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An Overview of Periodic Wireless Sensor Networks to The Internet of Things

Through developments in digital electronics and wireless technology, a variety of tiny devices have begun to be used in many aspects of everyday life. These devices can sense, compute and communicate. These typically consist of low-power radios, many smart sensors, and integrated CPUs. Such devices are utilized to establish a wireless sensor network (WSN) essential for the delivery of sensing services and monitoring of weather conditions. The concept of the Internet of things (IoT) is formed in conjunction with WSNs, where IoT can be described as an interconnection between recognizable devices in sensing and monitoring processes inside the internet networks. This paper offers a description of Periodic WSNs in general. It also offers an overview of the PWSN applications and challenges.


Introduction
The rapid advancement in embedded computing, wireless communications,   2 cooperating, and sending the data of monitored physical environment across multihop wireless networking. The sensed data by the sensor devices are transmitted to the base station for additional processing [2]. These sensors are constrained in terms of energy, processing, memory, sensing, and bandwidth [3,4]. Several applications use the WSN like agriculture, environmental, health care, buildings, industrial, and military applications [5.6]. In this article, we concentrate on a particular kind of WSNs that called periodic wireless sensor networks (PWSNs).
The process of collection and sending data by sensor nodes in PSN is done periodically. One of the major data providers in IoT is periodic wireless sensor networks (PWSNs). Thousands of sensors monitor a wide range of regions in such networks, where captured data is transmitted to the sink node [7,8].
A comprehensive description of PWSNs is provided in this article. The aims of this article are to assess the technology and features of PWSNs, review applications of PWSNs and include details on the challenges and future of PWSNs.
Section 2 begins with the definition of PWSNs and gives the PWSN 's design.
Section 3 offers details about the PWSNs applications and Section 4 discuss PWSN challenges. Section 5 discusses briefly the benefits and drawbacks of PWSNs and this article ended with a conclusion in section 7.

What is PWSN
After a particular change in climate and cataclysmic events in nature have happened around the world, the monitoring of environment is recommended for its significance, which is recently developing quickly as a one of the most important applications in WSNs. The applications of WSN can be categorized into three types. First, event-driven: in this type of application the sensor nodes send to the sink an alarm to inform him that a specific event happens, for example forest fire. Second, Query-driven: in this type of applications when the sensor nodes receive a query from the sink it will send the collected data, for example storage room. The third type is the periodic (or time-driven) [3,5] that we focus on in this article.
Periodic wireless sensor network (or time-driven) is defined as a network of wireless sensors in which the sensor nodes deploy to collect the data from the interested region in a periodic way, then, the updates from the PWSN are sent periodically to the sink (see Figure 1). Oppositely to different sorts of networks, PWSNs have an immense capacity of guaranteeing ongoing information gathering to the region of interest [6]. Several PWSNs applications (e.g. monitoring of environment and phenomena surveillance) use the periodic way to monitor certain conditions regularly. For example, the harsh environment uses the sensor nodes to gather the meteorological and hydrological measurable factors which surrounding it periodically such as pressure, light, humidity, speed and direction of wind, temperature, etc. Two main challenges in PSN, first, PSN has to provide adequate lifetime in order to satisfy application's needs. Second, data management is more difficult due to the huge amount of collected data by this network [7,8].  There are a lot of sensor nodes (i.e. hundreds) thrown in a field with the purpose to monitor light levels, temperature and soil moisture. In this application every sensor node captures a single data measurement every five minutes and at the end of every hour the collected data sends to the end user for further analysis via multihop communication [7,8].

Applications of PWSN
PWSNs may comprise of a wide range of sorts of sensors including infrared, visual, seismic, acoustics, thermal, magnetic and radar, which are characterized by their ability to observe the various environmental conditions, such as: pressure, movement, direction, temperature, light, humidity, speed, floor make-up, levels of noise, existence or nonexistence of a particular sorts of objects, and mechanical stress levels of connected objects [9]. Subsequently, an extensive variety of applications are potential as represented in Figure 3.
This range of applications incorporates homeland security [9], environment monitoring, structural health monitoring (e.g. bridges, tunnels and buildings), tracking target, monitoring of pipeline (e.g. water, oil, gas), monitoring of active analysis and prediction, wind speed and GPS data, battlefield monitoring and surveillance, [9], criminal surveillance in alleyways and roads, vehicular activity on roads [10]. It is anticipated that in the future all objects will be incorporated with sensors to make them intelligent. Intelligent objects can explore their environment, communicate with other intelligent objects and interact with humans. This anticipated opening the door to the new paradigm "Internet of Things (IoT)" which has increased the scope of PWSN demand [11].
IoT is a network of intelligent objects interconnected through a medium of communication. WSNs are expected to play an important role in IoT, since sensor nodes are the main building blocks of this concept [11]. In the era of Internet of cities and other intelligent environments, in remote metering and intelligent water provision, in efficient agriculture, in addition, in intelligent farming etc. [11].
In this section, some applications that rely on periodic sensor networks will be described as shown in Figure 4.

