1. Composition principle, exchange method and applicable scope of data communication
1. The principle of data communication
DTE is a data terminal. There are two types of data terminals: packet terminals (PT) and non-packet terminals (NPT). Packet terminals include computers, digital facsimile machines, intelligent user telegraph terminals (TeLetex), user sub-assembly and disassembly equipment (PAD), user packet switches, dedicated telephone exchanges (PABX), videotext access equipment (VAP), local area networks (LAN) and other special terminal equipments; non-packetized terminals include personal computer terminals, video teletext terminals, user telegraph terminals and other special terminals.
The data circuit is composed of a transmission channel and a data circuit terminal equipment (DCE). If the transmission channel is an analog channel, the DCE is usually a modem (MODEM). Its function is to convert analog signals and digital signals; if the transmission channel is a digital channel, The role of DCE is to achieve signal code and level conversion, and line connection control. In addition to the distinction between analog and digital transmission channels, there are also wired channels and wireless channels, dedicated lines and switching network lines. The switching network line needs to establish a connection through the call process, and then remove it after the communication is completed; the dedicated line connection does not need the above-mentioned call establishment and disconnection process because it is a fixed connection. The communication controller in the computer system is used to manage all communication lines connected to the data terminal. The central processor is used to process the data input by the data terminal equipment.
2. Exchange method of data communication
There are usually three exchange methods for data communication:
(1) Circuit switching
Circuit switching means that two computers or terminals use the same actual physical link when communicating with each other. This link is used for information transmission from beginning to end, and no other computers or terminals are allowed to share the circuit at the same time.
(2) Message exchange
Message exchange is to store the user's message in the switch's memory (memory or external storage). When the required output circuit is idle, the message is then sent to the switch or terminal to be received. This store-and-forward approach can improve the utilization of trunks and circuits.
(3) Packet switching
Packet switching is to divide the entire message sent by the user into several fixed-length data blocks (called packet or packet), and transmit these packets in a network in a store-forward manner. The first packet information is connected with the identification of the receiving address and the sending address. In a packet-switched network, the packet data of different users are transmitted using dynamic multiplexing technology, that is, the network has routing options, and the same route can have packets of different users being transmitted, so the line utilization rate is high.
3. The scope of application of various exchange methods
(1) Circuit-switching methods are commonly used in communication networks such as public telephone networks, public telegraph networks, and circuit-switched public data networks (CSPDN). The first two circuit switching methods are traditional methods; the latter method is basically similar to the public telephone network, but it uses four-wire or two-wire methods to connect users and is suitable for higher-speed data exchange. Because it is a dedicated public data network, its connection rate, working rate, subscriber line distance, line balance conditions, etc. are better than the public telephone network. Its advantages are strong real-time, low delay and low switching cost; its disadvantage is low line utilization. Circuit switching is suitable for the communication of long messages after one connection.
(2) The message exchange method is suitable for data communication between terminals with different rates, different protocols, and different code terminals, or one-to-multipoint same-text units. In this way, the network transmission delay is large, and a large amount of memory and external storage space is occupied, so it is not suitable for data communication that requires high system security and a small network delay.
(3) Packet switching is developed on the basis of the store_forward method, 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 transmission, and inter-computer communication. It has high transmission quality, low cost, and can communicate between terminals of different rates. The disadvantage is that it is not suitable for business applications that require high real-time performance and a large amount of information.
2. Classification of data communication
1. Wired data communication
(1) Digital data network (DDN)
The digital data network is composed of user loops, DDN nodes, digital channels, and network control management centers. Its network structure is shown in block diagram 2. DDN is a digital data transmission network composed of optical channels 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 that combines data communication technology, digital communication technology, optical migration communication technology, and digital cross-connect technology. The digital channel should include the connection line from the user to 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. The main features of DDN are:
â‘ High transmission quality and low bit error rate: The bit error rate of the transmission channel is small.
â‘¡High channel utilization.
â‘¢The clock system of the whole network must be kept in synchronization to ensure the transmission quality of the DDN circuit.
â‘£The rate of DDN's leased leased line service can be divided into 2.4-19.2kbit / s, N & TImes; 64kbit / s (N = 1-32); the maximum network access rate of users does not exceed 2Mbit / s.
⑤ DDN delay is small.
(2) Packet switching network
The packet switched network (PSPDN) is based on the CCITT X.25 recommendation, so it is also called X..25 network. It uses the store-and-forward method to divide the message sent by the user into data segments with a certain length, and add control information to each data segment to form a grouped group with addresses for transmission on the Internet. The most prominent advantage of the packet-switched network is that it can open multiple virtual channels on a circuit at the same time and be used by multiple users at the same time. The network has dynamic routing functions and advanced error detection functions, but the network performance is poor.
(3) Frame Relay Network
Frame relay network is usually composed of 3 parts 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 of different lengths in larger frame relay frames, and add addressing and control information to transmit on the Internet. Its functional characteristics are:
â‘ Use statistical multiplexing technology to allocate bandwidth as needed to provide users with shared network resources. Each line and network port can be shared by multiple endpoints according to information flow, which greatly improves the utilization rate of network resources.
â‘¡Using virtual circuit technology, only when the user prepares the data, the required bandwidth is allocated to the designated virtual circuit, and the bandwidth is dynamically allocated according to the packet in the network, so it is suitable for the use of bursty services.
â‘¢ Frame relay only uses a part of the physical layer and the link layer to perform its switching function, and uses the D channel connection where user information and control information are separated to implement information transmission in frame units, simplifying the processing of intermediate nodes. Frame Relay uses the reliable ISDN D channel link layer (LAPD) protocol, leaving functions such as flow control and error correction to intelligent terminals to complete, thus greatly simplifying the processing process and improving efficiency. Of course, the frame relay transmission line has high quality requirements, and its bit error rate should be less than 10 to the negative 8th power.
â‘£ Frame relay usually has a longer frame length than packet switching, reaching 1024-4096 bytes / frame, so its throughput is very high, and the rate it provides is 2048Mbit / s. The user rate is generally 9.6, 4.4, 19.2,? N & TImes; 64kbist / s (N = 1-31), and 2Mbit / s.
⑤) Frame Relay does not use the store_forward function, so it has the same advantages as fast packet switching. Its delay is less than 15ms.
2. Wireless data communication
Wireless data communication is also called mobile data communication, which is developed on the basis of wired data communication. Wired data communication relies on wired transmission, so it is only suitable for communication between fixed terminals and computers or computers. And mobile data communication transmits data through the propagation of radio waves, so it is possible to realize mobile communication in a mobile state. In a narrow sense, mobile data communication is wireless communication between computers or between computers and people. It interconnects with the wired data network to extend the application of the wired data network to mobile and portable users.
Nanning Nuoxin Technology Co., Ltd. , https://www.nx-vapes.com