What is the reliability of the communication channel. Telecommunications and communications

State exam

(State examination)

Question No. 3 “Communication channels. Classification of communication channels. Communication channel parameters. Condition for signal transmission over a communication channel ".

(Plyaskin)

Link. 3

Classification. 5

Characteristics (parameters) of communication channels. ten

Condition for signal transmission over communication channels. thirteen

Literature. 14

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 a 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 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, 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 necessary as a carrier of information; it itself does not carry information.

Fig. 1. Communication channel (option number 1)

Fig. 2 Communication channel (option no. 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-guide etc.) 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 non-linear, temporary and spatio-temporal.

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 the 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 Decimitre (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 the keying is carried out at a higher frequency).

6. By the value of the base of the radio signal

Ø broadband (B >> 1);

Ø narrow-band (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 the 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 exchange of messages is performed by the operator;

Ø automated - only information is entered manually;

Ø automatic - the process of message exchange is performed between an automatic device and a computer without the participation of an operator.

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 (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 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 in the form amplitude-frequency characteristic (AFC) and shows how the amplitude of the sinusoid at the output of the communication channel is attenuated 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 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 transformed 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 a certain maximum value that can occur in the input signals. Moreover, it is necessary 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 of the 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 if one knows 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 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.

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, noise level, type of coding and modulation.

5. Channel immunity: characterizes its ability to provide signal transmission in the presence of interference. It is customary to divide the interference into internal(represents thermal noise of the apparatus) and external(they are diverse and depend on the transmission medium). Channel immunity depends on 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 of 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: it is noise immunity, i.e. noise immunity.

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, represents only the physical medium for the propagation of signals, for example, a physical communication line.

From the source of the message (the speaking person), the message (speech) enters the input of the transmitting device (microphone). The transmitting device converts the message into signals that are fed to the input of the communication channel. At the output of the communication channel, the receiving device (telephone capsule), upon the received signal, reproduces the transmitted message, the latter being perceived by the message receiver (listening person). A transmitter, a communication channel, and a receiver form an information transmission system or a communication system.

According to their intended purpose, communication systems differentiate channels of tele-signalization, telemetry, telecontrol (telecommand), telegraph, telephone, sound broadcasting, facsimile, television broadcasting, etc.

Communication channels can take many forms, including storage-compliant channels that can carry messages as soon as a situation arises.

Examples of communication channels include:

• Connection between initiating and terminating circuit nodes
• A buffer on which messages can be placed and received
• Dedicated channel provided by the transmission medium either by physical separation, such as multi-pair cable, or by electrical separation, such as frequency division multiplexing or time division multiplexing
• The path for the movement of an electrical or electromagnetic signal is usually different from other parallel paths
• Part of the recording medium, such as a track or group of tracks, that allows reading or writing to a station or sound device
• In communication systems, the part that connects the data source and the data sink
• A specific radio frequency, pair or range of frequencies, usually indicated by a letter, number or code word and often allocated by international agreement
• A space in the Internet Relay Chat (IRC) network in which participants can communicate with one another

All of these communication channels share the property that they carry information that is carried across the channel by a signal.

An example of a communication channel is a specific radio frequency, a pair of frequencies, or a range of frequencies, usually indicated by a letter, number or code word and often allocated by international agreement. Marine VHF radio uses some 88 channels in the VHF band for bi-directional frequency modulated voice communications. Channel 16, for example, means 156.800 MHz.

Television channels are located at a frequency, the determining physical quantity of which is megahertz (MHz). Each channel is 6 MHz wide. In addition to these physical channels, television also has virtual channels. Wi-Fi (wireless network) is a communication channel consisting of unlicensed channels 1-13 in the range from 2412 MHz to 2484 MHz in 5 MHz steps.

