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| 1.0 GENERALITIES | |||||||||||||
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In recent years considerable progress has been made in the field of informatics, science that studies computers, and in the field of telecommunications, science that studies long-distance data communication. By their fusion derives telematics.
One branch of telematics is DATA COMMUNICATION, a technique consisting in long-distance transmission of digital informations between two computers through a communication channel.
The connection may occur between two computers, and then it takes the name of POINT TO POINT, but it may also occur among one central computer and many terminal units, and in this case it takes the name of CLIENT-SERVER.
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Physical quantities, like temperature, pressure, speed, brilliance, etc, can be detected by proper sensors and transmitted over a distance using electrical signals through connection lines. A line can transmit essentially two kinds of electrical signals: ANALOG or DIGITAL Analog signals are those that, vs. time, can assume all the values between the maximum and the minimum value allowed by the communication channel. A classical example is the microphone current that passes through the telephone duplex cable which connects one user to the other by the telephone network. The instantaneous values of this current are proportional to the pressure of the sound wave of the voice they have to transmit over a distance. A digital, or numeric signal on the other hand is a signal that can have only two values, or anyway only a discrete number of values, as it happens for computer generated data. Examples of analog and digital signals are represented below.
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The minimum quantity of information is the choice between only two possibilities that are usually indicated with 0 and 1, and it takes the name of BIT. Information stored in a digital signal is given by the number of bit necessary to represent it. To estimate the information in an analog signal we have to turn it into a digital one and to count the bits obtained after the conversion. |
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A numeric signal of period T can be expanded by Fourier series in a sum of infinite sinusoids of different amplitudes as shown in the picture below.
Random numeric signal can be expanded in to a Fourier integral and described in terms of its spectrum function as shown in the picture below.
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| Telecommunication channels used to transmit data are: telephone duplex cable, coaxial cable, optical fibre, free space.
Each of them allows the transmission of signals only in a finite range of frequencies. Therefore, channel bandwidth B is defined as the range of frequencies that a telecommunication channel is able to transmit. For instance, Italian telephone network usually transmits in a range of frequencies that goes from 300 Hz to 3.400 Hz according to the following picture:
To allow the transmission of a signal through a particular communication channel, we have to modulate the signal, in order to adapt its spectrum to that one of the channel to be crossed. |
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Data, generated by proper sensors or sent out by computers, have to be transmitted over a distance to be used by other users. widening:
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