Before talking about the major flaws in Ethernet, it is necessary to first understand the Ethernet communication mechanism. Ethernet refers to the IEEE802.3 standard, can be transmitted on the cable and twisted pair network. It first appeared in 1972, created by XeroxPARC. The current Ethernet uses a star and bus structure, the transmission rate of 10Mb / s, 100Mb / s, 1000Mb / s or higher. The main reason for the delay in Ethernet is the conflict because it uses CSMA / CD technology. In the traditional shared network, because the Ethernet so the site, using the same physical medium connected, which means that two devices simultaneously signal, there will be a conflict between the signals. In order to solve this problem, the Ethernet regulations, in a site to access the media, you must first monitor the network there are no other sites in the use of the media. , If there is a need to wait, this time there was a conflict. In order to reduce the probability of occurrence of conflict, Ethernet often used 1-sustained CSMA, non-persistent CSMA, P-persistent CSMA algorithm 2. As Ethernet is designed for office automation, and does not fully meet the requirements of the industrial environment and standards, the traditional Ethernet for industrial areas there are obvious shortcomings. But its cost is lower than the industrial network, high technology transparency, in particular, it follows the IEEE802.3 protocol for the field bus manufacturers to open the door, but to make Ethernet meet the requirements of the process, but also must overcome the following shortcomings:
First, the non-determinism of communication
Industrial control network is different from the ordinary data network, the biggest feature is that it must meet the control role of real-time requirements, that is, signal transmission should be fast enough and to meet the signal determinism. Real-time control often requires accurate and regular refresh of certain variable data. As the Ethernet using CSMA / CD mode, the network load is large, the network transmission uncertainty can not meet the real-time requirements of industrial control, so the traditional Ethernet technology is difficult to meet the control system requires accurate real-time communication requirements, As a "non - deterministic" network.
Second, the communication is not real-time
In industrial control systems, real-time can be defined as the responsiveness of the system's response time to an event. In other words, after an event occurs, the system must be reflected in a time that can be accurately foreseen. However, the real-time requirements for data transfer in the industry are very stringent, and data updates are often completed in tens of ms. And because of the existence of Ethernet CSMA / CD mechanism, when the conflict occurs, you have to re-send data, up to 16 times can try. It is clear that this mechanism for conflict resolution is at the expense of time. And even a drop line, even if only a few seconds of time, it is possible to cause the entire production to stop even the equipment, personal safety accident.
Commercial Ethernet does not have high stability and reliability, the traditional Ethernet at the beginning of the design is not designed for industrial applications, without taking into account the adverse conditions of industrial site environment, serious inter-line interference and machinery, climate, Dust and other conditions of the poor, and Ethernet anti-jamming (EMI) performance is very poor, when used in hazardous situations, do not have the essential safety performance; therefore the equipment of industrial reliability put forward higher requirements. At the same time in the production environment, industrial networks must have better reliability, recoverability, and maintainability. That is, to ensure that any component of a network system failure, will not lead to applications, operating systems, and even network system collapse and paralysis.
Third, the security issues
In the industrial production process, many of the site inevitably flammable, explosive or toxic gases, etc., for these industrial applications of intelligent devices and communications equipment, must take some explosion-proof technical measures to ensure the safety of industrial production The In the current technical conditions, the use of flameproof Ethernet system, explosion-proof measures more feasible, that is, through the Ethernet field equipment to increase security, airtight, sealing and other explosion-proof measures to make the field equipment itself, the ignition energy generated by the failure Do not leak, to ensure the safety of the system operation. For non-hazardous applications where there is no strict intrinsically safe requirements, you can not consider complex explosion protection measures. Industrial network security is another security issue that industrial Ethernet applications must consider. Industrial Ethernet can be the traditional three-tier network system, that is, information management layer, process monitoring layer, field equipment layer, integrated one, so that the data transmission rate faster, more real-time, and seamless integration with the Internet, To achieve the sharing of data, improve the operational efficiency of the factory. But also introduced a series of network security to the problem, the industrial network may be affected by the virus infection, hackers, illegal intrusion and illegal operations such as network security threats.
Fourth, the bus power supply problem
Line-powered (or bus-fed) means that the cables connected to the field devices not only transmit data signals but also provide power to the field devices. Ethernet from the beginning of the design did not take into account this issue, while the industrial site there is a lot of bus power demand.
Because there are many problems, ordinary commercial Ethernet is not directly used for industrial field control. In order to solve these problems Industrial Ethernet came into being. With the development and popularization of Internet technology, the improvement of Ethernet transmission rate and the development of Ethernet switching technology, the above problems of traditional commercial Ethernet are being solved in industrial Ethernet.
