. be used Although to locate the an distance intruder of indetection difficult may terrains be small, with the large vegetationinformation where provided visibility by the is limited. network can be used to locate an intruder in difficult terrains with large vegetationIn [12] another where visibility application is limited. of WSNs, developed to detect intruders, is introduced. The authors describedIn a[12] system another with application dense deployment of WSNs, ofdeveloped a large number to detect of intruders, miniaturized is introduced. wireless sensors,The authors which described a system with dense deployment of a large number of miniaturized wireless sensors, which are small, low cost, and consume low energy. This system utilizes acoustic and seismic sensors to are small, low cost, and consume low energy. This system utilizes acoustic and seismic sensors to provide detection information to the user and tracking and visual sensors in order to lower the false provide detection information to the user and tracking and visual sensors in order to lower the false alarm rate. alarm rate. 2.2. Health Applications 2.2. Health Applications In the health domain, WSNs utilize advanced medical sensors to monitor patients within a In the health domain, WSNs utilize advanced medical sensors to monitor patients within a healthcarehealthcare facility, facility, as as a a hospital hospital oror withinwithin their their home, home, as as well well as asto provide to provide real real time time monitoring monitoring of of patient’spatient’s vitals by utilizing wearable wearable hardware. hardware. In In Figure Figure 6,6 ,the the main main subcategories subcategories of of health health applicationsapplications of of WSNs WSNs namely namely patient patient wearable wearable monitoring,monitoring, home home assisting assisting systems, systems, and and hospital hospital patientpatient monitoring monitoring are are illustrated illustrated along along with with thethe typestypes of sensors that that are are most most commonly commonly used used in in them.them. WSNs WSNs that that have have been been developed developed forfor thesethese types of of health health applications applications are are examined examined in inthe the followingfollowing subsection. subsection.

Health

Applications

Patient Wearable Home Assisting Hospital Patient Monitoring Systems Monitoring

Motion Biomedical Seismic RF Sensors Sensors Sensors Sensors

Temperature Position Humidity Sensors Sensors Sensors

Figure 6. The subcategories of the health applications of WSNs and the sensors used in them. Figure 6. The subcategories of the health applications of WSNs and the sensors used in them.

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Appl. Syst. Innov. 2020, 3, 14 6 of 24 2.2.1. Patient Wearable Monitoring Appl.2.2.1.Health Syst. Patient Innov. monitoring Wearable2020, 3, 14 Monitoring applications can be combined with wearable hardware with embedded6 of 24 biomedical2.2.1. HealthPatient sensors monitoringWearable that Monitoring provide applications the patient’scan be combined health status with inwearable a remote hardware environment with embedded or within a healthcarebiomedical facility. sensors that provide the patient’s health status in a remote environment or within a Health monitoring applications can be combined with wearable hardware with embedded healthcareA healthcare facility. solution of this type, is examined in [13]. It uses real-time sensors incorporated biomedical sensors that provide the patient’s health status in a remote environment or within a in smartphonesA healthcare along solution with of a this barcode type, is system examined to providein [13]. It personalized uses real‐time medicinesensors incorporated care assistance. in healthcare facility. Thesmartphones mechanism along developed, with a performsbarcode system Electrocardiogram to provide personalized (ECG) monitoring medicine incare real assistance. time. Also, The the A healthcare solution of this type, is examined in [13]. It uses real‐time sensors incorporated in monitoringmechanism of developed, the blood glucoseperforms level, Electrocardiogram blood pressure, (ECG) and severalmonitoring kinds in ofreal diagnostics time. Also, could the be along with a barcode system to provide personalized medicine care assistance. The possiblemonitoring too, by of using the blood real-time glucose sensors. level, Thisblood system pressure, developed and several is illustrated kinds of indiagnostics Figure7. could be mechanism developed, performs Electrocardiogram (ECG) monitoring in real time. Also, the possible too, by using real‐time sensors. This system developed is illustrated in Figure 7. monitoring of the blood glucose level, blood pressure, and several kinds of diagnostics could be possible too, by using real‐time sensors. This system developed is illustrated in Figure 7.

FigureFigure 7. 7.An An illustration illustration of of the the real real timetime electrocardiogram (ECG) (ECG) monitoring monitoring environment environment [13]. [13 ]. Figure 7. An illustration of the real time electrocardiogram (ECG) monitoring environment [13]. AnAn alert alert portable portable tele-medical tele‐medical monitor monitor system (AMON) (AMON) is is proposed proposed in in [14], [14 aiming], aiming to toprovide provide continuous monitoring for high‐risk cardiac/respiratory patients. The system collects multiple vital continuousAn alert monitoring portable tele for‐ high-riskmedical monitor cardiac system/respiratory (AMON) patients. is proposed The system in [14], collects aiming multipleto provide vital signs, detects multi‐parameter medical emergencies and is connected to a cellular telemedicine center signs,continuous detects monitoring multi-parameter for high medical‐risk cardiac/respiratory emergencies and ispatients. connected The tosystem a cellular collects telemedicine multiple vital center (TMC). This system uses a wrist worn device, to monitor vital parameters of patients in order to (TMC).signs, Thisdetects system multi‐ usesparameter a wrist medical worn emergencies device, to monitorand is connected vital parameters to a cellular of telemedicine patients in center order to provide an integrated picture of their health condition. provide(TMC). an This integrated system pictureuses a wrist of their worn health device, condition. to monitor vital parameters of patients in order to provide2.2.2. Home an integrated Assisting pictureSystems of their health condition. 2.2.2. Home Assisting Systems The authors of [15] presented a homecare monitoring platform with internet remote connection 2.2.2.The Home authors Assisting of [15] Systems presented a homecare monitoring platform with internet remote connection to to assisted people and their environment. This platform can monitor and diagnose patients remotely, assistedThe people authors and of their [15] presented environment. a homecare This platform monitoring can platform monitor with and internet diagnose remote patients connection remotely, in real time in their home environment by the utilization of wearable or even surgically inserted to assisted people and their environment. This platform can monitor and diagnose patients remotely, in realbio‐ timesensors. in their A hybrid home model environment of the platform by the is utilizationproposed, with of wearable various levels or even in terms surgically of functionality, inserted bio- in real time in their home environment by the utilization of wearable or even surgically inserted sensors.combining A hybrid fixed and model mobile of the nodes. platform These is levels proposed, range withfrom varioussimple data levels acquisition in terms of of the functionality, assisted bio‐ sensors. A hybrid model of the platform is proposed, with various levels in terms of functionality, combiningperson to fixed primary and care mobile and nodes.emergency These nodes, levels up range to the from communication simple data and acquisition coordination of the with assisted an combining fixed and mobile nodes. These levels range from simple data acquisition of the assisted personappointed to primary help center care andas a hospital. emergency nodes, up to the communication and coordination with an person to primary care and emergency nodes, up to the communication and coordination with an appointedDuring help their center research, as a hospital. the authors of [16] investigated real‐time sensors for the diagnosis of appointed help center as a hospital. cardiacDuring patients their research, by using thesmartphone authors of and [16 wearable] investigated sensors. real-time The authors sensors introduced for the diagnosis this application of cardiac During their research, the authors of [16] investigated real‐time sensors for the diagnosis of patientsto cater by a using remote real time monitoring and wearable for severe sensors. cardiac The patients authors unable introduced to attend this a application routine checkup. to cater a cardiac patients by using smartphone and wearable sensors. The authors introduced this application remoteOne of real the time main monitoring reasons behind for severe this technology cardiac patients is to commercialize unable to attend it for athe routine benefit checkup. of those patients One of the to cater a remote real time monitoring for severe cardiac patients unable to attend a routine checkup. mainwho reasons are not behind financially this technology sound. Furthermore, is to commercialize the system it was for thedeveloped benefit ofin thosesuch a patients manner who that areany not One of the main reasons behind this technology is to commercialize it for the benefit of those patients kind of crisis phase is dealt with in terms of alerting messages that are automatically sent to the doctor. financiallywho are not sound. financially Furthermore, sound. Furthermore, the system was the developedsystem was in developed such a manner in such that a manner any kind that of any crisis An overview of the operation of this system is shown in Figure 8. phasekind is of dealt crisis with phase in is terms dealt of with alerting in terms messages of alerting that messages are automatically that are automatically sent to the doctor. sent to the An doctor. overview ofAn the overview operation of of the this operation system isof shownthis system in Figure is shown8. in Figure 8.

