Communication channels (CS) serve to transmit a signal and are a common link in any information transmission system.
By physical nature, communication channels are divided into mechanical, used for the transmission of material media, acoustic, optical and electrical transmitting, respectively, sound, light and electrical signals.
Electrical and optical communication channels, depending on the method of signal transmission, can be divided into wired, using physical conductors for signal transmission (electrical wires, cables, light guides), and wireless, using electromagnetic waves for signal transmission (radio channels, infrared channels).
According to the form of presentation transmitted information communication channels are divided into analog, through which information is transmitted in a continuous form, i.e. as a continuous series of values of some physical quantity, and digital, transmitting information presented in the form of digital (discrete, pulse) signals of various physical nature.
Depending on the possible directions of information transfer, communication channels are divided into simplex, allowing information to be transmitted in only one direction; half duplex, providing alternate transmission of information in both forward and reverse directions; duplex, allowing the transmission of information simultaneously in the forward and reverse directions.
Communication channels are switched, which are created from separate sections (segments) only for the time of transmission of information over them, and at the end of the transmission such a channel is liquidated (disconnected), and unswitched(highlighted) created on long time and having constant characteristics length, bandwidth, noise immunity.
Widely used in automated systems information processing and electrical control wire channels connections vary in bandwidth:
low speed information transfer rate in which from 50 to 200 bps. These are telegraph communication channels, both switched (subscriber telegraph) and non-switched;
medium speed, using analog (telephone) communication channels; the transmission speed in them is from 300 to 9600 bps, and in the new standards V.32 - V.34 of the International Consultative Committee for Telegraphy and Telephony (CCITT) and from 14400 to 56,000 bps;
high speed(broadband), providing an information transfer rate of over 56,000 bps.
To transfer information to low-speed and medium-speed CS the physical medium is usually wired communication lines: groups of either parallel or twisted wires called twisted pair. It consists of isolated conductors twisted together in pairs to reduce both electromagnetic crosstalk and signal attenuation during transmission at high frequencies.
For the organization of high-speed (broadband) CS are used various cables:
Shielded with twisted pairs of copper wires;
Unshielded with twisted pairs of copper wires;
coaxial;
Optical fiber.
STP cables(shielded with twisted pairs of copper wires) have good technical characteristics, but are inconvenient to use and expensive.
UTP cables(unshielded with twisted pairs of copper wires) are quite widely used in data transmission systems, in particular in computer networks.
There are five categories of twisted pairs: the first and second categories are used for low-speed data transmission; the third, fourth and fifth - at transmission speeds up to 16.25 and 155 Mbps, respectively. These cables are good technical specifications, relatively inexpensive, easy to use, do not require grounding.
Coaxial cable is a copper conductor coated with a dielectric and surrounded by a retinue of thin copper conductors shielding protective shell. Data transfer rate coaxial cable quite high (up to 300 Mbps), but it is not convenient enough to use and has a high cost.
Fiber optic cable(Fig. 8.2) consists of glass or plastic fibers with a diameter of several micrometers (light-guiding core) with a high refractive index p s, surrounded by insulation with a low refractive index n 0 and placed in protective polyethylene sheath. On fig. 8.2, a the distribution of the refractive index over the cross section of the fiber optic cable is shown, and in fig. 8.2, b- scheme of propagation of rays. The source of radiation propagating through a fiber optic cable is an LED or a semiconductor laser, the radiation receiver is a photodiode, which converts light signals into electrical signals. The transmission of a light beam through a fiber is based on the principle of total internal reflection of the beam from the walls of the light-guiding core, which ensures minimal signal attenuation.
Rice. 8.2. Propagation of rays along a fiber optic cable:
a- distribution of the refractive index over the cross section of the fiber optic cable;
b - ray propagation pattern
In addition, fiber optic cables protect transmitted information from external electromagnetic fields and high speed transmissions up to 1000 Mbps. Information encoding is carried out using analog, digital or pulse modulation of the light beam. An optical fiber cable is quite expensive and is usually used only for laying important trunk communication channels, for example, laid along the bottom Atlantic Ocean cable links Europe to America. In computer networks, fiber optic cable is used in the most critical areas, in particular, in the Internet. One thick backbone fiber-optic cable can simultaneously organize several hundred thousand telephone, several thousand video telephone and about a thousand TV channels connections.
High-speed CS organized on the basis of wireless radio channels.
