## 2 Digital Twin
### Data Representation  
Data representation enables components to understand data from different domains [49; 50; 51]. Several commonly used data representation protocols in DT are DT Definition Language (DTDL) [52], FIWARE [53], OPC Unified Architecture (OPC UA) [54; 55], and Feature-Based DT Framework (FDTF) [56]. In particular, DTDL is an open-standard platform proposed by Microsoft [52], which can realize data transmission within the system or between different systems. The summary of data representation protocols is listed in Table 1. \begin{table}
\begin{tabular}{p{113.8 pt} p{113.8 pt} p{113.8 pt}} \hline \hline
**Protocol Name** & **Protocol Type** & **Protocol Characteristics** \\ \hline DTDL & Data Representation & As an open-standard platform, it defines six characteristics of IoT components and enables seamless data transmission between different DTs. \\ \hline FIWARE & Data Representation & It supports DT data transmission and the processing of contextual information received from various IoT components. \\ \hline OPC UA & Data Representation & As a modeling framework, it can retrieve information from raw data, support data manipulation, and provide monitoring capabilities. \\ \hline FDTF & Data Representation & As a DT structure, it enables the DT system to share information based on the data link between DT components. \\ \hline CoAP & Communication & As a specialized web communication protocol based on the User Datagram Protocol (UDP), it is tailored for resource-restricted devices, supports the transmission of data via Hypertext Transfer Protocol (HTTP), and provides a publish and subscribe mechanism to simplify the process of obtaining continuous data from the sensor. \\ \hline MQTT & Communication & As a communication protocol based on Transmission Control Protocol (TCP), it enables lightweight way for IoT devices to communicate, provides reliable data transfer, and can establish a long-existing outgoing TCP protocol to enable transmission. \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Protocols of DTs. DTDL [52] defines the following six characteristics of IoT components: (i) _Interface:_ the basic information of a component (device ID, device type, display content, etc.). Different interfaces can inherit attributes from parent interfaces (e.g., charging station components can inherit some attributes of the parking lot such as addresses); (ii) _Telemetry:_ the data sent from any component in the IoT-based DT (the raw data from IoT sensors, the processed data generated by DT models, etc. ); (iii) _Property:_ the status of the components in the IoT-based DT (i.e., the device is read-only or read-write). This can also include the different states between different DT (whether the data between the two DTs are consistent, etc. ); (iv) _Command:_ operations that any DT can understand; (v) _Relationship:_ the connection between the DTs (e.g., the relationship between the house and the cleaner is cleaned up); and (vi) _Component:_ the entities that exist in the DT, including sensors, gateways, and digital systems. Based upon these six components, DTDL can be composed of multiple DTs with a unified structure and data, which enables seamless data transmission between different DTs. FIWARE [53] is also an open-source project designed to support DT data transmission and the processing of contextual information received from various IoT components. FIWARE developed a data modeling standard called FIWARE NGSI that describes the collection, processing, and change notification operations of contextual information. FIWARE NGSI is an object-oriented data protocol that uses context objects to represent physical/digital objects. NGSI provides a data model that enables data exchange between multiple DTs. It also uses NGSI-LD to represent the relationship between different objects and uses JSON-LD for encoding to unify the data format. OPC UA [54; 55] is a modeling framework that can retrieve information from raw data. OPC UA defines the data transmission and understanding model between different systems.