Figure 8. The system configuration of the remote patient monitoring application [16]. Figure 8. The system configuration of the remote patient monitoring application [16]. Figure 8. The system configuration of the remote patient monitoring application [16].

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Appl. Syst. Innov. 2020, 3, 14 7 of 24 2.2.3.Appl. Hospital Syst. Innov. Patient 2020, 3, 14 Monitoring (or Hospitalization) 7 of 24 2.2.3. Hospital Patient Monitoring (or Hospitalization) 2.2.3.Within Hospital healthcare Patient facilities, Monitoring such (or asHospitalization) a hospital, WSNs systems can be integrated to provide real Within healthcare facilities, such as a hospital, WSNs systems can be integrated to provide real time patient monitoring and emergency alerting for a more precise and quick response. A typical timeWithin patient healthcare monitoring facilities, and emergency such as a hospital, alerting WSNsfor a more systems precise can beand integrated quick response. to provide A typical real exampletimeexample patient of an of applicationmonitoring an application ofand this of emergency this kind kind is presentedis alerting presented for in ina [17 [17].more]. Specifically, Specifically, precise and the the quick authors authors response. described A typical a a WSN WSN applicationexampleapplication of where an where application wireless wireless sensorsof this sensors kind are are placedis presented placed within within in the[17]. the emergency Specifically, emergency rooms roomsthe authors of of JohnJohn described Hopkins a hospitalWSN hospital to monitorapplicationto monitor in realwhere in real time wireless time the the blood sensors blood oxygen areoxygen placed and and heart within heart rate therate of emergency of the the patients. patients. rooms The The of researchersJohnresearchers Hopkins collected hospital the the performanceto performancemonitor in statistics real statistics time of the the of bloodthe network network oxygen and and and despite despiteheart therate the di of difficultyffi theculty patients. of of the the The hospital hospital researchers environmentenvironment collected due duethe to to interferenceperformanceinterference and statistics radioand noise, of thenoise, the network application the application and despite of WSNs of theWSNs can difficulty improve can improve of the the operation hospital the operation environment of a healthcare of a healthcare due facility. to Theinterference devicefacility. developed The and device radio is developed depictednoise, the is in applicationdepicted Figure9. in ofFigure WSNs 9. can improve the operation of a healthcare facility. The device developed is depicted in Figure 9.

Figure 9. A monitor that can both display and transmit vital signs [17]. FigureFigure 9. 9.A A monitor monitor that that cancan both display and and transmit transmit vital vital signs signs [17]. [17 ]. 2.3. Environmental Applications 2.3.2.3. Environmental Environmental Applications Applications Environmental applications that demand continuous monitoring of ambient conditions at EnvironmentalhostileEnvironmental and remote applications applications areas can that be that demandimproved demand continuous with continuous the utilization monitoring monitoring of ofWSNs. ambientof ambient In Figure conditions conditions 10, the at hostile mainat andhostile remotesubcategories and areas remote can of environmental beareas improved can be withapplicationsimproved the utilization with of WSNs, the ofutilization namely WSNs. water Inof FigureWSNs. monitoring, 10 In, theFigure air main monitoring, 10, subcategories the main and of environmentalsubcategoriesemergency alerting,of applicationsenvironmental are depicted of applications WSNs, along namely with of WSNs, the water types namely monitoring, of sensors water monitoring, airthat monitoring, are typically air monitoring, and used emergency in andthem. alerting,emergencyWSNs are that depicted alerting, have been along are depicteddeveloped with the along types for thesewith of sensors thetypes types of that environmental of are sensors typically that applications used are typically in them. are used WSNs studied in that them. in have the beenWSNsfollowing developed that havesubsection. for been these developed types of environmental for these types applications of environmental are studied applications in the followingare studied subsection. in the following subsection.

Figure 10. The subcategories of the environmental WSN applications and the types of the sensors used in them. Figure 10. The subcategories of the environmental WSN applications and the types of the sensors used 2.3.1. WaterFigurein them. Monitoring 10. The subcategories of the environmental WSN applications and the types of the sensors used in them. Water, either for drinking or oceanic is an important factor in human lives, therefore the monitoring of water has a great academic interest for researchers as described below. The researchers in [18] designed a WSN application to evaluate the quality of fresh drinkable water. They designed a Cyber physical system (CPS) called PipeSense, which is an in-pipe system for

Appl. Syst. Innov. 2020, 3, 14 8 of 24 water monitoring that utilizes RFID (Radio Frequency Identification) based WSN. The network can provide information about water demand or and various repair information such as weak spots or pipe leakage. The in-pipe RFID sensors collect information from the system and send them to the data servers, where algorithms provide decision support. A WSN application for marine environment monitoring is presented in [19]. In order to prevent damage to the flora and fauna of a fish farm from feed and fecal waste, the authors designed an Underwater WSN (UWSN) with ground based wireless sensor nodes capable of monitoring the pollution of the farm. The sensor nodes are mobile in a limited space in order to measure a greater area.

2.3.2. Air Monitoring Air is a vital element for human lives and nowadays the of the atmosphere is a result of many modern human activities. WSNs can be utilized for air quality monitoring in occupied regions in order to prevent dangerous diseases and contaminations or risk the health of people. A characteristic example of an application of this kind is presented in [20], where an air quality monitoring application assisted by WSNs, called WSN-AQMS, is proposed. The specific system combines gas sensors along with Libelium waspmotes [21] to measure air quality parameters of gases such as ozone, CO, and NO2. The waspmotes monitor in real time the air quality and utilize the Zigbee protocol for data communication. The authors further introduced the Clustering Protocol for Air Sensor network (CPAS) in order to support the operation of this system

2.3.3. Emergency Alerting Proactive monitoring of the causes of natural disasters, can help to avoid these disasters or/and lower their cost. WSNs can be utilized for monitoring common disastrous causes in real time to provide proactive alerts in order to lower damage or even prevent disaster. Typical examples described in the rest of this subsection, are related to the monitoring of seismic activity, volcanic activity, forest fires, and tsunamis.

Seismic Activity Monitoring Earthquakes can cause enormous damage to an occupied region where they take place. WSNs can be utilized to monitor seismic activity in real time in order to take precautionary measures and enable the authorities to act in advance. A real time seismic activity monitoring system is presented. In [22], the authors designed a warning system for earthquakes in order to increase the time before an earthquake so as to take precaution measures. The authors deployed a WSN in the island of Mauritius, which has high seismic activity. The system monitors seismic activity by utilizing primary waves (P-waves) and estimates local velocity and the hypocenter’s location according to time delays in the arrival of the P-waves at the sensors.

Volcanic Activity Monitoring Volcanoes can cause enormous damage to nearby towns or cities when they are activated. Before a volcano erupts, there are many signs that a WSN system can measure, proactively, in order to inform nearby citizens about the eruption. When such a system is applied, citizens can protect their families and belongings by transporting them outside of the area of the eruption, preventing further damage. Below is an example of such a system. In [23], a WSN based system to monitor volcanic activity components is proposed. The system is low cost, flexible, and easy to deploy and to maintain for remote locations. The users of the system can choose GPS data synchronization when the sensor nodes have signal reception, or a specific algorithm when they have not, to collect accurate timestamps of each sample. Pieces of the equipment used, are shown in Figure 11. Appl. Syst. Innov. 2020, 3, 14 9 of 24

Appl.can Syst. choose Innov. 2020GPS, 3data, 14 synchronization when the sensor nodes have signal reception, or a specific9 of 24 algorithm when they have not, to collect accurate timestamps of each sample. Pieces of the equipment used, are shown in Figure 11.