Specifications

Uses the following channel characteristics

Immunity

Interference immunity A = 10 lg ⁡ P m i n s i g n a l P n o i s e (\ displaystyle A = 10 \ lg (P_ (min ~ signal) \ over P_ (noise)))... Where P m i n s i g n a l P n o i s e (\ displaystyle (P_ (min ~ signal) \ over P_ (noise)))- the minimum signal-to-noise ratio;

Channel volume

Channel volume V (\ displaystyle V) determined by the formula: V k = Δ F k ⋅ T k ⋅ D k (\ displaystyle V_ (k) = \ Delta F_ (k) \ cdot T_ (k) \ cdot D_ (k)),

where T k (\ displaystyle T_ (k))- the time during which the channel is busy with the transmitted signal;

For signal transmission through the channel without distortion, the channel volume V k (\ displaystyle V_ (k)) must be greater than or equal to the volume of the signal V s (\ displaystyle V_ (s)), that is . The simplest case of fitting the signal volume into the channel volume is to achieve the fulfillment of the inequalities Δ F k ⩾ Δ F s (\ displaystyle \ Delta F_ (k) \ geqslant ~ \ Delta F_ (s)), T k ⩾ T s (\ displaystyle T_ (k) \ geqslant ~ T_ (s))> and Δ D k ⩾ Δ D s (\ displaystyle \ Delta D_ (k) \ geqslant ~ \ Delta D_ (s))... Nevertheless, V k ⩾ V s (\ displaystyle V_ (k) \ geqslant ~ V_ (s)) can be performed in other cases, which makes it possible to achieve the required channel characteristics by changing other parameters. For example, as the frequency range decreases, the bandwidth can be increased.

Classification

There are many types of communication channels, among which wire communication channels (air, cable, fiber-optic, etc.) and radio communication channels (tropospheric, satellite, etc.) are most often distinguished. 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 propagation medium, communication channels are divided into wired, acoustic, optical, infrared and 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 linear and non-linear, temporal and space-time. It is possible to classify communication channels by frequency range.

Communication channel models

The communication channel is described by a mathematical model, the task of which is reduced to the definition of mathematical models of the output and input and S 1 (\ displaystyle S_ (1)), as well as establishing a connection between them, characterized by the operator L (\ displaystyle L), that is

S 2 = L (S 1) (\ displaystyle S_ (2) = L (S_ (1))).

Continuous channel models

Continuous channel models can be classified into a channel model with additive Gaussian noise, a channel model with undefined signal phase and additive noise, and a channel model with intersymbol interference and additive noise.

Ideal channel model

The ideal channel model is used when interference can be neglected. When using this model, the output signal is S 2 (\ displaystyle S_ (2)) is deterministic, that is

S 2 (t) = γ S 1 (t - τ) (\ displaystyle S_ (2) (t) = \ gamma ~ S_ (1) (t- \ tau))

where γ is a constant that determines the transmission coefficient, τ is a constant delay.

Channel model with undefined signal phase and additive noise

The channel model with an undefined signal phase and additive noise differs from the ideal channel model in that τ (\ displaystyle \ tau) is a random variable. For example, if the input signal is narrowband, then the signal S 2 (t) (\ displaystyle S_ (2) (t)) at the output of a channel with an undefined signal phase and additive noise is determined as follows:

S 2 (t) = γ (cos (θ) u (t) - sin (θ) H (u (t)) + n (t) (\ displaystyle S_ (2) (t) = \ gamma (cos (\ theta) u (t) -sin (\ theta) H (u (t)) + n (t)),

where it is taken into account that the input signal S 1 (t) (\ displaystyle S_ (1) (t)) can be represented as:

S 1 (t) = cos (θ) u (t) - sin (θ) H (u (t)) (\ displaystyle S_ (1) (t) = cos (\ theta) u (t) -sin (\ theta) H (u (t))),

where H () (\ displaystyle H ())- Hilbert transform, θ (\ displaystyle \ theta)- a random phase, the distribution of which is usually considered uniform over the interval

ISI and Additive Noise Channel Model

The model of a channel with intersymbol interference and additive noise takes into account the appearance of signal scattering in time due to the nonlinearity of the phase-frequency response of the channel and the limited bandwidth of the channel, that is, for example, when transmitting discrete messages through the channel, the value of the output signal will be affected by the channel responses not only the transmitted character, but also for earlier or later characters. In radio channels, the occurrence of intersymbol interference is influenced by the multipath propagation of radio waves.

Network technologies, communication channels and their main characteristics.

C ate:

Teach the basics of networking technology.

Develop cognitive interest.

To foster an information culture.

P checking homework.

