Direct communication channel. Communication lines and channels What are communication channels used for?

Characteristics

The following channel characteristics are used

Noise immunity

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

Channel volume

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

Where T k (\displaystyle T_(k))- time during which the channel is occupied by the transmitted signal;

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

Classification

There are many types of communication channels, among which the most common are wired communication channels (aerial, cable, fiber, etc.) and radio communication channels (tropospheric, satellite, 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 propagation medium, communication channels are divided into wired, acoustic, optical, infrared and radio channels.

Communication channels are also classified into

continuous (continuous signals at the input and output of the channel),
discrete or digital (at the input and output of the channel - discrete signals),
continuous-discrete (at the channel input - continuous signals, and at the output - discrete signals),
discrete-continuous (at the channel input there are discrete signals, and at the output there are continuous signals).

Channels can be linear and nonlinear, temporal and spatiotemporal. It is possible to classify communication channels by frequency range.

Communication channel models

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

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

Continuous channel models

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

Ideal Channel Model

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

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

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

Model of a channel with an uncertain signal phase and additive noise

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

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

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

S 1 (t) = c o s (θ) u (t) − s i n (θ) H (u (t)) (\displaystyle S_(1)(t)=cos(\theta)u(t)-sin(\ theta)H(u(t))),

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

Channel model with intersymbol interference and additive noise

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

In order to transmit various information, a medium for its distribution must initially be created, which is a set of lines or data transmission channels with specialized receiving and transmitting equipment. Lines, or communication channels, represent the connecting link in any modern data transmission system, and from an organizational point of view they are divided into two main types - lines and channels.

A communication line is a set of cables or wires, with the help of which communication points are connected to each other, and subscribers are connected to nearby nodes. At the same time, communication channels can be created in a variety of ways depending on the characteristics of a particular object and scheme.

What could they be?

They can be physical wired channels, which are based on the use of specialized cables, or they can also be wave channels. Wave communication channels are formed to organize all kinds of radio communications in a certain environment using antennas, as well as a dedicated frequency band. At the same time, both optical and electrical communication channels are also divided into two main types - wired and wireless. In this regard, optical and electrical signals can be transmitted through wires, ether, and many other methods.

In a telephone network, after a number is dialed, a channel is formed for as long as there is a connection, for example, between two subscribers, and also as long as a voice communication session is maintained. Wired communication channels are formed through the use of specialized compaction equipment, with the help of which it is possible to transmit information through communication lines over a long or short time, which is supplied from a huge number of different sources. Such lines include one or several pairs of cables at the same time and provide the ability to transmit data over a fairly long distance. Regardless of what types of communication channels are considered, in radio communications they represent a data transmission medium that is organized for a specific or simultaneously several communication sessions. If we are talking about several sessions, then in this case the so-called frequency distribution can be used.

What types are there?

Just like in modern communications, there are different types of communication channels:

Digital.
Analog.
Analog-digital.

Digital

This option is an order of magnitude more expensive compared to analogue ones. With the help of such channels, extremely high quality data transmission is achieved, and it is also possible to implement various mechanisms with the help of which absolute integrity of channels, a high degree of information security, and the use of a number of other services are achieved. In order to ensure the transmission of analogue information through digital technical communication channels, this information is initially converted into digital.

In the late 80s of the last century, a specialized digital network with integration of services appeared, better known to many today as ISDN. It is expected that such a network over time will be able to turn into a global digital backbone that connects office and home computers, providing them with a sufficiently high data transmission speed. The main communication channels of this type can be:

Fax machine.
Telephone.
Data transmission devices.
Specialized equipment for teleconferences.
And many others.

Such means can compete with modern technologies that are actively used today in cable television networks.

Other varieties

Depending on the transmission speed of communication channels, they are divided into:

Low speed. This category includes all kinds of telegraph lines, which are characterized by an extremely low (almost non-existent by today's standards) data transfer rate, which reaches a maximum of 200 bps.
Medium speed. There are analog telephone lines providing transmission speeds of up to 56,000 bps.
High-speed or, as they are also called, broadband. Data transmission via communication channels of this type is carried out at a speed of more than 56,000 bps.

Depending on the possibilities for organizing data transmission directions, communication channels can be divided into the following types:

Simplex. The organization of communication channels of this type provides the ability to broadcast data only in a certain direction.
Half duplex. Using such channels, data can be transmitted in both forward and reverse directions.
Duplex or full duplex. Using such feedback channels, data can be simultaneously transmitted in the forward and backward directions.

