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), physiological and psychological characteristics of human perception must be taken into account. This has important economic implications 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 properties 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 radio, 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 telephone, cannot carry all of the audio information that is transmitted in normal conversation, causing distortion and irregularities in the speaker's voice, as compared to normal, in-person speech.

電信指利用電子技術在不同的地點之間傳遞信息。電信包括不同種類的遠距離通訊方式，例如：無線電，電報，電視，電話，數(shù)據(jù)通訊以及計算機網(wǎng)絡通訊等。

組成通信系統(tǒng)的基本要素包括發(fā)信機，通道以及收信機。發(fā)信機負責將信息進行編碼或轉(zhuǎn)換成適合傳輸?shù)男盘?。信號通過信道傳輸至收信機。在傳輸過程中, 由于噪聲的存在，信號不可避免的會受到改變。收信機端試圖應用適當?shù)慕獯a手段從劣化的信號中恢復信息的原樣。描述信道的的一個重要指標是帶寬。

通信系統(tǒng)的結構可以是點對點，也可以是一點對多點，廣播則是一種特殊的一點對多點的傳播形式。

1948年，當時工作于貝爾實驗室的香農(nóng)發(fā)表了論文“通信的數(shù)學原理”，這一標志性的論文創(chuàng)建了分析通信系統(tǒng)的數(shù)學理論方法，也即信息論。信息論使我們可以根據(jù)信道的帶寬和信噪比特性推算其容量。

在論文發(fā)表的時代，電信系統(tǒng)主要是基于模擬電路系統(tǒng)。此后，隨著數(shù)字集成電路的大量普及，電信系統(tǒng)的設計可以充分利用信息論的理論指導提升性能。由此，數(shù)字信號處理也成為一個新的領域。

實際的通信信道不可避免的存在各種缺陷，其中包括：散彈噪聲、熱噪聲、延遲、非線性傳輸函數(shù)、快速衰落、帶寬限制以及信號反射等等。在一些當前的電信系統(tǒng)設計中，這些缺陷可以被利用來改善信道傳輸?shù)馁|(zhì)量。

現(xiàn)代通信系統(tǒng)普遍對于時間同步提出很高的要求。精確定時技術與通訊技術發(fā)展之間有著緊密的聯(lián)系。大多數(shù)現(xiàn)代廣域通訊系統(tǒng)都以原子鐘作為時間基準。

調(diào)變是指將信息轉(zhuǎn)換成適合遠距離傳播的模擬信號的處理過程。

主要數(shù)字信道編碼方式：漢明碼、格雷碼、二進制碼、Turbo碼。