The data is divided into data blocks with a certain length, and the identification information called the title is given to form an "information packet". The exchange mode of data transmission in packets is called packet switching. The packet switch stores the information packets sent from the sending terminal in the memory, and then sends them to the receiving terminal according to the header address information. This exchange is called storage exchange mode. Packet switching is based on "statistical time division multiplexing". In packet switching, under the premise that all terminals transmit data to each other at the same time, in order to reduce the suppression of trunk capacity, when traffic congestion may occur, traffic control must be carried out between communication nodes. Packet switching was put forward by the United States in 1960, and the proposal of X.25 was recognized in 1980 meeting, and the standard framework of packet switching was formulated. The physical layer of X.25 uses x.21; The data link layer is based on LAPB (Balanced Link Access Process), and all controls are carried out in the form of frames. Packet switching is a wide-area communication technology, which groups data and transmits them in a data sharing network without using a dedicated line. The difference between packet switching and circuit switching is that it uses a virtual circuit (datagram), that is, it consists of the bandwidth allocated by a certain * * * line network as needed. In packet-switched networks, there is no direct physical connection between the two parties exchanging data. This connection is just a logical connection. In a virtual line, a special path is established for each call, and all the packets of the call are transmitted through the network along this path. The transmitted data is divided into short packets and transmitted through the network respectively. At the destination, the packet is reassembled into the original format. Suppress data transmission. In addition, the correct switching of address information is a necessary condition for successful communication, so it is necessary to carry out error control on each communication node. Some advantages of packet switching: high line efficiency, because long-distance lines can be dynamically shared by multiple calls and users; Load handling: the network provides buffering capacity to handle temporary load increase without causing congestion; Data rate conversion: users exchange data at different rates; The cost is lower because many users can enjoy network resources. CSMA/CD (Carrier Sense Multiple Access/Collision Detection) In this protocol, each station listens to the bus and transmits only when the bus is idle. If a collision occurs, the packet will be retransmitted after a random time. CRC (Cyclic Redundancy Check) is a data transmission error detection function, which performs polynomial calculation on data and appends the obtained sum to the back of the frame. The receiving device also executes a similar algorithm. TM (Traffic Management) is a set of actions and operations that the network runs to ensure the operability of the network. TM is carried out in the form of communication flow control. ATM throughput management includes CAC, FRM, NRM, priority control (PC), traffic shaping (TS) and UPC, and its purpose is to maintain the required QoS. TDM (Time Division Multiplexing) is a device that distributes the available time on a composite line to its channels, usually staggered bits or characters from data terminals. WDM (Wavelength Division Multiplexing) transmits a series of optical carriers carrying information at intervals of 1 to several hundred nanometers along a single optical fiber in the optical frequency domain. It is also a multiplexing technology to separate optical carriers with different wavelengths by a certain method at the receiving end. It can be used to realize bidirectional transmission on a single optical fiber of the access network. SDM (Space Division Multiplexing) is a multiplexing method that uses space division to form different channels. For example, in an optical fiber access network, different optical fibers are used to transmit different types of uplink and downlink services.