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Channels and means of communication. Cable communication lines. What are the types?

Data transmission medium - communication lines (or channels) over which computers can exchange information.

If the network topology is not fully connected, different nodes are forced to use the same communication lines to transmit their data. In Fig. nodes A and B use common channel to send messages to the node IN, that is, the data transmission medium is used by several devices or network nodes. In this case the environment is called shared.

The computer is connected to the shared environment using a network adapter.

Depending on the data transmission medium used, communication lines are divided into:

Wired;

Cable;

Wireless.

Wired communication lines are built using telephone or telegraph wires. Such an environment has low data transfer rates and noise immunity, so when building a network, if possible, they prefer to use cable or radio.

However, at the moment there are quickly emerging technologies, allowing the use of electrical wires as communication lines. What makes such technologies attractive is the possibility of using already laid wires. Through these wires, energy is supplied to houses, apartments, offices, enterprises, etc., and information exchange can also take place in parallel.

Cable lines are built on the basis special cables, which are conductors enclosed in several layers of insulation.

Produced by industry big number types of cable, but three main types are used to build computer networks:

High frequency coaxial cables with copper core;

Cables based twisted pairs copper conductors;

Fiber optic (or fiber optic) cables. The following parameters are typical for cables:

Bandwidth - frequency range of signals transmitted by the cable;

Signal propagation delay;

Noise immunity of a cable - the degree of protection of a cable from interference and interference arising both in external environment, and on the internal conductors of the cable itself;

Attenuation is the degree of loss of signal power at the output of a communication line relative to the power at the input of this line.

Characteristic impedance (for electrical cables) - the total resistance that an electromagnetic wave encounters certain frequency when propagating along a homogeneous chain.

Coaxial cable - This is a central copper conductor enclosed in a metal braid (screen) and separated from it by a dielectric. The metal braid is usually covered with an outer insulating sheath (Fig. 1.). It serves to transmit information and also protects the internal core of the cable from interference caused by outsiders. electromagnetic fields, i.e. screens it.


Most often, coaxial cable is used in networks with a “common bus” topology.

Rice. 1. "Fat" (A) and "thin" (b) coaxial cable:

/ - central conductor; 2 - shielding braid; 3 - insulating

shell; 4 - dielectric

"Thick" coaxial cable (RG-8, RG-11) has

· characteristic impedance 50 Ohm,

diameter of central copper wire 2.17 mm

· external diameter is about 10 mm.

"Thin" coaxial cable (RG-58) has

· characteristic impedance 50 ohm

The diameter of the inner conductor is 0.89 mm

· external diameter is about 5 mm.

Cable on based on twisted pairs - These are several pairs of insulated copper conductors twisted in pairs and enclosed in a common dielectric shell.

Such a cable can be shielded (STP) or unshielded (UTP). In a shielded cable, each twisted pair placed in a metal braid. This helps to increase the noise immunity of the communication line, as well as improve protection against eavesdropping.

Unshielded twisted pair cables are currently the primary data transmission medium for non-optical technologies.

Depending on their characteristics, twisted pair cables are divided into five categories:

Category 3 cables(UTP 3) have a bandwidth of 16 MHz. Used for both data and voice transmission, so currently cable systems Many buildings are built on cable of the third category.

Category 4 cables(UTP 4) - an improved version of category 3 cable, 20 MHz bandwidth, increased noise immunity and low losses. In practice, it was rarely used, mainly where it was necessary to increase the length of a network segment.

Category 5 cables(UTP 5) are specially designed to support high speed technologies. Cable bandwidth category 5-100 MHz. Category 5 cable has now replaced Category 3 cable, and all new LAN technologies rely on it.

A special place is occupied cables of categories 6 and 7, which are released relatively recently and have a bandwidth of 200 and 600 MHz, respectively. Category 7 cables must be shielded; Category 6 may or may not be shielded. They are used in high-speed networks over longer lengths than Category 5 cables. These cables are significantly more expensive and are close in cost to fiber optic cables.

Fiber optic cable consists of one or more optical fibers (light guides) made of quartz glass and enclosed in a common protective shell.

-

Rice. 2. Types of fiber optic cable:

A - multimode fiber with a step change in refractive index; b - multimode fiber with a smooth change in refractive index; V - single mode fiber; 1 - mode 1; 2 - mode 2; 3 - glass shell; 4 - core

Each light guide consists of a glass core (central conductor) with a high refractive index, and a glass shell with a low refractive index. Due to this, light rays spread across the core, successively reflecting from the inner boundary of the glass shell.