FigureFigure 11. 11.A A depiction depiction of of a a geophone geophone sensorsensor with a a wireless wireless antenna antenna deployed deployed in inthe the field field [23]. [23 ].

ForestForest Fire Fire Prevention Prevention ForestForest fires fires can can destroy destroy both both animals animals andand vegetation, thus thus resulting resulting in in huge huge ecological ecological disasters. disasters. In orderIn order to to prevent prevent such such disastrous disastrous events events WSNsWSNs can be be deployed deployed within within forests forests and and monitor monitor in real in real timetime related related parameters parameters in in order order to to assist assist inin forestforest firefire prevention. One One example example of ofthis this type type of WSN of WSN applicationapplication is presentedis presented below. below. DueDue to theto the rapid rapid climate climate change change taking taking place, place, the the necessity necessity for properfor proper forest forest fire preventionfire prevention means means is extremely pressing. Such an example is described in [24], which combines traditional is extremely pressing. Such an example is described in [24], which combines traditional methods methods (patrols, watchtowers, satellite imaging) to a WSN array monitoring system using ZigBee (patrols, watchtowers, satellite imaging) to a WSN array monitoring system using ZigBee protocol [25]. protocol [25]. Related parameters as temperature, humidity, etc. are monitored in real time and sent Related parameters as temperature, humidity, etc. are monitored in real time and sent to a monitoring to a monitoring center in order to be analyzed. Then the system can make quick estimations of fire center in order to be analyzed. Then the system can make quick estimations of fire danger and inform danger and inform the authorities. the authorities. Tsunami Detection Tsunami Detection Another is the tsunami waves. It is crucial for coastal regions to be informed earlyAnother of such natural a catastrophe. disaster WSNs is the tsunamican be utilized waves. for It real is crucial time monitoring for coastal enabling regions to the be authorities informed to early of suchact proactively. a catastrophe. Below WSNs a Tsunami can be detection utilized WSN for real application time monitoring is presented. enabling the authorities to act proactively.In [26], Below a tsunami a Tsunami detection detection system WSN which application utilizes is a presented. WSN with underwater sensor nodes deployedIn [26], ain tsunami coastal regions detection is systemproposed. which In order utilizes to inform a WSN proactively with underwater the authorities, sensor nodes the system deployed in coastalutilizes regionsa lexical is resource proposed. messaging In order tosystem, inform called proactively SentiWordNet, the authorities, which is the able system to provide utilizes a lexicalinformation resource extracted messaging from system, the sensor called messages. SentiWordNet, which is able to provide information extracted from the sensor messages. 2.4. Flora and Fauna Applications 2.4. Flora and Fauna Applications Both flora and fauna domains are vital for every country. In Figure 12, the main subcategories of Bothflora floraand fauna and fauna applications domains of WSNs are vital which for everynamely country. are greenhouse In Figure monitoring, 12, the main crop subcategories monitoring, of floraand and livestock fauna applicationsfarming, are ofillustrated WSNs which along namelywith the are types greenhouse of sensors monitoring, that are most crop commonly monitoring, used and livestockin them. farming, are illustrated along with the types of sensors that are most commonly used in them. WSNs that have been developed for these types of flora and fauna applications are examined in the following subsection.

2.4.1. Greenhouse Monitoring An important sector of the agriculture domain involves greenhouses. Within them many crops can be grown to provide sustainable food while climate crops can be harvested all year round if certain conditions are applied within the greenhouse. Therefore, WSNs can be applied in greenhouse monitoring and control to improve their operation. Below are some examples of these applications Appl. Syst. Innov. 2020, 3, 14 10 of 24

In [27], a system of this type, called the Agricultural Environment Monitoring System (AEMS), is presented. It is an inexpensive and easy to apply system that can collect and monitor data related to crop growth, inside or outside a greenhouse via WSN sensors and CCTV cameras. The system gathers vital environmental parameters such as temperature, light intensity, humidity, air pressure, rainfall level,Appl. pH, Syst. and Innov. electrical 2020, 3, 14 conductivity (EC). 10 of 24

Figure 12. The subcategories of flora and fauna applications of WSNs and the types of the sensors used Figure 12. The subcategories of flora and fauna applications of WSNs and the types of the sensors in them. used in them. A relevant system developed for greenhouses which has energy management and indoor WSNs that have been developed for these types of flora and fauna applications are examined in climatethe following control capabilities, subsection. is introduced in [28]. The system monitors vital greenhouse parameters such as indoor luminance, temperature, and relative humidity, via sensor nodes. The indoor climate2.4.1. controlGreenhouse is possible Monitoring by the utilization of two fuzzy logic controllers, P (Proportional) and PD (Proportional-Derivative) that use the desired indoor climatic set-points. Furthermore, the system An important sector of the agriculture domain involves greenhouses. Within them many crops utilizes output actuations of heating units, motor-controlled windows and shading curtains, artificial can be grown to provide sustainable food while climate crops can be harvested all year round if lighting, etc. in order to achieve more precise greenhouse control. certain conditions are applied within the greenhouse. Therefore, WSNs can be applied in greenhouse monitoring and control to improve their operation. Below are some examples of these applications 2.4.2. Crop Monitoring In [27], a system of this type, called the Agricultural Environment Monitoring System (AEMS), is Withinpresented. the agricultureIt is an inexpensive domain, and the easy preservation to apply system of the crops that can plays collect a vital and role. monitor In order data to related provide a betterto crop environment growth, inside for theor outside crops, variousa greenhouse WSNs via applications WSN sensors can and be implemented.CCTV cameras. Crop The irrigationsystem andgathers fertilization vital environmental are some examples parameters of the such applications as temperature, that have light been intensity, designed humidity, as described air pressure, below. rainfallHidro level, Bus ispH, a system and electrical described conductivity in [29]. It (EC). is a remote controlled, automatic irrigation system for large areasA relevant of land appliedsystem developed at Jumilla (Murcia,for greenhouses Spain). which The system has energy divides management the deployment and indoor area into sevenclimate sub-regions, control capabilities, where each is has introduced a control in sector [28]. The for monitoring system monitors and controlling. vital greenhouse The control parameters sectors communicatesuch as indoor with luminance, each other temperature, and the central and relative controller humidity, via WLAN via sensor network. nodes. The indoor climate control is possible by the utilization of two fuzzy logic controllers, P (Proportional) and PD An automated fertilizer applicator system is built in [30], which consists of three modules, the (Proportional‐Derivative) that use the desired indoor climatic set‐points. Furthermore, the system input, the decision support, and the output modules. The input module can provide to the Decision utilizes output actuations of heating units, motor‐controlled windows and shading curtains, artificial Support System (DSS) module, GPS and real time sensor data by utilizing technology. With lighting, etc. in order to achieve more precise greenhouse control. these data the DSS is able to calculate quantity, application rate, and the spread pattern of the fertilizer that2.4.2. the outputCrop Monitoring module will provide to the crops. The authors of [31] developed and integrated an optimal fertilization decision support system Within the agriculture domain, the preservation of the crops plays a vital role. In order to using a wireless sensor LAN, IEEE 802.11 protocol (Wi-Fi) and a GIS analysis . Sensor nodes were provide a better environment for the crops, various WSNs applications can be implemented. Crop used to acquire vital data in real time such as soil moisture, conductivity, temperature, etc. Also, the irrigation and fertilization are some examples of the applications that have been designed as system used B/S a (browser/server) structure mode to provide high interactivity. Also, a GIS analysis described below. server wasHidro used Bus to is interpolatea system described the data in from [29]. It small is a remote experimental controlled, plots automatic to larger irrigation plots to system exploit for data large areas of land applied at Jumilla (Murcia, Spain). The system divides the deployment area into seven sub‐regions, where each has a control sector for monitoring and controlling. The control sectors communicate with each other and the central controller via WLAN network. An automated fertilizer applicator system is built in [30], which consists of three modules, the input, the decision support, and the output modules. The input module can provide to the Decision Support System (DSS) module, GPS and real time sensor data by utilizing bluetooth technology. With

Appl. Syst. Innov. 2020, 3, 14 11 of 24

these data the DSS is able to calculate quantity, application rate, and the spread pattern of the fertilizer that the output module will provide to the crops. The authors of [31] developed and integrated an optimal fertilization decision support system using a wireless sensor LAN, IEEE 802.11 protocol (Wi‐Fi) and a GIS analysis server. Sensor nodes Appl. Syst. Innov. 2020, 3, 14 11 of 24 were used to acquire vital data in real time such as soil moisture, conductivity, temperature, etc. Also, the system used B/S a (browser/server) structure mode to provide high interactivity. Also, a GIS reductionanalysis for server energy was conservation. used to interpolate An overview the data offrom the small overall experimental system architecture plots to larger is illustrated plots to in Figureexploit 13. data reduction for energy conservation. An overview of the overall system architecture is illustrated in Figure 13.

