Today, information is spreading so quickly that there is not always enough time to comprehend it. Most people rarely think about how and by what means it is transmitted, and even more so they do not imagine the scheme of information transmission.
Basic concepts
The transfer of information is considered to be the physical process of moving data (signs and symbols) in space. From the point of view of data transmission, this is a pre-planned, technically equipped event for the movement of information units in a set time from the so-called source to the receiver through an information channel, or a data transmission channel.
Data transmission channel - a set of means or medium for data distribution. In other words, this is the part of the information transmission scheme that ensures the movement of information from the source to the recipient, and under certain conditions and back.
There are many classifications of data transmission channels. If we single out the main ones, then we can list the following: radio channels, optical, acoustic or wireless, wired.
Technical channels of information transmission
Directly to technical data transmission channels are radio channels, fiber optic channels and cable. The cable can be coaxial or twisted pair. The former are an electrical cable with a copper wire inside, and the latter are twisted pairs of copper wires, insulated in pairs, in a dielectric sheath. These cables are quite flexible and easy to use. Fiber optic consists of fiber optic filaments that transmit light signals through reflection.
The main characteristics are bandwidth and noise immunity. Throughput is usually understood to mean the amount of information that can be transmitted over the channel in a certain time. And noise immunity is the parameter of channel resistance to external interference (noise).
Understanding Data Transfer
If you do not specify the area of \u200b\u200bapplication, the general scheme of information transmission looks simple, it includes three components: "source", "receiver" and "transmission channel".
Shannon's scheme
Claude Shannon, an American mathematician and engineer, was at the forefront of information theory. He proposed a scheme for transmitting information through technical communication channels.
It is not difficult to understand this scheme. Especially if you imagine its elements in the form of familiar objects and phenomena. For example, a source of information is a person on the phone. The handset will be an encoder that converts speech or sound waves into electrical signals. The data transmission channel in this case is the communication centers, in general, the entire telephone network leading from one telephone set to another. The subscriber's handset acts as a decoder. It converts the electrical signal back into sound, that is, into speech.
In this diagram of the information transfer process, the data is represented as a continuous electrical signal. This connection is called analog.
Coding concept
Coding is considered to be the transformation of information sent by a source into a form suitable for transmission over the communication channel used. The clearest example of coding is Morse code. In it, information is converted into a sequence of dots and dashes, that is, short and long signals. The receiving party must decode this sequence.
Modern technologies use digital communication. In it, information is converted (encoded) into binary data, that is, 0 and 1. There is even a binary alphabet. This relationship is called discrete.
Interference in information channels
There is also noise in the data transmission scheme. The concept of "noise" in this case means interference, due to which there is a distortion of the signal and, as a result, its loss. There are various reasons for the interference. For example, information channels can be poorly protected from each other. To prevent interference, various technical protection methods, filters, shielding, etc. are used.
K. Shannon developed and proposed to use the theory of coding to combat noise. The idea is that since noise occurs when information is lost, then the transmitted data must be redundant, but at the same time not enough to reduce the transmission rate.
In digital communication channels, information is divided into parts - packets, for each of which a checksum is calculated. This amount is sent with every packet. The receiver of information recalculates this amount and accepts the packet only if it matches the original one. Otherwise, the package is sent again. And so on until the sent and received checksums match.
Communication channel is a set of technical means and a physical medium capable of transmitting sent signals, which ensures the transfer of messages from the source of information to the recipient.
Channels are usually divided into continuous and discrete.
In the most general case, any discrete channel includes a continuous one as a component. If the influence of interfering factors on the transmission of messages in the channel can be neglected, then such an idealized channel is called channel without interference ... In such a channel, each input message uniquely corresponded to a certain output message and vice versa. If the influence of interference in the channel cannot be neglected, then when analyzing the features of transmitted messages over such a channel, models characterizing the operation of the channel in the presence of interference.