Wired

Wired communication channels include a mass of parallel or twisted copper wires, fiber-optic communication lines, and specialized coaxial cables. If we consider which communication channels use cables, it is worth highlighting several main ones:

Twisted pair. Provides the ability to transmit information at speeds up to 1 Mbit/s.
Coaxial cables. This group includes TV format cables, including both thin and thick. In this case, the data transfer speed already reaches 15 Mbit/s.
Fiber optic cables. The most modern and productive option. Communication channels for transmitting information of this type provide a speed of about 400 Mbit/s, which significantly exceeds all other technologies.

twisted pair

It consists of insulated conductors, which are twisted together in pairs in order to significantly reduce interference between pairs and conductors. It is worth noting that today there are seven categories of twisted pairs:

The first and second are used to provide low-speed data transmission, the first being a standard, well-known telephone wire.
The third, fourth and fifth categories are used to provide transmission speeds of up to 16, 25 and 155 Mbps, with different categories providing different frequencies.
The sixth and seventh categories are the most productive. We are talking about the ability to transmit data at speeds of up to 100 Gbit/s, which represents the most productive characteristics of communication channels.

The most common today is the third category. Focusing on various promising solutions regarding the need to constantly develop network capacity, the most optimal would be to use communication networks (communication channels) of the fifth category, which provide the speed of data transmission through standard telephone lines.

Coaxial cable

A specialized copper conductor is contained within a cylindrical shielding protective shell, which winds from fairly thin veins, and is also completely isolated from the conductor using a dielectric. This differs from a standard television cable in that it contains characteristic impedance. Through such information communication channels, data can be transmitted at speeds of up to 300 Mbit/s.

This cable format is divided into thin, which has a thickness of 5 mm, and thick - 10 mm. In modern LANs it is often customary to use a thin cable, since it is extremely easy to lay and install. The extremely high cost and difficult installation severely limit the possibilities of using such cables in modern information transmission networks.

Cable TV networks

Such networks are based on the use of a specialized coaxial cable, through which an analog signal can be transmitted over a distance of up to several tens of kilometers. A typical cable television network has a tree structure, in which the main node receives signals from a specialized satellite or through a fiber-optic link. Today, networks that use fiber-optic cable are actively used, with the help of which it is possible to serve large areas, as well as broadcast more voluminous data, while maintaining extremely high quality signals in the absence of repeaters.

With a symmetrical architecture, the return and forward signals are transmitted using a single cable in different frequency ranges, and at different speeds. Accordingly, the reverse signal is slower than the forward signal. In any case, using such networks, it is possible to provide data transfer speeds several hundred times higher compared to standard telephone lines, and therefore the latter ceased to be used a long time ago.

In organizations that install their own cable networks, symmetrical schemes are most often used, since in this case both forward and reverse data transmission is carried out at the same speed, which is approximately 10 Mbit/s.

Features of using wires

The number of wires that can be used to connect home computers and various electronics increases every year. According to statistics obtained during research by professional specialists, approximately 3 km of various cables are laid in a 150-meter apartment.

In the 90s of the last century, the British company UnitedUtilities proposed a rather interesting solution to this problem using its own development called DigitalPowerLine, better known today by the abbreviation DPL. The company proposed using standard power electrical networks as a medium to provide high-speed data transmission, transmitting packets of information or voice through ordinary electrical networks, the voltage of which was 120 or 220 V.

The most successful from this point of view is an Israeli company called Main.net, which was the first to release PLC (Powerline Communications) technology. Using this technology, voice or data transmission was carried out at speeds of up to 10 Mbit/s, while the information flow was distributed into several low-speed ones, which were transmitted at separate frequencies, and ultimately recombined into a single signal.

The use of PLC technology today is only relevant in conditions of data transmission at low speed, and therefore is used in home automation, various household devices and other equipment. Using this technology, it is possible to access the Internet at a speed of about 1 Mbit/s for those applications that require high connection speeds.

With a short distance between the building and the intermediate transceiver point, which is a transformer substation, the data transmission speed can reach 4.5 Mbit/s. This technology is actively used when forming a local network in a residential building or small office, since the minimum transmission speed makes it possible to cover a distance of up to 300 meters. Using this technology, it is possible to implement various services related to remote monitoring, security of objects, as well as management of object modes and their resources, which is included in the elements of an intelligent home.

Fiber optic cable

This cable is made from a specialized quartz core, the diameter of which is only 10 microns. This core is surrounded by a unique reflective protective shell, the outer diameter of which is approximately 200 microns. Data transmission is carried out by transforming electrical signals into light signals, using, for example, some kind of LED. Data encoding is carried out by changing the intensity of the light flux.

When transmitting data, the beam is reflected from the walls of the fiber, which ultimately arrives at the receiving end, with minimal attenuation. Using such a cable, an extremely high degree of protection from exposure to any external electromagnetic fields is achieved, and a fairly high data transfer rate is achieved, which can reach 1000 Mbit/s.

