## 3 IIoT Architecture  
### Overview of IoT and IIoT Architecture  
The origins of IIoT architecture can be traced back to the early designs of IoT architecture. In 2011, [PERSON] and [PERSON] [34] proposed a future IoT architecture called a U2 IoT model. The U2 IoT architecture works similar to a human nervous system that consists of unit IoT and ubiquitous IoT. The unit IoT serves as a local unit based on the man-like nervous model and is responsible for handling and managing diverse local IoTs. The ubiquitous IIoT follows the blueprint of social organization framework architecture and is responsible for integrating, managing, and controlling the collaboration among multiple IoT units across the industry, nationwide and worldwide. On the other hand, [PERSON] [35] worked on the concepts of web of things (WoT) by proposing an architecture that integrates the connection of \"things\" to the existing web services via existing web technologies. The proposed WoT architecture consists of five layers, includes accessibility, findability, sharing, composition and application layers. Subsequently, [PERSON] and [PERSON] [36] extended the WoT concepts into a hybrid distributed IoT architecture that consists of two distinct resource-oriented approaches: WoT and Tripe Space. WoT underlying a hypertext transfer protocol (HTTP) to interconnect the IoTs in the world wide web. Tripe Space applies semantic web protocol to exchange machine-processable data among the heterogeneous devices in the distributed local shared space. [PERSON] et al. [37] further customized the WoT \begin{table}
\begin{tabular}{p{113.8 pt} p{113.8 pt} p{113.8 pt}} \hline \hline
**Concerns** & **Conventional System** & **IIoT** \\ \hline Connected Nodes/Devices & Small to medium volume within the local networks & Billions of sensor nodes, actuators and automation devices connected \\ \hline Communication Networks & Homogenous & Heterogeneous \\ \hline System Scalability & Optional & High scalability \\  & & The design of IIoT security systems should consider the identification and authentication of an enormous scale of “things”, scalability of communication networks and security key distribution and revocation issues in future \\ \hline System Interoperability & Optional & High interoperability Diverse security mechanisms and defence systems over the distributed networks must be standardized and compatible with each other to communicate, exchange and process data securely \\ \hline Collected Data Types & Unified encoding scheme and data format, structured data & High volume (terabytes–zettabytes), High variety (diverse encoding scheme and format, structured data, unstructured data, semi-structured data, quasi-structure data) \\ \hline Data Processing Model & Moving data to process, moderate speed & Moving processing to data. In most industrial cases, high velocity necessitates real-time analytical processing \\ \hline Security and Privacy Concerns & Data-at-rest Data-in-memory Data-in-transit Data-in-transform & Data-at-rest Data-in-memory Data-in-transit Data-in-transform \\ \hline Authentication and Access Control Mechanisms & Centralized Approach & Distributed, decentralized approach Lightweight scheme \\ \hline \hline \end{tabular}
\end{table} 
Table 2: The difference between conventional systems and IIoT security concerns. architecture into the Smart Grid domain. Meanwhile, [PERSON] et al. [38] and [PERSON] et al. [39] proposed another IoT architecture based on software defined networks (SDN) that consists of three layers: infrastructure layer with interconnecting network devices; control layer that comprises of SDN controllers; and an application layer that includes the applications for configuring the SDN. On the other hand, several research projects such as IoT-A [40], iCore [41], Sensei [42], and COMPOSE [43] have proposed a reference architecture of IoT at a high abstraction level.