## 4 Results
### RQ7: Limitations of the Literature Review  
In this section, Table 9 explains the proposed solutions of the work conducted by various authors and the contributions with the limitations of their work are also described. The goal was to find research gaps in this area to help other researchers. The research gaps will allow researchers to develop solutions and new methods that could help fill the missing piece. \begin{table}
\begin{tabular}{p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt}} \hline \hline
**Reference** & **Title of Article** & **Proposed Solution** & **Limitations and Future Work** \\ \hline [PERSON] et al. [9] & Enhancement of relay nodes communication approach in WSN-IoT for underground coal mine & They designed relay node structures for a wireless sensor network and load balancing to improve network lifetime parameters. They designed an IoT-based WSN to provide advance warning of any natural disaster in coal mines. & There were several analysis parameters to analyze the networks, such as network lifetime, communication and transmission cost, energy consumption, and coverage of the whole area. & \\ \hline [PERSON] et al. [49] & Bio-inspired routing protocol for WSN-based smart grid applications in the context of Industry 4.0 & They designed a comprehensive, optimized, and QoS monitoring multi-hop network system for real-time data transmission in Industry 4.0. This self-optimized smart routing protocol (SIRP) was efficiently used for WSN-based SG applications. & \\ \hline [PERSON] et al. [52] & IoT and wireless sensor network-based autonomous farming robot & They developed a computer vision-based algorithm used for the classification of weed and a non-image. Wireless sensor nodes detect weed images through image processing methods and gather light, temperature, humidity, and moisture data. & \\ \hline [PERSON] et al. [53] & Efficient and secure three-party mutual authentication key agreement protocol for WSN in IoT environments & They proposed a practical and secure approach to merge IoT and WSN. Their scheme had high performance, low communication, and computational costs, low energy consumption, and provided effective authentication of the user in IoT. & \\ \hline [PERSON] et al. [102] & A secure IoT sensors communication in Industry 4.0 using blockchain technology & Wireless sensor network security improved using blockchain and compared security metrics. \&It ensured confidentiality and responsibility and tracked each sensor’s operation. The blockchain was used to store IoT artifacts and sensors. & \\ \hline [PERSON] et al. [103] & Prediction of satellite shadowing in smart cities with application to IoT & The technology had a minimal processing load. It was highly desirable to create a coverage map that can optimize network resources in satellites. & \\ \hline \hline \end{tabular}
\end{table} 
Table 9: Contributions and limitation of the literature. The limitation of the study is the effective coordination between IIoT where the network is heterogeneous for transmission of information. The absence of experimentation and optimization in predicting future locations or results are other essential aspects of DT. In this work, the authors develop a distributed blockchain-based security system integrated with the help of IoT and SDN. Blockchain is used for data security and confidentiality, while SDN-IoT incorporates sensor networks and IoT devices to improve the security services in Industry 4.0. Limitations of this study are that the developed model SDN-IoT was still in the initial stage, so it was not able to detect different types of risks, such as service denial (DoS) and flood attack and packet filtering. The developed system had no proper GUI, so the throughput, packet arrival time, and response time were rarely challenging to analyze.