Radio channel - it is a wireless communication channel laid over the air. A radio transmitter and a radio receiver are used to form a radio channel. Data transfer rates over the radio channel are practically limited by the bandwidth of the transceiver equipment. The radio wave range is determined by the frequency band of the electromagnetic spectrum used for data transmission. In table. 8.1 shows the ranges of radio waves and their corresponding frequency bands.
For commercial telecommunications systems, the most commonly used frequency bands 902 - 928 MHz and 2.40 - 2.48 GHz.
Wireless communication channels have poor noise immunity, but provide the user with maximum mobility and responsiveness.
Telephone lines the most extensive and widespread. They carry out the transmission of sound (tonal) and facsimile messages. On the basis of a telephone communication line, information and reference systems, systems Email and computer networks. On the base telephone lines analog and digital channels for information transmission can be created.
V analog phone lines telephone microphone converts sound vibrations to analog electrical signal, which is transmitted by subscriber line in ATS. The bandwidth required to transmit the human voice is approximately 3 kHz (range 300 Hz - 3.3 kHz). Call signaling is transmitted over the same channel as voice transmission.
V digital channels connections analog signal is sampled before input - converted to digital form: every 125 µs (sampling frequency is 8 kHz) current value analog signal displayed in 8-bit binary code.
Table 8.1
Radio wave bands and their corresponding frequency bands
Today, information is spreading so fast 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 do not imagine the scheme for transmitting information.
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 transfer, this is a pre-planned, technically equipped event for the movement of information units for set time from the so-called source to the receiver through an information channel, or data channel.
Data transmission channel - a set of means or a data distribution medium. In other words, this is that part of the information transfer scheme that ensures the movement of information from the source to the recipient, and under certain conditions, back.
There are many classifications of data transmission channels. If we highlight the main ones, we can list the following: radio channels, optical, acoustic or wireless, wired.
Technical channels of information transfer
Directly to technical channels data transmission includes radio channels, fiber optic channels and cable. The cable can be coaxial or twisted pair. The first is an electrical cable with copper wire inside, and the second - twisted pairs copper wires, insulated in pairs, located in a dielectric sheath. These cables are quite flexible and easy to use. An optical fiber consists of fiber optic strands that transmit light signals through reflection.
The main features are throughput and noise immunity. Bandwidth is usually understood as the amount of information that can be transmitted over the channel for certain time. And noise immunity is the parameter of channel stability to the effects of external interference (noise).
Understanding Data Transfer
If you do not specify the scope, general scheme information transmission looks simple, includes three components: "source", "receiver" and "transmission channel".
Shannon's scheme
Claude Shannon, an American mathematician and engineer, stood at the origins of information theory. He proposed a scheme for transmitting information through technical communication channels.
It is easy to understand this diagram. Especially if you imagine its elements in the form of familiar objects and phenomena. For example, the source of information is a person talking 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 communication nodes, in general, the entire telephone network leading from one telephone set to another. The subscriber's handset acts as the decoding device. It converts the electrical signal back into sound, i.e. into speech.
In this diagram of the information transfer process, data is represented as a continuous electrical signal. Such a connection is called analog.
The concept of coding
Coding is considered to be the transformation of information sent by the source into a form suitable for transmission over the communication channel used. Most clear example encoding 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.
V modern technologies using digital communications. In it, information is converted (encoded) into binary data, that is, 0 and 1. There is even a binary alphabet. Such a connection is called discrete.
Interference in information channels
Noise is also present in the data transmission scheme. The concept of "noise" in this case means interference, due to which the signal is distorted and, as a result, its loss. The reasons for interference can be different. For instance, information channels may be poorly protected from each other. To prevent interference, various technical ways protections, filters, shielding, etc.
K. Shannon developed and proposed for use the coding theory to combat noise. The idea is that if information is lost under the influence of noise, then the transmitted data should be redundant, but at the same time not so much as 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 transmitted along with each packet. The information receiver recalculates this sum and accepts the packet only if it matches the original one. Otherwise, the packet is sent again. And so on until the sent and received checksums match.
