## 5 Progress of Data Transmission Protocols for Wireless Communication in Solar PV Monitoring Systems
### ZigBee-Based Module
[PERSON] et al. [88] utilized point to point topology for data transmission with a web-based monitoring interface to develop a ZigBee-based solar PV monitoring system. The proposed system utilizing ZigBee modules was implemented with a web-based function as illustrated in Figure 12[88]. The test was conducted on 1.25 kW solar panels to extract 600 data points for calculating the AC voltage with a sampling rate of 60 milliseconds. Further research with regard to the measurement of efficiency and stability could be performed. In addition, the methodology could be utilized to identify the faults which could significantly increase the performance and efficiency of the solar PV monitoring system. [PERSON] et al. [91] suggested a method using a ZigBee module for solar PV array online monitoring and fault diagnosis. The system was evaluated using 2522 sample data. The fault diagnosis accuracy of the PV power plant was estimated to be 98.58%. More electrical and environmental parameters could be considered in future work to observe the faults effectively. [PERSON] [92] developed a solar PV system using ZigBee technology for PV module performance monitoring. Furthermore, a feed-forward compensation of network voltage was proposed to reduce the voltage fluctuation of the grid-connected current. The paper also proposed a method to reduce Total Harmonic Distortion (THD) in the grid-connected current through a simulation analysis. The algorithm developed by the authors could be further improved by involving a cloud-based system for real-time monitoring. [PERSON] et al. [93] developed a ZigBee-based low-cost solar PV monitoring system equipped with driving software for recording PV system parameters. The paper proposed a prototype system for a high voltage series-connected PV array in the range of (100-310) V and 3A as the maximum current with a sampling frequency of up to 14 samples/seconds. It is suggested that the proposed model under various conditions/interferences is examined to validate the performance of the system comprehensively. [PERSON] and [PERSON] [94] designed a solar PV monitoring system located in a remote location using ZigBee. The proposed system used the Python language to store the data in the Structured Query Language (SQL) database. Further research could be carried out by implementing the methodology at several locations. [PERSON] and [PERSON] [95] applied ZigBee topology as well to transfer data  
Figure 12: The overall setup of the ZigBee-based grid-connected photovoltaic system. to observe the panel angle to determine the maximum efficiency. The results indicated that the efficiency of the panel will be negatively affected if there is an increase in temperature of 3-5 \({}^{\circ}\)C after 20 \({}^{\circ}\)C. The suggested methodology could be implemented with a higher communication distance in order to read the values of current and voltage by utilizing sensors. Although ZigBee modules have countless practical applications, they have some negative points due to the non-conventional protocol design, complexity, the bandwidth constraints of the communication channel, and the signal processing techniques [96]. The protocol of ZigBee is inadequate for a large sensor network consisting of several large-scale clusters [97]. Moreover, the security aspect of ZigBee is weak and is prone to cyber-attacks which can be hacked or breached by professional hackers. The RF features in ZigBee are limited, creating a disruption of the signal by any obstacle. Furthermore, some other issues relating to ZigBee are signal interference, discrete communication, low range, and loss of signals in lifts, basements etc.