Key indicators of the communication system:

1) reliability of message transmission.

The degree of correspondence between the received and transmitted message is called the reliability of the transmission.

When transmitting discrete messages, reliability is determined by the error rate.

Where is the number of erroneously received message elements, and is the total number of message elements.

The frequency of errors is random.

When transmitting continuous messages, the difference between a transmitted and a received message is characterized by a random error.

received message, x(t)-received message;

Random interference at the output of the communication system.

The root mean square error (RMSE) criterion is often used.

The root mean square error is determined by:

Average interference power;

Average power of the useful signal.

P(- one-dimensional density of probabilistic noise.

Specified interference threshold.

Physically, this condition corresponds to the probabilistic absence of the so-called anomalous error, i.e. an error that may have a mismatch for the recipient.

For example: short-term system failure, impulse noise, etc.

2) noise immunity.

Transmission of information with the required reliability involves reliable operation communication systems, this is possible if the communication system is highly reliable, i.e. the ability of instruments and devices to perform their assigned functions for a long time and provide the necessary noise immunity - the ability to withstand the effects of interference.

Noise immunity depends on factors:

1) methods of practical implementation of the communication system;

2) element base;

3) production, technology of equipment;

4) operating conditions;

5)principles of building a communication system, etc.

The reliability of a communication system is quantified by the probability that the equipment will perform its functions within a given time.

Signal-to-noise ratio is a factor that evaluates the noise immunity of a communication system:

The lower the signal-to-noise ratio required, the higher the noise immunity of the communication system.

3) information transfer speed.

If the transmission of continuous messages is carried out in real time. However, it is often advisable to record a message and then transmit it at a speed that differs more or less from the time it was created. This allows efficient use of communication channels.

Numerically, the transmission speed is determined by the amount of information received from the sender to the recipient in 1 second. Measured in bits per second.

Speed ​​depends:

1) from the message and its statistical properties;

2) characteristics of the communication channel;

3) distortion and interference in the channel.

Very often, when transmitting discrete messages, the concept of technical transmission speed is used to describe the characteristics of the hardware of a communication system.

The maximum possible transmission speed is assessed by the channel capacity, which is numerically determined by the maximum amount of information transmitted over it in 1 second.

effective frequency band of the communication channel;

average interference power.

4) efficiency of the communication system.

To assess the quality of work, cost-related indicators are used.

1) energy;

2) frequency band;

3) cost of equipment;

4) weight and size, etc.

The set of properties characterizing the efficiency of a system from a cost point of view is called communication system efficiency.

To select a communication system based on efficiency, criteria are used, taking into account predetermined established restrictions on some parameters and characteristics of the communication system.

Unit cost criterion - These are the criteria in accordance with which communication systems are assessed by the cost of transmitting 1 bit of information at a given reliability.

Specific energy consumption, where

The signal energy at the receiver input spent on transmitting 1 bit;

Spectral Density interference.

Specific strip consumption, where

Equivalent communication system bandwidth;

R-baud rate (bit*sec).

The value can be considered as indicators of the performance of the communication system.

1.3. Classification of systems and information transmission lines.

Classification signs:

1) scope (telephone systems, data transmission, television, telemetry);

2) according to the form of the message (discrete, continuous);

3) by appearance line signal(continuous, pulse);

4) by operating frequency range and bandwidth (narrowband, broadband);

5) by type of communication (landline, mobile);

6) according to the principle of compaction and separation (time, frequency, code).

All communication systems are divided into two groups:

1) systems with free propagation of signals.

The level of signal scattering is proportional to the square of the distance between the transmitter and receiver (radio engineering).

2) systems with directional propagation of signals.

Forced signal propagation. Devices are used for this. The energy in them is not dissipated, but is absorbed by the guiding device. The systems are stable and ideal from the point of view of reliability. Ideal solution to the problem of electromagnetic compatibility - high throughput. However, these systems are very expensive and require the creation of amplifying relay points.

Problems:

1) electromagnetic compatibility problems, interference;

2)high efficiency, flexibility, mobility.

Systems with free propagation of signals are divided into:

1) systems with constant parameters - systems in which the parameters of the signal passing through the propagation medium do not undergo significant random changes, with the exception of the phase (radio relay communication systems, satellite communications– they operate in the centimeter wavelength range).

2) systems with random parameters– signal parameters change as it passes through the medium. These changes in the receiver or in systems with reflected or direct wave (shortwave signal systems undergo deep fading).

With a wavelength l=3-10 meters, radio signals are well reflected from the ionosphere, which allows them to spread over 2000 km.

At l<3 метров радиоволны распространяются в пределах видимости.

Wave classification:

The performance of any communication system is assessed primarily by the accuracy and speed of information transfer. The first determines the quality of transmission, the second - quantity. In a real communication system, the quality of transmission is related to the degree of distortion of the received message. These distortions depend on the properties and technical condition of the system, as well as on the intensity and nature of the interference. If the communication system is designed correctly and is technically sound, then irreversible distortion of messages is due only to the influence of interference. In this case, the quality of transmission is completely determined by the noise immunity of the system.

Under noise immunity understand the ability of a communications system to resist the harmful effects of interference on message transmission. Since the effect of interference manifests itself in the fact that the received message differs from the transmitted one, the noise immunity for a given interference can be characterized quantitatively the degree of correspondence of the received message to the transmitted one. This quantity is characterized by the term loyalty. The fidelity measure is chosen in different ways, depending on the nature of the message and the requirements of the recipient. It can be shown that transmission fidelity depends on the ratio of the average powers of the signal and interference (more often - the signal-to-noise ratio; English - signal-to-noise ratio - SNR; This relationship is usually denoted as S/N).

