Computer telecommunication systems called the exchange of information over a distance between several computers.
Computer communication channels can be classified according to the following criteria:
- According to the method of encoding information, it can be divided into digital and analog;
- According to the method of communication, they can be divided into dedicated and dial-up;
- According to the method of information transmission, they are divided into wired, wireless, and optical.
Analog- over analog channels, the information that is transmitted is presented in continuous form, that is, in the form of a continuous series of values of any physical quantity.
Digital- these are channels through which the transmitted information is transmitted in the form of digital (discrete, pulsed) signals of one or another physical nature.
Switched- these are channels created from separate sections only for the duration of the transmission of information through them; after the end of the communication session, such a channel is broken.
Dedicated channels- these are channels that are organized for a long time and have constant characteristics in terms of length and capacity.
The main characteristics of communication channels include information transmission speed, reliability, cost, and development reserves.
Information transfer speed is measured in bits/s and bauds. The number of changes in the signal information parameter per second is measured in baud.
Baud- this is the speed when one signal (for example, a pulse) is transmitted per second, regardless of the magnitude of its change. The unit of measurement bit/s corresponds to a single change in the signal in the communication channel and with simple signal encoding methods; when any change is only single, we can assume that: 1 baud = 1 bit/s; 1 Kbaud = 103 bps; 1 Mbaud = 106 bps, etc.
If a data element can be represented not by two, but by a large number of values of any signal parameter, the value of 1 baud will be more than 1 bit per second.
Reliability- transfer of information without loss or change. Transmitter and receiver are data transmission equipment that connect the source and receiver of information with a communication channel. Examples of data transmission equipment include modems, terminal adapters, network cards, etc.
To improve the quality of a signal transmitted over long distances, additional equipment is used: repeaters, switches, concentrators, routers, multiplexers.
The classification is based on these principles, taking into account the throughput of the communication channel:
- low-speed communication channels, the information transmission speed in them ranges from 50 to 200 bit/s;
- medium-speed communication channels, the transmission speed in them is from 300 to 9600 bps, and in new standards up to 56,000 bps;
- high-speed (broadband) communication channels providing information transmission speeds above 56,000 bps.
The speed characteristics of the channel largely depend on the cables used.
twisted pair- these are insulated copper wires, the usual diameter of which is 1 mm, twisted in pairs around one another in the form of a spiral. This allows you to reduce the electromagnetic interaction of several twisted pairs located nearby.
The most common application of twisted pair cable is a telephone line. Twisted pairs running over long distances are combined into a cable, which is covered with a protective coating. If the pairs of wires inside such cables were not twisted, then the signals passing through them would overlap each other. Telephone cables several centimeters in diameter can be seen strung on poles.
Twisted pair cables are used to transmit analog and digital signals. The bandwidth depends on the diameter and length of the wire, but over long distances it can reach several megabits per second.
There are two types of twisted pair:
- Unshielded twisted pair cables They have a fairly high throughput, are easy to use, do not require grounding and, due to their low price, are widely used. Unshielded twisted pair cable is not used in a local area network that processes sensitive information because it may increase field strength.
- Shielded twisted pairs have good technical characteristics, but are high in cost, rigid and inconvenient to operate, and require grounding. This type of cable is used mainly in networks with limited access to information.
Coaxial cable- means of data transmission. It is better shielded than twisted pair, so it can transmit data over longer distances at higher speeds. Two types of cables are widely used. One is used to carry only a digital signal, and the other type of cable is used to carry an analog signal.
Coaxial cable consists of an insulated, solid copper wire located in the center of the cable. A cylindrical conductor, usually made in the form of a fine copper mesh, is stretched over the insulation. It is covered with an outer protective layer of insulation (plastic shell). The design and special type of shielding of the coaxial cable provide high throughput and excellent noise immunity.
Coaxial cables for telecommunications are divided into two groups:
- “thick” coaxials;
- "thin" coaxials.
The thick coaxial cable has an outer diameter of 12.5 mm and a fairly thick conductor (2.17 mm), providing good electrical and mechanical characteristics.
The data transfer speed over a thick coaxial cable is up to 50 Mbit/s, but given the certain inconvenience of working with it and its significant cost, it is not always possible to use it in data networks.
A thin coaxial cable has an outer diameter of 5-6 mm, it is cheaper and more convenient to use, but the thin conductor in it (0.9 mm) causes worse electrical and mechanical characteristics. The data transfer speed over “thin” coaxial does not exceed 10 Mbit/s.
