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What is data transmission?

Source: Shangpin China | Type: website encyclopedia | Time: 2015-06-15
Data transmission refers to the process of transferring data between the data source (the starting point of transmission) and the data destination (the end point of transmission). Data transmission is a form of information transmission, mainly referring to computer Beijing website construction Relevant data transmission. According to the location of data source and data destination, data transmission can be divided into two categories: internal transmission and external transmission.
 
1. Internal transmission

The data source and data reside in the same computer, that is, the data exchange between the external equipment of the computer and the CPU is called intra computer transmission. In a computer, the devices often use bus to transmit data, and they are connected by interfaces. In a broad sense, interfaces refer to the connected parts between two relatively independent subsystems. Due to the relative independence between the host and various input/output (1/0) devices, they cannot be directly connected and must go through a conversion mechanism called I/O interface for short. In addition to realizing physical connection between equipment, 1/0 interface shall also have functions such as address decoding, information exchange, data buffer, data type conversion, etc.

The commonly used data transmission modes between the host and 1/0 device include program control mode, interrupt mode, DMA mode and offline transmission mode.

Program control mode: use the instructions in the program to control the data exchange between the external equipment and the processing unit.

Interrupt transmission: the external device that needs to exchange data with the processing unit sends an interrupt request to the processing unit, the processing unit responds to the interrupt request, suspends the running program, completes the data exchange by using the interrupt service subroutine, and returns to the suspended program after the exchange.

DMA (direct memory transfer): When a high-speed external device (such as a disk) requests to exchange data, the external device directly exchanges data with the memory.

Offline transmission: data exchange between devices that exchange data is conducted directly without the control of CPU.

2. External transmission

When the data source and data sink are far away, data transmission needs to be completed by means of public communication lines or dedicated lines, which is called external transmission. The data exchange between remote terminals and computers, and between computers, is external transmission. When external transmission is carried out, the transmission signal in the transmission line is a pulse representing data. When the distance is far (more than 1km), the signal transmitted in the line needs to be processed so that its frequency can be incorporated into the frequency band used by the line, which is called modulation. A modulator needs to be connected at the sending end of the line, and a demodulator needs to be connected at the receiving end of the line to restore the transmitted signal to the data pulse that can be recognized by the data receiving unit. The equipment which is also used as modulator and demodulator is called Modem. The data transmission line is divided into four types: low speed line, medium speed line, broadcast transmission and digital transmission.
  • Low speed line: improved from the original telegraph transmission line, including public transfer line and special line. The transmission rate is not greater than 200bps (200 binary bits per second).
  • Medium speed line: improved by telephone transmission line. The transmission rate of public transfer line of audio channel is 600-1200bps, and the transmission rate of special line is 2400 ^ - 9600bps
  • Broadcast transmission: use FM broadcast system to add equipment to broadcast data and broadcast programs together, which can be received by multiple receiving stations. Satellite relay can also be used. The maximum data transmission rate supported by the satellite transponder is 60 ^ - 120bps. Digital transmission: the transmission rate of audio channel can reach 56 000bps, and the transmission rate of coaxial cable is 1 5 ^ - 12Mbps, optical fiber communication transmission rate can reach 100 --- 1000Mbps.
In the process of data transmission, interference and equipment failure often cause errors in the transmitted data. In order to detect communication errors, redundant error detection codes are set in the transmission data. These redundant error detection codes are combined with the transmission data according to a certain mathematical relationship. Data can be detected when it does not meet the predetermined mathematical relationship due to errors in the transmission process. The wrong data transmission can be corrected by resending.
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