ADSL technology. How to choose an ADSL modem

ADSL technology

What is hidden behind this mysterious word:

ADSL is a data transmission technology that allows you to simultaneously use a regular telephone line for a telephone and for high-speed Internet. The telephone and ADSL channels do not affect each other. You can load pages, receive email, and talk on the phone at the same time. The maximum speed of the ADSL channel is up to 8 Mbit/s!

How does ADSL work?

A telephone or a regular modem at 14.4 kbit/s uses a low-frequency channel: usually the range of transmitted frequencies lies in the range of 0.6-3.0 kHz, a good telephone channel can transmit frequencies in the range of 0.2-3.8 kHz, which, under conditions of weak interference, allows you to increase the speed to 33.6 kbit/s c. On so-called digital PBXs, where an analog telephone signal is converted into a digital stream at a telephone exchange or node, the speed can be increased to 56.0 kbit/s. In practice, however, due to the imperfect quality of telephone lines, the actual speed is lower and rarely exceeds two tens of kilobits per second.
In conventional telephony, a so-called dial-up channel is used - a direct connection between subscribers is established by the telephone network for the entire duration of the communication session. Similarly, when you connect to the Internet, a direct connection is established between your modem and your ISP's modem. The telephone channel is busy with data transmission, so you cannot use the phone at this time.
The ADSL channel uses a higher frequency range. Even the lower limit of this range lies well above the frequencies used in the dial-up telephone channel. Naturally, the ADSL channel reaches through your telephone wire only to your PBX, then the paths of the switched and ADSL channels diverge: the switched channel goes to the telephone exchange, and the ADSL channel ends up in the digital network (for example, Ethernet LAN) of the provider. To do this, the provider's ADSL modem is installed directly at your telephone exchange. A very wide frequency band is used for data transmission, which practically makes it possible to reach a speed of 6 Mbit/s on a line of normal quality!
Unfortunately, not all telephone lines are suitable for ADSL. Before connecting the line, you must first check it. The main obstacles are the dual line and the security alarm.
It is not recommended to plug the ADSL modem into a telephone socket directly (without a splitter): the ADSL modem and telephone may interfere with each other. The modem and phone will not fail, but the connection will be unstable. To eliminate mutual influence, it is enough to install simple filters to separate low telephone and high ADSL frequencies. Filters are included with the ADSL modem and are called splitter and microfilter. A splitter is a special tee; one end connects to the telephone line, and the other two to the telephone and modem. The microfilter is connected to the line at one end and to the telephone at the other - useful for connecting parallel telephone sets.

The modern world is unthinkable without the Internet and computer networks. High-speed channels have entangled the world in a web - satellites, fiber optics, cables - the nerves and blood vessels of the worldwide information network. Giant speeds, huge traffic, high technologies... But for many years, high-speed channels with data transfer speeds above 1 megabit per second remained the lot of providers and large companies.
High technologies developed by leading Hi-Tech companies for high-speed data transfer have turned out to be a very expensive pleasure, having not only a huge cost of implementation, but also a high cost of ownership. To gain access to the Internet, ordinary users had to be content with ordinary, very common and cheap to operate Dial Up modems designed for use on analog telephone lines. And businesses, especially small ones, did not see the need to lay dedicated channels or provide themselves with satellite Internet - it was expensive and ineffective. What to download at high speeds - news, prices, documents, kilobyte drivers? For over two decades, Dial Up access rules the “last mile” - the very section along which information is delivered from the provider to the end user. Telephone lines, especially Russian ones, have become a barrier between users and providers who own high-speed data transmission channels. So we got an awkward picture - between cities, countries and continents, gigantic volumes of information were sent instantly, but on the last kilometer, on the last piece of telephone wire from the provider to the client, the speed dropped by orders of magnitude and the information came to the end user in uneven, torn portions, also with constant disconnectome.
For a long time, the capabilities of Dial Up modems suited many people. This technology, developed at the dawn of the computer era for analog telephone lines, has evolved extremely slowly and unhurriedly - over the past 15 years, data transfer speeds have increased from 14,400 Kbps to just 56,000 Kbps. For many years it seemed that this speed was enough for almost everything - downloading an HTML web page, a text document, a beautiful picture, a patch for a game or program, or drivers for new devices, the size of which for a number of years did not exceed several hundred kilobytes - all this did not took a long time and did not require high-speed connections. But life made its own adjustments.
The development of modern computer technologies, in addition to the increase in the frequency of central processors, the revolution in the field of three-dimensional graphics accelerators and the explosive increase in the capacity of information storage devices, has also led to a dramatic increase in the volume of transmitted information. Computer evolution, which followed the principle of “bigger, higher, faster,” led to programs and files increasing to monstrous sizes. For example, a Word document that has now become a standard is tens of times larger than a similar TXT file, the widespread introduction of 32-bit color has led to an increase in the size of pictures and video files many times over, high sound quality, and recently the bitrate of MP3 files has risen from the standard 128 Kbps to 192 Kbps, which also significantly affects the size. Yes, compression algorithms that have been significantly improved recently help to some extent, but this is still not a panacea. The sizes of drivers have recently increased to gigantic proportions, for example, Detonator FX from nVidia takes about 10 megabytes (even though two years ago they took only 2 megabytes), and the unified drivers for the nForce platform of the same company are already 25 megabytes and this the trend is capturing an increasing number of computer hardware manufacturers. But the main problem that makes Dial Up modems burn hot without giving them even a minute of rest is software patches or patches that correct errors in the software. The widespread introduction of rapid development tools has led to the mass release of crude, unoptimized programs. And why optimize the program if computer hardware is still redundant? Why engage in beta testing of a program if there is the Internet - it is enough to sell a crude program, then look at the list of the most frequently occurring problems and errors that users themselves compile when contacting support and then release a patch, after that another, a third, and so on ad infinitum . Involuntarily, we remember with nostalgia the times when the Internet was the lot of a select few, and programmers unspoiled by the World Wide Web licked their programs to the last bit, knowing that after their product went to the end user, nothing could be fixed. Programs were released much less frequently, but they worked like a Swiss watch. And now, looking sadly at, for example, the fourth (!) Microsoft patch for Windows 2000 with a size of 175 megabytes, you understand that using Dial Up access this lump cannot be drained even in a week, and how much will this patch cost if paid hourly? ! But there is also Microsoft Office and dozens of other programs that require correction. And there are gigantic deposits of music and videos on the Internet! I want to bite my elbow at the thought of all these treasures of information technology that are practically inaccessible to dialup specialists.
All these gloomy thoughts lead to the idea that Dial Up Internet access has outlived its usefulness and is urgently needed to be replaced. What can replace obsolete technologies? The already classic ISDN (Integrated Services Digital Network) and the relatively new satellite Internet immediately come to mind. They come immediately, but after much thought they both disappear. ISDN is eliminated due to the high cost of laying a dedicated channel, which is inappropriate in an apartment, and the high cost of ownership (subscription fee + payment for traffic). In principle, this type of access is possible when laying a home network, when several users share a high-speed channel and then distribute it throughout an apartment building via a local network. But as further material in the article will show, ISDN has a powerful competitor, negating all the advantages of this technology. Satellite Internet, of course, looks very attractive, but there are nuances, and not always pleasant ones. Yes, the satellite covers a large area of ​​the Earth’s surface, but you need to look at whether the satellite of the provider providing this service in your region is visible and at what angle it is visible; this determines what size satellite dish you will have to install. In addition, the satellite channel is still not very fast - the best of them provide about 400 Kbps towards the user (this is for ordinary users, of course, there are higher-speed options, but they are several orders of magnitude more expensive). Data is sent from the user to the provider by telephone, so the telephone line is just as busy as when using a Dialup modem. Satellite systems from different providers have a number of common disadvantages, such as the high cost of the equipment used and the complexity of its installation and configuration. In addition, satellite providers are, to put it mildly, not reliable enough. There are reasons for this, both objective (satellites do not last forever, a telecommunications satellite will fall into the dense layers of the atmosphere when they launch a replacement into the same orbit), and subjective ones - remember the fiasco of the NTV+ satellite Internet, which, it turns out, abandoned thousands of its users, leaving them with useless receivers.
It would be nice to have the same ISDN, but without any dedicated lines, but directly on a telephone copper cable. After all, a subscriber telephone line is nothing like a cable for a network. Yes, the quality is terrible, but it is possible to develop new technologies for sending data, convert everything into digital, modulate everything in a special way, correct errors that arise, and as a result get a broadband digital channel. So it turns out that all hope is for progress. And dreams and hopes turned out to be not at all fruitless - a holy place is never empty, and progress does not stand still - they received a technology that combines the best features of both Dial Up modems working on analog telephone lines and high-speed IDSN modems. Meet ADSL technology.

