State exam

(State examination)

Question No. 3 “Communication channels. Classification of communication channels. Communication channel parameters. Condition of signal transmission over the communication channel ".

(Plyaskin)

Link. 3

Classification. five

Characteristics (parameters) of communication channels. ten

Condition for signal transmission over communication channels. 13

Literature. fourteen

Link

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

The communication channel is designed to transmit signals between remote devices. Signals carry information intended for presentation to a user (person), or for use by computer applications.

The communication channel includes the following components:

1) transmitting device;

2) receiving device;

3) transmission medium of various physical nature (Fig. 1).

The information-carrying signal generated by the transmitter, after passing through the transmission medium, enters the input of the receiving device. Then the information is extracted 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 number 1)

Fig.2 Communication channel (option # 2)

Those. this (channel) is a technical device (technology + environment).

Classification

There will be exactly three types of classifications. Choose the taste and color:

Classification No. 1:

There are many types of communication channels, among which the most commonly distinguished channels wired communication ( aerial, cable, light-guideand others) 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 the change in the characteristics of the signals, depending on such phenomena occurring in the channel as fading and attenuation of signals.

By the type of distribution medium, communication channels are divided into:

Wired;

Acoustic;

Optical;

Infrared;

Radio channels.

Communication channels are also classified into:

Continuous (at the input and output of the channel - continuous signals),

Discrete or digital (discrete signals at the channel input and output),

· Continuous-discrete (continuous signals at the channel input, and discrete signals at the output),

· Discrete-continuous (discrete signals at the channel input, and continuous signals at the output).

Channels can be like linear and nonlinear, temporary and space-time.

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 specification of classification features is used.

Classification No. 2 (more detailed):

1. Classification by frequency range

Ø Kilometer (LW) 1-10 km, 30-300 kHz;

Ø Hectometric (SV) 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 Decimeter (UHF) 10-100 cm, 300-3000 MHz;

Ø Centimeter (CMB) 1-10 cm, 3-30 GHz;

Ø Millimeter (MMV) 1-10 mm, 30-300 GHz;

Ø Decimitre (DMMV) 0.1-1 mm, 300-3000 GHz.

2. By direction of communication lines

- directed (different conductors are used):

Ø coaxial,

Ø twisted pairs based on copper conductors,

Ø fiber optic.

- non-directional (radio links);

Ø line of sight;

Ø tropospheric;

Ø ionospheric

Ø space;

Ø radio relay (retransmission on decimeter and shorter radio waves).

3. By the type of transmitted messages:

Ø telegraph;

Ø telephone;

Ø data transmission;

Ø facsimile.

4. By the type of signals:

Ø analog;

Ø digital;

Ø impulse.

5. By the type of modulation (manipulation)

- In analog communication systems:

Ø with amplitude modulation;

Ø with single sideband modulation;

Ø with frequency modulation.

- In digital communication systems:

Ø with amplitude shift keying;

Ø with frequency shift keying;

Ø with phase shift keying;

Ø with relative phase shift keying;

Ø with tone shift keying (single elements manipulate the subcarrier oscillation (tone), after which the keying is performed at a higher frequency).

6. By the value of the base of the radio signal

Ø broadband (B \u003e\u003e 1);

Ø narrow-band (B "1).

7. By the number of simultaneously transmitted messages

Ø single-channel;

Ø multichannel (frequency, time, code division of channels);

8. In the direction of messaging

Ø one-sided;

Ø bilateral.
9. By order of message exchange

Ø simplex communication - two-way radio communication, in which transmission and reception of each radio station is carried out in turn;

Ø duplex communication - transmission and reception are carried out simultaneously (the most efficient);

Ø half-duplex communication - refers to the simplex, which provides for an automatic transition from transmission to reception and the possibility of re-asking the correspondent.

10. By methods of protection of transmitted information

Ø open communication;

Ø closed communication (classified).

11. By the degree of automation of information exchange

Ø non-automated - radio station control and messaging is performed by the operator;

Ø automated - only information is entered manually;

Ø automatic - the process of messaging is performed between an automatic device and a computer without operator participation.

