Abstract: Based on the history and current situation, it is concluded that the network technology of automated measurement and control system should follow the design principle of COTS (Commercial Off-the-Shelf Technology). Keywords: Distributed control system, fieldbus, ISO/OSI reference model, network product, switch 1 Historical Review After the emergence of telegraph, telephone, etc., there were communication networks for information transmission. Especially after the emergence of microcomputers, computers with different functions or different regions were connected to form networks. Digital communication technology has expanded from general single-machine (host to terminal) and multi-machine communication to computer network system. Automated measurement and control system is a distributed computer local area network that completes measurement and control functions. Its development process is as follows: (1) DCS began to appear in the mid-1970s. This was completed by large enterprises in the absence of a unified network standard. The network scale is about 32 to 64 nodes, the communication distance is about 1km, the main nodes are control stations and operation stations, the topology is mainly ring and bus type, the communication medium is mostly coaxial cable, and some use twisted pair, the communication rate is less than 1Mb/s. There are dedicated network components such as "communication card" in the control station and operation station. Representative products of this period include the DHW (Data Highway) bus of TDC2000 and the F bus of CENTUM, all of which use the "token bus" communication protocol. Data sharing has been achieved by workstation number. The representative products of this period are all analog instruments, and a few special equipment have serial ports such as RS-232/RS-422/RS-485, which can be connected to them. During this period, the communication protocols include the Synchronous Data Link Protocol (SDLC) proposed by IBM and others, the Advanced Data Link Protocol (HDLC), and the CCITT TX.25 of the International Telegraph and Telephone Information Committee. (2) During this period, in the development of office automation equipment, Xerox launched Ethernet in 1975. Later, many suppliers such as 3COM participated. Based on this, Ethernet LANs were formed and have been greatly developed in various general systems for handling sudden transactions. Ethernet uses Carrier Sense Multiple Access/Collision Detection (CSMA/CD) for data communication. (3) Based on the above two technologies, in February 1980, the IEEE (Institute of Electrical and Electronics Engineers) established a committee (hereinafter referred to as the IEEE 802 Committee) to develop local area network standards. In 1983, the ISO (International Organization for Standardization) adopted the Open Systems Interconnection (OSI) reference model, namely the ISO/OSI reference model. Based on this, in 1985, the IEEE 802 Committee established nine subcommittees (later increased to thirteen). IEEE 802.3 was responsible for CSMA/CD networks, IEEE 802.4 for token bus networks, and 802.5 for token ring networks. The other subcommittees were responsible for various related tasks. The work of these subcommittees later formed the ISO standards. These standards are collectively referred to as the IEEE 802 standards. (4) In the mid-to-late 1980s, based on the aforementioned ISO/OSI reference model and IEEE 802 standard, second-generation and third-generation DCS systems emerged. Their network characteristics included ensuring the continuity of the first-generation DCS network (i.e., interconnectivity), enabling interconnection of multiple device DCS systems and all workshops throughout the plant, and developing towards interconnection between the plant's control network and management network. During this period, the scale of the updated networks expanded, using fiber optics, increasing communication distances several times over, and communication speeds to 10Mb/s or higher. The number of workstation numbers covered was several times greater than before, and the number of workstation characters increased from 8 to 12 characters. Furthermore, the concept of a domain was formed. However, during this period, field instruments were still mainly analog instruments or HART standard instruments. Only the number of field instruments using remote I/O data communication was increasing, and the ability to communicate with PLCs, analyzers, and other digital devices was being enhanced. Interconnection between DCS systems from different manufacturers was also on the agenda, and heterogeneous network interconnection within DCS systems was gradually being realized, with the TDC3000 network structure being the most typical example. (5) In the 1980s, PLC programmable controllers were connected from individual controllers to form systems of small to medium scale. Around 1990, one or more PLCs were connected to one or more PCs (operator stations, with built-in HMI human-machine interface and configuration software or SCADA software) through RS-232/RS-485 serial ports to form a system. It adopted readily available network technologies, especially DDE or OPC data exchange software technology and IEC61131-3 standard configuration software, which greatly improved the openness and availability of PLC systems and made them a model of low-cost automation. It has now gradually transitioned to the industrial Ethernet network dominated by the early 21st century. Moreover, the CIP common industrial protocol led by Rockwell has been formed, and the three-layer communication network of DeviceNet/ControlNet/Ethernet/IP has been accepted. PLC has changed from having poor communication functions to being at the forefront of networking. (6) Fieldbus technology has formed a development boom in the 1990s. It adapts to the needs of field measurement and control in various industries, resulting in a situation where multiple standards coexist. FF H1/FF HSE are more suitable for process control, while Profibus and DeviceNet are more suitable for discrete control. The Foundation Fieldbus FF H1 adopts layers 1, 2, and 7 of the ISO/OSI communication model, as