The works of V. A. Kotelnikov and K. Shannon show that with a selected criterion and a given set of signals received with a certain interference ( white noise; white noise), There is a maximum (potential) noise immunity that cannot be exceeded by any reception method. A receiver that implements potential noise immunity is called optimal. At a certain intensity of interference, the probability of a reception error is lower, the more different the signals transmitting different messages are. The problem is choosing widely different signals to convey information. Transmission fidelity can be increased by increasing the complexity of modulation-demodulation methods and introducing noise-resistant message coding. Finally, the accuracy of transmission also depends on the method of receiving messages. It is necessary to select a reception method that best realizes the difference between signals at a given signal-to-noise ratio.

Another important indicator of a communication system is information transfer speed.

As already noted, the amount of transmitted information is usually measured in bits and bytes. Larger derived units of information volume (as well as computer memory capacity) are also widely used: kilobyte, megabyte, gigabyte, and also, more recently, terabyte and petabyte.

When determining the amount of information, a situation historically developed that with the names “bit” and “byte” the SI prefixes were (and are) incorrectly used (in accordance with the international standard IEC 60027-2, these units are used, for example, like this: instead of 1000 = 10 3 write 1024 = 2 10):

• 1 KB = 2 10 bytes = 1024 bytes;
• 1 MB = 2 20 bytes = 1024 KB;
• 1 GB = 2 30 bytes = 1024 MB = 1,048,576 KB, etc.

In this case, the designation “KB” is usually started with a capital letter, in contrast to the lowercase letter “k” to denote the multiplier 10 3.

Recall that the number of bits or bytes transmitted per second is the information transfer rate, which is defined in bits/s, baud or bytes/s. With increased transmission speed, it is defined in Kbit/s, Mbit/s, Gbit/s, KB/s, MB/s, GB/s, Kbaud, Mbaud, Gbaud, etc.

In recent years, the term “bitrate” ( bitrate), reflecting the amount of information transmitted per unit of time. Bitrate is commonly used to measure the effective transmission speed of useful information. Bitrate is expressed in bits per second |bit/s|, as well as derived values ​​with the prefixes kilo-, mega-, etc.

When using m-ary rather than binary symbols, the maximum amount of information that can be transmitted over a communication channel is log 2 m [bits]. Therefore, a discrete message source can provide maximum performance (output speed) of information [bit/s], not exceeding

Where T n - duration of one parcel; m- digital code base.

At m = 2 R H = 1 /T n and information transfer speed R H numerically equal technical speed v. At t > 2 possible information transfer rate R u > v. However, often in digital systems communication speed of information transfer R H This option occurs when not all parcels are used to transmit information, for example, if some of them are used for synchronization or for detecting and correcting errors (when using correction code).

As will be shown later, the maximum amount of information that can be transmitted by one binary symbol (“1” or “0”) is 1 bit. Theoretically, each symbol received at the input of a communication channel causes the appearance of one symbol at the output, so that the technical speed at the input and output of the channel is the same.

Compression of transmitted information. When transmitting information, there are two interrelated problems: eliminating redundant information and compressing the latter. Under redundancy understand the useless, superfluous piece of information when receiving, which is still impossible to use, and the consumer actually does not need it. Messages from almost any source are redundant. The fact is that the individual signs of the message are in a certain statistical relationship. So, in words of the Russian language, after two consecutive vowels, a consonant is more likely, and after three consecutive consonants, there will most likely be a vowel. Redundancy allows messages to be presented in a more economical form. The measure of possible reduction of a message without loss of information due to statistical relationships between its elements is determined by redundancy. The concept of “redundancy” applies not only to messages or signals, but also to language as a whole, code. For example, the redundancy of European languages ​​reaches 60-80%.

The reason for the appearance of redundancy is the insusceptibility of human organs to some part of the received information. For example, a television image can contain up to 16 thousand shades of one color, while human vision, sensitive to brightness, is insensitive to such a huge range of colors. At best, a person can distinguish up to several hundred color shades of the same color. Therefore, some color shades can be eliminated during transmission without a perceptible loss in the quality of the color image on the screen. The same can be said regarding the transmission of oral speech over a communication channel, upper frequency spectrum of which can be limited to a frequency of 3400 Hz without losing the meaning of the received message. Another very simple example - suppose that information about the values ​​of inductance I, capacitance WITH and resonant frequency/oscillatory circuit. In this case, it is possible to transmit only the values ​​of two quantities, for example, inductance and capacitance, to the channel, and calculate the resonant frequency at the receiving end using a well-known formula.

Eliminating redundancy in the original information allows fewer bits to be transmitted or stored. In information theory, K. Shannon proved a theorem (see below), according to which for a source without redundancy at R u (here WITH - capacity of the communication system), it is possible to find an encoding-decoding method in which it is possible to transmit messages over a communication channel with interference with an arbitrarily small error. The presence of redundancy in a message is often useful and even necessary, since it allows errors to be detected and corrected, i.e. increase the reliability of message reproduction. If message redundancy is not used to improve transmission reliability, it should be eliminated. For this purpose, special statistical coding is used, and the signal redundancy is reduced in relation to the message redundancy.

A universal indicator of a communication system is information efficiency c, characterizing the use of channel capacity r = RJC.

The timeliness of message transmission is determined by the acceptable delay, caused by the transformation of messages and signals, as well as the finite time of signal propagation along the communication channel (especially the propagation time is noticeable in satellite systems communications). It depends on two indicators: the nature and length of the channel and the duration of signal processing in the transmitting and receiving devices. The speed of information transmission and its delay in communication lines are independent characteristics.

The communication channel, as well as the transmitted signal, is characterized by three parameters: time Tk, during which information can be transmitted over the channel, dynamic range D K and channel bandwidth F K .

Iodine channel dynamic range understand the ratio of the permissible signal power to the power of interference present in the channel, expressed in decibels.

A generalized characteristic of a communication channel is its capacity(volume)

A necessary condition for undistorted transmission over the signal channel

Often the conversion of the primary signal into a high-frequency radio signal serves the purpose of matching transmitted signal with a channel. In the simplest case, the signal is matched with the channel in all three parameters:

If these conditions are met, the volume of the transmitted signal almost completely “fits” into the volume of the channel.

