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电信

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基本含义Top

  [拼音] [dian xin] 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Top

  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。
  通信系统的结构可以是点对点,也可以是一点对多点,广播则是一种特殊的一点对多点的传播形式。
  什么是电信
  电信是信息化社会的重要支柱。无论是在人类的社会、经济活动中,还是在人们日常生活的方方面面,都离不开电信这个高效、可靠的手段。
  “电信”是什么?国际电联的定义是:“使用有线电、无线电、光或其它电磁系统的通信。”按照这个定义,凡是发信者利用任何电磁系统,包括有线电信系统、无线电信系统、光学通信系统以及其它电磁系统,采用任何表示形式,包括符号、文字、声音、图像以及由这些形式组合而成的各种可视、可听或可用的信号,向一个或多个接收者发送信息的过程,都称为电信。它不仅包括电报、电话等传统电信媒体,也包括光纤通信、数据通信、卫星通信等现代电信媒体,不仅包括上述双向传送信息的媒体,也包括广播、电视等单向信息传播媒体。
  人类用电来传送信息的历史是由电报开始的。电报是一种以符号传送信息的方式,即所谓的数字方式,但1876年电话发明之后,在电信领域中,模拟方式便占了绝对优势。20世纪50年代,PCM技术的出现,使数字通信方式又看到了一丝曙光。但它的真正壮大和发展,以至一统天下,主要是依靠电子计算机的力量。因此,有人把“C&C”(即通信与计算机的融合)看成是人类信息史上一场新的革命的重要标志。今天我们在谈论“电信”时,已经少不了要谈到计算机在各个电信领域的广泛应用。它不仅在实现电信的智能化方面发挥了关键作用,而且,它的引入使电信不再是单纯“传送”信息,还增加了信息的处理和存储的功能。
  电信从采用金属导线的有线通信开始,到19世纪末出现了无线电报,使无线通信方式一时兴起。但由于无线通信存在的一些固有弱点在当时技术条件下一时难以克服,因而在很长一段时间里,它除了用于航海以及救灾等特定条件下外,一般都处于“备用”地位。由于技术的发展,不仅无线方式已东山再起,还出现了以光纤为传输媒体的全新通信方式。这不仅带来了电信传输方式的多样化,也为电信的高速化、个人化奠定了物质基础。
  由上可见,一百多年来,尽管电信的基本概念没有变,但它的外延却发生了沧桑巨变。电话的统治地位已经动摇,信息高速公路、互联网来了,它们对传统电信的观念和方式,都带来了前所未有的冲击。
形成Top

  1948年,当时工作于贝尔实验室的香农发表了论文“通信的数学原理”,这一标志性的论文创建了分析通信系统的数学理论方法,也即信息论。信息论使我们可以根据信道的带宽和信噪比特性推算其容量。
  在论文发表的时代,电信系统主要是基于模拟电路系统。此后,随着数字集成电路的大量普及,电信系统的设计可以充分利用信息论的理论指导提升性能。由此,数字信号处理也成为一个新的领域。
  实际的通信信道不可避免的存在各种缺陷,其中包括:散弹噪声、热噪声、延迟、非线性传输函数、快速衰落、带宽限制以及信号反射等等。在一些当前的电信系统设计中,这些缺陷可以被利用来改善信道传输的质量。
  现代通信系统普遍对于时间同步提出很高的要求。精确定时技术与通讯技术发展之间有着紧密的联系。大多数现代广域通讯系统都以原子钟作为时间基准。
  调变是指将信息转换成适合远距离传播的模拟信号的处理过程。
  主要数字信道编码方式:汉明码、格雷码、二进制码、Turbo码。
  Telecommunication
  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.


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