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 presupposes reliable operation of the communication system; this is possible if the communication system has high reliability, 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 of 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. The ideal solution to the problem of electromagnetic compatibility is 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 signal parameters passing through the propagation medium do not undergo significant random changes, with the exception of the phase (system radio relay communication, satellite communications - they operate in the centimeter wave range).

2) systems with random parameters - the parameters of the signal change as it passes through the medium. These changes in the receiver are either in reflected or direct wave systems (short wave systems - signals 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:

Block diagram of a single-channel communication system. Classification of communication systems

The set of technical means and distribution environment that ensures the transmission of messages from source to recipient is called telecommunication system.

When transmitting messages by a telecommunication system, the following operations are performed:

Converting a message coming from a message source (MS) into a primary telecommunication signal (hereinafter simply “primary signal”);

Conversion of primary signals into linear signals with characteristics consistent with the characteristics of the propagation medium (communication line);

Transmission route selection and switching;

Transmission of signals along the selected route;

Converting signals into messages.

Generalized block diagram of the system

telecommunications

IS – source of message (information);

PR 1 (PR -1) – converter (inverse converter) of the message into the primary signal;

SC – switching station, which represents a set of switching and control equipment that ensures the establishment of various types of connections (local, long-distance, international, incoming, outgoing and transit)

OS 1 (OS -1) – interface equipment that performs direct (inverse) conversion of primary signals into linear signals (secondary signals).

Telecommunication channel is a complex of technical means that ensures the transmission of messages between its source and recipient.

Transmission channel is a complex of technical means and propagation media that ensures the transmission of a primary telecommunication signal in a certain frequency band.

Transfer system is a complex of technical means and propagation media that ensures the transmission of a primary signal in a certain frequency band or at a certain transmission speed between switching stations.

Main characteristics of communication systems

When assessing the performance of a communication system, it is necessary, first of all, to take into account what accuracy of message transmission provides the system and with what speed information is transmitted. The first determines quality transmission, second - quantity.

Noise immunity for receiving messages characterizes the degree of correspondence between the transmitted and received messages, expressed in some quantitative measure. Noise immunity, is the ability of a system to withstand the harmful effects of interference. Noise immunity is assessed on the accuracy of message reception for a given signal-to-interference ratio (SNR) and depends both on the properties of the transmitted signals and on the method of reception. Loyalty reception is determined by the degree of similarity of the received and transmitted messages.

If the message is described by a continuous function a(t), then the deviation ε (t) received message ậ(t) from the transmitted A(t) is continuous:

(1.2.1)

and is often used as a measure of difference standard deviation(RMS):

, (1.2.2)

where the overbar denotes averaging over many realizations.

Information transfer rate R is called the average amount of information I, transmitted in this system per unit of time:

R[dv. units/sec.] = I/T, (1.2.4)

Where T– duration of information transfer.

Timeliness message transmission is determined by acceptable delay, conditioned by the transformation of messages and signals, as well as the finite time of signal propagation along the communication channel.

4 Basic parameters of signals and communication channels. A necessary condition for undistorted signal transmission

The communication channel is characterized in the same way as the signal by three main parameters:

- time T to, during which transmission is possible over the channel;

- dynamic range D to(the ratio of the permissible power of the transmitted signal to the interference power, expressed in decibels);

- channel bandwidth Fc.

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

(1.5.1)

A necessary condition for undistorted transmission of signals with volume over a channel is:

In the simplest case, the signal is matched with the channel in all three parameters, i.e. achieves the fulfillment of the following conditions:

Inequality (1.5.2) can also be satisfied when one or two of the inequalities (1.5.3) are not satisfied. This means that you can “trade” duration for spectral width, or spectral width for dynamic range, etc.

Along with the above basic parameters of the channel, its frequency properties are characterized by frequency transmission coefficient, and time properties - by impulse response h to (t,τ). It follows from clause 1.2.5 that these characteristics make it possible to describe the transformations of input signals in the time or frequency domain, carried out both by the channel as a whole and by its individual elements.

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 volume transmitted information It is customary to measure 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, the upper frequency of the 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 (the propagation time is especially noticeable in satellite communication systems). 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 the transmitted signal with the 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.

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 settings of 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 disorders can be caused incorrect work procedures for leveling the signal level 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 parameters subscriber line

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 to certify the operator’s cable system.

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 normal 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 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 an ADSL user received a better quality pair, while an ordinary telephone subscriber received a worse pair, 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 This is not the only case where it turns out to be difficult to separate the influence of technology and 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 very paradigm of NGN networks, the main goal of building broadband subscriber access networks is to provide users with the maximum possible data transmission bandwidth in 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. No exception ADSL technology. 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 regular phone, connected via a splitter, and broadband access to the NGN network is added. Depending on the configuration and type of ADSL modem, this may be USB interface to connect 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 already widely used by more than 5 million subscribers around the world.

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 side ADSL modem not only a computer is connected, but also a VoIP phone. 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 VoIP technologies allow 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 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