entrancePhysical transmission of data over communication lines. Communication lines What are physical communication lines
main function telecommunication networks (TCS) is to ensure information exchange between all subscriber systems of a computer network. The exchange is carried out through communication channels, which are one of the main components of telecommunication networks.
A communication channel is a set of physical medium (communication lines) and data transmission equipment (ATD) that transmit information signals from one network switching node to another or between a node switching and subscriber system.
In this way, a communication channel and a physical communication line are not the same thing. In the general case, on the basis of one communication line, several logical channels can be organized by time, frequency, phase, and other types of separation.
V computer networks are used telephone, telegraph, television, satellite communication networks. As communication lines, wired (air), cable, radio channels of terrestrial and satellite communications are used. The difference between them is determined by the data transmission medium. The physical transmission medium can be a cable, as well as the earth's atmosphere or outer space, through which electromagnetic waves propagate.
Computer networks use telephone, telegraph, television, satellite communication networks. As communication lines, wired (air), cable, radio channels of terrestrial and satellite communications are used. The difference between them is determined by the data transmission medium. The physical transmission medium can be a cable, as well as the earth's atmosphere or outer space, through which electromagnetic waves propagate.
Wired (overhead) communication lines- these are wires without insulating or shielding braids, laid between poles and hanging in the air. Traditionally, they serve to transmit telephone and telegraph signals, but in the absence of other possibilities, they are used to transmit computer data. Wired communication lines are characterized by low bandwidth and low noise immunity, so they are quickly replaced by cable lines.
cable lines include a cable consisting of conductors with insulation in several layers - electrical, electromagnetic, mechanical, and connectors for connecting various equipment to it. Three types of cable are mainly used in the CS: a cable based on twisted pairs of copper wires (this is a twisted pair in a shielded version, when a pair of copper wires is wrapped in an insulating screen, and unshielded, when there is no insulating wrap), coaxial cable (consists of an internal copper core and a braid separated from the core by a layer of insulation) and a fiber optic cable (consists of thin - 5-60 micron fibers through which light signals propagate).
Among the cable lines light guides have the best performance. Their main advantages are: high throughput (up to 10 Gbit/s and higher) due to the use of electromagnetic waves in the optical range; insensitivity to external electromagnetic fields and the absence of their own electromagnetic radiation, low labor intensity of laying an optical cable; spark, explosion and fire safety; increased resistance to aggressive environments; low specific gravity (ratio of linear mass to bandwidth); wide areas of application (creation of highways for collective access, communication systems for computers with peripherals local networks, in microprocessor technology, etc.).
Disadvantages of fiber optic communication lines: connecting additional computers to the light guide significantly weakens the signal, the high-speed modems necessary for the light guides are still expensive, the light guides connecting the computers must be supplied with converters of electrical signals into light and vice versa.
Radio channels of terrestrial and satellite communications generated by a transmitter and receiver of radio waves. Different types of radio channels differ in the frequency range used and the range of information transmission. Radio channels operating in the ranges of short, medium and long waves (HF, MW, LW) provide long-distance communication, but at a low data rate. These are radio channels where amplitude modulation of signals is used. Channels operating on the ultrashort wave (VHF) ranges are faster, they are characterized by frequency modulation of signals. Ultra-high-speed channels are those operating in the ultra-high frequency ranges (SHF), i.e. over 4 GHz. In the microwave range, signals are not reflected by the Earth's ionosphere, so a stable connection requires a direct line of sight between the transmitter and receiver. For this reason, microwave signals are used either in satellite channels or in radio relay channels, where this condition is met.
Communication line characteristics. The main characteristics of communication lines include the following: frequency response, bandwidth, attenuation, throughput, noise immunity, crosstalk at the near end of the line, reliability of data transmission, unit cost.
The characteristics of a communication line are often determined by analyzing its responses to some reference influences, which are used as sinusoidal oscillations of various frequencies, since they are often found in technology and can be used to represent any function of time. The degree of distortion of the sinusoidal signals of the communication line is estimated using the amplitude-frequency characteristic, bandwidth and attenuation at a certain frequency.
Frequency response(AFC) gives the most complete picture of the communication line, it shows how the amplitude of the sinusoid at the output of the line decays compared to the amplitude at its input for all possible frequencies of the transmitted signal (instead of the signal amplitude, its power is often used). Therefore, the frequency response allows you to determine the shape of the output signal for any input signal. However, it is very difficult to obtain the frequency response of a real communication line, therefore, in practice, other, simplified characteristics are used instead of it - the bandwidth and attenuation.
Link bandwidth represents a continuous range of frequencies in which the ratio of the amplitude of the output signal to the input signal exceeds a predetermined limit (typically 0.5). Therefore, the bandwidth determines the range of frequencies of a sinusoidal signal at which this signal is transmitted over the communication line without significant distortion. The bandwidth that has the greatest influence on the maximum possible information transfer rate over the communication line is the difference between the maximum and minimum frequencies of a sinusoidal signal in a given bandwidth. The bandwidth depends on the type of line and its length.
Distinctions should be made between bandwidth and the width of the spectrum of transmitted information signals. The spectrum width of transmitted signals is the difference between the maximum and minimum significant harmonics of the signal, i.e. those harmonics that make the main contribution to the resulting signal. If significant harmonics of the signal fall within the bandwidth of the line, then such a signal will be transmitted and received by the receiver without distortion. Otherwise, the signal will be distorted, the receiver will make mistakes in recognizing information, and, therefore, information will not be able to be transmitted with a given bandwidth.
attenuation- this is the relative decrease in the amplitude or power of the signal when a signal of a certain frequency is transmitted over the line.
Attenuation A is measured in decibels (dB, dB) and is calculated using the formula:
A \u003d 10? lg (P out / P in)
where P out, P in - signal power, respectively, at the output and at the input of the line.
For a rough estimate distortion of the signals transmitted over the line, it is enough to know the attenuation of the fundamental frequency signals, i.e. frequency, the harmonic of which has the highest amplitude and power. A more accurate estimate is possible if the attenuation at several frequencies close to the main one is known.
The throughput of a communication line is its characteristic, which determines (as well as the bandwidth) the maximum possible data transfer rate over the line. It is measured in bits per second (bps) as well as in derived units (Kbps, Mbps, Gbps).
Bandwidth communication line depends on its characteristics (frequency response, bandwidth, attenuation) and on the spectrum of transmitted signals, which, in turn, depends on the chosen method of physical or linear coding (i.e., on the method of presentation discrete information in the form of signals). For one coding method, the line may have one capacity, and for another - another.