FigureFigure 13. 13.A A structure structure diagram diagram of of the the integratedintegrated optimal fertilization fertilization decision decision support support system system [31]. [31 ]. Similarly, in [32] a scheme based on the collaboration of an integrated system for an automated Similarly, in [32] a scheme based on the collaboration of an integrated system for an automated irrigationirrigation management management with with an advancedan advanced novel novel routing protocol protocol for WSNs,for WSNs, was was proposed. proposed. This This system aimssystem at effi aimsciently at managingefficiently managing water supply water in cultivatedsupply in cultivated fields in an fields automated in an automated way. The systemway. The takes intosystem consideration takes into the consideration historical data the historical and the change data and in the change climate in values the climate to calculate values the to calculate quantity of waterthe thatquantity is needed of water for that irrigation. is needed for irrigation.

2.4.3.2.4.3. Livestock Livestock Farming Farming LivestockLivestock farming farming is is a a main main sector sector ofof thethe faunafauna domain. WSNs WSNs can can be be applied applied in invarious various tasks tasks suchsuch as as livestock livestock monitoring. monitoring. Below Below we we havehave collected some some examples examples of of these these applications. applications. In [In33 [33],], a sensor-based a sensor‐based system system developed developed to analyze to analyze the behaviorthe behavior of livestock of livestock animals animals and support and theirsupport farming their was farming proposed. was proposed. Green pastures Green pastures are used are byused cattle by cattle for grazing, for grazing, so grassso grass growth growth was analyzedwas analyzed through through photographic photographic sensors, sensors, so that so animalsthat animals can be can moved be moved towards towards them. them. Their Their major objectivemajor objective of work of was work to designwas to design rugged rugged hardware hardware that couldthat could be used be used outdoors outdoors for for modeling modeling both individualboth individual and herd and behavior herd behavior of animals. of animals. Although, Although, specially specially designed designed sensors sensors to monitor to monitor animal behavior,animal behavior, such as sleeping, such as sleeping, grazing, grazing, and ruminating, and ruminating, are used, are cattle used, monitoringcattle monitoring still poses still poses several challengesseveral challenges like radio like attenuation radio attenuation caused due caused to factors due to suchfactors as such animal as animal body, mobilitybody, mobility etc. etc. In [34], an inexpensive low power collar designed to assist livestock farming is proposed. The In [34], an inexpensive low power collar designed to assist livestock farming is proposed. The system developed uses a solar power relay router and two antennas placed so that the collar radio system developed uses a solar power relay router and two antennas placed so that the collar radio coverage is optimized. Also, a novel protocol, named Implicit Routing Protocol (IRP), was proposed coverage is optimized. Also, a novel protocol, named Implicit Routing Protocol (IRP), was proposed in in order to cope with the packet losses, which are caused due to the mobility of the animals. order toIn cope [35], with the theauthors packet describe losses, a which system are based caused on dueRFID to and the WSN mobility technologies, of the animals. which can be appliedIn [35 ],in themodern authors farm describe large‐scale a system management based systems. on RFID The and proposed WSN technologies, system can identify which and can be appliedtrack all in animals modern (i.e., farm pigs) large-scale that are present management within systems.the farm, and The monitor proposed their system health can as well identify as the and track all animals (i.e., pigs) that are present within the farm, and monitor their health as well as the environmental conditions. According to the authors this system, by the aforementioned parameters, can be used to prevent diseases, determine environment pollution, and inform on food safety issues.

2.5. Industrial Applications WSNs can be applied in various industrial applications to solve many related problems. In Figure 14, the main subcategories of industrial applications of WSNs namely logistics, robotics, and Appl. Syst. Innov. 2020, 3, 14 12 of 24 Appl. Syst. Innov. 2020, 3, 14 12 of 24 environmental conditions. According to the authors this system, by the aforementioned parameters, environmentalcan be used to conditions. prevent diseases, According determine to the authors environment this system, pollution, by the and aforementioned inform on food parameters, safety issues. can be used to prevent diseases, determine environment pollution, and inform on food safety issues. 2.5. Industrial Applications Appl.2.5. Syst. Industrial Innov. 2020 Applications, 3, 14 12 of 24 WSNs can be applied in various industrial applications to solve many related problems. In FigureWSNs 14, canthe mainbe applied subcategories in various of industrialindustrial applications applications of to WSNs solve namely many relatedlogistics, problems. robotics, Inand machineryFiguremachinery 14, health the health main monitoring monitoringsubcategories are are illustrated. of illustrated. industrial These Theseapplications specific specific categoriesof categories WSNs namely of of applications applications logistics, robotics, are studied and in in themachinery restthe of rest this of health subsection.this subsection. monitoring are illustrated. These specific categories of applications are studied in the rest of this subsection.

Figure 14. The subcategories of the industrial applications of WSNs and the types of the sensors used Figure 14. The subcategories of the industrial applications of WSNs and the types of the sensors used in them. Figurein them. 14. The subcategories of the industrial applications of WSNs and the types of the sensors used 2.5.1. Logisticsin them. 2.5.1. Logistics 2.5.1.The Logistics domain of logistics is an area of interest where WSNs can be applied, because many logistics The domain of logistics is an area of interest where WSNs can be applied, because many logistics systems need real time monitoring of various environmental parameters and better handling of systemsThe domain need real of logistics time monitoring is an area ofof interest various where environmental WSNs can beparameters applied, becauseand better many handling logistics of packages. These requirements can be fulfilled by combining the logistics systems with WSNs. Some systemspackages. need These real requirementstime monitoring can beof fulfilledvarious byenvironmental combining the parameters logistics systems and better with handling WSNs. Some of typespackages.types of these of theseThese applications applications requirements are describedare can described be fulfilled below. below. by combining the logistics systems with WSNs. Some typesThe of transportThe these transport applications logistics logistics are sector sector described requires requires below. low low cost cost andand high high-quality‐quality during during deliveries. deliveries. In [36], In [the36], the developmentdevelopmentThe transport and and logistics deployment sector of of arequires aWSN WSN based low based costsystem system and for high formonitoring‐ monitoringquality duringtransportation transportation deliveries. conditions, In conditions, [36], thesuch suchdevelopmentas temperature and deploymentand humidity, of awithin WSN based a cargo system container for monitoring travelling transportation via via both both a a trans trans-Atlantic conditions,‐Atlantic such cargo cargo vesselasvessel temperature and a and lorry a lorry is and described. is humidity, described. The withinThe main main ideaa cargo idea for for container the the deployment deployment travelling of of the thevia monitoring monitoringboth a trans system‐Atlantic is is depicted depicted cargo in Figurevesselin Figure 15and. It a 15. lorry is shownIt is is shown described. that that the Thethe use usemain of aof systemidea a system for of the of this deployment this kind, kind, can can of increase increasethe monitoring quality quality system by providing is depicted better better supervisioninsupervision Figure and15. It lowerand is shown lower the that costthe costthe by use reducingby reducingof a system losses losses of during this during kind, transportation. transportation. can increase quality by providing better supervision and lower the cost by reducing losses during transportation.