Under channel model the mathematical description of the channel is understood, allowing to calculate or evaluate its characteristics, on the basis of which methods of constructing communication systems are investigated without conducting experimental research.
The channel in which the probabilities of identifying the first signal with the second and the second with the first are the same is called symmetrical .
A channel whose input signal alphabet differs from the signal alphabet at its output is called channel with erasure.
The channel of transmission of the message from the source to the receiver, supplemented by a reverse channel, serves to increase the reliability of the transmission is called channel with feedback.
A communication channel is considered to be specified if the data on the message at its input is known, as well as the restrictions that are imposed on the input messages by the physical characteristics of the channels.
To characterize communication channels, two concepts of transmission rate are used:
1 – technical transmission speed, which is characterized by the number of elementary signals transmitted through the communication channel per unit of time, it depends on the properties of communication lines and on the speed of the channel equipment:
2 – information speed, which is determined by the average amount of information transmitted over the communication channel per unit of time:
Channel bandwidth is called the maximum speed of information transfer over this channel, achieved with the most perfect methods of transmission and reception.
Lecture number 8
Matching the physical characteristics of the communication channel and the signal
Each specific communication channel has physical parameters that determine the ability to transmit certain signals through this channel. Regardless of the specific type and purpose, each channel can be characterized by three main parameters:
Т К - channel access time [s];
F K - channel bandwidth [Hz];
Н К - permissible excess of the signal over the noise in the channel.
Based on these characteristics, an integral characteristic is used - channel volume.
Consider the following cases:
and) 
To assess the possibility of transmitting a given signal over a specific channel, you need to correlate the channel characteristics with the corresponding signal characteristics:
T C - signal duration [s];
F C - frequency band (spectrum width) of the signal [Hz];
H C - the level of excess of the signal over the interference.
Then we can introduce the concept signal volume .
Control
Communication, communications, radio electronics and digital devices
A communication channel is a system of technical means and a signal propagation medium for transmitting messages (not only data) from a source to a receiver (and vice versa). A communication channel, understood in a narrow sense (communication path), represents only the physical medium of signal propagation, for example, a physical communication line.
Question No. 3 “Communication channels. Classification of communication channels. Communication channel parameters. Condition of signal transmission over the communication channel ".
Link
Link - a system of technical means and a signal propagation medium for transmitting messages (not only data) from a source to a receiver (and vice versa). The communication channel, understood in the narrow sense (communication path ), represents only the physical medium of signal propagation, for example, a physical communication line.
The communication channel is designed to transmit signals between remote devices. Signals carry information intended for presentation to the user (person), or for use by computer applications.
The communication channel includes the following components:
- transmitting device;
- receiving device;
- transmission medium of various physical nature (Fig. 1).
The information-carrying signal generated by the transmitter, after passing through the transmission medium, enters the input of the receiving device. Further information is extracted from the signal and transmitted to the consumer. The physical nature of the signal is chosen so that it can propagate through the transmission medium with minimal attenuation and distortion. The signal is necessary as a carrier of information; it itself does not carry information.
Fig. 1. Communication channel (option number 1)
Fig.2 Communication channel (option # 2)
Those. this (channel) is a technical device (technology + environment).
Classification
There will be exactly three types of classifications. Choose the taste and color:
Classification No. 1:
There are many types of communication channels, among which the most commonly distinguishedchannels wired communication ( aerial, cable, light-guideand others) and radio communication channels (tropospheric, satellite and etc.). Such channels, in turn, are usually qualified based on the characteristics of the input and output signals, as well as on the change in the characteristics of the signals, depending on such phenomena occurring in the channel as fading and attenuation of signals.
By the type of distribution medium, communication channels are divided into:
- wired;
- acoustic;
- optical;
- infrared;
- radio channels.
