Plan:
- Introduction
- 1 Telecommunication classification
- 2 Types of communication
- 3 Signal
- 4 Communication line
- 5 Communication channel
- 6 Separation (compaction) of channels
- 7 Communication network
- 8 Standardization Literature
Introduction
Telecommunication- a method of transmitting information using electromagnetic signals, for example, by wire, fiber optic cable or radio.
The principle of telecommunication is based on the conversion of message signals (sound, text, optical information) into primary electrical signals. In turn, the primary electrical signals are converted by the transmitter into secondary electrical signals whose characteristics are in good agreement with the characteristics communication lines... Further, through the communication line, the secondary signals are fed to the input of the receiver. In the receiving device, the secondary signals are converted back into message signals in the form of sound, optical or text information.
1. Classification of telecommunications
By the type of information transmission, all modern telecommunication systems are conventionally classified into those intended for the transmission of sound, video, text.
Depending on the transmission medium, electrical, optical and radio communications are distinguished.
Depending on the purpose of the messages, the types of telecommunications can be qualified for the transmission of information of an individual and mass nature. In terms of time parameters, the types of telecommunications can be operating in real time either carrying out delayed delivery messages.
The main primary signals of telecommunication are: telephone, sound broadcasting, facsimile, television, telegraph, data transmission.
2. Types of communication
Depending on the data transmission medium, communication lines are divided into:
- satellite
- air
- terrestrial
- underwater
- underground
Depending on whether the sources / recipients of information are mobile or not, a distinction is made between stationary (fixed) and mobile communication (mobile, communication with mobile objects- SPO).
By the type of signal transmitted, a distinction is made between analog and digital communications. Analog communication is the transmission of a continuous signal (such as sound or speech). Digital communication is the transmission of information in discrete form (digital form). A digital signal by its physical nature is "analog", but this analog signal (pulse and discrete) is endowed with the properties of a number, as a result of which it becomes possible to use numerical methods for its processing.
Discrete messages can be transmitted by analog channels and vice versa. Currently, digital communication is replacing analog (digitization is taking place), since analog signals can be converted into discrete ones before being sent and restored after receiving without significant losses. The conditions ensuring the possibility of such a transformation are specified by the Kotelnikov theorem.
3. Signal
An analog signal is a physical quantity, the change (modulation) of which in space and time reflects the transmitted message. For example, changes in voltage (or current, frequency, phase, etc.) reflect the process of speech. The signal has the following measurements: height H (dynamic range), "width" F (spectrum width), length T (signal duration in time).
The volume of the signal is the product V = FHT. During signal transmission, measurements can be changed with or without volume retention. This is due to the following signal transformations:
- Restriction - removal from transmission of one or several parts of the signal without saving the information that was contained in the removed parts. For example, limiting the speech channel to the range of 300-3400 Hz (tone channel).
- Transformation - changing one or more dimensions by changing another or other dimensions while maintaining a constant volume (like a plasticine cube). For example, you can reduce transmission time by increasing the signal bandwidth or dynamic range, or both.
- Companding - includes two processes from which the name comes: compression (compression) and expansion (expansion). On the transmitting side, the signal is compressed in one or more dimensions, on the receiving side - recovery. For example, "biting out" pauses in speech on the transmitting side and restoring at the receiving side.
4. Communication line
Communication chain- conductors / fiber used to carry a single signal. In radio communication, the same concept has the name trunk... Distinguish cable chain- the chain in the cable and air circuit- suspended on supports.
Communication line(LAN) in the narrow sense is a physical medium through which information signals of data transmission equipment and intermediate equipment are transmitted. In a broad sense, a set of physical circuits and (or) linear paths of transmission systems that have common linear structures, their service devices and the same propagation medium (GOST 22348). Tract- a set of equipment and environments that form specialized channels that have certain standard indicators: frequency band, transmission speed, etc.
A line contains one or more communication chains (trunk). The signal acting on the line is called linear.
There are two main types of drugs:
- lines in the atmosphere (radio lines, RL);
- guiding transmission lines (communication lines).
5. Communication channel
To ensure the effective use of communication circuits on them with the help of channel forming equipment (CCE) are organized channels of connection... In some cases, the line, communication chain and communication channel coincide (one line, one circuit and one channel), in some cases the channel consists of several lines / circuits (both in series and in parallel). Channels can be nested within each other (group channel). A signal that "contains" several individual channels is called group signal... Channels can be divided into continuous (analog) and discrete (digital).
Communication channels in the direction of transmission are subdivided into:
- simplex- that is, allowing data transmission in only one direction, for example - radio broadcasting, television;
- half duplex one by one, for example - walkie-talkies;
- duplex- that is, allowing data transfer in both directions simultaneously, example is a telephone.