When encoding usually used is a change in some parameter of a periodic signal (for example, sinusoidal oscillations) - frequency, amplitude and phase, sinusoids or the sign of the potential of the pulse train. A periodic signal whose parameters change is called a carrier signal or a carrier frequency if a sinusoid is used as such a signal. If the received sinusoid does not change any of its parameters (amplitude, frequency or phase), then it does not carry any information.
The number of changes in the information parameter of the carrier periodic signal per second (for a sinusoid this is the number of changes in amplitude, frequency or phase) is measured in baud. The clock cycle of the transmitter is the period of time between adjacent changes in the information signal.
In general the line bandwidth in bits per second is not the same as the baud rate. Depending on the encoding method, it can be higher, equal or lower than the baud number. If, for example, with this coding method, a single bit value is represented by a pulse of positive polarity, and a zero value is represented by a pulse of negative polarity, then when transmitting alternately changing bits (there are no series of bits of the same name) physical signal during the transmission of each bit, it changes its state twice. Therefore, with this encoding, the line throughput is two times lower than the number of bauds transmitted over the line.
For throughput line is affected not only by physical, but also by the so-called logical coding, which is performed before physical coding and consists in replacing the original bit sequence of information with a new bit sequence that carries the same information, but has additional properties (for example, the ability for the receiving side to detect errors in received data or ensure the confidentiality of transmitted data by encrypting them). As a rule, logical coding is accompanied by the replacement of the original bit sequence by a longer sequence, which negatively affects the transmission time of useful information.
There is a certain connection between the line capacity and its bandwidth. With a fixed method of physical coding, the line capacity increases with an increase in the frequency of the carrier periodic signal, since this increase is accompanied by an increase in information transmitted per unit of time. But as the frequency of this signal increases, the width of its spectrum also increases, which is transmitted with distortions determined by the bandwidth of the line. The greater the discrepancy between the bandwidth of the line and the bandwidth of the transmitted information signals, the more the signals are subject to distortion and the more more likely to make mistakes in the recognition of information by the receiver. As a result, the speed of information transfer is less than one might expect.
C=2F log 2 M, (4)
where M is the number of different states of the information parameter of the transmitted signal.
In the Nyquist relation, which is also used to determine the maximum possible bandwidth communication line, the presence of noise on the line is not explicitly taken into account. However, its influence is indirectly reflected in the choice of the number of states of the information signal. For example, to increase the throughput of the line, it was possible to use not 2 or 4 levels, but 16 when encoding data. But if the noise amplitude exceeds the difference between adjacent 16 levels, then the receiver will not be able to stably recognize the transmitted data. Therefore, the number of possible signal states is actually limited by the signal power to noise ratio.
According to the Nyquist formula, the limit value of the channel capacity is determined for the case when the number of states of the information signal has already been selected, taking into account the possibilities of their stable recognition by the receiver.
Communication line noise immunity- this is its ability to reduce the level of interference created in the external environment on internal conductors. It depends on the type of physical medium used, as well as on the means of the line, shielding and suppressing interference. The most noise-resistant, insensitive to external electromagnetic radiation, are fiber-optic lines, the least noise-resistant - radio lines, cable lines occupy an intermediate position. The reduction of interference caused by external electromagnetic radiation is achieved by shielding and twisting the conductors.
Crosstalk at the near end of the line - determine the noise immunity of the cable to internal sources of interference. Usually they are evaluated in relation to a cable consisting of several twisted pair, when the mutual pickups of one pair on another can reach significant values and create internal interference commensurate with the useful signal.
Reliability of data transmission(or bit error rate) characterizes the probability of distortion for each transmitted data bit. The reasons for the distortion of information signals are interference on the line, as well as the limited bandwidth of its pass. Therefore, an increase in the reliability of data transmission is achieved by increasing the degree of noise immunity of the line, reducing the level of crosstalk in the cable, and using more broadband communication lines.
For conventional cable communication lines without additional means of error protection, the reliability of data transmission is, as a rule, 10 -4 -10 -6 . This means that, on average, out of 10 4 or 10 6 transmitted bits, the value of one bit will be corrupted.
Communication line equipment(data transmission equipment - ATD) is a border equipment that directly connects computers with a communication line. It is part of the communication line and usually operates at the physical level, providing the transmission and reception of a signal of the desired shape and power. Examples of ADFs are modems, adapters, analog-to-digital and digital-to-analog converters.
The DTE does not include the user's data terminal equipment (DTE), which generates data for transmission over the communication line and is connected directly to the DTE. A DTE includes, for example, a LAN router. Note that the division of equipment into APD and OOD classes is rather conditional.
On the lines of communication long distance, intermediate equipment is used, which solves two main tasks: improving the quality of information signals (their shape, power, duration) and creating a permanent composite channel (through channel) of communication between two network subscribers. In the LCN, intermediate equipment is not used if the length of the physical medium (cables, radio air) is not high, so that signals from one network adapter to another can be transmitted without intermediate restoration of their parameters.
V global networks high-quality signal transmission over hundreds and thousands of kilometers is ensured. Therefore, amplifiers are installed at certain distances. To create a through line between two subscribers, multiplexers, demultiplexers and switches are used.
The intermediate equipment of the communication channel is transparent to the user (he does not notice it), although in reality it forms complex network, called the primary network and serving as the basis for building computer, telephone and other networks.
Distinguish analog and digital communication lines, which use various types of intermediate equipment. In analog lines, intermediate equipment is designed to amplify analog signals that have a continuous range of values. In high-speed analog channels, a frequency multiplexing technique is implemented, when several low-speed analog subscriber channels are multiplexed into one high-speed channel. In digital communication channels, where rectangular information signals have a finite number of states, intermediate equipment improves the shape of the signals and restores their repetition period. It provides the formation of high-speed digital channels, working on the principle of time multiplexing of channels, when each low-speed channel is allocated a certain fraction of the time of the high-speed channel.
When transmitting discrete computer data over digital communication lines, the physical layer protocol is defined, since the parameters of the information signals transmitted by the line are standardized, and when transmitted over analog lines, it is not defined, since the information signals have free form and there are no requirements on the way the 1s and 0s are represented by the data communication equipment.
The following are used in communication networks re information transmission presses :
Simplex, when the transmitter and receiver are connected by one communication channel, through which information is transmitted in only one direction (this is typical for television communication networks);
Half-duplex, when two communication nodes are also connected by one channel, through which information is transmitted alternately in one direction, then in the opposite direction (this is typical for information-reference, request-response systems);
Duplex, when two communication nodes are connected by two channels (forward communication channel and reverse), through which information is simultaneously transmitted in opposite directions. Duplex channels are used in systems with decision and information feedback.
Switched and dedicated communication channels. In the TSS, there are dedicated (non-switched) communication channels and those with switching for the duration of information transmission over these channels.