Coaxial cables were widely used in telephone systems, but for long distance lines they are being replaced by fiber optic cables. However, coaxial cables are widely used for cable television.
Fiber optic cables Its structure resembles a twisted pair. The basis of the fiber optic cable is a glass core through which light propagates, surrounded by a solid core and placed in a protective shell with a diameter of 125 microns.
One cable can contain from one to several hundred such cores. The core is covered with a layer of glass with a lower refractive index than the core. It is designed to more reliably prevent light from escaping beyond the core.
The outer layer is a plastic shell that protects the glazing. The source of the light beam propagated through the fiber optic cable is a converter of electrical signals into optical signals, for example an LED or a semiconductor laser.
Information is encoded by changing the intensity of the light beam. The physical basis for transmitting a light beam along a fiber is the principle of total internal reflection of the beam from the walls of the fiber, which ensures minimal signal attenuation, the highest protection from external electromagnetic fields and high transmission speed. A fiber optic cable with a large number of fibers can carry a huge number of messages. At the other end of the cable, the receiving device converts the light signals into electrical signals.
The data transmission speed via fiber optic cable reaches 1000 Mbit/s, but it is very expensive and is used only for laying critical backbone communication channels. This cable connects the capitals and major cities of most countries of the world, as well as continents.
In computer networks and the Internet, fiber optic cable is used in their most critical areas. The possibilities of fiber optic channels are truly limitless: one thick backbone fiber optic cable can simultaneously organize several hundred thousand telephone channels, several thousand video telephone channels and about a thousand television channels.
Currently, wireless types of communication are becoming widespread: radio channels, infrared and millimeter waves.
Radio channel is a wireless communication channel laid over the air. A radio data transmission system includes a radio transmitter and a radio receiver tuned to the same radio wave range, which is determined by the frequency band of the electromagnetic spectrum used for data transmission.
Such a data transmission system is simply called a radio channel. Data transmission rates over a radio channel are practically unlimited (they are limited by the bandwidth of the transceiver equipment). High-speed radio access provides users with channels with transmission speeds of 2 Mbit/s and higher. In the near future, radio channels with speeds of 20-50 Mbit/s are expected.
Infrared and millimeter wave radiation without the use of a cable is widely used for communication over short distances. Remote controls for televisions and VCRs use infrared radiation. They are relatively directional, cheap and easy to install, but have one important drawback: infrared radiation does not pass through solid objects. On the other hand, the fact that infrared waves do not pass through walls is also positive. After all, this increases the security of the infrared system from eavesdropping compared to the radio system.
For this reason, the use of an infrared communication system does not require a government license, unlike radio communications (except for the ISM bands). Infrared communications are used in desktop computing systems (for example, to link laptops with printers), but still do not play a significant role in telecommunications.
Wireless communication channels have poor noise immunity, but provide the user with maximum mobility and efficiency of communication. In computer networks, wireless communication channels for data transmission are most often used where the use of traditional cable technologies is difficult or simply impossible.
But in the near future the situation may change - new Bluetooth wireless technology is being actively developed. Bluetooth is a technology for transmitting data over radio channels over short distances, allowing communication between wireless phones, computers and various peripherals even in cases where the line of sight requirement is violated.
Bluetooth was initially seen solely as an alternative to infrared connections between various portable devices. But now experts predict two directions for the widespread use of Bluetooth.
The first is home networks, which include various electronic equipment, in particular computers, televisions, etc. The second, much more important, direction is local area networks of offices of small firms, where the Bluetooth standard is positioned as a replacement for traditional wired technologies. The disadvantage of Bluetooth is the relatively low data transfer speed - it does not exceed 720 Kbps, so this technology is not capable of transmitting a video signal.
In Fig. 1 the following designations are adopted: X, Y, Z, W– signals, messages ; f– interference; PM- communication line; AI, PI– source and receiver of information; P– converters (coding, modulation, decoding, demodulation).
There are different types of channels, which can be classified according to various criteria:
1.By type of communication lines: wired; cable; fiber optic;
power lines; radio channels, etc.
2. By the nature of the signals: continuous; discrete; discrete-continuous (signals at the input of the system are discrete, and at the output are continuous, and vice versa).
3. In terms of noise immunity: channels without interference; with interference.
Communication channels are characterized by:
1. Channel capacity is defined as the product of the channel usage time T to, width of the frequency spectrum transmitted by the channel F to and dynamic range D to. , which characterizes the channel’s ability to transmit different signal levels
V k = T k F k D k.(1)
Condition for matching the signal with the channel:
Vc£ Vk ; Tc£ Tk ; Fc£ F k ; Vc£ Vk ; D c£ Dk.
