Communication lines and channels. Communication lines and data transmission channels Communication channels and means

Communication channels (CC) serve for signal transmission and are a common link in any information transmission system.

According to their physical nature, communication channels are divided into mechanical, used for transferring tangible media, acoustic, optical And electric, transmitting sound, light and electrical signals, respectively.

Electrical and optical communication channels, depending on the method of transmitting signals, can be divided into wired, which use physical conductors to transmit signals (electrical wires, cables, light guides), and wireless, which use electromagnetic waves to transmit signals (radio channels, infrared channels).

According to the form of presentation of the transmitted information, communication channels are divided into analog, through which information is transmitted in continuous form, i.e. in the form of a continuous series of values ​​of any physical quantity, and digital, transmitting information presented in the form of digital (discrete, pulsed) signals of various physical natures.

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 alternating transmission of information in both forward and reverse directions; duplex, allowing information to be transmitted simultaneously in the forward and reverse directions.

There are communication channels switched, which are created from separate sections (segments) only for the duration of the transmission of information through them, and upon completion of the transmission such a channel is eliminated (disconnected), and non-switched(dedicated), created for a long time and having constant characteristics in terms of length, throughput, and noise immunity.

Electrical wire communication channels widely used in automated information processing and control systems differ in throughput:

low speed, information transmission speed in which is from 50 to 200 bit/s. 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 Telegraph and Telephony Consultative Committee (ICITT) and from 14400 to 56,000 bps;

high speed(broadband), providing information transmission speeds of over 56,000 bps.

To transfer information to low-speed and medium-speed compressor stations the physical environment is usually wired communication lines: groups of either parallel or twisted wires called twisted pair It consists of insulated conductors twisted together in pairs to reduce both electromagnetic crosstalk and signal attenuation during transmission at high frequencies.

To organize high-speed (broadband) CS, various cables are used:

Shielded with twisted pairs of copper wires;

Unshielded with twisted pairs of copper wires;

Coaxial;

Fiber optic.

STP cables(shielded with twisted pairs of copper wires) have good technical characteristics, but are inconvenient to operate 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 of up to 16.25 and 155 Mbit/s, respectively. These cables have good technical characteristics, are relatively inexpensive, easy to use, and do not require grounding.

Coaxial cable It is a copper conductor coated with a dielectric and surrounded by a revolute of thin copper conductors with a shielding protective sheath. The data transfer speed over coaxial cable is quite high (up to 300 Mbit/s), but it is not easy 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-leading core) with a high refractive index ps, surrounded by low refractive index insulation n 0 and placed in a protective polyethylene shell. In Fig. 8.2, A shows the distribution of the refractive index over the cross-section of the fiber optic cable, and in Fig. 8.2, b- ray propagation diagram. The source of radiation propagated through a fiber optic cable is an LED or semiconductor laser, and 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 guide core, which ensures minimal signal attenuation.

Rice. 8.2. Beam propagation along a fiber optic cable:

A- distribution of the refractive index over the cross-section of the fiber-optic cable;

b - ray propagation diagram

In addition, fiber optic cables provide protection of transmitted information from external electromagnetic fields and high transmission speeds of up to 1000 Mbit/s. Information encoding is carried out using analog, digital or pulse modulation of the light beam. Fiber optic cable is quite expensive and is usually used only for laying important backbone communication channels, for example, a cable laid along the bottom of the Atlantic Ocean connects Europe with America. In computer networks, fiber optic cable is used in the most critical areas, in particular on the Internet. One thick backbone fiber optic cable can simultaneously organize several hundred thousand telephone, several thousand video telephone and about a thousand television communication channels.

High-speed compressor stations are organized on the basis of wireless radio channels.

Radio channel - This is a wireless communication channel laid over the air. To form a radio channel, a radio transmitter and a radio receiver are used. Data transmission rates over a 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 radio wave ranges and their corresponding frequency bands.