When using dedicated communication channels the transceiver equipment of communication centers is constantly connected to each other. This ensures a high degree of system readiness for information transfer, more high quality communications, support for a large amount of traffic. Due to the relatively high costs of operating networks with dedicated communication channels, their profitability is achieved only if the channels are fully loaded.
For switched communication channels, created only for the time of transmission of a fixed amount of information, are characterized by high flexibility and relatively low cost (with a small amount of traffic). The disadvantages of such channels are: loss of time for switching (to establish communication between subscribers), the possibility of blocking due to the busyness of individual sections of the communication line, lower communication quality, high cost with a significant amount of traffic.
- connection
Linear mounting device, not self-sustaining, working in tension and compression.
Construction terminology - binder
orff.
binder - CONNECTION
(eng. connection, relation, relationship) - the interdependence of the existence of objects, phenomena, actions, separated in space and / or time. With the identification of sustainable and necessary...
Big psychological dictionary - connection
CONNECTION, connections, O connections, v connections and (with someone to be) in connections, female
Explanatory Dictionary of Ushakov
1. That which binds
dependence, conditionality. "... Connection science and practice, connection theory and practice
their unity must become the guiding star of the party of the proletariat.” Stalin. Causal connection. logical connection
Install connection between phenomena. Connection between the parts of the whole. These questions are in connections between themselves
There can be no doubt about the mutual connections these questions. There is an undoubted connection between biography - due
with what. Book. Because of something, because of something, because of something. fits of anguish in connections
Phraseological Dictionary Fedorov
v connections with the fact that he is about to have to disappear from Verny (D. Furmanov. Mutiny). - in touch
adverb, number of synonyms: 3 hello 67 speak 14 hear 12
- with connections
adj., number of synonyms: 2 fancy 12 fancy 31
Dictionary of synonyms of the Russian language - connection
noun, number of synonyms: 2 burdens 17 constraint 34
Dictionary of synonyms of the Russian language - connections
noun, number of synonyms: 13 blat 8 close acquaintance with influential people 1 relationship 6 acquaintance 8 roof 49 paw 18 maza 15 relationship 6 subscription 7 hand 49 leverage 5 own hand 4 bonds 13
Dictionary of synonyms of the Russian language - connection
storage and transmission of information. initially connection was carried out with the help of messengers who transmitted messages
Technics. Modern Encyclopedia
transmitted in writing. This marked the beginning of the postal connections, which until the invention
optical telegraph in con. 18th century was the only kind connections. Opportunities connections significantly
electrical wired connection). In 1832, P. L. Schilling created the first practical
apparatus (telegraph connection). A. G. Bell in 1876 invented the telephone, thereby opening the era - in connection with
orff.
Lopatin's spelling dictionary
v connections with what) - connection
and, suggestion O connections, v connections and in connections, w.
Small Academic Dictionary
1.
Mutual relationship between someone or something.
Connection
between industry and agriculture. Connection science and production. Trading connections. Household connection
districts. Related connections.
Mutual dependence, conditionality.
Causal connection.
□
We want
only to say --- that all sciences are closely related to each other connections and that the strong acquisitions of one
V. Klassovsky.
Connection creativity of Petrov-Vodkin with the traditions of ancient Russian painting is obvious.
L. Mochalov - binder
BINDER-oh, -her.
Explanatory Dictionary of Kuznetsov
1. Book. Connecting, connecting. Be binder a link between someone, something
Trap binder thread of events.
2. Spec. Serving for binding, connection of individual particles. C is her substance. C materials. - Connection
in the direction of action (direct and reverse), by the type of processes that are determined by this connection
Forensic Encyclopedia
distinguish between: genetic (causal) connection; functional connection (connection between addicts
processes); volumetric connection(between objects that make up a set), substantial connection
between the properties of a thing and the thing itself as a whole); connection transformations (between non-direct
direct and reverse connections. Lit .: Eisman A.A. Expert opinion (Structure and scientific rationale). M., 1967. - binder
S / elm / y / yushch / y.
Morphemic spelling dictionary - Connection
1. A metal strip or wooden beam (1) penetrating the masonry and counteracting the expansion of the vaults.
Architectural Dictionary
2. Type of Russian hut (1), in which two living quarters are combined through a canopy into a rectangular volume.
(Terms of the Russian architectural heritage. Pluzhnikov V.I., 1995) - in connection with which
v connections with what union
Explanatory Dictionary of Efremova
Used when attaching an accessory part (which contains - binder
binder adj.
Explanatory Dictionary of Efremova
1. Binding, uniting something.
2. Serving for binding, connection of individual particles. - binder
adj., number of synonyms: 10 binder 16 unifying 5 player 61 sticky 10 sticky 28 uniting 29 mediating 5 binding 34 gluing 9 connecting 80
Dictionary of synonyms of the Russian language - connection
See tie
Dahl's Explanatory Dictionary - Connection
(chem.)
Encyclopedic Dictionary of Brockhaus and Efron
see Chemical structure or Structure. - BINDERS
BINDERS, a substance or two substances that have the ability to hold objects together
Scientific and technical dictionary
Natural binders, commonly called GLUES, are produced by boiling animal skins, bones
binder include EPOXY RESIN with a hardener that reacts with it, as well as THERMOSET and THERMOPLASTIC RESINS. - binders
BINDERS
Chemical Encyclopedia
continuous phases providing connectivity of discrete elements or filler particles - CONTACTS
CONTACTS- in building structures - frame elements of the building (structure) - providing
its spatial rigidity, as well as the stability of the main (bearing) structures. System connections usually - connection
CONNECTION-i, suggestion O connections, v connections and in connections; well.
Explanatory Dictionary of Kuznetsov
1. Relationship of mutual dependence, conditionality
connections together. // Consistency, consistency, harmony (in thoughts, presentation, etc.
Memories flashed by one after another without any connections. Achieved refinement and connections phrases.
2
between partners. Close, businesslike, mutually beneficial connections two countries. Friendship, family, love
family connections. Establish, strengthen, develop, break connections between countries. Maintain with. with a family - in connection with
v connections with a suggestion with creative see in connections co
Explanatory Dictionary of Efremova
Used to indicate a causal relationship - connection
Clutch, connecting link
Abramov's synonym dictionary
Cohesion of thoughts, concepts - association of ideas
see >> union
see also -> influential connection - Connections
In building structures, connecting elements that ensure the stability of the main (bearing) structures of the Frame and the spatial rigidity of the structure as a whole.
- connection
devices, networks of nodes and channels (lines) connections. Depending on the nature of the means used, it is divided
Geography. Modern Encyclopedia
One of the types connections is also a traditional mail delivering from one place to another
seal. Wired views connections: telegraph (invented in 1844), telephone (1876) and its varieties (teletype
telefax); wireless: radio (1895), television (1923), cellular connection(mobile
radiotelephones), satellite systems connections, global navigation systems; mixed view: computer networks - CONNECTION
CONNECTION, see CHEMICAL CONNECTION.