2.Information transfer rate – the average amount of information transmitted per unit of time.
3.
4. Redundancy – ensures the reliability of the transmitted information ( R= 0¸1).
One of the tasks of information theory is to determine the dependence of the speed of information transmission and the capacity of a communication channel on the parameters of the channel and the characteristics of signals and interference.
The communication channel can be figuratively compared to roads. Narrow roads – low capacity, but cheap. Wide roads provide good traffic capacity, but are expensive. Bandwidth is determined by the bottleneck.
The data transfer speed largely depends on the transmission medium in communication channels, which use different types of communication lines.
Wired:
1. Wired– twisted pair (which partially suppresses electromagnetic radiation from other sources). Transfer speed up to 1 Mbit/s. Used in telephone networks and for data transmission.
2. Coaxial cable. Transmission speed 10–100 Mbit/s – used in local networks, cable television, etc.
3. Fiber optic. Transfer speed 1 Gbit/s.
In environments 1–3, the attenuation in dB depends linearly on distance, i.e. power drops exponentially. Therefore, it is necessary to install regenerators (amplifiers) at a certain distance.
Radio lines:
1.Radio channel. Transfer speed 100–400 Kbps. Uses radio frequencies up to 1000 MHz. Up to 30 MHz, due to reflection from the ionosphere, electromagnetic waves can propagate beyond the line of sight. But this range is very noisy (for example, amateur radio communications). From 30 to 1000 MHz – the ionosphere is transparent and direct visibility is necessary. Antennas are installed at height (sometimes regenerators are installed). Used in radio and television.
2.Microwave lines. Transfer speeds up to 1 Gbit/s. Radio frequencies above 1000 MHz are used. This requires direct visibility and highly directional parabolic antennas. The distance between regenerators is 10–200 km. Used for telephone communications, television and data transmission.
3. Satellite connection. Microwave frequencies are used, and the satellite serves as a regenerator (for many stations). The characteristics are the same as for microwave lines.
2. Bandwidth of a discrete communication channel
A discrete channel is a set of means designed to transmit discrete signals.
Communication channel capacity – the highest theoretically achievable information transmission speed, provided that the error does not exceed a given value. Information transfer rate – the average amount of information transmitted per unit of time. Let us define expressions for calculating the information transmission rate and the throughput of a discrete communication channel.
When transmitting each symbol, an average amount of information passes through the communication channel, determined by the formula
I (Y, X) = I (X, Y) = H(X) – H (X/Y) = H(Y) – H (Y/X), (2)
Where: I (Y, X) – mutual information, i.e. the amount of information contained in Y relatively X;H(X)– entropy of the message source; H(X/Y)– conditional entropy, which determines the loss of information per symbol associated with the presence of interference and distortion.
When sending a message X T duration T, consisting of n elementary symbols, the average amount of transmitted information, taking into account the symmetry of the mutual amount of information, is equal to:
I(Y T, X T) = H(X T) – H(X T /Y T) = H(Y T) – H(Y T /X T) = n . (4)
The speed of information transmission depends on the statistical properties of the source, the coding method and the properties of the channel.
Bandwidth of a discrete communication channel
. (5)The maximum possible value, i.e. the maximum of the functional is sought over the entire set of probability distribution functions p (x).
The throughput depends on the technical characteristics of the channel (equipment speed, type of modulation, level of interference and distortion, etc.). The units of channel capacity are: , , , .
2.1 Discrete communication channel without interference
If there is no interference in the communication channel, then the input and output signals of the channel are connected by an unambiguous, functional relationship.
In this case, the conditional entropy is equal to zero, and the unconditional entropies of the source and receiver are equal, i.e. the average amount of information in a received symbol relative to the transmitted one is
I (X, Y) = H(X) = H(Y); H(X/Y) = 0.
If X T– number of characters per time T, then the information transmission rate for a discrete communication channel without interference is equal to
(6)Where V = 1/ – average transmission speed of one symbol.
Throughput for a discrete communication channel without interference
(7)Because the maximum entropy corresponds to equally probable symbols, then the throughput for uniform distribution and statistical independence of transmitted symbols is equal to:
. (8)Shannon's first theorem for a channel: If the information flow generated by the source is sufficiently close to the communication channel capacity, i.e.
then you can always find a coding method that will ensure the transmission of all source messages, and the information transmission rate will be very close to the channel capacity.
