Is a digital modulation of amplitude and phase, with 16 different levels.
Therefore, in this case, we have only one sinusoidal carrier, with only one frequency, but it is possible to transmit 16 different symbols, so that, since 2⁴ = 16, information brought by every symbol that arrives to destination is equal to 4 bits.
The 16 obtained symbols are divided in to two groups with 8 symbols in each group:

• a tract of sinusoid with a phase chosen among: 0° - 90° - 180° - 270° and amplitude of 3V or 5V;
• a tract of sinusoid with a phase chosen among: 45° - 135° - 225° - 315° and  amplitude of every configuration that arrives at destination is constituted by an angular phase and an amplitude, and it brings with it information of 4 bits according to the following diagram.

In this kind of transmission, data to be transmitted is organized in isolated characters that are put in line separately and in different moments.

Usually the character is composed of 8 bits.

A start bit puts the line out of its rest position, which is usually zero voltage. The stop bit, instead, puts it in the rest condition, as shown in the picture.

This kind of transmission, that is certainly very simple, is not the fastest, because we must add 2 bits every 8, only to allow transmission, and so it becomes 25% slower.
Moreover, since clock is not transmitted, in reception we shall generate a local clock that is released when the start bit arrives, and allows the interpretation of the received data.

Digital data, generated by a computer, pass through rs 232 c interface, and enters the modem that transforms them into FSK or PSK or QAM-PSK analog signals.
Then the modem amplifies them, filters them eliminating intermodulation products generated by the modulator and puts them into the telephone line through the differential translator.

This allows the signals, coming from the PC, to get in line without passing by the demodulator, it also allows to the signals coming from the remote modem, to enter the demodulator section of the modem, without passing by the modulator section.

Checking errors codes, as CYCLICAL CODES, when find a wrong bit in a block of transmitted bits, signal the error in the reception data, and transmit again the whole block. Therefore if there are many errors, then transmission will be very slowed down.
In cyclical codes the sequence of bit to be transmitted is considered as a polynomial and is divided by another particular polynomial called generator known both by the transmitter and the receiver. This operation is made after having adding at the end of the original data a number of zeros equal to the number of bits of the generator polynomial.
The result of this division is ignored. The zeros added before the transmission are replaced by the remainder that is transmitted together with the information data, as redundant element.

In reception we make at first the division between the received polynomial and the generator one; the former including the redundant remainder; if this is zero, then received data is right; on the contrary, we have a wrong data, which we can correct by transmitting again the wrong data.

Coaxial cable is an electrical line, used in the past, in place of the telephone duplex cable, for the transmission of telephone data, television data, or data when it was necessary to transmit big quantities of information on short or on long distances.
Nowadays coaxial cables often have been replaced by optical fibres that have better performances.

Usually telephone coaxial cables have a bandwidth in the order of some tens of MHz ad hence modulation rate in the order of mega baud and data rate in the order of megabit/sec.

STRUCTURE OF A COAXIAL CABLE

DCS 1,800 is a gsm too, but it works on the range of 1800MHz instead of 900MHz.
It is recent, it gives better performance in propagation, thanks to higher frequency.

The cells are small and so the system is particularly indicated for urban areas where it allows a higher number of connections than gsm 900.

These cellulars are all dual band, that means, that they can connect both to the 900MHz network, and to the 1,800MHz network, so that it is easier to find the line in case of a feeble field.

DECT system is used only in domestic environments.
The cells are much smaller than those of GSM, so that the transmission power does not exceed 10 mW.

These are electric or electronic devices with 6 terminals that connect a 2 wire user line to a 4 wire one between distant exchanges.

This device is necessary when the users are very far away from each other, and so we have to introduce signal amplifiers that are directional and do not allow  bi-directional amplification.

So we shall have two different tracts, but in each of them the signal will run in a single direction.

Differential translators work as shown in the picture below:

Radio bridges are those in which the base band is made by digital signals organized in PCM web.

Transmission systems of the digital kind have a lot of advantages in comparison with the analog ones. 
These are listed below.

• Possibility of regenerating the signals at a distance.
• Possibility of finding and correcting the errors in reception.
• Privacy of the transmission thanks to the coding used.
• Use of the same structures to transmit signals of different kinds such as TV, telephone, etc..
• Easier maintenance thanks to the limited control procedures to be made.

The disadvantages of the digital transmissions are the wider band width and the more complex circuits.

