Composition of communication system

Communication system is the general term for the technical system used to complete the information transmission process. Modern communication systems are mainly realized by the propagation of electromagnetic waves in free space or the transmission mechanism in guided media. The former is called a wireless communication system, and the latter is called a wired communication system.

Basic system

Generally consists of a source (sending device), a sink (receiving device) and a channel (transmission medium) and other components, known as the three elements of communication.

Method

The message (language, text, image or data) from the source is first transmitted by the terminal equipment (such as telephone, teletypewriter, Fax machine or data terminal equipment, etc.) is converted into an electrical signal, and then after being encoded, modulated, amplified or transmitted by the sending end equipment, the baseband signal is converted into a form suitable for transmission in the transmission medium; after being transmitted through the transmission medium, it is processed at the receiving end The receiving device performs inverse transformation and restores the message to the recipient. This kind of point-to-point communication is mostly two-way transmission. Therefore, there are originating and receiving devices at both ends of the communication object.

Classification

Communication systems can be divided into two categories according to the different transmission media used: ① Using metal conductors as transmission media, such as commonly used communication cables, etc. A communication system that uses cables as the transmission medium is called a wired telecommunications system; ② The communication system uses radio waves to propagate in the atmosphere, space, water, rock, soil and other transmission media to communicate. This communication system is called a radio communication system. Optical communication systems are also divided into "wired" and "wireless", and the transmission media they use are optical fibers and atmosphere, space or water respectively.

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Communication systems can be divided into telephone, telegraph, fax, data communication systems, etc. according to different communication services (that is, the type of information transmitted). Signals that change continuously in time are called analog signals (such as telephones); signals that are discrete in time and whose amplitude values ​​are also discrete are called digital signals (such as telegraphs). Analog signals can also be turned into digital signals through analog-to-digital conversion (including sampling, quantization and encoding processes). When the baseband signal transmitted in a communication system is an analog signal, the system is called an analog communication system; when the baseband signal transmitted is a digital signal, the communication system is called a digital communication system.

Communication systems all work in noisy environments. When designing an analog communication system, the minimum mean square error criterion is adopted, that is, the signal-to-noise ratio output by the receiving end is the largest. When designing a digital communication system, the minimum error probability criterion is adopted, that is, based on the selected transmission medium and the statistical characteristics of the noise, the best modulation system is selected, and the best signal and best receiver are designed.

Analog communication

Analog communication refers to a communication method that transmits analog signals from the source to the sink on the channel. Due to the resistance in the conductor, the direct transmission distance of the signal cannot be too far. The solution is to transmit the analog signal through the carrier wave. A carrier wave refers to a waveform that is modulated to transmit a signal, usually a high-frequency oscillating sine wave. In this way, modulating the analog signal on the carrier wave can transmit it much farther than direct transmission. Generally, the frequency of the sine wave is required to be much higher than the bandwidth of the modulation signal, otherwise aliasing will occur and the transmitted signal will be distorted.

Analog communication systems usually consist of signal sources, modulators, channels, demodulators, signal sinks and noise sources.

The advantage of analog communication is that it is intuitive and easy to implement, but it has poor confidentiality and weak anti-interference ability. Since the spectrum of signals transmitted in analog communication channels is relatively narrow, channel utilization can be improved through multiplexing.

Digital communication

Digital communication refers to a communication method that transmits digital signals from the source to the sink on the channel. Compared with analog communication, its advantages are: strong anti-interference ability and no noise accumulation; it can carry out long-distance transmission and ensure quality; it can adapt to various communication business requirements and facilitate comprehensive processing; the transmitted binary digital signal can be directly It is received and processed by the computer; it is easy to implement using large-scale integrated circuits, and communication equipment is conducive to integration; it is easy to perform encryption processing, and security is easier to ensure.

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