Figure 15. Depiction of the basic configuration of a WSN based system for transport logistics Figure 15. Depiction of the basic configuration of a WSN based system for transport logistics Figureapplications 15. Depiction [36]. of the basic configuration of a WSN based system for transport logistics applicationsapplications [36 [36].]. The application of WSNs in Cold Chain Logistics (a continuous temperature-controlled supply chain) can greatly improve the monitoring and management of these chains [37]. WSNs are suitable for real time monitoring of many environmental parameters and provide accurate data collection that meets the demand of Cold Chain Logistics. The researchers used a Zigbee ad hoc network model to build the system’s framework. By using fuzzy control decision, the environmental parameters are maintained in a stable range and with Maximum Similarities Multiple Characteristic Recognition (MSMCR) the safety of cold-chain food is ensured. Appl. Syst. Innov. 2020, 3, 14 13 of 24

Another application of WSNs developed for Cold Chain Logistics is studied in [38]. This system focuses only on aquatic products and their transportation. In order to develop such a system, the authors proposed a WSN integrated with Compressed Sending (CS), in order to combat heavy data traffic from the real time sensor data transmission. Also, CS provides a low complexity approximation which assists storage, transmission, processing, and meets the recourse constraints of WSNs. A WSN application in logistics that utilizes GPS technology is described in [39]. This system monitors in real-time the status of the goods and has embedded a terminal, which is used to locate the goods and a cloud services platform, which is used to identify the recipient.

2.5.2. Robotics Nowadays there are many applications that combine WSNs and robots. Robots can cooperate and combat some of the major problems of WSNs, such as sensor node mobility, node redeployment, travelling salesman, etc. Typical WSN applications of this kind are presented below. A robotic navigation method proposed in [40] provides road maps for the robot to traverse. It uses a WSN with sensors, designed to provide sophisticated maps of their sensing areas. Specifically, each sensor constructs a map, based on the traversable area sensed. Then, all sensor maps are combined to create one large map. Once the road maps are generated, the sensors are used to sense areas of interest for the robot to travel to. The robot then considers all possible roads to take and selects the most efficient path available in the network. If an area becomes hazardous for the robot, the network can reconfigure the road map, and remove this hazardous area from the list of available paths. One of the most difficult tasks concerning WSNs is the maintenance of a projected network. A combination of WSNs and robotics was researched in [41], where a robotic network servicing system, named Randomized Robot assisted Relocation of Static Sensors (R3S2) was developed. In R3S2, robots move around a network which is contained within a virtual grid. The robot moves to the least recently visited grid point, searching for sensing holes in the network. When an area that is not being covered by a sensor is detected, the robot will find a node which has overlapping coverage with other nodes and move it to the uncovered area. In addition, if the robot discovers redundant sensors, it will move the nodes to cover a greater area. A different approach is proposed in [42], where a mobile robot is used to transfer data from a widespread network between nodes that are out of reach from one another for various reasons. This is the so-called travelling salesman problem (TSP)—within a WSN. By having a robot traveling among nodes which are out of each other’s wireless communication range, it allows the network to be widespread while also saving power by using the robot for data muling.

2.5.3. Machinery Health Monitoring The objective of machinery health monitoring is to examine the performance of various types of technical equipment and to either detect or predict the occurrence of faults that are obstructive or even catastrophic for their operation. In [43], a WSN is developed in order to perform energy usage evaluation and condition monitoring for electric machines. The motor efficiency and health condition are estimated non- intrusively by using wireless nodes that monitor the motor terminal quantities (i.e., line voltages, line currents, and temperature, with no interference with the operation of the electric machines. In [44], a WSN based monitoring system of oil and gas pipelines, named REMONG, is proposed. In order to detect the existence of leakages, the system uses wireless sensor nodes which monitor the pressure and temperature of the pipeline fluid at several points of interest on the pipelines which are stretched over large geographical areas. In [45] a WSN designed to enhance safety in industrial machinery consisting of a main vehicle and an attached trailer is proposed. A 3D accelerometer and a 3D magnetometer, incorporated in a sensor system device, monitor the trailer operating conditions and the corresponding data are wirelessly transmitted to a processing unit which executes a stability control algorithm. A vibrational energy Appl. Syst. Innov. 2020, 3, 14 14 of 24

In [44], a WSN based monitoring system of oil and gas pipelines, named REMONG, is proposed. In order to detect the existence of leakages, the system uses wireless sensor nodes which monitor the pressure and temperature of the pipeline fluid at several points of interest on the pipelines which are stretched over large geographical areas. In [45] a WSN designed to enhance safety in industrial machinery consisting of a main vehicle Appl. Syst. Innov. 2020, 3, 14 14 of 24 and an attached trailer is proposed. A 3D accelerometer and a 3D magnetometer, incorporated in a sensor system device, monitor the trailer operating conditions and the corresponding data are harvestingwirelessly system transmitted developed, to a processing converts kinetic unit which energy executes from trailer a stability natural control vibrations algorithm. to electrical A vibrational energy for theenergy system harvesting power supply.system developed, converts kinetic energy from trailer natural vibrations to electrical energy for the system power supply. 2.6. Urban Applications 2.6. Urban Applications The variety of sensing abilities offered by WSNs also provides an opportunity to gain an The variety of sensing abilities offered by WSNs also provides an opportunity to gain an unprecedented level of information about a target area, be it a room, a building, or outdoors. WSNs unprecedented level of information about a target area, be it a room, a building, or outdoors. WSNs are indeed a tool to measure the spatial and temporal features of any phenomena within an urban are indeed a tool to measure the spatial and temporal features of any phenomena within an urban environment, providing a limitless number of applications. The most popular applications of WSNs environment, providing a limitless number of applications. The most popular applications of WSNs in thein the urban urban domain domain are are related related to to smart smart homes, homes, smart smart cities,cities, transportation systems, systems, and and structural structural healthhealth monitoring, monitoring, as as depicted depicted in in Figure Figure 16 16 and and further further describeddescribed in the the rest rest of of this this subsection. subsection.

Urban

Applications

Smart Smart Transportation Structural Health Homes Cities Systems Monitoring

Gas & Accelero- Motion Smoke Visual Optical Corrosion meters Sensors Sensors Sensors Sensors Sensors

Biomedical Pressure Acoustic Strain Sensors Sensors Sensors Sensors

FigureFigure 16. 16.The The subcategories subcategories of of the the urbanurban applications of of WSNs WSNs and and the the types types of the of thesensors sensors used used in in them.them. 2.6.1. Smart Cities 2.6.1. Smart Cities In largeIn large cities cities monitoring monitoring of of various various parameters parameters is crucial for for the the optimal optimal life life of of the the citizens. citizens. WSNsWSNs have have a huge a huge amount amount of of applications applications to to provide provide realreal time data to to the the authorities authorities for for the the optimal optimal functionfunction of aof city. a city. Specifically, Specifically, increased increased transportation transportation ofof people creates creates problems problems and and time time wastage, wastage, whenwhen a large a large number number of of vehicles vehicles is is heading heading towards towards common common destinations. WSNs WSNs can can be be utilized utilized for for monitoringmonitoring in orderin order to to reduce reduce tra traffic,ffic, indicate indicate car car parking parking spots,spots, etc. [46]. [46]. In [In47 ][47] a WSN-based a WSN‐based intelligent intelligent car car parking parking systemsystem is proposed. Specifically, Specifically, within within a car a car park park area,area, each each parking parking lot lot is equipped is equipped with with one one inexpensive inexpensive sensor sensor node, node, which which detects detects the the occupation occupation or vacancyor vacancy of this of parking this parking lot. lot. A base A base station station collects collects data data relative relative to to thethe status of of all all parking parking lots lots and and sendssends periodic periodic reports reports to to a database.a database. This This database database cancan be accessed by by the the upper upper layer layer management management systemsystem in orderin order to to execute execute several several tasks, tasks, such such as as vacantvacant parking lots lots discovery, discovery, security security supervision, supervision, automatedautomated toll, toll, or/ or/andand statistic statistic report report creation. creation. In [In48 ],[48], a system a system to to measure measure and and classify classify road road tra trafficffic basedbased on WSN is is proposed. proposed. The The nodes nodes of of this system have integrated magnetic sensors and are deployed along the road in order to provide this system have integrated magnetic sensors and are deployed along the road in order to provide real time traffic monitoring. This configuration has been proposed as an easy and cheap to implement real time traffic monitoring. This configuration has been proposed as an easy and cheap to implement alternative to inductive coils that are traditionally deployed in roads for basic traffic control tasks. The architecture adopted in the system proposed is graphically presented in Figure 17. In [49], a system for road traffic control near flooded tunnels is proposed. The specific system, consists of a WSN and a centralized control system. The analogue output signals of sensors, located in the interior of two underground tunnels, are converted to digital ones which are next transmitted through serial-to-Ethernet converters to an access point. Following a corresponding procedure the signals reach a programmable logic controller, which monitors the level of water in the interior of two Appl. Syst. Innov. 2020, 3, 14 15 of 24

Appl. Syst. Innov. 2020, 3, 14 15 of 24 underground tunnels and accordingly not only regulates the water drainage of flooded tunnels by usingalternative pumps, to but inductive also performs coils that tra areffic traditionally control of vehicles deployed approaching in roads for the basic entrance traffic to control the tunnels. tasks. The architecture adopted in the system proposed is graphically presented in Figure 17.