In a number of cases, inequality (1.2) can be satisfied even when one or two of the inequalities (1.3) are not satisfied. This means that you can “trade” duration for spectral width or spectral width for dynamic range, etc. Let's look at an example.

Example 1.1

Let a telephone signal recorded on a tape recorder with a spectrum width of 3.4 kHz be transmitted through a communication channel whose bandwidth is 340 Hz. This can be done by playing back the signal at five times the speed at which it was recorded. In this case, all frequencies of the original signal will decrease by five times, but the transmission time will also increase by the same amount. The received signal is also recorded on a tape recorder, and then, by playing it back at five times the speed, the original signal can be restored with high accuracy. Similarly, a signal can be transmitted faster if the channel bandwidth is wider than the signal spectrum.

However, the greatest interest is in the possibility of exchanging the dynamic range of a communication channel for bandwidth. It turns out that with the introduction of pulse-code modulation types (see Chapter 2), it is possible to transmit a message with a dynamic range of, for example, 60 dB over a channel in which the signal exceeds the interference by only 30 dB. In this case, the channel bandwidth is used several times wider than the message spectrum.

2. COMMUNICATION SYSTEMS AND THEIR MAIN CHARACTERISTICS

2.1. Basic concepts and definitions

The object of transmission in any communication system is a message carrying some information.

In message transmission systems, the semantic content of the concepts of information and message are very close.

In general, information is understood as a set of information about any events, phenomena or objects. To transmit or store information, various signs (symbols) are used to express (represent) information in some form. These can be letters, numbers, gestures and drawings, mathematical or musical symbols, words and phrases of human speech, various implementations of forms of electrical vibrations, etc.

A message refers to the form in which information is presented. In other words, a message is something that is to be transmitted. The set of possible messages with their probabilistic characteristics is called ensemble of messages. The message source selects messages from the ensemble. The selection process is random; it is not known in advance what message will be transmitted. A distinction is made between discrete and continuous messages.

Discrete messages are formed as a result of the sequential output of individual elements - characters - by the source. Many different signs are called alphabet of message source, and the number of characters is volume of the alphabet. In particular, signs can be letters of a natural or artificial language that satisfy certain rules relationships.

Messages intended for processing in computer information systems, is usually called data.

The message is a sequence of states source of information unfolding in time. Depending on whether the set of states of the information source is countable, finite (with the power of the alphabet M) or receiving its states from a certain continuum of possible values, sources are divided into

discrete and continuous (analog). Under discrete a source of information is understood to be some object that at certain points in time receives one of M states of a discrete set. A continuous source at each moment of time can take one of an infinite number of its states. The concept of a message source is introduced accordingly, and all possible sources can be divided into discrete and continuous.

To transmit a message over a distance, there must be some kind of carrier, a material carrier. As such, static or dynamic means and physical processes are used. Physical

the process used as a message carrier and displaying the message being transmitted is called a signal.

The display of a message is ensured by a change in any physical quantity characterizing the process. This value is

information parameter of the signal.

Signals, like messages, can be continuous or discrete. The information parameter of a continuous signal over time can take on any instantaneous values ​​within certain limits. The continuous signal is often called analog. A discrete signal is characterized by a finite number of information parameter values. Often this parameter takes only two values.

In telecommunication systems, electrical signals are used as a carrier used to transmit messages over a distance, since they have the highest speed of propagation (approaching the speed of light in a vacuum - 3,108 m/s).

Any physical process that changes in accordance with the transmitted message can be used as a signal. It is important that the signal is not the physical process itself, but a change in individual parameters of this process. These changes are determined by the message that this signal carries.

In many cases, the signal reflects temporary processes occurring in some system. Therefore, the description of a particular signal may be some function of time. Having defined this function one way or another, we define the signal. However, such a complete description of the signal is not always required. To solve a number of problems, more than general description in the form of several generalized parameters characterizing the basic properties of the signal, similar to how it is done in transportation systems.

The technology of information transmission is, in essence, the technology of transporting (transmitting) signals through communication channels. Therefore, it is advisable to determine the signal parameters that are basic from the point of view of its transmission. These parameters are signal duration, dynamic range and spectrum width.

Every signal, considered as a time process, has a beginning and an end. That's why signal duration T is its natural parameter that determines the time interval within which the signal exists.

The characteristics of a signal within the interval of its existence are the dynamic range and the rate of change of the signal.

Dynamic range is defined as the ratio of the highest instantaneous signal power to the lowest:

Ä =10 lg P c max , (dB).

P cmin

The dynamic range of the speaker's speech is 25 ÷ 30 dB, vocal

ensemble – 45 ÷ 55 dB, symphony orchestra – 65 ÷ 75 dB.

IN In real conditions, interference always occurs. Satisfactory transmission requires that the minimum signal power exceed the interference power. The signal-to-noise ratio characterizes the relative strength of the signal. Usually the logarithm of this ratio is determined, which is called the excess of the signal over the noise. This excess is taken as the second parameter of the signal. The third parameter issignal spectrum width F. This value gives an idea of ​​the rate of change of the signal within the interval of its existence. The signal spectrum can extend over a very large frequency band. However, for most signals it is possible to specify the frequency band within which its main energy is concentrated. This band determines the width of the signal spectrum.

IN In communications technology, the signal spectrum is often deliberately limited. This is due to the fact that the equipment and communication line have a limited bandwidth. The spectrum is limited based on the permissible signal distortion. For example, when telephone communication Two conditions must be met: that the speech be intelligible and that the correspondents can recognize each other by voice. To meet these conditions, the spectrum of the speech signal can be limited to a band from 300 to 3400 Hz. Transmission of a wider range of speech in this case is impractical, as this leads to technical complications and increased costs.