For commercial telecommunications systems, the frequency ranges most often used are 902 - 928 MHz and 2.40 - 2.48 GHz.

Wireless communication channels have poor noise immunity, but provide the user with maximum mobility and speed of response.

Telephone lines most branched and widespread. They transmit audio (tone) and fax messages. Information and reference systems, e-mail systems and computer networks were built on the basis of a telephone communication line. Analogue and digital information transmission channels can be created on the basis of telephone lines.

IN analog telephone lines a telephone microphone converts sound vibrations into an analog electrical signal, which is transmitted via the subscriber line to the telephone exchange. The required bandwidth for human voice transmission is approximately 3 kHz (range 300 Hz -3.3 kHz). Call signals are transmitted over the same channel as voice transmission.

IN digital communication channels The analog signal is sampled before input - converted into digital form: every 125 μs (sampling frequency is 8 kHz), the current value of the analog signal is displayed in 8-bit binary code.

Table 8.1

Radio wave ranges and corresponding frequency bands

Control

Communication, communication, radio electronics and digital devices

A communication channel is a system of technical means and a signal propagation medium 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 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 for transmitting a signal over a communication channel.”

Link

Link a system of technical means and a signal propagation environment for transmitting messages (not only data) from a source to a recipient (and vice versa). Communication channel, understood in a narrow sense ( communication path ), represents only the physical signal propagation medium, 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 application programs.

The communication channel includes the following components:

1. transmitting device;
2. receiving device;
3. transmission medium of various physical nature (Fig. 1).

The signal generated by the transmitter and carrying information, after passing through the transmission medium, arrives at the input of the receiving device. Next, the information is separated 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 No. 1)

Fig.2 Communication channel (option No. 2)

Those. this is (channel) technical device (technique + environment).

Classification

There will be exactly three types of classifications. Choose according to taste and color:

Classification No. 1:

There are many types of communication channels, the most common of which arechannels wired communications ( aerial, cable, fiber etc.) and radio communication channels (tropospheric, satelliteand etc.). Such channels, in turn, are usually qualified based on the characteristics of the input and output signals, as well as on changes in the characteristics of the signals depending on such phenomena occurring in the channel as fading and attenuation of signals.

Based on 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 channel continuous signals),
• discrete or digital (discrete signals at the input and output of channel ),
• continuous-discrete (at the channel input there are continuous signals, and at the output there are discrete signals),
• discrete-continuous (at the input of the channel there are discrete signals, and at the output there are continuous signals).

Channels can be like linear and nonlinear, temporary and spatiotemporal.

Possible classification of communication channels by frequency range.

Information transmission systems are single-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 detailing of classification features is used.

Classification No. 2 (more detailed):

1. Classification according to the range of frequencies used

• Kilometer (DV) 1-10 km, 30-300 kHz;
• Hectometric (HW) 100-1000 m, 300-3000 kHz;
• Decameter (HF) 10-100 m, 3-30 MHz;
• Meter (MV) 1-10 m, 30-300 MHz;
• UHF (UHF) 10-100 cm, 300-3000 MHz;
• Centimeter wave (CMW) 1-10 cm, 3-30 GHz;
• Millimeter wave (MMW) 1-10 mm, 30-300 GHz;
• Decimimiter (DMMV) 0.1-1 mm, 300-3000 GHz.
  1. According to the direction of communication lines
     • directed ( different conductors are used):
• coaxial,
• twisted pairs based on copper conductors,
• fiber optic.
  • omnidirectional (radio links);
• line of sight;
• tropospheric;
• ionospheric
• space;
• radio relay (retransmission on decimeter and shorter radio waves).
  1. 
     By type of messages transmitted:
• telegraph;
• telephone;
• data transmission;
• facsimile.
  1. By type of signals:
• analog;
• digital;
• pulsed.
  1. By type of modulation (manipulation)
     • In analog communication systems:
• with amplitude modulation;
• with single-sideband modulation;
• with frequency modulation.
• In digital communication systems:
• with amplitude manipulation;
• with frequency shift keying;
• with phase shift keying;
• with relative phase shift keying;
• with tone keying (single elements manipulate a subcarrier waveform (tone), followed by manipulation at a higher frequency).
  1. According to the radio signal base value
• broadband (B>> 1);
• narrowband (B»1).