The theorem does not answer the question of how to carry out coding.
Example 1. The source produces 3 messages with probabilities:
p 1 = 0,1; p 2 = 0.2 andp 3 = 0,7.
Messages are independent and are transmitted in a uniform binary code ( m = 2 ) with a symbol duration of 1 ms. Determine the speed of information transmission over a communication channel without interference.
Solution: The source entropy is equal to
[bit/s].
To transmit 3 messages with a uniform code, two digits are required, and the duration of the code combination is 2t.
Average signal speed
V =1/2 t = 500 .
Information transfer rate
C = vH = 500 × 1.16 = 580 [bit/s].
2.2 Discrete communication channel with interference
We will consider discrete communication channels without memory.
Channel without memory is a channel in which each transmitted signal symbol is affected by interference, regardless of what signals were transmitted previously. That is, interference does not create additional correlative connections between symbols. The name “no memory” means that during the next transmission the channel does not seem to remember the results of previous transmissions.
Communication lines are used to transmit signals between interacting systems in computer networks.
In the narrow sense of the term communication line(transmission link, link) refers to the physical medium over which signals are transmitted between two end systems. Signals are generated by special technical means (transmitters, amplifiers, multiplexers, etc.) related to network equipment.
Transmission medium (transmission medium) or physical environment- a material substance through which signals are propagated.
Computer networks use two types of transmission media: cable and wireless.
Rice. 3.1 Types of transmission media
The basis of wireless transmission media is the earth's atmosphere or outer space through which electromagnetic waves propagate. Cable transmission media uses various types of cables: coaxial, fiber optic, twisted pair. The transmission of signals in them is carried out using electrical (electric current) or optical (light) signals.
In a broad sense, the term « communication line" in the field of computer networks means link.
Link (channel, data link) is a set of one or more physical transmission media and channel-forming (network) equipment that provide data transfer between interacting systems in the form of signals corresponding to the type of physical medium.
In this context, the terms "communication line" and "communication channel" are synonymous.
Rice. 3.2 Link
Distinguish physical (physical link) And brain teaser (logical link) channels. A physical communication channel is a means of transmitting signals between interacting systems. Depending on the type of signals transmitted and the physical medium used for their distribution, physical channels are divided into electric(twisted pair, coaxial cable), optical(fiber optic cable) and wireless(radio channels, infrared channels, etc.).
Logical channels are established between protocols of any layers of the OSI model of interacting systems and define the path along which data is transmitted from a source to a receiver through one or a sequence of physical channels.
When laying several logical channels in a physical channel, the resources of the physical channel are distributed between the logical channels using methods multiplexing.
Rice. 3.3 Physical and logical communication channels
Communication channels (lines) can be classified based on the following characteristics:
● by type of physical environment;
● by type of presentation of transmitted information;
● in the direction of data transmission;
● by lifetime;
● by connection method;
● by bandwidth.
Depending on the type of presentation of transmitted information, channels are divided into analog, designed for transmitting analog signals and discrete, used for transmitting discrete (digital) signals.
Depending on the direction of data transmission, channels are distinguished:
● simplex(simplex), in which the transfer is carried out only in one direction;
● half duplex(half-duplex alternately forward and reverse;
● duplex(duplex), in which the transmission is carried out simultaneously in two directions - forward and reverse.
Rice. 3.4 Simplex channel
Rice. 3.5 Half duplex channel
Rice. 3.6 Duplex channel
Channels can also be classified according to the time of availability for the subscriber. Channels between end systems that are available for long-term data transmission due to a permanently existing connection with specified characteristics are called highlighted or non-switched. Communication channels through which data transfer is possible only after establishing a connection between interacting systems are called dial-up or temporary. In this case, the channel will exist only during the communication session, i.e. time required for data transfer.
According to the connection method, channels are divided into: "point-to-point" (point-to-point), "point-to-multipoint" (point-to-multipoint),"multipoint"» ( multipoint). A point-to-point link connects only two nodes or two communicating systems. A point-to-multipoint link connects one central system (node) to a group of other systems (nodes). A multipoint link allows a group of nodes or systems to connect to each other.
An important characteristic of a communication channel is its bandwidth (bandwidth). Depending on the bandwidth (the difference between the limiting frequencies of the passband) and the method of signal transmission, channels are divided into baseband (baseband channel) And broadband (broadband channel).