X lesson od:

Network technology is a consistent set of standard protocols and software and hardware (for example, network adapters, drivers, cables and connectors), sufficient to build a computer network.
Today the Internet is the interconnection of a large number of networks. Each network consists of tens and hundreds of servers. The servers are directly interconnected by various communication lines: cable, terrestrial radio communication, satellite radio communication. A large number of computers and local computer networks, which are network clients, are connected to each server. Clients can connect to the server not only via direct lines, but also via regular telephone channels.
Communication channels refers to the technical means that allow the transmission of data at a distance. In the context we are considering, communication channels will be calledcommunication means for transferring information between remote computers ... Conventional communication channels (telephone, telegraph, satellite, etc.) can be used as technical means of transmitting information. Nowadays, communication channels built specifically for the transmission of digital information are considered to be more progressive means. These include, for example, fiber optic networks.

The main characteristics of communication channels arethroughput andnoise immunity ... Throughput reflects the ability of a channel to transmit a given number of messages per unit of time. This parameter depends on the physical properties of the communication channel. In other words,throughput is the amount of data transmitted by the modem per unit of time, excluding additional service information, such as start and stop bits, initial end records of Stokes, etc.
Immunity sets the parameter of the level of distortion of the transmitted information. In order to avoid changes or loss of information during its transmission, special methods are used to reduce the effect of noise.

Classification of computer communication channels:

by coding method:digital andanalog ;

by the way of communication:allocated (persistent connection) anddial-up (temporary connection);

by signal transmission method:cable (twisted pair, coaxial cable, fiber optic, optical (fiber), radio relay, wireless, satellite;telephone , radio (radio relay, satellite).

Twisted pair consists of two insulated wires twisted together. Twisting the wires reduces the effect of external electromagnetic fields on the transmitted signals.

Coaxial cable in comparison with twisted pair, it has a higher mechanical strength, noise immunity.

Fiber optic cable - an ideal transmission medium, it is not affected by electromagnetic fields and itself has practically no radiation.

Communication lines:
Radio relay communication lines (RRL) are designed to transmit signals in the ranges of decimeter, centimeter and millimeter waves. The transmission is carried out through a system of repeaters located at a line-of-sight distance.

Wireless networking equipment is designed to transmit information over radio channels between computers, network and other specialized devices.

Satellite communication lines operate in 9-11 frequency ranges and, in the future, in optical ranges. In these systems, the signal from the earth station is sent to a satellite containing transceiver equipment, where it is amplified, processed and sent back to the earth, providing communication over long distances and covering large areas.

Communication channels are divided intosimplex andduplex ... In one case, information is transmitted in only one direction, which is a less efficient means. In another case, information is transmitted in two directions, and several messages can be transmitted simultaneously.

As a physical process that transfers data at a distance, usesignals ... This process can be influenced by various phenomena that createinterference (for example, it can be a voltage of an extraneous origin that appears in communication channels and limits the transmission range of useful signals).

Depending on the source of occurrence and on the nature of their impact, interference is divided into:

own communication channel interference;

mutual created by the influence of channels on each other;

external - from extraneous electromagnetic fields.

Practice has shown that getting rid of noise (interference) is impossible due to natural (unavoidable) causes of their occurrence. Then the idea of ​​searching for the possibility of protection in the transmitted text itself was proposed (K.E. Shannon). The best way was to use redundant code. The function of protecting information during transmission over communication channels includes three components:the confirmation , error detection andnotification about them, return in original condition. The information is encoded accordingly, along with the main content, information about the size of the transmitted information is transmitted. When information is received, the information about the length of the message is checked against the initial state; if the values ​​do not match, a signal is sent to the information transfer point about the need to re-send.

Proxy server - an intermediate, transit web server used as an intermediary between the browser and the final web server. The main reason for using a proxy server is to save the volume of information transfer and increase the access speed due to caching. For example, if most of the company's employees often use the same web server containing the current exchange rate, this information will be stored in a proxy, and thus pages will be requested from the original server only 1 time. When using a proxy, the company only needs one public IP address.

Protocol - a set of rules governing the format and procedures for the exchange of information between two independent processes or devices.

Network protocol - a set of rules and agreements used in the transfer of data.