7. By the number of simultaneously transmitted messages

• single-channel;
• multi-channel (frequency, time, code division of channels);
  

8. By direction of message exchange

• one-sided;
• bilateral.
  9. By order of message exchange
• simplex communicationtwo-way radio communication, in which the transmission and reception of each radio station is carried out alternately;
• duplex communicationtransmission and reception are carried out simultaneously (the most efficient);
• half-duplex communicationrefers to simplex, which provides for an automatic transition from transmission to reception and the possibility of asking the correspondent again.

10. Methods of protecting transmitted information

• open communication;
• closed communication (classified).

11. According to the degree of automation of information exchange

• non-automated control of the radio station and exchange of messages is performed by the operator;
• automated only information is entered manually;
• automatic the process of messaging is carried out between an automatic device and a computer without operator participation.

Classification No. 3 (something may be repeated):

1. As intended

Telephone

Telegraph

Television

- broadcasting

2. By transmission direction

- simplex (transmission in one direction only)

- half-duplex (transmission alternately in both directions)

- duplex (simultaneous transmission in both directions)

3. According to 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 in both time and level)

5. By 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 formamplitude-frequency response (AFC) And shows how the amplitude of a sinusoid at the output of a communication channel attenuates compared to the amplitude at its input for all possible frequencies of the transmitted signal. The normalized amplitude-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, 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. To experimentally check the amplitude-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 be found in the input signals. Moreover, the frequency of the input sinusoids needs to be changed in small steps, which means the number of experiments should be large.

- ratio of the spectrum of the output signal to the input
Bandwidth

Fig.4 Normalized amplitude-frequency response of the channel

1. Bandwidth: is a derived characteristic from the frequency response. It represents a continuous range of frequencies for which the ratio of the amplitude of the output signal to the input exceeds some predetermined limit, that is, the bandwidth determines the range of signal frequencies at which this signal is transmitted through a communication channel without significant distortion. Typically, the bandwidth is measured at 0.7 from the maximum frequency response value. Bandwidth has the greatest influence on the maximum possible speed of information transmission over a communication channel.
2. Attenuation: is defined as the relative decrease in amplitude or power of a signal when a signal of a certain frequency is transmitted over a channel. Often, when operating a channel, the fundamental frequency of the transmitted signal is known in advance, that is, the frequency whose harmonic has the greatest amplitude and power. Therefore, it is enough to know the attenuation at this frequency to approximately estimate the distortion of the signals transmitted over the channel. More accurate estimates are possible with knowledge of 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.

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 channel length, for example, attenuation 0.1 dB/meter.

1. 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:Kbit/s, Mbit/s, Gbit/s. The transmission speed depends on the channel bandwidth, noise level, type of coding and modulation.
2. Channel noise immunity: characterizes its ability to provide signal transmission in conditions of interference. Interference is usually divided into internal (representsthermal noise of equipment) and external (they are diverse anddepend on the transmission medium). The noise immunity of the channel depends on hardware and algorithmic solutions for processing the received signal, which are embedded in the transceiver device.Noise immunitytransmission of signals through the channelmay be increased due to coding and special processing signal.
3. Dynamic range: logarithm of the ratio of the maximum power of signals transmitted by the channel to the minimum.
4. Noise immunity:This is noise immunity, i.e.e. noise immunity.

Condition for transmitting signals over communication channels.

A channel is essentially a filter. In order 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 given formulas

channel bandwidth, or frequency band that the channel can miss with normal signal attenuation;

dynamic range, equal to the ratio of the maximum permissible signal level in a channel to the level of interference normalized for this type of channel;

time during which the channel is used for data transmission;

the width of the frequency spectrum of the signal, 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;

signal duration, or time of its existence.

