ADSL technology

What is hidden behind this cryptic word:

ADSL is a data transmission technology that allows you to simultaneously use a regular telephone line for both your telephone and high-speed Internet. The telephone and ADSL channels do not affect each other. You can simultaneously download pages, receive mail and talk on the phone. The maximum speed of the ADSL channel is up to 8 Mbps!

How does ADSL work?

A telephone or an ordinary 14.4 kbps modem uses a low-frequency channel: usually the transmitted frequency range is in the range 0.6-3.0 kHz, a good telephone channel can transmit frequencies in the 0.2-3.8 kHz range, which, provided there is weak interference, allows you to raise the speed to 33.6 kbps c. On the so-called digital automatic telephone exchanges, where an analog telephone signal is converted into a digital stream at a telephone exchange or a node, the speed can be brought up to 56.0 kbps. In practice, however, due to the imperfect quality of telephone lines, the real speed turns out to be 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. Likewise, when you connect to the Internet, a direct connection is established between your modem and the provider's modem. The telephone channel is busy with data transmission, so you cannot use the telephone at this time.
ADSL uses a higher frequency range. Even the lower limit of this range lies much higher than the frequencies used in the dial-up telephone channel. Naturally, the ADSL channel goes through your telephone wire only to your PBX, then the paths of the dial-up and ADSL channels diverge: the dial-up channel goes to the telephone exchange, and the ADSL channel goes to the provider's digital network (for example, Ethernet LAN). To do this, the provider's ADSL modem is installed directly at your telephone exchange. For data transmission, a very wide frequency band is used, which practically makes it possible to achieve 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 twin line and burglar alarms.
It is not recommended to plug the ADSL modem directly into the telephone socket (without a splitter): the ADSL modem and the telephone can 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 the simplest filters to separate low telephone and high ADSL frequencies. The filters are included with the ADSL modem and are called splitter and microfilter. A splitter is a special tee, one end of which connects to the telephone line, and the other two to the telephone and modem. The microfilter is connected at one end to the line, the other to the telephone - useful for connecting parallel telephones.