Communication channels are also classified into:
- continuous (continuous signals at the input and output of the channel),
- discrete or digital (discrete signals at the input and output of the channel),
- continuous-discrete (continuous signals at the channel input, and discrete signals at the output),
- discrete-continuous (discrete signals at the channel input, and continuous signals at the output).
Channels can be likelinear and nonlinear, time and space-time.
Possible classification of communication channels by frequency range.
Information transmission systems aresingle-channel and multi-channel ... The type of system is determined by the communication channel. If a communication system is built on the same type of communication channels, then its name is determined by the typical name of the channels. Otherwise, the specification of classification features is used.
Classification No. 2 (more detailed):
- Classification by frequency range
- Kilometer (LW) 1-10 km, 30-300 kHz;
- Hectometric (SV) 100-1000 m, 300-3000 kHz;
- Decameter (HF) 10-100 m, 3-30 MHz;
- Meter (MV) 1-10 m, 30-300 MHz;
- Decimeter (UHF) 10-100 cm, 300-3000 MHz;
- Centimeter (CMB) 1-10 cm, 3-30 GHz;
- Millimeter (MMV) 1-10 mm, 30-300 GHz;
- Decimitre (DMMV) 0.1-1 mm, 300-3000 GHz.
- By direction of communication lines
- directed ( different conductors are used):
- By direction of communication lines
- coaxial,
- twisted pairs based on copper conductors,
- fiber optic.
- non-directional (radio links);
- line of sight;
- tropospheric;
- ionospheric
- space;
- radio relay (retransmission on decimeter and shorter radio waves).
By the type of transmitted messages:
- telegraph;
- telephone;
- data transmission;
- facsimile.
- By the type of signals:
- analog;
- digital;
- pulse.
- By the type of modulation (manipulation)
- In analog communication systems:
- By the type of modulation (manipulation)
- with amplitude modulation;
- with single sideband modulation;
- with frequency modulation.
- In digital communication systems:
- with amplitude shift keying;
- with frequency shift keying;
- with phase shift keying;
- with relative phase shift keying;
- with tone shift keying (single elements manipulate the subcarrier wave (tone), after which the keying is performed at a higher frequency).
- By the value of the base of the radio signal
- broadband (B \u003e\u003e 1);
- narrowband (B "1).
7. By the number of simultaneously transmitted messages
- single-channel;
- multichannel (frequency, time, code division of channels);
8.
In the direction of messaging
- one-sided;
- bilateral.
9. By order of message exchange - simplex communication- two-way radio communication, in which transmission and reception of each radio station is carried out in turn;
- duplex communication- transmission and reception are carried out simultaneously (the most efficient);
- half-duplex communication- refers to the simplex, which provides for an automatic transition from transmission to reception and the possibility of re-asking the correspondent.
10. By methods of protection of transmitted information
- open communication;
- private communication (classified).
11. By the degree of automation of information exchange
- non-automated - radio station control and message exchange is performed by the operator;
- automated - only information is entered manually;
- automatic - the message exchange process is performed between an automatic device and a computer without operator participation.
Classification number 3 (something can be repeated):
1. By appointment
Telephone
Telegraph
Television
- broadcasting
2. By transfer direction
- simplex (transmission in one direction only)
- half duplex (transmission alternately in both directions)
- duplex (simultaneous transmission in both directions)
3. By the nature of the communication line
Mechanical
Hydraulic
Acoustic
- electrical (wired)
- radio (wireless)
Optical
4. By the nature of the signals at the input and output of the communication channel
- analog (continuous)
- discrete in time
- discrete by signal level
- digital (discrete both in time and in level)
5. By the number of channels per communication line
Single channel
Multichannel
And another drawing here:
Fig. 3. Classification of communication lines.
