1. :
degrade [dɪ'greɪd] - (), ,
degradation [ˌdegrə'deɪʃ(ə)n] ; ,
signal degradation -
propagate ['prɔpəgeɪt]
emission [ɪ'mɪʃ(ə)n ], [iː-] - ,
attenuation [ətenju'eɪʃ(ə)n] - ,
devise [dɪ'vaɪz] - , (, );
dissipation [dɪsɪ'peɪʃ(ə)n] - ,
plot - ; ;
loss -
ease [i:z] - ,
installation [ɪnstə'leɪʃ(ə)n] - ;; ;
maintenance ['meɪnt(ə)nən(t)s] - , ;
availability [əveɪlə'bɪlətɪ] -
radiate [reɪdieɪt] - ()
2. :
Every telecommunications system involves the transmission of an information-bearing electromagnetic signal through a physical medium that separates the transmitter from the receiver. All transmitted signals are to some extent degraded by the environment through which they propagate. Signal degradation generally falls into three types: noise, distortion, and attenuation (reduction in power). Noise is the presence of random, unpredictable, and undesirable electromagnetic emissions that can mask the intended information signal. Distortion is any undesired change in the amplitude or phase of any component of an information signal that causes a change in the overall waveform of the signal. Both noise and distortion are commonly introduced by all transmission media, and they both result in errors in reception.
Various modulating and encoding schemes have been devised to provide protection against the errors caused by channel distortion and channel noise. However, even powerful signals suffer some degree of attenuation as they pass through the transmission medium. The principal cause of power loss is dissipation, the conversion of part of the electromagnetic energy to another form of energy such as heat. In communications media, channel attenuation is typically expressed in decibels (dB) per unit distance. Attenuation of zero decibels means that the signal is passed without loss; three decibels means that the power of the signal decreases by one-half. The plot of channel attenuation as the signal frequency is varied is known as the attenuation spectrum, while the average attenuation over the entire frequency range of a transmitted signal is defined as the attenuation coefficient.
Channel attenuation is an important factor in the use of each transmission medium. Along with noise and distortion, it can influence the choice of one medium over another. Transmission efficiency which is generally viewed as the amount of signal degradation created by the use of a particular transmission medium, is the most important factor to consider when choosing transmission media. Though cost, ease of installation and maintenance, and availability, also count.
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As it has been previously noted, modern telecommunications systems employ three main transmission media: wire, radio, and optical.
In wire transmission an information-bearing electromagnetic wave is guided along a wire conductor to a receiver. Propagation of the wave is always accompanied by a flow of electric current through the conductor. Since all practical conductor materials are characterized by some electrical resistance, part of the electric current is always lost by conversion to heat, which is radiated from the wire. This dissipative loss leads to attenuation of the electromagnetic signal, and the amount of attenuation increases linearly with increasing distance between the transmitter and the receiver.
Optical communication employs a beam of modulated monochromatic light to carry information from transmitter to receiver. The light spectrum spans a tremendous range in the electromagnetic spectrum, extending from the region of 10 terahertz (104 gigahertz) to 1 million terahertz (109 gigahertz). This frequency range essentially covers the spectrum from far infrared (0.3-mm wavelength) through all visible light to near ultraviolet (0.0003-mm wavelength). Propagating at such high frequencies, optical wavelengths are naturally suited for high-rate broadband telecommunication. For example, amplitude modulation of an optical carrier at the near-infrared frequency of 300 terahertz by as little as 1 percent yields a transmission bandwidth that exceeds the highest available coaxial cable bandwidth by a factor of 1,000 or more.
In radio transmission a radiating antenna is used to convert a time-varying electric current into an electromagnetic wave or field, which freely propagates through a nonconducting medium such as air or space. In a broadcast radio channel, an omnidirectional antenna radiates a transmitted signal over a wide service area. In a point-to-point radio channel, a directional transmitting antenna is used to focus the wave into a narrow beam, which is directed toward a single receiver site. In either case the transmitted electromagnetic wave is picked up by a remote receiving antenna and reconverted to an electric current.
3. :
random ['rændəm] - ,
span ,
yield [jiːld] - , ( -.)
essentially [ɪ'senʃ(ə)lɪ] - , ; ;
directional antenna -
omnidirectional antenna -
4. :
1. How many types does signal degradation fall into? 2. What do noise and distortion result in? 3. Give the definition of dissipation. 4. What are the factors to consider when choosing transmission media? 5. What main transmission media do modern telecommunications systems employ? 6. Why are noise and distortion undesirable in signals? 7. In wire transmission an information-bearing electromagnetic wave isnt guided along a wire conductor to a receiver, is it?
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5. :
1. The degradation of signals is caused by the environment through which they propagate. 2. Attenuation does not influence signal reception. 3. The choice of transmission medium depends purely upon its cost and availability. 4. Optical media use a rather narrow range in electromagnetic spectrum. 5. Radio transmission doesn't require any conductiong medium.
6. , :
; ; ; ; ; ; ; ; ; ; .
7. :
Information-bearing electromagnetic signal; intended information signal; channel attenuation; attenuation spectrum; dissipative loss; high-rate broadband telecommunication; far-infrared and near-infrared frequency; point-to-point radio channel.
8. :