A more general physical characteristic of a signal is the volume of the signal:

If ν ≤ 1, then the signals are called narrowband (simple). If ν >> 1, then – broadband (complex).

Under natural conditions, signals created and received by living beings propagate throughout their habitat. This environment can be called message transmission channel. Let us note right away that even

V In such a simple transmission system, it is typical that there is interference in the channel, i.e. signals generated by extraneous sources. With the emergence of the need for rapid transmission of messages over long distances, people began to need to use various devices (“ technical means"). In modern transmission systems

V used as physical storage media electric currents or voltage, as well as electromagnetic vibrations.

When transmitting messages, there is a need to use such technical means as sensors - converters of various

physical processes into low-frequency electric currents called primary signals(for example, microphone, videocon); devices for encoding discrete messages, used both for the purpose of matching the power of the source alphabet M and the number of code symbols used in the transmission channel, and for the purpose of ensuring high transmission reliability; devices for modulating high-frequency signal carriers with primary signals. Since the recipient perceives the message, as a rule, in the form that is presented at the output of the original source, the transmission system requires such technical means as a demodulator, decoder, which carry out inverse conversion high-frequency signals into analogues of primary, low-frequency signals into analogues of original messages (for example, using a speaker, kinescope, etc.).

2.2. Communication systems

The set of technical means (hardware and software) and the distribution environment required to transmit a message from the source to the recipient is called a communication system. In functional diagrams and their implementations, nodes such as an encoder and a modulator are combined in a transmitting device; similarly, the demodulator and decoder are combined into a single device - the receiver. A typical functional diagram, including the main components of the communication system, is shown in Fig. 1.2. The communication line specified here, in many cases identified with the transmission channel, is designed to transmit signals with the minimum possible loss of their intensity from the transmitter to the receiver. In electrical communication systems, a communication line, in particular, is a pair of wires, a cable or a waveguide; in radio communication systems, it is a region of space in which electromagnetic waves propagate from the transmitter to the receiver.

The interference w(t), which is inevitably present in the communication system, is localized in the communication line, leading to random, unpredictable distortion of the shape of the transmitted signal.

Rice. 2.1. Generalized structural scheme telecommunication systems

The receiver processes the received signal x (t), distorted by interference, and reconstructs the transmitted message u (t) from it. Typically, the receiver performs the opposite operations to those performed at the transmitter.

A communication channel is usually called a set of technical means used to transmit a message from a source to a consumer. These means are a transmitter, a communication line and a receiver.

The communication channel together with the source and consumer form information transmission and processing system. Distinguish discrete message transmission systems(for example, telegraph communication system) and continuous message transmission systems(radio broadcasting systems, television, telephony, etc.). There are also mixed-type communication systems in which continuous messages are transmitted by discrete signals. Such systems include, for example, pulse-code modulation systems.

When transmitting messages one way from sender to recipient, or point-to-point, a point-to-point one-way communication channel is used. If the source and recipient alternately change places, then for the exchange of signals it is necessary to use an alternate two-way communication channel, allowing transmission in both one and the opposite direction (half-duplex mode). Greater opportunities for exchange are provided by a simultaneous two-way communication channel, which ensures simultaneous transmission of signals in opposite directions (duplex mode).

A communication system is called multichannel if it provides mutually independent transmission of several messages over one common communication channel.

If it is necessary to exchange messages between many senders and recipients, called in this case users or subscribers, it is necessary to create message transmission systems (MTS) with a large number communication channels. This leads to the concept of a message transmission and distribution system (MTDS), i.e. communication systems in a broad sense. Such a system is usually called a communications (telecommunications) network, information network or messaging network. An example of a SPRS is a fully connected network (Fig. 1.1), where end points (EP) are connected to each other according to the “each to each” principle.

Fig.2.2. Fully connected information transmission network

This network is non-switched, and communication between subscribers is carried out via permanently assigned (non-switched) channels. The distribution of information in such networks is ensured by special access methods or procedures for controlling the transfer of information, which serve to notify which subscribers will exchange messages. With an increase in the number of subscribers in a multipoint network, delays in the transmission of information increase significantly, and in fully connected networks the number of communication lines and the volume of equipment significantly increases. The solution to these problems is associated with the use of switched networks SPRS, where subscribers communicate with each other not directly, but through one or more switching nodes (SM).

Thus, a switched SPRS is a collection of OP, switching nodes and communication lines connecting them.

The main task of modern SPRS is to provide a wide range of users (people or organizations) with a variety of information services, which include, first of all, the effective delivery of messages from one point to another, satisfying the requirements for speed, accuracy, latency, reliability and cost.

The statistical characteristics of the call flow are studied using queuing theory methods, in particular teletraffic theories. This theory makes it possible to establish requirements for switching devices and the number of lines that guarantee satisfactory communication quality for a given percentage of failures or waiting time.

For example, the load on the telephone network depends on the number, time of occurrence and duration telephone conversations.

Load intensity is understood as the mathematical expectation of the incoming load per unit of time (in telephony – 1 hour).

Erlang (1 hour lesson) is taken as a unit of measurement of load intensity. The load changes throughout the day; the hour of greatest load is called CHN. Each subscriber on average provides a load in the range of 0.06 ...

0.15 Earl. Based on these values, the telephone network and its switching systems are calculated.

The source of information in the communication system (see Fig. 2.1) is the sender of the message, and the consumer is its recipient. In some information transmission systems, the source and consumer of information can be a person, while in others - various kinds of automatic devices, computers, etc.

Converting a message into a signal involves three operations:

– conversion from non-electrical to electrical form;

– primary coding;

– transformation in order to match the characteristics of the signal with the characteristics of the communication channel.

These three operations can be independent or combined.

At the first stage, the message is converted using sensors into an electrical quantity - a primary signal.

The main primary telecommunication signals are: telephone (voice), sound broadcasting, facsimile, television, telegraph, data transmission (for example, text entry from a keyboard).

