## 5 The Technologies
### Cyber-Physical Production Systems  
A CPS is a foundational technology in Industry 4 ecosystems, which can be implemented in a broad range of industries [2,18]. CPPSs are the application of CPSs in a manufacturing context [2,37,41], a key enabler for achieving smart manufacturing [16].  
The formation of a CPPS can demonstrate the established interconnections crossing all levels of manufacturing systems [7]. Information transparency can then be achieved by collecting and visualising the production data via wired and/or wireless networks [2,97]. Finally, the analysed and interpreted data can be used for enabling decentralised decision-making strategies to achieve the intelligence of the system [7,115].  
The development of CPPSs can significantly enhance the flexibility and adaptability of the systems, which in turn enables the desired batch-size-of-one production model [3,21]. For example, [PERSON] et al. [64] discussed that scalability and modularity are the main considerations while building the Proto Learning Factory to enhance the changeability of the facility. Similarly, [PERSON] and [PERSON] [116] pointed out that changeability can also be facilitated by digitising the physical processes. A digitised production system can help the deployment of the desired scalability and modularisation [103,117].  
CPPSs can be applied to core manufacturing operations as well as supporting systems such as storage [101,118], and material handling and intralogistics systems [2]. The structure of a CPPS includes the manipulators, control stations, material handling systems, and data management systems [39,42,119].  
There are three main types of machines that are employed, as shown in the papers reviewed for this research, namely CNC machines [2,3,7,11,63,120], collaborative robots [7,9,11,63], and 3D Printers [7,11,41]. A centralised database can be used to interconnect manufacturing processes and business processes [38].  
Communication techniques play a significant role in a CPPS. In the reviewed papers, there are a number of communication systems/protocols that were considered in building Industry 4 facilities. MTConnect and OPC UA are the most commonly used communication standards for building CPPSs [61]. Alternatively, [PERSON], [PERSON], and [PERSON] [42] discussed the use of a virtual network computing (VNC) connection and Modbus in the CPPS of the SEPT learning factory. The use of the TCP and user datagram protocol (UDP) and a comparison between them were discussed in building a CPPS learning factory [7].  
Lastly, ERP software, as the top tier in the vertical integration of a CPPS, has also been discussed in the reviewed articles, as it is used when people design CPPSs in Industry 4 facilities. [PERSON] and [PERSON] [6] used SAP, a top-level ERP solution, to establish the vertical integration in their learning factory. However, due to budget constraints, low-cost ERP software, called CANIAS, was considered in building the Ostfalia learning and innovation factory [80]. Similarly, [PERSON] et al. [40] have employed MS SQL-based ERP software (called Venio ERP) as the top layer of the vertical integration in their learning factory.