I believe that when it comes to Industrial Internet of Things (IIoT) communication protocols, everyone can easily come up with a few, such as RS-232, RS-485, and so on DeviceNet、Modbus RTU、Modbus TCP、Ethernet/IP、PROFINET， Or TCP, UDP, OPC UA, MQTT, but what are the differences and connections between them? Cutting without sorting is still messy! Below, we will take you through the fog and see the clouds. We suggest that you bookmark this article!
To quickly master the differences and connections among numerous protocols, beginners need to first understand one thing, which is the OSI model.
The OSI (Open Systems Interconnection) model is a standardized reference model for communication protocols, published by the International Organization for Standardization (ISO) in 1984, aimed at providing a universal reference framework for computers and communication devices to achieve interoperability and interoperability between different vendors. This model divides computer network communication into seven levels, each providing different functions and services. Data is gradually abstracted and encapsulated from lower to upper levels, and standardized protocols and services are defined between each layer through interfaces, allowing devices from different vendors to communicate with each other.
The OSI model also plays an important role in the industrial Internet of Things, as in the application of IIoT, devices need to exchange data and communicate with each other, and the selection and implementation of communication protocols must follow the principles of the OSI reference model. For example, fieldbus communication protocols such as Profibus and Foundation Fieldbus are designed based on the OSI reference model, and they perform different functions at different OSI levels with a certain degree of interoperability.
In addition, in industrial IoT applications, standardized protocols such as Ethernet, TCP/IP, HTTP, MQTT, etc. are also involved. These protocols are designed based on the OSI reference model, and their different levels and functions are closely related to the OSI reference model. Therefore, the OSI model is a very important standardized reference model in the industrial Internet of Things, which provides guidance and support for the communication and interaction of industrial IoT devices.
The seven levels are:
1. Physical Layer: Responsible for transmitting raw bit streams, defining the characteristics of electrical appliances, physical devices, and transmission media, such as voltage, current, encoding, and physical interfaces.
2. Data Link Layer: responsible for frame synchronization, flow control, and error checking of data. Its main task is to convert the original bit stream into logically meaningful data frames.
3. Network Layer: Responsible for selecting the appropriate path between the source host and the target host, and forwarding and addressing data packets through routers to achieve interconnectivity between different networks.
4. Transport Layer: Provides reliable end-to-end data transmission, including data segmentation, transmission control, error recovery, and traffic control.
5. Session Layer: Responsible for establishing, managing, and maintaining sessions and connections between applications, including session establishment, maintenance, and termination.
6. Presentation Layer: Responsible for data format conversion, encryption and decryption, compression and decompression, and data description, enabling correct interpretation of data between different systems.
7. Application Layer: Provides users with various network application services, such as email, file transfer, remote login, and web browsing.
Having said so much, the following is our focus! Knocking on the blackboard! In the field of industrial automation, communication protocols can usually be divided into multiple levels, some of which correspond to the levels of the OSI reference model, as shown below:
1. Physical Layer: Responsible for transmitting data signals, commonly used industrial physical layer protocols include RS-232, RS-485, Profibus PA, Foundation Fieldbus, etc.
2. Data Link Layer: responsible for data framing, error detection and correction, flow control, etc. Common industrial data link layer protocols include Profibus DP, DeviceNet, Modbus RTU, etc.
3. Network Layer: Responsible for transmitting data packets from the source address to the destination address, forwarding and addressing them through routers. Common industrial network layer protocols include Modbus TCP, Ethernet/IP, PROFINET, etc.
4. Transport Layer: Provides reliable end-to-end data transmission, commonly used industrial transport layer protocols include TCP, UDP, ISO Transport Service on top of TCP (ISO over TCP), etc.
5. Session Layer: Responsible for establishing, managing, and maintaining sessions and connections between applications. Common industrial session layer protocols include OPC UA, MQTT, etc.
6. Presentation Layer: Responsible for data format conversion, encryption and decryption, compression and decompression, etc. Common industrial presentation layer protocols include OPC UA, DDS, etc.
7. Application Layer: Provides users with various network application services. Common industrial application layer protocols include OPC UA, Modbus TCP, PROFINET IO, Ethernet/IP, etc.
It should be noted that the classification and hierarchical division of industrial protocols are not completely uniform, and different manufacturers and organizations may also use different classification methods and levels. The above is just a common classification method.