Characteristics (parameters) of communication channels
- Channel transfer function: is presented asamplitude-frequency characteristic (AFC)and shows how the amplitude of the sinusoid decays at the output of the communication channel in comparison with the amplitude at its input for all possible frequencies of the transmitted signal. The normalized frequency response of the channel is shown in Fig. 4. Knowing the amplitude-frequency response of a real channel allows you to determine the shape of the output signal for almost any input signal. To do this, it is necessary to find the spectrum of the input signal, transform the amplitude of its constituent harmonics in accordance with the amplitude-frequency characteristic, and then find the shape of the output signal by adding the converted harmonics. For experimental verification of the amplitude-frequency characteristic, it is necessary to test the channel with reference (equal in amplitude) sinusoids over the entire frequency range from zero to some maximum value that can occur in the input signals. Moreover, it is necessary to change the frequency of the input sinusoids in small steps, which means that the number of experiments should be large.
- - the ratio of the spectrum of the output signal to the input
- bandwidth
Fig. 4 Normalized channel amplitude-frequency response
- Bandwidth: is a derived characteristic from the frequency response. It is a continuous range of frequencies for which the ratio of the output signal amplitude to the input signal exceeds a certain predetermined limit, that is, the bandwidth determines the range of signal frequencies at which this signal is transmitted through the communication channel without significant distortion. Typically the bandwidth is measured at 0.7 times the maximum frequency response. The bandwidth has the greatest impact on the maximum possible data transfer rate over the communication channel.
- Attenuation: is defined as the relative decrease in the amplitude or power of a signal when a signal of a certain frequency is transmitted over a channel. Often, during channel operation, the fundamental frequency of the transmitted signal is known in advance, that is, the frequency whose harmonic has the highest amplitude and power. Therefore, it is enough to know the attenuation at this frequency in order to approximately estimate the distortion of the signals transmitted over the channel. More accurate estimates are possible by knowing the attenuation at several frequencies corresponding to several fundamental harmonics of the transmitted signal.
Attenuation is usually measured in decibels (dB) and is calculated using the following formula: where
- signal power at the channel output,
- signal power at the channel input.
The attenuation is always calculated for a specific frequency and is related to the channel length. In practice, the concept of "linear attenuation" is always used, i.e. signal attenuation per unit of channel length, for example, attenuation 0.1 dB / meter.
- Transmission speed: characterizes the number of bits transmitted over the channel per unit of time. It is measured in bits per second -bit / s , as well as derived units:Kbps, Mbps, Gbps... The transmission rate depends on the channel bandwidth, the noise level, the type of coding and modulation.
- Channel immunity: characterizes its ability to provide signal transmission in the presence of interference. Interference is usually divided intointernal (representsthermal noise from equipment) and external (they are diverse anddepend on the transmission medium). The channel immunity depends on the hardware and algorithmic solutions for processing the received signal, which are embedded in the transceiver.Immunity transmission of signals through the channelcan be increased at the expense encoding and special processing signal.
- Dynamic range: logarithm of the ratio of the maximum power of the signals transmitted by the channel to the minimum.
- Interference immunity: this is noise immunity, i.e.e. noise immunity.
Condition for the transmission of signals over communication channels.
The channel is essentially a filter. For the signal to pass through it without distortion, the volume of this channel must be greater than or equal to the signal (see figure).
Mathematically, the condition can be written as follows:, where
; (1)
In the above formulas
- the channel bandwidth, or the frequency band that the channel can pass with the normalized signal attenuation;
- dynamic range equal to the ratio of the maximum allowable signal level in the channel to the level of interference, normalized for this types of channels;
- the time during which the channel is used for data transmission;
- the width of the signal frequency spectrum, i.e. the interval on the frequency spectrum scale occupied by the signal;
- dynamic range equal to the ratio of the average signal power to the average interference power in the channel;
- the duration of the signal, or the time of its existence.
Another form of writing a condition (expanded):
P. S .: The parameter "Channel volume" in some sources is also indicated as one of the parameters of the communication channel, but not everywhere. The mathematical formula is given above in (1).