FigureFigure 17. 17.A A depiction depiction ofof thethe various setups setups where where WSNs WSNs can can be be deployed deployed [48]. [48 ].

A WSNIn [49], based a system system for road for street traffic lighting control monitoring near flooded and tunnels control is proposed. is presented The in specific [50]. This system, system enablesconsists the of remote a WSN controland a centralized of street lightingcontrol system. lamps, The by analogue using Doppler output sensorssignals of to sensors, allow for located vehicle detection.in the interior The light of two intensity underground of the lamps tunnels, is increased, are converted to a to preset digital level, ones in which the presence are next of transmitted approaching vehiclesthrough and serial reduced‐to‐Ethernet in the absenceconverters of to them. an access point. Following a corresponding procedure the signals reach a programmable logic controller, which monitors the level of water in the interior of 2.6.2.two Smart underground Homes tunnels and accordingly not only regulates the water drainage of flooded tunnels byIn using the erapumps, of informatics, but also performs various traffic systems control can of improve vehicles human approaching lives. Wirelessthe entrance sensor to the networks tunnels. can be appliedA WSN in the based indoor system environment for street lighting as in smart monitoring homes and where control machine is presented to machine in [50]. communications This system enables the remote control of street lighting lamps, by using Doppler sensors to allow for vehicle take place. Two typical examples are the indoor localization and motion monitoring and the monitoring detection. The light intensity of the lamps is increased, to a preset level, in the presence of of the indoor air quality. approaching vehicles and reduced in the absence of them. Specifically, the monitoring of indoor air quality (IAQ), which is a term that refers to the air quality of a2.6.2. building, Smart Homes has become, with the rapid urbanization process that is taking place, one of the most important topics of WSN urban applications. Modern people spend a great part of their life within In the era of informatics, various systems can improve human lives. Wireless sensor networks a building, thus the air quality plays a critical role for their health, safety, and comfort. In order to can be applied in the indoor environment as in smart homes where monitor IAQ the authors of [51] developed a WSN application that utilizes wireless nodes equipped communications take place. Two typical examples are the indoor localization and motion monitoring with gas sensors for sensing and communicating wirelessly in real time. and the monitoring of the indoor air quality. Similarly, the WSN based IAQ monitoring system, which is described in [52], uses sensor nodes Specifically, the monitoring of indoor air quality (IAQ), which is a term that refers to the air thatquality measure of a building, temperature, has become, relative with humidity, the rapid and urbanization concentration process of carbon that is taking dioxide place, in classrooms one of the in ordermost to important correlate the topics level of of WSN indoor urban air qualityapplications. with theModern students’ people level spend of performance a great part inof their their studies. life withinAnother a building, popular thus application the air quality of indoor plays a localizationcritical role for and their motion health, monitoring safety, and comfort. that uses In WSNs, order is describedto monitor in [ 53IAQ]. The the authors authors developed of [51] developed a system, a whichWSN isapplication used to estimate that utilizes the position wireless of nodes a person withinequipped an indoor with gas environment. sensors for Thesensing system and utilizescommunicating mobile sensorwirelessly nodes in real worn time. by the moving persons and a networkSimilarly, of the static WSN sensor based nodes IAQ atmonitoring known locations. system, which To calculate is described the person’s in [52], uses position, sensor the nodes system gathersthat measure data from temperature, the nodes relative in a central humidity, PC and and applies concentration Monte Carlo of carbon based dioxide estimation in classrooms algorithms in to trackorder the to moving correlate persons the level in of real indoor time. air quality with the students’ level of performance in their studies. Another popular application of indoor localization and motion monitoring that uses WSNs, is 2.6.3.described Transportation in [53]. The Systems authors developed a system, which is used to estimate the position of a person within an indoor environment. The system utilizes mobile sensor nodes worn by the moving persons The transportation of vehicles within the urban environment is an important factor for the safety and a network of static sensor nodes at known locations. To calculate the person’s position, the system and proper function of public roads. In the modern era various researchers have proposed many gathers data from the nodes in a central PC and applies Monte Carlo based estimation algorithms to systems in the transportation domain, which by utilizing WSNs can improve the safety of roads track the moving persons in real time. and provide information during the use of them by drivers. Below we present some relative WSN applications2.6.3. Transportation applied in Systems the transportation domain. In [54] the authors describe a WSN application for VANETs. This application using Secure Multimedia Broadcast Framework (SMBP) is to monitor a persons’ way of driving by utilizing a

Appl. Syst. Innov. 2020, 3, 14 16 of 24 Appl. Syst. Innov. 2020, 3, 14 16 of 24 The transportation of vehicles within the urban environment is an important factor for the safety and Theproper transportation function of ofpublic vehicles roads. within In thethe modernurban environment era various is researchers an important have factor proposed for the safetymany andsystems proper in the function transportation of public domain, roads. Inwhich the modernby utilizing era WSNsvarious can researchers improve the have safety proposed of roads many and Appl.systemsprovide Syst. Innov. ininformation the2020 transportation, 3, 14 during domain,the use whichof them by byutilizing drivers. WSNs Below can we improve present the some safety relative of roads WSN and16 of 24 provideapplications information applied induring the transportation the use of them domain. by drivers. Below we present some relative WSN applicationsIn [54] theapplied authors in the describe transportation a WSN domain. application for VANETs. This application using Secure processingMultimediaIn [54] unit theBroadcast on theauthors vehicle, Framework describe which a analyzes(SMBP)WSN application is data, to monitor collected for aVANETs. persons’ from the Thisway sensors applicationof driving in order byusing to utilizing calculate Secure a the costMultimediaprocessing of the driver’s unit Broadcast on insurance. the vehicle,Framework The which described (SMBP) analyzes is system to data, monitor iscollected utilized a persons’ from in order the way sensors to of determine driving in order by how toutilizing calculate much a the driverprocessingthe cost detects of theunit nearby driver’s on the vehicles, vehicle,insurance. drives which The with analyzesdescribed caution, data, system followscollected is utilized the from tra in theffi orderc sensors law, to or determine in uses order a mobile to how calculate much device. Thethethe sensing drivercost of system detectsthe driver’s nearby includes insurance. vehicles, a static Thedrives sensor described with board caution, system that follows provides is utilized the the traffic in sensed order law, to dataor determine uses via a cellularmobile how device. networkmuch to thetheThe driver control sensing detects centersystem nearby and includes a vehicles, Global a static drives Position sensor with board System caution, that/Universal followsprovides the Mobilethe traffic sensed law, Telecommunications data or uses via cellulara mobile network device. System (GPSTheto/ UMTS)the sensing control module system center inincludes and order a Global toa static connect Position sensor to the board System/Universal Internet. that provides The basic Mobilethe configuration sensed Telecommunications data via of cellular SMBP network is System depicted in Figureto(GPS/UMTS) the control18. module center andin order a Global to connect Position to the System/Universal Internet. The basic Mobile configuration Telecommunications of SMBP is depicted System (GPS/UMTS)in Figure 18. module in order to connect to the Internet. The basic configuration of SMBP is depicted in Figure 18.