Environmental Monitoring
The primary area of applications in which PWSNs are extensively utilized is environmental monitoring. Although the main focus of environmental monitoring is to monitor the environment in its different types, it can be defined as observing and studying the natural environment and phenomena so that these environments are well understood. PWSN-based environmental monitoring incorporates meteorological monitoring, natural catastrophic events observing, traffic control, animal behavior monitoring, intelligent home automation, and so forth [9].
Georgia implemented one of the PWSN-based environmental monitoring application called "Georgia Automatic Environmental Monitoring Network (GAEMN)", which intended for monitoring the weather attributes. In this application, the time interval in which the data is collected by the sensor nodes is every 1 second, while at the end of each period of 15 minutes the data is summarized and sent. When the sink receives the data, processes it and then sends it through the Internet [12]. Later, the data is analyzed in a cybernetic infrastructure system.

Water Monitoring
There are two reasons that have been motivated researchers to explore the aquatic environment over the last decade: first, the area covered by the oceans is more than 70% of the Earth's surface and almost 10% of this area has been explored. Second, the importance of this type of environment for diverse humanitarian purposes such as food production, transportation, defense and adventure, and so forth. So, researchers used sensor networks in the aquatic environment because they are considered one of the most important types of technology that can be used in this environment. Applications of PWSNs in aquatic environment contain monitoring of water features, tsunami and seismic, oil spills, pollution control and climate recording, assisted navigation, and marine life studies [9]. One of the PWSN-based project for underwater is ARGO [20]. In this project more than 3000 sensor nodes have been deployed, which are scattered around the worldwide oceans to gather in a periodic fashion salinity, velocity and temperature readings from a depth of 2000 meters. Sensory data gathered are transmitted to the satellite periodically every 10 days while the nodes are always on the surface.
In Australia, a group of researchers [21] deployed a small periodic sensor network containing nine sensor nodes designed to monitor Burdekin delta waters.
Another example of projects based on the periodic sensor network was developed by [24] as a flood monitoring and detection system.

Industrial Monitoring
Other areas where PWSNs are used are in the field of industrial applications.
This type of network has attracted the attention of both academics and industrialists, as these networks provide continuous monitoring, control and analysis of industrial operations, as well as contribute significantly to locate the best performance of operations. Among the problems covered by PWSN-based industrial applications are: air pollution, structural condition monitoring, production performance monitoring, assessment and enhancement. [9].
A good example of PWSN-based industrial applications is the real fusion project [25]. In this project, two periodic sensor networks have been developed: the first network aims to monitor bulk materials for seven days in five silos in a factory. While the second network, which was built of nine sensors, was aimed to monitor the different environmental conditions in an industrial warehouse and for ten weeks, for example, temperature and humidity. After the end of each period, the access points direct the data to a server situated in the head office of factory.
Another examples of PWSN-based industrial applications are: a project developed to improve the production of oil reservoirs [26]; A system was developed and deployed by [27] for use in detecting damage on buildings; at last, the authors in [28] have been considered the noise pollution in urban regions utilizing PWSN.

Healthcare Monitoring
Continuous monitoring has become a condition for providing better health care to an expanding number of patients, whether in hospital or at home. Today, PWSNs play a pivotal role in the applications of health care as a low-cost effective solution. The work of the networks of the periodic sensor is to monitor the The authors in [29] proposed a PWSN-based health care system aiming of monitoring the activities of older people. Through ongoing monitoring of their activities, the developed system network analyzes the sensed data, discovers abnormal activities and warnings when necessary. Another interesting example of PWSN-based health application is HipGuard [30]. This application is a system designed to monitor patients (who replaced the hip) at home for eight to twelve weeks after surgery.

Challenges in PWSN
One of the distinguish things for PWSN is getting data from its sensor nodes members, which are deployed in large numbers over wide zones, before are sent to the sink periodically. In this type of networks there will be many challenges (as illustrated in figure 5) that affect the design, efficiency and performance of the PWSNs. In the next sections the details of the principle challenges in PWSN is given (part of these challenges may be common in the conventional WSNs)  Figure 5. PWSN Challenges.

Deployment
Sensors deployment for the purpose of collecting information about a particular phenomenon in the area of interest may be represent the first challenge in PWSNs because they represent the first stage in the network life cycle [9]. In PWSNs the deployment process of sensor nodes can be categorized into two types: one of them is deterministic and the second is random. In the first deployment method, the human or robot is used to put the sensor nodes one by one at planned, specific positions in the field of sensor. While, in the second method the aircraft, shell, rocket maybe used to drop the sensor nodes at random locations based on the requirements of sensing locations [9]. Furthermore, even though some applications allow the deployment of sensors in a deterministic manner, but random deployment is often used in most cases for practical reasons like time and cost. In PWSN applications, sensors ought to cover the whole region as well as requires connection with the communication network [31].