6. Separation (compaction) of channels
and Modulation.
Creation of several channels on one communication line is ensured by their diversity in frequency, time, codes, address, wavelength.
- frequency division of channels (FDM, FDM) - division of channels by frequency. A certain frequency range is allocated to each channel.
- time division of channels (TDM, TDM) - division of channels in time. Each channel is allocated a time slice (timeslot).
- code division of channels (QKK, CDMA) - division of channels by codes. Each channel has its own code, the imposition of which on the baseband signal allows you to highlight the information of a specific channel.
- spectral division of channels (SRK, WDM) - division of channels by wavelength.
It is possible to combine methods, for example CHRK + VRK, etc.
7. Communication network
Data transmission network
Telecommunication network (system) - a set of terminal devices, communication lines and communication nodes operating under a single control. For example: computer network, telephone network.
In general, the communication system consists of:
- terminal equipment(TOE, terminal device, terminal device) source and destination of the message, and
- signal conversion devices(OOI) at both ends of the line.
The terminal equipment provides the primary processing of the message and signal, the transformation of messages from the form in which they are provided by the source (speech, image, etc.) into the signal (on the side of the source, sender) and back (on the side of the receiver), amplification, etc. P.
Signal conversion devices can protect the signal from distortion, shaping the channel (s), matching the group signal (signal of several channels) with the line on the source side, recovering the group signal from a mixture of the useful signal and interference, dividing it into individual channels, error detection and correction on the recipient's side. Modulation is used to form the group signal and match with the line.
The communication line may contain signal conditioning devices such as amplifiers and regenerators... The amplifier simply amplifies the signal along with the interference and transfers it further, it is used in analog transmission systems(ASP). Regenerator ("re-receiver") - performs signal recovery without interference and re-shaping of the linear signal, is used in digital transmission systems(DSP). Amplification / regeneration points are serviceable and non-serviceable (OUP, NUP, ORP and NRP, respectively).
In DSP, terminal equipment is called DTE (data terminal equipment, DTE), MTP is called DCE ( data link termination equipment or line terminal equipment, DCE). For example, in computer networks, the role of the DTE is played by the computer, and the DCE is the modem.
8. Standardization
In the world of communications, standards are extremely important because communications equipment must be able to communicate with each other. There are several international organizations that publish communication standards. Among them:
- International Telecommunication Union (eng. International Telecommunication Union , ITU) is one of the UN agencies.
- Institute of Electrical and Electronics Engineers (eng. Institute of Electrical and Electronics Engineers , IEEE).
- Special Commission for Internet Development (eng. Internet Engineering Task Force , IETF).
In addition, standards are often (usually de facto) determined by the leaders of the telecommunications equipment industry.
A.P. Salnikov
THEORY
ELECTRIC COMMUNICATION
Lecture notes
Part 1
SAINT PETERSBURG
UDC 621.391.1
A.P. Salnikov Theory of electrical communication: Lecture notes, part 1 / SPbSUT. –SPb., 2002. –93 p .: ill.
It is intended for students studying the discipline "Theory of electrical communication".
Contains general information about communication systems, description of deterministic signal models. Transformations of signals in typical functional units of communication systems (modulators and detectors of various types, multipliers and signal frequency converters) are considered.
Control questions for all sections are given for self-testing of their assimilation and recommendations for conducting accompanying experimental research in a virtual training laboratory for the TPP course.
The material corresponds to the current curriculum for the TPP course.
Responsible editor M.N. Chesnokov
© Salnikov A.P., 2002
© Published by St. Petersburg State University
telecommunications them. prof. M.A. Bonch-Bruevich, 2002
Editor I.I. Szczęsnyak
ЛР No. dated 02. Signed to print. 02
Volume 8,125 academic-ed. l. Shooting gallery. 200 copies Zach.
RIO SPbGUT. 191186, St. Petersburg, nab. R. Moiki, 61
General information about communication systems
Information, messages, signals
Under information understand the totality of any information about phenomena, objects, etc. Posts represent a material form of existence of information and can have a different physical nature. Signals in electrical communication, there are processes (functions of time) of an electrical nature, through which messages are transmitted over a distance. The general and the different in these fundamental concepts of communication theory are explained in Table 1.1. It also indicates possible converters of messages into signals, which are called signal sensors.
Table 1.1.
Text messages are sequences of symbols from some finite set ( a i ) (language) with known alphabet size m... Conversion of this kind of messages into a signal can be carried out, for example, by a computer keyboard by sequentially coding individual message characters k- bit combinations of 0 and 1, which correspond to two different voltage levels.
