Principle of the hottest Industrial Ethernet switc

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Industrial Ethernet switching technology principle

1. Introduction

Industrial Ethernet switching technology solves the limitations of fieldbus network compared with graphene, liquid metal and other new materials. Each Ethernet device can enjoy high bandwidth alone, thus alleviating the problems of insufficient bandwidth and network bottleneck, and laying a solid foundation for richer and more powerful automation applications in the future. This paper mainly discusses the basic principles of switching technology

2. Switching mechanism

switching is a general term of technology that sends the information to meet the requirements to the objects by means of equipment automatic completion according to the needs of information transmission at both ends of the communication. The switch in a broad sense is a kind of equipment that completes the information exchange function in the communication system

in the network system, the concept of exchange is an improvement on the shared working mode. Hub is a shared device. The hub itself cannot identify the destination address. When device a in a local area transmits data to device B, the data packets are transmitted by broadcasting on the hub based network. Each device determines whether to receive the data packets by verifying the address information in 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. If there is a collision, you have to retry. This method is to share network bandwidth

the switch forwards the data frame according to the MAC (media access control) address of the data frame. When the switch forwards a data frame, the following rules shall be followed:

if the destination MAC address of the data frame is a broadcast address or a multicast address, it shall be 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 will be forwarded to the corresponding port according to the address table

if the destination MAC address of the data frame is on the same port as the source address of the data frame, it will discard the data frame and the exchange will not occur

the switch has a high bandwidth back bus and internal switching matrix. All ports of the switch are connected to the back bus. Through the switch address table, the switch only allows 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 wrong packets and wrong packets, and avoid sharing conflicts

In the switch address table of the

switch, a table entry is mainly composed of a MAC address and the switch port number where the address is located. The whole address table is generated by dynamic self-learning method, that is, 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 table item has a time mark to indicate the storage time period of the table item. If the address table entry is still not referenced within a certain time range, 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 transmit data between multiple port pairs at the same time. Each port can be regarded as an independent segment, and the network devices connected to it enjoy all the bandwidth independently without competing with other devices

3. Switching mode

at present, there are mainly the following three switching technologies:

1. Port switch

port switching first appeared in slot hubs. The backplane of such hubs is usually divided into multiple Ethernet segments. When there is no bridge or router connection, they cannot communicate with each other. After the module is inserted, it is usually allocated to a certain backplane segment. Port switching is used to allocate and balance the module ports among multiple backplane segments. According to the degree of support, port switching can be divided into:

module switching: segment migration of the entire module

port group switching: generally, the ports on the module are divided into several groups, and each group of ports allows segment migration

port level switching: supports migration of each port between different segments. This switching technology is based on the first layer of OSI. It has the advantages of flexibility and load balancing. However, it does not change the characteristics of shared transmission media, so it cannot be called a real switching

2. Frame switch

frame switch is the most widely used local switching technology at present. It provides a parallel transmission mechanism by differentiating the traditional transmission media, so as to reduce the conflict domain and obtain high bandwidth. Generally, there are three processing methods:

cut through

the Ethernet switch adopting the cut through switching method can be understood as a vertical and horizontal line matrix switch between ports. When a data packet is detected at the input port, it checks the packet header of the packet, obtains the destination address of the packet, starts the internal dynamic lookup table to convert it into the corresponding output port, connects it at the intersection of input and output, and connects the data packet directly to the corresponding port to realize the switching function. Because it only checks the packet header (usually only 14 bytes) of the data packet and does not need storage, the cut in method has the advantages of small delay and fast switching speed. The so-called latency refers to the time that a packet enters a network device and leaves the device

it has three main disadvantages: first, because the contents of data packets have not been saved by the Ethernet switch, it is unable to check whether the transmitted data packets are wrong and cannot provide error detection capability; Second, because there is no cache, I/O ports with different rates can not be connected directly, and it is easy to lose packets. If you want to connect to a high-speed network, such as providing a fast Ethernet (100Base-T), FDDI or ATM connection, you cannot simply "connect" the input/output ports, because there is a speed difference between the input/output ports, and a cache must be provided; Thirdly, when the ports of the Ethernet switch increase, the switching matrix becomes more and more complex, and it becomes more and more difficult to implement

fragment free

this is a solution between pass through and store and forward. Before forwarding, it first checks whether the length of the data packet is enough for 64 bytes (512 bit). If it is less than 64 bytes, it indicates that it is a false packet (or residual frame), and then discards the packet; If it is larger than 64 bytes, the packet is sent. The data processing speed of this method is faster than the store and forward method, but slower than the pass through method. However, it can avoid the forwarding of residual frames, so it is widely used in low-end switches

switches using this type of switching technology generally use a special cache. This cache is a first in first out FIFO (first in first out) in which bits enter from one end and then come out from the other end in the same order. When a frame is received, it is stored in FIFO. If the frame ends with a length less than 512 bits, the contents (residual frames) in the FIFO will be discarded. Therefore, there is no residual frame forwarding problem existing in ordinary direct forwarding switches, which is a very good solution. The data packets will be cached and saved before forwarding. The fatigue and wear test results of the device show that ensuring that the collision fragments do not propagate through the network can greatly improve the network transmission efficiency

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 to be sent through the lookup table, and then send the packet. For this reason, the store and forward mode has a large delay in data processing, which is its deficiency. However, it can detect the error of data packets entering the switch, and support the exchange between input/output ports of different speeds, which can effectively improve the network performance. Another advantage is that this switching mode supports the conversion between ports with different speeds and keeps the high-speed ports and low-speed ports working together. The implementation method is to store 10Mbps low-speed packets, and then forward them to the port through 100Mbps rate

3. Cell switching (ATM switch)

atm uses cell switching with a fixed length of 53 bytes. Because the length is fixed, it is easy to implement with hardware. ATM adopts a special non differential connection to ensure the concentricity of the experiment, operates in parallel, and establishes multiple nodes simultaneously through a switch, but it will not affect the communication ability between each node. On the basis of the original resource accumulation, ATM water will also allow 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 circuit for multiplexing, which can greatly improve the channel utilization. ATM technology is widely used in Telecom backbone lines

4. Conclusion

it can be seen from the above that the switch adopting the storage and forwarding mode of frame switching technology best meets the accuracy, economy and real-time requirements of industrial automation. In the field of industrial automation communication, industrial Ethernet switching technology will be the mainstream in the future. (end)

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