ADSL - what is it?

Let's start with the name: ADSL stands for Asymmetric Digital Subscriber Line.
This standard is part of a whole group of high-speed data transmission technologies under the general name xDSL, where x is a letter characterizing the speed of the channel, and DSL is the abbreviation already known to us Digital Subscriber Line - digital subscriber line. The name DSL was first used back in 1989, when the idea of ​​digital communications using a pair of copper telephone wires instead of specialized cables first arose. The imagination of the developers of this standard is clearly lame, so the names of the technologies included in the xDSL group are quite monotonous, for example HDSL (High data rate Digital Subscriber Line - high-speed digital subscriber line) or VDSL (Very high data rate Digital Subscriber Line - very high-speed digital subscriber line). All other technologies in this group are much faster than ADSL, but require the use of special cables, while ADSL can work on ordinary copper pair, which is widely used when laying telephone networks. The development of ADSL technology began in the early 90s. Already in 1993, the first standard for this technology was proposed, which began to be implemented in telephone networks in the USA and Canada, and since 1998, ADSL technology has gone into the world, as they say.
In general, in my opinion, it is still premature to bury the copper subscriber line, which consists of two wires. Its cross-section is quite sufficient to ensure the passage of digital information over quite significant distances. Just imagine how many millions of kilometers of such wire have been laid throughout the Earth since the appearance of the first telephones! Yes, no one has lifted distance restrictions; the higher the speed of information transmission, the shorter the distance it can be sent, but the problem of the “last mile” has already been solved! Thanks to the use of high-tech DSL, adapted to a copper pair, on the subscriber telephone line, it became possible to use these millions of kilometers of analog lines to organize cost-effective high-speed data transfer from the provider, who owns a thick digital channel, to the end user. The wire, once intended exclusively for providing analog telephone communication, with a slight movement of the hand turns into a broadband digital channel, while maintaining its original responsibilities, since owners of ADSL modems can use the subscriber line for traditional telephone communication while simultaneously sending digital information. This is achieved due to the fact that when using ADSL technology on the subscriber line to organize high-speed data transmission, information is transmitted in the form of digital signals with significantly higher frequency modulation than that usually used for traditional analogue telephone communications, which significantly expands the communication capabilities of existing telephone lines.

ADSL - how does it all work?

How does ADSL work? What ADSL technologies make it possible to turn a pair of telephone wires into a broadband data transmission channel? Let's talk about this.
To create an ADSL connection, two ADSL modems are required - one at the provider and another at the end user. Between these two modems there is a regular telephone wire. The connection speed may vary depending on the length of the “last mile” - the further you are from the provider, the lower the maximum data transfer speed.

Data exchange between ADSL modems takes place at three frequency modulations sharply spaced apart from each other.

