AN ANALYSIS OF VARIOUS FUNCTION IN WIRELESS SENSOR NETWORK APPLIED IN PRECISION AGRICULTURE

: Precision Agriculture is the idea of instantaneous observation of environmental and field parameters like soil moisture, humidity, temperature, soil PH etc. and transfer of the values of those parameters to the remote central server, which on the basis of these values, takes appropriate action to control the use of different resources like pesticides, water, fertilizer etc. in an optimized way in order to maximize the crop yield in terms of both quality and quantity. In Precision Agriculture, the farmers monitor and analyze the values of the field parameters in real time by using the Wireless Sensor Network (WSN) technology with the help of a varied range of sensors. This information further helps in making the exact judgment for controlling the resource allocation. This survey paper explores different existing methods, communication technologies and protocols related to sensor-based data collection, monitoring and control system. It also reviews the application of WSN in Precision Agriculture environment and successfully points out the future scope of the work in this domain.


INTRODUCTION
The main strength of India's economic development is measured by the agricultural growth and development. In upcoming years' old-fashioned way of farming will not be able to solve the problem of ever-increasing demands for the increasing population. Newer techniques need to be adopted to increase the agricultural productivity [1]. It is also important to develop eco-friendly technologies in a sustainable way. The application of WSN technology can be highly beneficial for remote measurement of parameters such as plant growth condition, including temperature, humidity, atmospheric pressure, soil moisture and water level. The wireless monitoring and controlling system can enhance the crop quality and productivity [2,3]. The technological advancement in agricultural field gives birth to a new concept called precision agriculture (PA). PA is mainly based on the two types of techniques, which are "Map Based Precision Agriculture" & "Sensor Based Precision Agriculture". On Map Based Precision Agriculture, various techniques like Geographic Information System (GIS),Global Positioning System(GPS), Remote Server (RS) technology, Soil sampling etc. are used to observe and control field parameters. GPS recognizes the exact location of the field, which will be helpful for observing, & controlling WSN based systems [4]. GIS technology helps us to collect the remotely sensed data, report of the crop inspection, soil survey maps and soil nutrient levels that are stored layer wise in remote central server [5]. However, the main disadvantage is that we cannot get the real time information from a map-based PA Techniques. In this approach, the prescribed map is stored on the server and accessed when required. So, in the meantime, if any changes occur in field variables, then it cannot be fetched. To overcome this problem, sensor-basedPA techniques can be used where we can get real time data. The Sensor is a device that can sense different types of physical parameters. Various sensor nodes are used to sense environmental and field variables. A simple sensor network consists of micro-controller, memory, analog to digital converter, transceiver & sensors, which arecapable of collecting sensory information, performing some kind of processing and communicating with other connected nodes in the network [2]. Sensor Network can be deployed for a wide range of applications. Currently its application has increased in agronomy. Observing crops for identifying environmental & field conditions and detecting the diseases also play an important role in successful farming. WSN technology can help us to decrease the work force, effort and time required for observing an agricultural environment. The application of WSN for crop field monitoring & controlling of resource utilization offers a tremendous possibility for massive profits for the farmers and society as a group [5].This survey paper reviews the existing works, finds their weaknesses and explores the scope of the future work in this field.

WIRELESS SENSOR NETWORK FOR PRECISION AGRICULTURE (PA)
The Wireless Sensor Network (WSN) consists of distributed autonomous sensors to observe and control field & environmental parameters. It then submits the data to the remote server through the gateway node. The availability of smarter, smaller and inexpensive sensors solves the problem of monitoring of environmental parameters. The wireless sensors, connected with the applications, work in collaboration with the Wireless Node. In the early days, monitoring system depended heavily on the sample collection of the field parameters and laboratory analysis. Therefore, it was impossible to retrieve and articles are ferences of ed in these cluded that opology to , each node nsures the also handle s of mesh application.
connected to the base station and can access r condition to the user dule so that m [18,20]. ogies and munication m. A Wi-Fi based smart wireless network (Fig. 5) consists of sensor nodes (WSN802G), gateway (wireless router), remote user and server with the database. The sensor nodes connect to the gateway through standard 802.11 b/g/n protocol [10,25]. In a first time, authentication of the sensors is done with a standard WPA2-PSK protocol in the gateway [10,18]. The gateway is further connected with the internet. The collected data are stored in remote server over the internet. User can access the data from the server through the internet.

