1. IntroductionSensornetworks are dense of many small, inexpensive devices, called sensor nodes,which are distributed to perform a specific action or an application-orientedtask. 1 Each sensor node can only do a limited amount of processing. However,when they are coordinated with the information from a huge number of othernodes, they are capable of measuring a physical environment in great details.2 It can collect, process, analyse and disseminate data anytime anywhere. Overthe past few decades, traditional sensor network had been dominating in variousdomains in term of data collection and functions monitoring. However, when it requires thousands ormillions of sensor nodes spanning across a huge area, it will need miles ofshielded cable connections, which are costly and time consuming to build.
3Hence, this motivates a huge effort in industrial investment and researchactivities in Wireless Sensor Network (WSN) since the last decade. 4AWSN requires almost no infrastructure. 5 The exact location of sensor nodesdo not need to be predetermined or engineered. 6 It can be placed at harshand non-reachable environment such as mountains, over the sea and deep forest.It is flexible as additional work station can be added at an ad-hoc condition.The implementation cost is incredibly lower compared to traditional sensornetwork.
7WSNs are promisingapproach for a variety of applications in biomedical health monitoring, wildlife tracking, natural disaster predicting, smart building, monitoring safety of buildings,and industrial application. Detailed application will be further explored inthe review later on.Thisliterature research aimed to provide an overview of sensor network. These arethe few main topics that will be covered and discussed: To discuss the history and evolution of sensor network To understand the technology behind sensor network To discover the application of sensor network in today’s world To identify the limitation and security issue of sensor networkInvestigationon the potential use of large sensor network on the monitoringthe behaviour of an engineering structure accurately during commissioning andoperation with a specific focus on vibration monitoring will be analysed in the discussion.2. History & Evolution ofSensor Network Similar to most ofthe technologies, research and development on sensor network started withmilitary purpose. SoundSurveillance System (SOSUS), a system of acoustic sensor on the ocean bottom,was deployed to detect quiet Soviet submarines at strategic locations duringCold War.
More refined acoustic networks were developed over the next few yearsfor submarine surveillance. 8 Theresearch on Distributed Sensor Network (DSN) was carried out by the United States Defence Advanced Research Projects Agency(DARPA) in early 1980s for US military. At that time, the Advanced ResearchProjects Agency Network (ARPANET) had been in operation for several years, withabout 200 hosts at research institutes and universities. DSNs were assumed tohave a lot of spatially distributed low-cost sensing nodes, collaborating witheach other but operated autonomously, with information being routed towhichever node that can best use the information.
8 The size of the sensorswere huge (approximately the size of shoes box or bigger) and thus theapplications were limited. Even though the researchers of DSN back then had thevision of WSN in mind, but the technology was not quite ready yet. Hence, theearliest DSNs were not tightly associated with wireless connectivity. 9 Advancement of in communication,microelectromechanical and computing technology in late 1990s have caused a newwave of research in WSNs. The significant shift in WSNs research has attractedlarge investment and international attention.
Researches were focus ondeveloping sensor nodes that are cheaper, smaller, and suitable for highlydynamic ad hoc and resource-constrained environments. Hence, many new civilianapplications of sensor networks such as vehicular sensor network, environmentmonitoring and body sensor network have emerged. 10 SensIT 11, an initiative researchprogram launched by DARPA has provided the current sensor networks with newcapabilities such as dynamic querying and tasking, ad hoc networking,multi-tasking and reprogramming.
Currently, WSNs have been believedas one of the most important technologies in 21st century. 12 China hasincluded WSNs in their national strategic research programmes 13 which leadsto the acceleration and commercialization of WSNs as well as emerging of manynew technology companies such as Crossbow Technology and Dust Networks. Theadoption of WSNs also drove the growth of Internet of Things (IoT). The integrationof cloud technologies and WSN technologies are the key elements of industrialIoT. Both short range and long range of industrial IoT landscapes have emerged.14 According to ON WORLD, by 2021, there will be33 million installed wireless devices used for industrial sensing and assettracking applications. Wireless tracking, sensing, equipment control andassociated services will reach $35 billion over the next five years forindustrial agriculture, construction, automation and related markets. 14 Figure 2.
1: GlobalInstalled Industrial WSN Devices by Market Segment (2016-2021) 14 Over the past few years, research onLow PowerWide Area Network (LPWAN) has been rolled out. It can communicate at a longerrange (up to 30 kilometres), lower cost and minimal maintenance. Hence, LPWAN potentiallywill be another evolution from the sensor network technology. 14 3. Technology of Sensor Networks3.1 Hardware Structure of a Sensor NodeA sensor node is made up of four basic componentssuch as sensing unit, processing unit, transceiver unit and a power unit whichis shown in Figure 3.