Scientific and technical dictionary - for the reason that
v connections with the fact that the union
Explanatory Dictionary of Efremova
Used when attaching the subordinate part of a complex - in connection with
v connections with the suggestion with creative
Explanatory Dictionary of Efremova
see in connections With - connection
connection well. local
Explanatory Dictionary of Efremova
That which binds, burdens; burden. - connection
connection well.
Explanatory Dictionary of Efremova
1. Mutual relations between someone, something.
|| Commonality, mutual understanding, internal - binder
Binder, binder, binder, binders, binder, binder, binder, binders binder, binder, binder, binder, binder, binder, binder, binders, binder binder, binder, binders, binder, binder, binder, binder, binders, binder binder, binder, binders, binder, binder, binder, binders, binder, binding, binder, binding
Zaliznyak's grammar dictionary - binder
BINDER, binder, binder(Books). incl. effective present temp. from bind, the same as binding. Binder link. Binders threads.
Explanatory Dictionary of Ushakov - CONNECTION
CONNECTION- in philosophy - the interdependence of the existence of phenomena separated in space
Big encyclopedic dictionary
and in time. Connections classified according to the objects of knowledge, according to the forms of determinism (unambiguous
connection (connection offspring, connection transformations) - in the direction of action (direct and reverse
by the type of processes that this connection (connection functioning, connection development, connection
management) - according to the content, which is the subject connections (connection, providing the transfer of matter - binder
BINDER, oh, her (book). Connecting, connecting. Binder link.
Explanatory dictionary of Ozhegov - connections
Wide ~
Dictionary of Russian Idioms - connections
connection (connections)
Michelson's Phraseological Dictionary
(inosk.) - friendship, acquaintance (intimate relationship)
Wed "Without friends, yes without connections
shtetl. He had no powers and had no connections.
Turgenev. Dog. - before communication
adverb, number of synonyms: 12 arivederche 15 bye 26 be healthy 83 bye 31 see you 39 goodbye 58 see you soon 25 goodbye 39 see you 18 call 1 happily 57 good luck 19
Dictionary of synonyms of the Russian language - Connection
and reverse (see Reverse connection). The methodology of Structuralism arises as a result of awareness
Great Soviet Encyclopedia
29; Zinoviev A. A., On the definition of the concept connections, "Questions of Philosophy", 1960, No. 8; Novinsky
I. I., Concept connections in Marxist Philosophy, M., 1961; Shchedrovitsky G. P., Problems of methodology of system
which marked the beginning of the postal connections(See Postal connection), which during the slave and feudal
see Wired connection). The creator of the electric telegraph (1832) was P. L. Schilling. In 1837 S. Morse - connection
orff.
Lopatin's spelling dictionary
connection, -and - connections
connections pl.
Explanatory Dictionary of Efremova
Meeting influential people. - connection
CONNECTION, and about connections, v connections and in connections, w.
Explanatory dictionary of Ozhegov
1. (in connections). Relationship of mutual dependence
conditionality, commonality between something. C. theory and practice. Causal s.
2. (in connections). Close communication between
something Friendly s. Strengthen international connections.
3. (in connections and in connections). Love relationship
cohabitation. Love s. To be in connections with someone.
4. pl. Close acquaintance with someone, providing
support, patronage, benefit. Have connections in influential circles. Large connections.
5. (in connections - connection
See tie
Dahl's Explanatory Dictionary - binder
oh, her. book.
Small Academic Dictionary
1.
incl. present from tying.
2. in value adj.
serving for connections, connections of smth.
Binder substance. Binder link.
- connection
1) bag
Russian-Crimean Tatar Dictionary
connection theory with practice - nazariyenen ameliyat arasındaki bağ (alâqa)
2) (close
communication) alâqa, bag, munasebet
friendly connections- dostane munasebetler
3) alâqa
telegraphic connection- telegraf alâqası
without connections- bagsIz - connection
1) (relationship, connection) katena (-), mapatanisho pl., mfungamano (mi-), muambatano (mi-), mwambisho (mi-), ufungamano singular, uhusiano (ma-), mwamali (mi-), muoano (mi-) ver.;
Russian-Swahili dictionary
connections - mafungamano pl., maingiliano pl. - connection
bunch, connection
Russian-Bulgarian dictionary
vrazka w
- telephone connection
- v connections With... - in connection with
Because of, in view of, in connection with, in connection with, in light of, owing to, as a result of, on the grounds of in connection with
- in this regard Complete Russian-English Dictionary
- connections Russian-Mongolian dictionary
- for the reason that
V souvislosti s tim
Russian-Czech dictionary - connections Russian-Czech dictionary
- in connection with
V connections With
Russian-Hebrew Dictionary
בְּהֶקשֵר ל-; לְרֶגֶל - middleware Complete Russian-English Dictionary
- binder Russian-Lithuanian dictionary
- connection
Jungtis (-ies) (3) (chem.)
Russian-Lithuanian dictionary
sąraiša (1) (technical)
sarysis (1)
rysys (4)
sasaja (1) - in connection with...
v connection() and () with...
Russian-Bulgarian dictionary
vvv vrazka with ... - Connection Russian-Turkish Dictionary
- binder
prich. 1. bağlayıcı; 2. məc. əlaqələndirən.
Russian-Azerbaijani dictionary - connections
pl. h.
Russian-German dictionary
(communication, relationships) Beziehungen pl; Kontakte pl (contacts)
cultural connections- kulturelle beziehungen
international connections- international contact - in connection with
The rules are worked out in connection with the multiplication of negative numbers.
It is important to consider these findings in the context of the evolutionary relationship of life on our planet to the presence of mercury. - in this regard
V teto souvislosti
Russian-Czech dictionary - be in touch Russian-Czech dictionary
- and communication Russian-Czech dictionary
- due
(due to) por causa de, por motivo de; (on the occasion of something) por ocasiao
Russian-Portuguese Dictionary - binder
prince
Russian-Portuguese Dictionary
de ligação; agregativo
- binder link - connection
relações fpl; (connectivity) ligação f, coerência f; those ligadura f
Russian-Portuguese Dictionary
- v connections - binder
Lep
Russian-Czech dictionary
lepidlo
pojidlo
pojivo
spojivo - connections
noun; pl. ties
Complete Russian-English Dictionary - connection
Someone-something with someone-something n. female kind
Russian-Ukrainian dictionary
biol.
zv "language of names of people. kind - connection
I
Russian-English scientific and technical dictionary
see bilateral connection
II
see violate connections; transverse connections; gap connections
split connections; connection with triple connections
III
see also relationship; dependency between; close
connection; establish a relationship between
Ship-to-ship and ship-to-shore communications ...