Another form of writing a condition (expanded):

P. S .: The “Channel volume” parameter in some sources is also indicated as one of the communication channel parameters, 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 main function of an information system is the storage of information and its transfer in space. The set of technical means for transmitting messages from source to consumer is called a communication system. These means are a transmitting device, a communication line and a receiving device. Sometimes the concept of communication system includes the source and consumer of messages.

The block diagram of the simplest communication system is shown in Figure 2. Here the starting point is the source of the message. The source can produce continuous or discrete messages. The source of messages and the recipient in some communication systems can be a person, in others - various types of devices (automatic machine, computer, etc.). Transmission of messages over a distance is carried out using any material medium (paper, magnetic tape, etc.) or a physical process (sound or electromagnetic waves, current, etc.).

The source of information or message is the physical object, system or phenomenon that forms the message being transmitted.

A message is a value or change in some physical quantity that reflects the state of an object (system or phenomenon). As a rule, primary messages - speech, music, images, environmental measurements, etc., are functions of time - f (t) or other arguments - f (x, y, z) non-electrical nature (acoustic pressure, temperature, brightness distribution on a certain plane, etc.).

Fig.2. Block diagram of the communication system.

Each i - the source message is an arbitrary sequence of alphabetic elements
(
,
, ...,) length m , where the superscript of the elements is the sequence number, and the subscript only means the place of the letter in the message, but not its type.

At m = 1 the message is one letter, that is, there is such a message basic message . In general, when m > 1 the same letter may appear in a message repeatedly. A common property of an elementary message is its indivisibility into smaller messages.

Finite set of messages X c given the probability distribution on it p ( x ) is called a discrete message ensemble and is denoted by ( X , p ( x )}.

A device that converts a message into a signal is called a transmitting device, and a device that converts a received signal into a message is called receiving device.

Using a converter in the transmitting device, the message A, which can have any physical nature (image, sound vibration, etc.), is converted into a primary electrical signal b(t). In telephony, for example, this operation comes down to converting sound pressure into a proportionally varying electrical current of the microphone. In telegraphy, encoding is first performed, as a result of which the sequence of message elements (letters) is replaced by a sequence of code symbols (0, 1 or dot, dash), which is then converted into a sequence of direct current electrical pulses using a telegraph apparatus.

In the transmitter the primary signal b(t) (usually low frequency) is converted into a secondary (high frequency) signal u(t), suitable for transmission over the channel being used. This is done through modulation.

The conversion of a message into a signal must be reversible. In this case, using the output signal, it is possible, in principle, to restore the input primary signal, i.e., to obtain all the information contained in the transmitted message. Otherwise, some information will be lost during transmission, even if the signal reaches the receiving device without distortion.

The physical process that displays (carries) the message being transmitted is called a signal.

A signal is a material and energy form of information representation. In other words, a signal is a carrier of information, one or more parameters of which, changing, display a message.

The information-message-signal chain is an example of the processing required where the source of information is located. On the information consumer side, processing is carried out in the reverse order: “signal – message – information”.

Any transformation of a message into a specific signal by establishing a one-to-one correspondence between them is broadly called encoding.

Coding can include processes of conversion and sampling of continuous messages (analog-to-digital conversion), modulation (manipulation in digital communication systems) and coding itself in the narrow sense of the word. The reverse operation is called decoding.

A communication line is the medium used to transmit signals from a transmitter to a receiver.

In electrical communication systems, this is a cable or waveguide; in radio communication systems, it is a region of space in which electromagnetic waves propagate from the transmitter to the receiver. During transmission, the signal may be distorted and interfere with n(t).

The receiving device processes the received vibration z(t)=u(t)+n(t), which is the sum of the incoming distorted signal u(t) and interference n(t), and restores the message from it , which reflects the transmitted message with some error a. In other words, the receiver must, based on the fluctuation analysis z(t) determine which of the possible messages was transmitted. Therefore, the receiving device is one of the most critical and complex elements of the communication system.