The baseband channel is characterized by simplicity and low cost of implementation, and therefore is widely used in local networks (the word “BASE” in the names of Ethernet physical layer specifications (for example, 10BASE-T, 100BASE-FX, 1000BASE-SX) indicates baseband transmission) . The signal over the baseband channel is transmitted in the base frequency band, i.e. without carrier modulation, with the entire bandwidth used to transmit only one signal.
Unlike a baseband channel, the entire bandwidth of a wideband channel is divided between several logical channels using multiplexing techniques, allowing signals to be transmitted simultaneously and independently between multiple pairs of communicating systems. Broadband access technologies (for example, xDSL, PowerLine (PLC), 3G (UMTS), 4G (LTE)) are used to organize connection to a range of services offered by telecom operators.
Signals
Data transmission over communication channels is carried out using their physical representation - electrical (electric current), optical (light) or electromagnetic signals.
If we consider the signal as a function of time, then it can be:
● analog (continuous) - its value changes continuously over time;
● digital (discrete) - having a finite, usually small number of values.
Rice. 3.7 Analog signal
Rice. 3.8 Digital signal
The signals used to transmit the data stream must be informative, i.e. have some variable parameters that will allow the receiver to identify the received data. This signal is often used harmonic signal.
Harmonic signal- these are harmonic oscillations that spread over time in space, which carry information or some kind of data. Harmonic vibrations- these are oscillations in which a physical (or any other) quantity changes over time according to a sinusoidal or cosine law.
The harmonic signal carries information in the form of three parameters: amplitudes, phases And frequencies and is described by the formula:
where A is the signal amplitude; ω – circular frequency: ω=2πf (f– linear frequency: f=1/T , the reciprocal of the period T); φ 0 – initial phase of the harmonic signal; t- time.
Rice. 3.9 Harmonic signal
To ensure high data transfer rates, frequency is important: the higher the frequency, the faster the transfer speed.
Time function y(t) can be arbitrary and have different frequencies.
Let us recall the theory of harmonic analysis and the Fourier transform. French scientist J.B. Fourier proved that any change in time of some function can be approximated as a finite or infinite sum of a series of harmonic oscillations with different amplitudes, frequencies and initial phases.
In other words, any periodic signal (analog or digital), described by a complex function of time, can be represented as an infinite or finite sum of simple harmonic oscillations (harmonics) with frequencies that are multiples of the fundamental frequency ω=2π/T:
Where i– harmonic number; A i– amplitude, φi– initial phase, ωi– angular frequency i th harmonics; t- time.
First harmonic ω 1 is called the first or fundamental harmonic of the signal, all other harmonics are called higher. In this case, the frequency of each subsequent harmonic is greater than the previous one ω 1 < ω 2 < ω 3 ….. < ω ∞.
Periodic signal This type of influence is called when the signal shape is repeated after a certain time interval T, which is called the period.
Rice. 3.10 Formation of a signal from the sum of the first 4 harmonics and spectral amplitude diagram of a periodic signal
A set of harmonic oscillations, the sum of which makes up the original signal, forms frequency spectrum this signal, i.e. the range of frequencies that make up a given signal.
There are practically no signals in nature that would have an infinite spectrum. The predominant part of the energy of real signals is concentrated in a limited region (band) of frequencies, and the signal itself is represented as a finite sum of harmonic oscillations. In this case, the signal spectrum y(t) is defined as the difference between the frequencies of the upper and lower harmonics: f n -f 1, where n< ∞.
From the set of harmonics that make up the signal, they isolate and distinguish amplitude And phase range. The amplitude spectrum is the set of amplitudes of all harmonics, which is usually represented by a diagram in the form of a set of vertical lines, the lengths of which are proportional (on a selected scale) to the amplitude values of harmonic oscillations, and the place on the horizontal axis is determined by the frequency (harmonic number) of this component. The amplitude and phase spectrum uniquely determine the signal. However, for many practical problems it is sufficient to limit ourselves to the amplitude spectrum.
When transmitting a signal over a communication channel, its shape is distorted due to unequal deformation of harmonics of different frequencies. This occurs because the physical parameters of the communication channel differ from ideal ones. The signal is affected by factors such as attenuation, noise and interference. However, the main factor influencing the signal shape is the bandwidth of the communication channel. In order to transmit a signal without significant distortion, the communication channel must have bandwidth not less than the frequency spectrum width transmitted signal.
Rice. 3.11 Influence of physical parameters of the transmission medium on the signal
State exam
(State examination)
Question No. 3 “Communication channels. Classification of communication channels. Communication channel parameters. Condition for transmitting a signal over a communication channel.”