Audio messages represent changes in air pressure at a given point in space in time p(t). With the help of a microphone, they are converted into an alternating electrical signal u(t), which in a certain sense is a copy of the message and differs from it only in physical dimension.
Video messages can be viewed as the distribution of brightness on the surface of an object b(x, y), the still image of which needs to be transmitted over a distance (phototelegraph), or a more complex process b(x, y, t) (black and white television). A characteristic feature of the transmission of video messages is the need to transform the multidimensional functions describing them into a one-dimensional signal u(t). This is achieved by using scanning devices (SD) in video signal sensors for element-wise conversion of the brightness of individual points of objects into the level of an electrical signal using photoelectric cells (PV) or other photoelectric converters.
Signal classification
According to the relative width of the spectrum, signals are divided into low-frequency (also called low-frequency, video, broadband signals) and high-frequency (high-frequency, radio, narrow-band, band-pass signals).
For LF signals Δ F/F cp> 1, where
Δ F = F max– F min is the absolute width of the signal spectrum,
F cf = ( F max + F min) / 2 - average frequency of the signal spectrum,
F max - maximum frequency in the signal spectrum,
F min is the minimum frequency in the signal spectrum.
For RF signals Δ F/F Wed<< 1.
As a rule, the primary signals at the output of the sensors are low-frequency. It is useful to remember the frequency ranges in which the spectra of typical signals in communication and broadcasting systems are located:
1) telephone - 300 ÷ 3400 Hz (standard tone frequency channel),
2) broadcasting - from 30-50 Hz to 6-15 kHz,
3) television - 0 ÷ 6 MHz (for the broadcast image decomposition standard adopted in Russia).
By their nature, signals are distinguished between deterministic and random. Deterministic signals are considered known at each point in the time axis. In contrast, the values random (stochastic) signals at each moment of time are a random variable with varying probability. Obviously, deterministic signals, due to their complete definiteness, cannot carry any information. It is convenient to use them in theory to analyze various functional units ( UGH), but in practice as test signals for measuring unknown parameters and characteristics of individual links of the paths of communication systems.
In terms of shape, signals can be divided into four types, shown in Table 1.2.
Table 1.2.
| Time t | |||
| continuous | discrete | ||
  The values u(t)
| Continuous | u(t) analog 1 t | u(t) t |
| Discrete | u(t)
 t
| u(t) digital 4  t
|
 |
Signal ( 1 ), continuous in time and states, is called analog... Signal ( 4 ), discrete in time and states, - digital... These signals are most often used in various nodes of communication systems. Accordingly, distinguish analog and digital FU by the form of signals at their inputs and outputs. Possibility of converting analog signal to digital using analog-to-digital converter (ADC) and vice versa - with the help digital-to-analog converter (DAC)... Graphic symbols (UGO) of these typical FUs are shown in Fig. 1.1.
Signals can be considered as objects of transportation through communication channels and can be characterized by basic parameters, such as
- signal duration T With,
- width of its spectrum F c,
- dynamic range 
, where
and - maximum and minimum
instantaneous signal strength.
They also use a more general characteristic - signal volume At the intuitive level, it is obvious that the larger the volume of the signal, the more informative it is, but the higher the requirements for the quality of the channel for its transmission.
Classification of communication systems
By the type of transmitted messages, they are distinguished:
1) telegraphy(text transmission),
2) telephony(voice transmission),
3) phototelegraphy(transmission of still images),
4) the television(transfer of moving images),
5) telemetry(transmission of measurement results),
6) telecontrol(transmission of control commands),
7) data transmission(in computing systems and ACS).
By frequency range - in accordance with the ten-day division of the ranges of electromagnetic waves from myriameter(3 ÷ 30) kHz to decimillimeter(300 ÷ 3000) GHz.
By appointment - broadcasting(high-quality transmission of speech, music, video from a small number of message sources to a large number of recipients) and professional(connected), in which the number of sources and recipients of messages is of the same order.
The following modes of operation of the CC are distinguished:
1) simplex(signal transmission in one direction),
2) duplex(simultaneous transmission of signals in forward and reverse directions),
3) half duplex(alternate transmission of signals in the forward and reverse directions).
Let's clarify the term we have already used link. It is customary to understand it as a part of the SS between points A on the transmitting side and B on the receiving side. Depending on the choice of these points, in other words, according to the type of signals at the input and output, channels are distinguished:
1) continuous,
2) discrete,
3) discrete-continuous,
4) continuous-discrete.