There are three main types of protocols that work in different networks and with different operating systems: Novell IPX (Inter Packet Exchange), TCP / IP, NetBEUI (Network BIOS User Interface).
Transmission Control Protocol / Internet Protocol (TCP / IP) is a set of protocols developed for the Internet and became its foundation. TCP guarantees that every byte sent reaches its destination without loss. IP assigns local IP addresses to physical network addresses, thereby providing the address space with which routers operate.

The TCP / IP family includes:

Telnet protocol, which allows remote terminals to connect to remote hosts (computers);

domain addressing system DNS, which allows users to address network nodes using a symbolic domain name instead of a numeric IP address;

file transfer protocol FTP, which defines the mechanism for storing and transferring files;

Hypertext Transfer Protocol HTTP.

Questions and tasks

What is called network technology?

What are communication channels?

What are the main characteristics of communication channels?

Give the classification of communication channels.

What is a proxy server?

What are protocols?

What is the function of TCP / IP?

Homework : synopsis.

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

By their physical nature, communication channels are divided into mechanical, used for the transmission of tangible media, acoustic, optical and electrical transmitting sound, light and electrical signals, respectively.

Electrical and optical communication channels, depending on the method of signal transmission, can be subdivided into wired, using physical conductors for signal transmission (electrical wires, cables, optical fibers), and wireless, using electromagnetic waves for signal transmission (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 a continuous form, i.e. in the form of a continuous series of values ​​of some physical quantity, and digital, transmitting information presented in the form of digital (discrete, impulse) signals of various physical nature.

Depending on the possible directions of information transmission, 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 backward directions; duplex, allowing the transmission of information simultaneously in the forward and backward directions.

Communication channels are dial-up, which are created from separate sections (segments) only for the time of transmission of information through them, and at the end of the transmission, such a channel is eliminated (disconnected), and non-commutated(highlighted), created for a long time and having constant characteristics in length, bandwidth, noise immunity.

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

low speed, information transfer rate in which 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 bit / s, and in the new standards V.32 - V.34 of the International Consultative Committee on Telegraphy and Telephony (CCITT) and from 14400 to 56000 bit / s;

high speed(broadband), providing information transfer rates in excess of 56,000 bit / s.

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

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

Shielded with twisted pairs of copper wires;

Unshielded with twisted-pair copper wires;

Coaxial;

Fiber optic.

STP cables(shielded with twisted-pair copper wires) have good technical characteristics, but are inconvenient to use and expensive.

UTP cables(unshielded with twisted pairs of copper wires) are 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 transfer rates 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 is a copper conductor, covered with a dielectric and surrounded by a twisted of thin copper conductors with a shielding protective sheath. The data transfer rate over coaxial cable is 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 strand) with a high refractive index n s, surrounded by low-refractive insulation n 0 and placed in a protective polyethylene sheath. In fig. 8.2, a shows the distribution of the refractive index over the cross-section of the fiber-optic cable, and Fig. 8.2, b- ray propagation scheme. A light-emitting diode or a semiconductor laser is a source of radiation propagated through a fiber-optic cable, a radiation receiver is a photodiode, which converts light signals into electrical ones. 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, due to which the minimum signal attenuation is ensured.

Rice. 8.2. Fiber Optic Beam Propagation:

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

b - ray propagation scheme

In addition, fiber-optic cables provide protection of transmitted information from external electromagnetic fields and high transmission rates up to 1000 Mbps. Information is encoded using analog, digital or pulse modulation of a light beam. Fiber-optic cable is quite expensive and is usually used only for laying critical trunk 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, in 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 CS are organized on the basis of wireless radio channels.

Radio channel - it is a wireless communication channel laid over the air. To form a radio channel, a radio transmitter and a radio receiver are used. The data transmission rates over the radio channel are practically limited by the bandwidth of the transceiver equipment. The radio wavelength range is determined by the frequency band of the electromagnetic spectrum used for data transmission. Table 8.1 shows the ranges of radio waves and the corresponding frequency bands.

For commercial telecommunication systems, the most commonly used frequency bands 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 responsiveness.

Telephone communication lines the most ramified and widespread. They carry out the transmission of audio (tone) and fax messages. Information systems, e-mail systems and computer networks were built on the basis of the telephone line. On the basis of telephone lines, analog and digital channels of information transmission can be created.

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

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

Table 8.1

Radio wave ranges and corresponding frequency bands