Information transfer scheme. Information transmission channel. Information transfer rate.

There are three types of information processes: storage, transmission, processing.

Data storage:

· Information carriers.

· Types of memory.

· Information storage.

· Basic properties of information storages.

The following concepts are associated with information storage: information carrier (memory), internal memory, external memory, information storage.

A storage medium is a physical medium that directly stores information. Human memory can be called random access memory. Learned knowledge is reproduced by a person instantly. We can also call our own memory internal memory, since its carrier - the brain - is inside us.

All other types of information carriers can be called external (in relation to a person): wood, papyrus, paper, etc. Information storage is information organized in a certain way on external media intended for long-term storage and permanent use (for example, document archives, libraries, card indexes). The main information unit of the repository is a certain physical document: a questionnaire, a book, etc. The organization of a repository is understood as the presence of a certain structure, i.e. orderliness, classification of stored documents for the convenience of working with them. The main properties of information storage: the amount of stored information, storage reliability, access time (i.e., the time to search for the required information), the availability of information protection.

Information stored on computer memory devices is commonly called data. Organized data storages on external memory devices of a computer are usually called databases and data banks.

Data processing:

· General scheme of information processing.

· Statement of the processing task.

· Executor of processing.

· Algorithm of processing.

· Typical tasks of information processing.

Information processing scheme:

Initial information - processing executor - summary information.

In the process of information processing, a certain information problem is solved, which can be preliminarily stated in the traditional form: a certain set of initial data is given, and some results are required. The very process of moving from the initial data to the result is the processing process. The object or subject performing the processing is called the executor of the processing.

To successfully perform information processing, the performer (person or device) must know the processing algorithm, i.e. sequence of actions that must be performed in order to achieve the desired result.

There are two types of information processing. The first type of processing: processing associated with obtaining new information, new content of knowledge (solving mathematical problems, analyzing the situation, etc.). The second type of processing: processing associated with changing the form, but not changing the content (for example, translating text from one language to another).

An important type of information processing is coding - the transformation of information into a symbolic form, convenient for its storage, transmission, processing. Coding is actively used in technical means of working with information (telegraph, radio, computers). Another type of information processing is data structuring (adding a certain order to the information storage, classification, cataloging of data).

Another type of information processing is the search in a certain information storage for the necessary data that satisfy certain search conditions (query). The search algorithm depends on how the information is organized.

Transfer of information:

· Source and receiver of information.

· Information channels.

· The role of the sense organs in the process of human perception of information.

· The structure of technical communication systems.

· What is encoding and decoding.

· The concept of noise; noise protection techniques.

· Information transfer rate and channel bandwidth.

Information transfer scheme:

Information source - information channel - information receiver.

Information is presented and transmitted in the form of a sequence of signals, symbols. From the source to the receiver, a message is transmitted through some material medium. If technical means of communication are used in the transmission process, then they are called information transmission channels (information channels). These include telephone, radio, TV. The human senses play the role of biological information channels.

The process of transferring information through technical communication channels follows the following scheme (according to Shannon):

The term "noise" refers to all kinds of interference that distorts the transmitted signal and leads to loss of information. Such interference, first of all, arises for technical reasons: poor quality of communication lines, insecurity from each other of various streams of information transmitted over the same channels. To protect against noise, various methods are used, for example, the use of various kinds of filters that separate the useful signal from the noise.

When discussing the topic of measuring the speed of information transmission, you can use the reception of analogy. Analog - the process of pumping water through water pipes. Here, pipes are the channel for transferring water. The intensity (speed) of this process is characterized by water consumption, i.e. the number of liters pumped per unit of time. In the process of transmitting information, the channels are technical communication lines. By analogy with the water supply system, we can talk about the information flow transmitted through the channels. Information transfer rate is the information volume of a message transmitted per unit of time. Therefore, the units of measurement of the information flow rate: bit / s, byte / s, etc. information process transmission channel

Another concept - the capacity of information channels - can also be explained with the help of the "water-pipe" analogy. You can increase the flow of water through the pipes by increasing the pressure. But this path is not endless. If too much pressure is applied, the pipe may burst. Therefore, the limiting water consumption, which can be called the throughput of the water supply system. Technical data communication lines have a similar limit of data transmission rate. The reasons for this are also physical.