Fig. 5: Wi-Fi based Wirweless Sensor network
This wireless technology [1] helps in real time monitoring of the agricultural field. The field and environmental parameters like water level, temperature, light, atmospheric pressure, humidity, etc. are collected dynamically and stored in server for further processing. The parameters are analyzed in the server to take farm management decisions about resource utilization. Proper decision helps to increase productivity and quality. The sensing modules used in this system can help to increase the battery life cycle as it uses low power (3.0 to 3.63v) sensor for data collection.

Bluetooth Based Wireless Sensor Network
Wi-Fi based distributed wireless sensor network has some disadvantages like costly hardware for communication technology, time consuming installation and difficulties in maintenance. To overcome these disadvantages,Bluetooth based WSN can be used in PA. Bluetooth based system ( Fig. 6) ensures dynamic, cost effective re-location and low power consumption [11,22,24].

Fig. 6:Bluetooth Based Wireless Sensor Network
A Bluetooth based WSN consists of sensor nodes, gateway, server and remote users. The sensing nodes are distributed over the field in an ad-hoc manner to collect data. Each node has three parts like memory for data logging, communication unit and battery. These nodes transfer data to the gateway node using Bluetooth (IEEE 802.15.1) at 2.4 GHz frequency. Further data are transferred to the base station (server) through the internet. The necessary analysis and decisions are made at the base station to monitor the environmental and field conditions [3]. The power consumption of this module is low as Bluetooth consumes very low power for transmission. The system is cost effective as Bluetooth is a low-priced technology [5]. But it suffers from low communication range and poor data security.

Radio Frequency Based Wireless Sensor Network
Another important wireless communication technology used in PA is Radio Frequency (RF) based WSN [16,26]. An architecture of radio frequency based WSN is given in Fig.  7. The architecture has four main parts -sensor nodes, gateway, server and remote users. The sensor nodes connect to the gateway through the RF communication module. Gateway is connected to the server over the internet.  Agricultural D [9] ce the crop p er environmen the water requi tem is shown i ing WSN [10] n, and yield and m was fixed at ility of the w ed accordingly ted in Fig. 14  In 2015, Pascual et al. [13] proposed a model, which could monitor temperature, relative humidity and soil moisture wirelessly using ZigBee protocol. Data were collected in a remote computing unit through XBee. The data were analyzed in MATLAB software. The system architecture ( Fig. 15) consisted of soil moisture, humidity and temperature sensor, XBee unit and Arduino. But, one main drawback of the system is its use of the costly transmission device (XBee). Maurya et al. [14] proposed an energy efficient routing protocol to maximize the lifetime of the sensor network. Sensor nodes continuously sense temperature and soil moisture content of agricultural field and check whether the value of these parameters exceeds the desired threshold level or not. If the value exceeds the limit, then a data packet will be sent to the base station. Control decisions are taken based on the reception of this data packet. Thus, the system reduces the continuous transmission and consequently the power consumption for communication. The limitation of the proposed protocol is that the base station stays idle in case the sensed value does not cross the threshold value for the parameters.
In 2015, Mr. Gaikwad S. V. et al. [15] proposed a monitoring system using WSN to observe environmental parameters like NPK, temperature and Humidity, etc. The system lacks in generating control decisions for field condition. The given Fig. 16explains the architecture. The reviewed articles are summarized in Table 3 with the proposed schemes and their drawbacks. Variable rate irrigation using Bluetooth Data overlapping, low transmission range, not secure as unlicensed band is used for data transmission [7] Field parameter monitoring system System cannot analyze data to control field parameters [8] Paddy crop and fieldmonitoring system No mechanism to resend the lost packet [1] Wi-Fi based sensing module to collect environmental and field data.
High Power consumption [9] Disease detection and monitoring system Only disease detection but no control mechanism [10] Irrigation control system by using Wireless Moisture Sensor Network (WMSN) Technology No other environmental and field parameters are considered except soil moisture level [11] A geometric relation between a sensor and its surrounding space points No information about deployment strategy and scheduling algorithm [12] Automated monitoring system Device fixed at a particular angle and distance thus lacks in monitoring the crop from other sides [13] Field & environmental parameters monitoring and controlling system Since the model used XBee for communication, the system is not cost effective [14] An energy efficient network layer routing protocol Base station cannot retrieve any data until the values of parameters cross the threshold level. As a result, base station remains idle for a long time [15] Monitoring system to notice the environmental and field parameters.
System only observed and stored the parameters and did not provide any control decision

CLASSIFICATION OF EXISTING SYSTEM
The literatures can be classified according to the design of experiments. There are two types of study found in literature, one is simulation based system and another is an implementation based system. Simulation based system can be further divided into three classes according to the type of simulator. Implementation based system is also classified into three classes. The given figure (Fig.17) represents these classifications of WSN applied in PA.