As can be seen from the figure, voice frequencies (1) are not involved at all in receiving/transmitting data, and are used exclusively for telephone communications. The data reception frequency band (3) is clearly demarcated from the transmitting band (2). Thus, three information channels are organized on each telephone line - an outgoing data transmission stream, an incoming data transmission stream and a regular telephone communication channel. ADSL technology reserves a 4 KHz frequency band for the use of regular telephone service or POTS - Plain Old Telephone Service (plain old telephone service - sounds like "good old England"). Thanks to this, a telephone conversation can actually be carried out simultaneously with reception/transmission without reducing the speed of data transfer. And if there is a power outage, telephone communication will not disappear anywhere, as happens when using ISDN on a dedicated channel, which, of course, is an advantage of ADSL. It must be said that such a service was included in the very first specification of the ADSL standard, being the original highlight of this technology.
To increase the reliability of telephone communications, special filters are installed that extremely effectively separate the analog and digital components of communication from each other, without excluding joint simultaneous operation on one pair of wires.
ADSL technology is asymmetrical, like Dial Up modems. The speed of the incoming data flow is many times higher than the speed of the outgoing data flow, which is logical, since the user always uploads more information than he transfers. Both the transmission and reception speeds of ADSL technology are significantly higher than those of its closest competitor ISDN. Why? It would seem that the ADSL system does not work with expensive special cables, which are ideal channels for data transmission, but with ordinary telephone cable, which is as perfect as walking to the moon. But ADSL manages to create high-speed data transmission channels over a regular telephone cable, while showing better results than ISDN with its own dedicated line. This is where it turns out that the engineers of Hi-Tech corporations do not eat their bread in vain.
High reception/transmission speed is achieved by the following technological methods. First, the transmission in each of the modulation zones shown in Figure 2 is in turn divided into several more frequency bands - the so-called bandwidth sharing method, which allows several signals to be transmitted on one line simultaneously. It turns out that information is transmitted or received simultaneously through several modulation zones, which are called carrier frequency bands - a method that has long been used in cable television and allows you to watch several channels over one cable using special converters. The technique has been known for twenty years, but only now are we seeing its application in practice to create high-speed digital highways. This process is also called frequency division multiplexing (FDM). When using FDM, the reception and transmission ranges are divided into many low-speed channels, which provide data reception/transmission in parallel mode.
Oddly enough, when considering the method of dividing bandwidth, a widespread class of programs such as Download manager comes to mind as an analogy - they use the method of splitting them into parts to download files and simultaneously downloading all these parts, which makes it possible to use more efficiently link. As you can see, the analogy is direct and differs only in implementation; in the case of ADSL, we have a hardware option not only for downloading, but also for sending data.
The second way to speed up data transfer, especially when receiving/sending large volumes of the same type of information, is to use special hardware-implemented compression algorithms with error correction. Highly efficient hardware codecs that allow on-the-fly compression/decompression of large amounts of information are one of the secrets of ADSL speeds.
Thirdly, ADSL uses an order of magnitude larger frequency range compared to ISDN, which makes it possible to create a significantly larger number of parallel information transmission channels. For ISDN technology, the standard frequency range is 100 KHz, while ADSL uses a range of about 1.5 MHz. Of course, long-distance telephone lines, especially domestic ones, attenuate the reception/transmission signal modulated in such a high-frequency range quite significantly. So at a distance of 5 kilometers, which is the limit for this technology, the high-frequency signal is attenuated by up to 90 dB, but at the same time continues to be reliably received by ADSL equipment, which is required by the specification. This forces manufacturers to equip ADSL modems with high-quality analog-to-digital converters and high-tech filters that could catch a digital signal in the jumble of chaotic waves that the modem receives. The analog part of the ADSL modem must have a large dynamic range of reception/transmission and low noise level during operation. All this undoubtedly affects the final cost of ADSL modems, but still, compared to competitors, the costs of ADSL hardware for end users are significantly lower.

How fast is ASDL technology?

Everything is learned by comparison; you cannot evaluate the speed of a technology without comparing it with others. But before that, you need to take into account several features of ADSL.
First of all, ADSL is an asynchronous technology, that is, the speed of receiving information is much higher than the speed of transmitting it from the user. Therefore, two data rates must be taken into account. Another feature of ADSL technology is the use of high-frequency signal modulation and the use of several lower-speed channels lying in a common field of receive and transmit frequencies for simultaneous parallel transfer of large volumes of data. Accordingly, the “thickness” of the ADSL channel begins to be influenced by such a parameter as the distance from the provider to the end user. The greater the distance, the more interference and the greater the attenuation of the high-frequency signal. The frequency spectrum used is narrowed, the maximum number of parallel channels is reduced, and the speed decreases accordingly. The table shows the change in the capacity of data reception and transmission channels when the distance to the provider changes.

In addition to distance, the data transfer speed is greatly influenced by the quality of the telephone line, in particular the cross-section of the copper wire (the larger the better) and the presence of cable outlets. On our telephone networks, traditionally of poor quality, with a wire cross-section of 0.5 square meters. mm and an ever-distant provider, the most common connection speeds will be 128 Kbit/s - 1.5 Mbit/s for receiving data going to the user and 128 Kbit/s - 640 Kbit/s for sending data from the user at distances of 5 kilometers. However, as telephone lines improve, ADSL speed will increase.

to be continued...

Recorded by

For comparison, let's look at other technologies.

Dial Up modems, as you know, are limited to a maximum data reception speed of 56 Kbps, a speed that I, for example, have never achieved on analog modems. For data transfer, their speed is a maximum of 44 Kbps for modems using the v.92 protocol, provided that the provider also supports this protocol. The usual data sending speed is 33.6 Kbps.
The maximum ISDN speed in dual-channel mode is 128 Kbps, or, as you can easily calculate, 64 Kbps per channel. If the user calls on an ISDN phone, which is usually supplied with the ISDN service, then the speed drops to 64 Kbps, since one of the channels is busy. Data is sent at the same speeds.
Cable modems can provide data transfer rates from 500 Kbps to 10 Mbps. This difference is explained by the fact that the cable bandwidth is simultaneously distributed among all connected users on the network, therefore, the more people there are, the narrower the channel for each user. When using ADSL technology, the entire channel bandwidth belongs to the end user, making the connection speed more stable compared to cable modems.
And finally, dedicated digital lines E1 and E3 can show data transfer rates in synchronous mode of 2 Mbit/s and 34 Mbit/s, respectively. The performance is very good, but the prices for wiring and maintaining these lines are exorbitant.

Glossary.

Subscriber line- a pair of copper wires running from the ATC to the user's phone. You can also find its English designation - LL (Local Loop). Previously it was used exclusively for telephone conversations. With the advent of Dial Up modems, it has long served as the main channel for accessing the Internet; now it is used for the same purposes by ADSL technology.

Analog signal- a continuous oscillatory signal, characterized by such concepts as frequency and amplitude. Analog signals at specified frequencies are used to control telephone connections, such as a busy signal. A simple telephone conversation is a type of analog signal with constantly changing frequency and amplitude parameters.

Digital signal- a digital signal, in contrast to an analogue one, is intermittent (discrete), the value of the signal changes from minimum to maximum without transition states. The minimum value of the digital signal corresponds to the state “0”, the maximum value “1”. Thus, when digitally transmitting information, binary code is used, which is the most common code in computers. A digital signal, unlike an analog one, cannot be distorted even in conditions of strong noise and interference on the line. In the worst case, the signal will not reach the end user, but the error correction system, which is present in the vast majority of digital communications equipment, will detect the missing bit and send a request to resend the damaged piece of information.

Modulation- the process of converting data into a signal of a specific frequency, intended for transmission over a subscriber line, over a special cable or, for wireless systems, over radio waves. The process of converting the modulated signal back is called demodulation.

Carrier frequency- a special high-frequency signal of a certain frequency and amplitude, separated from other frequencies by silent bands.

Cable modems- modems using cables from existing cable television networks. These networks are public networks, that is, the data transfer speed strongly depends on the number of users simultaneously on the network. Therefore, although the maximum speed of cable modems reaches 30 Mbit/s, in practice it is rarely possible to get more than 1 Mbit/s.
P.S. If any terms in the article are unclear to you, please write, the glossary will be expanded.