1. Classification and characteristics of the communication channel
Link Is a set of means for transmitting 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 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 input of the system 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 transmitted 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 transfer 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 Mbit / s - 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; 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 (sometimes regenerators are 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 each symbol is transmitted, 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 equal to:
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 of measurement of 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 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 \u003d 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 bandwidth 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.
, where is an arbitrarily small quantity,
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 \u003d 0.1; p 2 \u003d 0.2 and p 3 \u003d 0.7.
Messages are independent and are transmitted in a uniform binary code ( m \u003d 2) with a symbol duration of 1 ms. Determine the speed of information transmission over the communication channel without interference.
Decision: The source entropy 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 \u003d 1/2t = 500 .
Information transfer rate
C \u003d vH \u003d 500 x 1.16 \u003d 580 [bit / s].
2.2 Discrete communication channel with interference
We will consider discrete communication channels without memory.
Channel without memory A channel is called 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.
In the presence of interference, the average amount of information in the received message symbol - Y, relative to the transmitted - X equally:
.
For message symbol X T duration T,consisting of n elementary symbols the average amount of information in the received message symbol - Y T relative to the transmitted - X T equally:
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 \u003d 2320 bit / s
The throughput of a continuous noisy channel is determined by the formula

=2322 bps.
Let us prove that the information capacity of a continuous channel without memory with additive Gaussian noise under the limitation on the peak power is not more than the information capacity of the same channel with the same value of the limitation on the average power.
The expected value for a symmetric uniform distribution

Mean square for symmetrical uniform distribution

Variance for symmetrical uniform distribution

Moreover, for a uniformly distributed process.
Differential entropy of a signal with uniform distribution
.
The difference between the differential entropies of a normal and uniformly distributed process does not depend on the variance
= 0.3 bit / sample
Thus, the throughput and capacity of the communication channel for a process with a normal distribution is higher than for a uniform one.
Determine the capacity (volume) of the communication channel
V k \u003d T k C k \u003d 10 × 60 × 2322 \u003d 1.3932 Mbit.
Let's determine the amount of information that can be transmitted in 10 minutes of channel operation
10× 60× 2322\u003d 1.3932 Mbps.
Tasks

1. Messages composed of the alphabet are transmitted to the communication channel x 1, x 2 and x 3with probabilities p (x 1) \u003d 0.2; p (x 2) \u003d 0.3 and p (x 3) \u003d 0.5.
The channel matrix looks like:
wherein .
Calculate:
1. Entropy of the information source H (X) and receiver H (Y).
2. General and conditional entropy H (Y / X).
3. Loss of information in the channel during transmission to characters ( k \u003d 100).
4. The amount of information received during transmission to characters.
5. Baud rate, if the transmission time of one character t \u003d 0.01 ms.
2. Alphabet characters are transmitted over the communication channel x 1, x 2, x 3and x 4 with probabilities. Determine the amount of information received during the transmission of 300 symbols, if the effect of interference is described by the channel matrix:
.
3. Determine the loss of information in the communication channel when transmitting equiprobable symbols of the alphabet, if the channel matrix has the form
.
t \u003d 0.001 sec.
4. Determine the loss of information when transmitting 1000 characters of the source alphabet x 1, x 2 and x 3 with probabilities p \u003d 0.2; p \u003d 0.1 and p () \u003d 0.7if the influence of interference in the channel is described by the channel matrix:
.
5. Determine the amount of information received when transmitting 600 symbols, if the probabilities of the appearance of symbols at the source output X are equal: and the effect of interference during transmission is described by the channel matrix:
.
6. Messages consisting of symbols of the alphabet are transmitted to the communication channel, while the probabilities of the appearance of symbols of the alphabet are:
The communication channel is described by the following channel matrix:
.
Determine the baud rate if the transmission time of one character ms.
7.Signals are transmitted through the communication channel x 1, x 2 and x 3 with probabilities p \u003d 0.2; p \u003d 0.1 and p () \u003d 0.7.The effect of interference in the channel is described by the channel matrix:
.
Determine the total conditional entropy and the share of information loss that falls on the signal x 1(partial conditional entropy).
8. Alphabet characters are transmitted over the communication channel x 1, x 2, x 3and x 4 with probabilities.
Channel noise is specified by the channel matrix
.
Determine the bandwidth of the communication channel, if the transmission time of one symbol t \u003d 0.01 sec.
Determine the amount of information received when transmitting 500 symbols, if the probabilities of the appearance of symbols at the receiver input Y are equal:, and the effect of interference during transmission is described by the channel matrix:
.