In order for the received message to most accurately correspond to the transmitted one, it is advisable to transmit signals in discrete form. Analog signals are converted to discrete values ​​by a quantization process in which a continuous range of signal values ​​is subdivided into discrete domains so that all signal values ​​falling within one of these domains are replaced by a single discrete value. In this case, quantization takes place not only according to some signal parameter, for example, amplitude, but also according to time.

The second stage of converting a message into a signal - encoding - consists of converting letters, numbers, signs into certain combinations of elementary discrete symbols, called code combinations or words. The rule for this transformation is called a code. The purpose of coding, as a rule, is to coordinate the source of messages with communication channels, providing either the maximum possible speed of information transmission or a given noise immunity. Coordination is carried out taking into account the statistical properties of the message source and the nature of the impact of interference.

At the third stage, the primary signals u (t) are converted into signals convenient for transmission over a communication line (in shape, power, frequency, etc. These operations are performed in the transmitter. In the simplest case, the transmitter may contain an amplifier of the primary signals or only a filter , limiting the band of transmitted frequencies. In most cases, the transmitter is a carrier (carrier) generator and a modulator. The modulation process consists of controlling the carrier parameters with the primary signal u (t). At the transmitter output we obtain a modulated signal s (u, t).

An information transmission system is called multichannel if it provides mutually independent transmission of several messages over one common communication channel.

A communication channel can be characterized in the same way as a signal, by three parameters: the time during which the channel transmits, the dynamic range and the channel bandwidth. For undistorted signal transmission, the channel capacity V k must be no less than the signal volume.

The common features of the various channels are as follows. Firstly, most channels can be considered linear. In such channels, the output signal is simply the sum of the input signals (superposition principle). Secondly, at the channel output, even in the absence of a useful signal, there is always noise. Thirdly, when a signal is transmitted over a channel, it undergoes a time delay and attenuation in level. And finally, in real channels There are always signal distortions caused by channel imperfections.

The signal at the channel output can be written in the following form:

x (t) = µ s (t − τ) + w (t),

where s (t) – signal at the channel input; w (t) – interference; µ and τ are quantities characterizing the attenuation and delay time of the signal.

2.3. Main indicators of the quality of functioning of the communication system

Based on the purpose of any telecommunication system – transfer of information from source to consumer – it is possible to evaluate the performance of the system by two indicators: the quality and quantity of transmitted information. These indicators are inextricably linked.

The quality of transmitted information is usually assessed by the reliability (fidelity) of message transmission. Quantitatively, reliability is characterized by the degree of correspondence of the received message to the transmitted one. A decrease in reliability in a communication channel occurs due to interference and distortion. But since the distortion in the channel can, in principle, be compensated and in properly designed channels they are quite small, the main reason for the decrease in reliability is interference. Thus, the fidelity of message transmission is closely related to noise immunity systems, i.e. its ability to resist the interfering effects of extraneous signals. The more noise-resistant a system is, the higher the transmission fidelity it provides for given characteristics of interfering influences and a certain power of transmitted signals reflecting the state of the source. The quantitative measure of credibility is chosen differently depending on the nature of the message.

If the message is a discrete sequence of elements from some finite set, the influence of interference is manifested in the fact that instead of the actually transmitted element, some other element may be received. This event is called an error. As a quantitative measure of reliability, we can take the error probability p or any increasing function of this probability.

An indirect measure of quality can be an assessment of the degree of distortion of the form of received data. standard signals(edge ​​distortion, crushing, front fluctuations, etc.). These distortions are also normalized for discrete channels. There are simple relationships for converting waveform distortion into error probability.

When transmitting continuous messages, the degree of correspondence of the received message v (t) to the transmitted u (t) can be a certain value ε, which is the deviation of v from u. The standard deviation criterion is often adopted, expressed by the relation:

ε 2 = 1 T ∫ [ v (t ) − u (t ) ] 2 dt . T0

The standard deviation ε 2 takes into account the influence on the received message ν (t) of both interference and all kinds of distortions (linear, nonlinear).

The reliability of transmission depends on the signal-to-interference power ratio. The higher this ratio, the lower the probability of error (the greater the reliability).

For a given noise intensity, the probability of error is less, the more the signals corresponding to different elements of the message differ from each other. The challenge is to select signals with large differences for transmission.

Reliability also depends on the method of reception. It is necessary to choose a reception method that best realizes the difference between signals for a given signal-to-interference ratio. A properly designed receiver can increase the signal-to-interference ratio quite significantly.

An indirect assessment of the quality of transmission of continuous messages is given by the characteristics of channels (frequency, amplitude, phase, level of interference, etc.), by some parameters of signals and interference (distortion rate, signal-to-interference ratio, etc.), by subjective perception messages. The quality of telephone communications, for example, can be assessed by speech intelligibility.

There are significant differences between discrete and continuous message transmission systems. In analog systems, any, even no matter how small, interfering effect on the signal that causes distortion of the modulated parameter always entails the introduction of a corresponding error into the message. In discrete message transmission systems, an error occurs only when the signal is reproduced (identified) incorrectly, and this occurs only with relatively large distortions.

In the theory of noise immunity developed by V.A. Kotelnikov shows that for a given coding and modulation method there is a maximum (potential) noise immunity, which in a real receiver can be achieved, but cannot be surpassed. A receiving device that realizes potential noise immunity is called an optimal receiver.

Along with reliability (noise immunity), the most important indicator of the operation of a communication system is transmission speed. In discrete message transmission systems, the speed is measured by the number of transmitted binary symbols per second R. For one channel, the transmission speed is determined by the relation

R = 1 log 2 m,

where T is the duration of an elementary signal; m – code base. For m = 2 we have R = 1/T = v, Baud.

The maximum possible transmission speed R max is usually called

system throughput. The capacity of an analog message transmission system is estimated by the number of simultaneously transmitted telephone conversations, radio broadcasts or television programs and so on.