Fig. 17: Classification of Existing System
In [13] the authors proposed a simulation based WSN for PA application. They used MATLAB 7.0 software for the simulation. In [28] the authors implemented a WSN for energy efficient data transmission among sensor nodes using the Tossim simulation tool. In [29] the authors proposed a simulation based WSN for PA application. They used theQualNet software for the simulation.
In [22] the authors implemented a WSN for collecting data and monitoring the field parameters using GSM and ubiquitous wireless sensor networks. In [7] the authors proposed an implementation based WSN for PA application. They used Ethernet with Radio Frequency for implementing the project in real life. In [23,27] the authors implemented a WSN for soil moisture monitoring system in the field usingZigbee. In [21,26] the authors proposed an implementation based WSN for PA application. They used Radio Frequency for the automated irrigation control system. In [3,5,11,22,24,25] the authors implemented a WSN for environmental parameter monitoring and data collection system in the field using Bluetooth. In [25,28] the authors proposed an implementation based WSN for PA application. They used Wi-Fi for controlling the water level of the field.
This analysis helps to figure out the most frequently used hardware for PA applications. Bluetooth with Ethernet is the most common combination used in the reviewed literatures for data collection. RF with Ethernet module are used very rarely in these articles. The cost of RF module is lower than all other modules, so RF with Ethernet can be a costeffective solution for future application.

COMPARISON STUDY OF REVIEWED ARTICLES
The existing WSN approaches are compared in Table 4 based on their applications, characteristics, schemes, technology and drawbacks. Table 3 represents the papers with their proposed schemes and existing drawbacks in their system. Table 4 compares the reviewed articles based on their technical details. From the table, we can observe that WSN is applied in PA for irrigation control, crop monitoring, field &environmental parameter monitoring system etc. The major contributions of the reviewed literatures are classified based on their proposed system, technology, sensors, frequency band, range of communication, battery life, cost and power consumption. The limitation of each study is described in the "Remarks" column.
In the crop monitoring system proposed in [1,23,27] soil moisture, temperature, wind speed, wind direction, humidity, rain gauge, water and pH sensors were used. The sensor nodes communicated with each other and transmitted the data to the gateway through the Zigbee technology. Whereas in case of real time monitoring system [18,25,28] although the same type of sensors was used but data transmission was done through the Wi-Fi technology. Due to the use of Wi-Fi, the module supports easy configuration and communication among deployment nodes. This technology also provides secure data transmission. But the power consumption is comparatively higher than other technology-based systems. Therefore, it is not suitable for covering large area with a significantly high number of nodes. On the other hand, although Zigbee technology provides secure data transmission with low power consumption, but it causes high cost. To provide the costeffective module, Bluetooth technology is used [22,24] in monitoring and data collection system. This technology is used with all types of major sensors as it is cost effective and requires low power consumption. The major drawbacks of these systems are low communication range and data overlapping. To overcome low communication range, RF technology was introduced in the automated irrigation control system [21,26] for controlling the soil moisture. However, it also suffers from the reconfiguration problem while adding new nodes.
Thus, it can be noticed that each technology-based system suffers from certain types of problems. None of these systems offer completes solution with respect to costand communication range and power consumption. Consequently, there is immense research scope to design a secure automated monitoring and controlling system considering all spatial and temporal variables involved in agricultural pursuit. Further research is also required to find the applicability of low-cost LORA technology in PA, to overcome the power consumption problem and low communication range.
From this comparison, we can conclude that a low cost WSN with extended lifetime is required to monitor spatial and temporal variables and their interdependency

CHALLENGES OF SENSOR BASED MONITORING IN PA
The development of WSN initiates new research movements in the Precision Agricultural domain. WSN with tiny, selfcontrolling sensor nodes, low cost equipment, and scalability makes it suitable for agricultural automation. However, there are several open issues related to the deployment of WSN that makes this automation a challenging task. Some of these challenges are listed below.

Power Consumption and Battery Life
The time required from sowing to harvesting is 3-4 months.
The system of WSN needs to be sustained throughout this period for real time monitoring. During these periods, frequent battery replacement is not feasible and cost effective. Therefore, power consumption of the sensor network must be low to ensure extended battery life. Hence, high performance battery or renewable energy source such as small solar panels, rechargeable battery etc. is required.
In case of multi-hop WSN the rate of power consumption is increased due to the large number of communication links. Therefore, new schemes must be developed to optimize the power consumption in order to increase the overall network lifetime.