Bandwidth of continuous communication channel
(14)
For a discrete communication channel, the maximum value of the transmission rate corresponds to equally probable characters of the alphabet. For a continuous communication channel, when the target is the average signal strength, the maximum speed is obtained using a normal centered random signal.
If the signal is centered ( m x \u003d 0) i.e. without a constant component in this case, the rest power is zero ( P 0 \u003d 0). The condition of centeredness provides maximum variance for a given average signal power
If the signal has a normal distribution, then the a priori differential entropy of each sample is maximum.
Therefore, when calculating the throughput of a continuous channel, we assume that a continuous signal with a limited average power is transmitted over the channel - P c and additive noise ( y \u003d x + f) also with limited average power - P n type of white (Gaussian) noise.

The dissemination of information occurs in the process of its transmission.

When transmission of informationthere are always two objects - a source and a receiver of information. These roles can change, for example, during a dialogue, each of the participants acts either as a source or as a receiver of information.

Information flows from source to receiver through a communication channel, in which it must be associated with some material medium.To convey information, the properties of this medium must change over time. So the light bulb, which is on all the time, conveys information only that some process is going on. If you turn on and off the light bulb, you can transmit a variety of information, for example, using Morse code.

When people talk, the information carrier is sound waves in the air. In computers, information is transmitted using electrical signals or radio waves (in wireless devices). Information can be transmitted using light, a laser beam, a telephone or postal system, a computer network, etc.

Information comes through the communication channel in the form of signals that the receiver can detect with the help of his senses (or sensors) and "understand" (decode).

Signal- This is a change in the properties of the carrier, which is used to transfer information.

Examples of signals are a change in the frequency and volume of a sound, a flash of light, a change in voltage on contacts, etc.

A person can receive signals only with the help of his senses. To transmit information, for example, using radio waves, you need auxiliary devices: a radio transmitter that converts sound into radio waves, and a radio receiver that performs the reverse conversion. They allow you to empower a person.

It is impossible to convey a lot of information with a single signal. Therefore, most often not a single signal is used, but a sequence of signals, that is message.It is important to understand that a message is only a "shell" for transmitting information, and information is contentmessages. The receiver must itself "extract" information from the received signal sequence. You can accept the message, but not accept the information, for example, by hearing speech in an unknown language or by intercepting the encryption.

The same information can be conveyed through different messages, for example, through spoken language, by means of a note, or by means of a flag semaphore that is used in the navy. At the same time, the same message can carry different information for different receivers. So the phrase "It is raining in Santiago", broadcast in 1973 on military radio frequencies, for the supporters of General A. Pinochet served as a signal for the beginning of a coup d'etat in Chile.

Thus, information is presented and transmitted in the form of a sequence of signals, symbols. From the source to the receiver, a message is transmitted through some material medium. If technical means of communication are used in the transmission process, then they are called information transmission channels (information channels). These include telephone, radio, TV. The human senses play the role of biological information channels.