FigureFigure 18. 18.A A representation representation of of the the Secure Secure MultimediaMultimedia Broadcast Broadcast Framework Framework (SMBP) (SMBP) [54]. [54 ]. Figure 18. A representation of the Secure Multimedia Broadcast Framework (SMBP) [54]. In [In55 [55]] a system a system is proposed,is proposed, which which analyzes analyzes the the performance performance ofof a vehicle on the the road road by by utilizing utilizing a telematicsa telematicsIn [55] WSN. a WSN.system The system The is proposed, system utilizes utilizes which deployed analyzesdeployed sensor the sensor nodes performance nodes on the on roadof the a vehicle androad a and sinkon the a node sink road on node by the utilizing on roadside. the Thearoadside. sensorstelematics nodesThe WSN. sensors detect The thesystemnodes passing detectutilizes vehicles the deployed passing and transmitsensor vehicles nodes data and on to transmit the sinkroad node.dataand ato Furthermore,sink the node sink onnode. the when an eventroadside.Furthermore, occurs The awhen packetsensors an iseventnodes created, occurs detect the a sinkpacketthe passing node is created, implements vehicles the sinkand a timestamp nodetransmit implements todata it andto a thetimestamp forwards sink node. it to to it the comFurthermore,and node, forwards which whenit utilizes to the an com aevent level-based node, occurs which a static packet utilizes routing is created, a level protocol.‐ basedthe sink Next, static node the routing implements com nodeprotocol. forwards a timestamp Next, thisthe comto packet it to theandnode user, forwards forwards which it this analyzesto thepacket com the to node, the provided user, which which information utilizes analyzes a level in the order‐based provided to static determine information routing various protocol. in order metrics Next, to determine ofthe the com road suchnodevarious as aforwards vehicle’s metrics this of speed, packetthe road lane to thesuch vehicle user, as a densitywhich vehicle’s analyzes etc. speed, A depictionthe lane provided vehicle of theinformation density overall etc. system in Aorder depiction configuration to determine of the is shownvariousoverall in Figure systemmetrics 19 configuration of. the road such is shown as a invehicle’s Figure 19.speed, lane vehicle density etc. A depiction of the overall system configuration is shown in Figure 19.

Figure 19. A graphical depiction of the WSN based transportation monitoring system [55]. FigureFigure 19. 19.A A graphical graphical depiction depiction of of thethe WSNWSN based transportation transportation monitoring monitoring system system [55]. [55 ].

In recent years, various WSN have been deployed within cities resulting in the formation of small WSN based islands. In [56] the authors describe an integration of these existing islands deployed along the roadside, in order to be utilized for post-accident management and to prevent accidents. Furthermore, the authors utilize two wireless standards i.e., IEEE 802.15.4 and IEEE 802.11p due to the need for separate roadside sensor units. In order to prevent accidents, the WSN islands constantly detect road conditions and transmit information to the vehicles passing by the area, which aggregate Appl. Syst. Innov. 2020, 3, 14 17 of 24 Appl. Syst. Innov. 2020, 3, 14 17 of 24 In recent years, various WSN have been deployed within cities resulting in the formation of smallIn WSN recent based years, islands. various In WSN [56] havethe authors been deployed describe within an integration cities resulting of these in theexisting formation islands of deployedsmall WSN along based the islands.roadside, In in [56] order the to authors be utilized describe for post an‐ accidentintegration management of these existingand to preventislands deployed along the roadside, in order to be utilized for post‐accident management and to prevent Appl.accidents. Syst. Innov. Furthermore,2020, 3, 14 the authors utilize two wireless standards i.e., IEEE 802.15.4 and IEEE17 of 24 802.11paccidents. due Furthermore, to the need forthe separate authors roadside utilize two sensor wireless units. standardsIn order to i.e., prevent IEEE accidents, 802.15.4 andthe WSNIEEE islands802.11p constantly due to the detect need roadfor separate conditions roadside and transmit sensor informationunits. In order to the to vehiclesprevent passingaccidents, by thethe WSNarea, thesewhichislands data. aggregate constantly Then the these vehicledetect data. road that Then conditions has the this vehicle information, and transmitthat has transmits thisinformation information, warning to the transmits vehicles messages passingwarning to nearby by messages the vehicles area, in ordertowhich nearby for aggregate thevehicles data these in to order be data. disseminated for Then the data the vehicleto along be disseminated that the has road. this along The information, post-accident the road. transmits The post gathered ‐warningaccident information messagesgathered is restricted,informationto nearby by vehicles the is authors,restricted, in order in by order for the the to authors, data be used to bein only disseminatedorder by to authorized be used along only personnel the by road. authorized suchThe post as insurance personnel‐accident gathered companies, such as information is restricted, by the authors, in order to be used only by authorized personnel such as criminologicalinsurance companies, teams, or criminological other authorities. teams, or Moreover, other authorities. according Moreover, to the authors, according the to the post-accident authors, insurance companies, criminological teams, or other authorities. Moreover, according to the authors, managementthe post‐accident can function management properly can by function communicating properly withby communicating the WSN islands with on the the WSN roadside, islands without on the post‐accident management can function properly by communicating with the WSN islands on thethe use roadside, of VANETs. without The the overall use of system VANETs. operation The overall is illustrated system operation in Figure is 20 illustrated. in Figure 20. the roadside, without the use of VANETs. The overall system operation is illustrated in Figure 20.

FigureFigure 20. 20.An An illustration illustration of of the the WSN WSN deployed for for an an Intelligent Intelligent Transport Transport system system [56]. [56 ]. Figure 20. An illustration of the WSN deployed for an Intelligent Transport system [56]. TheThe researchers researchers of of [57 [57],], presented presented a mechanisma mechanism that that controls controls the the speed speed of of a a vehicle vehicle byby utilizingutilizing an infrastructure-to-vehiclean infrastructureThe researchers‐to‐vehicle of communication [57], communicationpresented a viamechanism WSNs.via WSNs. Inthat the Incontrols describedthe described the speed system system of thea vehicle tratheffi trafficc by lights utilizing lights utilize long-rangeutilizean infrastructure long RFID‐range tags,‐ toRFID‐vehicle which tags, communication can which measure can measure a via vehicle’s WSNs. a vehicle’s speed In the and speeddescribed communicate and systemcommunicate withthe traffic the with vehicle’s lights the onboardvehicle’sutilize RFIDlong onboard‐range tags, RFID toRFID avoid tags, tags, to collisions. avoidwhich collisions. can The measure onboard The onboarda vehicle’s RFID RFID tags speed tags are areand able able communicate to to provide provide feedback feedbackwith the to actuators,tovehicle’s actuators, which onboard which can RFID change can tags,change the to longitudinal avoid the longitudinal collisions. speed The speed by onboard controlling by controlling RFID varioustags arevarious able parameters to parameters provide of feedback the of vehicle the suchvehicleto as actuators, braking such as andwhich braking throttling. can andchange Furthermore,throttling. the longitudinal Furthermore, the authors speed the created by authors controlling a fuzzycreated logicvarious a fuzzy algorithm parameters logic enablingalgorithm of the the vehicle such as braking and throttling. Furthermore, the authors created a fuzzy logic algorithm automaticenabling control the automatic of the vehicle. control of Structural the vehicle. elements Structural of theelements system of developedthe system developed are shown are in Figureshown 21. inenabling Figure the21. automatic control of the vehicle. Structural elements of the system developed are shown in Figure 21.