(Plyaskin)
Link. 3
Classification. 5
Characteristics (parameters) of communication channels. 10
Condition for transmitting signals over communication channels. 13
Literature. 14
Link
Link- a system of technical means and signal propagation environment for transmitting messages (not only data) from source to recipient (and vice versa). Communication channel, understood in a narrow sense ( communication path), represents only the physical signal propagation medium, for example, a physical communication line.
The communication channel is designed to transmit signals between remote devices. Signals carry information intended for presentation to the user (person) or for use by computer application programs.
The communication channel includes the following components:
1) transmitting device;
2) receiving device;
3) transmission medium of various physical nature (Fig. 1).
The signal generated by the transmitter and carrying information, after passing through the transmission medium, arrives at the input of the receiving device. Next, the information is separated from the signal and transmitted to the consumer. The physical nature of the signal is chosen so that it can propagate through the transmission medium with minimal attenuation and distortion. The signal is necessary as a carrier of information; it itself does not carry information.
Fig.1. Communication channel (option No. 1)
Fig.2 Communication channel (option No. 2)
Those. this (channel) is a technical device (technology + environment).
Classification
There will be exactly three types of classifications. Choose according to taste and color:
Classification No. 1:
There are many types of communication channels, the most common of which are channels wired communications ( aerial, cable, fiber etc.) and radio communication channels (tropospheric, satellite and etc.). Such channels, in turn, are usually qualified based on the characteristics of the input and output signals, as well as on changes in the characteristics of the signals depending on such phenomena occurring in the channel as fading and attenuation of signals.
Based on the type of distribution medium, communication channels are divided into:
Wired;
Acoustic;
Optical;
Infrared;
Radio channels.
Communication channels are also classified into:
· continuous (continuous signals at the input and output of the channel),
· discrete or digital (at the input and output of the channel - discrete signals),
continuous-discrete (at the channel input there are continuous signals, and at the output there are discrete signals),
· discrete-continuous (discrete signals at the channel input, and continuous signals at the output).
Channels can be like linear And nonlinear, temporary And spatiotemporal.
Possible classification communication channels by frequency range .
Information transmission systems are single-channel And multichannel. The type of system is determined by the communication channel. If a communication system is built on the same type of communication channels, then its name is determined by the typical name of the channels. Otherwise, the detailing of classification features is used.
Classification No. 2 (more detailed):
1. Classification according to the range of frequencies used
Ø Kilometer (DV) 1-10 km, 30-300 kHz;
Ø Hectometric (HW) 100-1000 m, 300-3000 kHz;
Ø Decameter (HF) 10-100 m, 3-30 MHz;
Ø Meter (MV) 1-10 m, 30-300 MHz;
Ø UHF (UHF) 10-100 cm, 300-3000 MHz;
Ø Centimeter wave (SMV) 1-10 cm, 3-30 GHz;
Ø Millimeter wave (MMW) 1-10 mm, 30-300 GHz;
Ø Decimilimeter (DMMV) 0.1-1 mm, 300-3000 GHz.
2. According to the direction of communication lines
- directed ( different conductors are used):
Ø coaxial,
Ø twisted pairs based on copper conductors,
Ø fiber optic.
- omnidirectional (radio links);
Ø line of sight;
Ø tropospheric;
Ø ionospheric
Ø space;
Ø radio relay (retransmission on decimeter and shorter radio waves).
3. By type of messages transmitted:
Ø telegraph;
Ø telephone;
Ø data transmission;
Ø facsimile.
4. By type of signals:
Ø analog;
Ø digital;
Ø pulse.
5. By type of modulation (manipulation)
- In analog communication systems:
Ø with amplitude modulation;
Ø with single-band modulation;
Ø with frequency modulation.
- In digital communication systems:
Ø with amplitude manipulation;
Ø with frequency shift keying;
Ø with phase manipulation;
Ø with relative phase shift keying;
Ø with tonal keying (single elements manipulate a subcarrier oscillation (tone), after which manipulation is carried out at a higher frequency).
6. According to the radio signal base value
Ø broadband (B>> 1);
Ø narrowband (B»1).
7. By the number of simultaneously transmitted messages
Ø single-channel;
Ø multi-channel (frequency, time, code division of channels);
8. By direction of message exchange
Ø one-sided;
Ø
bilateral.
9. By order of message exchange
Ø simplex communication- two-way radio communication, in which the transmission and reception of each radio station is carried out alternately;
Ø duplex communication- transmission and reception are carried out simultaneously (the most efficient);
Ø half-duplex communication- refers to simplex, which provides for an automatic transition from transmission to reception and the possibility of asking the correspondent again.