Altogether, however, digital systems are more convenient than analog ones, so that today most radio bridges are digital.

ASK modulation is used only in bridges with small capacity, while PSK and especially QAM - PSK, are used in all other cases.

If data to be transmitted by radio bridges are telephone data, then the telephone exchange will transmit its data already organized in P.C.M. technique, that is already multiplexed.

Every channel of PCM is usually coded in AMI code or in HDB 3 code.

Cable for telephonic use, constituted by two unipolar insulated conductors in PVC.

Often these are white and red and are protected by a sheath, in PVC.
Conductors, usually have a diameter of 0,4 mm and of 0,6 mm, when they are used for short connections, or of 1,2 mm, when they connect points as far as some Kilometers distance.
They are used to connect the user telephone to the nearest telephone exchange.

These are normalized by ISO 2110 standard.
The 1 logic is represented by a voltage level lower than – 3V and called mark.
The 0 logic is represented by a voltage level higher than + 3V and called space.
Most frequently used values are – 12V for 1 logic and +12V for 0 logic.
Therefore negative logic is employed.
The load resistance value must be chosen between 3 KW and 7 KW.
Connector has got a male part that must be connected to the DTE and a female part that must be connected to the DCE

Correction errors codes allow, once an error has been found in a block of bits, to correct it.

These are, for instance, REDUNDANCE OF BLOCK CODES.

the sequence of bits to be transmitted is divided into characters of seven bit each, that are put then to form a table like the one in the following picture.

Then we have to count the number of bit for each line and, if they are odd number, we add a bit of parity 1, if they are an even number, we add a bit of parity 0.

The same thing has to be done for the columns adding a bit of parity for each column including the one formed by bits of parity.

Then we have to transmit the new characters, now enriched by bits of parity, in a sequential way to the receiver who, once received them, recompose them recreating the original matrix.

Now checks of the parity will be made for each line and for each column.

If bits result to be an even number the receiver will delete all the added redundant bits of line, column and blocks. Then he puts in a sequential order the remaining bits containing information, transmitting them to the decoder.

the other hand, if one of the transmitted bits is wrong, then the bits of the column and of the line of the wrong bit will be odd instead of even.

When the wrong bit will be found by the cross of the line and of the column with odd number bit, channel decoding will automatically correct it. It is always possible because the bit can have only the values 1 or 0.

Every level of the transmission protocols receives information and commands from lower level and send them to the higher, or vice versa, without interpreting them exactly, but only carrying out only the instructions of its competence.

The two systems are actually connected between them by a telephone line, but it is just as if every level talked to its correspondent level of the other elaboration system.
The protocols of level 1-2-3-4 make compatible electrical signals that flow through the line and go from a PC to the other.

The protocols of level 5-6-7 instead, carry out data elaboration functions applying a particular software of the connection.

LEVEL 7 APPLICATION – This one is the highest level of the OSI model, the beginning and the end of all other levels and the only one directly accessible by the user.

It is constituted by applicative procedures necessary to develop elaboration work that the user must carry out in the software, in the archives, in the application packages he is using.

User applies directly these procedures, also if they are physically placed in a very far PC, as it can happens, for example, surfing in Internet.

LEVEL 6 PRESENTATION – It allows the exact interpretation of the data exchanged between two unities, so that are exactly interpretable by the users, separately by formats, code, control characters, compression criteria, and so on.
It is hence like an interpreter, put between two users, who presents them the same frame.

LEVEL 5 SESSION – It starts up the logical connection to the called user, and deactivates it on caller’s request, without modifying higher level.
Therefore it can chose the kind of connection, full duplex or half duplex, by a dialogue to the correspondent protocol of the called user.

LEVEL 4 TRANSPORT – It manages the transmission of digital data between the two users fixing the length of the data to be transmitted, it finds wrong arrived data and corrects them or calls for second transmission.

LEVEL 3 network – It manages the transmission between two users among the various possible connections of the network.
It divides the message in several packages that it numbers progressively, it decides their path in telematic network providing to its invoice.
In reception, the original message is recomposed by the many packages.

LEVEL 2 LINE – This protocol manages the right transfer of the message across the line.

This level deals with the formation of the web, it generates synchronization and calls for a second transmission of wrong bits.

Typical examples of these protocols are: HDLC; SDLC; BSC.