Communication channels can be characterized by analogy with signals with the following three parameters:
– access time ,
– bandwidth ,
– dynamic range 
[dB],
where is the maximum allowable power
signal in the channel,
Is the power of the channel's own noise.
The generalized parameter of the channel is its capacity
An obvious prerequisite for matching the signal and the channel is the fulfillment of the inequality V c< V To.
It is less obvious that this condition is also sufficient and it is not at all necessary to achieve a similar agreement in terms of particular parameters (duration, spectrum, dynamic range), since it is possible to “exchange” the signal spectrum width for its duration or dynamic range.
Control questions
1. Give definitions to the concepts of information, message, signal. What are the connections and differences between them?
2. Give examples of messages of different physical nature and corresponding signal sensors.
3. How are messages described by multidimensional functions converted into signals? Give examples.
4. Classify signals according to the characteristics of their shape and spectrum.
5. On what basis are LF and HF signals distinguished?
6. By what criterion are analog and digital signals and FUs distinguished?
7. Specify the main parameters of the signals.
8. Draw block diagrams of communication systems for:
Transmission of discrete messages,
Transmission of continuous messages,
· Transmission of continuous messages over digital channels.
9. Indicate the purpose of the following FU communication systems:
Source encoder and channel encoder,
Modulator,
Demodulator,
· Channel decoder and source decoder.
10. What is common and different in the tasks solved by the SPDS and SPNS demodulators?
11. What communication systems do you know:
By the type of transmitted messages,
By the range of frequencies used,
By appointment,
· By operating modes?
12. Give a definition of the term "communication channel". What classification of communication channels do you know?
13. Specify the main parameters of communication channels.
14. Formulate the conditions for the coordination of signals and communication channels.
To consolidate the results obtained in sections 1.1 and 1.2. knowledge, it is useful to complete laboratory work No. 14 "Acquaintance with PDS systems" (from the list of topics of the virtual training laboratory) in full. This work is for informational purposes and allows you to observe all the main processes of receiving, converting and receiving signals in systems for transmitting discrete messages (Fig. 1.3). You should pay attention to the oscillograms and spectrograms of signals at the outputs of typical FUs (source encoder when choosing different types of interface, channel encoder when choosing different noise-immune codes, modulator with different types of modulation, demodulator and decoder) included in the PDS systems, and compare your representations obtained during the study of the section.
It is recommended, based on the results of observation of signals at different points of the SPDS path, to classify them, determine their main parameters, and also select different types of channels in the SPDS (continuous, discrete, discrete-continuous and continuous-discrete). It is also useful to get a visual representation of the function of each FU SPDS.
To consolidate the information obtained about the difference between LF and HF signals and fill them with practical content, it is advisable to conduct research within the framework of laboratory work No. 4 "Modulated signals". When choosing as primary LF signals of different shapes, pay attention not only to the difference between oscillograms and spectrograms of primary (LF) and modulated (HF) signals, but also to the features that combine them when using different types of modulation (Fig. 1.4).
When performing these works, it is not necessary to strictly adhere to the tasks contained in them. Use the capabilities of the VL resources to conduct research at your own discretion and desire.
Spaces
Signals are, first of all, processes, i.e. functions of time x(t) existing on a limited interval T(in theory it is possible T→ ∞). They can be depicted graphically (Fig.2.1) and described by an ordered sequence of values at separate points in time t k
(vector string).
Different signals differ in shape (set of values x(t k)). Instead of a complex collection of points on a curve x(t) in a simple area - two-dimensional space, you can enter into consideration more complex spaces (signal spaces), in which each signal is represented by the simplest element - a point (vector).
In mathematics, space is understood as a set of objects (of any physical nature) endowed with some common property. The properties with which it is advisable to endow signal spaces should reflect the most essential properties of real signals, such as their duration, energy, power, etc.
Metric spaces
The first property that we will give the signal space is called the metric.
Metric space Is a set with a suitably defined distance between its elements. This distance itself, as well as the way it is determined, is called metric and denote. The metric must be functional, i.e. mapping of any pair of elements and a set to the real axis, satisfying the intuitive requirements (axioms):
1) (equality for ),
2) 
,
3) (triangle axiom).
It should be noted that metrics can be set in different ways and, as a result, different spaces can be obtained for the same elements.
Examples of metrics:
1) 
,
2) 
Euclidean metric,
3) 
Euclidean metric.
Linear spaces
We will improve the structure of the signal space, endowing it with simple algebraic properties inherent in real signals, which can be algebraically added and multiplied by numbers.