An information transmission channel is a set technical means, providing the transmission of electrical signals from one point to another. The channel inputs are connected to the transmitter, and the outputs are connected to the receiver. In modern digital systems communication, the main functions of the transmitter and receiver are performed by the modem. One of the main characteristics of the channel is the speed of information transfer. The maximum possible rate of information (data) transmission over a communication channel under fixed constraints is called the channel capacity, denoted by C and has the dimension bit/s. V general case channel capacity can be determined by the formula: (8.22) where I is the amount of information transmitted over time T. As a measure of the amount of information, we take R. Hartley's measure, defined as the logarithm of the possible states of the object b. (8.23) To find I, we use the Kotelnikov theorem, which proves that a signal that does not contain frequencies above P in its spectrum can be represented by 2P independent values per second, the totality of which completely determines this signal. This procedure called analog to digital conversion, was considered in Chap. 6. It consists of two stages - time sampling, i.e., representing the signal in the form of n samples taken over a time interval 1 = 1 / (2P), and level quantization, i.e., representing the signal amplitude by one of t possible values. Let us determine the number of different messages that can be composed of n elements that take any of m different fixed states. From an ensemble of n elements, each of which can be in one of m fixed states, one can compose m a various combinations, i.e., 1 = m". Then: (8.24) During the time T, the number of samples n = T / 1 = 2RG. If there were no noise, then the number m of discrete signal levels would be infinite. In the case of noise, the latter determines the degrees the distinguishability of individual signal amplitude levels.Since power is an average characteristic of the amplitude, the number of distinguishable signal levels in terms of power is (P e + P w) / P w), and in amplitude, respectively: 
Then the channel capacity is: 
(8.25) So, the channel capacity is limited by two values: channel bandwidth and noise. Relation (8.25) is known as the Hartley-Shannon formula and is considered the main one in information theory. The frequency band and signal power are included in the formula in such a way that for C = const, when narrowing the band, it is necessary to increase the signal power, and vice versa. The main characteristics of communication channels include: ■ frequency response (AFC); ■ bandwidth; ■ attenuation; * throughput; ■ reliability of data transmission; ■ noise immunity. To determine the characteristics of a communication channel, an analysis of its response to a certain reference impact is used. Most often, sinusoidal signals are used as a reference. different frequencies. The frequency response shows how the amplitude of the sinusoid changes at the output of the communication line compared to the amplitude at its input for all frequencies transmitted signal. The bandwidth is the range of frequencies for which the ratio of the amplitude of the output signal to the input signal exceeds a certain specified limit (for a power of 0.5). This bandwidth defines the frequency range of a sinusoidal signal at which this signal is transmitted over the communication line without significant distortion. Bandwidth affects the maximum possible speed of information transmission over the communication line. Attenuation - is defined as a relative decrease in the amplitude or power of the signal when a signal of a certain frequency is transmitted over a communication line. Attenuation I is usually measured in decibels (dB) and is calculated by the formula: where P out is the signal power at the line output; P in - signal power at the line input. The line throughput characterizes the maximum possible data transfer rate over the communication line and is measured in bits per second (bps), as well as in derived units of Kbps, Mbps, Gbps. Line throughput is affected by physical and logical coding. Presentation method discrete information in the form of signals transmitted to the communication line is called physical linear coding. The spectrum of the signal and, accordingly, the bandwidth of the line depend on the chosen encoding method. Thus, for one or another coding method, the line may have a different bandwidth. If the signal changes so that only two of its states can be distinguished, then any change in it will correspond to the smallest unit of information - a bit. If the signal changes so that more than two states can be distinguished, then any change in it carries several bits of information. Number of changes information parameter carrier vibration ( periodic signal) per second is measured in bauds. The line bandwidth in bits per second is generally not the same as the number of bauds. It can be either higher or lower than the baud rate, and this ratio depends on the encoding method. If the signal has more than two distinct states, then the throughput in bps will be higher than the number of bauds. For example, if the information parameters are the phase and amplitude of a sinusoid, and there are 4 phase states (0, 90, 180 and 270) and two amplitude values, then information signal has eight distinct states. In this case, a modem operating at 2400 baud (with clock frequency 2400 Hz), transmits information at a speed of 7200 bps, since three bits of information are transmitted with one signal change. When using a signal with two different states, the opposite picture can be observed. This occurs when each bit in the sequence is encoded with several changes in the information parameter of the carrier signal for reliable information recognition by the receiver. For example, when encoding a one bit value with a pulse of positive polarity, and a zero value of a bit with a pulse of negative polarity, the signal changes its state twice during the transmission of each bit. With this coding method, the line throughput is two times lower than the number of bauds transmitted over the line. The throughput is affected by logical encoding, which is performed before the physical one and