This is the characteristic of packet switching There are three kinds of switching***: packet switching, message switching, and circuit switching (1) Circuit switching: Since circuit switching has to establish a physical path between the communicating parties that is exclusively used by both parties before communication (the switching equipment and links between the communicating parties are connected segment by segment), it has the following advantages and disadvantages. Advantages: ①The delay in transmitting data is very small because the communication line is exclusive to the users of both sides of the communication, and the data is transmitted directly. ② Once the physical path between the two sides of the communication is established, the two sides can communicate at any time, real-time. ③The two parties transmit data in the order of sending when communicating, and there is no out-of-order problem. Circuit switching is suitable for transmitting both analog and digital signals. ⑤ Circuit switching is simple in terms of switching equipment (switches, etc.) and control. Disadvantages: ① The average connection establishment time of circuit switching is too long for computer communication. ② After the circuit switching connection is established, the physical path is occupied exclusively by the two communicating parties, and even if the communication line is free, it cannot be used by other users, so the channel utilization is low. ③When circuit switching, the data goes straight to the terminals of different types, specifications and rates, and it is difficult for them to communicate with each other, and it is also difficult to carry out error control in the process of communication. (2) Message switching: Message switching is the unit of data exchange, which carries information such as destination address and source address, and adopts store-and-forward transmission at the switching node, thus it has the following advantages and disadvantages: Advantages: ① Message switching does not need to set up a dedicated communication line for the two sides in advance, and there is no connection establishment delay, so that the user can send a message at any time. Because of the store-and-forward transmission method, it has the following advantages: a. It is easy to set up code checking and data retransmission facilities in the message exchange, plus the exchange node also has the path selection, so it is possible to do that when a certain transmission path fails, it can re-select another path to transmit the data to improve the reliability of the transmission; b. It is easy to realize the code conversion and rate matching in the store-and-forward, and even the sender and receiver can not be in the available state at the same time. Both sides can not be available at the same time. This facilitates communication between computers of different types, specifications and speeds; c. Provides multi-target service, i.e., a message can be sent to multiple destinations at the same time, which is difficult to realize in circuit switching; d. Allows the establishment of data transmission priority, so that messages with high priority are converted first. ③The two sides of communication do not possess a communication line fixedly, but partially possess this physical path at different times one section at a time, thus greatly improving the utilization rate of the communication line. Disadvantages: ① As the data enter the exchange node, it has to go through the process of storage and forwarding, which causes forwarding delay (including receiving messages, checking the correctness, queuing, sending time, etc.), and the larger the communication volume of the network, the greater the delay caused by the network, so the real-time nature of the message switching is poor, and it is not suitable for transmitting the data of the real-time or interactive services. ②The message exchange is only applicable to digital signals. ③ Since there is no limit to the length of the message, and each intermediate node has to receive the entire incoming message in its entirety, and when the output line is not idle, it may also have to store a few complete messages waiting to be forwarded, requiring a larger buffer for each node in the network. In order to reduce costs, reduce the capacity of the buffer memory of the node, sometimes to wait for the forwarding of the message exists on disk, further increasing the transmission delay. (3) Packet switching: Packet switching still adopts the store-and-forward transmission method, but it divides a long message into several shorter packets first, and then sends these packets (carrying the source and destination addresses and numbering information) one by one, so that packet switching, in addition to the advantages of message, has the following advantages and disadvantages in comparison with message switching: Advantages: (1) Accelerates the transmission of data in the network. Because packets are transmitted one by one, it is possible to make the store operation of the latter packet parallel to the forwarding operation of the former packet, and this pipelined transmission method reduces the transmission time of messages. In addition, the buffer required for transmitting a packet is much smaller than the buffer required for transmitting a message, so that the chance of waiting for transmission due to insufficient buffer and the waiting time is bound to be much less. ② Simplified storage management. Because the length of the packet is fixed, the size of the corresponding buffer is also fixed, the management of the memory in the switching node is usually simplified to the management of the buffer, which is relatively easy. (iii) Reduces the error rate and the amount of retransmitted data. Because the packet is shorter, the error rate is inevitably reduced, and the amount of data retransmitted each time is greatly reduced, which not only improves the reliability, but also reduces the transmission delay. Because of the short packet, it is more suitable for the priority strategy, which is convenient for the timely transmission of some urgent data, so for the bursty data communication between computers, packet switching is obviously more suitable. Disadvantages: ① Although packet switching has less transmission delay than message switching, there still exists store-and-forward delay, and its node switch must have stronger processing capability. ② Packet switching, like message switching, adds information such as source and destination address and packet number to each packet, which increases the amount of transmitted information by about 5% to 10%, reduces the communication efficiency to a certain extent, increases the processing time, makes the control complicated and increases the delay. ③ When packet switching adopts datagram service, out-of-order, lost or duplicate packets may occur, and when the packets arrive at the destination node, it is necessary to sort the packets according to the number and other work, which increases the trouble. If virtual circuit service is used, there is no out-of-order problem, but there are three processes: call establishment, data transmission and virtual circuit release. In short, if the amount of data to be transmitted is very large, and its transmission time is much larger than the call time, it is more appropriate to use circuit switching; when the end-to-end path has a lot of segments of the link composition, it is more appropriate to use packet switching to transmit data. From the point of view of improving the channel utilization of the entire network, message switching and packet switching is better than circuit switching, of which packet switching is less delay than message switching, especially suitable for bursty data communication between computers.