ADSL Technology (by Jeff Newman)
ADSL technology (Asymmetric Digital Subscriber Line) is one of the types of xDSL technologies that provide users with a broadband transmission medium between network nodes relatively close to each other at an affordable price.
Research and development in ADSL was fueled by investments from telephone companies that, unlike conventional broadcast television, wanted to deliver on-demand video programming to users. Advances in the development of ADSL technology have made it suitable not only for digital television broadcasting, but also for a variety of other high-speed interactive applications, such as Internet access, delivery of corporate information to remote offices and branch offices, and on-demand audio and video information. Under the best operating conditions and acceptable distances, ADSL technology can transmit data at speeds of up to 6 Mbit/s in the forward direction (in some versions, up to 9 Mbit/s) and 1 Mbit/s in the reverse direction.

ADSL equipment transmits data approximately 200 times faster than conventional analog modems, which have an average sustained transmission speed of about 30 Kbps, and in the same physical distribution environment.

Employees of Network Computing magazine tested ADSL modems manufactured by Amati Communications (ATU-C and ATU-R), Aware (Ethernet Access Modem) and Paradyne (5170/5171 ADSL Modem) in the MCI Developers Lab and assessed the advantages of their performance and disadvantages of ADSL technology.

As a result, when testing ADSL devices with a fairly large load, no significant flaws were identified, so from an engineering point of view, this technology is ready for implementation. Considering that the cost of equipment and services for any technology decreases as it is introduced, it makes sense to begin negotiations with telephone companies now.

No additional wiring is needed.

The main advantage of ADSL technology is that it uses twisted pair copper wires, which are widely used today. In addition, in this case there is no need for expensive upgrading of switches, laying additional lines and their termination, as is the case with ISDN. ADSL technology also allows you to work with existing telephone terminal equipment. Unlike ISDN, which relies on dial-up connections (its rates depend on call duration and circuit usage), ADSL is a leased-circuit service.

Signals are transmitted over a pair of wires between two ADSL modems installed at a remote network node and at the local PBX. A network ADSL modem converts digital data from a computer or some other device into an analog signal suitable for transmission over twisted pair cables. To check parity, redundant bits are inserted into the transmitted digital sequence. This ensures reliable delivery of information to the telephone exchange, where this sequence is demodulated and checked for errors.

However, it is not at all necessary to bring the signal to the telephone exchange. For example, if branch offices are located within a small town, use pairs of wires laid between them. In this case, the “remote” ADSL modem operating in receiving mode and the “central” transmitting ADSL modem can be connected by copper wire without any additional intermediate elements between them. The connection of offices separated by long distances from one another, provided that each of them is located relatively close to “its own” PBX, is carried out using trunk lines provided by telephone companies.

The use of ADSL technology allows you to send several types of data at different frequencies simultaneously. We were able to select the best transmission frequency for each specific application (for data, voice and video). Depending on the encoding method used in a particular ADSL implementation, the signal quality is affected by the length of the connection and electromagnetic interference.

When using a line for data transmission and telephony together, the latter will work without additional power supply, as is necessary in the case of ISDN. In the event of a power failure, regular telephony will continue to operate, receiving current supplied to the line by the telephone company. However, ADSL modems must be connected to AC power to transmit data.

Most ADSL devices are designed to work together with a frequency sharing device used in Plain Old Telephone Service (POTS) called a frequency splitter. These functional features of ADSL give it a reputation as a reliable technology. It is also harmless, since in the event of an accident it does not have any effect on the operation of telephony. ADSL seems like a pretty basic technology, and in essence it is. Installing and running it is not difficult. Simply connect the device to the network and phone line, and leave the rest to the telephone company.

However, this technology has some features that you need to consider when creating and operating your network. For example, ADSL devices can be affected by certain physical factors inherent in the transmission of signals over a pair of wires. The most important of these is line attenuation. In addition, the reliability and capacity of the data transmission channel can be affected by significant electromagnetic interference on the cable, especially from the telephone company network itself.

Types of Line Coding

ADSL modems use three types of line coding, or modulation: Discrete Multitone (DMT), Carrierless Amplitude/Phase (CAP), and the rarely used Quadrature Amplitude Modulation (QAM). Modulation is necessary to establish a connection, transmit signals between two ADSL modems, rate negotiation, channel identification and error correction.

DMT modulation is considered the best because it provides more flexible bandwidth control and is easier to implement. For the same reason, the American National Standards Institute (ANSI) adopted it as a standard for line coding of ADSL channels.

However, many disagree that DMT modulation is better than CAP, so we decided to try them both. And although the modems used in our tests were early implementations, they all worked perfectly. As a result, we were convinced of the following: ADSL modems based on DMT are indeed more stable in signal transmission and can operate over long distances (up to 5.5 km).

It should be noted that users only need to worry about the channel linear coding method in the area between modems (for example, from your office to the service provider's PBX). If these devices are used on packet switching networks, such as the Internet, worrying about possible conflicts between network nodes is not your concern.

For testing, we used a copper pair with 24-gauge wire, which has a signal attenuation of 2-3 dB for every 300 m. According to the specification, the length of the ADSL line should not exceed 3.7 km (attenuation about 20 dB), but good ADSL Modems can operate reliably over much longer distances. We also found that the actual range of most modems exceeds 4.6 km (26 dB). DMT-based ADSL modems operated at the maximum possible distance under our conditions - 5.5 km - at speeds of 791 Kbit/s in the forward direction and 582 Kbit/s in the reverse direction (the measured signal attenuation in the line was 31 dB).

Both CAP-based ADSL modems operated at speeds of 4 Mbit/s in the forward direction and 422 Kbit/s in the reverse direction over a distance of 3.7 km. At a lower speed (2.2 Mbit/s), only one modem worked at a distance of 4.6 km.

In addition to those just described, we carried out tests in which we reproduced real conditions on the lines, for example, we checked the work with bridge taps, often used in telephony. A spur bridge is an open telephone line that extends away from the main line. Typically, this additional line is not used and therefore does not create additional crosstalk on the main line, but does significantly increase its attenuation. It is therefore surprising that some modems tested worked fine with a spur line length of 1.5 km and a main line length of 3.7 km. When the length of the main line increased to 4.6 km, the reliability of signal transmission became below the permissible level only if the length of the branch line was increased to 300 m.

Electromagnetic interference

Electromagnetic interference at the near and far ends (Near-End Crosstalk - NEXT; Far-End Crosstalk - FEXT) of a line is a form of electromagnetic interference that distorts the signal in the ADSL channel and thus negatively affects its decoding. This type of interference can occur at either end of the connection if there is a line running adjacent to the ADSL line that carries extraneous signals, such as T1 or another ADSL line.