Using a fiber optic cable, it is possible to simultaneously organize the operation of several hundred thousand telephone, videotelephone, and television channels. If we talk about other advantages inherent in such cables, it is worth noting the following:

Extremely high difficulty of unauthorized connection.
The highest degree of protection against any fire.
Sufficiently high data transfer speed.

However, if we talk about what disadvantages such systems have, it is worth highlighting that they are quite expensive and necessitate the transformation of light lasers into electric ones and vice versa. The use of such cables in the vast majority of cases is carried out in the process of laying trunk communication lines, and the unique properties of the cable have made it quite common among providers providing the organization of the Internet network.

Switching

Among other things, communication channels can be switched or non-switched. The first ones are created only for a certain time while data needs to be transferred, while non-switched ones are allocated to the subscriber for a specific period of time, and have no dependence on how long the data was transferred.

WiMAX

Such lines, unlike traditional radio access technologies, can also operate on a reflected signal that is not in the direct line of sight of a particular base station. Expert opinion today clearly agrees that such mobile networks offer enormous prospects for users compared to fixed WiMAX, which is intended for corporate customers. In this case, information can be transmitted over a fairly large distance (up to 50 km), while the characteristics of communication channels of this type include speeds of up to 70 Mbit/s.

Satellite

Satellite systems involve the use of specialized microwave antennas that are used to receive radio signals from any ground stations, and then relay the received signals back to other ground stations. It is worth noting that such networks involve the use of three main types of satellites located in medium or low orbits, as well as geostationary orbits. In the vast majority of cases, it is customary to launch satellites in groups, since, moving away from each other, they provide coverage of the entire surface of our planet.

Characteristics

The following channel characteristics are used

Noise immunity

Noise immunity. Where is the minimum signal-to-noise ratio;

Channel volume

The channel volume is determined by the formula: ,

where is the time during which the channel is occupied by the transmitted signal;

To transmit a signal through a channel without distortion, the volume of the channel must be greater than or equal to the volume of the signal, i.e. . The simplest case of fitting the signal volume into the channel volume is to achieve the fulfillment of the inequalities , > and . However, it can be performed in other cases, which makes it possible to achieve the required channel characteristics by changing other parameters. For example, as the frequency range decreases, the bandwidth can be increased.

Classification

Based on the type of propagation medium, communication channels are divided into wired, acoustic, optical, infrared and radio channels.

Communication channels are also classified into

continuous (continuous signals at the input and output of the channel),
discrete or digital (at the input and output of the channel - discrete signals),
continuous-discrete (at the channel input - continuous signals, and at the output - discrete signals),
discrete-continuous (at the channel input there are discrete signals, and at the output there are continuous signals).

Channels can be either linear or nonlinear, temporal or spatiotemporal. It is possible to classify communication channels by frequency range.

Communication channel models

The communication channel is described by a mathematical model, the task of which is reduced to defining the mathematical models of the output and input and, as well as establishing a connection between them, characterized by the operator, i.e.

Continuous channel models

Ideal Channel Model

The ideal channel model is used when the presence of interference can be neglected. When using this model, the output signal is deterministic, i.e.

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

Model of a channel with an uncertain signal phase and additive noise

A channel model with an uncertain signal phase and additive noise differs from an ideal channel model in that it is a random variable. For example, if the input signal is narrowband, then the output signal of a channel with an uncertain signal phase and additive noise is defined as follows:

where it is taken into account that the input signal can be represented in the form:

where is the Hilbert transform, is a random phase, the distribution of which is usually considered uniform over the interval.

Channel model with intersymbol interference and additive noise

The model of a channel with intersymbol interference and additive noise takes into account the appearance of signal scattering in time due to the nonlinearity of the phase-frequency characteristic of the channel and the limitation of its bandwidth, i.e. for example, when transmitting discrete messages over a channel, the value of the output signal will be affected by the channel's responses not only to the transmitted symbol, but also to earlier or later symbols. In radio channels, the occurrence of intersymbol interference is influenced by the multipath propagation of radio waves.

Models of discrete communication channels

To specify a discrete channel model, it is necessary to determine a set of input and output code symbols, as well as a set of conditional probabilities of output symbols for given input .

Models of discrete-continuous communication channels

There are also models of discrete-continuous communication channels

Notes

Literature

Zyuko A. G., Klovsky D. D., Korzhik V. I., Nazarov M. V., Theory of electrical communication / Ed. D. D. Klovsky. - Textbook for universities. - M.: Radio and communication, 1999. - 432 p. -

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:

V c £ V k ; T c £ Tk ; F c £ F k ; V c £ V k ; D c £ Dk.

2.Information transfer rate – the average amount of information transmitted per unit of time.

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.

, where is an arbitrarily small value,

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 and p 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.

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.