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Reposted from: Shanghai Zhao Yue Communication Technology Co., Ltd. Author: Luan Lili-MEXON. The emergence of fieldbuses has played a significant role in enabling device-oriented automation systems. However, such dedicated real-time communication networks have drawbacks such as high costs, low speeds, and limited support for various applications. Coupled with the diversity of bus communication protocols, different bus products cannot be interconnected, shared, or made interoperable, which thus imposes severe limitations on the further development of fieldbus-based industrial networks. With the development of Ethernet technology, and in particular the emergence of high-speed Ethernet, Ethernet was able to overcome its own limitations and enter the industrial sector as industrial Ethernet, thereby allowing people to use Ethernet devices to replace expensive industrial networking equipment. 1. The main defects of Ethernet Before discussing the main defects of Ethernet, it is necessary to first understand its communication mechanism. Ethernet refers to a network that adheres to the IEEE802.3 standard and can transmit data over optical fibers and twisted pairs. It first appeared in 1972, created by XeroxPARC. Current Ethernet uses star and bus topologies, with transmission speeds of 10 Mb/s, 100 Mb/s, 1000 Mb/s or higher. The main cause of latency in Ethernet is collisions, which arise from its use of CSMA/CD technology. In traditional shared networks, since all stations in Ethernet are connected using the same physical medium, this means that when two devices transmit signals at the same time, there is interference between those signals. To solve this problem, Ethernet stipulates that before a station accesses the medium, it must first check whether any other stations on the network are using that medium at the same time. , if it exists then one must wait, and this is when a conflict occurs. To reduce the likelihood of conflicts, Ethernet often employs algorithms such as 1-continuous CSMA, non-continuous CSMA, and P-continuous CSMA. Since Ethernet was designed with office automation in mind and does not fully meet the requirements of industrial environments and standards, there are significant drawbacks to using traditional Ethernet in industrial applications. However, its cost is lower than that of industrial networks, and its technical transparency is high; in particular, its adherence to the IEEE802.3 protocol facilitates work for various fieldbus manufacturers. Yet, to make Ethernet meet process requirements, the following shortcomings must be overcome: 1.1 Determinism Since Ethernet’s MAC layer protocol is CSMA/CD, this protocol leads to conflicts on the network, especially when the network load is high. In an industrial network, if there are numerous collisions, data must be retransmitted multiple times, which **increases** the uncertainty in inter-network communication. In industrial control networks, such uncertainty from one point to another inevitably leads to a decline in the system’s control performance. 1.2 Real-time performance In industrial control systems, real-time can be defined as the measurability of the system’s response time to a certain event. In other words, after an event occurs, the system must respond within a time frame that can be accurately predicted. However, industrial applications have very strict requirements regarding the real-time nature of data transmission, with data updates often being completed within dozens of milliseconds. Also due to the CSMA/CD mechanism inherent in Ethernet, when a collision occurs, the data must be resent, with up to 16 attempts allowed. It is obvious that this conflict-resolution mechanism comes at the cost of time. Moreover, even a brief interruption in connection, lasting just a few seconds, can lead to the cessation of entire production processes, as well as safety hazards related to equipment and personnel. 1.3 Reliability Since Ethernet was not designed with industrial network applications in mind from the beginning. When applied in industrial settings, it faces harsh operating conditions and severe inter-wire interference, all of which inevitably lead to a reduction in its reliability. In a production environment, industrial networks must possess good reliability, recoverability, and maintainability. In other words, it ensures that the failure of any component within a network system does not lead to the collapse or disruption of the applications, operating system, or even the entire network system. 