Depending on the nature of light propagation, fiber optic cable is divided into (Fig. 2):

Single mode fiber;

Multimode fiber with step change in refractive index;

Multimode fiber with a smooth change in refractive index.

Beam fashion is the angle of reflection of the beam in the core.

Single-mode cables use cores with a very small diameter - 8-9 microns, which is comparable to the wavelength of light, so only one mode can exist in such a cable.

Single-mode 9/125 µm cable is common on the market. In this designation, 9 µm corresponds to the diameter of the optical fiber core, and 125 µm to the diameter of the glass cladding.

Producing glass fibers with such a small diameter is difficult technological process, which makes single-mode cable quite expensive. However, its characteristics, compared to cheaper multimode cables, are significantly higher, which makes it possible to use it when transmitting data over long distances.

Multimode fiber uses wider cores, making it cheaper than single-mode cable. The most common multimode cables are 50/125 µm and 62.5/125 µm. In a core of this diameter, light can travel along different paths, reflecting at different angles - there is more than one mode of beam.

Multiple modes lead to transmission pulse dispersion and beam interference, which ultimately leads to deterioration of cable performance. Therefore, multimode cables are mainly used for data transmission to short distances(up to 2000 m) at speeds of no more than 1 Gbit/s.

When transmitting over fiber optic cables, semiconductor lasers or LEDs are used as the light source. The wavelength of the light they emit is usually 850, 1300 or 1550 nm, which corresponds to certain “transparency windows” of the fiber itself.

Topic 1.4: Basics of local networks

Topic 1.5: Basic technologies of local networks

Topic 1.6: Basic software and hardware components of a LAN

Local networks

1.2. Environment and methods of data transmission in computer networks

1.2.2. Communication lines and data channels

To build computer networks, communication lines are used that use different physical media. The following physical media are used in communications: metals (mainly copper), ultra-transparent glass (quartz) or plastic and ether. The physical transmission medium may be twisted pair cable, coaxial cable, fiber optic cable, and the surrounding environment.

Communication lines or data lines are intermediate equipment and physical media over which data are transmitted. information signals(data).

Several communication channels (virtual or logical channels) can be formed in one communication line, for example, by frequency or time division of channels. A communication channel is a means of one-way data transfer. If a communication line is exclusively used by a communication channel, then in this case the communication line is called a communication channel.

A data transmission channel is a means of two-way data exchange, which includes communication lines and data transmission (reception) equipment. Data transmission channels connect information sources and information receivers.

Depending on the physical medium of data transmission, communication lines can be divided into:

  • wired communication lines without insulating and shielding braids;
  • cable, where communication lines such as twisted pair cables, coaxial cables or fiber optic cables are used to transmit signals;
  • wireless (terrestrial and satellite radio channels) using for signal transmission electromagnetic waves, which spread over the airwaves.

Wired communication lines

Wired (overhead) communication lines are used to transmit telephone and telegraph signals, as well as to transmit computer data. These communication lines are used as trunk communication lines.

Analog and digital channels data transmission. Transmission speed over wire lines is "plain old" telephone line"(POST - Primitive Old Telephone System) is very low. In addition, the disadvantages of these lines include noise immunity and the possibility of simple unauthorized connection to the network.

Cable communication lines

Cable communication lines have a rather complex structure. A cable consists of conductors enclosed in several layers of insulation. IN computer networks Three types of cables are used.

twisted pair(twisted pair) - a communication cable, which is a twisted pair of copper wires (or several pairs of wires) enclosed in a shielded sheath. Pairs of wires are twisted together to reduce interference. Twisted pair cable is quite noise-resistant. There are two types of this cable: UTP unshielded twisted pair and STP shielded twisted pair.

This cable is characterized by ease of installation. This cable is the cheapest and most common type of communication, which is widely used in the most common local networks with Ethernet architecture, built on a star topology. The cable connects to network devices using an RJ45 connector.

The cable is used to transmit data at speeds of 10 Mbit/s and 100 Mbit/s. Twisted pair cable is usually used for communication over a distance of no more than a few hundred meters. The disadvantages of twisted pair cable include the possibility of simple unauthorized connection to the network.

Coaxial cable(coaxial cable) is a cable with a central copper conductor that is surrounded by a layer of insulating material in order to separate the central conductor from the outer conductive shield (copper braid or layer of aluminum foil). The outer conductive screen of the cable is covered with insulation.