Literature
1. http://edu.dvgups.ru/METDOC/ENF/BGD/BGD_CHS/METOD/ANDREEV/WEBUMK/frame/1.htm;
2. http://supervideoman.narod.ru/index.htm.
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The transfer of information occurs from the source to the recipient (receiver) of the information. The sourceinformation can be anything: any object or phenomenon of animate or inanimate nature. The process of transferring information takes place in a certain material environment that separates the source and recipient of information, which is called channel transmission of information. Information is transmitted through the channel in the form of a sequence of signals, symbols, signs, which are called message. Recipient information is an object that receives a message, as a result of which certain changes in its state occur. All of the above is shown schematically in the figure.
Transfer of information
A person receives information from everything that surrounds him through the senses: hearing, sight, smell, touch, taste. A person receives the greatest amount of information through hearing and sight. Sound messages are perceived by ear - acoustic signals in a continuous medium (most often in air). Vision perceives light signals that transfer the image of objects.
Not every message is informative for a person. For example, although a message in an incomprehensible language is transmitted to a person, it does not contain information for him and cannot cause adequate changes in his state.
The information channel can be either natural (atmospheric air, through which sound waves are transported, sunlight reflected from the observed objects), or be artificially created. In the latter case, we are talking about technical means of communication.
Technical information transmission systems
The first technical means of transmitting information over a distance was the telegraph, invented in 1837 by the American Samuel Morse. In 1876, American A. Bell invents the telephone. Based on the discovery of electromagnetic waves by the German physicist Heinrich Hertz (1886), A.S. Popov in Russia in 1895 and almost simultaneously with him in 1896 by G. Marconi in Italy, radio was invented. Television and the Internet appeared in the 20th century.
All of the listed technical methods of information communication are based on the transmission of a physical (electrical or electromagnetic) signal over a distance and are subject to some general laws. These laws are studied by communication theory, which arose in the 1920s. The mathematical apparatus of communication theory - mathematical communication theory, developed by the American scientist Claude Shannon.
Claude Elwood Shannon (1916-2001), USA
Claude Shannon proposed a model of the process of transmitting information through technical communication channels, represented by a diagram.
Technical information transmission system
Encoding here means any transformation of information coming from a source into a form suitable for its transmission over a communication channel. Decoding - reverse transformation of the signal sequence.
The operation of such a scheme can be explained using the familiar process of talking on the phone. The source of information is the speaking person. The encoder is the telephone handset microphone, with the help of which sound waves (speech) are converted into electrical signals. The communication channel is the telephone network (wires, telephone switchboards through which the signal passes). The decoding device is a telephone receiver (earpiece) of the listening person - the receiver of information. Here, the incoming electrical signal turns into sound.
Modern computer systems for transmitting information - computer networks - work on the same principle. There is an encoding process that converts a binary computer code into a physical signal of the type that is transmitted over a communication channel. Decoding consists in converting the transmitted signal back into computer code. For example, when using telephone lines in computer networks, the encoding-decoding functions are performed by a device called a modem.
Channel bandwidth and information transfer rate
Developers of technical information transmission systems have to solve two interrelated problems: how to ensure the highest speed of information transmission and how to reduce the loss of information during transmission. Claude Shannon was the first scientist to tackle these problems and create a new science for that time - information theory.
K. Shannon defined a method for measuring the amount of information transmitted through communication channels. He introduced the concept channel bandwidth, as the maximum possible speed of information transfer. This speed is measured in bits per second (as well as kilobits per second, megabits per second).
The bandwidth of a communication channel depends on its technical implementation. For example, the following communication means are used in computer networks:
Telephone lines,
Electrical cable communication,
Fiber optic cable communication,
Radio communication.
The throughput of telephone lines is tens, hundreds of Kbit / s; the throughput of fiber-optic lines and radio communication lines is measured in tens and hundreds of Mbit / s.