The electromagnetic field emitted by some wires interferes with other wires and causes data transmission errors. For the modems we tested, the impact of an adjacent busy T1 line on the data flow transmitted over the ADSL line was minimal, and the quality of signal transmission over the ADSL and T1 lines did not deteriorate. This impact on the PBX is likely to be exacerbated if multiple T1 lines and multiple ADSL lines are interleaved with each other. When laying ADSL channels, the telephone company must take into account this mutual influence of the lines.

Another interference that occurs when transmitting a signal over an ADSL line is amplitude modulation (AM) noise. It is similar to the noise that occurs on a line passing near high-power electrical appliances, such as refrigerators and laser printers, or near high-power motors installed in an elevator shaft. MCI engineers conducting modem tests applied a pulse voltage of up to 5 V to a twisted pair cable running parallel to our ADSL line, but the bit error level remained at an acceptable level. In fact, such an effect on modems in our tests could be neglected.

In our opinion, there is about a year left before widespread adoption of ADSL technology in public networks. In the meantime, it is under development and the possibility of its use is being assessed. However, ADSL technology is already used in the networks of corporations and small towns. Many companies have begun to produce products for ADSL. The wide bandwidth and noise resistance of the first versions of ADSL modems that participated in our tests confirmed their high reliability. Now, when upgrading your network and increasing the number of users, ADSL technology can no longer be neglected.

What is ADSL (another article)
ADSL (Asymmetric Digital Subscriber Line) is one of the high-speed data transmission technologies known as DSL (Digital Subscriber Line) technologies, collectively referred to as xDSL.
The name DSL technologies originated in 1989, when the idea of ​​using analog-to-digital conversion at the subscriber end of the line first appeared, which would improve the technology of data transmission over twisted pair copper telephone wires. ADSL technology was developed to provide high-speed access to interactive video services (video on demand, video games, etc.) and equally fast data transfer (Internet access, remote LAN access and other networks).

So what is ADSL? First of all, ADSL is a technology that allows you to turn twisted pair telephone wires into a high-speed data transmission path. The ADSL line connects two ADSL modems that are connected to the telephone cable (see figure). In this case, three information channels are organized - a “downward” data transmission stream, an “upstream” data transmission stream and a regular telephone communication channel. The telephone communication channel is allocated using filters, which ensures that your phone will work even if the ADSL connection fails.
ADSL is an asymmetric technology - the speed of the “downstream” data flow (that is, the data that is transmitted towards the end user) is higher than the speed of the “upstream” data flow (in turn, transmitted from the user to the network.
To compress large amounts of information transmitted over twisted pair telephone wires, ADSL technology uses digital signal processing and specially created algorithms, advanced analog filters and analog-to-digital converters.
ADSL technology uses a method of dividing the bandwidth of a copper telephone line into several frequency bands (also called carriers). This allows multiple signals to be transmitted simultaneously on one line. When using ADSL, different carriers simultaneously carry different parts of the transmitted data. This is how ADSL can provide, for example, simultaneous high-speed data transmission, video transmission and fax transmission. And all this without interrupting regular telephone communication, which uses the same telephone line.
Factors affecting the data transfer speed are the condition of the subscriber line (i.e., the diameter of the wires, the presence of cable outlets, etc.) and its length. Signal attenuation in a line increases with increasing line length and increasing signal frequency, and decreases with increasing wire diameter. In fact, the functional limit for ADSL is a subscriber line with a length of 3.5 - 5.5 km. Currently, ADSL provides downstream data speeds of up to 8 Mbit/s and upstream data speeds of up to 1.5 Mbit/s.

Do you need an ADSL line?

It's up to you to decide, but to help you make the right decision, let's look at the benefits of ADSL.

First of all, high data transfer speed.
In order to connect to the Internet or a data network, you do not need to dial a phone number. ADSL creates a broadband data transmission channel using an existing telephone line. After installing ADSL modems, you get a permanent connection. A high-speed data link is always ready to go - whenever you need it.
ADSL technology allows full use of line resources. Typical telephone communications use about one hundredth of the telephone line's bandwidth. ADSL technology eliminates this "disadvantage" and uses the remaining 99% for high-speed data transmission. In this case, different frequency bands are used for different functions. For telephone (voice) communications, the lowest frequency region of the entire line bandwidth is used (up to approximately 4 kHz), and the entire remaining band is used for high-speed data transmission.
ADSL opens up completely new possibilities in those areas where it is necessary to transmit high-quality video signals in real time. These include, for example, video conferencing, distance learning and video on demand. ADSL technology allows you to provide services with data transfer rates that are more than 100 times faster than the fastest analog modem currently available (56 Kbps) and more than 70 times faster than ISDN data transfer rates (128 Kbps).
We should not forget about costs. ADSL technology is effective from an economic point of view, if only because it does not require the installation of special cables, but uses existing two-wire copper telephone lines. That is, if you have a connected telephone at home or in the office, you do not need to lay additional wires to use ADSL.
The subscriber has the opportunity to flexibly increase speed without changing equipment, depending on his needs.
Based on materials from the Verkhnevolzhsky branch of Centrotelecom.

ADSL and SDSL

Asymmetrical and symmetrical DSL lines

Residential users, limited by 56.6 Kbps dial-up connections, want access to broadband applications, while businesses, with their expensive T-1/E-1 Internet connections, want to reduce their costs. The best technology allows you to solve problems using existing equipment. Where possible, you should switch to Digital Subscriber Line (DSL).

DSL technology allows you to connect the user's premises with the central office (Central Office, CO) of the service provider over existing copper telephone lines. If the lines meet the established requirements, then using DSL modems the transmission speed can be increased from the mentioned 56.6 Kbps to 1.54 Mbps or more. However, the main disadvantage of DSL lines is that their usability largely depends on the distance to the service provider's site.

DSL is not a one-size-fits-all technology; it comes in many varieties, although some may not be available in your local area. DSL options typically follow one of two basic designs, although they may differ in specific characteristics. Two main models - asymmetric (Asymmetric DSL, ADSL) and symmetric (Symmetric DSL, SDSL) digital subscriber line - stood out in the early stages of technology development. In the asymmetric model, preference is given to data flow in the forward direction (from the provider to the subscriber), while in the symmetric model, the flow rate in both directions is the same.

Individual users prefer ADSL, while organizations prefer SDSL. Each system has its own advantages and limitations, the roots of which are in a different approach to symmetry.