LEVEL 1 phYSICAL – This is the lowest protocol that fixes electrical and mechanical characteristics of the DCE/DTE connection.

Electrical or electronic circuits that allow the passage of some frequencies and prevent the passage of others.
According to the selection they make on frequencies, they are divided in:

• HIGH PASS
• LOW PASS
• BAND PASS
• BAND ELIMINATE

response corresponds to the following scheme:

According to their constitution, they are divided in:

• active
• passive

In telephone exchanges, there are many filters of different kinds.

The range of frequencies of the uhf is the same of that used by the ETACS, so that the two systems have to share out the channels to avoid mutual disturbs.

Therefore frequencies used are:

• 890 – 915MHz in uplink, that is from cellular to SRB
• 935 – 960MHz in downlink, that is from srb to cellular.

FSK is the digital modulation used with a modulation index 0,5 of gmsk kind.
Therefore transmissions of GSM take up two frequency bands 25MHz large that are multiplexed in fdma in 124 channels of 200 KHz each.

Every channel of 200 KHz then is multiplexed in TDMA in 8 time slots that together constitute the web of gsm .

Altogether we obtain:

duplex useful telephone channels.

For the 9 pins version of the RS 232 C, corresponding to the European standard V.24 ITU, in the table the description of the connection of every pin.

Most of the telecommunications satellites move in circular equatorial orbits, and, complete one orbit in 24 hours, they seem to be stationary in the sky.

Thanks to this fact, we can point at them our fixed parabolic antennas of the satellite radio bridges.

A satellite that turns around the earth is subject to the force of gravity that attracts it towards earth, but it is also subject to the centrifugal force that tends to force it into space.

For the satellite to keep turning in the same orbit, these two forces have to be exactly equal.

This is the first cellular system completely digital used in Italy.

This digital system has many advantages in respect of the analog ETACS not more used:

• the possibility of finding and correcting transmission errors;
• almost total immunity to disturbance, so transmission is cleaner. So we have a smaller signal/noise ratio (S/N) which gives many advantages: the possibility of transmitting at lower power; the possibility of using the same frequencies at short distances contemporarily; and the possibility of having a higher number of users in the same country;
• using digital modulation allows the use of GSM systems, not only for telephone transmission, but also for data communication, for FAXES, for connections among PCs, with Internet and with data bases to realize main services;
• telephone transmissions are reserved because they are coded, and not so clear as was the ETACS ones.
• possibility to personalize the SIM card in which are stored the user’s data and can be used with any other GSM cellular;

Analogue micro phonic signal, is converted in digital one sampling it by using Shannon theorem.

Because telephone band width is 4 KHz, at a frequency of fc = 2B = 8 KHz
we generate 8.000 samples per second by micro phonic signal and everyone is coded by 13 bits, obtaining a data rate of:

These bits, put in a coder/compressor are compressed in 13 Kbit /sec without loosing nothing in information features.

In 20 msec are constituted some blocks. They are composed by a number of bits:

During GSM transmission, we measure continuously the strength of the field, due to the various channels, and we choose the channel with best field and S/N ratio.

So we obtain that transmission frequency is continuously variable.

Adding this feature to the scrambling, and to the coding, we understand how in GSM standard is impossible to decode an intercepted transmission.

The network of the networks.

Nowadays it is the widest data transmission network, that connects thousands of local networks, diffused all over the world, to millions of users who daily use it.
The network of the networks, was born in USA as imitation of the military net called ARPAnet (created with defensive purposes in the Sixties).
It developed more and more during the eighties, starting by a basic structure called NSFNET.

Initially some American universities used to connect to it in order to exchange information in real time for the research or for students’ use.
Since then, the growth of INTERNET has been exponential all over the world especially in Europe and Italy.
Universities, public and private companies, schools, professional men, craftsmen, shops and private citizens use internet all over the world to diffuse information, data, advertisements, propaganda, publicity etc. outward.

Connection to internet, is possible by an external or internal modem of a computer, using the telephone line.
It is possible to connect to internet at any hour of the day and, especially, during the night when lines are more free and telephone rates are lower.
The connection may happen thanks to a provider that is an intermediary between the private citizen who looks for the access to the net and all the advertisers.

The provider allows the private citizen to pay the call with a city rate, because today it is present in all the biggest cities, so that it is possible a cheaper connection than in the past.
Internet allows hyper textual and multimedia connections in real time, making possible to download by the net programs, data, images, pictures, films, information, according to the surfing system WWW.