Classification number 3 (something can be repeated):

1. By appointment

Telephone

Telegraph

Television

Broadcasting

2. By transfer direction

Simplex (transmission in one direction only)

Half duplex (alternate transmission in both directions)

Duplex (transmit simultaneously in both directions)

3. By the nature of the communication line

Mechanical

Hydraulic

Acoustic

Electrical (wired)

Radio (wireless)

Optical

4. By the nature of the signals at the input and output of the communication channel

Analog (continuous)

Discrete in time

Discrete by signal level

Digital (discrete and in time and in level)

5. By the 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: is presented as amplitude-frequency characteristic (AFC)and shows how the amplitude of the sinusoid at the output of the communication channel decays in comparison with the amplitude at its input for all possible frequencies of the transmitted signal. The normalized 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, transform the amplitude of its constituent harmonics in accordance with the amplitude-frequency characteristic, and then find the shape of the output signal by adding the converted harmonics. For experimental verification of 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 a certain maximum value that can occur in the input signals. Moreover, it is necessary to change the frequency of the input sinusoids with a small step, which means that the number of experiments should be large.

- the ratio of the spectrum of the output signal to the input
- bandwidth

Fig. 4 Normalized channel amplitude-frequency response

2. Bandwidth: is a derived characteristic from the frequency response. It is a continuous range of frequencies for which the ratio of the amplitude of the output signal to the input signal exceeds a certain predetermined limit, that is, the bandwidth determines the range of signal frequencies at which this signal is transmitted through the communication channel without significant distortion. Typically, the bandwidth is measured at 0.7 times the maximum frequency response. The bandwidth has the greatest impact on the maximum possible data transfer rate over the communication channel.

3. Attenuation: is defined as the relative decrease in the amplitude or power of a signal when a signal of a certain frequency is transmitted over a channel. Often, during channel operation, the fundamental frequency of the transmitted signal is known in advance, that is, the frequency whose harmonic has the highest amplitude and power. Therefore, it is enough to know the attenuation at this frequency in order to approximately estimate the distortion of the signals transmitted over the channel. More accurate estimates are possible by knowing 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 strength at the channel input.

The 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 of 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: Kbps, Mbps, Gbps... The transmission rate depends on the channel bandwidth, noise level, type of coding and modulation.

5. Channel immunity: characterizes its ability to provide signal transmission in the presence of interference. Interference is usually divided into internal (represents thermal noise from equipment) and external (they are diverse and depend on the transmission medium). Noise immunity of the channel depends on the hardware and algorithmic solutions for processing the received signal, which are embedded in the transceiver. Immunity transmission of signals through the channel can be increased at the expense encoding 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. Interference immunity: it is noise immunity, i.e. noise immunity.

In fig. 1 the following designations are adopted: 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 that can be classified according to different criteria:

1.By the 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 system input are discrete, and at the output are continuous, and vice versa).

3... For noise immunity: channels without interference; with interference.

Communication channels are characterized by:

1. Channel capacity defined as the product of the channel usage time T to, bandwidth of frequencies passed by the channel F toand dynamic range D to ... , which characterizes the channel's ability to transmit different signal levels

V to \u003d T to F to D to.(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 transfer rate - the average amount of information transmitted per unit of time.

3.

4. Redundancy - ensures the reliability of the transmitted information ( R \u003d 0¸1).

One of the tasks of information theory is to determine the dependence of the information transmission rate and communication channel capacity on the channel parameters and characteristics of signals and interference.

The communication channel can be figuratively compared to roads. Narrow roads - low traffic, but cheap. Wide roads are good traffic but expensive. The bandwidth is determined by the "bottleneck".

The data transfer rate largely depends on the transmission medium in the communication channels, which are various types of communication lines.

Wired:

1. Wired- twisted pair (which partially suppresses electromagnetic radiation from other sources). Transfer rates up to 1 Mbps. Used in telephone networks and for data transmission.

2. Coaxial cable.Transmission speed 10-100 Mbps - used in local networks, cable TV, etc.

3... Fiber optic.The transmission rate is 1 Gbps.

In environments 1–3, the attenuation in dB is linear with distance, i.e. the power drops exponentially. Therefore, after a certain distance it is necessary to install regenerators (amplifiers).

Radio lines:

1.Radio channel.The transmission speed is 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). From 30 to 1000 MHz - the ionosphere is transparent and line of sight is required. Antennas are installed at a height (regenerators are sometimes installed). Used in radio and television.

2.Microwave lines.Transfer rates up to 1 Gbps. Radio frequencies above 1000 MHz are used. This requires line-of-sight and highly directional parabolic antennas. The distance between the regenerators is 10–200 km. Used for telephony, television and data transmission.