A communication channel is a set of means that ensure the transmission of a signal from a certain point A of the system to point B(Fig. 3).

Points A And IN can be chosen arbitrarily, as long as a signal passes between them. Part of a communications system located up to a point A, is the signal source for this channel.

Rice. 3. Communication channel.

The channel, as a source of interference, has some influence on the transmitted signal. The receiver's task is to isolate the transmitted message from the noisy signal and send it to the consumer.

Communication channels are classified according to various criteria, including mathematical description (continuous and discrete channels, continuous and discrete time).

If the signals arriving at the input of a channel and received from its output are discrete in state, then the channel is called discrete. If these signals are continuous, then the channel is called continuous. There are also discrete-continuous and continuous-discrete channels, the input of which receives discrete signals, and the output receives continuous signals, or vice versa. From the above it is clear that the channel can be discrete or continuous, regardless of the nature of the messages being transmitted. Moreover, in the same communication system, both discrete and continuous channels can be distinguished. It all depends on how the points are chosen A And IN channel input and output.

In this tutorial we will look at discrete communication channel .

If the harmful effect of interference in the channel can be neglected, then a model in the form of an idealized channel called channel without interference. In an ideal channel, each message at the input uniquely corresponds to a certain ratio at the output and vice versa. When the requirements for reliability are high and the ambiguity of the relationship between messages is neglected x And y is unacceptable, a more complex model is used - a channel with noise.

The simplest class of channel models is formed by discrete channels without memory; they are defined as follows. The input is a sequence of letters (elements) from a finite alphabet, let
, output is a sequence of letters of the same or another alphabet, say
. Finally, each letter of the output sequence depends statistically only on the letter at the corresponding position in the input sequence, and is determined by a given conditional probability
, defined for all letters alphabet at the input and all letters at the exit. An example is a binary symmetric channel (Fig. 4), which is a discrete channel without memory with binary sequences at the input and output, in which each symbol of the sequence at the input is reproduced correctly at the output of the channel with a certain probability 1-q and changes with probability q noise to the opposite symbol. In general, in a discrete channel without memory, transition probabilities exhaust all known information about how the input signal, interacting with noise, forms the output signal.

Rice. 4. Binary symmetrical channel.

A much wider class of channels, channels with memory, form channels in which the input signals are sequences of letters from finite alphabets, but in which each output letter can statistically depend not only on the corresponding letter of the input sequence.

"

A communication line and a communication channel are not the same thing.

Communication line(LS) - this is physical environment, through which information signals are transmitted. Several communication channels can be organized in one communication line by means of time, frequency code and other types of division - then they talk about logical (virtual) channels. If a channel completely monopolizes a communication line, then it can be called a physical channel and in this case coincides with the communication line. Although it is possible, for example, to talk about an analog or digital communication channel, it is absurd to talk about an analog or digital communication line, because a line is only a physical medium in which communication channels of various types can be formed. However, even when talking about a physical multi-channel line, it is often called a communication channel. L Ss are an obligatory link in any information transmission system.

Rice. 15. 2. Classification of Communication channels

The classification of communication channels (CC) is shown in Fig. 15. 2. According to the physical nature of drugs and CS based on them, they are divided into:

mechanical - used to transfer material storage media

acoustic - transmit a sound signal;

optical - transmit a light signal;

electric - transmit an electrical signal.

Electrical and optical CS can be:

wired, using wire communication lines (electrical wires, cables, light guides, etc.) to transmit signals;

wireless (radio channels, infrared channels, etc.), using electromagnetic waves propagating over the air to transmit signals.

According to the form of presentation of the transmitted information, CS are divided into:

analog- analogue channels transmit information presented in continuous form, that is, in the form of a continuous series of values ​​of any physical quantity;

digital- information is transmitted via digital channels, presented in the form of digital (discrete, pulsed) signals of one or another physical nature.