System capacity R max should not be confused with

communication channel capacity C (see Chapter 4). The throughput of a communication system is a technical concept that characterizes the equipment used, while the throughput of a channel determines the potential capabilities of the channel for transmitting information. In real systems, the transfer rate R always less than channel capacity WITH. Information theory proves that when R ≤ C it is possible to find such transmission methods and corresponding reception methods in which the reliability of the transmission can be made as great as desired.

From what has been discussed it follows that the quantity and quality of information transmitted in a communication channel is mainly determined by interference in the channel. Therefore, when designing and operating communication systems, it is necessary to achieve not only small distortions of the received primary signal, but also a specified excess of the signal over the interference. Usually the signal-to-interference ratio for received primary signals is normalized.

An important characteristic of a communication system is latency. The delay is understood as the maximum time elapsed between the moment a message is sent from the source to the input of the transmitting device and the moment the restored message is issued by the receiving device. The delay depends, firstly, on the nature and length of the channel, and secondly, on the duration of processing in the transmitting and receiving devices.

Control questions

1. What is meant by message and signal?

2. Draw a functional diagram of the information transmission system.

3. What is a communication channel? What types of channels do you know?

4. How is a continuous message converted into a signal?

5. What is transmission integrity and how is it quantified?

6. Define the main characteristics of a signal?

7. What is modulation?

8. How is the transmitted message restored at the receiver?

9. What parameters determine the quality of information transmission and the amount of information transmitted?

10. What is meant by communication system throughput?

Lecture 3

Factors that determine the quality parameters of ADSL connections

Factors influencing ADSL quality parameters

Our study of ADSL technology is purely practical and focused on the study of measurement methods.

For this reason, in the book we will be interested not so much in the operating principles of ADSL systems, but in those factors that determine the quality parameters of the ADSL network and, ultimately, the technological and commercial success of the technology as a whole.

In this small section, based on the above information about ADSL technology, we will try to identify factors that characterize ADSL quality parameters.

In order to highlight the groups of factors that interest us, let us return to Fig. 1.8.

As follows from the figure, the ADSL user connection diagram contains three objects: a modem, a DSLAM and a subscriber pair section.

We are less interested in individual parameters of a modem or DSLAM than in the parameters of these devices as a technological pair.

Consequently, two groups of factors influencing ADSL quality parameters can be distinguished.

Influence from the modem-DSLAM pair. Influence of subscriber cable pair parameters.

Let's study these factors separately.

Impact of endpoints and DSLAMs

The principles of operation of a modem-DSLAM pair discussed above show that the parameters of such devices can influence the overall parameters of ADSL access quality. There are several factors at play here.

ADSL technology provides for technological independence of the parameters of DSLAM and modem; these devices can be of different manufacturers. Any inconsistencies in the modem-DSLAM pair should affect the quality of ADSL access.

The inconsistency factor at the “handshake” level may manifest itself in the fact that the modem and DSLAM may not establish the most efficient mode of operation and data exchange.

At the connection diagnostic level, the inconsistency factor can lead to incorrect setting equalizers and echo cancellers, which will affect the transmission speed parameters. Here there may be a factor of disruption in the operation of only one device.

For example, the procedure for setting up an echo canceller in the modem may turn out to be incorrect and violations may occur.

Similar violations can be caused by incorrect operation of signal level equalization procedures in DSLAM, etc.

Similarly, problems can be caused by inconsistencies at the channel diagnostic level. Here, violations in the negotiation process of encoding schemes and any failures in the operation of SNR diagnostic algorithms can lead to deterioration in the quality of the ADSL connection.

Looking ahead, we note that the diagnosis of all of the listed factors can only be realized in the process of complex studies of devices using compliance test methods. These techniques are too complex to operate and too expensive.

Influence of subscriber line parameters

The most interesting factor for operation, which directly affects ADSL quality parameters, are the parameters of the subscriber cable pair.

Since the subscriber cable and its parameters are not introduced by ADSL technology from the outside, but are already available to the operator in the form and condition in which it lived before the NGN era, this contains the weakest element of the ADSL technological chain. And although it is impossible to equate cable measurements with ADSL measurements, subscriber pair measurements account for more than 50% of all operational measurements in the initial stages of ADSL implementation.

Let's briefly consider what subscriber line parameters may be critical for ADSL quality. Each of the listed parameters is given in more detail in Chapter 4.

Basic parameters of subscriber cables

Let's start with the general (or basic) parameters of subscriber cables. These include all those parameters that have historically been used for certification cable system operator.

It can be argued that this is a group of parameters and methods of their analysis, the same for any subscriber cables, despite their type and method of use.

Indeed, if there is a metal cable, then it has resistance, capacitance, insulation parameters, and all of the listed parameters do not depend on the purpose for which the cable is laid. It can be used for regular telephone communication, for ADSL, for radio system, etc.

And all applications require a certain set of parameters to judge the quality of the subscriber pair.

That is why such parameters are called basic.

The basic parameters of a subscriber pair are fully described in regulatory documents and are well known.

The main basic parameters include:

presence of direct/alternating voltage on the line; subscriber loop resistance; subscriber loop insulation resistance; capacitance and inductance of the subscriber loop; complex resistance of a line at a certain frequency (line impedance); symmetry of the pair in the sense of ohmic resistance.

The values ​​of the listed parameters determine the quality of the subscriber pair, and on this basis we can say that they are important for certification of cables for ADSL.

Specialized cable parameters

As shown above, ADSL transmission parameters are influenced not so much by the basic parameters of the subscriber pair, but by the parameters of the subscriber cable as a channel for transmitting 256DMT/QAM signals.

In this case, an important group of parameters is related directly to the transmission procedure, which includes parameters such as signal distortion, signal attenuation, various types of noise and external influences on the line.

Since this group of parameters is directly related to the area of ​​application of the ADSL cable, they are called specialized.