3. Satellite connection... Microwave frequencies are used, and the satellite serves as a regenerator (and for many stations). The characteristics are the same as for microwave lines.

2. Bandwidth of a discrete communication channel

A discrete channel is a collection of means for transmitting discrete signals.

Communication channel bandwidth - the highest theoretically achievable information transfer rate, 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 transfer rate and the bandwidth of the discrete communication channel.

When transmitting each character, on average, the amount of information passes through the communication channel, determined by the formula

I (Y, X) \u003d I (X, Y) \u003d H (X) - H (X / Y) \u003d H (Y) - H (Y / X), (2)

where: I (Y, X) -mutual information, i.e. the amount of information contained in Yrelatively 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 noise and distortion.

When sending a message X Tduration T, composed of n elementary symbols, the average amount of transmitted information, taking into account the symmetry of the mutual amount of information, is:

I (Y T, X T) \u003d H (X T) - H (X T / Y T) \u003d H (Y T) - H (Y T / X T) \u003d n. (4)

The information transfer rate depends on the statistical properties of the source, the encoding method and the properties of the channel.

Discrete communication channel bandwidth

. (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 (speed of the equipment, type of modulation, level of interference and distortion, etc.). The units for measuring the 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 linked by an unambiguous, functional relationship.

In this case, the conditional entropy is zero, and the unconditional entropies of the source and receiver are equal, i.e. the average amount of information in the received symbol relative to the transmitted one is

I (X, Y) \u003d H (X) \u003d H (Y); H (X / Y) \u003d 0.

If a X T - the number of characters per time T, then the information transfer rate for a discrete communication channel without interference is

(6)

where V = 1/ - average bit rate of one symbol.

Bandwidth for a discrete communication channel without interference

(7)

Because the maximum entropy corresponds for equiprobable symbols, then the capacity for uniform distribution and statistical independence of the transmitted symbols is:

. (8)

Shannon's first channel theorem: If the information flow generated by the source is close enough to the bandwidth of the communication channel, i.e.

then you can always find such a coding method that will ensure the transmission of all messages from the source, and the information transfer rate will be very close to the channel capacity.

The theorem does not answer the question of how to carry out the coding.

Example 1. The source generates 3 messages with probabilities:

p 1 = 0,1; p 2 \u003d 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 the communication channel without interference.

Decision: The entropy of the source is

[bit / s].

To transmit 3 messages with a uniform code, two bits are required, while the duration of the code combination is 2t.

Average signal rate

V =1/2 t = 500 .

Information transfer rate

C = vH = 500 × 1.16 \u003d 580 [bit / s].

2.2 Discrete communication channel with interference

We will consider discrete communication channels without memory.

Channel without memory is called a channel in which each transmitted signal symbol is affected by interference, regardless of which signals were transmitted earlier. That is, interference does not create additional correlations between symbols. The name "without memory" means that during the next transmission, the channel does not seem to remember the results of previous transmissions.

Communication channels (CS) serve for signal transmission and are a common link in any information transmission system.

By their physical nature, communication channels are divided into mechanical, used for the transmission of tangible media, acoustic, optical and electrical, transmitting respectively sound, light and electrical signals.

Electrical and optical communication channels, depending on the method of signal transmission, can be subdivided into wired, using physical conductors for signal transmission (electrical wires, cables, optical fibers), and wireless, using electromagnetic waves for signal transmission (radio channels, infrared channels).

According to the form of presentation of the transmitted information, communication channels are divided into analogthrough which information is transmitted in a continuous form, i.e. in the form of a continuous series of values \u200b\u200bof some physical quantity, and digital, transmitting information presented in the form of digital (discrete, pulse) signals of various physical nature.

Depending on the possible directions of information transmission, communication channels are divided into simplex, allowing information to be transmitted in only one direction; half duplexproviding alternate information transfer in both forward and backward directions; duplex, allowing the transmission of information simultaneously in forward and backward directions.

Communication channels are dial-up, which are created from separate sections (segments) only for the time of transmission of information through them, and at the end of the transmission, such a channel is eliminated (disconnected), and non-commutated (highlighted), created for a long time and having constant characteristics along the length, bandwidth, noise immunity.