2. Solutions for Industrial Applications of Ethernet In order to address the three major shortcomings of Ethernet and the special requirements of industrial environments for industrial networks, various methods have been employed to improve Ethernet’s performance and quality in order to meet the needs of these industries. The following are several solutions: 2.1 Switching technology To address network congestion when the Ethernet load is high, Ethernet switches can be used. It employs effective collision domain segmentation techniques for shared local area networks. The various collision domains are connected using switches in order to reduce the collision problems and erroneous transmissions caused by the CSMA/CD mechanism. This helps to minimize conflicts and increase the determinism of the system, but it is costly, with certain delays in the allocation and buffering processes. 2.2 High-Speed Ethernet We know that the greater the load on a network, the higher the probability of collisions. Data shows that when the load on a network is below 36%, conflicts hardly occur; at a load of less than 10%, the collision rate for 10M Ethernet is once every five years. The collision probability for 100M Ethernet is once every 15 years. However, after exceeding 36%, the probability of conflicts increasing with rising load grows at a geometric rate. Clearly, increasing the communication speed of Ethernet can effectively reduce the load on the network. Fortunately, high-speed Ethernet variants with communication rates of 100 M/S and 1 G/S are now available. Coupled with thorough design and control over the number of network nodes and traffic volume in the system, Ethernet can certainly be used as an industrial network. 2.3 IEEE1588 Synchronization Mechanism IEEE1588 defines a protocol for precise synchronized clocks (PTP) in measurement and control networks, related to network communication, local computation, and task allocation. This protocol is not exclusive, but it is particularly suitable for Ethernet-based technologies, with precision in the microsecond range. It uses a time stamp to synchronize local time. Even when there are certain fluctuations in the generation of the synchronization control signal during network communication, the precision it achieves still meets the requirements. This makes it particularly suitable for Ethernet-based systems. By adopting this technology, the Ethernet TCP/IP protocol can operate in high-precision network control systems with minimal modifications. In the regional bus, the precision it achieves far exceeds that of various existing systems. Furthermore, there are significant advantages to using Ethernet TCP/IP protocol-based network technologies at all levels of an enterprise. A simple system that includes the IEEE1588 synchronization mechanism consists of at least one master clock and multiple slave clocks. If there are multiple potential master clocks, the active master clock will be determined by an optimized master clock algorithm. All clocks continuously compare their clock properties with those of the master clock; if a new clock is added to the system or the existing master clock loses connection to the network, the other clocks will determine a new master clock. If multiple PTP subsystems need to be interconnected, this must be achieved through boundary clocks. One port of the boundary clock serves as a slave port to connect with the subsystem, providing a clock standard for the entire system. Therefore, the master clock of this subsystem is the original master clock of the entire system. The other ports of the boundary clock serve as master ports, through which synchronization information is transmitted to the subsystems. The port of the boundary clock is a regular clock for the subsystem. The precise network synchronization protocol defined by IEEE1588 achieves high synchronization within the network, eliminating the need for dedicated synchronization communication when assigning control tasks, thereby separating the communication timing pattern from the application execution timing pattern. Thanks to high-precision synchronization, the fluctuations in data transmission time inherent to Ethernet technology are reduced to an acceptable level that does not affect control precision. One of the major advantages of IEEE1588 is that its standard is highly representative and open. Due to its openness, many suppliers of control systems have now adopted this standard in their products. Moreover, manufacturers of different devices follow the same standards, which ensures good synchronization among their products as well. 3. Typical Industrial Ethernet With the rapid development of Ethernet technology, its 80% market share, and the obvious shortcomings of fieldbuses, various manufacturers in the industrial control sector have begun to develop industrial Ethernet solutions that are suitable for their own industrial products and offer good compatibility. One of the most widely used industrial Ethernet standards is SIMATIC NET, developed by the German company Siemens. It provides open communication systems suitable for various control levels in industrial environments; these communication systems are all based on ** and international standards, and comply with the ISO/OSI network reference model. The main architecture of SIMATIC NET industrial Ethernet consists of network hardware, network components, topology, gateway processors, and SIMATIC NET software. 