Procedurally specialized parameters differ from basic ones in that any measurements of these parameters are always based on line frequency testing techniques.

According to these methods, to diagnose a subscriber cable, you should apply a specialized test signal (impact) and analyze the quality of the passage of such a signal along the line (response).

Specialized options include:

cable attenuation;

wide-band noise and signal-to-noise ratio (SNR); amplitude-frequency response (AFC); near-end crosstalk (NEXT); far-end crosstalk (FEXT); impulse noise; return losses; symmetry of the pair in the sense of uneven transmission characteristics.

Irregularities in the cable

The third factor that directly affects ADSL quality parameters at the subscriber cable level is the presence of inhomogeneities in the cable.

Any inhomogeneities in the subscriber cable negatively affect transmission parameters.

As an illustration of the processes occurring in the transmission system, Fig. 3.1 shows a parallel tap, which is a fairly common phenomenon on the domestic network.

In the case of transmitting a wideband signal through a parallel tap, the transmitted signal is first branched and then reflected from the unmatched end of the tap.

As a result, on the receiver side, two signals - direct and reflected - are superimposed on each other, and the reflected signal can be considered as noise. Since the noise signal in the case shown in Fig. 3.1 has the same structure as a regular signal, its influence is maximum on the transmission quality parameters.

Rice. 3.1. Parallel tapping and its impact on ADSL transmission parameters

The level of destructive influence of the reflected signal will directly depend on the level of reflection at the tap. From signal theory, the higher the frequency of the transmitted signal, the higher the reflection level.

As a result, any broadband transmission systems are very sensitive to any inhomogeneities in the cable. In the case of ADSL, the sensitivity to inhomogeneities is slightly compensated by the adaptive adjustment of the modem-DSLAM pair, so that the presence of taps does not negate the possibility of transmission.

But in the case of a tap, the ADSL transmission speed drops sharply, which allows equipment manufacturers and system engineers to put forward requirements that no inhomogeneities in the ADSL cable be allowed.

Crosstalk

The concept of transient attenuation is less clear from the point of view of the nature of the appearance of this factor, but it better reflects the measurement method. Therefore, in practice both concepts are used.

The fourth factor influencing ADSL transmission parameters in a cable is the factor of mutual influence of subscriber cables on each other.

Methodologically, the parameters of mutual influence are called transient interference, or transient attenuation.

Fig.3.2. Crosstalk NEXT and FEXT

There are two parameters of transient interference (Fig. 3.2).

near-end coupling loss (i.e., the effect of the near-end transmitter on the near-end receiver); far-end crosstalk (i.e., the effect of a distant transmitter on a near-end receiver).

Nominally, FEXT and NEXT refer to specialized parameters of the cable pair. But the role of this parameter is so unique that it requires separate consideration and research.

Suffice it to say that, despite the existence of the concepts NEXT and FEXT for decades, there is no general methodology for measuring these parameters, and in the conditions of NGN subscriber networks it can hardly be built.

For example, the mutual influence of one pair on another can potentially exist, but not manifest itself in any way as long as one pair carries telephony and the other ADSL.

But as soon as you connect a new ADSL subscriber, this influence can “kill” the quality of communication in both pairs.

The same applies to interference from external sources of electromagnetic radiation - in the general case, it is impossible to predict their manifestation on an individual pair.

The following types of possible crosstalk can be identified as the most important for ADSL quality parameters.

The influence of an ADSL subscriber on another ADSL subscriber. Impact of AM radio frequencies on ADSL. Influence of external electromagnetic interference. Impact from digital transmission systems (E1, HDSL, etc.).

The potential impact of ADSL on the quality of traditional telephony has been discussed for a long time. The reason for discussing this topic was complaints from traditional telephony subscribers about the deterioration of communication quality in the process of mass introduction of ADSL.

Although the theory of using splitters excludes the influence of ADSL on the telephone network, complaint statistics showed a stable relationship between the level of ADSL implementation and the number of complaints.

Special studies have shown that there really is no crosstalk between the telephone network and ADSL, and complaints are largely due to the activities of the operators themselves.

To provide better quality ADSL services, operators switched pairs so that the ADSL user received the best quality pair, while regular phone The average subscriber received the pair worse, which led to an assessment of the negative role of ADSL.

By the way, this example shows that in the process of mass adoption of ADSL, purely technical factors are strongly mixed with social, historical and administrative factors. As shown in Chapter 7, this example is not the only case where it is difficult to separate the influence of technology from other processes in the operating system.

Some ADSL Applications

Now, from a general analysis of ADSL technology, let's move on to considering some options for using this technology in NGN subscriber access networks.

As follows from the NGN network paradigm itself, the main goal of building broadband subscriber access networks is to provide users with the maximum possible data transmission bandwidth to the transport network. The range of services provided to the user depends on this, and the success of the implementation of NGN depends on the effectiveness of the implementation of new services, because it is for their sake that a new technical revolution is taking place.

Thus, the topic of services is fundamental to the study of any issues related to NGN. ADSL technology is no exception. In this section we will look at options for using ADSL on a modern network, which should complement our understanding of the place of this technology in a modern communication system.

Individual connection

The simplest application of ADSL technology is the individual use of broadband access to provide services to an individual user.

The undoubted advantage of ADSL is that it offers very effective method migration of subscribers from the telephone network to the NGN network.

Let us recall that for this you only need to install splitters at both ends of the subscriber line, thereby separating data traffic and telephone traffic, and then connect an ADSL modem on the user side and a DSLAM on the station side.

Fig.3.3. Subscriber individual connection diagram

As a result of this migration process, ADSL technology becomes individually oriented. It is aimed at individual subscribers of the telephone network and offers to connect them to the NGN network at minimal cost. Accordingly, ADSL is most often used in individual connection mode (Fig. 3.3).

As shown in the figure, in the case of an individual subscriber connection to ADSL, the task is to provide a single user with broadband access.