The modern world cannot be imagined without the Internet and computer networks. High-speed channels have entangled the world with cobwebs - satellites, fiber optics, cables - nerves and blood vessels of the worldwide information network. Gigantic speeds, gigantic traffic, high technologies ... But at the same time, for many years, high-speed channels with data transfer rates exceeding 1 megabit per second remained the lot of providers and large companies.
High technologies developed by leading hi-tech companies for high-speed data transmission have proved to be very expensive, with not only a huge implementation cost, but also a high cost of ownership. Ordinary users to get access to the Internet had to be content with ordinary, very widespread and cheap 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 establish satellite Internet for themselves - it was expensive and ineffective. What to download at high speeds - news, prices, documents, kilobyte drivers? For more than two decades, Dial Up access has been governed by the "last mile" rules - the very site by which information is delivered from the provider to the end user. Telephone lines, especially Russian ones, have become a wall in the way 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 ragged portions, besides with constant disconnect.
For a long time, the possibilities of Dial Up modems suited many. This technology, developed at the dawn of the computer era for analog telephone lines, has evolved extremely slowly and slowly - over the past 15 years, data transfer rates have increased from 14,400 Kbps to only 56,000 Kbps. For many years, it seemed that this speed was enough for almost everything - download 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 did not exceed several hundred kilobytes for a number of years. was time consuming and did not require high-speed connections. But life has made its own adjustments.
The development of modern computer technologies, in addition to an increase in the frequency of central processors, a revolution in the field of 3D graphics accelerators and an explosive increase in the capacity of information storage devices, has also led to a dramatic increase in the volume of information sent. Computer evolution proceeded according to the principle of "more, higher, faster", led to the fact that programs and files have grown to monstrous sizes. For example, the now standard Word document is dozens 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 at times, high sound quality, and recently the bitrate of MP3 files has risen from the standard 128 Kbps to 192 kbps, which also noticeably affects the size. Yes, to some extent, the compression algorithms that have been significantly improved in recent years help, but this is still not a panacea. The sizes of drivers have recently grown to gigantic sizes, for example, Detonator FX from nVidia takes about 10 megabytes (despite the fact that two years ago they took only 2 megabytes), and unified drivers for the nForce platform from the same company are already 25 megabytes and this the trend is capturing an increasing number of computer hardware manufacturers. But the main trouble that makes Dial Up modems heat up, not giving them even a minute of rest, is software patches or patches that correct errors in software. The widespread adoption of rapid development tools has led to the massive release of raw, unoptimized software. And why optimize a program if the computer hardware is still redundant? Why engage in beta testing a program if there is an Internet network - it is enough to sell a raw program, then look at a list of the most common problems and errors that users themselves will compose when contacting support and then release a patch, after which another, a third, and so on ad infinitum ... Inevitably, with nostalgia, one recalls the times when the Internet was the lot of a handful of the elite, and programmers who were not spoiled by the worldwide network licked their programs to the last byte, 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, sadly looking at, for example, the fourth (!) Microsoft patch for Windows 2000, 175 megabytes in size, you understand that Dial Up access won't drain this lump even in a week, and how much will this patch cost for hourly payment ! And then there is Microsoft Office and dozens of other programs that need fixing. And the 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, which are practically not available to dial-up specialists.
All these gloomy reflections lead to the thought that Dial Up Internet access has outlived its usefulness and urgently needs replacement. What can replace obsolete technologies? The classic ISDN (Integrated Services Digital Network) and the relatively new satellite Internet immediately come to mind. Then they come immediately, but after long reflections, both disappear. ISDN disappears due to the high cost of laying a dedicated channel, which is inappropriate in an apartment and the high cost of ownership (subscription fee + traffic charges). In principle, this type of access is possible when laying a house network, when several users jointly create a high-speed channel for themselves, and then distribute it through an apartment building through a local network. But as the further material of the article will show, ISDN has a powerful competitor that negates 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 captures a large area of ​​the Earth's surface, but you need to look to see if the satellite of the provider providing this service in your region is visible and from what angle it is seen, it depends on what size the satellite dish you have to install. In addition, the satellite channel is still not very fast - the best of them provide about 400 Kbps towards the user (for ordinary users, of course, there are higher-speed options, but they are several orders of magnitude more expensive). The transfer of data from the user to the provider is carried out by telephone, thus, the telephone line is as busy as when using a Dialup modem. Satellite systems of different providers have a number of common disadvantages, which are the high cost of the equipment used and the complexity of its installation and configuration. In addition, satellite providers, to put it mildly, are not reliable enough. There are reasons for this, both objective (satellites are not eternal, a telecommunications satellite will fall into the dense layers of the atmosphere, when they still bring a replacement to the same orbit), and subjective ones - remember the fiasco of the satellite Internet NTV +, which, it turns out, threw 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 the telephone copper cable. After all, a subscriber telephone line is not a cable for the network. Yes, the quality is terrible, but you can develop new technologies for transferring data, translate everything into digital, modulate everything in a special way, correct the errors that arise and get a broadband digital channel as a result. So it turns out that all the 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 - we got a technology that combines the best features of both Dial Up modems operating on analog telephone lines and high-speed IDSN modems. Meet - ADSL technology.

What is ADSL?

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 the letter characterizing the channel speed, and DSL is the already known abbreviation Digital Subscriber Line - digital subscriber line. For the first time the name DSL sounded back in 1989, it was then that the very idea of ​​digital communications appeared for the first time using a pair of copper telephone wires instead of specialized cables. The imagination of the developers of this standard is clearly lame, so the names of the technologies included in the xDSL group are rather 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 of this group are much faster than ADSL, but at the same time require the use of special cables, while ADSL can work on ordinary copper pair, which is widely used in laying telephone networks. The development of ADSL technology began in the early 90s. Already in 1993, the first standard of this technology was proposed, which began to be implemented in the telephone networks of the USA and Canada, and since 1998 ADSL technology has gone, as they say, into the world.
In general, it is still premature for us to bury a copper subscriber line consisting of two wires. Its cross section is quite sufficient to ensure the passage of digital information over fairly long distances. Imagine how many millions of kilometers of such a wire have been laid throughout the Earth since the appearance of the first telephones! Yes, no one has canceled the distance restrictions, the higher the information transfer rate, 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 DSL technologies on the subscriber telephone line, adapted to the copper pair, these millions of kilometers of analog lines have become possible to use for the organization of cost-effective high-speed data transmission from the provider, which owns a thick digital channel, to the end user. The wire, once intended solely for providing analog telephone communication, with a slight movement of the hand turns into a broadband digital channel, while retaining its original responsibilities, since the owners of ADSL modems can use the subscriber line for traditional telephone communication simultaneously with the transfer of 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 a significantly higher frequency modulation than that usually used for traditional analog telephone communication, which significantly expands the communication capabilities of existing telephone lines.

