## 3 IIoT Architecture
### Overview of IoT and IIoT Architecture
A step closer to real-world industry implementation, several researchers [44; 45; 46; 47; 48] and vendors (i.e., Finnode, ThingWorx and Xively) use cloud technologies to tackle the IIoT heterogeneity issues and scalability services. These cloud-centric IIoT architectures use a centralized or decentralized cloud platform to process and manage the aggregated data from heterogeneous networks such as RFID, WSN, and body area network (BAN). These cloud-based IIoT platforms also provide API interfaces for industries to develop their IIoT applications. Researchers [49; 50; 51] recently attempted to integrate blockchain technologies in solving the decentralized issues of cloud-based IIoT architecture. Whether these blockchain-based architectures are practicable to support a large scale of things with their constrained resources in real-world industry implementation needs to be further investigated. Generally, the initial widely accepted IIoT architecture is constructed based on the three-layer architecture [6; 52], namely the perception, network, and application layers. The perception layer consists of the \"things\" identification and sensing technologies to collect and exchange the data. The network layer enables the communication and data transmission between the perception layer and the application layer. In most cases, it also involves data aggregation and curation process. Lastly, the application layer confluxes the data aggregated and virtualises the analysed result based on society, business and government demands. Different business interests reflect various IIoT applications for this layer, such as smart cities, intelligent health and smart transport. As three-layer architecture confronted the interoperability and scalability problem to well-suit into existing Internet and telecommunication networks, [PERSON] et al. [53] extended the three-layer architecture into five-layer architecture by proposing a new business layer that resides on the top of the application layer and further dividing the previous network layer into processing layer and transport layer. The transport layer is responsible for transmitting the data generated from the perception layer into the processing layer. The processing layer focuses on processing, storing, and performing analytical works based on the application layer's demand. While the application layer consists of diverse IIoT applications customized to each industry requirement, the business layer monitors these applications' release and charging, conducts research on business and profit models, and controls privacy issues. Subsequently, ITU [52] proposed an IIoT reference architecture that consists of four layers: device layer, network layer, service and application support layer and application layer. The device layer is responsible for capturing and uploading data directly or indirectly via communication networks or gateway protocol, such as controller area network (CAN) bus, ZigBee and Bluetooth. The network layer is capable of handling network and transport connectivity. The service and application support layer aimed to provide a support function for various IIoT applications includes data curation, processing or storage. The application layer consists of IIoT applications. Thereafter, Cisco [54] proposed a seven-layer IIoT reference architecture comprising physical devices and controllers, connectivity, edge or fog computing, data accumulation, data abstraction, application, collaboration and processes layers. The physical devices and controllers layer includes various endpoints that can generate data, be queried and managed. The connectivity layer refers to the communication and connectivity either between devices, local networks or across the networks globally. Transforming network data flows into an appropriate data format for high-level data processing and storage occurred in the edge or fog computing layer.