## 4 The Design Principles
### Interconnection  
In the Industry 4 paradigm, machines, devices, and systems are interconnected and capable of exchanging data with each other within the manufacturing context, which is the cornerstone of Industry 4 ecosystems [91; 92]. CPSs facilitate seamless M2M communication and data exchange within smart manufacturing environments [17; 93]. Building CPSs with cross-domain networks in smart manufacturing contexts is a consequential solution for gaining interconnectability in compliance with Industry 4 visions [28; 43].  
The design principle of interconnection can be interpreted as building interoperability in smart manufacturing systems, which is a way to enhance the performance of system integration, scalability, flexibility, and security level of the systems [90; 91; 94; 95]. Achieving the desired bi-directional M2M and machine-to-human (M2H) communications heavily relies on enabling data sharing in real time, which is the central principle of interoperability.  
Industrial communication protocols play significant roles in supporting instant data exchange for building interoperability in manufacturing systems [96]. In a broad range of applications, the Industrial Ethernet Transmission Control Protocol/Internet Protocol (TCP/IP) is used for building the networks in smart manufacturing contexts [7; 16]. On top of that, OPC Unified Architecture (OPC UA) and Message Queuing Telemetry Transport (MQTT) are the most commonly used industrial communication protocols for enabling communications for both hardware and software platforms [97; 98; 40; 99].  
From a different perspective, the interconnection can also be presented as the foundation for enabling systems integration alongside value chains in smart manufacturing contexts [83]. The horizontal, vertical, and end-to-end integrations are the facts that distinguish the Industry 4 paradigm from its three predecessors [83]. Well-established integrations of Information Technology and Operational Technology (IT/OT) components in manufacturing contexts are an essential Industry 4 expectation [59].  
With the support of advanced IT infrastructure, horizontal and vertical integration enable flexible production models across the value chain and organisational levels [46]. Horizontal connection links all field-level devices, vertical connection permeates through all organisational levels, and there is also cross-linkage between physical assets and their virtual representation [88].  
From a local perspective, horizontal integration targets the linking of machines and devices at the shop floor level to enable communication between machines over various field bus technologies, e.g., Modbus, ProfilBus, and CANopen [6; 24]. From a global point of view, horizontal integration is achieved through the whole value chain [35].  
Vertical integration starts from the Enterprise Resource Planning (ERP) level to the field level via Industrial Ethernet systems to enhance the real-time data transfer and acquisition [6; 43]. Fieldbus and industrial communication techniques are commonly used to enable vertical integration, crossing all levels of the manufacturing system, to establish a backbone for Industry 4 systems [76]. Vertical integration can also enable the digitisation of manufacturing processes [17].  
In smart manufacturing contexts, end-to-end integration refers to the fusion of all production processes alongside the entire value chain, from the raw material suppliers to the end customers [100]. The connection enables seamless data acquisition through all stages of the product lifecycle, achieving greater efficiency, flexibility, and customisation [7].  
For example, an online product configurator is developed in the Textile Learning Factory 4.0 to allow choice and ordering of personalised products [12].