# Communication Protocols of an Industrial Internet of Things Environment: A Comparative Study
## 1 Introduction  
The Internet of Things (IoT) is an emerging technology that represents a cost-effective, scalable, and reliable ecosystem, proposed for many applications, including smart city sectors [1; 2], consumer devices [3; 4], industrial environments [5; 6], Internet of vehicles [7; 8], multimedia [9; 10], and 5G systems [11; 12]. The IoT platform, from the communications perspective, consists of a TCP/IP network and standard protocols [13]. Standard protocols primarily include advanced message queuing protocol version 1.0 (AMQP 1.0) [14; 15; 16], message queuing and telemetry transport (MQTT) [17], constrained application protocol (CoAP) [18], extensible messaging and presence protocol (XMPP) [19], and JavaScript object notation (JSON) [20]. The TCP/IP networks include Wi-Fi [21], Internet, Intranet, and modern mobile networks. The employment of a communication infrastructure and protocol depends on the field of application, the timing requirements, and the data transmission rates [13]. Therefore, the requirements of consumer electronics are different from those of smart city sectors and industrial applications. In the industry context, Industrial Internet of Things (IIoT) [22; 23], Industry 4.0 [24; 25], Smart Manufacturing [26; 27], and Smart Factory [28; 29] are all terminologies for the emerging industrial environments that employ information and communication technologies (ICTs), including IoT platforms, while maintaining industry requirements. The term IIoT refers to the use of IoT technologies in many fields of application such as manufacturing, factories, transportation, gas, oil, and electric grids. However, most industrial and SCADA-like (supervisory control and data acquisition) systems of these fields employ proprietary communication protocols and ICTs, which lead to closed industrial systems. Hence, customers are stuck to a single vendor, costs are high, and interoperability is not fulfilled. In this paper, standard IoT protocols are introduced and compared. Then, MQTT is chosen for machine-to-machine (M2M) communications to complement the MODBUS TCP [30; 31; 32] operations in an IIoT environment. This environment integrates an event-based message-oriented protocol, i.e., MQTT, with a polling-based request-response protocol, intended for industrial applications, i.e., MODBUS TCP. The study shows that the MODBUS TCP and MQTT can coexist together, and in parallel, within the same IIoT environment. While industrial requirements of control and monitoring can be met via the MODBUS TCP using the request-response model, the MQTT protocol complements its operation by fulfilling the IoT requirements, using the publish-subscribe pattern for M2M communications. The IoT protocol works in parallel with the MODBUS TCP and relays industrial data to an Internet-based server for remote monitoring, analysis, and archiving. To solve the interoperability problem, industrial data are formatted using the MODBUS message format and transferred in the payload of the MQTT publish messages. In fact, the IoT protocols cannot replace the MODBUS protocol in most industrial applications, especially when an optimized message structure is required in the application layer. A second scenario is proposed here, which is to consider the MODBUS TCP as an IoT protocol, and hence build the IIoT environment with the MODBUS TCP only. This solution is efficient and complies with most industrial applications; however, it only depends on the request-response model. The choice of solution is totally dependent on the requirements of the industrial application. The developed environment can be employed in many industrial applications, including Internet SCADA, Internet-based monitoring, and industrial control systems. Hence, customers are not stuck to a single vendor, costs are reduced, and interoperability is maintained. The remainder of the paper is organized as follows.