Depending on the possible directions of information transfer, there are:

simplex CS that allows information to be transmitted only in one direction;

half duplex CS, providing alternating transmission of information in forward and reverse directions;

duplex CS, allowing the transmission of information simultaneously in both forward and reverse directions.

Finally, communication channels can be:

switched;

non-switchable.

Switched channels are created from separate sections (segments) only for the duration of information transmission through them; upon completion of transmission, such a channel is eliminated (disconnected).

Non-switched(dedicated) channels are created for a long time and have constant characteristics in length, throughput, and noise immunity.

By capacity they can be divided into:

low speed CS, the information transmission speed in which is from 50 to 200 bit/s; these are telegraph CS, both switched (subscriber telegraph) and non-switched;

medium speed CS, for example analog (telephone) CS; the transmission speed in them is from 300 to 9600 bps, and in the new standards V 90-V. 92 International Telegraph and Telephone Consultative Committee (ICITT) and up to 56,000 bps

high speed(broadband) CS providing information transmission speeds above 56,000 bps.

It should be especially noted that telephone CS is narrower than telegraph CS, but the data transmission speed through it is higher due to the mandatory presence of a modem, which significantly reduces F from the transmitted signal. With simple encoding, the maximum achievable data transfer rate over analog channels does not exceed 9600 baud = 9600 bit/s. The complex protocols for encoding transmitted data currently used use not two, but several signal parameter values ​​to display a data element and allow data transmission rates over analog telephone lines to be achieved at 56 kbit/s = 9600 baud.

For digital CS organized on the basis of telephone lines, the data transmission speed, due to a decrease in F s and an increase in H s of the digitized signal, can also be higher (up to 64 kbit/s), and when multiplexing several digital channels into one in such a composite CS, the transmission speed can double, triple, etc.; There are similar channels with speeds of tens and hundreds of megabits per second.

Physical environment information transmission in low- and medium-speed CS are usually wired communication lines: groups of either parallel or twisted (“twisted pair”) wires.

To organize broadband CS, various cables are used, in particular:

unshielded with twisted pairs of copper wires (Unshielded Twisted Pair - UTP);

shielded with twisted pairs of copper wires (Shielded Twisted Pair - STP);

fiber optic (Fiber Optic Cable - FOC);

coaxial (Coaxial Cable - CC);

wireless radio channels.

twisted pair- these are insulated conductors, twisted together in pairs to reduce crosstalk between conductors. Such a cable, usually consisting of a small number of twisted pairs (sometimes even two), is characterized by less signal attenuation when transmitting at high frequencies and less sensitivity to electromagnetic interference than a parallel pair of wires.

UTP cables most often used in data transmission systems, in particular in computer networks. There are five categories of UTP twisted pairs: the first and second categories are used for low-speed data transmission; the third, fourth and fifth - at transmission speeds of up to 16, 25 and 155 Mbit/s, respectively (and when using the Gigabit Ethernet technology standard on twisted pair, introduced in 1999, and up to 1000 Mbit/s). With good technical characteristics, these cables are relatively inexpensive, they are easy to use, and do not require grounding.

STP cables have good technical characteristics, but are high in cost, rigid and inconvenient to operate, and require screen grounding. They are divided into types: Type 1, Type 2, Type 3, Type 5, Type 9. Of these, Type 3 determines the characteristics of an unshielded telephone cable, and Type 5 determines the characteristics of a fiber-optic cable. The most popular cable is Type 1 IBM standard, consisting of two pairs of twisted wires shielded with a conductive braid that is supposed to be grounded. Its characteristics roughly correspond to those of a Category 5 UTP cable.

Coaxial cable It is a copper conductor coated with a dielectric and surrounded by a revolute of thin copper conductors with a shielding protective sheath. Coaxial cables for telecommunications are divided into two groups:

thick coaxials;

thin coaxials.