The protocol used by Internet is HTTP that allows to transfer by the net and into the net data in hyper textual form.
Frames and pictures that the net shows, are realized with a programming language called HTML.

It is a phenomenon that shows itself in reception of digital data when, because of the high Bitrate, of the disturbs of the line, of the limited bandwidth, an error occurs in the interpretation of the data.
In this case, we study the eye diagram, that shows itself considering all the possible deformed signals that the line can make flow, comparing them to the original shape of the wave, according to the following schemes.

Local nets are telecommunication nets for data communication, of modest extent and high rate.

Usually their extent doesn’t go over the town ambit, but they can be created also in the interior of a single building or of a single room.
They often are closed networks and are organized in a business model, so that they aren’t standardized but have several and different protocols.

Usually they have good performances because they have been built on a specific request; moreover they have low noise rate and high bitrate.
Usually channels implied for communication are duplex telephone cable, coaxial cable, optical fibre.

Network is composed by branches and knots.
Computers are to be put in the knots, where many branches meet.
Network configuration can have the shape of a star, or of a ring or of a bus.

By the name of Samuel Morse, American painter, (1791 – 1872) inventor of the telegraph and of the alphabet that by him took its name.
Morse code is constituted by lines and points, and has been used for a long time for the telegraphic communication on short and long distance.

This kind of connection is frequent when many terminals refer to a single central computer of the head company, as happens, for example, in the case of banks, which have got many automatic counters, situated in different parts of the country.

From these, it must be possible, by using cards, to carry out (different kind) of  banking transactions, without going to the bank, which may be distant, but always in the same town.

The offices of Land Registry, of the Town Hall, of the Provincial Administration, of the Local Health Units, of big companies such as FIAT or ITALTEL, often have a terminal for each office and a single central processing unit, that is the data base, to which all the terminals refer.

It is possible to transmit different symbols chosen among many and not only two kinds of symbols.
This method takes the name of MULTIPLE LEVELS CODING, although different levels of voltage actually are not always transmitted, but often only a number of characters, different one by the other, bigger than two.
Information related to the choice between only two levels is of one bit, but if the choice is among M different and equiprobable symbols, information will be:

The maximum modulation rate, that is the quantity of transmitted symbols per second, in a bandwidth channel B is, according to NYQUIST criterion:

But , if every symbol arrived at destination is chosen among G , then information obtained at destination, in the same period is:

v is called G , is measured by Bit /sec and, as it is clear, is bigger or equal to MODULATION RATE vb or BAUDRATE that, instead, is measured by symbols/sec.
Precisely, if the levels or the symbols, are more than two, BITRATE results bigger than BAUDRATE and, if the levels are equal to M, formula that links these two quantities is:

Let’s suppose, for example, to use a code in which we transmit four different levels of voltage instead of two, in this case, for each arriving level, information will be of two bits and not of only one.

In fact, to distinguish four combinations, we need two bits and since the arriving time of one voltage level is always the same, because fixed by the same NYQUIST’s criterion, we shall obtain that, while modulation rate remains the same, transmission rate instead, will double.
In this way, increasing the number of voltage levels, it is possible to increase the quantity of information that arrives at destination in the same time.
But increasing the number of levels, keeping the same maximum voltage, implies that each level becomes smaller and smaller, till in reception it will be no more distinguishable by noise, always present, as shown in the following picture in which there are examples of two levels coding, four levels coding, and eight levels coding.

A limit to the increasing of the levels, exists, and it is fixed analytically, by a SHANNON’s formula:

In this formula, C is called CHANNEL CAPACITY, is measured in BIT/SEC, and it indicates the maximum theoretical Bitrate over which in reception bits are confused with noise;

• B indicates bandwidth of the considered channel and it is measured in Hz;
• S is the power of the signal and it is measured in Watt;
• N is the power of the noise and it is measured in Watt;
• S/N is the ratio SIGNAL/NOISE and it is dimensionless.

Data are generated by computers in NRZ code, which will be transformed in RZ code, because this contains, in its spectrum, the carrier allowing, in reception, the regeneration of the latter.

In line coding, instead, one of the most used codes is the three levels AMI code, in which to the bit 0 corresponds always the level 0, to bit 1 instead correspond alternately levels +1 e -1, so that the average value of the signal is always zero. Moreover in the case that in reception we have to regenerate the original clock, but we have a long sequences of zeros, we use HDB3 code, which has got three levels as well.