1.Figure3.1: The component of a sensor node 15 Sensing units generally consists of 2 subunits: sensor andanalogue-to-digital converter (ADC). Based on the observed phenomenon, theanalogue signals produced by the sensors are converted to digital signals bythe ADC, and transmit into the processing unit. 6 Processing unit isusually associated with a small storage unit. It can manage the processes thatmake the sensor node collaborate with the other sensor nodes to carry out theassigned sensing tasks. 15 Theprocessing unit of a sensor node determines to most of the energy consumption andcomputational capabilities of a sensor node.
16 The requirement of storagesize in term of fast and non-volatile memory can varies depending on theoverall sensor network structure. 17 Transceiverunit is responsible of connection and communication between sensor nodes in anetwork. There are a few choices of wireless transmission media such as RadioFrequency (RF), Laser and Infrared. RF based communication is widely usedbecause it fits to most of WSN applications. 10 Power unit is the mostcritical component in a sensor node.
Power is consumed by the other 3 units andcan be stored in batteries or capacitors for WSNs. Energy harvesting techniqueis an alternative solution to solve the lifetime problem of a sensor node.Energy can be harvested from the environment (solar, vibration, waves) or otherenergy sources (body heat, foot strikes, fingers motion) and convert it toelectric energy to power the sensor node. 18 Position finding systemmay be required since most of the sensor networks require high locationaccuracy and routing techniques. Mobilizerwill be needed when it requires to move sensor nodes to execute assigned tasks.6 3.2 Operating systems of SensorNetwork Overthe years, there are various Operating Systems (OSes) emerging in the sensornetwork community such as TinyOS, Contiki, SOS,Mantis OS, Nano-RK, RETOS and LiteOS. 6 OS’srole is to build reliable applications that are efficient and safe.
10 Theseoperating systems are increasingly mature and widely used in real-worldapplications. Due to resource constrain in hardware platforms and dynamicallychanging environment, development of WSN applications still remain as achallenge. The choice of the operating system for WSN is critical to mitigatingthese challenges. 19 Comparison will be doneon the 3 most commonly used sensor network OSes: TinyOS, Contiki and LiteOS.20 TinyOSTinyOS is a tiny component-basedoperating system specifically designed for sensor network 10.
This operatingsystem follows an event-driven programming model 21 and implemented in a specialprogramming language called NesC 22. The endorsement of NesC by TinyOSconsumes less resources and reduces development complexity. These features haveled to the widespread adoption of TinyOS in the WSN domain. Full applicationmust be replaced to perform software reconfiguration as static optimizationdoes not preserved component structure after compilation. 23 ContikiOS Contiki OS is a lightweight and flexible operating for tiny network sensors that provide IPcommunication. Contiki OS kernel is event-driven and use C as the programminglanguage which allows it to be highly portable. The system supportmultithreading and Protothreads (a thread-like concurrency model).
24 Contiki achieved softwarereconfiguration through a dynamic linking and loading module. 25 LiteOSLiteOS ismulti-threaded operating system that provides Unix-like abstraction to wirelesssensor network. LiteOS support LiteC++ (an extension of C) and allows dynamicloading, online debugging, and file system assisted communication stacks.
Itsupports software reconfiguration through a separation between kernel and userapplication. 26 Table 3.1: Comparison between TinyOS, Contiki andLiteOS 20 3.
3 Sensor Network Architecture Figure 3.2: Sensor nodes scatter in asensor field. 6Thesensor nodes are normally scattered in a sensor field as shown in Figure 3.2.All the scattered sensor nodes in the sensor field are able to collect data androute data back to the sink. Data are routed back to the sink by a multi-hopinfrastructure-less architecture through the sink as shown in Figure 3.
2. Thesink is able to communicate with the task manager node through the Internet orsatellite. 6Figure 3.3: The sensor networkprotocol. 29 The protocol stack used by all sensornodes and the sink is shown in Figure 3.3.
This protocol stack integrates data with networking protocols, promotescooperative efforts of sensor nodes, combines power and routing awareness, and communicatespower efficiently through the wireless medium. 5 The protocol stackconsists of 5 layers (application layer, transport layer, networklayer, data link layer, physical layer) and 3 planes (power managementplane, mobility management plane, task management plane).