Electric wire communication channels widely used in automated information processing and control systems differ in their bandwidth:

low speed,information transfer rate in which from 50 to 200 bit / s. These are telegraph communication channels, both switched (subscriber telegraph) and non-switched;

medium speed,using analog (telephone) communication channels; the transmission speed in them is from 300 to 9600 bit / s, and in the new standards V.32 - V.34 of the International Consultative Committee on Telegraphy and Telephony (CCITT) and from 14400 to 56000 bit / s;

high speed(broadband), providing information transfer rates in excess of 56,000 bit / s.

To transfer information to low-speed and medium-speed compressor stations the physical medium is usually wired communication lines: groups of either parallel or twisted wires called twisted pair. It consists of insulated conductors twisted in pairs to reduce both electromagnetic crosstalk and signal attenuation during transmission at high frequencies.

To organize high-speed (broadband) KS, various cables are used:

Shielded with twisted-pair copper wires;

Unshielded with twisted-pair copper wires;

Coaxial;

Fiber optic.

STP cables(shielded with twisted-pair copper wires) have good technical characteristics, but are inconvenient to use and expensive.

UTP cables(unshielded with twisted pairs of copper wires) are widely used in data transmission systems, in particular in computer networks.

There are five categories of twisted pairs: the first and second categories are used for low-speed data transmission; the third, fourth and fifth - at transfer rates up to 16.25 and 155 Mbit / s, respectively. These cables have good technical characteristics, are relatively inexpensive, easy to use, and do not require grounding.

Coaxial cableis a copper conductor covered with a dielectric and surrounded by a twisted of thin copper conductors with a shielding protective sheath. The data transfer speed over coaxial cable is quite high (up to 300 Mbps), but it is not convenient enough to use and has a high cost.

Fiber optic cable(Fig. 8.2) consists of glass or plastic fibers with a diameter of several micrometers (light-guide core) with a high refractive index n s,surrounded by low-refractive insulation n 0and placed in a protective polyethylene sheath. In fig. 8.2, andshows the distribution of the refractive index over the cross section of the fiber optic cable, and Fig. 8.2, b- ray propagation scheme. A light-emitting diode or a semiconductor laser is a source of radiation propagated through a fiber-optic cable, a radiation receiver is a photodiode, which converts light signals into electrical ones. The transmission of a light beam through a fiber is based on the principle of total internal reflection of the beam from the walls of the light-guiding core, due to which the minimum signal attenuation is ensured.

Fig. 8.2. Fiber Optic Beam Propagation:

and- the distribution of the refractive index over the cross-section of the fiber-optic cable;

b -ray propagation scheme

In addition, fiber optic cables provide protection of the transmitted information from external electromagnetic fields and high transmission rates up to 1000 Mbps. Information is encoded using analog, digital or pulse modulation of a light beam. Fiber-optic cable is quite expensive and is usually used only for laying critical trunk communication channels, for example, a cable laid along the bottom of the Atlantic Ocean connects Europe with America. In computer networks, fiber optic cable is used in the most critical areas, in particular, in the Internet. One thick backbone fiber optic cable can simultaneously organize several hundred thousand telephone, several thousand video telephone and about a thousand television communication channels.

High-speed CS are organized on the basis of wireless radio channels.

Radio channel -it is a wireless communication channel laid over the air. To form a radio channel, a radio transmitter and a radio receiver are used. The data transmission rates over the radio channel are practically limited by the bandwidth of the transceiver equipment. The radio wavelength range is determined by the frequency band of the electromagnetic spectrum used for data transmission. Table 8.1 shows the ranges of radio waves and the corresponding frequency bands.

For commercial telecommunication systems, the most commonly used frequency bands are 902 - 928 MHz and 2.40 - 2.48 GHz.

Wireless communication channels have poor noise immunity, but provide the user with maximum mobility and responsiveness.

Telephone communication linesmost branched and widespread. They carry out transmission of sound (tone) and fax messages. Information and reference systems, e-mail systems and computer networks have been built on the basis of the telephone line. On the basis of telephone lines, analog and digital channels of information transmission can be created.

IN analog telephone lines a telephone microphone converts sound vibrations into an analog electrical signal, which is transmitted over the subscriber line to the PBX. The bandwidth required for human voice transmission is approximately 3 kHz (300 Hz -3.3 kHz range). Call signals are transmitted over the same channel as voice transmission.

IN digital communication channels the analog signal is sampled before input - converted into digital form: every 125 μs (sampling frequency is 8 kHz) the current value of the analog signal is displayed in 8-bit binary code.