Linear space L over the field F call many elements 
, called vectors, for which two operations are specified - addition of elements (vectors) and multiplication of vectors by elements from the field F(called scalars). Without going into mathematical details, in what follows, under the field of scalars we mean the sets of real numbers R(real space case L) or complex numbers WITH(the case of a complex space L). These operations must satisfy the system of linear space axioms.
1. Closedness of operations of addition and multiplication by a scalar:
2. Addition properties:
associativity,
commutability.
3. Properties of multiplication by a scalar:
Associativity,
the distributivity of the sum of vectors,
distributiveness of the sum of scalars.
4. the existence of a zero vector.
5.existence against
in the opposite vector.
A vector formed by summing several vectors with scalar coefficients
are called linear combination(variety). It is easy to see that the set of all linear combinations of vectors for different a i(without affecting) also forms a linear space called linear shell for vectors.
The set of vectors is called linearly independent if equality
is possible only for all a i= 0. For example, on the plane, any two noncollinear vectors (not lying on one straight line) are linearly independent.
The system of linearly independent and nonzero vectors forms in space L basis, if
.
This single set of scalars (a i) corresponding to a specific vector is called coordinates(projections) on the basis.
Thanks to the introduction of the basis, operations on vectors turn into operations on numbers (coordinates)
If in linear space L can be found n linearly independent vectors, and any n+ 1 vectors are dependent, then n – dimension space L(dim L = n).
Normalized spaces
Our next step in improving the structure of the signal space is to combine geometric (typical for metric spaces) and algebraic (for linear spaces) properties by introducing a real number characterizing the "size" of an element in space. This number is called the norm vectors and denote.
As a norm, you can use any mapping of a linear space onto a real axis that satisfies the following axioms:
3) 
.
conclusions
1. The mathematical apparatus for spectral analysis of periodic signals is the Fourier series.
2. The spectra of periodic signals are discrete (linear), represent a set of amplitudes and phases of harmonic oscillations (components) of frequencies along the axis at intervals of Δ f = f 1 = 1/T.
3. The Fourier series is a special case of the generalized Fourier series when used as a basis
or .
Spectra of T-finite signals
Time-limited signals are called T-finite signals. By definition, they cannot be periodic and, therefore, Fourier series expansion is not applicable to them.
To obtain an adequate description of such signals in the frequency domain, the following technique is used. At the first stage, from a given signal x(t) starting at the point t 1 and end at point t 2 go to signal x P ( t), which is a periodic repetition x(t) on an infinite time axis with a period. Signal x P ( t) can be expanded in a Fourier series
,
where 
.
Let us introduce into consideration the current frequency and spectral density of the amplitudes 
.
Then 
.
Original signal x(t) can be obtained from x P ( t) as a result of the passage to the limit T®¥ .
, , å ® ò , ,
Thus, to describe the spectrum of a finite signal, we come to the integral Fourier transform known in mathematics:
- direct,
- the opposite.
In this case (and in the future), the complex function was written in the form, as is customary in the scientific and technical literature.
It follows from the obtained relations that the spectrum of the T-phi- nite signal is continuous. It is a collection of an infinite number of spectral components with infinitesimal amplitudes, continuously following the frequency axis. Instead of these infinitesimal amplitudes, a spectral function (spectral amplitude density) is used
where is the amplitude spectrum,
- phase spectrum.
conclusions
1. The integral Fourier transform is the mathematical apparatus for spectral analysis of T-finite signals.
2. The spectra of T-finite signals are continuous and described by continuous functions of frequency in the form of the modulus of the spectral density of amplitudes (amplitude spectrum) and its argument (phase spectrum).
Fourier transform properties
1. Direct and inverse Fourier transforms are linear operators therefore, the principle of superposition is at work. If, then 
.
2. The direct and inverse Fourier transforms are one-to-one.
3. Lag property.
If, then
(in this case, substitutions are used:).
4. Spectral function of the δ-function.
Using the general expression for the spectral function and the filtering property of the δ-function, we obtain
.
5. Spectral function of a complex harmonic signal .
(2.5)
Using one of the definitions of the δ-function 
and performing interchange in it t and w (or f), we get
Classification of telecommunication systems by purpose (types of transmitted messages) and type of signal propagation medium
Communication, communications, electronics and digital devices
The classification of telecommunication systems is very diverse, but it is mainly determined by the types of transmitted messages by the medium of propagation of telecommunication signals and the methods of distribution of message switching in the network Fig. 2 Classification of telecommunication systems by the types of transmitted messages and the medium of distribution The following communication systems are distinguished by the type of transmitted messages: telephone voice transmissions telegraph transmissions text facsimile transmissions of still images TV and sound broadcasting transmission of moving images and ...
