Principles of Industrial Ethernet Switching Technology
Abstract: Industrial Ethernet switching technology solves the performance limitations of fieldbus networks and lays a solid foundation for future richer and more powerful automation applications.
[Keywords] Industrial Ethernet Switching Technology Industrial Ethernet Switch Industrial Ethernet
1 Introduction
Industrial Ethernet switching technology solves the performance limitations of fieldbus networks. Each Ethernet device can enjoy high bandwidth exclusively, thereby alleviating the problem of insufficient bandwidth and network bottlenecks, and laying a solid foundation for future richer and more powerful automation applications. . This article mainly discusses the basic principles of switching technology.
2. Exchange mechanism
Exchange is a technical term for the method of automatically completing equipment according to the needs of transmitting information at both ends of the communication, and sending the information that needs to be transmitted to the objects that meet the requirements. A generalized exchange is a device that completes the information exchange function in a communication system.
In the network system, the exchange concept is proposed to improve the shared working mode. The HUB hub is a shared device. The HUB itself cannot recognize the destination address. When an A device in a local area network transmits data to the B device, the data packets are transmitted in a broadcast manner on the HUB-based network. The device determines whether to receive by verifying the address information of the packet header. In other words, in this working mode, only one set of data frames can be transmitted on the network at the same time, and if a collision occurs, you must retry. This way is to share network bandwidth.
The switch performs data frame forwarding according to the MAC (Media Access Control) address of the data frame. When forwarding data frames, the switch follows the following rules:
If the destination MAC address of the data frame is a broadcast address or a multicast address, it is forwarded to all ports of the switch (except the source port);
If the destination MAC address of the data frame is a unicast address, but this address is not in the address table of the switch, it will also be forwarded to all ports of the switch (except the source port);
If the destination MAC address of the data frame is in the address table of the switch, it is forwarded to the corresponding port according to the address table;
If the destination MAC address of the data frame and the source address of the data frame are on the same port, it will discard the data frame and the exchange will not occur.
The switch has a very high bandwidth back bus and internal switching matrix. All ports of the switch are connected to this back bus. Through the switch address table, the switch only allows the necessary network traffic to pass through the switch. The filtering and forwarding of the switch can effectively isolate the broadcast storm, reduce the occurrence of error packets and error packets, and avoid sharing conflicts.
In the exchange address table of the switch, an entry is mainly composed of a MAC address and the switch port number where the address is located. The entire address table is generated using a dynamic self-learning method. When the switch receives a data frame, the source address and input port of the data frame are recorded in the exchange address table. Each address entry has a time stamp that indicates the time period in which the entry is stored. If the address table entry is still not referenced within a certain time frame, it will be removed from the address table. Therefore, the most effective and accurate address-port information is always maintained in the exchange address table.
The switch can perform data transmission between multiple port pairs at the same time. Each port can be regarded as an independent network segment, and the network devices connected to it independently enjoy the full bandwidth without competing with other devices.
3. Exchange method
There are currently three main exchange technologies:
1. Port switch
Port switching first appeared in slot-type hubs. The backplane of this type of hub is usually divided into multiple Ethernet segments. When connected without a bridge or router, they cannot communicate with each other. After the module is inserted, it is usually assigned to a network segment of a backplane. Port switching is used to distribute and balance the ports of the module among multiple network segments of the backplane. According to the degree of support, port switching can be subdivided into:
Module exchange: migrate the entire module to the network segment.
Port group exchange: Usually the ports on the module are divided into several groups, and each group of ports allows network segment migration.
Port-level switching: each port can be migrated between different network segments. This switching technology is based on the first layer of OSI, which has the advantages of flexibility and load balancing capabilities, but it does not change the characteristics of the shared transmission medium, so it cannot be called a real exchange.