10. Methods of protecting transmitted information
Ø open communication;
Ø closed communication (secret).
11. According to the degree of automation of information exchange
Ø non-automated - control of the radio station and exchange of messages is performed by the operator;
Ø automated - only information is entered manually;
Ø automatic - the process of messaging is carried out between an automatic device and a computer without operator participation.
Classification No. 3 (something may be repeated):
1. By purpose
Telephone
Telegraph
Television
Broadcasting
2. By transmission direction
Simplex (transmission in one direction only)
Half-duplex (transmission alternately in both directions)
Duplex (simultaneous transmission in both directions)
3. According to the nature of the communication line
Mechanical
Hydraulic
Acoustic
Electrical (wired)
Radio (wireless)
Optical
4. By the nature of the signals at the input and output of the communication channel
Analogue (continuous)
Discrete in time
Discrete by signal level
Digital (discrete in both time and level)
5. By number of channels per communication line
Single channel
Multichannel
And another drawing here:
Fig.3. Classification of communication lines.
Characteristics (parameters) of communication channels
1. Channel transfer function: presented in the form amplitude-frequency response (AFC) and shows how the amplitude of the sinusoid at the output of the communication channel attenuates in comparison with the amplitude at its input for all possible frequencies of the transmitted signal. The normalized amplitude-frequency response of the channel is shown in Fig. 4. Knowing the amplitude-frequency response of a real channel allows you to determine the shape of the output signal for almost any input signal. To do this, it is necessary to find the spectrum of the input signal, convert the amplitude of its constituent harmonics in accordance with the amplitude-frequency characteristic, and then find the shape of the output signal by adding the converted harmonics. To experimentally check the amplitude-frequency response, it is necessary to test the channel with reference (equal in amplitude) sinusoids over the entire frequency range from zero to some maximum value that can be found in the input signals. Moreover, the frequency of the input sinusoids needs to be changed in small steps, which means the number of experiments should be large.
-- ratio of the spectrum of the output signal to the input
- bandwidth
Fig.4 Normalized amplitude-frequency response of the channel
2. Bandwidth: is a derived characteristic from the frequency response. It represents a continuous range of frequencies for which the ratio of the amplitude of the output signal to the input exceeds some predetermined limit, that is, the bandwidth determines the range of signal frequencies at which this signal is transmitted through a communication channel without significant distortion. Typically, the bandwidth is measured at 0.7 from the maximum frequency response value. Bandwidth has the greatest influence on the maximum possible speed of information transmission over a communication channel.
3. Attenuation: is defined as the relative decrease in amplitude or power of a signal when a signal of a certain frequency is transmitted over a channel. Often, when operating a channel, the fundamental frequency of the transmitted signal is known in advance, that is, the frequency whose harmonic has the greatest amplitude and power. Therefore, it is enough to know the attenuation at this frequency to approximately estimate the distortion of the signals transmitted over the channel. More accurate estimates are possible with knowledge of the attenuation at several frequencies corresponding to several fundamental harmonics of the transmitted signal.
Attenuation is usually measured in decibels (dB) and is calculated using the following formula: 
, Where
Signal power at the channel output,
Signal power at the channel input.
Attenuation is always calculated for a specific frequency and is related to the channel length. In practice, the concept of “linear attenuation” is always used, i.e. signal attenuation per unit channel length, for example, attenuation 0.1 dB/meter.
4. Transmission speed: characterizes the number of bits transmitted over the channel per unit of time. It is measured in bits per second - bit/s, as well as derived units: Kbit/s, Mbit/s, Gbit/s. The transmission speed depends on the channel bandwidth, noise level, type of coding and modulation.
5. Channel noise immunity: characterizes its ability to provide signal transmission in conditions of interference. Interference is usually divided into internal(represents thermal noise of equipment) And external(they are diverse and depend on the transmission medium). The noise immunity of the channel depends on hardware and algorithmic solutions for processing the received signal, which are embedded in the transceiver device. Noise immunity transmission of signals through the channel may be increased due to coding and special processing signal.
6. Dynamic range : logarithm of the ratio of the maximum power of the signals transmitted by the channel to the minimum.
7. Noise immunity: This is noise immunity, i.e. noise immunity.
CHANNELS OF CONNECTION
1. Classification and characteristics of the communication channel
A communication channel is a set of means designed to transmit signals (messages).
To analyze information processes in a communication channel, you can use its generalized diagram shown in Fig. 1.