Deployment
Deployment means placing the actual sensors in a realworld environment. Proper deployment of the sensors is a critical area that needs to be addressed with proper care in order to increase network lifetime, reduce communication cost and communication delay and increase the overall effectiveness of the network. The system can be implemented using star topology with single-hop communication. The system architecture consists of base station and PC, as a server. The server can monitor and display the data. For large deployment of huge numbers of nodes, cluster tree based multi-hop communication may be more appropriate which needs further investigation.

Localization
The important function of a sensor network is to retrieve and forward data to the destination node. In real time, sensors are deployed from the airborne system without knowing the actual position in remote places. There may be no supporting transportation to reach that position. In this scenario, physical locations of the sensors need to be determined by localization algorithm. Hence, effective localization technique is required for the WSN.

Synchronization
In case of wireless networks, synchronization is very important for basic communication. The synchronization related problem depends upon the four parts: send time, access time, propagation time, and receive time. In agriculture environment, clock synchronization is required for analyzing the data and participating in the monitoring system to take correct control decision at right time.

Security
Security is essential for reliable data transmission. Sometimes data retrieved from the sensors cannot be transmitted to the base station in its original form. During transmission, some error can be generated or mixed up with the original data. To protect the data from various malicious activities, certain security issues need to be carefully handled.

Data Integrity
It ensures that data packets are correctly transferred from the sender to the destination and cannot be changed in the middle. It ensures the assurance of the accuracy and consistency of data over its entire life-cycle. This problem may be solved by using secure tunnel through VPN functionality.

Data Confidentiality
Confidentiality is the specific mechanism to ensure the protection of data from harmful intruders. It allows only the authorized users to access sensitive and protected data. During short-or long-range communication over the medium, confidentiality is one of the most important issues in network security which can be solved by the application of appropriate cryptographic mechanisms.

Data Authentication
Without proper authentication, the data may be hijacked, accessed or changed by any unauthorized person or authority. This problem may be solved by using a secret key to compute a message authentication code for all communicated data.

Real time
The health of crops and yield depends on climate conditions, like humidity, intensity of illumination, temperature, etc.
Monitoring the weather conditions in hourly or daily basis are very laborious to the farmers. For these reasons, WSN deployed in the field needs to be activated at a particular time gap for real time monitoring of the spatial and temporal variability of the agricultural field. Thus, real-time monitoring systems need to be developed to enhance the crop productivity and plant growth and avoid disasters in farms.

Cost
The important part of designing WSN infield is the hardware and software cost. The sensor node designed for any agricultural application needs to be cost effective so that the farmers can easily afford it. The design cost of the module should be decided according to the country market. So designing a cost effective WSN is really a challenging task.

Storage &Data Management
The data management is an important part of PA. The large amount of data collected from several sensors in agricultural field needs to be stored for further analysis which is a challenging job. The modern cloud environment may serve this requirement for a high volume of storage in an efficient manner. The researcher should also pay attention to the (i) data collection schemes, (ii) data analysis method, (iii) sensor types, (iv) semantic sensor networking, (v) big data, and (vi) complex event processing for effective data management. The integration of IoT and software-defined network holds a huge promise for an efficient new methodology involving the deployment, monitoring, and designing of a WSN.

CONCLUSION
This survey provides a useful insight into the applications, topology, communication technologies and protocols of WSN used in PA. WSN based PA is helpful for farmers to enhance the productivity and quality. It is more applicable than the traditional agricultural practice. The wireless monitoring and control improve the effectiveness and efficiency of resources used, which leads to the sustainable development. Wide ranges of sensors are available in the market, which are useful in the field of agriculture. With the introduction of the wireless sensor network, we can accumulate the data in the gateway and can process it to obtain some meaningful results with the help of these sensors. This survey helps to find out the type of sensors used in PA, topology to deploy the sensors over the field and communication technology for data transmission. The reviewed literatures are compared with respect to their key findings to figure out the future research areas. The challenges that need to be addressed for an effective PA application are also discussed in this survey.

FUTURE SCOPE
Data collection, management and control of resources, maintaining inter-dependability among spatial-temporal variability using wireless sensor network is a challenging task. From the literature review, it is clear that no literature considers all the major types of variability's in the field and environment. Therefore, there is immense research scope to design a secure automated monitoring and controlling system considering all spatial and temporal variables involved in agricultural pursuit. Further research is also required to find the applicability of low-cost LORA technology in PA, to overcome the power consumption problem and low communication range.