Text 2. Wireless Media
11. Выучите следующие слова и выражения:
comprise [kəm'praɪz]- включать; заключать в себе, содержать ,составлять
destination[ˌdestɪ'neɪʃ(ə)n] - назначение, место назначения
curvature ['kɜːvəʧə]- кривизна
repeater - повторитель, промежуточная станция; промежуточный усилитель (линии связи)
transponder [tran'spɒndə, trɑːn-] - ретранслятор
cease [siːs]- переставать (делать что-л.), прекращать
geosynchronous [ˌdʒiːə(ʊ)'sɪŋkrənəs] - геосинхронный
geostationary [ dʒiːə(ʊ)ˈsteɪʃ(ə)n(ə)ri] - геостационарный
overlap [ˌəuvə'læp] - частично перекрывать; заходить один за другой
'footprint' - зона обслуживания (в спутниковой связи)
option ['ɔpʃ(ə)n]- выбор, альтернатива, (возможный) вариант
diffuse [dɪ'fjuːs] - (adj) разбросанный, распространённый, рассеянный
[dɪ'fjuːz] - (v) рассеивать
2. Прочитайте и устно переведите текст:
Unlike wired transmission media wireless transmission relies on electromagnetic waves. These waves require no physical medium, they radiate by including a current in a transmitting antenna and then travel through the air or free space using infrared, radio, or microwave signals.
To understand wireless technology, a basic understanding of the radio frequency (RF) spectrum is required. The RF spectrum is a part of the electromagnetic spectrum in which a variety of commonly used devices (including television, AM and FM radios, microwave radios, cell phones, and others) operate. The electromagnetic spectrum has been used for communications for over 100 years, and it comprises an infinite number of frequencies, from AM radio at 1 MHz to the cellular/PCS band at 2 GHz.
Frequencies are measured in cycles per second, or Hertz, which are inversely related to wavelength. At low frequencies wavelengths are long, while at higher frequencies wavelengths are very short. Given an equal power level, the longer the wavelength, the greater the distance the signal can travel. Whereas low-frequency signals (such as AM radio) can be transmitted for hundreds of miles, high-frequency signals (such as infrared) can travel only a few feet.
Radio waves are easy to generate and are omnidirectional, but have low transmission rates. Also, depending on their frequency, radio waves either cannot travel very far, or are absorbed by the earth. In some cases, though, High Frequency (HF) waves are reflected back to earth by the ionosphere (a layer of the atmosphere).
Microwaves are high frequency radio waves that travel in straight lines through the air. Microwave links are widely used to provide communication when it is impractical or too expensive to install physical transmission media. With microwave transmission, a source can be directly focused on its destination without interfering with neighboring transmissions. Because they travel in straight lines, though, the curvature of the earth can interfere with the microwave transmitters; the solution to this is the addition of repeaters between the source and destination to redirect the data path. Microwaves are used for long distance communication, cellular phones, garage door openers, and much more.
Satellite transmission is microwave transmission in which one of the stations is a satellite orbiting the earth. A microwave beam is transmitted to the satellite from the ground. This beam is received and retransmitted (relayed) to the predetermined destination. The receiver and transmitter in satellites is known as transponder.
A satellite operates in specific frequency ranges. Bands are grouped in pairs such as 4/6 GHz, where the number refers to downlink / uplink frequencies. Normally there are many microwave bands assigned by letter: P, L, C, X, K, Q, V, and W. Most of the bands have a subband such as Ku, Ka, Kt, Kp, Ce, Cz, etc. The optimum frequency range for satellite transmission is in the range 1 to 10 GHz.
As a satellite moves across the sky, communication is possible for only a short time: as it drops below the horizon, communication ceases until it later appears above the other horizon. To always provide communications, modern telecommunication satellites are positioned approximately 22,300 miles (over 35,888 km) above the equator and arrange a satellite's velocity synchronous with the earth's rotation. This is called geostationary, or geosynchronous orbit. Only three satellites are needed to provide coverage of the entire earth with small overlapping areas.
Another wireless telecommunications technology is the low-earth orbit (LEO) satellite system, where satellites communicate directly with handheld telephones on the Earth. Because these satellites are relatively low (less than 900 miles (or 1450 km) above), they move across the sky quite rapidly, so sophisticated ground equipment must be used to track a satellite, increasing the cost of the system. Satellites in this orbital range have a very small 'footprint', so lots of them are required to enable world wide communication (35 or more).
Infrared light is used for close-range communication, such as remote controls or a computer mouse, because it does not pass through objects well. This is also a plus because infrared communications in one room do not interfere with the infrared communications in another room. Infrared communication is more secure than other options, such as radio, but it cannot be used outside due to interference by the Sun.
Lasers are also used for wireless communications. It is a relatively low cost way to connect two buildings' LANs, but it has drawbacks: it is difficult to target on the destination's receiver because the beam is so small. Laser light also diffuses easily in poor atmospheric conditions, such as rain, fog, or intense heat.
3. Ознакомьтесь с дополнительными словами и выражениями:
ionosphere [aɪ'ɔnəsfɪə] - ионосфера (верхние слои атмосферы, от 50-80 км)
horizon [hə'raɪz(ə)n]- горизонт
to target ['tɑːgɪt]- предназначать
4. Ответьте на следующие вопросы:
1. What is the difference between wired transmission media and wireless transmission? 2. How are frequencies measured? 3. What is the relation between the wavelength of a signal and the distance it can travel? 4. Is it difficult to generate radio waves? 5. When are microwave links used? 6. How do radio waves propagate? 7. What drawbacks do lasers have? 7. What are the main features of communication with the help of satellites?