Compared with industrial Ethernet, Ethernet has the following four defects
Before talking about the main deficiencies of Ethernet, it is necessary to understand the communication mechanism of Ethernet. Ethernet refers to the network that follows the IEEE802.3 standard and can be transmitted on fiber optic cables and twisted pairs. It first appeared in 1972 and was created by XeroxPARC. Currently, Ethernet adopts star and bus structure, and the transmission rate is 10Mb / s, 100Mb / s, 1000Mb / s or higher. The main reason for the delay of Ethernet is collision, the reason is that it uses CSMA / CD technology. In a traditional shared network, all sites in Ethernet are connected by the same physical medium, which means that when two devices send out signals at the same time, there will be conflicts between signals. In order to solve this problem, Ethernet stipulates that before a site accesses the media, it must first monitor whether other sites are using the media at the same time. , If you have to, you have to wait, and a conflict has occurred. In order to reduce the probability of collisions, Ethernet often uses the algorithm of 1-persistent CSMA, non-persistent CSMA, and P-persistent CSMA. Because Ethernet is designed for office automation and does not fully meet the requirements of the industrial environment and standards, there are still obvious defects in using traditional Ethernet in the industrial field. However, its cost is lower than that of industrial networks, and its technical transparency is high. In particular, it follows the IEEE802.3 protocol and opens up a convenient door for fieldbus manufacturers. However, in order for Ethernet to meet the technical requirements, the following defects must also be overcome:
(1) Uncertainty of communication
The biggest characteristic of the industrial control network is different from the ordinary data network is that it must meet the control function for real-time
The requirement is that the signal transmission should be fast enough and meet the certainty of the signal. Real-time control often requires accurate and regular refreshing of data for certain variables. Because Ethernet adopts CSMA / CD mode, when the network load is large, the uncertainty of network transmission cannot meet the real-time requirements of industrial control, so traditional Ethernet technology is difficult to meet the real-time requirements of accurate and regular communication required by the control system, and has been regarded It is a "non-deterministic" network.
(2) Non-real-time communication
In industrial control systems, real-time can be defined as the testability of the system's reaction time to an event. That is to say, after an event occurs, the system must respond within an accurately predictable time frame. However, the industry has strict requirements on the real-time nature of data transmission, and often the data update is completed within tens of ms. Also because of the CSMA / CD mechanism of Ethernet, when a conflict occurs, the data must be retransmitted. You can try up to 16 times. Obviously this conflict resolution mechanism comes at the expense of time. And even if it is dropped, even a few seconds, it may cause the entire production to stop or even equipment, personal safety accidents.
(3) Commercial Ethernet does not have high stability and reliability
The traditional Ethernet was not designed for industrial applications at the beginning of the design. It did not take into account the harsh working conditions of the industrial site environment, severe line-to-line interference, and the harsh conditions of machinery, climate, dust, etc. (EMI) performance is very poor, and it does not have intrinsically safe performance when it is used in hazardous situations; therefore, it places higher requirements on the industrial reliability of the equipment. At the same time, the industrial network must have good reliability, recoverability, and maintainability in the production environment. That is to ensure that when any component in a network system fails, it will not lead to the collapse and paralysis of applications, operating systems, and even network systems.
(3) Security issues
In the industrial production process, flammable, explosive or toxic gases are inevitably present in many sites. For the smart devices and communication equipment used in these industrial sites, certain explosion-proof technical measures must be taken to ensure the safe production of industrial sites . Under the current technical conditions, it is feasible to adopt explosion-proof and explosion-proof measures for the Ethernet system. That is, by adopting explosion-proof measures such as increased safety, airtightness, and potting for the Ethernet field devices, the ignition energy generated by the failure of the field device itself No leakage, to ensure the safety of system operation. For non-hazardous occasions without strict intrinsic safety requirements, complex explosion-proof measures may not be considered. Network security of industrial systems is another security issue that industrial Ethernet applications must consider. Industrial Ethernet can integrate the traditional three-layer network system of the enterprise, namely the information management layer, process monitoring layer, and field device layer, so that the data transmission rate is faster, the real-time performance is higher, and it can be seamlessly integrated with the Internet. Realize the sharing of data and improve the operation efficiency of the factory. But at the same time, it also attracts a series of network security issues. Industrial networks may be threatened by network security including virus infection, hacker's illegal invasion and illegal operation.
(4) Bus power supply problem
Bus power supply (or bus feed) means that the cable connected to the field device not only transmits data signals, but also provides working power to the field device. Ethernet has not considered this problem from the beginning of the design, and there are a lot of bus power supply requirements in the industrial field.
Because of the above problems, ordinary commercial Ethernet cannot be directly used for industrial site control. In order to solve these problems, industrial Ethernet came into being.
With the development and popularization of Internet technology, the increase of Ethernet transmission rate and the exchange of Ethernet technology
With the development, the above problems of traditional commercial Ethernet are being solved in industrial Ethernet.
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