Noise, noise protection
The term "noise" refers to all kinds of interference that distorts the transmitted signal and leads to loss of information. Such interference primarily occurs for technical reasons: poor quality of communication lines, insecurity from each other of various streams of information transmitted over the same channels. Sometimes, when talking on the phone, we hear noise, crackling, interfering with understanding the interlocutor, or the conversation of completely different people is superimposed on our conversation.
The presence of noise leads to the loss of transmitted information. In such cases, noise protection is required.
First of all, technical methods are used to protect communication channels from the effects of noise. For example, using shielded cable instead of bare wire; the use of various kinds of filters that separate the useful signal from noise, etc.
Claude Shannon was designed coding theorygiving methods of dealing with noise. One of the important ideas of this theory is that the code transmitted over the communication line should be redundant... Due to this, the loss of some part of the information during transmission can be compensated. For example, if you are hard to hear while talking on the phone, then by repeating each word twice, you have a better chance that the other person will understand you correctly.
However, you cannot make the redundancy too large. This will lead to delays and higher communication costs. Coding theory allows you to get a code that is optimal. In this case, the redundancy of the transmitted information will be the minimum possible, and the reliability of the received information - maximum.
In modern digital communication systems, the following technique is often used to combat the loss of information during transmission. The whole message is split into chunks - packages... For each package, the check sum (the sum of binary digits) that is sent with this packet. At the place of reception, the checksum of the received packet is recalculated and, if it does not match the initial sum, the transmission of this packet is repeated. This will continue until the initial and final checksums match.
Considering the transfer of information in propaedeutic and basic computer science courses, first of all, this topic should be discussed from the position of a person as a recipient of information. The ability to receive information from the surrounding world is the most important condition for human existence. The human senses are the information channels of the human body that connect a person with the external environment. On this basis, information is divided into visual, sound, olfactory, tactile, and gustatory. The rationale for the fact that taste, smell and touch carry information to a person is as follows: we remember the smells of familiar objects, the taste of familiar food, and we recognize familiar objects by touch. And the contents of our memory are stored information.
Students should be told that in the animal world the informational role of the senses is different from that of a human. The sense of smell plays an important information function for animals. The heightened sense of smell of service dogs is used by law enforcement agencies to search for criminals, detect drugs, etc. The visual and sound perception of animals differs from that of humans. For example, bats are known to hear ultrasound, while cats see in the dark (from a human perspective).
Within the framework of this topic, students should be able to give specific examples of the process of transferring information, determine for these examples the source, receiver of information, the channels used for transferring information.
When studying computer science in high school, students should be introduced to the basic provisions of technical communication theory: the concepts of coding, decoding, information transfer rate, channel bandwidth, noise, noise protection. These questions can be considered within the framework of the topic “Technical means of computer networks”.
State exam
(State examination)
Question No. 3 “Communication channels. Classification of communication channels. Communication channel parameters. Condition of signal transmission over the communication channel ".
(Plyaskin)
Link. 3
Classification. five
Characteristics (parameters) of communication channels. ten
Condition for signal transmission over communication channels. 13
Literature. fourteen
Link
Link - a system of technical means and a signal propagation medium for transmitting messages (not only data) from a source to a receiver (and vice versa). The communication channel, understood in the narrow sense ( communication path), represents only the physical medium of signal propagation, for example, a physical communication line.
The communication channel is designed to transmit signals between remote devices. Signals carry information intended for presentation to the user (person), or for use by computer applications.
The communication channel includes the following components:
1) transmitting device;
2) receiving device;
3) transmission medium of different physical nature (Fig. 1).
The information-carrying signal generated by the transmitter, after passing through the transmission medium, enters the input of the receiving device. Further information is extracted from the signal and transmitted to the consumer. The physical nature of the signal is chosen so that it can propagate through the transmission medium with minimal attenuation and distortion. The signal is necessary as a carrier of information, it itself does not carry information.
