This paper introduces the composition principle, exchange mode and application scope of data communication. The classification of data communication, and look forward to the bright application prospect in the future.

Application scope and application prospect of subject word data communication synthesis principle

Data communication is a new communication method combining communication technology and computer technology. To transmit information between the two places, there must be a transmission channel. There are wired data communication and wireless data communication according to different transmission media. But they all connect data terminals with computers through transmission channels, so that data terminals in different places can share software, hardware and information resources.

I. Composition principle, exchange mode and application scope of data communication

Composition principle of 1. data communication

The principle of data communication is shown in the block diagram 1. DTE is a data terminal in the figure. There are two types of data terminals: packet terminal (PT) and non-packet terminal (NPT). Packet terminals include computers, digital fax machines, intelligent telex terminals (Teletex), user packet assembly and disassembly equipment (PAD), user packet switches, private telephone exchanges (PABX), videotex access equipment (VAP), local area networks (LAN) and other special terminal equipment. Non-packet terminals include personal computer terminals, video terminals, telex terminals and other special terminals. The data circuit consists of transmission channel and data circuit terminal equipment (DCE). If the transmission channel is an analog channel, DCE is usually a modem whose function is to convert analog signals and digital signals. If the transmission channel is a digital channel, the function of DCE is to realize the conversion between signal type and level, as well as line connection control. In addition to the difference between analog and digital, there are wired channels and wireless channels, private lines and switching networks. Switched network lines should be connected through the call process, and then removed after the communication is over; Because the private line connection is a fixed connection, there is no need for the above-mentioned call setup and disconnection process. The communication controller in the computer system is used to manage all communication lines connected with the data terminal. The CPU is used to process the data input by the data terminal equipment.

2. Exchange mode of data communication

There are usually three ways to exchange data communication:

(1) circuit switching

Circuit switching means that two computers or terminals use the same physical link when communicating with each other. This link is used for information transmission from beginning to end in communication, and other computers or terminals are not allowed to have circuits at the same time.

(2) Message exchange

Message exchange stores the user's message in the memory (internal memory or external memory) of the switch, and then sends the message to the switch or terminal for receiving when the required output circuit is idle. This store-and-forward formula can improve the utilization ratio of trunk lines and circuits.

(3) packet switching

Packet switching is to divide the whole message sent by users into several fixed-length data blocks (called packets or packets) and transmit these packets in the network in a store-and-forward manner. The first packet information is associated with the identification of the receiving address and the sending address. In the packet-switched network, the packet data transmission of different users adopts dynamic multiplexing technology, that is, the network has routes, and the same route can be transmitted by different users, so the line utilization rate is high.

3. Scope of application of various exchange methods

(1) circuit-switched mode is usually used in communication networks such as public telephone network, public telegraph network and circuit-switched public data network (CSPDN). The first two circuit switching modes are traditional; The latter method is basically similar to the public telephone network, but it connects users with four or two lines and is suitable for high-speed data exchange. Because it is a dedicated public data network, its connection rate, working rate, subscriber line distance and line balance conditions are better than those of the public telephone network. Its advantages are strong real-time, small delay and low switching cost. Its disadvantage is low line utilization. Circuit switching is suitable for long message communication after one connection.

(2) The message exchange mode is suitable for data communication with different rates, different protocols and different codes, and can be stored, forwarded from terminal to terminal or from one point to multiple points. This method is not suitable for data communication with high system security requirements and small network delay, because it has a long network transmission delay and occupies a lot of memory and external storage space.

(3) Packet switching is developed on the basis of store-and-forward mode, but it has the advantages of circuit switching and message switching. It is suitable for conversational computer communication, such as database retrieval, graphic information access, e-mail delivery and inter-computer communication. It has high transmission quality and low cost, and can communicate between terminals with different speeds. Its disadvantage is that it is not suitable for services with high real-time requirements and large amount of information.

Second, the classification of data communication

1. Wired data communication

(1) digital data network

Digital data network consists of subscriber loop, DDN node, digital channel and network control management center, and its network structure is shown in block diagram 2. DDN is a digital data transmission network composed of optical fiber or digital microwave, satellite and other digital channels and digital cross-multiplexing equipment. It can also be said that DDN is a digital communication network integrating data communication technology, digital communication technology, optical communication technology and digital cross-connection technology. The digital channel should include the connecting line between the user and the network, that is, the transmission of the user loop should also be digital, but there are actually ordinary cables and twisted pair cables, but the transmission quality is not as good as before. The main features of DDN are:

① High transmission quality and low bit error rate: the bit error rate of the transmission channel is low.

② The channel utilization rate is high.

③ The whole network clock system should be synchronized to ensure the transmission quality of DDN circuit.

(The dedicated line service rate of DDN can be divided into 2.4- 19.2kbit/s, n× 64kbit/s (n =1-32); The maximum user access rate shall not exceed 2mbit/s.

⑤ DDN delay is small.

(2) packet switching network

Packet switched network (PSPDN) is based on CCITT X.25 proposal, so it is also called X.25 network. It divides the message sent by users into data segments with a certain length by means of store and forward, and adds control information to each data segment to form a packet combination group with address, which is transmitted on the Internet. The most prominent advantage of packet-switched network is that it can simultaneously open multiple virtual paths on one circuit for multiple users to use at the same time. The network has dynamic routing function and advanced error detection function, but the network performance is poor.

(3) Frame Relay Network

Frame relay network usually consists of frame relay access equipment, frame relay switching equipment and public frame relay service network, as shown in block diagram 3. Frame Relay network is developed from packet switching technology. Frame relay technology is to encapsulate user data groups with different lengths in a larger frame relay frame, and then transmit them on the network after adding addressing and control information. Its functional features are as follows:

(1) Using statistical multiplexing technology, bandwidth is allocated as needed to provide users with * * * constant network resources, and each line and network port can be * * * constant according to the information flow from multiple terminals, which greatly improves the utilization rate of network resources.

(2) Using virtual circuit technology, the required bandwidth is allocated to the designated virtual circuit only when the user prepares data, and the bandwidth is dynamically allocated according to the grouping in the network, which is suitable for the use of unexpected services.

③ Frame Relay only uses a part of the physical layer and the link layer to perform its switching function, and uses D-channel connection where user information and control information are separated to transmit information in units of frames, which simplifies the processing of intermediate nodes. Frame Relay adopts the reliable link layer (LAPD) protocol of ISDN D channel, leaving the functions of flow control and error correction to intelligent terminals, which greatly simplifies the processing process and improves efficiency. Of course, the quality of frame relay transmission line is very high, and its bit error rate should be less than the negative 8 power of 10.

④ The frame length of frame relay is usually longer than that of packet switching, reaching 1024-4096 bytes/frame, so its throughput is very high, and the rate it provides is 2048 mbit/s. The user rates are generally 9.6, 4.4, 19.2 and n× 64 kbist/s (n =/. ..

⑤) Frame Relay does not use store-and-forward function, so it has some advantages similar to fast packet switching. The time delay is less than15ms.

2. Wireless data communication

Wireless data communication, also called mobile data communication, is developed on the basis of wired data communication. Wired data communication depends on wired transmission, so it is only suitable for wired communication between fixed terminals and computers. However, mobile data communication transmits data through the propagation of radio waves, so mobile communication can be realized in a moving state. Mobile data communication in a narrow sense is wireless communication between computers or between computers and people. It extends the application of wired data network to mobile and portable users through interconnection with wired data network.