FigureFigure 21. 21.A A depiction depiction of of the the various various sensorssensors deployed on on a a car car for for experimenting experimenting [57]. [57 ]. Figure 21. A depiction of the various sensors deployed on a car for experimenting [57]. 2.6.4. Structural Health Monitoring

The aim of Structural Health Monitoring (SHM) in buildings and other types of civil engineering substructures, is to monitor their integrity and to detect the existence and the extent of damages in the materials or/and the structure of their bodies. The use of wireless sensors facilitates the accomplishment of tasks of this kind both in a periodic basis and after critical events (e.g., earthquakes). Appl. Syst. Innov. 2020, 3, 14 18 of 24

A WSN application of this kind is presented in [58]. Specifically, a WSN incorporating nodes with appropriate sensors, such as accelerometers and strain gauges, was developed to monitor the restoration works carried out in an old church building located in Italy, which was damaged after an earthquake. The system monitored the on the fly response of the structure to vibrations and enabled the transmission of alert signals when needed. In [59] the design, development, and deployment of a, WSN based, structural health monitoring platform for a stadium located in the USA is described. By using vibration sensing, the system collects real time, data during athletic and other major events to verify the structural behavior of the stadium, in correlation with the behavior of the audience. Another structural health monitoring system which makes use of a WSN is presented in [60]. This system is designed, implemented, deployed, and tested to monitor the structural performance of a bridge in China by sensing vibration signals that are produced under various conditions in specific points of interest located at the body of the bridge.

3. Discussion There is no doubt that WSNs enjoy remarkable capabilities which make them ideal for an ever- extending range of applications. On the other hand, the operation of WSNs comes up with serious problems. Some of them are application dependent. For instance, in military applications the required physical dimensions and weight of nodes are application dependent. For instance, in some surveillance applications the sensor nodes have to be extremely small in order to be undercover, while in many other military applications, physical dimensions and weight of the nodes are not considered to be important restrictions. On the other hand, nodes in such applications should definitely have an adequately extensive communication range (maybe 1 km), while the area to be covered is several square kilometers. Also, communication should ≥ attain optimal throughput, reliability, security, and resistance to jamming and intervention. Moreover, nodes should be robust enough to resist severe ambient conditions. Similarly, the WSN should be tolerant to the loss of a certain quantity of nodes. In health applications, the physical dimensions and weight of the nodes have to be as small as possible particularly in the cases where they are wearable. Conversely, there is not any need for an extended communication range of nodes or area covered. Communication should be fault tolerant, fast, and reliable while jamming should be definitely avoided because the transmission of data is absolutely vital when time critical alert information is sent. In flora and fauna applications, the limitations, if any, for physical dimensions and weight of the nodes are application dependent. For instance, in WSNs used in livestock farming, nodes which are implanted under the skin of animals must be as small as possible while in agricultural applications such limitations are usually absent. Also, in flora and fauna applications, nodes should be robust enough to stand ambient conditions. Similarly, the WSNs should be tolerant to the loss of a certain quantity of nodes. In addition, in some cases there is need for an extended communication range of nodes and area covered. The volume of data transferred is usually high, but the communication standards that should be met are not very high. In environmental monitoring the physical dimensions and weight of the nodes are not considered to be the first requisite. Instead, the construction of the nodes has to be extremely robust in order to tolerate severe ambient conditions. Also, both the communication range and the area covered should be adequately wide. Additionally, communication should be resistant to jamming because emergency alerts should be transmitted without delay. Moreover, the WSN should be tolerant to the loss of a certain quantity of nodes. In industrial applications most tasks are time critical while in the industrial environment the presence of electromechanical interference is remarkable. Therefore, communication should achieve optimal throughput, reliability, and resistance to jamming and interference. It may also be necessary to Appl. Syst. Innov. 2020, 3, 14 19 of 24 operate under strict security standards. The communication range and the area covered depends on the nature of the specific application and as do the physical dimensions and weight of the nodes. In the urban domain, there are different conditions that must be fulfilled in indoor and outdoor applications. Precisely, in WSNs for indoor use nodes have to be of relatively small physical dimensions and weight. Usually there is no need for the communication range and the area covered to be wide. Conversely, communication should attain high security to protect privacy and should resist interference caused by other home appliances. On the other hand, in outdoor urban applications, the physical dimensions and weight of nodes are of minor importance. Yet, both the communication range and the area covered need to be wide. Also, the communication should achieve high levels of throughput, security, reliability, and resistance to jamming and intervention due to the huge volume of data transmitted. Moreover, nodes should be robust enough to ambient conditions. Also, the WSN should be tolerant to the loss of a certain quantity of nodes. The abovementioned considerations, regarding the required features that WSNs should have per type of application, are synoptically presented in Table1.

Table 1. Required specifications of Wireless Sensor Networks (WSNs) per type of application.

Required Specifications Type of Application Node Node Communication Communication Communication Communication Network Weight and Robustness Range Throughput Reliability Security Tolerance Dimensions Application Military Very High Wide Very High Very High Very High Very High dependent Health Small High Small Very High Very High High High Application Flora and Fauna High Wide Medium Medium Low High dependent Of minor Environmental Very High Wide Very High High High Very High importance Application Application Industrial Very High Very High Very High High Very High dependent dependent Indoor Small Medium Small Medium Very High Very High High Urban Of minor Outdoor[M1] Very High Wide Very High Very High Very High Very High importance

Additionally, apart from the aforementioned issues that are application dependent, the operation of WSNs is also obstructed due to general issues, such as difficulties of wireless communication and weaknesses of the nodes. Specifically, sensor nodes of WSNs suffer from extremely strict energy constraints. This is because their energy is typically supplied by batteries which are usually impractical to be either recharged or replaced, since the locations of sensor nodes are usually difficult or even impossible to reach. Therefore, the attainment of energy conservation is a vital issue for WSNs. For this reason, energy inefficiencies that exist at every one of the five layers of the protocol stack of sensor nodes must be eliminated. Given that data transmission is by far the most energy consuming task of nodes, power control schemes [61], data aggregation schemes that decrease the size of data transferred [62], [63] and energy efficient routing protocols [64] have been proposed. Likewise, in applications in which multimedia data are transmitted, the use of compression and restoration schemes provide a substantial reduction of communication load [65,66]. Additionally, the presence of excessive data traffic in a specific region of a WSN causes network congestion which obstructs data transmission, generates packet losses, and decreases the network throughput. For this reason, methodologies for congestion avoidance [67–69], congestion control [70,71], and load balancing [72,73] are used. Furthermore, each time that a node is disconnected from the rest of the network, due to, malfunction, damage, or energy depletion the communication for the remainder becomes more difficult and the communication cost is increased. For this reason, methods for the preservation of network connectivity are essential to be applied [74,75]. Moreover, it is anticipated that the nodes achieve the best exploitation of their sensing range and their communication range in order to cover as much of the network area. This is why coverage maximization methods Appl. Syst. Innov. 2020, 3, 14 20 of 24 are considered [76,77]. Moreover, in WSN applications where multimedia data are transmitted, the attainment of high Quality of Service (QoS) is a necessity. Furthermore, the accomplishment of energy efficient routing in WSNs is often influenced by other issues such as the QoS attained [78,79]. Last but not least, in most WSN applications, the data transmitted within the network must be protected from any unauthorized use. For this reason, security preservation schemes are used [80,81].

4. Conclusions The usage of WSNs already provides remarkable advantages for various domains of human activity. Thanks to the continuous evolution of technology both the capabilities of sensor nodes will keep expanding and their manufacturing costs will become lower. This is the reason why the range of WSN applications is expected to carry on growing. In this article, the utilization of WSNs in specific domains, namely military, environmental, flora and fauna, health, industrial, and urban, was examined via the investigation of corresponding typical examples, both novel and well-known ones. From this examination, it became evident that the usage of WSNs not only provides numerous advantages in specific domains when compared against the relative means and methods that were traditionally used, but it also introduces novel applications. Additionally, for various applications both the problems and solutions developed, were identified and discussed. The combinational utilization of relative methodologies and tools will assist both the enhancement of existing applications and the development of novel ones. On the other hand, certain problems that obstruct the usage of WSNs, such as energy limitations, congestion, connectivity loss, inadequate coverage, low QoS, and susceptible security, will remain at the center of scientific research.

Author Contributions: All authors have equally contributed to the work reported. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Conflicts of Interest: The authors declare no conflict of interest.

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