3.1 Basic Types and Network Hardware of SIMATIC NET Industrial Ethernet There are two types of SIMATIC NET industrial Ethernet: 10Mbit/s industrial Ethernet and 100Mbit/s industrial Ethernet. It is a unit-level and control-level transmission network that utilizes carrier-based transmission technology, based on IEEE802.3 and the CSMA/CD medium access method. In Siemens industrial Ethernet, the commonly used physical transmission media are shielded twisted pair (TP), industrial shielded twisted pair (ITP), and fiber optics. TP connections are commonly used for end-to-end connections. A Data Terminal Equipment (DTE) is connected directly to the port of the network connection element, and it is responsible for amplifying and forwarding signals. In SIMATIC NET industrial Ethernet, these network connection components are OLM (Optical Link Module), ELM (Electrical Link Module), OSM (Optical Switch Module), and ESM (Electrical Switch Module). The DTE is connected to the connection element via a TP or ITP cable. 3.2 SIMATIC NET Industrial Ethernet Network Components The SIMATIC NET Industrial Ethernet network components include the Industrial Ethernet link templates OLM and ELM, as well as the Industrial Ethernet switches OSM/ESM and ELS, and the Industrial Ethernet link module OMC. Among them, the OLM (Optical Link Module) has 3 ITP interfaces and 2 BFOC interfaces. The ITP interface can connect three terminal devices and network segments, while the BFOC interface can connect two optical devices (such as OLMs), with a speed of 10 Mbit/s. The ELM (Electrical Link Module) has 3 ITP interfaces and one AUI interface. Network devices can be connected to the LAN via the AUI interface at a speed of 10 Mbit/s. On standard OSMs, the electrical interfaces (TP/ITP) are 10/100 Mbit/s adaptive and have adaptive wiring order. The fiber optic interface is a 100 Mbit/s full-duplex BFOC interface, suitable for multimode fiber connections. The maximum distance between two OSMs is 3 km. A maximum of 50 OSMs can be connected on the same network segment, resulting in an extended range of 150 km. It also features addressing*, address deletion, the ability to set the transmission baud rate (10 or 100 Mbit/s), and adaptive functions, which simplify network configuration and enhance network scalability. Furthermore, in accordance with the IEEE802.1Q standard, OSM/ESM also supports VLANs (Virtual Local Area Networks), which provide VLAN priority tags for packets. It assigns data priority levels from low to high (0-7), with packets that lack a destination address being considered low-priority frames. 3.3 Topology of SIMATIC NET industrial Ethernet 3.3.1 Bus topology In the bus topology of OLM or ELM, DTE devices can be connected to OLM or ELM via ITP cables and interfaces. Each OLM or ELM has three ITP interfaces. OLMs can be connected via optical cables, with a maximum of 11 units that can be cascaded. The ELMs can be connected to each other using ITP XP standard cables, with a maximum of 13 units capable of being cascaded. ESMs can be connected via TP/ITP cables to form a network of the overall type. Any port can be used as a cascading port. The distance between two ESMs must not exceed 100m, and a maximum of 50 ESMs can be connected in the entire network. 3.3.2 Ring Topology OLM can connect the ends of a bus network using optical cables, thereby forming a ring network. A maximum of 11 OLMs can be cascaded across the entire network, and compared to bus-type networks, the redundant ring topology enhances the reliability of data exchange. OSM/ESM can also form a ring network topology, and they possess network redundancy management capabilities. Through the DIP switches, any OSM/ESM in the network can be set as the redundant manager. Thus, a redundant ring network can be formed, in which ports 7 and 8 on the OSM/ESM serve as the fiber-optic connection ports for the ring network. As a redundancy manager, OSM monitors the status of ports 7 and 8; once a network interruption is detected, it reconstructs the entire network by switching it to a backup channel, ensuring that data transmission is not interrupted. The network reconfiguration time is less than 0.3S. 3.4 Ring Network Redundancy In Siemens industrial Ethernet, each OSM/ESM (except OSM TP22 and ESM TP40) is equipped with a standby-sync interface. Using a pair of OSM/ESMs, the standby master station and standby slave station are set via DIP switches. By connecting the spare interfaces with ITP XP standard cables, this pair of OSM/ESMs can be used to redundantly connect to another ring network. The