well as the user layer. It has withstood long-term testing in practical engineering in terms of two-wire power supply, explosion protection, electromagnetic interference protection, lightning protection, redundancy, field control, interoperability, and interchangeability. FF HSE, based on seamless connection with FF H1, uses high-speed Ethernet (HSE) to complete the network COTS (Commercial Off-the-shelf Technology) task for the central control room level or monitoring layer. It adopts network products such as switches and TCP/UDP/IP protocols at the transport and network layers, ensuring the system's openness and operability. The development of fieldbus technology is inseparable from the digitalization, networking, and intelligentization of field instruments. Currently, the production of field instruments such as transmitters used in process industries is 10% FF, 40% HART, and 50% analog instruments. Therefore, it is necessary to pay attention to the upgrading of field detection instruments and actuators in order to popularize fieldbus technology. (7) The development of the Internet (global wide area network) and mobile communication, multimedia technology, personal computers and operating system network support functions that emerged before the 1990s have had a profound impact on the digital communication technology of automated measurement and control systems. Due to the popularization of the Internet and the rise of the "network access industry", the prices of network products such as switches, integrators, and Category 5 twisted copper cables have decreased. The application technology of Ethernet and Internet protocol suite (including TCP/IP, etc.) has penetrated into various network devices. The transmission mode has developed from baseband to carrier, broadband and other modes. The network speed has increased from 10Mb/s to 100Mb/s, 1000Mb/s, etc. Therefore, the "industrial Ethernet" craze has emerged in automated measurement and control systems. Furthermore, due to the emphasis on the deterministic and real-time characteristics of automated measurement and control systems, some people have modified commercial Ethernet to form various real-time Ethernet systems. However, this is just a process, and the key is whether the cost-effectiveness can be accepted by users. In short, the trend of developing towards a flattened network for automated measurement and control systems, and even "e-network to the end", is obvious, but it will take time. (8) The emergence of the fourth generation DCS is adapted to the development of fieldbus technology and integrated control technology in the early 21st century. Currently, all 11 master control systems certified by the Fieldbus Foundation's HIST interoperability test are 4th generation DCS systems. Furthermore, the "Federation Fieldbus Systems Engineering Guidelines" state that "all master FF functions, including engineering, configuration, maintenance, and operational performance, should be compatible and seamlessly integrated with traditional analog or discrete I/O, intelligent HART and dedicated I/O, bus-based I/O, and FF systems. It is recommended to avoid using standalone software tools, displays, or programs that are only for FF and incompatible with traditional systems." This provides a solution to the viewpoint of "FCS replacing DCS," namely, a path of mutual integration, complementarity, and a win-win situation for both DCS and FCS. This is thanks to network technology, which enables such complex communication requirements to be perfectly integrated under cost-effective conditions. Of course, the master control systems mentioned above all reach a higher level of FF HSE. The communication technology of 4th generation DCS converges with Ethernet and TCP/UDP/IP protocols, ensuring compatibility and data sharing with factory management networks, gradually forming an enterprise Intranet. 2. The Open Systems Interconnection (OSI) Reference Model reflects the working principle of computer network communication, segmenting the communication process and correspondingly dividing network functions into different logical and physical layers. Different functional layers complete different communication stages. The OSI model consists of seven layers: Application Layer, Presentation Layer, Session Layer, Transport Layer, Network Layer, Data Link Layer, and Physical Layer. A comparison between it and the FF H1 and FF HSE communication models is shown in Figure 1. The basic method for implementing the functions of each layer of a network node is to implement lower-layer protocols in hardware and higher-layer protocols in software. Currently, OSI functions are implemented using integrated circuits such as "dedicated chips." Ethernet is a good example, as is LonWorks technology. Its Neuron chip integrates three CPUs, embedding the relevant seven-layer communication protocols of the OSI model, including compatibility with multiple communication media, NeuronC control language, and network variables, enabling interoperability. This has led to its widespread application in local area networks such as intelligent buildings, serving as a successful example. 3. Classification of Network Products If we consider all interface cards, connectors, and communication media that connect terminal nodes to cables and other communication media, as well as nodes specifically designed for communication, as network products, and further consider office or commercial network products and industrial management and control network products (including fieldbus), and also classify communication software, it becomes a complex task. Currently, based on the functions of low-level network interconnection devices, the main types are repeaters, bridges, routers, and gateways; all of these are active products. Passive products include T-type branches, passive hubs, terminators, cables, and optical fibers. In actual networks, existing physical layer standard connectors such as RS-232 and RS-485 are widely used, solving many simple communication needs, and their commercial forms are diverse. In practical networks, products also include interface cards, interface modules, and linking devices, especially connectors, hubs, and switches for commercial Ethernet. As network products, wireless digital communication devices such as modems and data radios are already widely used in China, and it is hoped that this will change with the development of digital communication services related to commercial mobile communication technology. The popularization of embedded system technology, especially the use of 32-bit ARM9 processors in network products, will improve the intelligence, price, and size of network products, which also promotes the development of network technology. 