Fig. 1. Communication channel (option number 1)
Fig.2 Communication channel (option # 2)
Those. this (channel) is a technical device (technology + environment).
Classification
There will be exactly three types of classifications. Choose the taste and color:
Classification No. 1:
There are many types of communication channels, among which the most commonly distinguished channels wired communication ( aerial, cable, light-guideand others) and radio communication channels (tropospheric, satellite and etc.). Such channels, in turn, are usually qualified based on the characteristics of the input and output signals, as well as on the change in the characteristics of the signals, depending on such phenomena occurring in the channel as fading and attenuation of signals.
By the type of distribution medium, communication channels are divided into:
Wired;
Acoustic;
Optical;
Infrared;
Radio channels.
Communication channels are also classified into:
Continuous (at the input and output of the channel - continuous signals),
Discrete or digital (at the input and output of the channel - discrete signals),
· Continuous-discrete (continuous signals at the channel input, and discrete signals at the output),
· Discrete-continuous (discrete signals at the channel input, and continuous signals at the output).
Channels can be like linear and nonlinear, temporary and space-time.
Possible classification communication channels by frequency range .
Information transmission systems are single-channel and multichannel... The type of system is determined by the communication channel. If a communication system is built on the same type of communication channels, then its name is determined by the typical name of the channels. Otherwise, the specification of classification features is used.
Classification No. 2 (more detailed):
1. Classification by frequency range
Ø Kilometer (LW) 1-10 km, 30-300 kHz;
Ø Hectometric (SV) 100-1000 m, 300-3000 kHz;
Ø Decameter (HF) 10-100 m, 3-30 MHz;
Ø Meter (MV) 1-10 m, 30-300 MHz;
Ø Decimeter (UHF) 10-100 cm, 300-3000 MHz;
Ø Centimeter (CMB) 1-10 cm, 3-30 GHz;
Ø Millimeter (MMV) 1-10 mm, 30-300 GHz;
Ø Decimiters (DMMV) 0.1-1 mm, 300-3000 GHz.
2. By direction of communication lines
- directed (different conductors are used):
Ø coaxial,
Ø twisted pairs based on copper conductors,
Ø fiber optic.
- non-directional (radio links);
Ø line of sight;
Ø tropospheric;
Ø ionospheric
Ø space;
Ø radio relay (retransmission on decimeter and shorter radio waves).
3. By the type of transmitted messages:
Ø telegraph;
Ø telephone;
Ø data transmission;
Ø facsimile.
4. By the type of signals:
Ø analog;
Ø digital;
Ø impulse.
5. By the type of modulation (manipulation)
- In analog communication systems:
Ø with amplitude modulation;
Ø with single sideband modulation;
Ø with frequency modulation.
- In digital communication systems:
Ø with amplitude shift keying;
Ø with frequency shift keying;
Ø with phase shift keying;
Ø with relative phase shift keying;
Ø with tone shift keying (single elements manipulate the subcarrier oscillation (tone), after which they are keyed at a higher frequency).
6. By the value of the base of the radio signal
Ø broadband (B \u003e\u003e 1);
Ø narrowband (B "1).
7. By the number of simultaneously transmitted messages
Ø single-channel;
Ø multichannel (frequency, time, code division of channels);
8. In the direction of messaging
Ø one-sided;
Ø
bilateral.
9. By order of message exchange
Ø simplex communication - two-way radio communication, in which transmission and reception of each radio station is carried out in turn;
Ø duplex communication - transmission and reception are carried out simultaneously (the most efficient);
Ø half-duplex communication - refers to the simplex, which provides for an automatic transition from transmission to reception and the possibility of re-asking the correspondent.
10. By methods of protection of transmitted information
Ø open communication;
Ø closed communication (classified).
11. By the degree of automation of information exchange
Ø non-automated - radio station control and messaging is performed by the operator;
Ø automated - only information is entered manually;
Ø automatic - the process of messaging is performed between an automatic device and a computer without operator participation.