For example, this could be the subscriber's apartment. In this case, the subscriber is left with a regular phone connected through a splitter, and broadband access to the NGN network is added. Depending on the configuration and type of ADSL modem, it can be a USB interface for connecting one computer or Ethernet, to which you can even connect a home local network. In turn, computers or IPTV devices can be installed on the home local network to provide broadcast television signals.

VoDSL technology

A new application in relation to traditional ADSL services is associated with the development of voice transmission technology in packet networks (Voice over IP, VoIP). Currently, VoIP has become very widespread. An example is the Skype service, which is widely used by more than 5 million subscribers worldwide.

If there is a potential for voice over data, another application of ADSL could be the provision of VoIP services. This service can be called voice over ADSL, or VoDSL.

The service diagram is shown in Fig. 3.4. On the user's side, not only the computer, but also the VoIP phone are connected to the ADSL modem. On the station side, after the DSLAM, an access switch (BRAS) is installed, which allocates the VoIP traffic and forwards it to the VoIP/PSTN telephone gateway, so that the VoIP traffic is converted into regular telephone traffic and goes out to the public network.

Call" href="/text/category/koll/" rel="bookmark">collective use of ADSL

The VoDSL services discussed above have another interesting application, namely the ability to share one ADSL connection.

As shown above, modern technologies VoIP allows you to install ADSL on the user side Additional Phone. But no one forbids connecting several VoIP phones instead of one phone, and creating a local network instead of one computer (Fig. 3.5). In this case, we get an entire network for a small office on one ADSL.

This approach to using ADSL promises great promise for this technology. For example, a small company rents a new office and traditionally asks itself the question of how to ensure communication with the outside world. If the office space was previously an apartment, then it has only one telephone. And that’s when an ADSL solution can come to the rescue. It is enough to connect to a single pair of ADSL, and the office will have the required number of telephones and a fairly wide “pipe” to the Internet.

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Fig.3.6. Integrated broadband access network and the place of ADSL in it

ATM adaptation level is AAL2, data packets are also converted into an ATM cell stream (adaptation level AAL5). In other words, the IAD performs the task of multiplexing speech and data streams into virtual circuits (VCs) for transmission over a DSL line, as well as serving as a bridge or traffic router local networks Ethernet while simultaneously supporting a sufficient number of voice connections.

Already now the use of IAD to create corporate networks very

popular within the framework of mass ADSL implementation projects in Moscow and St. Petersburg. As the “internetization” of small and medium-sized businesses and ADSL networks develops, the proposed usage scheme will continue to find its customers.

Bibliography

1. Baklanov ADSL/ADSL2+: theory and practice of application. - M.: Metrotek, 2007.

Control questions

List the factors influencing ADSL quality parameters. How do end devices and DSLAMs affect ADSL quality parameters? List and describe the basic parameters of the subscriber cable. List and describe specialized cable parameters. How cable inhomogeneities affect ADSL. How does parallel tapping in the cable affect ADSL transmission parameters? Describe the terms “crosstalk and crosstalk attenuation.” Draw a diagram of the occurrence of transient interference. Name and characterize the parameters of transient interference. Name the most important types of crosstalk. Draw a diagram of an individual ADSL subscriber connection. Draw a diagram of the VoDSL service organization. Draw a diagram of a collective connection to ADSL. What is IAD and what functions does it perform? Draw an integrated broadband access network and the place of ADSL in it

Communication system characterized by a set of parameters. Those of them that are related to the quality of the system by a monotonic dependence are called indicators of system quality. The larger (smaller) the value of the quality indicator, the better (worse) the system, other things being equal.

When designing a system, a large number of quality indicators and parameters are taken into account in accordance with a pre-founded optimality criterion. The best (optimal) system is considered to be the one that corresponds to the largest (smallest) value of a certain objective function of quality indicators. Quality indicators and parameters of communication systems are conventionally divided into:

— information (noise immunity, speed, throughput and delay of information transmission);

— technical and economic (cost, dimensions, weight);

— technical and operational indicators (average time trouble-free operation, operating temperature range, etc.).

Let's highlight indicators, characterizing communication system from the point of view of information transfer.

Noise immunity is one of the main indicators of the quality of a communication system. Noise immunity for a given interference is characterized by transmission fidelity—the degree to which the received message corresponds to the transmitted message. When transmitting continuous messages, the measure of fidelity is the standard deviation between the received a"(t) and the transmitted a(t) messages:

Where T - the time during which the message is received.

Primary signal b(t) associated with the message a(t) linear dependence, i.e.

b(t) = ka(t),

Where k - conversion factor.

where the asterisk indicates the signal estimate, which differs from this signal by the amount of error.

The smaller the standard deviation, the higher the noise immunity.

The measure of fidelity can also be the probability that the error ε will not exceed a predetermined valueε 0:

The greater this probability, the higher the noise immunity.

The measure of fidelity of discrete message transmission is probability of error. The lower this probability, the greater the noise immunity.

The maximum noise immunity possible for given transmission conditions is called potential noise immunity.

Another important indicator of the quality of a communication system is its throughput, those. the maximum transmission speed Rmax allowed by this system. It is determined by the number N channels of this system and capacity C communication channel:

For a discrete communication channel without interference

Where T— duration of transmission of one symbol; m - volume of the alphabet. (Hereinafter, notation of the form logx denotes the operation of binary logarithm log 2 x.)

For a continuous communication channel

WITH= Flog(l + P With / P w) ,

Where F - channel bandwidth; R c - signal power; R w - noise power.

Transfer speed (as well as throughput) is measured in bits per second

Transmission delay— this is the time from the moment the message is transmitted in the transmitter until the restored message is issued at the output of the receiver. It depends on the length of the communication channel and the duration of signal transformations in the transmitter and receiver. Transmission delay is one of the most important indicators of the quality of a communication system.

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