## 2 Digital Twin
### Communication Protocols  
The communication protocol realizes the information transmission between IoT devices, and between IoT systems [49; 50; 51]. There are some representative communication protocols common across IoT systems, including Constrained Application Protocol (CoAP) [57; 58], OASIS Standard Message Passing Protocol (MQTT) [59], Modbus TCP/IP Protocol [60; 61; 62; 63; 64], and Ultra Reliable Low Latency Communication (URLLC) [65]. The summary of communication protocols are listed in Table 1.  
CoAP [57; 58] is a specialized web communication protocol based on the User Datagram Protocol (UDP), tailored for resource-restricted devices (battery-powered IoT devices, etc.). CoAP can be used to handle communication between IoT devices. It is designed to be compatible with the transmission of data via Hypertext Transfer Protocol (HTTP) and is  
\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 Modbus TCP/IP & Communication & As a communication protocol based on Transmission Control Protocol (TCP), it realizes the connection between industrial devices, provides reliable data transfer, and contains built-in checksum protection. \\ \hline URLLC & Communication & As a communication protocol, it tends to achieve low latency and reliability in the transmission process between IoT devices. \\ \hline \hline \end{tabular}
\end{table}
Table 1: _Cont._more compatible with Web-based applications. CoAP can provide a publish and subscribe mechanism to simplify the process of obtaining continuous data from the sensor.  
MQTT [59] is the communication protocol based on Transmission Control Protocol (TCP). It is a lightweight way for IoT devices to communicate with each other through MQTT messages. These messages are lightweight due to their optimized headers, so that the use of network resources can be reduced. The MQTT protocol includes MQTT clients and MQTT brokers. The MQTT client can be IoT sensors, and the MQTT broker can be deployed in an IoT gateway, which is used to manage data communication between sensors and control commands to IoT actuators. Since MQTT is based on the TCP protocol, it can provide reliable data transfer. Furthermore, IoT sensors may need to establish a connection with the IoT gateway within a certain period of time. In this case, with the help of MQTT brokers, IoT sensors can establish a long-existing outgoing TCP protocol to enable transmission.  
Modbus TCP/IP [60; 61; 62; 63; 64] is a communication protocol used in the industrial field to realize the connection between industrial devices. The protocol was initially proposed to provide communication between Programmable Logic Controllers (PLCs). Modbus TCP/IP uses port 502 for data transfer. Meanwhile, the protocol contains built-in checksum protection, which can ensure the reliability of the data transmission required in IoT systems. With the aid of Modbus TCP/IP protocol, reliable operations can be supported and applied to the smart grid, smart manufacturing, and other IoT systems.  
URLLC [65] is a communication protocol that can achieve low latency and reliability in the transmission process between IoT devices. Generally speaking, URLLC is based on the 5G wireless protocol, which can reduce the delay to 0.5 ms. In order to reduce the delay, URLLC uses a mini-slot for smaller time resource units, removes the scheduling mechanism of direct channel access, supports asynchronous upload, and adopts dynamic scheduling algorithms such as hybrid automatic repeat request (HARQ). In order to improve reliability, URLLC utilizes a channel estimation mechanism, a transmission diversity mechanism, and a bit error rate reduction mechanism. With the aid of URLLC, IoT sensors can exchange information faster and more reliably.