In very - connection
f
Russian-Finnish dictionary
1) yhteys
2) yhteys, liikenne, viestintä
telephone connection- puhelinyhteys
facilities connections
viestivälineet, yhteysvälineet
3)pl connections yhteydet
cultural connections- kulttuuriyhteydet
v connections with this - täman yhteydessä - binder Russian-Czech dictionary
- connections
pl
Russian-Portuguese Dictionary
(dating) relações fpl; empenhes mpl; (blat) pistolão m fam bras; (amorous) ligação f (amorosa); (means of communication) telecomunicações fpl; military ligações e transmissões
In the receiver, the secondary signals are converted back into message signals in the form of sound, optical or textual information.
Etymology
The word "electrocommunication" comes from new.-lat. electricus and others - Greek. ἤλεκτρον (electro, shiny metal; amber) and the verb "knit". The synonym is the word "telecommunications" (eng. telecommunication, from fr. télécommunication), used in English-speaking countries. Word telecommunication, in turn, comes from the Greek tele-(τηλε-) - "far" and from lat. communicatio - message, transfer (from lat. communico - I make it general), that is, the meaning of this word includes non-electric types of information transfer (using optical telegraph, sounds, fire on watchtowers, mail).
Telecommunication classification
Telecommunications is the object of study of the scientific discipline electrical communication theory.
According to the type of information transfer, all modern systems Telecommunications are conditionally classified into those intended for the transmission of sound, video, text.
Depending on the purpose of the messages, the types of telecommunications can be classified as intended for the transmission of information of an individual and mass nature.
In terms of time parameters, types of telecommunications can be operating in real time or implementing delayed delivery messages.
The main primary telecommunication signals are: telephone, sound broadcasting, facsimile, television, telegraph, data transmission.
Communication types
- Cable lines - electrical signals are used for transmission;
- Radio communication - radio waves are used for transmission;
- LW, MW, HF and VHF communication without the use of repeaters
- Satellite communications - communications using space repeater(s)
- Radio relay communication - communication using terrestrial repeater(s)
- Cellular communication - radio relay communication using a network of ground base stations
- Fiber optic communication - light waves are used for transmission.
Depending on the engineering way communication line organizations are divided into:
- satellite;
- air;
- ground;
- underwater;
- underground.
- Analog communication is the transmission of a continuous signal.
- digital communication is the transmission of information in discrete form (digital form). A digital signal, by its physical nature, is analog, but the information transmitted with its help is determined by a finite set of signal levels. For processing digital signal numerical methods are applied.
Signal
V general view The communication system includes:
- terminal equipment: terminal equipment, terminal device (terminal), terminal device, source and recipient of the message;
- signal conversion devices(UPS) from both ends of the line.
The terminal equipment provides primary processing of the message and signal, conversion of messages from the form in which they are provided by the source (speech, image, etc.) into a signal (on the side of the source, sender) and back (on the side of the recipient), amplification, etc. P.
Signal conditioning devices can provide signal protection from distortion, channel (s) shaping, group signal matching (signal of several channels) with a line on the source side, group signal recovery from a mixture of useful signal and noise, its separation into individual channels, error detection and correction on the recipient's side. Modulation is used to form a group signal and match with the line.
The communication link may contain signal conditioning devices such as amplifiers and regenerators. The amplifier simply amplifies the signal along with interference and transmits further, is used in analog transmission systems(ASP). Regenerator ("receiver") - produces signal recovery without interference and re-formation of a linear signal, used in digital transmission systems(CSP). Amplifying / regeneration points are serviced and unattended (OUP, NUP, ORP and NRP, respectively).
In the DSP, the terminal equipment is called DTE (data terminal equipment, DTE), UPS - DCE ( data link termination equipment or communication line terminal equipment, DCE). For example, in computer networks, the role of the OOD is performed by a computer, and the DCE is a modem.
Standardization
Standards in the world of communication are extremely important, as communication equipment must be able to communicate with each other. There are several international organizations that publish communication standards. Among them:
- International Telecommunication Union (English) International Telecommunication Union, ITU) is one of the UN agencies.
- (English) Institute of Electrical and Electronics Engineers, IEEE).
- Internet Development Special Commission Internet Engineering Task Force, IETF).
In addition, standards are often (usually de facto) determined by the leaders of the telecommunications equipment industry.
Communication line generally consists of a physical medium through which electrical information signals are transmitted, data transmission equipment and intermediate equipment. Synonymous with the term communication line(line) is a term link(channel).
The physical transmission medium can be a cable, that is, a set of wires, insulating and protective sheaths and connectors, as well as the earth's atmosphere or outer space through which electromagnetic waves propagate.
Depending on the data transmission medium, communication lines are divided into the following:
§ wired (air);
§ cable (copper and fiber-optic);
§ radio channels of terrestrial and satellite communications.
Wired (overhead) communication lines are wires without any insulating or shielding braids, laid between poles and hanging in the air. Such communication lines traditionally carry telephone or telegraph signals, but in the absence of other possibilities, these lines are also used to transmit computer data. The speed qualities and noise immunity of these lines leave much to be desired. Today, wired communication lines are rapidly being replaced by cable ones.
cable lines are quite complex structures. The cable consists of conductors enclosed in several layers of insulation: electrical, electromagnetic, mechanical, and possibly also climatic. In addition, the cable can be equipped with connectors that allow you to quickly connect various equipment to it. There are three main types of cable used in computer networks: twisted-pair copper cables, copper-core coaxial cables, and fiber optic cables.
A twisted pair of wires is called twisted pair. Twisted pair exists in a shielded version , when a pair of copper wires is wrapped in an insulating screen, and unshielded , when there is no insulating wrap. Twisting wires reduces the influence of external interference on the useful signals transmitted over the cable.
Coaxial cable has an asymmetric design and consists of an inner copper core and a braid separated from the core by a layer of insulation. There are several types of coaxial cable that differ in characteristics and applications - for local networks, for global networks, for cable television, etc.
fiber optic cable consists of thin fibers through which light signals propagate. This is the highest quality type of cable - it provides data transmission at a very high speed (up to 10 Gb / s and higher) and, better than other types of transmission medium, provides data protection from external interference.
Radio channels of terrestrial and satellite communications generated by a transmitter and receiver of radio waves. There are a large number of different types of radio channels, differing both in the frequency range used and in the channel range. The ranges of short, medium and long waves (KB, SV and DV), also called amplitude modulation ranges (Amplitude Modulation, AM) by the type of signal modulation method used in them, provide long-distance communication, but at a low data rate. More high-speed are channels operating on the ultrashort wave (VHF) ranges, which are characterized by frequency modulation, as well as ultra-high frequency ranges (microwave or microwaves).