2. Frame switch
Frame switching is currently the most widely used LAN switching technology. It provides a parallel transmission mechanism by micro-segmenting traditional transmission media to reduce collision domains and obtain high bandwidth. There are generally three processing methods:
Cut-through
The Ethernet switch adopting the straight-through switching mode can be understood as a line matrix telephone switch that crosses each port vertically and horizontally. When it detects a data packet at the input port, it checks the header of the packet, obtains the destination address of the packet, starts the internal dynamic lookup table to convert to the corresponding output port, connects at the intersection of input and output, and passes the data packet directly to The corresponding port realizes the switching function. Because it only checks the header of the data packet (usually only 14 bytes), no storage is required, so the cut-in method has the advantages of small delay and fast exchange speed. The so-called latency refers to the time it takes for a data packet to enter a network device to leave the device.
Its shortcomings mainly have three aspects: first, because the content of the data packet has not been saved by the Ethernet switch, it is impossible to check whether the transmitted data packet is wrong, and it cannot provide error detection capabilities; Input / output ports with different rates are directly connected, and it is easy to lose packets. If you want to connect to a high-speed network, such as providing Fast Ethernet (100BASE-T), FDDI, or ATM connections, you cannot simply "connect" the input / output ports because there are speed differences between the input / output ports, Cache must be provided; third, when the ports of Ethernet switches increase, the switching matrix becomes more and more complex, the more difficult it is to implement.
Fragment Free
This is a solution between straight-through and store-and-forward. It checks whether the length of the data packet is enough to be 64 bytes (512 bit) before forwarding. If it is less than 64 bytes, it means that it is a fake packet (or called a frame), then discard the packet; if it is greater than 64 bytes, then Send the package. The data processing speed of this method is faster than that of the store-and-forward method, but slower than that of the straight-through method. However, because it can avoid the forwarding of residual frames, it is widely used in low-end switches.
Switches that use this type of switching technology generally use a special type of cache. This kind of buffer is a first-in-first-out FIFO (First In First Out), where bits enter from one end and then exit from the other end in the same order. When the frame is received, it is saved in the FIFO. If the frame ends with a length of less than 512 bits, the content (residual frames) in the FIFO will be discarded. Therefore, there is no residual frame forwarding problem that exists in ordinary pass-through forwarding switches, which is a very good solution. Data packets will be cached before being forwarded to ensure that collision fragments do not propagate through the network, which can greatly improve network transmission efficiency.
Store-and-Forward
Store and forward (Store and Forward) is one of the most widely used technologies in the network field. The controller of the Ethernet switch first caches the data packets coming from the input port, first checks whether the data packets are correct, and filters out conflicting packet errors. After confirming that the packet is correct, take out the destination address, find the output port address you want to send through the lookup table, and then send the packet out. Because of this, the store-and-forward method has a large delay in data processing, which is its shortcoming, but it can perform error detection on the data packets entering the switch and can support the exchange between input / output ports of different speeds Improve network performance. Another advantage is that this switching method supports the conversion between different speed ports and keeps the high-speed ports and low-speed ports working together. The way to achieve this is to store the 10Mbps low-speed packet and then forward it to the port through the 100Mbps rate.
3. Cell switching (ATM switch
ATM uses a fixed-length 53-byte cell exchange. Due to the fixed length, it is easy to implement in hardware. ATM uses a dedicated non-differential connection, running in parallel, and establishing multiple nodes at the same time through a switch, but does not affect the communication capabilities between each node. ATM also allows multiple virtual links to be established between the source node and the target node to ensure sufficient bandwidth and fault tolerance. ATM uses statistical time division circuits for multiplexing, so it can greatly improve the channel utilization. ATM technology is widely used in telecommunication backbone lines.
4. Concluding remarks As can be seen from the above, the switch adopting the frame switching technology in store and forward mode is most in line with the requirements of industrial automation accuracy, economy and real-time. In the field of industrial automation and communication, the use of industrial Ethernet switching technology will be the mainstream of the future.
Detailed introduction: Industrial Ethernet Technical Manual
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