involves replacing the bits of the original information with a new bit sequence that carries the same information, but has additional properties (detecting codes, encryption). In this case, the distorted bit sequence is replaced by a longer sequence, so the channel capacity is reduced. In the general case, the relationship between the bandwidth of a line and its maximum possible bandwidth is determined by relation (8.25). From this relation it follows that although theoretical limit there is no increase in line capacity (with a fixed bandwidth), in practice such a limit exists. It is possible to increase the line capacity by increasing the transmitter power or reducing the interference power. However, an increase in transmitter power leads to an increase in its dimensions and cost, and noise reduction requires the use of special cables with good protective screens and noise reduction in communication equipment. The channel capacity is the maximum rate value. To achieve this transmission rate, the information must be encoded in the most efficient way. The assertion that such coding is possible is the most important result of Shannon's information theory. Shannon proved the fundamental possibility of such efficient coding, without, however, determining specific ways of its implementation. (Note that in practice, engineers often talk about channel capacity, meaning by this the real, and not the potential transmission rate.) The efficiency of communication systems is characterized by the parameter, equal to speed transmission of information I per unit bandwidth G, i.e. I / R. To illustrate the existing possibilities for creating effective systems connections in fig. 8.12 shows graphs of the dependence of the efficiency of information transfer when various types M-ary discrete amplitude, frequency and phase modulation(in addition to binary modulation, modulation with 4, 8, 16 and even 32 positions of the modulated parameter is also used) from the ratio of the energy of one bit to the noise power spectral density (Eo/Mo). The Shannon boundary is also shown for comparison. Comparison of the curves shows, in particular, that the most efficient transmission is phase-shifted. discrete modulation, however, at a constant signal-to-noise ratio, the most popular type of 4PSK modulation is three times worse than potentially achievable. The reliability of data transmission characterizes the probability of distortion for each transmitted bit data. The reliability indicator is the probability of erroneous reception of the information symbol - R. 1 OR 
Rice. 8.12. Efficiency of digital communication systems: 1 - Shannon boundary; 2 - M-ary FMn; 3 - M-ary AMn; 4 - M-ary FSK The value of R osh for communication channels without additional funds error protection is, as a rule, 10 4 ... 10 6 . In fiber-optic communication lines, Р osh is 10 "9. This means that at Р osh = 10 4, on average, out of 10,000 bits, the value of one bit is distorted. Bit distortions occur both due to the presence of interference on the line, and due to distortion waveform limited by the bandwidth of the line.Therefore, to improve the reliability of the transmitted data, it is necessary to increase the degree of noise immunity of the lines, as well as to use more broadband communication lines.An indispensable part of any channel is the communication line - the physical medium that ensures the flow of signals from the transmitter to the receiver. Depending on the data transmission medium, communication lines can be: ■ wired (overhead) ■ cable (copper and fiber optic) ■ terrestrial and satellite communications (wireless channels connections). Wired communication lines are wires laid between supports without any shielding or insulating braids. Noise immunity and data transfer rate in these lines is low. As a rule, telephone and telegraph signals are transmitted through such communication lines. 8.3.1.
The transfer of information occurs from the source to the recipient (receiver) of information. source information can be anything: any object or phenomenon of living or inanimate nature. The process of information transfer takes place in some material environment that separates the source and recipient of information, which is called channel transfer of information. Information is transmitted through a channel in the form of a certain 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. Perceived by ear audio messages- acoustic signals in a continuous medium (most often - in the air). Vision perceives light signals that carry the image of objects.
Not every message is informative for a person. For example, a message in an incomprehensible language, although transmitted to a person, does not contain information for him and cannot cause adequate changes in his state.
An information channel can either be of a natural nature (atmospheric air through which sound waves are transmitted, sunlight reflected from observed objects), or be artificially created. V last 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, the American A. Bell invents the telephone. Based on the discovery by the German physicist Heinrich Hertz electromagnetic waves(1886), A.S. Popov in Russia in 1895 and almost simultaneously with him in 1896 G. Marconi in Italy, radio was invented. Television and the Internet appeared in the twentieth 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 certain general laws. The study of these laws is communication theory that emerged in the 1920s. Mathematical apparatus of communication theory - mathematical theory of communication, developed by the American scientist Claude Shannon.
Claude Elwood Shannon (1916–2001), USA
Claude Shannon proposed a model for 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 - inverse transformation of the signal sequence.
The operation of such a scheme can be explained by the familiar process of talking on the phone. The source of information is talking man. An encoder is a handset microphone that converts sound waves (speech) into electrical signals. The communication channel is the telephone network (wires, switches of telephone nodes through which the signal passes). The decoding device is a handset (headphone) of the listening person - the receiver of information. Here the incoming electrical signal is converted into sound.