ABOUT ASYMMETRY

ADSL technology is actively penetrating the market for high-speed connections for private users, where it competes with cable modems. Fully satisfying the appetites of home users in their “walks” on the WWW, ADSL provides data transfer speeds from 384 Kbps to 7.1 Mbps in the main direction and from 128 Kbps to 1.54 Mbps in the reverse direction.

The asymmetric model fits well with the way the Internet works: large amounts of multimedia and text are transmitted in the forward direction, while the level of traffic in the reverse direction is negligible. ADSL costs in the US typically range from $40 to $200 per month, depending on expected data speeds and service level guarantees. Cable modem-based service is often less expensive, about $40 per month, but the lines are shared between customers, as opposed to dedicated DSL.

Figure 1. An asymmetric digital subscriber line carries data at frequencies from 26 to 1100 kHz, while the same copper cable can carry analog voice in the range from 0 to 3.4 kHz. Symmetrical DSL (SDSL) occupies the entire frequency range of a data line and is not compatible with analog voice signals.

The carrier line is capable of supporting ADSL along with analog voice by allocating digital signals to frequencies outside the normal telephone signal spectrum (see Figure 1), which requires the installation of a divider. To separate telephone frequencies at the lower end of the audio spectrum from the higher frequencies of ADSL signals, the divider uses a low-pass filter. The available ADSL bandwidth remains intact regardless of whether analog frequencies are used. To support maximum ADSL speeds, splitters must be installed both at the user premises and at the central site; they do not require power and therefore will not interfere with “vital” voice service in the event of a power loss.

Determining ADSL speeds is more of an art than a science, although they do decrease at fairly predictable intervals. Providers provide the best possible service, with results highly dependent on the distance to the central hub. Typically, “best possible” means that providers guarantee 50% throughput. Attenuation and interference such as crosstalk become significant over lines longer than 3 km, and over distances greater than 5.5 km they can render lines unsuitable for data transmission.

At distances up to 3.5 km from the central node, ADSL speeds can reach 7.1 Mbit/s in the forward flow direction and 1.5 Mbit/s in the subscriber-to-CO direction. However, DSL Reports editor Nick Braak believes that the upper limit is unattainable in practice. Braak states, “In fact, speeds of 7.1 Mbps are impossible to achieve, even in laboratory conditions.” At distances greater than 3.5 km, ADSL speed is reduced to 1.5 Mbit/s in the forward direction and to 384 Kbit/s from subscriber to CO; As the length of the subscriber line approaches 5.5 km, the speed drops even more significantly - to 384 Kbit/s in the forward direction of flow and to 128 Kbit/s in the reverse direction.

Service contracts for ADSL services may contain a clause regarding the user's refusal to connect to home networks or Web servers. However, DSL technology itself does not prevent the connection of home local networks. For example, even if an ISP provides a single IP address to a customer, through Network Address Translation (NAT), multiple users can share that single IP address.

One DSL connection is enough for a home with many computers. Some DSL modems have a built-in DSL concentrator, as well as specialized devices called "residential gateways" that act as bridges between the Internet and home networks.

ADSL uses two ADSL modulation schemes: Discrete Multitone (DMT) and Carrierless Amplitude and Phase (CAP).

DMT provides for dividing the spectrum of available frequencies into 256 channels in the range from 26 to 1100 kHz, 4.3125 kHz each.

CONNECTING A COPPER LINE TO ATU-R

So, we have a central node, a copper cable with twisted pairs and a remote site. What to connect to what?

A so-called remote transmission unit (ADSL Transmission Unit-Remote, ATU-R) is installed at the customer’s site. Originally referring only to ADSL, "ATU-R" now refers to the remote device for any DSL service. In addition to providing DSL modem functionality, some ATU-Rs can perform bridging, routing, and time division multiplexing (TDM) functions. On the other side of the copper cable line, at the central node, there is an ADSL Transmission Unit-Central Office (ATU-C), which coordinates the channel from the CO side.

A DSL provider multiplexes multiple DSL subscriber lines into one high-speed backbone network using a DSL Access Multiplexer (DSLAM). Located at the central node, the DSLAM aggregates data traffic from multiple DSL lines and feeds it into the service provider's backbone, and the backbone then delivers it to all destinations on the network. Typically, DSLAM is connected to an ATM network via PVCs with Internet Service Providers and other networks.

G.LITE: ADSL WITHOUT DIVIDER

A modified version of ADSL, known as G.lite, eliminates the need to install a splitter on the customer's premises.

The throughput of G.lite is significantly lower than ADSL speeds, although it is many times higher than the notorious 56.6 Kbps. Throughput is reduced as a result of potentially increased interference, with additional interference introduced by remote control.

Using DTM, the same modulation method used in ADSL, G.lite supports maximum speeds of 1.5 Mbps upstream and 384 Kbps upstream.

ITU Recommendation G.992.1, also known as G.dmt, was first published in 1999, along with G992.2, or G.lite. G.lite equipment appeared on the market in 1999 and was cheaper than ADSL, mainly due to the fact that the provider's technicians did not need to travel to the customer for installation and troubleshooting. It's difficult for service providers to justify spending hundreds of dollars on a single landline connection with a $49 subscription fee, so any cost-reducing modification is met with extreme enthusiasm by the market.

DSL FOR BUSINESS

Businesses have completely different needs than home users, making a balanced SDSL line a natural choice for office applications.

Corporate upstream bandwidth can quickly become depleted due to heavy Web server traffic and employees sending large volumes of PDFs, PowerPoint presentations, and other documents. Outgoing traffic can equal or even exceed incoming traffic. Providing round-trip speeds of about 1.5 Mbps in North America and 2.048 Mbps in Europe, ADSL links resemble T-1/E-1 connections, the dominant architectural component corporate networks worldwide.

If an ADSL line uses unoccupied frequencies and does not conflict with analog voice frequencies, then SDSL occupies all available spectrum. In SDSL, voice compatibility is sacrificed for full-duplex data transmission. No divider, no analog voice signals - nothing but data.

As a viable alternative to T-1/E-1 traffic, SDSL has attracted the attention of Competitive Local Exchange Carriers (CLECs) as a means of providing value-added services. In general, SDSL services are typically distributed by CLECs, but ILECs typically use HDSL to implement T-1 service. At optimal conditions SDSL can rival T-1/E-1 in data transfer speeds and has three times the speeds of ISDN (128 Kbps) at maximum distances. Figure 2 shows the dependence of speeds on distance in the case of SDSL: the greater the distance, the lower the speeds; in addition, parameters vary depending on the equipment supplier.

SDSL uses an adapted 2 Binary, 1 Quaternary (2B1Q) modulation scheme borrowed from ISDN BRI. Each pair of binary digits represents one four-digit character; two bits are sent in one hertz.