The process of transferring information through technical communication channels follows the following scheme (according to Shannon):

Information transfer is possible using any information coding language, understandable to both the source and the receiver.

Encoder - a device designed to transform the original message of the information source into a form convenient for transmission.

Decoder - a device for converting the encoded message into the original one.

Example. During a telephone conversation: the source of the message is the speaking person; an encoder - a microphone - converts the sounds of words (acoustic waves) into electrical impulses; communication channel - telephone network (wire); a decoding device - that part of the tube that we bring to our ear, here the electrical signals are again converted into sounds we hear; the receiver of information is a listening person.

Claude Shannon developed a special coding theory that provides methods for dealing with noise. One of the important ideas of this theory is that the code transmitted over the communication line must be redundant. Due to this, the loss of some part of the information during transmission can be compensated. However, you cannot make the redundancy too large. This will lead to delays and higher communication costs. In other words, in order for the content of a message distorted by interference to be reconstructed, it must be redundantthat is, it must contain "extra" elements, without which the meaning is still restored. For example, in the message "Vlg vpdt to Kspsk mr", many will guess the phrase "Volga flows into the Caspian Sea", from which all vowels have been removed. This example suggests that natural languages \u200b\u200bcontain a lot of "superfluous", their redundancy is estimated at 60-80%.

When discussing the topic of measuring the speed of information transmission, you can use the reception of analogy. Analog - the process of pumping water through water pipes. Here, pipes are the channel for transferring water. The intensity (speed) of this process is characterized by water consumption, i.e. the number of liters pumped per unit of time. In the process of transmitting information, the channels are technical communication lines. By analogy with the water supply system, we can talk about the information flow transmitted through the channels. The information transfer rate is the information volume of the message transmitted per unit of time. Therefore, the units of measurement of the information flow rate: bit / s, byte / s, etc.

Another concept - the capacity of information channels - can also be explained with the help of the "water-supply" analogy. You can increase the flow of water through the pipes by increasing the pressure. But this path is not endless. If too much pressure is applied, the pipe may burst. Therefore, the limiting water consumption, which can be called the throughput of the water supply. Technical data communication lines have a similar limit of data transmission rate. The reasons for this are also physical.

Using Internet resources, find answers to questions:

Exercise 1

1. What is the process of transferring information?

Transfer of information - the physical process by which the movement of information is carried out in space. They wrote down the information on disk and transferred it to another room. This process is characterized by the following components:

2. General scheme of information transfer

3. List the communication channels known to you

Link (eng. channel, data line) is 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). The communication channel, understood in the narrow sense ( communication path), represents only the physical medium of signal propagation, for example, a physical communication line.

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

4. What is telecommunications and computer telecommunications?

Telecommunications (Greek tele - into the distance, far away and Latin communicatio - communication) is the transmission and reception of any information (sound, image, data, text) over a distance through various electromagnetic systems (cable and fiber-optic channels, radio channels and other wired and wireless channels communication).

Telecommunication network is a system of technical means through which telecommunications are carried out.

Telecommunication networks include:
1. Computer networks (for data transmission)
2. Telephone networks (voice transmission)
3. Radio networks (transmission of voice information - broadcast services)
4. Television networks (voice and image transmission - broadcast services)

Computer telecommunications - telecommunications, the terminal devices of which are computers.

The transfer of information from a computer to a computer is called synchronous communication, and through an intermediate computer, which allows accumulating messages and transmitting them to personal computers as requested by the user, is called asynchronous.

Computer telecommunications are starting to take root in education. In higher education, they are used to coordinate scientific research, efficient exchange of information between project participants, distance learning, and consultations. In the school education system - to improve the efficiency of students' independent activities related to various types of creative work, including educational activities, based on the widespread use of research methods, free access to databases, exchange of information with partners both domestically and abroad.