Modern computer systems transmission of information - computer networks operate on the same principle. There is an encoding process that converts binary computer code into physical signal of the type that is transmitted over the communication channel. The decoding is inverse transformation transmitted signal into computer code. For example, when using telephone lines in computer networks, the functions of encoding and decoding are performed by a device called a modem.
Channel capacity and information transfer rate
Developers technical systems transmission of information, two interrelated tasks have to be solved: how to ensure top speed transmission of information and how to reduce the loss of information during transmission. Claude Shannon was the first scientist who took on the solution of these problems and created a new science for that time - information theory.
K.Shannon determined the method of measuring the amount of information transmitted over communication channels. They introduced the concept channel bandwidth,as the maximum possible information transfer rate. This speed is measured in bits per second (as well as kilobits per second, megabits per second).
The throughput of a communication channel depends on its technical implementation. For example, computer networks use the following means of communication:
telephone lines,
Electrical cable connection,
fiber optic cabling,
Radio communication.
Throughput of telephone lines - tens, hundreds of Kbps; the throughput of fiber optic lines and radio communication lines is measured in tens and hundreds of Mbps.
Noise, noise protection
The term "noise" refers to various kinds of interference that distort the transmitted signal and lead to loss of information. Such interference primarily arises for technical reasons: poor quality communication lines, insecurity from each other of various information flows transmitted over the same channels. Sometimes, while talking on the phone, we hear noise, crackling, which make it difficult to understand 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 necessary.
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 developed coding theory, which gives methods for dealing with noise. One of the important ideas of this theory is that the code transmitted over the communication line must 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 when talking on the phone, then by repeating each word twice, you have a better chance that the interlocutor will understand you correctly.
However, you can not make the redundancy too large. This will lead to delays and higher communication costs. Coding theory allows you to get a code that will be optimal. In this case, the redundancy of the transmitted information will be the minimum possible, and the reliability of the received information will be the maximum.
V modern systems digital communications To combat the loss of information during transmission, the following technique is often used. The whole message is divided into portions - packages. For each package is calculated check sum(sum binary digits) that is sent with this packet. The checksum is recalculated at the receiving end. received package and, if it does not match the original amount, the transfer this package repeats. This will continue until the initial and final checksums match.
Considering the transfer of information in propaedeutic and basic courses Informatics, 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 - essential condition human existence. The human sense organs are the information channels of the human body, which connects a person with external environment. On this basis, information is divided into visual, auditory, 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, we recognize familiar objects by touch. And the content of our memory is stored information.
Students should be told that in the animal world the informational role of the senses is different from the human one. important information function performs sense of smell for animals. The heightened sense of smell of service dogs is used law enforcement 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, and cats are known to see in the dark (from a human perspective).
Within the framework of this topic, students should be able to lead concrete examples the process of information transfer, to determine for these examples the source, receiver of information, used information transmission channels.
When studying computer science in high school, students should be introduced to the basic provisions of the technical theory of communication: the concepts of coding, decoding, information transfer rate, channel capacity, noise, noise protection. These issues can be considered within the framework of the topic “Technical means of computer networks”.
State exam
(state examination)
Question number 3 “Communication channels. Classification of communication channels. Communication channels parameters. Condition for signal transmission over a communication channel.
(Plyaskin)
Link. 3
Classification. 5
Characteristics (parameters) of communication channels. 10
The condition for signal transmission over communication channels. thirteen
Literature. 14
Link
Link- a system of technical means and a signal propagation environment for transmitting messages (not just data) from a source to a recipient (and vice versa). A communication channel understood in a narrow sense ( communication path), represents only physical environment propagation of signals, for example, physical line connections.
The communication channel is designed to transmit signals between remote devices. Signals carry information intended to be presented to the user (human) or to be used application programs COMPUTER.
The communication channel includes the following components:
1) transmitting device;
2) receiving device;
3) transmission medium of various physical nature (Fig.1) .
The information-carrying signal formed by the transmitter, after passing through the transmission medium, is fed to the input of the receiving device. Further, the 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 needed as a carrier of information, it does not carry information itself.
Fig.1. Communication channel (option No. 1)
Fig.2 Communication channel (option No. 2)
Those. this (channel) - technical device(technology + environment).
Classification
There will be exactly three types of classifications. Choose your taste and color:
Classification #1:
There are many types of communication channels, among which the most common are wired channels communications ( air, cable, light guide etc.) and radio channels (tropospheric, satellite and etc.). Such channels, in turn, are usually qualified on the basis of the characteristics of the input and output signals, as well as the change in the characteristics of the signals depending on such phenomena occurring in the channel as fading and attenuation of signals.