Thick The coaxial cable has an outer diameter of 12.5 mm and a fairly thick conductor (2.17 mm), providing good electrical and mechanical characteristics. The data transfer speed over a thick coaxial cable is quite high (up to 50 Mbit/s), but given the certain inconvenience of working with it and its significant cost, it cannot always be recommended for use in data networks. Thin The coaxial cable has an outer diameter of 5-6 mm, it is cheaper and more convenient to use, but the thin conductor in it (0.9 mm) causes worse electrical (transmits a signal with acceptable attenuation over a shorter distance) and mechanical characteristics. Recommended data transfer rates over “thin” coax do not exceed 10 Mbit/s.

The basis fiber optic cable constitute "internal subcables" - glass or plastic fibers with a diameter of 5 (single-mode) to 100 (multi-mode) microns, surrounded by a solid filler and placed in a protective sheath with a diameter of 125-250 microns. A single cable may contain from one to several hundred of these "internal subcables". The cable, in turn, is surrounded by filler and covered with a thicker protective sheath, inside of which one or more power elements are laid, which ensure the mechanical strength of the cable.

The optical signal propagates along a single-mode fiber (their diameter is 5-15 microns), almost without being reflected from the walls of the fiber (it enters the fiber parallel to its walls), which ensures a very wide bandwidth (up to hundreds of gigahertz per kilometer). Many signals propagate along a multimode fiber (its diameter is 40-100 microns), each of which enters the fiber at its own angle (its own mode) and, accordingly, is reflected from the walls of the fiber in different places (bandwidth of multimode fiber is 500-800 MHz/ km).

The source of the light beam propagated through the fiber optic cable is a converter of electrical signals into optical signals, for example an LED or a semiconductor laser. Information is encoded by changing the intensity of the light beam. The physical basis for transmitting a light beam along a fiber is the principle of total internal reflection of the beam from the walls of the fiber, which ensures minimal signal attenuation, the highest protection from external electromagnetic fields and high transmission speed. A fiber optic cable with a large number of fibers can carry a huge number of messages. At the other end of the cable, the receiving device converts the light signals into electrical signals. The data transmission speed via fiber optic cable is very high and reaches 1000 Mbit/s, but it is very expensive and is usually used only for laying critical backbone communication channels. Such a cable connects the capitals and major cities of most countries of the world, and a cable laid along the bottom of the Atlantic Ocean connects Europe with America. The fiber optic cable connects St. Petersburg with Moscow, the Baltic and Scandinavian countries, in addition, it is laid in metro tunnels and connects all districts of the city. In computer networks, fiber-optic cable is used in their most critical areas, in particular on the Internet. The possibilities of fiber optic channels are truly limitless: one thick backbone fiber optic cable can simultaneously organize several hundred thousand telephone channels, several thousand video telephone channels and about a thousand television channels.

Radio channel is a wireless communication channel laid over the air. A data transmission system (DTS) over a radio channel includes a radio transmitter and a radio receiver tuned to the same radio wave range, which is determined by the frequency band of the electromagnetic spectrum used for data transmission. Often this SPD is simply called a radio channel. Data transmission rates over a radio channel are practically unlimited (they are limited by the bandwidth of the transmitting and receiving equipment). High-speed radio access provides users with channels with transmission speeds of 2 Mbit/s and higher. In the near future, radio channels with speeds of 20-50 Mbit/s are expected. Table 15. 1 shows the names of radio waves and the corresponding frequency bands.

Table 15. 1. Radio wave bands

For commercial telecommunication systems, the frequency ranges 902-928 MHz and 2.4-2.48 GHz are most often used (in some countries, such as the USA, at low levels of radiation power - up to 1 W - it is allowed to use these ranges without state licensing).

Wireless communication channels have poor noise immunity, but provide the user with maximum mobility and efficiency of communication. In computer networks, wireless communication channels for data transmission are most often used where the use of traditional cable technologies is difficult or simply impossible. But in the near future the situation may change - new Bluetooth wireless technology is being actively developed.