In Fig. 1 the following designations are accepted: X, Y, Z, W – signals, messages; f – interference; LS – communication line; AI, PI – source and receiver of information; P – converters (coding, modulation, decoding, demodulation).
There are different types of channels, which can be classified according to various criteria:
1. By type of communication lines: wired; cable; fiber optic;
power lines; radio channels, etc.
2. By the nature of the signals: continuous; discrete; discrete-continuous (signals at the input of the system are discrete, and at the output are continuous, and vice versa).
3. In terms of noise immunity: channels without interference; with interference.
Communication channels are characterized by:
1. Channel capacity is defined as the product of the time of use of the channel Tk, the width of the frequency spectrum transmitted by the channel Fk and the dynamic range Dk, which characterizes the ability of the channel to transmit different signal levels
V k = T k F k D k. (1)
Condition for matching the signal with the channel:
V c £ V k ; T c £ T k ; F c £ F k ; V c £ V k ; D c £ D k .
2. Information transmission speed - the average amount of information transmitted per unit of time.
3. The throughput of a communication channel is the highest theoretically achievable speed of information transfer, provided that the error does not exceed a given value.
4. Redundancy – ensures the reliability of the transmitted information (R = 0¸1).
One of the tasks of information theory is to determine the dependence of the speed of information transmission and the capacity of a communication channel on the parameters of the channel and the characteristics of signals and interference.
The communication channel can be figuratively compared to roads. Narrow roads – low capacity, but cheap. Wide roads provide good traffic capacity, but are expensive. Bandwidth is determined by the bottleneck.
The data transfer speed largely depends on the transmission medium in communication channels, which use different types of communication lines.
Wired:
1. Wired - twisted pair (which partially suppresses electromagnetic radiation from other sources). Transfer speed up to 1 Mbit/s. Used in telephone networks and for data transmission.
2. Coaxial cable. Transmission speed 10–100 Mbit/s – used in local networks, cable television, etc.
3. Fiber optic. Transfer speed 1 Gbit/s.
In environments 1–3, the attenuation in dB depends linearly on distance, i.e. power drops exponentially. Therefore, it is necessary to install regenerators (amplifiers) at a certain distance.
Radio lines:
1. Radio channel. Transfer speed 100–400 Kbps. Uses radio frequencies up to 1000 MHz. Up to 30 MHz, due to reflection from the ionosphere, electromagnetic waves can propagate beyond the line of sight. But this range is very noisy (for example, amateur radio communications). From 30 to 1000 MHz – the ionosphere is transparent and direct visibility is necessary. Antennas are installed at height (sometimes regenerators are installed). Used in radio and television.
2. Microwave lines. Transfer speeds up to 1 Gbit/s. Radio frequencies above 1000 MHz are used. This requires direct visibility and highly directional parabolic antennas. The distance between regenerators is 10–200 km. Used for telephone communications, television and data transmission.
3. Satellite communications. Microwave frequencies are used, and the satellite serves as a regenerator (for many stations). The characteristics are the same as for microwave lines.
2. Bandwidth of a discrete communication channel
A discrete channel is a set of means designed to transmit discrete signals.
The throughput of a communication channel is the highest theoretically achievable speed of information transfer, provided that the error does not exceed a given value. Information transmission rate is the average amount of information transmitted per unit of time. Let us define expressions for calculating the information transmission rate and the throughput of a discrete communication channel.
When transmitting each symbol, an average amount of information passes through the communication channel, determined by the formula
I (Y, X) = I (X, Y) = H(X) – H (X/Y) = H(Y) – H (Y/X), (2)
where: I (Y, X) – mutual information, i.e. the amount of information contained in Y relative to X; H(X) – entropy of the message source; H (X/Y) – conditional entropy, which determines the loss of information per symbol associated with the presence of interference and distortion.
When transmitting a message X T of duration T, consisting of n elementary symbols, the average amount of transmitted information, taking into account the symmetry of the mutual amount of information, is equal to:
I(Y T , X T) = H(X T) – H(X T /Y T) = H(Y T) – H(Y T /X T) = n. (4)
The speed of information transmission depends on the statistical properties of the source, the coding method and the properties of the channel.
Bandwidth of a discrete communication channel
. (5)
The maximum possible value, i.e. the maximum of the functional is sought over the entire set of probability distribution functions p(x).
The throughput depends on the technical characteristics of the channel (equipment speed, type of modulation, level of interference and distortion, etc.). The units of channel capacity are: , , , .
