First, the structure of data communication, exchange methods and scope of application
1. The principle of data communication
DTE is a data terminal. Data terminals include packet type terminals (PT) and non-packet type terminals (NPT). The grouped terminal has a computer, a digital fax machine, a smart telex terminal (TeLetex), a user subassembly device (PAD), a user packet switch, a dedicated telephone exchange (PABX), a viewable access device (VAP), a local area network. Various dedicated terminal devices such as (LAN); non-grouping terminals include various dedicated terminals such as personal computer terminals, viewable text terminals, and telex terminals.
The data circuit is composed of a transmission channel and a data circuit terminal device (DCE). If the transmission channel is an analog channel, the DCE is usually a modem (MODEM), and 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 pattern and level conversion, as well as line connection control. In addition to the distinction between analog and digital, the transmission channel is divided into a wired channel and a wireless channel, a dedicated line and a switched network line. The switching network line needs to establish a connection through the call process, and then removes after the communication ends; the dedicated line connection does not require the above call setup and disconnection process because it is a fixed connection. A 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 data input by the data terminal device.
2. Data communication exchange method
There are usually three ways to exchange data:
(1) Circuit switching
Circuit switching means that two computers or terminals use the same physical link when communicating with each other. The link is used for information transmission from beginning to end, and other computers or terminals are not allowed to simultaneously share the circuit.
(2) Message exchange
The message exchange is to store the user's message in the memory of the switch (memory or external memory). When the required output circuit is idle, the message is sent to the switch or terminal to be received. This way of storing_forwarding can improve the utilization of trunks and circuits.
(3) Packet switching
Packet switching is to divide the entire message sent by the user into a fixed-length data block (called packet or packet), and transmit the packets in the network in a storage-forwarding manner. The first group information is linked with the identifier of the receiving address and the sending address. In the packet switched network, packet data of different users are transmitted by dynamic multiplexing technology, that is, the network has routing, and the same route can be transmitted by packets with different users, so the line utilization is high.
3. 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 (CSPDNs). The first two circuit switching methods are traditional; the latter method is basically similar to the public telephone network, but it is connected to the user in a four-wire or two-wire manner, and is suitable for data exchange at a higher rate. Because it is a dedicated public data network, its connection rate, working rate, subscriber line distance, and line balance conditions are superior to the public telephone network. The advantages are strong real-time performance, low delay, and low switching cost; the disadvantage is that the line utilization rate is low. Circuit switching is suitable for long message communication after one connection.
(2) The message exchange mode is applicable to data communication for storing and forwarding between terminals of different rates, different protocols, different code terminals or one-to-many points. Because of this method, the network transmission delay is large, and it occupies a large amount of memory and external storage space, and thus is not suitable for data communication requiring high system security and small network delay.
(3) Packet switching is developed on the basis of the storage_transfer mode, but it has the advantages of circuit switching and message exchange. It is suitable for conversational computer communication, such as database retrieval, graphic information access, email delivery and inter-computer communication. It has high transmission quality, low cost and can communicate between terminals at different rates. The disadvantage is that it is not suitable for business use with high real-time requirements and large amount of information.
Second, the classification of data communications
Wired data communication
(1) Digital Data Network (DDN)
The digital data network consists of a subscriber loop, a DDN node, a digital channel, and a network control management center. The network structure is shown in block 2. DDN is a digital data transmission network composed of optical channels such as optical fibers or digital microwaves, satellites, 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 user-to-network connection line. That is, the transmission of the subscriber loop should also be digital, but in fact there are ordinary cables and twisted pairs, 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 small.
2 channel utilization is high.
3 requires the clock system of the whole network to keep in sync to ensure the transmission quality of the DDN circuit.
The rate of 4DDN leased line services can be divided into 2.4-19.2kbit/s, N&TImes; 64kbit/s (N=1-32); the user access rate is up to 2Mbit/s.
The 5DDN delay is small.
(2) Packet switched network
The Packet Switched Network (PSPDN) is based on the CCITT X.25 recommendation, so it is also known as the X..25 network. It adopts the store-and-forward method. The message sent by the user is divided into data segments with a certain length, and control information is added to each data segment to form a group of groupings with addresses, which are transmitted on the network. The most prominent advantage of the packet switching network is that multiple virtual paths can be opened simultaneously on one circuit for simultaneous use by multiple users. The network has dynamic routing function and advanced error detection function, but the network performance is poor.
(3) Frame Relay Network
The frame relay network is usually composed of a frame relay access device, a frame relay switching device, and a common frame relay service network 3, as shown in block 3. The Frame Relay network was developed from packet switching technology. Frame relay technology encapsulates user data groups of different lengths into larger frame relay frames, and transmits them on the network after addressing and control information. Its features are:
1Using statistical multiplexing technology, bandwidth is allocated on demand, and users are provided with network resources of common prosperity. Each line and network port can be shared by multiple endpoints according to information flow, which greatly improves the utilization of network resources.
2 Using virtual circuit technology, only when the user prepares the data, the required bandwidth is allocated to the specified 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.
The 3-frame relay uses only a part of the physical layer and the link layer to perform its switching function, and implements the information transmission in units of frames by using the D channel connection in which the user information and the control information are separated, which simplifies the processing of the intermediate node. Frame Relay adopts the reliable link layer (LAPD) protocol of ISDN D channel, which leaves the functions of flow control and error correction to the intelligent terminal, which greatly simplifies the processing and improves the efficiency. Of course, the quality of the frame relay transmission line is very high, and the error rate should be less than 10 minus 8 power.
The 4-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 provided is 2048 Mbit/s. User rates are typically 9.6, 4.4, 19.2, N&TImes; 64 kbist/s (N = 1-31), and 2 Mbit/s.
5) Frame Relay does not use the store_forward function, and thus has some of the same advantages as fast packet switching. The delay is less than 15ms.
2. Wireless data communication
Wireless data communication, also known as mobile data communication, is based on wired data communication. Wired data communication relies on wired transmission and is therefore only suitable for communication between a fixed terminal and a computer or computer. Mobile data communication transmits data by the propagation of radio waves, and thus 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 extends the use of wired data networks to mobile and portable users by interconnecting with wired data networks.
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