电信（telecommunication）：指利用电子技术在不同的地点之间传递信息。电信包括不同种类的远距离通讯方式，例如：无线电，电报，电视，电话，数据通讯以及计算机网络通讯等。构成 组成通信系统的基本要素包括发信机，通道以及收信机。发信机负责将信息进行编码或转换成适合传输的信号。信号通过信道传输至收信机。在传输过程中, 由于噪声的存在，信号不可避免的会受到改变。收信机端试图应用适当的解码手段从劣化的信号中恢复信息的原样。描述信道的的一个重要指标是带宽。
Telecommunication refers to communication over long distances. In practice, something of the message may be lost in the process. Telecommunication covers all forms of distance and/or conversion of the original communications, including radio, telegraphy, television, telephony, data communication and computer networking.
The elements of a telecommunication system are a transmitter, a medium (line) and possibly a channel imposed upon the medium (see baseband and broadband as well as multiplexing), and a receiver. The transmitter is a device that transforms or encodes the message into a physical phenomenon; the signal. The transmission medium, by its physical nature, is likely to modify or degrade the signal on its path from the transmitter to the receiver. The receiver has a decoding mechanism capable of recovering the message within certain limits of signal degradation. Sometimes, the final receiver is the human eye and/or ear (or in some extreme cases other sensory organs) and the recovery of the message is done by the brain (see psychoacoustics.)
Telecommunication can be point-to-point, point-to-multipoint or broadcasting, which is a particular form of point-to-multipoint that goes only from the transmitter to the receivers.
One of the roles of the telecommunications engineer is to analyse the physical properties of the line or transmission medium,and the statistical properties of the message in order to design the most effective encoding and decoding mechanisms.
When systems are designed to communicate through human sensory organs (mainly those for vision and hearing), physiolo gical and psychological characteristics of human perception must be taken into account. This has important economic implicati ons and engineers must research what defects can be tolerated in the signal and not significantly degrade the viewing or hearing experience.
Examples of human (tele)communications In a simplistic example, consider a normal conversation between two people. The message is the sentence that the speaker decides to communicate to the listener. The transmitter is the language areas in the brain, the motor cortex, the vocal cords, the larynx, and the mouth that produce those sounds called speech. The signal is the sound waves (pressure fluctuations in air particles) that can be identified as speech. The channel is the air carrying those sound waves, and all the acoustic prop-erties of the surrounding space: echoes, ambient noise, reverberation. Between the speaker and the listener, there might be other devices that do or do not introduce their own distortions of the original vocal signal (for example a telephone, a HAM r-adio, an IP phone, etc.) The receiver is the listener's ear and auditory system, the auditory nerve, and the language areas in the listener's brain that will decode the signal into information and filter out background noise.
All channels have. Another important aspect of the channel is called the bandwidth. A low bandwidth channel, such as a t-elephone, cannot carry all of the audio information that is transmitted in normal conversation, causing distortion and irregula-rities in the speaker's voice, as compared to normal, in-person speech.