In the microwave range (above 4 GHz), signals are no longer reflected by the Earth's ionosphere, and stable communication requires a line of sight between the transmitter and receiver. Therefore, such frequencies use either satellite channels or radio relay channels, where this condition is met.
In computer networks today, almost all the described types of physical data transmission media are used, but fiber-optic media are the most promising. Today, both backbones of large territorial networks and high-speed communication lines of local networks are being built on them.
A popular medium is also twisted pair, which is characterized by an excellent ratio of quality to cost, as well as ease of installation. With the help of twisted pair, end subscribers of networks are usually connected at distances up to 100 meters from the hub. Satellite channels and radio communications are used most often in cases where cable communications cannot be used - for example, when passing the channel through a sparsely populated area or to communicate with a mobile network user.
Even when considering the simplest network of only two machines, one can see many of the problems inherent in any computer network, including problems connected with physical transmission signals over communication lines , without the solution of which any kind of connection is impossible.
In computing, data is used to represent binary code . Inside the computer, data ones and zeros correspond to discrete electrical signals. The representation of data as electrical or optical signals is called coding. . There are various ways to encode the binary digits 1 and 0, for example, potential a way in which one voltage level corresponds to one, and another voltage level corresponds to zero, or impulse a method when pulses of different or one polarity are used to represent numbers.
Similar approaches can be used to encode data and transfer it between two computers over communication lines. However, these communication lines differ in their electrical characteristics from those that exist inside a computer. The main difference between external communication lines and internal ones is their much longer length , as well as in the fact that they pass outside the shielded housing in spaces often subject to strong electromagnetic interference. All this leads to much higher distortions. rectangular pulses(for example, "filling up" the fronts) than inside the computer. Therefore, for reliable recognition of pulses at the receiving end of the communication line, when transmitting data inside and outside the computer, it is not always possible to use the same speeds and coding methods. For example, the slow rise of the pulse front due to the high capacitive load of the line requires the transmission of pulses at a lower speed (so that the leading and trailing edges of neighboring pulses do not overlap and the pulse has time to grow to the required level).
Used in computer networks both potential and impulse encoding of discrete data , as well as a specific way of representing data that is never used inside a computer - modulation(Fig. 3). When modulating, discrete information is represented by a sinusoidal signal of the frequency that the existing communication line transmits well.
Potential or pulse coding is used on high quality channels, while sinusoidal modulation is preferred when the channel introduces severe distortion into the transmitted signals. Typically, modulation is used in wide area networks when transmitting data over analog telephone links, which were designed to transmit voice in analog form and are therefore not well suited for direct transmission of pulses.
Used to convert data from one form to another modems. Term "modem" - short for modulator/demodulator. A binary zero is converted, for example, to a low frequency signal, and a unit is converted to a high frequency signal. In other words, by converting the data, the modem modulates the frequency of the analog signal (Fig. 4).
The number of wires in the communication lines between computers also affects the method of signal transmission.
Data transfer can occur in parallel (Fig. 5) or sequentially (Fig. 6).
To reduce the cost of communication lines in networks, they usually strive to reduce the number of wires and because of this they use not parallel transmission of all the bits of one byte or even several bytes, as is done inside a computer, but serial, bit-by-bit transmission, requiring only one pair of wires.
When connecting computers and devices, three different methods are also used, denoted by three different terms. The connection is: simplex, half duplex and full duplex(Fig. 7 ).
A simplex connection is said to be when data moves in only one direction. A half-duplex connection allows data to travel in both directions, but in different time, and finally, a duplex connection is when data travels in both directions at the same time.
Rice. 7. Examples of data flows.
Another important concept is connection switching.
Any communication networks support some way of switching their subscribers among themselves. These subscribers can be remote computers, local networks, fax machines, or simply interlocutors communicating using telephone sets. It is practically impossible to provide each pair of interacting subscribers with their own non-switched (i.e. permanent connection) physical communication line, which they could exclusively “own” for a long time. Therefore, in any network, some method of subscriber switching is always used, which ensures the availability of available physical channels simultaneously for several communication sessions between network subscribers.
Connection switching allows network hardware to share the same physical link between many devices. The two main ways to switch a connection are - circuit switching and packet switching.
Switching circuits creates a single continuous connection between two network devices. While these devices are communicating, no other device can use this connection to transfer its own information - it is forced to wait until the connection is free.
A simple example of a circuit switch is an A-B type switch that connects two computers to one printer. To allow one of the computers to print, you turn a toggle switch on the switch, establishing a continuous connection between the computer and the printer. A point-to-point connection is formed . As shown in the figure, only one computer can print at the same time.
Rice. 6Switching circuits
Most modern networks, including the Internet, use packet switching. Data transfer programs in such networks divide data into pieces called packets. In a packet-switched network, data can travel in one packet at a time, or in multiple packets. The data will arrive at the same destination, even though the paths they took may be completely different.
To compare two kinds of connections in a network, let's assume that we interrupted the link in each of them. For example, by disconnecting the printer from the manager in fig. 6 (by moving the toggle switch to position B), you have deprived him of the ability to print. A circuit-switched connection requires an uninterrupted communication link.
Rice. 7. Packet switching
Conversely, data in a packet-switched network can move in different ways. This is seen in fig. 7. Data doesn't necessarily follow the same path between office and home computers, breaking one of the links will not result in a loss of connection - the data will simply go the other way. Packet switched networks have many alternative routes for packets.
Packet switching is a subscriber switching technique that has been specifically designed to carry computer traffic efficiently.
The essence of the problem lies in pulsating nature of traffic , which is generated by typical network applications. For example, when accessing a remote file server, the user first browses the contents of that server's directory, which involves a small amount of data transfer. It then opens the required file in text editor, and this operation can create quite an intensive data exchange, especially if the file contains large graphic inclusions. After displaying several pages of the file, the user works with them locally for a while, which does not require any network transfer at all, and then returns modified copies of the pages to the server - and this again generates heavy network data transfer.
The traffic ripple ratio of an individual network user, equal to the ratio of the average data exchange intensity to the maximum possible, can be 1:50 or 1:100. If for the described session to organize channel switching between the user's computer and the server, then most of the time the channel will be idle. At the same time, the switching capabilities of the network will be used and will not be available to other network users.
In packet switching, all messages transmitted by the network user are broken up at the source node into relatively small parts, called packets. A message is a logically completed piece of data - a request to transfer a file, a response to this request containing the entire file, etc.