5. What is the bandwidth of the information transmission channel?
Bandwidth - metric characteristic showing the ratio of the limiting number of passing units (information, objects, volume) per unit of time through a channel, system, node.
In computer science, the definition of bandwidth is usually applied to a communication channel and is determined by the maximum amount of information transmitted / received per unit of time.
Bandwidth is one of the most important factors for users. It is estimated by the amount of data that the network can transfer at the limit per unit of time from one device connected to it to another.

The speed of information transfer depends largely on the speed of its creation (source performance), methods of encoding and decoding. The highest possible information transfer rate in a given channel is called its bandwidth. The channel capacity, by definition, is the information transfer rate when using the "best" (optimal) source, encoder and decoder for a given channel, therefore, it characterizes only the channel.

The transfer of information occurs from the source to the recipient (receiver) of the information. The sourceinformation can be anything: any object or phenomenon of animate or inanimate nature. The process of transferring information takes place in a certain material environment that separates the source and recipient of information, which is called channel transmission of information. Information is transmitted through the channel in the form of a sequence of signals, symbols, signs, which are called message. Recipient information is an object that receives a message, as a result of which certain changes in its state occur. All of the above is shown schematically in the figure.

Transfer of information

A person receives information from everything that surrounds him through the senses: hearing, sight, smell, touch, taste. A person receives the greatest amount of information through hearing and sight. Sound messages are perceived by ear - acoustic signals in a continuous medium (most often in air). Vision perceives light signals that transfer the image of objects.

Not every message is informative for a person. For example, although a message in an incomprehensible language is transmitted to a person, it does not contain information for him and cannot cause adequate changes in his state.

The information channel can be either natural (atmospheric air, through which sound waves are transported, sunlight reflected from the observed objects), or be artificially created. In the latter case, we are talking about technical means of communication.

Technical information transmission systems

The first technical means of transmitting information over a distance was the telegraph, invented in 1837 by the American Samuel Morse. In 1876, American A. Bell invents the telephone. Based on the discovery of electromagnetic waves by the German physicist Heinrich Hertz (1886), A.S. Popov in Russia in 1895 and almost simultaneously with him in 1896 by G. Marconi in Italy, radio was invented. Television and the Internet appeared in the 20th century.

All of the listed technical methods of information communication are based on the transmission of a physical (electrical or electromagnetic) signal over a distance and are subject to some general laws. These laws are studied by communication theory, which arose in the 1920s. The mathematical apparatus of communication theory - mathematical communication theory, developed by the American scientist Claude Shannon.

Claude Elwood Shannon (1916-2001), USA

Claude Shannon proposed a model of the process of transmitting information through technical communication channels, represented by a diagram.

Technical information transmission system

Encoding here means any transformation of information coming from a source into a form suitable for its transmission over a communication channel. Decoding - reverse transformation of the signal sequence.

The operation of such a scheme can be explained using the familiar process of talking on the phone. The source of information is the speaking person. The encoder is the telephone handset microphone, with the help of which sound waves (speech) are converted into electrical signals. The communication channel is the telephone network (wires, switches of telephone nodes through which the signal passes). The decoding device is a telephone receiver (earpiece) of the listening person - the receiver of information. Here, the incoming electrical signal turns into sound.

Modern computer systems for transmitting information - computer networks - work on the same principle. There is an encoding process that converts a binary computer code into a physical signal of the type that is transmitted over a communication channel. Decoding consists in converting the transmitted signal back into computer code. For example, when using telephone lines in computer networks, the encoding-decoding functions are performed by a device called a modem.

Channel bandwidth and information transfer rate

Developers of technical information transmission systems have to solve two interrelated problems: how to ensure the highest speed of information transmission and how to reduce the loss of information during transmission. Claude Shannon was the first scientist to tackle these problems and create a new science for that time - information theory.