Classification of telecommunication systems by purpose (types of transmitted messages) and type of signal propagation medium.
The classification of telecommunication systems is very diverse, but is mainly determined by the types of transmitted messages, the propagation medium of telecommunication signals and the methods of distribution (switching) of messages in the network (Fig. 1.2.2).
Figure 1.2.2 - Classification of telecommunication systems by type
transmitted messages and distribution media
By the type of transmitted messages, the following communication systems are distinguished: telephone (voice transmission), telegraph (text transmission), facsimile (still image transmission), television and sound broadcasting (moving image and sound transmission), telemetry, telecontrol and data transmission.
By design, telephone and television systems are divided into broadcasting, characterized by a high degree of artistic reproduction of messages, and professional, with a special application (office communications, industrial television, etc.). In the telemetry system, the measured physical quantity (temperature, pressure, speed, etc.) is converted with the help of sensors into a primary electrical signal entering the transmitter. At the receiving end, the transmitted physical quantity or its changes are separated from the signal and observed or recorded using recording devices. In the telecontrol system, commands are transmitted to automatically perform certain actions.
Data transmission systems, ensuring the exchange of information between computing facilities and objects of automated control systems, differ from telegraph ones by higherspeed and fidelity of information transfer.
Depending on the propagation medium of signals, a distinction is made between systems (lines) of wire communication (air, cable, fiber-optic, etc.) and radio communication. Cable communication systems are the backbone of long-distance communication networks; signals are transmitted through them in the frequency range from tens of kHz to hundreds of MHz. Fiber-optic communication lines (FOCL) are very promising. They allow in the range from 600 to 900 GHz (0.5 ... 0.3 microns) to provide a very high bandwidth (hundreds of television or hundreds of thousands of telephone channels). Along with wired communication lines, radio lines of various ranges (from hundreds of kHz to tens of GHz) are widely used. These lines are more economical and indispensable for communication with mobile objects. The most widespread for multichannel radio communication are radio relay lines (RRL) of the meter, decimeter and centimeter ranges at frequencies from 60 MHz to 40 GHz. A type of RRL are tropospheric lines using reflections from tropospheric irregularities. Satellite communication lines (SLS) are increasingly used - RRL with a repeater on the satellite. Frequency ranges from 4 to 6 and from 11 to 27.5 GHz are allocated for these communication lines (systems). Long range with one repeater on the satellite, flexibility and the possibility of organizing global communications are important advantages of SLS.
The frequency ranges of electromagnetic oscillations used in radio communication systems are presented in table. 1.2.1.
Table 1.2.1 - Frequency range of electromagnetic oscillations,
used in radio communication systems
Communication systems can operate in one of three modes:
Simplex - the transmission of messages is carried out in one direction from the source to the recipient;
Duplex - provides the possibility of simultaneous transmission of messages in the forward and backward direction;
Half duplex - the exchange of messages is carried out in turn.
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By their nature, messages can be discrete-valued (or discrete) and continuous-valued (or continuous).
Discrete-valued messages are those that take a finite or countable number of values. Typical examples of such messages are alphanumeric text, consisting of letters, numbers, and punctuation marks.
If the set of messages is continuous, then such messages are called continuous. Such messages include speech, motion picture, etc.
To transmit messages of various physical nature (speech, image, digital data) via radio channels, it is necessary to convert them into electrical oscillations, called primary signals. There must be a one-to-one correspondence between the message and the signal, which makes it possible to obtain at the receiving point
transmitted message. For example, the sound pressure during the transmission of voice messages is converted by a microphone into an electrical voltage.
Electrical signals that are analogs of continuous-valued messages are called analog. Primary electrical signals corresponding to discrete-valued messages are called digital. The process of converting discrete-valued messages into digital signals is called encoding. When coding, each message from the ensemble is put in a one-to-one correspondence with a code combination of unit elements of a digital signal, which is called a primary code. Electrical impulses are usually used as single elements of code combinations, which have quite definite values of the amplitude - the representing (information) parameter of a digital signal. The number of distinct values of the representing parameter used to construct the codewords determines the base of the code. Depending on the value of the code base T distinguish between binary t = 2, ternary t = 3 and, in general, m-personal codes. In digital messaging systems, binary codes are usually used, in which the values of the amplitude of single pulses are usually identified with the symbols 1 and 0. The symbols of the elements of the code combinations 1 and 0 are called bits. The use of binary codes allows you to use
in communication equipment, standard elements of digital technology. Analog signals can be converted to pulse and digital signals. Conversion of an analog signal to a pulse signal is achieved by sampling it in time in accordance with the sampling theorem. Converting an analog signal to digital is achieved by sampling it in time and quantizing it in terms of level. The levels of the quantized samples can be converted to codewords of a digital signal.
