Telecommunication Media

Every telecommunications system involves the transmission of an information-bearing electromagnetic signal through a physical medium that separates the transmitter from the receiver. The major transmission media are metal wire, terrestrial and satellite radio, and optical fibre.

All transmitted signals are to some extent degraded by the environment through which they propagate. Signal degradation can take many forms, but generally it falls into three types: noise, distortion, and attenuation. 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. The relative impact of these factors on reliable communication depends on the rate of information transmission, on the desired fidelity upon reception, and on whether communication must occur in "real time" as in two-wav must occur in "real time" as in two-way voice telephony and video teleconferencing.

Various modulating and encoding schemes have been devised to provide protection against the errors caused by channel distortion and channel noise. In addition to these signal-processing techniques, protection against reception errors can be provided by boosting the power of the transmitter, thus increasing the signal-to-noise ratio. However, even powerful signals suffer some degree of attenuation, or reduction in power, 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 mat the signal is passed without loss; three decibels means that the power of the signal decreases by one-half. Channel attenuation is an important factor in the use of each transmission medium.

Упражнение I. Определите, является ли утверждение:

Information-bearing electromagnetic signal is transmitted through a physical medium that separates the transmitter from the receiver.

1) ложным 2) в тексте нет информации 3) истинным

Упражнение II. Определите, является ли утверждение:

Signals passing through the transmission medium, part of the electromagnetic energy is converted to another from of energy.

1) в тексте нет информации 2) истинным 3) ложным

Упражнение III. Определите, является ли утверждение:

The information carrying capacity of each medium differs greatly.

1) истинным 2) в тексте нет информации 3) ложным

Упражнение IV. Определите, является ли утверждение:

Signal degradation falls into two categories: noise and distortion.

1) ложным 2) в тексте нет информации 3) истинным

Упражнение V. Укажите, какой части текста (1, 2, 3, 4) соответствует следующая информация:

Means of protection against the errors caused by channel distortion and channel noise.

1) 3 2) 1 3) 2 4) 4

Упражнение VI. Укажите, какой части текста (1, 2, 3, 4) соответствует следующая информация:

Factors influencing reliable communication.

1) 4 2) 3 3) 1 4) 2

Упражнение VII. Ответьте на вопрос.

What so noise and distortion result in?

1. They lead to video teleconferencing.

2. They lead to errors in information reception.

3. They lead to increasing the signal-to-noise ratio.

4. They lead to transmission media.

Упражнение VIII. Определите основную идею текста.

1. Major transmission media used in telecommunication

2. Environment through which all transmitted signals propagate

3. Various modulating and encoding schemes

4. Types of signal degradation which may limit the information carrying capacity