Energy Consumption
The challenge frequently connected with PWSN design is the limited energy of sensor nodes which is the main source of its work [11], and the deployment of sensor nodes in a different environments which cannot be reached by human  12 (because of physical dangers, e.g. military applications, or because of use necessities (imposed by the requirements of application) e.g. monitoring the volcanos). Ordinarily, the sensor nodes are operated by a battery, which after a while needs to be recharged or changed when its capacity runs out. In some situations, neither one of the options is fitting, where it is difficult or impossible to change or recharge the sensor batteries, due to cost restrictions or deployment of sensor nodes in a harsh, hostile or remote area [32]. On the other side, it may be simply disposed of whenever the source of its energy is exhausted. For batteries that cannot be charged, the sensor node must be capable of working until the task time is over or supplanted the battery. The application type determines the length of the task time, the lifetime of PWSN must be sufficiently long in order to satisfy the application prerequisites, for instance, the sensor nodes used in applications that monitor the movements of glacial must have the ability to work for years whereas the sensor nodes used in the battlefield can operate just for hours or days [11]. As a result, the frequently most essential challenge for a PWSN is the efficiency of energy.

Security
The other challenge facing the PWSN is security, where Periodic sensor networks are not limited to military applications but can be used in other applications such as surveillance, monitoring (e.g. building, habitat), smart homes and in systems which are critical like airports and hospitals [32]. It is known that most types of networks need security, but the need is increasing in sensor networks due to lack of resources and nature of wireless. Because sensor nodes resources are limited (such as energy and memory), make these networks requires nontraditional security solutions because they are unfeasible to these networks (i.e. as it is completely inappropriate because of the unique characteristics and application  13 requirements for sensor networks). Then again, the physical security of the sensors ought to be constantly guaranteed [33]. Due to sensor nodes are deployed in large areas that are often unsafe and non-controlled, the sensor nodes will be vulnerable to many different types of risks and attacks by intruders or simply possible by animals. Thus, it is crucial to build up an entire system that considers the physical/data security of the sensors and the necessities of the monitored application [33].

Scalability/density
The challenge of scalability arises from the density diffusion of sensors, although these large numbers of sensors can cause redundancy and improve the network in terms of fault tolerance. There are usually hundreds or thousands of sensors diffused for the purpose of sensing a certain physical phenomenon [9]. This redundant volume is considered necessary for the purpose of ensuring the reliability of sensor readings that work collaboratively. As a result of these massive data, several problems will arise in the PWSNs: data redundancy increases dramatically, collision of packets in the network, expanding the consumption of energy, a complex task for decision makers. Therefore, when developing networking protocols, they must be capable to deal with large numbers of sensors in an efficient manner [9]. Hence, the techniques of data collection and data aggregation have gotten a great consideration in PWSN so as to decrease the volume of produced data.

Routing
Among the other challenges that can be exposed to the networks of periodic sensors are how to find routes between the sink and sensor nodes and maintain these routes. There are many constraints imposed on routing protocols design,

Coverage
Another challenge that is important for the periodic sensor networks is to maintain the maximum possible coverage for the area of interest. After being the deployment of sensor nodes, we must ensure that the sensing coverage satisfactory during every period of the lifetime of network. Otherwise, a few applications used in periodic sensor networks, such as environmental monitoring, allow some flexibility on the subject of network coverage. Other applications, such as industrial or military surveillance, which are critical applications, require full coverage of the interested area. Thus, when taking into consideration the problem of coverage, sensor scheduling strategies have proven to be an effective mechanism for periodic sensor networks [36,37]. Because certain security leaks that may arise due to energy-saving measures, malicious attackers can attack PWSNs. In addition, surroundings like space and walls, etc., affect PWSNs.

6.
The scope of future in PWSNs more work needs to be conducted. For instance, they need to analyze the results of very high node densities. This is necessary not to overlook the growing use of PWSN apps and the anticipated complexity of reaching similar devices through the entire network [1,11].
In fact, IoT is anticipated in the immediate future to have a significant effect on our lives. PWSNs would be embedded in IoT, with numerous sensor nodes entering the Network. sensors should collaborate with other nodes to make sense of and track the world.
Throughout the immediate future, as seen in Fig 6, IoT is going to make the interaction between the environment and people more. Thus, their use of areas can increase significantly and constantly. Of starters, smart driver devices that are commonly implemented in the immediate future would be feasible to warn drivers

Conclusion
A PWSN is a network of nodes operating collaboratively to observe the world certain because they are considered as a primary source of data.