To transmit a message in the transmission path, the primary signal is converted into a radio signal using modulation or keying.
Modulation is called the process of changing the parameters of the radio frequency oscillation in accordance with the change in the information parameter of the primary signal (message).
The unmodulated harmonic signal is called the carrier. The energy of the primary signals is concentrated mainly in the low-frequency region, therefore, the spectra of the primary signals are transferred to the high-frequency region by modulating the high-frequency carrier (carrier) in the transmitter with the primary signal. The average carrier frequency is much larger than the message bandwidth.
In radio communication systems, the transmitted message modulates one or a set of parameters of the high-frequency carrier. The carrier parameters changed during modulation are called informative parameters. The informative parameter of the high-frequency carrier determines the name of the modulation type. The number of possible types of modulation for a given type of carrier is determined by the number of its parameters.
As a carrier, high-frequency harmonic oscillations, pulse sequences, complex composite sequences, etc. are used.
In single-channel radio communication systems, direct modulation of a harmonic carrier with a transmitted message is most often carried out. The signal in such systems has one modulation stage. In this case, three main types of harmonic carrier modulation are possible: amplitude (AM), frequency (FM) and phase (FM). Varieties of amplitude modulation are suppressed carrier dual-band modulation (DSM) and single-sideband modulation (OM).
Frequency modulation and phase modulation are usually considered two types of angle modulation.
The modulation of an RF signal by a primary pulse signal (pulse train) is called pulse modulation. When used as a carrier of a periodic sequence of pulses of a certain shape, four main types of pulse modulation are distinguished: amplitude-pulse, pulse-width, phase-pulse and frequency-pulse. With pulse modulation in transmitters of radio communication systems, a second stage is required, in which the high-frequency oscillation is modulated with a sequence of pulses. As a result, a number of two-stage types of modulation are obtained: amplitude-pulse-amplitude modulation, phase-pulse-amplitude modulation, etc.
In multichannel systems, the transmitted message modulates an intermediate carrier - a subcarrier, which in turn modulates the carrier. In this case, the signal is generated using two modulation stages: the first is determined by the subcarrier modulation method, and the second is determined by the carrier modulation method. In frequency and phase division multiplexing systems, a harmonic waveform is used as the subcarrier, in time division systems, a sequence of pulses, and in code division systems, a coded sequence of pulses.
If the primary signals of continuous messages are presented in
analog form, they are directly fed to the modulator. In the digital representation of continuous messages, the set of coding and modulation operations, similar to the same operations in the transmission of discrete messages, is called pulse code modulation(PCM).
In the process of modulation, the spectrum of the primary signal is transferred to a given frequency domain, which allows placing the spectra of signals of various radio communication systems in an orderly manner in each frequency range allocated for radio communication.
The modulation of a radio frequency waveform by a primary digital signal is called manipulation.
Thus, the transmitted message can be sent to the input of the communication channel in the form of an analog, pulse or digital primary signal. In the transmitter, using modulation or keying, the primary signal is converted into a radio signal used to transmit a message over a communication line. The classification of messages and signals is shown in Fig. 2.2.
By the type of radio signals, all radio communication systems are divided into three groups: analog signal transmission systems (analog radio communication systems); digital signal transmission systems (digital radio communication systems); pulse signal transmission systems (pulse radio communication systems). Aviation radio stations provide the ability to transmit and receive several types of messages: voice, telegraph and various data.
Historically, various types of telecommunications have developed independently of each other for a long period of time. All types of telecommunications deal with electrical signals of different nature and parameters, therefore, each type in its development focused on the creation of its own channels, systems and even its own network. The structure of the network was chosen in accordance with the peculiarities of the distribution of message flows, typical for a particular type of telecommunication. As a result, several independent networks have been formed. The means of communication from which the networks were created turned out to be scattered. Already in the early 1960s. it became clear that interconnection of networks should become a promising direction for the development of telecommunications. First of all, it was required to combine homogeneous networks within each type of telecommunication, and then isolated networks of individual types of telecommunications.
The need to transmit electrical signals in coinciding directions made it possible to raise the question of combining separate transmission systems in coinciding directions into a single transmission system. A transmission system is a set of technical means that allows the formation of independent electrical channels through which telecommunication signals are transmitted.
Finally, one of the most important prerequisites leading to the merger of networks is the similarity of the functions performed by different switching systems and consists in organizing the transmission paths of messages for their delivery from the sender to the recipient.