K. Shannon defined a method for measuring the amount of information transmitted through communication channels. He introduced the concept channel bandwidth, as the maximum possible speed of information transfer. This speed is measured in bits per second (as well as kilobits per second, megabits per second).

The bandwidth of a communication channel depends on its technical implementation. For example, the following communication means are used in computer networks:

Telephone lines,

Electrical cable communication,

Fiber optic cable communication,

Radio communication.

The throughput of telephone lines is tens, hundreds of Kbit / s; the throughput of fiber-optic lines and radio communication lines is measured in tens and hundreds of Mbit / s.

Noise, noise protection

The term "noise" refers to all kinds of interference that distorts the transmitted signal and leads to loss of information. Such interference primarily occurs for technical reasons: poor quality of communication lines, insecurity from each other of various streams of information transmitted over the same channels. Sometimes, when talking on the phone, we hear noise, crackling, interfering with understanding the interlocutor, or the conversation of completely different people is superimposed on our conversation.

The presence of noise leads to the loss of transmitted information. In such cases, noise protection is required.

First of all, technical methods of protecting communication channels from the effects of noise are used. For example, using shielded cable instead of bare wire; the use of various kinds of filters that separate the useful signal from noise, etc.

Claude Shannon was designed coding theorygiving methods of dealing with noise. One of the important ideas of this theory is that the code transmitted over the communication line should be redundant... Due to this, the loss of some part of the information during transmission can be compensated. For example, if you are hard to hear while talking on the phone, then by repeating each word twice, you have more chances that the other person will understand you correctly.

However, you cannot make the redundancy too large. This will lead to delays and higher communication costs. Coding theory allows you to get a code that is optimal. In this case, the redundancy of the transmitted information will be the minimum possible, and the reliability of the received information will be maximum.

In modern digital communication systems, the following technique is often used to combat the loss of information during transmission. The whole message is split into chunks - packages... For each package, the check sum (the sum of binary digits) that is sent with this packet. At the place of reception, the checksum of the received packet is recalculated and, if it does not match the initial sum, the transmission of this packet is repeated. This will continue until the initial and final checksums match.

Considering the transfer of information in propaedeutic and basic computer science courses, first of all, this topic should be discussed from the position of a person as a recipient of information. The ability to receive information from the surrounding world is the most important condition for human existence. The human senses are the information channels of the human body that connect a person with the external environment. On this basis, information is divided into visual, sound, olfactory, tactile, and gustatory. The rationale for the fact that taste, smell and touch carry information to a person is as follows: we remember the smells of familiar objects, the taste of familiar food, and we recognize familiar objects by touch. And the contents of our memory are stored information.

Students should be told that in the animal world the informational role of the senses is different from that of a human. The sense of smell plays an important information function for animals. The heightened sense of smell of service dogs is used by law enforcement agencies to search for criminals, detect drugs, etc. The visual and sound perception of animals differs from humans. For example, bats are known to hear ultrasound, while cats see in the dark (from a human perspective).

Within the framework of this topic, students should be able to give specific examples of the process of transferring information, determine for these examples the source, receiver of information, the channels used for transferring information.

When studying computer science in high school, students should be introduced to the basic provisions of technical communication theory: the concepts of coding, decoding, information transfer rate, channel capacity, noise, noise protection. These questions can be considered within the framework of the topic “Technical means of computer networks”.

Channel information transfer with decisive feedback

graduate work

1.2.1 Methods of information transmission over communication channels

Transfer of information with repetition (accumulation). This transmission method is used to improve the reliability in the absence of a reverse channel, although there are no fundamental restrictions on its use even in the presence of feedback. This is sometimes classified as stacked message reception. The essence of the method consists in transmitting the same message several times, memorizing the received messages, comparing them element by element and composing a message, including the elements selected "by the majority." Suppose that the same codeword 1010101 is transmitted three times. In all three transmissions, it was interfered with and was corrupted:

The receiver compares three received symbols bit by bit and puts down those symbols (under the line), the number of which prevails in this digit.