Bluetooth is a technology for transmitting data over radio channels over short distances, allowing communication between wireless phones, computers and various peripherals even in cases where the line of sight requirement is violated.

Commonly used and already quite well-known are the connections of electronic equipment to each other using an infrared communication channel. But these connections require line of sight. For example, the TV remote control cannot be used if there is at least a sheet of newsprint between you and the TV.

Initially Bluetooth was considered solely as an alternative to the use of infrared connections between various portable devices. But now experts predict two directions for the widespread use of Bluetooth. The first direction is home networks, which include various electronic equipment, in particular computers, televisions, etc. The second, much more important direction is local area networks of offices of small companies, where the Bluetooth standard can replace traditional wired technologies.

The disadvantage of Bluetooth is the relatively low data transfer rate - it does not exceed 720 kbps, so this technology is not capable of transmitting a video signal.

Telephone lines are the most extensive and widely used. Telephone communication lines transmit audio (tone) and fax messages; they are the basis for the construction of information and reference systems, e-mail systems and computer networks.

Both analogue and digital information transmission channels can be organized via telephone lines. Let us consider this issue, due to its high relevance, in a little more detail.

The "Primitive Old Telephone System", in the English abbreviation POTS (Primitive Old Telephone System), consists of two parts: the backbone communication system and the subscriber access network to it. The simplest option for subscribers to access the backbone system is to use a subscriber analog communication channel. Most telephone sets are connected to an automatic telephone exchange (PBX), which is already an element of the trunk system, in this way.

A telephone microphone converts sound vibrations into an analog electrical signal, which is transmitted over the subscriber line to the telephone exchange. The required bandwidth for human voice transmission is approximately 3 kHz, ranging from 300 Hz to 3.3 kHz. When you pick up the handset, an “off-hook” signal is generated, informing the telephone exchange about the call, and if the telephone exchange is not busy, the desired telephone number is dialed, which is transmitted to the telephone exchange in the form of a sequence of pulses (with pulse dialing) or in the form of a combination of audio frequency signals (with tone dialing). The conversation ends with an "on-hook" signal generated when the handset is put down. This type of calling procedure is called "in band" because the call signals are transmitted over the same channel as the voice transmission.

Did you know, What is a thought experiment, gedanken experiment?
This is a non-existent practice, an otherworldly experience, an imagination of something that does not actually exist. Thought experiments are like waking dreams. They give birth to monsters. Unlike a physical experiment, which is an experimental test of hypotheses, a “thought experiment” magically replaces experimental testing with desired conclusions that have not been tested in practice, manipulating logical constructions that actually violate logic itself by using unproven premises as proven ones, that is, by substitution. Thus, the main task of the applicants of “thought experiments” is to deceive the listener or reader by replacing a real physical experiment with its “doll” - fictitious reasoning on parole without the physical verification itself.
Filling physics with imaginary, “thought experiments” has led to the emergence of an absurd, surreal, confused picture of the world. A real researcher must distinguish such “candy wrappers” from real values.

Relativists and positivists argue that “thought experiments” are a very useful tool for testing theories (also arising in our minds) for consistency. In this they deceive people, since any verification can only be carried out by a source independent of the object of verification. The applicant of the hypothesis himself cannot be a test of his own statement, since the reason for this statement itself is the absence of contradictions in the statement visible to the applicant.

We see this in the example of SRT and GTR, which have turned into a kind of religion that controls science and public opinion. No amount of facts that contradict them can overcome Einstein’s formula: “If a fact does not correspond to the theory, change the fact” (In another version, “Does the fact not correspond to the theory? - So much the worse for the fact”).

The maximum that a “thought experiment” can claim is only the internal consistency of the hypothesis within the framework of the applicant’s own, often by no means true, logic. This does not check compliance with practice. Real verification can only take place in an actual physical experiment.

An experiment is an experiment because it is not a refinement of thought, but a test of thought. A thought that is self-consistent cannot verify itself. This was proven by Kurt Gödel.