Data Communication Basics

I. composition principle, exchange method and applicability of data communication
　　
1. Principles of data communication
　　
DTE is a data terminal.
Data terminals are divided into two categories: group-type terminals (PT) and non-group-type terminals (treaty. Group terminals include computers, digital fax machines, smart user telegraph terminals (teletex), user group device (PAD), user group switch, and dedicated telephone switch (PABX) various dedicated terminal devices, such as Visual Graphic access device (VAP) and LAN.

Group terminals include personal computer terminals, visual text terminals, user telegraph terminals, and other specialized terminals.
　　
A Data circuit consists of a transmission channel and a data circuit terminal device (DCE). If the transmission channel is a analog channel, DCE is usually a modem ), it is used to convert analog and digital signals. If the transmission channel is a digital channel, DCE is used to convert the signal code type and level, and to control the connection of lines. In addition to analog and digital channels, transmission channels can also be divided into wired and wireless channels, dedicated lines, and exchange network lines. The exchange network line needs to establish a connection through the call process, and then remove it after the communication ends. Because the leased line connection is a fixed connection, the above call establishment and disconnection process are not required. Communication Controllers in computer systems are used to manage all communication lines connected to data terminals. The central processor is used to process the data input by the data terminal device.
　　
2. Data communication exchange methods
　　
Generally, there are three exchange methods for data communication:
　　
(1) Circuit Switching
　　
Circuit Switching refers to the use of the same physical link when two computers or terminals communicate with each other, other computers or terminals are not allowed to share the circuit at the same time.
　　
(2) Packet Exchange
　　
Message exchange stores user messages in the memory (memory or external storage) of the switch. When the output circuit is idle, the message is sent to the switch or terminal to be received. This storage _ forwarding method can improve the utilization of the trunk line and circuit.
　　
(3) group exchange
　　
A group switch divides an entire packet sent by a user into a fixed-length data block (called a group or a package), and transmits these groups in the network as a storage _ forwarding method. The first group information is identified by the Receiving address and Sending address. In the group exchange network, the grouped data of different users are transmitted dynamically using reusable technology, that is, the network has a route selection, and the same route can have different user groups for transmission, therefore, the line utilization is high.
　　
3. Applicability of various exchange methods
　　
(1) circuit switching is usually used in public telephone networks, public telegraph networks, and public data networks (cspdn) for circuit switching. The first two circuit switching methods are traditional. The latter method is similar to that of the public telephone network, but it is connected to the user by four or two lines. This method is suitable for high-speed data exchange. Because it is a dedicated public data network, its connection rate, working rate, user line distance, line balancing conditions and so on are superior to the public telephone network. The advantage is strong real-time performance, low latency, and low switching costs. The disadvantage is that the line utilization is low. Circuit Switching is suitable for communication of long packets after one connection.
　　
(2) The packet exchange mode is suitable for data communication that stores and forwards data at different rates, protocols, terminals of different code terminals, or one point to multiple points in the same file unit. Because of this method, the network transmission latency is large and it occupies a large amount of memory and external storage space. Therefore, it is not applicable to data communication that requires high system security and low network latency.
　　
(3) group switching is developed based on the storage _ forwarding method, but it has the advantages of circuit switching and packet switching. It is suitable for conversational computer communication, such as database retrieval, graphic information access, email transmission, and inter-computer communication. It has high transmission quality and low costs, it can also communicate with terminals at different rates. Its disadvantage is that it is not suitable for businesses with high real-time requirements and a large amount of information.
　　
Ii. Data Communication Classification
　　
1. Wired Data Communication
　　
(1) Digital Data Network (DDN)
　　
The digital data network consists of user loops, DDN nodes, digital channels, and network control management centers. The network structure is shown in figure 2. DDN is a digital data transmission network consisting of optical fiber, digital microwave, satellite, and other digital channels and digital cross-multiplexing devices. It can also be said that DDN is a digital communication network that combines data communication technology, digital communication technology, light migration communication technology, and digital cross-connection 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 in fact there are also common cables and twisted pair wires, but the transmission quality is not as good as before. DDN has the following features:
　　
① High transmission quality and low bit error rate: the bit error rate of the transmission channel must be small.
　　
② High channel utilization.
　　
③ Ensure that the clock system of the entire network is synchronized to ensure the transmission quality of the DDN circuit.
　　
④ The rate of DDN leased line services can be divided into 2.4-19.2 kbit/s, n × 64kbit/S (n = 1-32 ); the maximum inbound rate is 2 Mbit/s.
　　
⑤ Low latency of DDN.
　　
(2) group exchange network
　　
The Group Exchange Network (pspdn) is based on CCITT X.25 recommendations, so it is also called the X .. 25 network. It uses the storage and forwarding method to divide the packets sent by users into data segments with a certain length, and adds control information to each data segment to form a group with addresses, transmission over the Internet. The most prominent advantage of the group exchange network is that multiple virtual channels can be opened simultaneously on one circuit and used by multiple users at the same time. The network has dynamic routing selection function and advanced error detection function, however, the network performance is poor.
(3) Frame Relay Network
　　
A frame relay network is usually composed of three parts: a frame relay access device, a frame relay switching device, and a public Frame Relay Service Network, as shown in figure 3. Frame Relay Network is developed from the group exchange technology. Frame Relay Technology encapsulates user data groups of different lengths in large frame frames, and adds addressing and control information to them for online transmission. Features:
　　
① The Statistical reuse technology is used to allocate bandwidth on demand and provide users with common network resources. Each line and network port can be distributed by multiple endpoints by information flow, greatly improving the utilization of network resources.
　　
② The virtual circuit technology is used. Only when the user prepares data can the required bandwidth be allocated to the specified virtual circuit, and the bandwidth is dynamically allocated in the network according to the group, therefore, it is suitable for the use of burstable services.
　　
③ Frame Relay only uses part of the physical layer and link layer to implement its exchange function. It uses the D-Channel Connection of user information and control information separation to implement information transmission in frame units, it simplifies the processing of intermediate nodes. Frame Relay uses reliable isdn d-Channel link layer (LAPD) protocol, which leaves traffic control, error correction, and other functions to smart terminals. This greatly simplifies the processing process and improves efficiency. Of course, the quality of Frame Relay Transmission Lines is very high, and the error rate should be less than 10 to the power of 8.
　　
④ Frame length is usually the length of the frame group, which is 10-2048 bytes/frame. Therefore, the throughput is very high and the provided speed is Mbit/s. The user speed is generally 9.6, 4.4, 19.2, n × 64kbist/S (n = 1-31), and 2 Mbit/s.
　　
(⑤) Frame Relay does not use the _ forward function, so it has the same advantages as fast group exchange. The latency is less than 15 ms.
　　
2. Wireless Data Communication
　　
Wireless Data Communication, also known as mobile data communication, is developed on the basis of wired data communication. Wired data communication relies on wired transmission, so it is only suitable for communication between a fixed terminal and a computer or a computer. Mobile Data Communication transmits data through the spread of radio waves, which may enable mobile communication in the mobile status. In a narrow sense, mobile data communication is a wireless communication between computers or between computers. It connects to the wired data network to extend the application of the wired data network to mobile and portable users.