There are two types of coaxial cable: thin coaxial cable with a diameter of 5 mm and thick coaxial cable with a diameter of 10 mm. A thick coaxial cable has less attenuation than a thin one. The cost of coaxial cable is higher than the cost of twisted pair and network installation is more difficult than with twisted pair.

Coaxial cable is used, for example, in local networks with Ethernet architecture, built using a “common bus” topology.

Coaxial cable is more noise-resistant than twisted pair and reduces its own radiation. Bandwidth – 50-100 Mbit/s. The permissible length of the communication line is several kilometers. Unauthorized connection to coaxial cable is more difficult than to twisted pair cable.

Cable fiber optic communication channels. Fiber optic is a silicon or plastic based optical fiber encased in a low refractive index material that is enclosed by an outer sheath.

Optical fiber transmits signals in only one direction, so the cable consists of two fibers. At the transmitting end of the fiber optic cable, conversion of the electrical signal into light is required, and at the receiving end, the reverse conversion is required.

The main advantage of this type of cable is its extremely high level of noise immunity and absence of radiation. Unauthorized connection is very difficult. Data transfer speed 3Gbit/s. The main disadvantages of fiber optic cable are the complexity of its installation, low mechanical strength and sensitivity to ionizing radiation.

Wireless (terrestrial and satellite radio channels) data transmission channels

Radio channels of terrestrial (radio relay and cellular) and satellite communications are formed using a transmitter and receiver of radio waves and belong to the technology wireless transmission data.

Radio relay data transmission channels

Radio relay communication channels consist of a sequence of stations that are repeaters. Communication is carried out within line of sight, the range between neighboring stations is up to 50 km. Digital radio relay lines communications (CRRS) are used as regional and local systems communications and data transmission, as well as for communication between cellular base stations.

Satellite data links

IN satellite systems Microwave antennas are used to receive radio signals from ground stations and relay these signals back to ground stations. Satellite networks use three main types of satellites, which are located on geostationary orbits, medium or low orbits. Satellites are usually launched in groups. Spaced apart from each other, they can provide coverage of almost the entire surface of the Earth. Job satellite channel data transmission is shown in the figure


Rice. 1.

It is more expedient to use satellite communications to organize a communication channel between stations located at very long distances, and the ability to serve subscribers in the most inaccessible points. The throughput is high – several tens of Mbit/s.

Cellular data channels

Cellular radio channels are built on the same principles as cellular telephone networks. Cellular communication is a wireless telecommunications system consisting of a network of terrestrial base transceiver stations and a cellular switch (or mobile switching center).

Base stations are connected to a switching center, which provides communication both between base stations and with other telephone networks and with global network Internet. In terms of its functions, the switching center is similar to a regular PBX wired communication.

LMDS (Local Multipoint Distribution System) is a standard for cellular networks for wireless information transmission for fixed subscribers. The system is built according to cellular principle, one base station allows you to cover an area with a radius of several kilometers (up to 10 km) and connect several thousand subscribers. The BSs themselves are connected to each other by high-speed terrestrial communication channels or radio channels. Data transfer speed up to 45 Mbit/s.

WiMAX radio data channels(Worldwide Interoperability for Microwave Access) are similar to Wi-Fi. WiMAX, unlike traditional radio access technologies, also operates on a reflected signal, beyond line of sight base station. Experts believe that mobile networks WiMAX opens up much more interesting prospects for users than fixed WiMAX, intended for corporate customers. Information can be transmitted over distances of up to 50 km at speeds of up to 70 Mbit/s.

MMDS radio data channels(Multichannel Multipoint Distribution System). These systems are capable of serving an area within a radius of 50-60 km, while direct visibility of the operator’s transmitter is not necessary. The average guaranteed data transfer rate is 500 Kbps - 1 Mbps, but up to 56 Mbps per channel can be provided.

Radio data transmission channels for local networks. The wireless communication standard for local area networks is Wi-Fi technology. Wi-Fi provides connection in two modes: point-to-point (for connecting two PCs) and infrastructure connection (for connecting several PCs to one access point). Data exchange speed is up to 11 Mbit/s with a point-to-point connection and up to 54 Mbit/s with an infrastructure connection.

Bluetooht radio data channels is a technology for transmitting data over short distances (no more than 10 m) and can be used to create home networks. Data transfer speed does not exceed 1 Mbit/s.

Types of data transmission channels and their characteristics

Used in computer networks Data transmission channels are classified according to a number of characteristics.

Firstly, according to the form of information presented in the form of electrical signals, channels are divided into digital and analog.

Secondly, according to the physical nature of the data transmission medium, communication channels are distinguished: wired (usually copper), optical (usually fiber optic), wireless (infrared and radio channels).

