Amplitude Shift Keying

The modulation speed, vb, indicates the number of symbols arriving at destination per second, and is measured in BAUD.

Not all the transmitted bits arrive at destination in a perfectly intelligible way because of the thermal noise, the interferences, the cross talk, the switching central noise, limited bandwidth, etc.

Therefore we define BER, the number of wrong bits arrived at destination, divided by the total number of transmitted bits.

In the telephone field we can accept BER values of 10^-3, while in data communication between two PCs this value can’t be lower than 10^–7.

One of the cause of the BER is the intersymbolic interference.

The minimum quantity of information is the choice between two possibilities, usually indicated as 0 e 1 and takes the name of BIT which is the fusion of the two words: BInary digiT.
Information doesn’t exist if an event must necessarily happen as well as if it is impossible that the event happens.
Actually in both cases, there is no chance to choose, so that the same concept of information is absent.

The speed of transmission v indicates the total quantity of information arriving at destination and is measured in BIT/sec.

Cellular repeater.

We call communication channel a physical medium able to allow that the information flow from the source to the receiver.

In the telecommunication field we use the following channel:

• TELEPHONE DUPLEX CABLE
• COAXIAL CABLE
• OPTICAL FIBRE
• WAVEGUIDE
• TERRESTRIAL RADIO BRIDGE
• SATELLITE RADIO BRIDGE

Devices of the telephone exchanges that count the units of time in order to make the invoice to charge it on the bill.

There is one of them for every user.

Noise of the telephone communication, due to the passage of telephone signal from a line to another, causing the listening of the conversation, of one or more users by other users.
Therefore there will be no privacy that, instead, has to be guaranteed by the Telephone Company.
It may be of two kinds:

• PARA CROSS TALK
• TELE CROSSTALK

The first if the exchange of the telephone signal, happens next to the near user, the second if the exchange of the telephone signal, happens next to the far user.

DCE =Data Communication Equipement
Usually we use this term for the modem, that is the device that allows the data communication from a computer to the other by telephone line

This usually is a PC or, however, any terminal able to transmit or to receive data, hence by the big computer of the NASA, for example, to the little connected by modem to internet

Logarithmic unit very used in many field of telecommunications.
For example, the gain G = 10.000 of an antenna, measured in dB, becomes:

Frequency Division Multiplexing
System of telephonic multiplex, that allows to transmit several calls at the same time, on the same cable, relocating in frequency in the base band.

Frequency Shift Keying

Hyper Text Markup Language

hyper text trasfer protocol

It is the science that deals with the elaboration of information on a digital level, especially by the use of computers.
It is constituted by all the different scientific and technical aspects that are specifically to be applied to the gathering and processing of the data.

Nowadays for information we mean any thing is able to eliminate an uncertainty.
Therefore, the more improbable is the new we transmit, the more information we give, while if the event is very likely to happen, than we won’t transmit much information.
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 which is the fusion of the two words: BInary digiT.
In general, quantity Q of information included in the choice of one among M equiprobable possibilities is:

Telephone noise, in which electromagnetic energy of different origin, gets in a telephone channel and disturbs listening of the users.

International Standard Organization / Open System Interconnection

Limiters are telephone exchange devices, that avoid the signal to go over the maximum allowed level, so that the voltage level at the telephone receivers is tolerable to the human ear without any kind of disturbance.

Microwaves are electromagnetic waves with wavelengths that go from 1 mm to 1 meter. They are used both in terrestrial and satellite radio bridge transmissions, connections to space probes, radars, for television, for telephone and data transmission.
They need parabolic antennas, they expand in straight line, and are reflected by the high layers of the ionosphere.

Modulation is an analog or digital procedure, applied to its informative signal, in order to adapt it to the transmission channel or to allow multiplation with other signals.

These are telephone exchange devices that translate the frequency of the telephone signal, in order to obtain the FDM multiplation of 12 channels on the same line.
The most common modulator in the telephone field, is the balanced modulator, that exists in different versions.
It is used essentially for amplitude modulation with suppressed carrier DSB.

Mobile Service Switching Centre

Noise is a form of unwelcome energy, that arrives at destination together with useful signal, preventing the interpretation of the latter.

Personal Computer
These are small computers, of easy handling, accessible price that are used for business or private purposes, research, calculations, writing, text storage, data filing etc.
It is essentially constituted by a Central Unit where we find the motherboard on which are assembled: the CPU (Central processing unit), the RAM (random access memory), the ROM (read only memory), the cache memory (to speed up the processing), the video and audio cards, the modem, the controllers.
In the central unit, there are also the floppy reader, the CD reader (Compact Disk), the hard disk, the feeder.
The central unit is connected to the monitor, that visualizes texts, pictures and films; to the speakers for the sound stereophonic reproduction; keyboard; 
to the printer with pins, or ink jet, or laser; to the scanner, for the digitisation of the pictures; to the joystik (control lever), for video games; to the mouse, for the interaction with the screen; to the microphone, for the audio recording, and to a jack to input music from recorders, keyboards, hi-fi or television.

Pulse Code Modulation
This is a digital modulation technique used in the field of fixed and mobile telephony, and it allows multiplexing of several telephone channels on a single transmission medium.
In European Telephone PCM, 32 channels are multiplexed, 2 of them are service channels and 30 of them are vocal channels.

Phase Shift Keying

Poli - Vynil - Chloride
Plastic material used as an electrical insulator of the conductors.

Space Division Multiplexing
After hand switching, there were electromechanical switchers, also called selectors, in which switching was carried out by the SDM technique.
This technique allows electric connection of the two telephone circuits only for the time necessary for communication.
Nowadays, Space Division Multiplexing in Italy is no longer used because of new digital techniques.

This is the set of frequencies that constitute the signal.
The spectrum can be according to Fourier’s series, with lines if the signal is periodic, or with bands if the signal is aperiodic.

Telephone switching is a set of procedures that are carried out by telephone exchanges, in order to connect two users of the same national telephone network.
These can be, usually two telephones, but they can be also PCs, or big computers or databases, teleprinters, etc.
Originally switching was of manual kind, carried out by switchboard operators on user’s calls, by cords and jacks to put into panels with as many holes as the users connected to the called exchanges.
The SDM technique today is almost extinguished and replaced finally by digital auto switchers, electronic devices with integrated circuits, directed by exchange computers working in multiprocessing.

Time Division Multiplex
It is a digital technique of telephone multiplexing, which consists in putting several telephone signals in turn in the same cable and allowing hence the transmission in the same cable.

Set of the frequencies that can flow through a telephone line.

Usually they vary from 300 Hz a 3.400 Hz according to a choice on the condition of good intelligibility of spoken speech between two callers.

The above mentioned limitation is useful because it allows the complete use of telephone bandwidth channel, with FDM method.

Transmission capacity of a radio bridge is the quantity of information that it is able to transmit, and is measured by the number of telephone channels each of those is 4 KHz large.

For example, a radio bridge with a 96 KHz bandwidth has got the following transmission capacity:

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