SDSL lines are better suited to the needs of organizations than ADSL to the needs of residential users. While cable modem providers lure residential customers with lower prices than ADSL, SDSL offers the same speeds as T-1/E-1 for significantly less money. The standard price range for T-1 is $500 to $1,500, depending on distance, and the equivalent SDSL range is $170 to $450. The lower the cost of SDSL services, the lower the guaranteed data transfer speed.

LET'S BE CLARITY

Signal quality is affected by many changing factors, many of which are not exclusive to DSL. However, some of the devices that once made our lives easier on switched networks are now hindering the use of digital subscriber lines.

Crosstalk. Emitted by bundles of wires converging at the service provider's central hub Electric Energy produces interference known as Near-End Crosstalk (NEXT). As signals move between channels on different cables, the line's capacitance drops. "Near end" means that the interference is coming from an adjacent pair of cables in the same area.

Separating the DSL and T-1/E-1 lines greatly reduces the negative impact of crosstalk, but there is no guarantee that the service provider will choose to implement this type of implementation.

EXT has a double - Far-End Crosstalk, FEXT, the source of which is in another pair of cables, at the far end of the line. As for DSL, the degree of influence on such lines by FEXT is significantly lower than by NEXT.

Linear attenuation. Signal strength decreases as it travels along a copper cable, especially for signals at high data rates and high frequencies. This imposes very significant limitation for the use of DSL over long distances.

Low-impedance wiring can minimize signal attenuation, but any given provider may find the required cost unjustified. Thick wires have less resistance than thin wires, but they are more expensive. The most popular cables are 24 gauge (about 0.5 mm) and 26 gauge (about 0.4 mm); The lower attenuation of the 24 caliber makes it suitable for use over long distances.

Load inductors. In a time when public switched telephone networks (PSTN) carried only voice calls, inductors helped extend the length of telephone lines—a very laudable goal. The problem today is that they negatively impact DSL functionality.

The fact that load inductors cut frequencies above 3.4 kHz to improve voice frequency transmission makes them mutually incompatible with DSL. Potential DSL subscribers will not be able to receive DSL service while the inductors remain on the copper cable sections.

Shunted branches. If the telephone company is not going to completely disconnect the unused section of wiring, it will shorten it by installing a shunted tap. This practice didn't bother anyone much until demand for DSL began to grow rapidly. Shunts greatly impact the suitability of a line for DSL support and often simply need to be removed before the DSL line can be qualified for use.

Echo cancellation. The echo canceller allows signal transmission in only one direction at a time. The devices block potential echoes but make two-way communications impossible. To disable echo canceller, modems can send a 2.1 kHz response signal at the start of a call.

Fiber optic cable. Distance restrictions and noise interference are not the only pitfalls to DSL adoption. If the subscriber line uses fiber optics, then this route is not suitable for DSL. Fiber optics supports digital transmission, but DSL lines were designed with analog copper wiring in mind. Local links in the future will be based on a hybrid fiber/twisted pair approach, with small copper runs to the nearest fiber node.

SPEECH OVERDUCTION

Everyone would like to reduce the cost of local (and, indirectly, long-distance) voice transmission using Voice over DSL (VoDSL). ADSL supports analog voice frequencies by carrying digital data at higher frequencies, but VoDSL follows an alternative course. VoDSL converts speech from analog to digital and transmits it as part of its digital payload.

Both ADSL and SDSL support VoDSL, but G.lite is considered unsuitable for this task.

to be continued...

A choice based solely on price can end up being a disappointment. The lower the monthly fee, the less accessible the service will be.

Another important point regarding DSL, like any other communication channel, is security. Unlike cable modems, DSL users receive dedicated connections that are not affected by the activity of other users. Neighbors do not occupy the same lines at the same time as you, as is the case with cable modems, which is certainly a plus in terms of security. However, both technologies may be at risk of intrusion and denial of service attacks due to persistent connections and fixed IP addresses.

If data transmission systems could someday turn into living organisms, then the copper “twisted pair” would be the most durable of them. The last mile is a large and growing market, particularly sensitive to affordable technologies with high supported throughput.

Free, unlimited, broadband access for everyone is not possible in our lifetime, but if you are considering purchasing DSL services, you are going in the right direction.

Speed ​​and modulation.
ADSL connection speed.

First:
That the unit of information is a byte; there are 8 bits in one byte. Thus, when you download files, keep in mind that if your download speed is shown as, for example, 0.8 Mb/s (Megabytes per second), then the real speed is 0.8x8 = 6.4 Mbps (Megabits per second) !

Second:
The higher the speed set, the greater the likelihood of connection instability! The most stable speed is 6144 Kbps incoming and 640 Kbps outgoing with G.DMT modulation. For the Internet, high speed is not needed in principle - you simply will not feel the difference between 6144 Kbps and 24000 Kbps. However, when using the IP-TV service, you need to know that one channel occupies a bandwidth of 4-5 megabits per second. Therefore, if you want to watch IP-TV and have an Internet connection at the same time, please note that for the Internet the channel width will decrease by the amount indicated above. In addition, if for some reason you need to download information simultaneously into several streams, it also makes sense for you to ask to increase the speed.
Although you can ask to increase or decrease the speed by calling technical support on 062 (this is done immediately!).

What are the characteristics of modulations.
Question: What are the characteristics of modulations?
Answer:
G.dmt is an asymmetric DSL modulation based on DMT technology, which provides data transmission speeds towards the user up to 8 Mbit/s, and away from the user up to 1.544 Mbit/s.

G.lite is a modulation based on DMT technology, which provides data transfer rates towards the user up to 1.5 Mbit/s, and away from the user up to 384 Kbit/s. "

ADSL - modulation provides data transmission speeds towards the user up to 8 Mbit/s, and towards the user up to 768 Kbit/s.

T1.413 is a discrete asymmetric multitone modulation, which is based on the G.DMT standard. Accordingly, the speed limit is approximately the same as in G.dmt modulation.

ADSL2+

Just three years ago, many would have thought that ADSL technology was changing the world. Makes available fantastic speeds hitherto unknown to dial-up Internet users. But, as they say, you quickly get used to everything good, and you want more.

A rather funny situation has developed in our country. When there was a boom in ADSL providers all over the world and virtually no interest in home networks ETTH (Ethernet To The Home), in our country such networks began to be actively built. At the moment, the whole world is slowly beginning to realize that the development of multimedia and especially High-Definition (HD) content is greatly limited by the speed capabilities of xDSL networks, and in Russia ETTH is already available in all major cities. Thus, we seemed to have stepped over one stage of network development (ADSL providers developed in parallel with ETTH, but there was no obvious dominance) and found ourselves among the leaders. At least in something! But today we will not discuss this at all. As you know, ADSL technology already exists in the second version and even in 2+. We will talk about their differences from a technical point of view and prospects in the Internet providing market.