There is another method of transferring information with accumulation, in which not a symbol-by-symbol comparison is made, but a comparison of the entire combination as a whole. This method is easier to implement, but produces poorer results.

Thus, the high noise immunity of the method of transmitting information with repetition (accumulation) is based on the fact that the signal and interference in the channel do not depend on each other and change according to different laws (the signal is periodic, and the interference is random), therefore, the repeated combination in each transmission, as generally will be distorted in different ways. As a result, at the reception, the accumulation, that is, the summation of the signal, increases in proportion to the number of repetitions, while the amount of interference increases according to a different law. If we assume that the noise and the signal are independent, then the mean squares are summed up and the mean square of the sum increases in proportion to the first degree. Therefore, for n repetitions, the signal-to-noise ratio increases by a factor of n, and this occurs without increasing the signal power. However, this is achieved by increasing the complexity of the equipment and increasing the transmission time or frequency band in the event that the signal is transmitted at several frequencies simultaneously in time. In addition, with dependent errors and bursts of errors, the noise immunity of the system decreases.

Information transfer with feedback. Noise immunity of open-loop transmission (OSS) is provided by the following methods: error-correcting coding, transmission with repetition, simultaneous transmission over several parallel channels. In PBOS, error correction codes are usually used, which is associated with high redundancy and complexity of the equipment. Feedback transmission (PIC) largely eliminates these disadvantages, since it allows the use of less error-resistant codes, which, as a rule, have less redundancy. In particular, error detection codes can be used. The advantage of the return channel is also the ability to monitor the performance of the object receiving information.

With PIC, the concept of a direct channel is introduced, i.e. channel from the transmitter to the receiver, for example, a command signal is transmitted from the control point (CP) to the controlled point (CP). In this case, the return channel will be the transmission of a message from the control panel to the control unit about the receipt of the command signal, and on the return channel both a message can be transmitted only that the signal has been received at the control panel input (in this case, only the passage of the signal through the communication channel is monitored), and information about the complete execution of the command. Feedback is also possible, giving information about the phased passage of the command signal along the receive path.

Let's consider some types of transmission with feedback.

Information feedback transmission (IOS). If the message is transmitted in the form of an anti-interference code, then in the encoder this code can be converted into an anti-interference code. However, since this is not usually necessary, the encoder is a register for converting simple parallel code to serial. Simultaneously with the transmission over the direct channel, the message is stored in a storage device on the transmitter (Fig. 1.1a). At the controlled point, the received message is decoded and also stored in the storage. However, the message is not immediately transmitted to the recipient: first, it arrives through the return channel to the control point. In the CP comparison scheme, the received message is compared with the transmitted one. If the messages match, then the “Confirmation” signal is generated and subsequent messages are transmitted (sometimes, before the next message is sent to the control center, the “Confirmation” signal is first sent that the previous message was received correctly and information can be transmitted from the storage device to the recipient). If the messages do not match, which indicates an error, the "Erase" signal is generated. This signal locks the key to stop the transmission of the next message and is sent to the control panel to destroy the message recorded in the drive. After that, the CP is re-transmitting the message recorded in the drive.

Fig.1.1a. Method of transferring information from IOS.

In systems with ITS, the leading role belongs to the transmitting part, since it determines the presence of an error, the receiver only informs the transmitter about which message it received. There are various options for transmission with IOS. So, there are systems with IOS, in which the transmission of signals occurs continuously and stops only when an error is detected: the transmitter sends the "Erase" signal and repeats the transmission. Systems with ITS, in which all information transmitted to the control panel is transmitted through the reverse channel, are called systems with relay feedback. In some systems with IOS, not all information is transmitted, but only some characteristic information about it (receipts). For example, informational symbols are transmitted via the forward channel, and control symbols are transmitted via the reverse channel, which will be compared at the transmitter with pre-recorded control symbols. There is an option in which, after checking the message received on the return channel and detecting an error, the transmitter can either repeat it (duplicate message) or send additional information necessary for correction (correction information). The number of repetitions can be limited or unlimited.

