Paper Keywords: data communication; Principle; classify
Abstract: With the wide popularization of computer technology and the development of computer telematics application, data communication came into being, which realized the transmission between computers and between computers and terminals. Due to the change of different business requirements and the development of communication technology, data communication has gone through different development processes.
Data communication is a communication system with "data" as its business. Data are predetermined numbers, letters or symbols with certain meanings and their combinations. With the rapid development of computer technology and communication technology, and their mutual penetration and combination, data communication is a new communication mode, which is the product of the combination of computer and communication. With the widespread popularization of computer technology and the development of computer telematics application, data communication came into being, which realized the transmission between computers and between computers and terminals. Due to the change of different business requirements and the development of communication technology, data communication has gone through different development processes.
1 Communication system transmission mode
Cable communication: twisted pair, coaxial cable, etc. Local telephone and long-distance communication. Modulation mode: single sideband /FDM. Digital baseband transmission technology of PCM time division multiplexing based on coaxial cable. Optical fiber will gradually replace coaxial cable.
Microwave relay communication: coaxial, easy to set up, less investment and short cycle. Analog telephone microwave communication mainly adopts single sideband/FM /FDM modulation, and the communication capacity is 6000 channels. Digital microwave adopts BPSK, QPSK and QAM modulation technologies. Multi-level modulation technologies such as 64QAM and 256QAM are adopted to improve microwave communication capability, and 1920~7680 PCM digital telephone can be transmitted in 40M channel.
Optical fiber communication: Optical fiber communication is based on the long-distance transmission of laser in optical fiber, which has the characteristics of large communication capacity, long communication distance and strong anti-interference. At present, it is used for local, long-distance and trunk transmission, and the user optical fiber communication network is gradually developing. At present, based on long-wave laser and single-mode fiber, the number of calls per fiber exceeds 10000, and the communication fiber strength of the fiber itself is very huge. In the past few decades, optical fiber communication technology has developed rapidly, and various equipment applications have emerged, such as access equipment, photoelectric conversion equipment, transmission equipment, switching equipment, network equipment and so on. Optical fiber communication equipment consists of photoelectric conversion unit and digital signal processing unit.
Satellite communication: long communication distance, large transmission capacity, large coverage area, no geographical restrictions and high reliability. At present, mature technologies include analog modulation, frequency division multiplexing and frequency division multiple access. Digital satellite communication adopts digital modulation, time division multiplexing and time division multiple access.
Mobile communication: GSM, CDMA. Key technologies of digital mobile communication: modulation technology, error correction coding and digital speech coding.
2 composition principle of data communication
Data terminals (DTE) can be divided into two categories: packet terminals (PT) and non-packet terminals (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, videotex 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. Transmission channels are divided into analog and digital channels, wired and wireless channels, private lines and switched network lines. 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.
3 Classification of data communication
3. 1 wired data communication
Digital data network. Digital data network consists of subscriber loop, DDN node, digital channel and network control management center. 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 pairs, but the transmission quality is not as good as before.
Packet switching network. Packet switched network (PSPDN) is based on CCITTX.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 group with an 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.
Frame relay network. Frame relay network usually consists of frame relay access equipment, frame relay switching equipment and public frame relay service network. 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.
3.2 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 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 mobile state. Mobile data communication in a narrow sense is wireless communication between computers or between computers and people. Through interconnection with wired data network, it extends the application of wired data network to mobile and portable user 4 networks and their protocols.
4. 1 computer network
A computer network is a collection of two or more computers interconnected by optical cables, twisted-pair telephone lines or wireless channels. Through the network, users can enjoy network resources, such as documents, programs, printers and modems. According to geographical location, computer networks can be divided into Internet, WAN, MAN and LAN. Internet is the largest Internet in the world; Wide area network generally refers to the network connecting various regions of a country. WAN is generally distributed between 100- 1000 km; MAN, also known as MAN, generally covers a city, and Fiona Fang does not exceed10-100km; The geographical distribution of LAN is relatively small, such as a building, or a unit, a school, or even a big room.
Local area network (LAN) is the most widely used computer network at present. A unit can use multiple LANs, such as financial department using LAN to manage financial accounts, labor and personnel department using LAN to manage personnel files, all kinds of talent information and so on.
4.2 network protocol
Network protocol is a language used for network conversation between two computers. There are many kinds of network protocols, including character-oriented protocol, bit-oriented protocol and byte counting protocol, but TCP/IP protocol is the most commonly used one. It is suitable for large networks composed of multiple LANs and small networks that do not need routing. TCP/IP protocol is characterized by its open architecture and easy management.
TCP/IP is actually a collection of standard network protocols and related protocols, including transmission control protocol and InternetProtocol. TCP protocol is used to transfer data between applications, and IP protocol is used to transfer data between programs and hosts. Because TCP/IP is cross-platform, it has become the standard connection protocol of the Internet. The network protocol is divided into the following four layers: the network interface layer is responsible for receiving and sending physical frames; Network layer: responsible for communication between adjacent nodes; Transport layer: responsible for the communication from the starting point to the end point; Application layer: provides applications such as file transfer and e-mail. Data must be transmitted from one computer to another in the form of TCP/IP protocol, and the data must be processed by the above four layers of communication software before being transmitted in the physical network.
The current IP protocol consists of 32-bit binary numbers. For example, 202.0.96. 133 represents the IP address used by computers connected to the Internet, which is unique in the whole Internet.
5 conclusion
In short, with the continuous development of Internet technology, data communication technology will be more and more widely used. Today, with the emergence of new technologies and equipment for data communication, it is particularly important to learn, understand and master data communication technology.