Table 8.1

Radio wave ranges and corresponding frequency bands

To organize data transfer, you must use communication lines and channelsthat carry out communication between computers, telephones, telegraphs and other means of communication.

The transmitted information resides in a physical medium that can consist of various types of cables and wires, as well as the surrounding space.

How do communication channels differ from communication lines?

Despite the fact that both concepts are often identified, they have some differences that you need to know about in order to build correct information communication.

Through the channels, communication is transmitted in one direction or in two, if the exchange occurs between the receiver and the transmitter.

Communication lines, in turn, are formed from the connection of several channels, and there can also be only one channel in them.

There are such communication lines:

• Wired;


• Cable;


• Wireless.

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

Wire (overhead) communication lines

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

The disadvantages of such a connection include a relatively low signal transmission rate and a low degree of immunity from interference.

It is also possible a banal unauthorized connection of unscrupulous subscribers, which leads to a decrease in the quality of data transmission and financial losses of broadcasting companies.

Cable communication lines

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

For data exchange in computer networks, the following types of cables are used:

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


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


• Fiber optic cable - similar in structure to coaxial, but instead of a copper conductor, this cable uses thin fiberglass, the role of internal insulation is played by a plastic or glass shell that does not allow light to go out, it forms a complete internal reflection. It is noteworthy that signals can pass through the fiber exclusively in one direction, for this reason they are located in pairs in cables. The installation of such communication lines is very laborious, the cable itself is quite sensitive to damage, but at the same time it provides the highest signal transmission speed up to 3 Gbps. If a fiber optic cable is used, an electrical-to-light converter must be used on the transmission side, and a light-to-electrical converter on the receiving side.

Wireless communication channels

Lines and channels of communication can be built on the work of wireless terrestrial or satellite radio channels.

Radio relay channels are a group of repeater stations that are located in a certain order at a certain distance from each other.

Stations and repeaters are used in the field of cellular communications and to transmit other types of signals within the same city or region.

Satellite communication is provided by satellites, which are located in the earth's orbit and are repeaters. The signal from the ground transmitting station goes to the satellite, and from the satellite it is transmitted to the ground receiving station.

This method of communication makes it possible to provide communication for the inhabitants of the most remote parts of the planet, since satellites are often launched not one by one, but in groups.

All repeaters are located in orbit at some distance from each other, therefore together they can cover almost the entire globe.

Examples of communication lines and channels at the exhibition

You can find out what lines and communication channels modern companies use at the specialized exhibition "Communication", which will take place at the Expocentre Fairgrounds.

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

Read our other articles:

The main function of the information system is the storage of information and its transfer in space. The set of technical means for transmitting messages from source to consumer is called a communication system. These means are a transmitter, a communication line, and a receiver. Sometimes the concept of a communication system includes the source and consumer of messages.

The structural diagram of the simplest communication system is shown in Figure 2. Here the source of the message is the starting point. The source can generate continuous or discrete messages. The source of messages and the recipient in some communication systems can be a person, in others - various kinds of devices (automaton, computer, etc.). The transmission of messages over a distance is carried out using some material medium (paper, magnetic tape, etc.) or a physical process (sound or electromagnetic waves, current, etc.).

The source of information or message is a physical object, system or phenomenon that forms the transmitted message.

A message is a value or change in some physical quantity that reflects the state of an object (system or phenomenon). Typically, primary messages - speech, music, images, environmental measurements, etc., are functions of time - f (t) or other arguments - f (x, y, z) non-electrical nature (acoustic pressure, temperature, brightness distribution on a certain plane, etc.).

Fig. 2. Block diagram of the communication system.

Each i - the th source message is an arbitrary sequence of alphabet elements
(
,
, ...,) length m , where the superscript of the elements is the sequence number, and the subscript only means the position of the letter in the message, but not its kind.

When m = 1 the message is one letter, that is, there is such a message elementary message ... In the general case, for m > 1 the same letter may appear in the message repeatedly. A common property of an elementary message is its indivisibility into smaller messages.

Finite set of messages X c the probability distribution given on it p ( x ) is called a discrete ensemble of messages and is denoted ( X , p ( x )}.

A device that converts a message into a signal is called a transmitter, and a device that converts a received signal into a message is called receiving device.