4. About Switches In recent years, switches have been widely used in the networks of newly built central control rooms in process industry enterprises. The stacked switch diagrams are particularly prominent, proving that the monitoring layer of automated measurement and control system networks has adopted switches in many industries. A switch is a multi-bus switching matrix interconnected, meaning each port is connected to a high-bandwidth backplane bus. Besides encapsulating data packets for forwarding and reducing collision domains, switches also provide functions such as broadcast storm isolation and network management. Industrial Ethernet switches, in particular, should be adaptable to industrial environments, provide dual power supplies to avoid data loss caused by a single power source, support ring networks for network redundancy, pass security certifications, and ensure rapid recovery after outages. It is precisely because of these added functions that their price is slightly higher than commercial Ethernet products. Currently, industrial Ethernet switches from companies like Hirschmann and Excellence are widely used. As for whether commercial switches can be used in industrial applications, it should be determined on a case-by-case basis. For example, it is not uncommon for many industrial enterprises to use brand-name Dell computers as operator stations instead of general industrial control computers in system integration. 5 System Integration Technology and Network Technology The foundation of automated measurement and control systems is 3C (computer, control, communication) and 1I (integration). Currently, the 1S (solution) goal has been proposed, driven by the integration of electrical control, instrumentation control, and management control. System integration technology and network technology complement each other, working together to achieve the goal of sharing data and information over the network. The term "system integration" became popular in the late 1980s and early 1990s. Now, the distinction between "Integrated System" and "System Integration" is gradually blurring, demonstrating the increasing proficiency of system integrators and end-users in network technology, and the growing demand for interface cards for interconnecting various networks or fieldbuses. Dozens of interface card products and multi-protocol gateways (Modbus, Ethernet, TCP, IP, Profibus) from companies like Woodhead and Prosoft can meet the needs of system integration. Driven by three standards—OPC (Object Linking Embedded for Process Control), EDDL and FDT (regarding interoperability), and IEC 61131-3 (regarding configuration)—system integration technology is further advancing. The ISO 15745 standard, released in 2003, addresses the two fundamental issues of application requirements and interfaces in system integration. The standard is officially titled: Industrial Automation Systems and System Integration—Open Systems Application Integration Framework. It is divided into four parts, with Part 1 proposing the Open Systems Application Integration Framework (AIF). The network of automated measurement and control system often involves its second, third and fourth parts. For example, the Foundation Fieldbus FF H1/FF HSE is related to the third and fourth parts. Therefore, it is a combination of heterogeneous systems. In many heterogeneous systems, the monitoring layer adopts the Ethernet standard of the fourth part. The star structure centered on the switch is more commonly used. Moreover, redundancy technology, security technology, quality service, etc. have been greatly improved. The real-time response time of industrial Ethernet has been reduced to 5-10ms. This can meet the requirements of control systems (including safety systems) in process industries and discrete industrial control systems (except for a few motion control systems with high synchronization requirements). Therefore, it can be said with certainty that practice has proven that the use of fieldbuses such as FF H1 in the field layer and general industrial Ethernet in the monitoring layer is a mature network technology and will be widely adopted in the future. 6 Conclusion (1) COTS can be used as a design principle in military information systems. Therefore, general automated measurement and control systems should pay more attention to the application of COTS, reduce the price of network products, and make the "auxiliary equipment" of fieldbus universal. (2) Classify the network requirements of automated measurement and control systems. First, promote COTS in data acquisition systems; promote different fieldbus technologies for different industries; in the process of promoting industrial Ethernet, engineering applications and development should be given equal importance. (3) Digitalization, intelligence and networking of testing instruments and actuators are the foundation. New products in this area should be vigorously developed in China. At the same time, testing centers and certification centers should be established in China to carry out standardization work, ensure openness, operability and interchangeability, and strive to become internationally recognized products. (4) Network technology should be further popularized in the automation industry. Although there are already more than a dozen related books, it is still very important to hold training courses for users. The cooperation between the China Instrument and Control Society and Beijing Synthetic Network Company has set an example for serving users in this regard and is worth promoting.