Classification number 3 (something can be repeated):
1. By appointment
Telephone
Telegraph
Television
Broadcasting
2. By transfer direction
Simplex (transmission in one direction only)
Half duplex (alternate transmission in both directions)
Duplex (transmit simultaneously in both directions)
3. By the nature of the communication line
Mechanical
Hydraulic
Acoustic
Electrical (wired)
Radio (wireless)
Optical
4. By the nature of the signals at the input and output of the communication channel
Analog (continuous)
Discrete in time
Discrete by signal level
Digital (discrete and in time and in level)
5. By the number of channels per communication line
Single channel
Multichannel
And another drawing here:
Fig. 3. Classification of communication lines.
Characteristics (parameters) of communication channels
1. Channel transfer function: is presented as amplitude-frequency characteristic (AFC)and shows how the amplitude of the sinusoid at the output of the communication channel decays in comparison with the amplitude at its input for all possible frequencies of the transmitted signal. The normalized frequency response of the channel is shown in Fig. 4. Knowing the amplitude-frequency response of a real channel allows you to determine the shape of the output signal for almost any input signal. For this, it is necessary to find the spectrum of the input signal, transform the amplitude of its constituent harmonics in accordance with the amplitude-frequency characteristic, and then find the shape of the output signal by adding the converted harmonics. For experimental verification of the amplitude-frequency characteristic, it is necessary to test the channel with reference (equal in amplitude) sinusoids over the entire frequency range from zero to some maximum value that can occur in the input signals. Moreover, it is necessary to change the frequency of the input sinusoids in small steps, which means that the number of experiments should be large.
- the ratio of the spectrum of the output signal to the input
- bandwidth
Fig. 4 Normalized channel frequency response
2. Bandwidth: is a derived characteristic from the frequency response. It is a continuous range of frequencies for which the ratio of the amplitude of the output signal to the input signal exceeds a certain predetermined limit, that is, the bandwidth determines the range of signal frequencies at which this signal is transmitted through the communication channel without significant distortion. Typically, the bandwidth is measured at 0.7 times the maximum frequency response. The bandwidth has the greatest impact on the maximum possible data transfer rate over the communication channel.
3. Attenuation: is defined as the relative decrease in the amplitude or power of a signal when a signal of a certain frequency is transmitted over a channel. Often, during channel operation, the fundamental frequency of the transmitted signal is known in advance, that is, the frequency whose harmonic has the highest amplitude and power. Therefore, it is enough to know the attenuation at this frequency in order to approximately estimate the distortion of the signals transmitted over the channel. More accurate estimates are possible by knowing the attenuation at several frequencies corresponding to several fundamental harmonics of the transmitted signal.
Attenuation is usually measured in decibels (dB) and is calculated using the following formula: 
where
Signal power at the channel output,
Signal strength at channel input.
The attenuation is always calculated for a specific frequency and is related to the channel length. In practice, the concept of "linear attenuation" is always used, i.e. signal attenuation per unit of channel length, for example, attenuation 0.1 dB / meter.
4. Transmission speed: characterizes the number of bits transmitted over the channel per unit of time. It is measured in bits per second - bit / s, as well as derived units: Kbps, Mbps, Gbps... The transmission rate depends on the channel bandwidth, the noise level, the type of coding and modulation.
5. Channel immunity: characterizes its ability to provide signal transmission in the presence of interference. Interference is usually divided into internal (represents thermal noise from equipment) and external (they are diverse and depend on the transmission medium). Channel immunity depends on the hardware and algorithmic solutions for processing the received signal, which are embedded in the transceiver. Immunity transmission of signals through the channel can be increased at the expense encoding and special processing signal.
6. Dynamic range : logarithm of the ratio of the maximum power of the signals transmitted by the channel to the minimum.
7. Interference immunity: this is noise immunity, i.e. noise immunity.