Messages can be of arbitrary length, from a few bytes to many megabytes. In contrast, packets can also typically be of variable length, but within narrow limits, such as 46 to 1500 bytes. Each packet is provided with a header that specifies the address information needed to deliver the packet to the destination host, as well as the packet number that will be used by the destination host to assemble the message.
Packets are transported on the network as independent information blocks. Network switches receive packets from end nodes and, based on address information, transmit them to each other, and ultimately to the destination node.
Packet network switches differ from circuit switches in that they have an internal buffer memory for temporary storage of packets if the output port of the switch is busy transmitting another packet at the time the packet is received. In this case, the packet is for some time in the queue of packets in the buffer memory of the output port, and when it reaches the queue, it is transferred to the next switch. This data transfer scheme allows you to smooth traffic ripples on backbone connections between switches and thus use them in the most efficient way to increase the throughput of the network as a whole.
Indeed, for a pair of subscribers, it would be most effective to provide them with a switched communication channel for their sole use, as is given in circuit-switched networks. With this method, the interaction time of a pair of subscribers would be minimal, since data would be transmitted without delay from one subscriber to another.
A packet-switched network slows down the process of interaction of a particular pair of subscribers. However, the total amount of computer data transmitted by the network per unit of time with packet switching technology will be higher than with circuit switching technology.
Usually, if the provided access speed is equal, a packet-switched network turns out to be 2-3 times cheaper than a circuit-switched network, that is, a public telephone network.
Each of these schemes circuit switching (circuit switching) or packet switching (packet switching)) has its advantages and disadvantages, but according to the long-term forecasts of many experts, the future belongs to packet switching technology, as it is more flexible and versatile.
Circuit-switched networks are well suited for constant-rate data switching, when the switching unit is not a single byte or data packet, but a long-term synchronous data stream between two subscribers.
Both packet-switched networks and circuit-switched networks can be divided into two classes on a different basis - networks with dynamic switching and networks with constant switching.
In the first case, the network allows a connection to be established at the initiative of the network user. Switching is performed for the duration of the communication session, and then (again, at the initiative of one of the interacting users), the connection is broken. In general, any network user can connect to any other network user. Typically, the connection period between a pair of users during dynamic switching ranges from several seconds to several hours and ends when certain work is performed - transferring a file, viewing a page of text or image, etc.
In the second case, the network does not provide the user with the ability to perform dynamic switching with another arbitrary network user. Instead, the network allows a pair of users to order a connection for an extended period of time. The connection is established not by the users, but by the personnel maintaining the network. The time for which permanent switching is established is usually measured in several months. The always-switched mode in circuit-switched networks is often referred to as a service. dedicated or leased channels.
Examples of networks that support dynamic switching mode are telephone networks public, local networks, Internet.
Some types of networks support both modes of operation.
Another problem to be solved in signaling is the problem mutual synchronization of the transmitter of one computer with the receiver of another . When organizing the interaction of modules inside the computer, this problem is solved very simply, since in this case all modules are synchronized from a common clock generator. The problem of synchronization when connecting computers can be solved different ways, both by exchanging special clock pulses over a separate line, and by using periodic synchronization with predetermined codes or pulses of a characteristic shape that differs from the shape of the data pulses.
Asynchronous and synchronous transmission. When data is exchanged at the physical layer, the unit of information is a bit, so the physical layer means always maintain bit-by-bit synchronization between the receiver and the transmitter.
However, if the quality of the communication line is poor (usually this applies to telephone switched channels), to reduce the cost of equipment and increase the reliability of data transmission, additional funds byte-level synchronization.
This mode of operation is called asynchronous or start-stop. Another reason for using this mode of operation is the presence of devices that generate data bytes at random times. This is how the keyboard of a display or other terminal device works, from which a person enters data for processing by a computer.
V asynchronous mode each data byte is accompanied by special start and stop signals. The purpose of these signals is, firstly, to notify the receiver of the arrival of data and, secondly, to give the receiver enough time to perform some timing-related functions before the next byte arrives.
The described mode is called asynchronous because each byte can be slightly offset in time relative to the bitwise cycles of the previous byte.
The tasks of reliable exchange of binary signals represented by the corresponding electromagnetic signals in computer networks are solved by a certain class of equipment. In local networks, this network adapters, and in global networks - data transmission equipment, which includes, for example, the considered modems. This equipment encodes and decodes each information bit, synchronizes the transmission of electromagnetic signals over communication lines, checks the correctness of the transmission by the checksum, and can perform some other operations.
Control questions:
3. What communication lines are used in computer networks?
4. What lines of communication are the most promising?
5. How are binary signals transmitted on the network? What is modulation?
6. What is the modem used for?
7. What is serial and parallel data transmission?
8. What is a simplex, half duplex and full duplex connection?
9. What is connection switching?
10. What are the two main ways to switch a connection?
11. What is packet switching and what is its advantage?
12. When is it appropriate to use circuit switching?
13. Explain the concepts of asynchronous and synchronous data transfer?
The main types of communication lines are divided into wired and wireless. In wired communication lines, the physical medium through which signals propagate forms a mechanical link between the receiver and transmitter. Wireless communication lines are characterized by the fact that there is no mechanical connection between the transmitter and receiver, and the information carrier is electromagnetic waves that propagate in the environment.
Wired communication lines
By design features, wire lines are divided into:
air, which are wires without any insulating or shielding sheaths, laid between poles and hanging in the air;
cable, which consist of conductors enclosed, as a rule, in several layers of insulation.
By overhead lines Communications traditionally carried telephone or telegraph signals, but in the absence of other possibilities, these lines are used to transmit computer data. The speed characteristics and noise immunity of these lines leave much to be desired. Wired communication lines are quickly being replaced by cable ones.
Cable electrical communication lines are divided into three main types: a cable based on twisted pairs of copper wires, a coaxial cable with a copper core, and also a fiber optic cable.
A twisted pair of wires is called a twisted pair. The wires are twisted to eliminate mutual influence between electric currents in conductors. Twisted pair exists in a shielded version, when a pair of copper wires is wrapped in an insulating screen, and unshielded, when there is no insulating sheath. One or more twisted pairs are bundled into sheathed cables.
Unshielded twisted pair has a wide range of applications. It is used in both telephone and computer networks. Currently, UTP cable is a popular medium for transmitting information over short distances [about 100 meters]. Twisted pair cables are divided into 5 categories according to electrical and mechanical characteristics. In computer networks, cables of categories 3 and 5 are widely used, which are described in the American standard EIA / TIA-568A.
