## 5 The Technologies
### Industrial Internet-of-Things  
IIoT, as the application of IoT in the industrial sector, is the foundation of the ongoing digital transformation in the manufacturing industry [155; 156]. In the Industry 4 paradigm, all components of hardware and software are expected to be connected to achieve seamless communication in a smart manufacturing environment [21].  
Analysing the technical aspects, there are two considerations while building an IIoT system in the development of Industry 4 facilities, including data collection and data transfer [41]. Data collection relies on the employment of smart sensors embedded in manufacturing systems [111]. Data transfer requires the establishment of industrial communication with integrated data management strategies [78].  
RFID is well-known as the first IIoT device, which is designed for capturing data at the factory floor layer [39; 40; 42; 155]. RFID technology has been well-developed to fit into ERP and MESs to make factories smarter [157]. Also, RFID systems should be able to withstand the tough conditions in manufacturing environments, such as high temperature and humidity, and exposure to chemicals [59].  
With the advantages of low costs and simple configuration, RFID tags have been used extensively in the intra-logistic system of manufacturing settings to track production progress [107; 158]. Captured data can then be sent to relevant systems, such as ERP, MESs, Product Data Management (PDM), and Computer-Aided Process Planning (CAPP), via cabled or wireless networks [40; 157]. However, RFID has limitations in metal- and liquid-enclosed environments, such as machine tooling systems [105]. Also, the performance of RFID devices can vary, depending on the brands and models [157]. Thus, special considerations need to be made when building the system, including radio frequency deconfiction, read/write time, memory type, capacity [157; 158], etc.  
In terms of building industrial communication, IIoT systems support the majority of common industrial communication protocols such as OPC UA [88; 107] and MQTT [42; 99; 132]. The Advanced Message Queuing Protocol (AMQP) and Open DDS protocol are also options [42]. OPC UA is the most promising industrial communication protocol to build the interoperability in smart manufacturing systems [97; 98]. The OPCUA protocol is designed to support multiple operating systems and hardware platforms. Also, it is suitable for both simple and complex systems while supporting a broad range of devices.  
In addition, 5G technology has a massive capability to boost the development of IIoT in smart manufacturing environments [63]. It enables high security and low latency in massive data transmission, which lays the foundation for the edge cloud computing services [63, 99]. The employment of 5G technology can also enhance the wireless connectivity of the systems and has shown its advantages in large data transmission situations [150, 159, 160].  
Moreover, [PERSON], [PERSON], and [PERSON] [134] discussed the potential of using Light Fidelity (LiFi) to replace Wi-Fi on the factory floor. The advantages are clear, such as higher frequency and bandwidth, enhanced data security, less interference, etc.  
Despite the advantages of the aforementioned technologies, the sockets of IEEE 488 and RS 232 are still often used for field devices to establish the interconnection in current industrial sites [161]. Connection via USB and RJ45 ports is much more commonly established for simple projects in the development of academic Industry 4 facilities [105]. Moreover, QR codes and bar codes still have critical positions in the systems of Industry 4 learning factories [39, 42, 56].