General concepts

Let's briefly refresh our memory on the main distinguishing features of ADSL technology. It belongs to the xDSL family of standards designed to provide high data transfer speeds over existing telephone lines. Despite the fact that ADSL is far from the fastest technology in the xDSL family, it is the one that has become most widespread in the world due to the optimal combination of speed and range.

The ADSL channel is asymmetrical, that is, the upstream (from the user to the provider) and downstream (in the opposite direction) flows are not equivalent. Moreover, the equipment on both sides is different. On the user side it is a modem, and on the provider side it is a DSLAM (ADSL switch).

Despite the fact that only three versions of ADSL are widely known (ADSL, ADSL2 and ADSL2+), there are actually many more specifications. I suggest taking a look at the table where all the main ADSL standards are presented. By and large, the specifications differ in operating frequencies and are needed to ensure that ADSL technology can operate on various types of telephone lines. For example, Annex A uses a frequency band starting from 25 kHz and ending at 1107 kHz, while Annex B operating frequencies start at 149 kHz. The first was developed for data transmission over public telephone networks (PSTN or POTS, in English), and the second was intended to work together with ISDN networks. In our country, Annex B is most often used in apartments with security alarms, which also use frequencies above 20 kHz.

Table

Different ADSL standards to work on different lines

ANSI T1.413-1998- Issue 2 ADSL

ITU G.992.1- ADSL (G.DMT)

ITU G.992.1- Annex A ADSL over POTS

ITU G.992.1- Annex B ADSL over ISDN

ITU G.992.2- ADSL Lite (G.Lite)

ITU G.992.3/4- ADSL2

ITU G.992.3/4- Annex J ADSL2

ITU G.992.3/4- Annex L RE-ADSL2

ITU G.992.5- ADSL2+

ITU G.992.5- Annex L RE-ADSL2+

ITU G.992.5- Annex M ADSL2+M

ADSL2

Due to what? ADSL2 faster? According to the developers, there are 5 key differences: Improved modulation mechanism, reduced overhead in transmitted frames, more efficient encoding, reduced initialization time and improved DSP performance. Let's sort it out in order.

As you know, ADSL uses quadrature amplitude modulation (QAM) with orthogonal frequency division multiplexing (OFDM). Without going into technical details, at a glance, the situation is something like this: the available bandwidth (fits into the frequency range 25-1107 kHz) is divided into channels (25 for transmission and 224 for reception); Each channel transmits a portion of the signal, which is modulated using QAM; Then the signals are multiplexed using fast Fourier transform and transmitted to the channel. On the reverse side, the signal is received and processed in the reverse order.

QAM, depending on the quality of the lines, encodes words of varying depths and sends them to the channel at a time. For example, the QAM-64 algorithm used in ADSL2 uses 64 states to send an 8-bit word at a time. Moreover, ADSL uses the so-called equalizing mechanism - this is when the modem constantly evaluates the quality of the line and adjusts the QAM algorithm to a greater or lesser word depth to achieve greater speed or better communication reliability. Moreover, equalizing works for each channel separately.

In fact, everything described above took place in the first version of ADSL, however, the reworking of modulation and coding algorithms made it possible to work more efficiently on the same communication lines.

To improve performance over long distances, the developers have also reduced redundancy, which was previously fixed at 32 kbps. Now this value can vary depending on the state of the physical environment from 4 to 32 kbit/sec. And although this is not so critical at high speeds, at long distances, when it becomes possible to use only low bit rates, this somehow increases throughput.

ADSL2+

It would seem that so many changes in ADSL2 compared to the first ADSL allowed the speed to increase by only 1.5 times. What did they come up with in ADSL2+ to increase the throughput of the downlink channel by 2 times compared to ADSL2 and 3 times compared to ADSL? Everything is banal and simple - the frequency range has expanded to 2.2 MHz, which made a twofold increase in speed real.

In addition to this, in ADSL2+ implemented the ability to combine ports (port bonding). Thus, by combining two lines into one logical channel, you will get a throughput of 48/7 Mbit/s. This, of course, is rare, but if there are two telephone numbers in the apartment, this is quite possible. Or, as an option, you can get double the speed on one physical line if you use a cable with two copper pairs, crimped with an RJ-14 connector.

Instead of a conclusion

What would you like to say finally? The advantages of the new standards are, in fact, more than obvious. From the point of view of the average user, this is an increase in the speed threshold, which “pulled up” the ADSL speed to the level of cable networks. Purely nominally, both are capable of transmitting HD content. But as practice shows, where high-quality ETTH has reached, ADSL and cable companies are gradually beginning to lose ground, feeling at ease only in the absence of serious competition. It would seem, why do we need such high speeds, since in many regions of our country the mass transition from dial-up access to broadband is just beginning? According to some forecasts, by 2010 traffic prices will decrease by 3-4 times. And if the speed of the incoming channel (ADSL2+ - 24 Mbit/s) has a significant reserve, then the low speed of the return channel (ADSL - 1 Mbit/s, ADSL2+ - 3.5 Mbit/s) greatly limits ADSL users. For example, one of the main advantages of ETTH networks - internal resources - is technically possible to implement in ADSL, but the relatively low upload speed is a serious obstacle to fast internal file exchange between users. This also affects the efficiency of work in peer-to-peer networks, where users of large ETTH providers can often download files at speeds close to 100 Mbit/s.

Of course, ADSL has a future, and its “overclocked” versions will allow you to freely use fast Internet for a couple of years for sure. And what will happen next? Wait and see.

Glossary

Modulation– change in parameters (phase and/or amplitude) of a modulated oscillation (high-frequency) under the influence of a control (low-frequency) signal.
Quadrature Amplitude Modulation (QAM) - with this type of modulation, information is encoded in the signal by changing both its phase and amplitude, which allows you to increase the number of bits in a symbol.

Symbol– signal state per unit time.
Fourier multiplexing is the decomposition of a carrier signal, which is a periodic function, into a series of sines and cosines (Fourier series) with subsequent analysis of their amplitudes.

Frame– a logical block of data starting with a sequence indicating the beginning of the frame, containing service information and data, and ending with a sequence indicating the end of the frame.

Redundancy– the presence in a message of a sequence of symbols that allows it to be written more briefly, using the same symbols using coding. Redundancy increases the reliability of information transfer.