According to 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 (at the input of the channel - discrete signals, and at the output - continuous signals).
Channels can be linear and non-linear, temporary and spatio-temporal.
Possible classification communication channels by frequency range .
Information transmission systems are single-channel and multichannel. The system type is determined by the communication channel. If the 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 according to the frequency range used
Ø Kilometer (LW) 1-10 km, 30-300 kHz;
Ø Hectometric (CB) 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 (SMW) 1-10 cm, 3-30 GHz;
Ø Millimeter (MMV) 1-10 mm, 30-300 GHz;
Ø Decimal (DMMV) 0.1-1 mm, 300-3000 GHz.
2. According to the 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 type transmitted messages:
Ø telegraph;
Ø telephone;
Ø data transfer;
Ø facsimile.
4. Type of signals:
Ø analog;
Ø digital;
Ø impulse.
5. By type of modulation (manipulation)
- V analog systems connections:
Ø with amplitude modulation;
Ø with single-sideband modulation;
Ø with frequency modulation.
- In digital communication systems:
Ø with amplitude manipulation;
Ø with frequency shift keying;
Ø with phase keying;
Ø with relative phase shift keying;
Ø with tone keying (single elements manipulate the subcarrier oscillation (tone), after which manipulation is carried out at a higher frequency).
6. By the value of the radio signal base
Ø broadband (B>> 1);
Ø narrowband (B "1).
7. By the number of simultaneously transmitted messages
Ø single-channel;
Ø multichannel (frequency, time, code division channels);
8. By message direction
Ø unilateral;
Ø
bilateral.
9. In order of message exchange
Ø simplex communication- two-way radio communication, in which the transmission and reception of each radio station is carried out in turn;
Ø duplex communication - transmission and reception is carried out simultaneously (the most efficient);
Ø half duplex- refers to the simplex, which provides for automatic transition from transmission to reception and the possibility of questioning the correspondent.
10. By ways of protecting transmitted information
Ø closed communication (secret).
11. According to the degree of automation of information exchange
Ø non-automated - radio control and messaging is performed by the operator;
Ø automated - only information is entered manually;
Ø automatic - the messaging process is carried out between automatic device and computers without the participation of the operator.
Classification No. 3 (something may be repeated):
1. By appointment
Telephone
Telegraph
TV
Broadcasting
2. By direction of transmission
Simplex (transmission in one direction only)
Half duplex (transmission alternately in both directions)
Duplex (transmit simultaneously in both directions)
3. By the nature of the line of communication
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
1. Channel transfer function: presented in the form amplitude-frequency characteristic (AFC) and shows how the amplitude of the sinusoid at the output of the communication channel decays compared to 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 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, convert 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 frequency response, 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, you need to change the frequency of the input sinusoids with a small step, 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 frequency response of the channel
2. Bandwidth: is a derivative characteristic of the frequency response. It is a continuous range of frequencies for which the ratio of the amplitude of the output signal to the input exceeds a certain predetermined limit, that is, the bandwidth determines the frequency range of the signal at which this signal is transmitted over the communication channel without significant distortion. Typically, the bandwidth is measured at 0.7 of the maximum frequency response. The bandwidth to the greatest extent affects the maximum possible speed of information transfer 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 the operation of the channel, 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 sufficient 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 if the attenuation at several frequencies corresponding to several fundamental harmonics of the transmitted signal is known.
Attenuation is usually measured in decibels (dB) and is calculated using the following formula: 
, where
Signal strength at the channel output,
Signal strength 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 "specific attenuation" is always used, i.e. signal attenuation per unit channel length, for example, attenuation of 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 - bps, as well as derived units: Kbps, Mbps, Gbps. The transmission rate depends on the channel bandwidth, noise level, coding type and modulation.
5. Channel noise immunity: characterizes its ability to provide signal transmission under interference conditions. Interference is divided into internal(represents thermal noise of equipment) and external(they are varied and depend on the transmission medium). The noise immunity of the channel depends on the hardware and algorithmic solutions for processing the received signal, which are embedded in the transceiver. Noise immunity signaling through a channel can be increased at the expense coding and special processing signal.
6. Dynamic Range : log ratio maximum power signals transmitted by the channel to the minimum.
7. Noise immunity: this is noise immunity, i.e. noise immunity.