In this code, very used in the field of digital telephony, in addition to the alternative correspondence of +1 and –1 to the original bit 1, as in AMI code, if we have more then three consecutive zeros, the fourth bit zero is represented by a +1 or –1 (called violation) just to assure the possibility, in reception, to rebuild the original clock.

Optical fibers are wires of glass of cylindrical shape, extremely thin used in many fields of technique , of medicine, of astronomy, of optics, of telecommunications and even of art and interior decoration.
They are made of glass or, rarely, of plastic, but in every case they are very transparent to the light.
The ones used in telecommunications are crossed from one end to the other, by light pulses in the infrared field, and hence they are invisible to human eye.
Their performances are better than those of coaxial cables that preceded them in the same use till twenty years ago.
In fact they allow, for example, to transfer 12.000 telephone calls at the same time in a single fiber, they are completely insensible to electromagnetic waves, and to their interferences, they are very light and very cheap, they have very low attenuation, so we can have distances of 200 Kilometres without any amplification.

They are constituted by a central part called core and an external part called cladding and by a protection made by pvc.

Radio bridges use, for transmissions, frequencies in the field of the microwaves, and so with waves lengths in the field of the centimetres, so that we have to use parabolic antennas, that are strongly directive. They have a very high gain, typically in the field of 40/50 dB.

Parabolic antennas use, electromagnetic high frequency waves beam very narrow and concentrated so that they allow the transmission of high quantity of information.
Radio bridges powers, are of few watt for few kilometres distances.

It is constituted by all the telephones, the telephone lines, the telephone exchanges, and by the transmission and reception systems.
The telephone exchanges are divided into:

• LOCAL EXCHANGES
• SECTOR EXCHANGES
• DISTRICT EXCHANGES
• COMPARTMENT EXCHANGES
• NATIONAL EXCHANGES

Users telephones are directly connected to the local exchanges.

LOCAL EXCHANGES are connected in the shape of a star to the nearest sector exchange.
SECTOR EXCHANGES are connected in the shape of a star to the nearest district exchange.

DISTRICT EXCHANGES are connected in the shape of a star to the nearest compartment exchange.

COMPARTMENT EXCHANGES are connected together in the shape of a net among them.
NATIONAL EXCHANGES, have seats in Rome, Milan and Palermo and allow the connection with foreign countries.

All this is shown below.

Usually COMPARTMENT EXCHANGES coincide with the capital of the region.

Analogously, the DISTRICT EXCHANGES coincide with the capital of the province.

The SECTOR EXCHANGES coincide with the city.

The LOCAL EXCHANGES coincide with the quarter.

Information managed by a computer, is made of bits, and correspondent electrical signals, are of digital kind as the ones in the picture below.

On the other hand, by studying the serial development and the Fourier’s integral, we know that the spectrum of a random signal, constituted by rectangular discrete pulses, as the one shown in the previous picture, includes a continuous component and a theoretically infinite bandwidth, as shown in the following picture.

In this one, the frequency f₀ is equal to:

But the Public Switched Telephone Network, as we have already said, has got a limited bandwidth, of 300 Hz – 3400 Hz

So that a signal of this kind can’t pass through it, without, at least a series of necessary modifications that takes the name of:
modulations
Modulations are operations applied to the signal to be transmitted, to adapt it to the selected line. In the particular case of sound modems, applied modulations are operations in which a digital signal, as the one generated by a computer, must be transformed in an analogue one to flow in the public switched telephone network.

On a GEOSTATIONARY satellite for telecommunications there are different devices which carry out different jobs.

Some of these devices are propulsors.

Moreover there are the solar panels which always have to be pointed at the sun, in order to have the batteries recharged.

In synchronous transmission, a digital carrier is generated, also called clock, usually in the modem, to synchronize data transmission, so that the centre of the transmitted bit will correspond to the falling front of the clock, and the beginning or the end of the bit to the rising front of the clock as represented in the picture.

The digital carrier gives the bit clock time, indicating with its falling front, the instant in which the reception signal must be tested with half of the bit level, to allow its interpretation.

This kind of interface allows data transmission on a distance longer than 15 meters allowed by the null modem cable , up to a maximum distance in the order of one Kilometre, without using the modem or the public switched telephone line.
RS 485 A interface connects a computer to a teledetecting switchboard even if on a long distance, because the line (formed by four wires) is telefeeded.

