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Modulation & demodulation

MODEM is short for mo dulator/ dem odulator. Modulation is the conversion of digital bit streams into analog telemetry, suitable for transmission over phone lines. Demodulation is the reversal of that process.

Remembering the diagrams presented earlier, what was at the time obviously just fluctuations in the voltage on a line can also be considered a square wave of constant amplitude (peaks are always +5V DC, valleys are always 0V DC) and varying period or frequency (how long the signal stays high or low). So, the changes in amplitude tell us whether a one or a zero is present, and the changes in period/frequency give us the pattern of ones and zeros, the actual intelligence of the message.

As we discussed earlier, we specified 0V DC as " low" for simplicity, but really we would probably have made " low" the opposite of " high, " or -5V DC.

As you can see, nothing has changed. We're still sending the same message, " 101101110100, " it is still taking 12 seconds, and we're still representing ones and zeros by playing with the voltage on the line. The only difference is the specified value used to represent a zero. In fact, we could just as easily have chosen -5V DC to represent a one and +5V DC a zero. The serial communications standard, " RS-232, " actually specifies that a binary one is represented by negative voltage. According to RS-232, any voltage from -3V DC to -15V DC will be acceptable as a one. If you get a seventy-five-foot cable to connect your two machines, you will get the infamous “unpredictable results”.

Square waves are nice, neat and easily understandable. They are either one value or another. Voice transmission facilities are not based on square waves, though. They are based on sine waves. Sine waves are not quite so neat. They look like this:

Voice transmission is achieved by manipulating an existing sine wave, or carrier. Obviously, converting a square wave into a sine wave for addition to an existing carrier is not a simple task.

Modems take a signal from a computer and encode it for transmission. Some examples of encoding techniques are “amplitude shift keying”, “frequency shift keying”, and “phase shift keying”. Modems encode a square-wave-oriented signal into a signal that can be added onto a carrier, and the other modem decodes it on the other end using the reverse of whichever technique was used to do the encoding.


Vocabulary notes:

reversal – инверсия

fluctuations – колебания

square waves – меандры

sine waves – синусоидные волны

amplitude shift keying – амплитуда переключения регистров

frequency shift keying – частота переключения регистров

phase shift keying – стадия переключения регистров

VIII. Make up a 100-word essay on “How does modem work? ” using information from these texts.

IX. Translate the following sentences into Russian paying attention to Complex Subject:

1. These elements are known to have been found two decades ago. 2. Semiconductors are shown to be good detectors of radio waves. 3. Metal is known to conduct electricity. 4. Electrical charges are known to be positive and negative. 5. This engineer appears to have presented some interesting data. 6. The problem of new generators is likely to be one of the most essential. 7. This method of switching is sure to be chea­per. 8. All the details of the plan are supposed to have been explained to you. 9. The chemist is expected to obtain a new substance. 10. They are considered to carry out their research in time. 11. The origin of the word 'robot' is said to have appeared first in a play of a Czech play­wright, Karel Capec. 12. Four atomic power stations are assumed to be built for experimental purposes. 14. The improvement of the technolo­gical processes is supposed to ensure lower cost of power. 15. Long transmission lines are known to be necessary for the transfer of electric energy over long distances. 16. Some types of reactors are known to pro­duce more nuclear fuel during their operation than they consume.


VIII. Choose the sentences with Complex Subject, translate them into Russian:

1. The engineers were glad to have obtained such good results. 2. Our task is to study well. 3. This system is expected to have wide applica­tion. 4. For him to have done these experiments is a great success. 5. Heat causes the liquid to evaporate. 6. Russia is known to have put into operation the first atomic station in 1954. 7. The engineer made his assistants check the results many times. 8. The apparatus to be assembled is very complicated. 9. The idea to use this new substance is not new. 10. The application of this device in our experiment is certain to give better results.


IX. Translate the following sentences into Russian paying attention to the emphatic construction it is (was)... that (who):

1. It is electronics that produced radar. 2. It was Einstein who provid­ed a new conception of time, space and gravitation. 3. It was A.S.Po­pov who invented the radio. 4. It was from radio that the subject of electronics was born. 5. It was radioelectronics that produced cyberne­tics, cosmonautics and nuclear physics. 6. It was in the laboratory that I found him. 7. It was D.I.Mendeleyev who formulated the Periodic Law. 8. It was in the Soviet Union that the first atom-driven ice-breaker " Lenin" was made. 9. It was in 1868 that D.I.Mendeleyev formulated the Periodic Law of Elements. 10. It is automation that improves work­ing conditions. 11. It was in 1944 that the first relay machine was completed. 12. It is the programme that ensures the execution of all ope­rations assigned to the computers. 13. It was in the 20th century that electronic computers appeared.


UNIT 9 “Internet”


I. Read and translate the following international words:

Сommunication; segment; to form; commercial firms; company; administrative organization; to control; structure; system; protocol; credit card companies; global; automatic; final; formatted documents; original; addressee.


II. Read the text “Internet” (1) and find English equivalents for the following Russian words and word combinations:

Большое количество маршрутизаторов; изменить всю систему; требовать; действовать одновременно; сетевые протоколы; цифровые сигналы; датаграмма; удалённый компьютер; простые текстовые сообщения.



Computers, switches, routers, wires and satellites. We often talk about the Internet as if it is only hardware - large numbers of computers, switches, routers, wires and satellite data communication links. Indeed, the Internet consists of various national and international data communication backbone segments, which form the high-speed core of the network, and many sub-networks connecting offices and workers within international, governmental and governmental organizations, educational and research institutions and a rapidly growing number of commercial firms; online service providers such as and Internet service providers (ISPs) and through them large numbers of small companies and individuals. A salient feature is that no 'central' organization 'runs' this hotchpotch of networks. Of course, there are administrative organizations that control regional and national backbone segments, and others that handle the registration of unique names for individual websites. However, all these administrative structures loosely organized, and there is no central control. What is more, new sub-networks can be added without having to reconfigure the whole system.

Key Internet Protocols: ТСР/IР. Every form of communication — whether between people or between computers — requires protocols to support it. Within the Internet, dozens of such protocols operate simultaneously. The strength of the Internet protocol suite is that it can be placed on the top of wide variety of communication networks which are operated on the basis of different network protocols. It should be noted here that the suite of Internet protocols is not the only one that is being used. For instance, public telecom operators, banks and credit card companies use different suites of protocols to operate the global network of automatic teller machines while using the data communication cable networks.


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