Using a converter in the transmitting device, the message and, which can be of any physical nature (image, sound vibration, etc.), is converted into a primary electrical signal b(t). In telephony, for example, this operation is reduced to converting sound pressure into a proportionally varying electric current of the microphone. In telegraphy, coding is first performed, as a result of which the sequence of message elements (letters) is replaced by a sequence of code symbols (0, 1 or dot, dash), which is then converted into a sequence of direct current electric pulses using a telegraph apparatus.

In the transmitter, the primary signal b(t) (usually low frequency) turns into a secondary (high frequency) signal u(t) suitable for transmission over the channel being used. This is done through modulation.

The transformation of a message into a signal must be reversible. In this case, based on the output signal, it is possible, in principle, to restore the input primary signal, i.e., to obtain all the information contained in the transmitted message. Otherwise, some information will be lost during transmission, even if the signal reaches the receiving device without distortion.

The physical process representing (carrying) the message being transmitted is called a signal.

A signal is a material and energy form of information presentation. In other words, a signal is a carrier of information, one or more parameters of which, changing, display a message.

The information-message-signal chain is an example of the processing needed where the information comes from. On the side of the information consumer, processing is carried out in the reverse order: “signal - message - information”.

Any transformation of a message into a specific signal by establishing a one-to-one correspondence between them is broadly called encoding.

Coding can include processes of converting and sampling continuous messages (analog-to-digital conversion), modulation (manipulation in digital communication systems) and coding itself in the narrow sense of the word. The reverse operation is called decoding.

A communication link is the medium used to carry signals from a transmitter to a receiver.

In electrical communication systems, this is a cable or waveguide, in radio communication systems, an area of \u200b\u200bspace in which electromagnetic waves propagate from a transmitter to a receiver. Signal transmission may be distorted and interfering with n(t).

The receiver processes the received waveform z(t)=u(t)+n(t), which is the sum of the incoming distorted signal u(t) and interference n(t), and restores the message from it , which reflects the transmitted message with some error a... In other words, the receiver should, based on the fluctuation analysis z(t) determine which of the possible messages was transmitted. Therefore, the receiving device is one of the most critical and complex elements of the communication system.

A communication channel is a set of means that ensure the transmission of a signal from some point A of the system to point B (fig. 3).

Points AND and IN can be selected arbitrarily, as long as the signal passes between them. The part of the communication system located up to the point AND, is the signal source for this channel.

Fig. 3. Communication channel.

The channel as a source of interference has some influence on the transmitted signal. The tasks of the receiver are to extract the transmitted message from the noisy signal and send it to the consumer.

Communication channels are classified according to various criteria, including mathematical description (continuous and discrete channels, continuous and discrete time).

If the signals arriving at the input of the channel and received from its output are discrete in states, then the channel is called discrete. If these signals are continuous, then the channel is called continuous. There are also discrete-continuous and continuous-discrete channels, at the input of which discrete signals are received, and continuous signals are taken from the output, or vice versa. It can be seen from the above that the channel can be discrete or continuous, regardless of the nature of the messages being transmitted. Moreover, in the same communication system, both discrete and continuous channels can be distinguished. It all depends on how the points are selected AND and IN channel input and output.

In this tutorial, we will consider discrete communication channel .

If the harmful effect of interference in the channel can be neglected, then the analysis uses a model in the form of an idealized channel, called channel without interference... In an ideal channel, each input message has a unique output ratio and vice versa. When accuracy requirements are high and disregard for ambiguity between messages x and y unacceptable, a more complex model is used - a noisy channel.

The simplest class of channel models is formed by discrete channels without memory; they are defined as follows. The input is a sequence of letters (elements) from a finite alphabet, let
, output - a sequence of letters of the same or a different alphabet, say
... Finally, each letter of the output sequence depends statistically only on the letter at the corresponding position in the input sequence, and is determined by a given conditional probability
defined for all letters input alphabet and all letters at the exit. An example is a binary symmetric channel (Fig. 4), which is a discrete memory-free channel with binary sequences at the input and output, in which each symbol of the sequence at the input with some probability 1-q is reproduced at the output of the channel correctly and with probability q changes noise to the opposite symbol. In the general case, in a discrete memoryless channel, the transition probabilities exhaust all known information about how an input signal interacts with noise to form an output signal.

Fig. 4. Binary balanced channel.

A much wider class of channels - channels with memory, form channels in which the input signals are sequences of letters from finite alphabets, but in which each letter at the output can statistically depend not only on the corresponding letter of the input sequence.

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