It is the communication at a distance of any kind of information, such as pictures, sound, films, texts, digital data, software, by the usual communication channels: telephone duplex cable; coaxial cable; optical fibres; radio bridges; telecommunications satellites.
In particular, telecommunications comprehend: fixed and mobile telephony, (transmission at a distance of oral communications), telegraphy, (transmission of written messages), data communication, (communication by digital signals which en code different kinds of data, generally intended for computers),faxsimile transmission (communication of pictures), video telephony, (communication of video and voice of the correspondents).

It is the integration, on a national and international scale, of computers with several terminals. These are connected by telephone and telegraph lines or radio waves, after the introduction of proper devices for encoding and decoding of  information. This integration is made in order to create a user net of services which allows interactive access to data bases, reproduction of picture and text at distance, (faximile), transmission and reception of pictures, (teletypewriting, etc), simultaneous conversation among many users (teleconference).

When we make a telephone connection on a switched line, the user cannot know which kind of connection has been used, because the telephone signal can run in different ways, but it always transmits the same information.
In order to work, the telephone signal has to be perfectly adapted to the communication medium, and this is possible by adapting the spectrum and the shape of the signal to the channel, so that the signal results protected from noise, distortions, cross talk and other kind of disturbance.

In the following picture we can observe how it is possible to connect two telephones using different kind of channels.

Moreover it is possible to use different channels depending on the time of the connection opening. This happens when two exchanges are connected both by radio bridges and optical fibres, and in this case, the connection is chosen according to which of the two lines is free at that time.

The transponder is the repeater, that is in a telecommunication satellite.
It receives signals from a single station on the Earth, and broadcasts them again over a much larger surface of the Earth, so that a lot of countries can receive them.

A flow Diagram of a telecommunications satellite equipement, is as follows:

Telecommunications satellites have several transponders that allow the management several thousand telephone channels and of dozens of television channels.
Up link is the tract from the Earth towards the satellite, and downlink is the tract from the satellite to the Earth.

Some multiplexing procedures similar to FDM and TDM techniques, (used on the Earth) are used to allow the optimal use of the transponder of the satellites.

These techniques for the access to the transponder and to its use are essentially of four kinds:

• FDMA frequency division multiple access;
• TDMA time division multiple access;
• SDMA space division multiple access;
• CDMA code division multiple access.

It is the oldest asynchronous transmission protocol of start-stop kind, in which we transmit a single symbol as it is created by the transmitting device without any time correlation among the different emissions.
Every single character, represented by a byte, is preceded by a start bit and followed by one or two stop bits, while a bit of parity can be added to check the errors.

Start bit puts the line out of its rest position, which is usually zero voltage. The stop bit, instead, puts it in the rest condition.

The start bit has the same duration as the other bits, while the stop bit has to be longer than the others, just to be more clearly intelligible.

Used codes are7 bits ASCII, or 8 bits ASCII, or 8 bits EBCDIC.

This transmission protocol is very common. It decomposes the file to be transmitted in many blocks, all of the same length, and composed of five different fields: one data block, and four control ones.
Here is the transmission scheme.
The transmitting station transmits the first block to the receiving station, which verifies its integrity and signals to the transmitting station the free errors data arriving.
The transmitting station then transmits the second block and so on.
If there is an error in reception, the receiving station signals it to the transmitting station which will retransmit the wrong block.
The block consists of five fields transmitted in the following time order:

• Header, it contains the ASCII SOH character (Start OF Header)and it is the start of a block. It consists of a single byte.
• Order number of the block expressed in binary numeration, which goes from 0 to 255, hence the numeration starts again from 0. It consists of only one byte.
• Complement to 1 of the block number. It consists of a single byte.
• Data, consisting by 128 bytes of the file to be transmitted.

This protocol is better than the XMODEM.
It is faster, because every transmitted byte is 1 KByte long instead of 128 byte, but it is better to use it on low noise lines because every error found causes the new transmission of 1 KByte instead of 128 byte.
The error control system is CRC.

This protocol is very common because it allows the multiple transmission of the file, of its name, of length and of date creation. It is also very protected from errors.
It can vary the data rate depending on the quality of the connection line, and it can also recover incomplete files because of the interruption of the connection.