The process of implementing message exchange is as follows.
(1) If a user needs to send a message, the information to be sent needs to be added to the packet header, including the destination address and the source address, and the formed packet is sent to the switch. When the communication controller in the switch detects that a user line has a message input, it sends an interrupt request to the central processing unit, and sends the message to the memory word by word.
(2) After receiving the message, the central processing unit can process the message, such as analyzing the message header, discriminating and determining the route, etc., and then transferring the message to the external large-capacity memory, waiting for an idle output line.
(3) Once the line is idle, the message is transferred from the external memory to the internal memory and sent to the line via the communication controller.
"Store-and-forward": The message exchange mode first stores the entire message by the switch, and then performs the necessary processing when there is a line idle.
1) Do not monopolize the line, data of multiple users can share one line by storing and queuing.
2) The process of establishing a wireless path improves the utilization of the line.
3) Support multi-point transmission (one message is transmitted to multiple users, only need to add "address field" in the message, and the intermediate node copies and forwards according to the address field).
4) The intermediate node can convert the data format to facilitate the collection of the receiving site.
5) Added error detection function to avoid unnecessary transmission of erroneous data.
Message exchange: Message exchange is a unit in which packets are exchanged for data. The message carries information such as the destination address and source address, and uses the store-and-forward transmission method at the switching node. Therefore, it has the following advantages and disadvantages:
Message exchange advantages:1 Message exchange does not require a dedicated communication line to be established in advance for the communication parties. There is no connection establishment delay, and the user can send the message at any time.
2 Due to the use of store-and-forward transmission, it has the following advantages:
a. It is convenient to set the code check and data retransmission facilities in the message exchange, and the switch node also has the path selection, so that when one transmission path fails, the other path is re-selected to transmit data, thereby improving the transmission. reliability;
b. It is easy to implement transcoding and rate matching in store-and-forward, and even the transmitting and receiving parties can be in a different state at different times. This facilitates communication between computers of different types, specifications, and speeds;
c. Provide multi-target service, that is, a message can be sent to multiple destination addresses at the same time, which is difficult to implement in circuit switching;
d. Allow the priority of data transmission to be established, and the packets with higher priority are preferentially converted.
3 The two communicating parties do not occupy a communication line at a fixed time, but occupy the physical path in a part of the time at different times, thereby greatly improving the utilization rate of the communication line.
Message exchange disadvantages:1Because the data enters the switching node, it has to go through the process of storage and forwarding, which causes forwarding delay (including receiving packets, checking the correctness, queuing, sending time, etc.), and the larger the traffic of the network, the time The larger the delay, the poorer the real-time exchange of messages, which is not suitable for transmitting data in real-time or interactive services.
2 Message exchange is only applicable to digital signals.
3 Because there is no limit on the length of the message, each intermediate node must completely receive the entire received message. When the output line is not idle, it may also need to store several complete messages for forwarding, requiring each in the network. The node has a large buffer. In order to reduce the cost and reduce the capacity of the buffer memory of the node, it is sometimes necessary to store the message waiting to be forwarded on the disk, further increasing the transmission delay.
Packet switching still uses the store-and-forward transmission method, but divides a long message into several shorter packets, and then sends these packets (carrying source, destination address, and number information) one by one, so the packet switching has a report. In addition to the advantages of the text, it has the following advantages and disadvantages compared with message exchange:
Packet switching advantages:1 accelerates the transmission of data in the network. Since the packets are transmitted one by one, the storage operation of the latter packet can be paralleled with the forwarding operation of the previous packet. This pipelined transmission method reduces the transmission time of the packet. In addition, the buffer required to transmit a packet is much smaller than the buffer required to transmit a message, so the probability of waiting for transmission due to insufficient buffers and the waiting time are necessarily 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.
3 Reduce the probability of error and the amount of data retransmitted. Because the packet is shorter, the probability of error is inevitably reduced, and the amount of data retransmitted each time is greatly reduced, which not only improves reliability but also reduces transmission delay.
4 Because the packet is short, it is more suitable for adopting the priority policy, which is convenient for transmitting some urgent data in time. Therefore, for burst data communication between computers, packet switching is obviously more suitable.
Packet switching disadvantages:1 Although packet switching has less transmission delay than packet switching, there is still a store-and-forward delay, and its node switch must have more processing power.
2 Packet switching is the same as message exchange. Each packet is added with information such as source, destination address and packet number, which increases the amount of information transmitted by about 5% to 10%, which reduces communication efficiency to a certain extent. The processing time makes the control complicated and the delay increases.
3 When packet switching uses datagram service, out-of-sequence, loss or repeated packets may occur. When the packet arrives at the destination node, it is necessary to sort the packets by number, which increases the trouble. If the virtual circuit service is used, there is no problem of out-of-order, but there are three processes of call setup, data transmission and virtual circuit release.
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