Third, according to the method of dividing the medium between messages, the above-mentioned channels with time division (TDM) and frequency division (FDM) are distinguished.

One of the main characteristics of a channel is its throughput(information transmission rate), determined by the channel bandwidth and the method of encoding data in the form of electrical signals. Information speed is measured by the number of bits of information transmitted per unit of time. Along with information they operate baud (modulation) speed , which is measured in butts , that is, the number of changes in a discrete signal per unit time. It is the baud rate that is determined by the bandwidth of the line. If one change in the value of a discrete signal corresponds to several bits, then the information rate exceeds the baud rate.

Indeed, if at the baud interval (between adjacent signal changes) N bit, then the number of signal gradations is 2 N. For example, with a number of gradations of 16 and a speed of 1200 baud, one baud corresponds to 4 bits/s and the information speed is 4800 bits/s.
As the length of the communication line increases, the attenuation of the signal increases and, consequently, the bandwidth and information speed decrease.

Maximum possible information speed V related to bandwidth F communication channel using the Hartley-Shannon formula. It is assumed that one change in signal value occurs per log 2 k bit where k– number of possible discrete signal values. Since the speed V= log 2 k/t, Where t- duration of transient processes approximately equal to 3 T B, a T B = 1 / (2pF), then:

V= 2F log 2 k, bit/s,

Where k≤ 1+A(A – signal/noise ratio).

Wired communication lines in computer networks they are represented by coaxial cables and twisted pairs of wires.

The following coaxial cables are used: “thick” with a diameter of 12.5 mm and “thin” with a diameter of 6.25 mm. A “thick” cable has less attenuation and better noise immunity, which makes it possible to work over long distances, but it bends poorly, which makes it difficult to lay connections indoors, and is more expensive than a “thin” one.

There are shielded STP(Shielded Twist Pair) and unshielded UTP(Unshielded Twist Pair) pairs of wires. More often, unshielded pairs are used, which have several categories (types).

More advanced are unshielded twisted pairs of categories 5 and 6. A pair of category 5 is used at frequencies up to 100 MHz. The conductor in it is made of copper conductors with a diameter of 0.51 mm, wound using a certain technology and enclosed in a heat-resistant insulating sheath. Connection lengths in high-speed UTP LANs usually do not exceed 100 m.

Examples of category 6 and 7 pairs are cables manufactured by PIC. They contain 4 pairs of wires, each with its own color of polyethylene insulation. In Category 6 cable, the sheath has a diameter of 5 mm and the copper conductors have a diameter of 0.5 mm. The attenuation in this cable at 100 MHz is about 22 dB. In a category 7 cable, each pair is additionally enclosed in shielding aluminum foil, the sheath diameter is increased to 8 mm, the attenuation at 100 MHz is about 20 dB, at 600 MHz - 50 dB.

Twisted pair cables are sometimes called balanced line in the sense that the two wires of the line transmit the same signal levels (relative to ground), but with different polarities. During reception, a signal difference is perceived, called a paraphasic signal. Common mode interference is self-compensating.

Fiber optic communication lines(FOCL) have a significant advantage over wire lines. They are indispensable for transmitting information over long distances, as well as in high-speed backbone channels of corporate and territorial networks.

Structurally, the fiber-optic line is a quartz core with a diameter of 10 microns, covered with a reflective shell with an outer diameter of 125...200 microns. Typical characteristics of fiber-optic lines are operation at waves of 0.83...1.55 microns, attenuation 0.7 dB/km, frequency band up to 2 GHz.

Limit distances D for data transmission via fiber-optic lines
(without relay) depend on the radiation wavelength l: at l= 850 nm
D= 5 km, and at l= 300 nm – D= 50 km. However, as the radiation wavelength decreases, the cost of the equipment increases significantly.

An example of a data transfer medium between mainframes, workstations, pools peripheral devices can serve as a Fiber Channel environment on a fiber-optic line, providing speeds from 133 to 1062 Mbit/s at distances up to 10 km. For comparison - according to the standard SCSI interface speed is 160 Mbit/s at distances of no more than tens of meters between workstation and a disk drive.

New standards for high-speed data transmission include the digital synchronous hierarchy standard SDH(Synchronous Digital Hierachy). In SDH networks, fiber-optic lines are used as data transmission lines. The standard establishes the structure of frames into which the stream of transmitted data is divided. This structure is called a transport module.