Category 3 cable is designed for low speed data transmission. For it, the attenuation at a frequency of 16 MHz is determined and should not be lower than 13.1 dB with a cable length of 100 meters. Category 5 twisted-pair cable is characterized by an attenuation of at least 22 dB for a frequency of 100 MHz with a cable length of not more than 100 meters. The frequency of 100 MHz was chosen because the cable of this category is intended for high speed transmission data, the signals of which have significant harmonics with a frequency of approximately 100 MHz.
All UTP cables, regardless of their category, are available in 4-pair configuration. Each of the four pairs has a specific color and twist pitch. The advantages of UTP cable include:
cable flexibility, which simplifies the installation of a communication line;
low cost with a sufficiently high throughput [up to 1 Gb / s].
The disadvantages of unshielded twisted pair cable are:
low noise immunity;
hard limitation of cable length.
Shielded twisted pair cable STP well protects the transmitted signals from interference, and also radiates less electromagnetic waves to the outside. However, the presence of a grounded shield increases the cost of the cable and complicates its laying, as it requires high-quality grounding. The STP cable is mainly used to transmit discrete information, and voice is not transmitted over it.
The primary standard that defines STP parameters is the IBM proprietary standard. In this standard, cables are not divided into categories, but into types. Type 1 is roughly equivalent to Category 5 UTP. It consists of 2 pairs of stranded copper wires shielded with a conductive braid that is grounded. The IBM type 2 cable is a type 1 cable with 2 pairs of unshielded voice wire added. Not all IBM standard types are STP.
A coaxial cable consists of two concentric conductors isolated from each other, of which the outer one looks like a tube. Due to this design, the coaxial cable is less susceptible to external electromagnetic influences, so it can be used at higher data rates. In addition, these cables, due to the relatively thick central core, are characterized by minimal attenuation of the electrical signal, which allows information to be transmitted over sufficiently long distances. The bandwidth of a coaxial cable can be more than 1 GHz/km and the attenuation can be less than 20 dB/km at 1 GHz.
There are a large number of types of coaxial cables used in networks various types- telephone, television and computer. Two types of cables are used in LANs: thin coaxial cable and thick coaxial cable.
The thin coaxial cable has an outer diameter of about 5 mm, and the diameter of the central copper wire is 0.89 mm. This cable is designed to transmit signals with a spectrum of up to 10 MHz at a distance of up to 185 meters.
The thick coaxial cable has an outer diameter of about 10 mm, and the diameter of the central copper wire is 2.17 mm. This cable is designed to transmit signals with a spectrum up to 10 MHz at a distance of up to 500 meters.
A thin coaxial cable has worse mechanical and electrical characteristics than a thick coaxial cable, but due to its flexibility, it is more convenient for installation.
A coaxial cable is several times more expensive than a twisted pair cable, and in terms of characteristics it is inferior, in particular, to a fiber optic cable, so it is less and less used in building a communication system of computer networks.
Fiber optic cables consist of center conductor light [core] - a glass fiber surrounded by another layer of glass - a shell with a lower refractive index than the core. Spreading through the core, the rays of light do not go beyond its limits, being reflected from the shell. Each glass fiber only transmits signals in one direction.
Depending on the distribution of the refractive index and on the size of the core diameter, there are:
multimode fiber with stepped refractive index;
multimode fiber with smooth change refractive index;
single mode fiber.
A single-mode cable uses a center conductor of very small diameter, commensurate with the long wavelength of light - from 5 to 10 microns. In this case, almost all rays propagate along the optical axis of the core without being reflected from the cladding. The bandwidth of a single-mode cable is very wide - up to hundreds of gigahertz per kilometer. The production of thin high-quality fibers for a single-mode cable is a complex technological process, which makes the cable quite expensive.
Multimode cables use wider inner cores that are easier to manufacture technologically. The standards define the two most common multimode cables: 62.5/125 µm and 50/125 µm, 62.5 µm or 50 µm is the diameter of the center conductor, and 125 µm is the diameter of the outer conductor.
In multi-mode cables, there are multiple light beams in the inner conductor at the same time reflecting off the outer conductor. The angle of reflection of the conductor is called the mode of the beam. Multimode cables have a narrower bandwidth - from 500 to 800 MHz/km. The narrowing of the band occurs due to the loss of light energy during reflections, as well as due to the interference of rays of different modes.
As sources of light emission in fiber-optic cables, the following are used:
LEDs;
lasers.
LEDs can emit light with a wavelength of 0.85 and 1.3 microns. Laser emitters operate at wavelengths of 1.3 and 1.55 µm. The speed of modern lasers makes it possible to modulate the light flux with frequencies of 10 GHz and higher.
Fiber optic cables have excellent electromagnetic and mechanical characteristics, their disadvantage is the complexity and high cost of installation work.
Wireless communication lines
The table provides information about the ranges of electromagnetic oscillations used in wireless channels connections.
Radio channels of terrestrial and satellite communications are formed using a transmitter and receiver of radio waves. Radio waves are electromagnetic oscillations with a frequency f less than 6000 GHz [with a wavelength l greater than 100 microns]. The relationship between wavelength and frequency is given by
f = c/lambda where c = 3*10 8 m/s is the speed of light in vacuum.
To transmit information, radio communication is used primarily when cable communication is not possible - for example:
when the channel passes through a sparsely populated or hard-to-reach area;
to contact mobile subscribers such as a taxi driver, an ambulance doctor.
The main disadvantage of radio communication is its weak noise immunity. This primarily applies to the low-frequency ranges of radio waves. The higher the operating frequency, the greater the capacity [number of channels] of the communication system, but the smaller the limit distances at which direct transmission between two points is possible. The first of the reasons gives rise to a trend towards the development of new higher frequency ranges. However, radio waves with a frequency exceeding 30 GHz are operable for distances of no more than or on the order of 5 km due to the absorption of radio waves in the atmosphere.
For transmission over long distances, a chain of radio relay [relay] stations is used, separated from each other by a distance of up to 40 km. Each station has a tower with a receiver and transmitter of radio waves, receives a signal, amplifies it and transmits it to the next station. Directional antennas are used to increase the signal strength and reduce the effect of interference.
Satellite communication differs from radio relay in that an artificial satellite of the Earth acts as a repeater. This type of communication provides a higher quality of the transmitted information as it requires a smaller number of intermediate nodes on the way of information transmission. Often a combination of radio relay and satellite communications is used.
Infrared radiation and radiation in the millimeter range are used on short distances in blocks remote control. The main disadvantage of radiation in this range is that it does not pass through the barrier. This disadvantage is also an advantage when the radiation in one room does not interfere with the radiation in another. This frequency does not require permission. This is an excellent channel for transmitting data indoors.
The visible range is also used for transmission. Typically, the light source is a laser. Coherent radiation is easily focused. However, rain or fog spoil the deal. The transmission can be spoiled even by convection currents on the roof that occur on a hot day.