All this gave rise to the need for the construction and development of various telecommunication networks, taking into account the prospect of their merging into a single communication network.
Given these circumstances, at the end of the 1960s. It was decided to create a Unified Automated Communication Network (EASC) in the country, which would unite all telecommunication networks, regardless of their departmental affiliation.
The creation of the EASC was based on the unification of disparate and numerous small networks into national networks of each type of telecommunication, and then into a single network for the purpose of joint use of certain technical means, and primarily transmission and switching systems.
However, at the end of the 20th century, the course of development of technical progress, in particular, the widespread introduction of modern telecommunication technologies into the country's communication network, as well as historical changes in the political and economic structure of Russia, predetermined the creation of a new concept of building a communication network.
The interconnected communication network of the Russian Federation (RF VSS) is part of the country's infrastructure and is a set of networks, services and communication equipment located and operating in the country. It is designed to meet the needs of the population, public authorities and administration, defense, security, law and order, as well as users of all categories in telecommunication services.
All communication networks included in the unified telecommunication network (ESE) of the Russian Federation can be classified according to several criteria (Fig. 2.7):
Rice. 2.7 Classification of communication networks of the Russian Federation
In technical terms, the functioning of the RF ARIA is based on the principles and structures, according to which the entire communication network of the country is divided into two interrelated components: the primary network and the secondary network.
The primary network is a collection of all channels without their subdivision by purpose and type of communication. It includes lines and channel-forming equipment.
The secondary network consists of single-purpose channels (telephone, telegraph, broadcasting, data transmission, television, etc.), formed on the basis of the primary network. The secondary network includes switching nodes, endpoints, and circuits dedicated to the primary network.
In addition to the accepted division of the ESE networks into primary and secondary, another two-level division is possible, according to functional purpose: into a transport network and an access network.
The transport communication network consists of intercity and zonal (regional) communication networks. The access network (subscriber network or subscriber access network) is a local network. The transport network is intended for the transmission of high-speed (broadband) message streams and their accumulation.
The access network consists of subscriber lines (on metal or optical cables or radio channels) with subscriber terminal devices of local switching stations connected to them, connecting their transmission lines and transmission lines to the nodes of the transport network.
Telecommunication management network is a special network that provides management of telecommunication networks and their services by organizing interconnection with the components of various telecommunication networks based on common interfaces and protocols standardized by the International Telecommunication Union.
The telecommunication management network provides unified management of digital networks included in the RF ARS.
Based on the territorial characteristics and purpose, primary and secondary networks are subdivided into backbone (intercity - for secondary networks), intrazonal (zonal) and local networks, as well as international networks.
Backbone communication networks are technologically interconnected long-distance telecommunication networks formed between the center of the Russian Federation and the centers of the constituent entities of the Federation, as well as the centers of the constituent entities of the Federation among themselves.
Zonal (regional) communication networks are technologically interconnected telecommunication networks formed within the territory of one or several constituent entities of the Federation.
Local communication networks - technologically interconnected telecommunication networks, formed within the administrative or otherwise defined territories, not related to regional communication networks. Local networks are subdivided into urban and rural.
Backbone, intra-zone and part of the local digital superimposed primary networks are the basis of the transport digital communication network in Russia. Local and primary networks in the section "local node - terminal device" in accordance with the new terminology are an access network (Fig. 2.8).
Rice. 2.8 The principle of construction of the primary network of the ESE
The structure of the primary network takes into account the administrative division of the territory of the country. The entire territory of Russia is divided into zones, which, as a rule, coincide with the territory of regions, territories, and sometimes republics.
Each ESE channel provides transmission of telecommunication signals.
A telecommunication service is an organizational and technical structure based on a communication network (or a set of communication networks) that provides communication services to users in order to meet their needs for a certain set of telecommunication services. There are three types of telecommunication services: voice services, documentary telecommunication services and multimedia services.
The classification of telecommunication services is shown in Figure 2.9.
Rice. 2.9 Telecommunication services
Traditional communication networks (public switched telephone networks - PSTN, data transmission networks (DTS)) are characterized by a narrow specialization. For each type of communication there is a separate network that requires maintenance, while the free resources of one network cannot be used by another network. from numerous superimposed secondary networks, to ensure the introduction of new services with different requirements for the volume of transmitted information and the quality of its transmission.
The multiservice network forms a single information and telecommunications structure that supports all types of traffic (data, voice, video) and provides all types of services (traditional and new, basic and additional) at any point, at any time, in any set and volume.
The basic services of a multiservice network include traditional transmission and access services:
Additional services include the following:
The need to create multiservice networks is dictated by the emerging telecommunication services market.