backup master station and slave stations are connected via ITP XP9/9 standard cables. When the backup master station channel fails, the backup slave station connection channel takes over ； When the backup master station channel returns to normal, the backup master station will notify the backup slave station, and the backup slave station will cease operating. And the time for the entire network reconstruction is less than 0.3m. 3.5 SIMATIC NET Industrial Ethernet Communication Processors The commonly used SIMATIC NET industrial Ethernet communication processors (CPs) include the CP243-1 series, CP343-1 series, and CP443-1 series of processors used in S7 PLC stations, as well as network cards for use in PCs; these processors offer Ethernet interfaces such as ITP, RJ45, and AUI. They connect PLCs or PCs to industrial Ethernet at a speed of 10/100 Mbit/s. The CP series of templates are used for S7 series PLCs to communicate over industrial Ethernet. With these templates, it is easy to connect S7 series PLCs via Ethernet, and the STEP7-Micro/WIN32 software can be utilized to remotely configure, program, and diagnose the S7 series PLCs over Ethernet. Additionally, through the CP series, the PLCs in the S7 series can be connected to each other via Ethernet, as well as communicate with OPC servers on PCs. 3.6 SIMATIC NET Industrial Ethernet Software SIMATIC NET Industrial Ethernet Software includes SIMATIC NET V6.2 and OPC (OLE for Process Control). The SIMATIC NET software offers a unified Windows interface, and it also integrates and enhances various functions; in particular, it provides the APC (Advanced PC Configuration) tool, which allows a PC to function as a node in the overall system control structure and to communicate with other PLC nodes. Additionally, it offers OPC Server capabilities as well as data processing functions. OLE (Object Linking and Embedding) is itself an application based on Microsoft COM technology, while the OPC interface is an open and unified software interface based on OLE. OPC does not rely on any single manufacturer; almost all industrial control software and hardware manufacturers have integrated OPC interfaces. As a result, device communication between different hardware manufacturers can be carried out through this unified OPC interface, thereby avoiding difficulties in data exchange caused by differences in communication protocols among various device manufacturers. SIMATIC NET OPC features a server/client architecture; the program interfaces through which clients access the server include automated interfaces and user-defined interfaces, with only the custom interfaces enabling access to fault alarm and trigger event messages. The structure of SIMATIC NET OPC is hierarchical, namely OPC server --- OPC group --- OPC item, and all OPC data access is based on this structure. 4. Prospects and Future of Industrial Ethernet Industrial Ethernet attracts an increasing number of manufacturers in the manufacturing sector thanks to its unique advantages of low cost, high efficiency, high scalability, and high intelligence. On the one hand, with so many manufacturers developing industrial Ethernet technology, if no unified standards are established, similar to what happens with fieldbuses, there will be numerous standards and poor compatibility, which in turn will hinder the development of industrial Ethernet. That is precisely why the international community has begun working on establishing a standard for industrial Ethernet. At the third meeting of the IEC/SC65C/WGs, the working group responsible for drafting standards for industrial Ethernet, held in Beijing in May 2004, we were able to see an initial version of international standards for industrial Ethernet. These standards were finalized in August 2005; after further rounds of feedback collection in February and December 2006, they were officially released in the second half of 2007. This transformed the series of standards from IS standards to IEC standards 6. On the other hand, the rapid advancement of Ethernet and communication technologies has also driven further development in industrial Ethernet technology. Now, industrial Ethernet technology is beginning to evolve in the direction of real-time industrial Ethernet and wireless industrial Ethernet. In particular, B&R in Austria has developed true real-time Ethernet (Ethernet Powerlink), and in the near future, a new generation of industrial Ethernet components designed for future industrial networks will also appear. Due to Ethernet’s reputation of being able to reach all the way down to the control layer of devices on-site in enterprises, as industrial Ethernet technology continues to develop, it will replace the current fieldbus-based industrial networks and become the dominant technology in industrial networking. This post was last edited by wopale3 on 2009-3-17 10:37.]