Test

Communication, communication, radio electronics and digital devices

Communication channel - system technical means and a signal propagation medium for transmitting messages (not just data) from source to destination (and vice versa). Communication channel, understood in in the narrow sense(communication path), represents only the physical medium of signal propagation, for example, physical line communications.

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 to be presented to the user (person), 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 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 (channel) technical device(technology + 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 arewired channels 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 appearance transmitted messages:
  • 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 systems communications:
  • 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;
  • multichannel (frequency, time, code division channels);


8. By direction of message exchange

  • one-sided;
  • bilateral.
    9. By order of message exchange
  • simplex communication two-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 communication refers 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

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 messaging process is carried out between automatic device and computers 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 both in 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 transmitted signal. The normalized amplitude-frequency response of the channel is shown in Fig. 4. Knowledge of amplitude-frequency response 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 amplitude-frequency characteristic, it is necessary to test the channel with reference (equal in amplitude) sinusoids over the entire frequency range from zero to some maximum value that may occur in 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 bps , 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 ratio the maximum permissible signal level in the 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. Mathematical formula 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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To organize data transfer you must use lines and communication channels, which communicate between computers, telephones, telegraphs and other means of communication.

The transmitted information is located in a physical medium, which may consist of various types cables and wires, as well as the surrounding area.

What is the difference between communication channels and communication lines?

Despite the fact that both concepts are often identified, they have some differences that you need to know about to build a correct information communication. Through channels, communication is transmitted in one direction or in two if the exchange occurs between the receiver and the transmitter. Communication lines, in turn, are formed from the connection of several channels; they can also have only one channel.

The following communication lines exist:

  • Wired;

  • Cable;

  • Wireless.

Let's take a closer look at each type of line and learn about their capabilities, advantages and disadvantages.

Wired (overhead) communication lines

These lines can be used to carry telegraph, telephone or computer signals. They consist of wires through which data is exchanged. This type of communication is suitable for transmitting digital and analog signals, because its popularity is quite high.

The disadvantages of such a connection include the relatively low signal transmission speed and low degree of immunity from interference. It is also possible for unscrupulous subscribers to simply connect without permission, which leads to a decrease in the quality of data transmission and financial losses for broadcasters.

Cable communication lines

The structure of the cable may be different, but basically they all consist of groups of conductors that are treated with reliable insulation.

The following types of cables are used for data exchange in computer networks:

  • Twisted pair - consists of two wires made of copper, which are twisted together and covered with an unshielded or shielded sheath. This method of connecting conductors helps to increase noise immunity; it is possible that several twisted pairs of wires are contained in one cable at once. This connection is the cheapest and most accessible; installation of cables is quite simple, which leads to unauthorized connections to networks by the same unscrupulous subscribers.

  • Coaxial cable - consists of a central conductor, the role of which is played by a copper wire, and a conductive screen, most often aluminum foil or copper braid is used as it. Between the main conductor and the screen there is an insulating material, outer part The screen is also covered with insulation. This connection method is more expensive and time-consuming, because unauthorized connections less. Such lines are characterized by good immunity from interference and high speed of information transfer.

  • Fiber optic cable is similar in structure to coaxial cable, but instead of a copper conductor, this cable uses thin glass fiber, the role of internal insulation is performed by a plastic or glass shell, which does not allow light to escape, it forms complete internal reflection. It is noteworthy that signals can pass through fiber only in one direction, which is why they are arranged in pairs in cables. Installation of such communication lines is very labor-intensive; the cable itself is quite sensitive to damage, but at the same time it provides highest speed signal transmission up to 3 Gbit/s. If a fiber optic cable is used, an electrical to light converter must be used on the transmission side, and a converter must be used on the receiving side light signal to electric.

Wireless communication channels

Communication lines and channels can be built on the operation of wireless terrestrial or satellite radio channels. Radio relay channels are a group of repeater stations that are located in a certain order at a certain distance from each other. They are used in the field of cellular communications and for transmitting other types of signals within one city or region.

Satellite communications are provided by satellites that are located in earth orbit and act as relays. The signal from the ground transmitting station goes to the satellite, and from the satellite it is transmitted to the ground receiving station.

This method of communication makes it possible to provide communications to residents of the most remote parts of the planet, since satellites are most often launched not one at a time, but in groups. All repeaters are located in orbit at some distance from each other, so together they can cover almost the entire globe.

Examples of lines and communication channels at the exhibition

Find out which lines and communication channels are used modern companies, you can at a specialized exhibition that will take place at the Expocentre Fairgrounds.

The exhibition will be dedicated to new products in the IT field. The event will feature the latest technical solutions to ensure communication.