The return channel is used to determine if retransmission of information is necessary. In systems with ITS, an increase in the reliability of transmission is achieved by repeating information only in the presence of an error, while in systems without feedback (when transmitting with accumulation), repetition is carried out regardless of the distortion of the message. Therefore, in systems with IOS, information redundancy is much less than in systems with PBOS: it is minimal in the absence of distortions and increases in the event of errors. In systems with IOS, the quality of the return channel should be no worse than the quality of the forward channel in order to avoid distortions that can increase the number of repetitions.

Decisive feedback transmission (POC). The message transmitted from the transmitter via the direct channel is received at the receiver (Fig. 1.1b), where it is stored and checked in the decoding device (decoder). If there are no errors, then the message is sent from the storage device to the recipient of information, and through the return channel a signal is sent to the transmitter to continue further transmission (continuation signal). If an error is detected, the decoder issues a signal that erases information in the drive. The recipient does not receive the message, but through the return channel, the transmitter receives a signal to re-request or retransmit the transmission (a signal to repeat or re-ask). At the transmitter, the repetition signal (sometimes called the decisive signal) is extracted by the receiver of the decision signals, and the switching device disconnects the encoder's input from the information source and connects it to the storage device, which allows the transmitted message to be repeated. The repetition of the message can occur several times before it is correctly received.

Fig.1.1b. Method of transferring information from ROS.

When transmitting from POC, the error is determined by the receiver. To do this, the transmitted message must be encoded with an anti-jamming code, which allows the receiver to select the allowed combination (message) from the unresolved ones. This means that the transmission from the POC is redundant. The fidelity of transmission in POC systems is determined by code selection and protection of the repetition and continuation decision signals. The latter does not present any particular difficulties, since these signals carry one binary unit of information and can be transmitted by a sufficiently noise-resistant code.

Systems with POC, or systems with repeated demand, are subdivided into systems with waiting for a decision signal and systems with continuous information transmission.

In systems with waiting, the transmission of a new codeword or repetition of the transmitted one occurs only after the request signal arrives at the transmitter.

In systems with continuous transmission, information is continuously transmitted without waiting for a request signal. At the same time, the transmission speed is higher than in systems with waiting. However, after an error is detected, a re-request signal is sent over the reverse channel, and during the time of arrival at the transmitter, some new message will already be transmitted from the latter. Therefore, systems with continuous transmission must be complicated by the appropriate blocking of the receiver so that it does not receive information after an error is detected.

To compare the efficiency of an open-loop system, which uses the Hamming code with one error correction, and a system with POC, using a simple code, the concept of an efficiency factor is introduced. This factor takes into account the reduction in the probability of erroneous reception and the cost of achieving it, the gain in error protection (if these codes are used), the relative reduction in the transmission rate and circuit redundancy associated with the use of different codes. The final comparison showed that, in contrast to the system without feedback, which uses a complex code, the system with POC gives a gain of 5.1 times. The high efficiency of POC systems has ensured their widespread use.

A comparative analysis of the reliability of the transmission of systems with IOS and ROS showed that:

1) systems with IOS and ROS provide the same transmission reliability with the same total energy consumption of signals in the forward and reverse channels, provided that these channels are symmetric and have the same level of interference;

2) systems with ITS provide a higher transmission reliability than Systems with ROS with relatively weak interference in the return channel as opposed to the forward one. In the absence of interference in the return channel, systems with IOS provide error-free message transmission over the main channel;

3) in case of strong interference in the return channel, higher reliability is provided by systems with ROS;

4) with bursts of errors in the forward and reverse channels, systems with IOS provide a higher reliability.

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