ADSL - How Does It All Work?

How does ADSL work? What technologies can ADSL use 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 from the provider and one from the end user. Between these two modems there is a regular telephone wire. The connection speed can vary depending on the length of the "last mile" - the farther from the provider, the lower the maximum data transfer speed.

Data exchange between ADSL modems is carried out on three sharply separated frequency modulations.

As can be seen from the figure, voice frequencies (1) are completely not involved in receiving / transmitting data, and are used exclusively for telephone communication. The frequency band for receiving data (3) is clearly delimited 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 4KHz bandwidth for regular telephone service or POTS - Plain Old Telephone Service. Thanks to this, a telephone conversation can actually be carried out simultaneously with reception / transmission without reducing the speed of data transfer. And in the event of a power outage, telephone communication will not disappear anywhere, as is the case when using ISDN on a dedicated channel, which is undoubtedly the advantage of ADSL. I must say 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 communication, special filters are installed that extremely effectively separate the analog and digital components of communication between themselves, not excluding the joint simultaneous work on the same pair of wires.
ADSL technology is asymmetric, like Dial Up modems. The speed of the incoming data stream is several times higher than the speed of the outgoing data stream, which is logical, since the user always downloads more information than transmits. Both the transmission speed and the reception speed of ADSL technology are significantly higher than that 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 an ordinary telephone cable, which is ideal, like walking to the moon. But ADSL manages to create high-speed data channels over a regular telephone cable, while showing better results than ISDN with its own dedicated line. Here it turns out that engineers of Hi-Tech corporations do not eat their bread in vain.
High speed of reception / transmission is achieved by the following technological methods. First, the transmission in each of the modulation zones shown in Figure 2 is in turn subdivided into several more frequency bands - the so-called bandwidth sharing method, which allows several signals to be transmitted over one line at the same time. 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 we see its application in practice to create high-speed digital highways. This process is also called Frequency Division Multiplexing (FDM). When using FDM, the transmission and reception ranges are divided into many low-speed channels, which in parallel provide data reception / transmission.
Oddly enough, but when considering the method of sharing the bandwidth to mind, as an analogy comes such a widespread class of programs as the Download manager - they use the method of splitting them into parts to download files and downloading all these parts at the same time, which makes it more efficient to use link. As you can see, the analogy is direct and differs only in implementation, in the case of ADSL we have a hardware version and not only for uploading, but also for sending data.
The second way to speed up data transfer, especially when receiving / sending large amounts of information of the same type, is to use special hardware-implemented compression algorithms with error correction. Highly efficient hardware codecs that allow compressing / decompressing large amounts of information on the fly - this is one of the secrets of the speed shown by ADSL.
Thirdly, ADSL uses an order of magnitude wider frequency range than ISDN, which makes it possible to create a significantly larger number of parallel information transmission channels. For ISDN, the standard is 100 kHz, while ADSL uses about 1.5 MHz. Of course, long-distance telephone lines, especially domestic ones, attenuate the transmit / receive signal modulated in such a high-frequency range very 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 it still continues to be confidently received by the 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 the digital signal in the chaotic jumble that the modem receives. The analog part of the ADSL modem should 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 all the same, compared to competitors, the cost of ADSL hardware for end users is much lower.

How fast is ASDL technology?

Everything is learned in comparison, it is impossible to assess the speed of technology without comparing it with others. Before doing that, however, there are several ADSL features to consider.
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, there are two baud rates to consider. Another feature of ADSL technology is the use of high-frequency signal modulation and the use of several lower-speed channels lying in the common field of transmit and receive frequencies for simultaneous parallel transmission of large amounts 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 stronger the attenuation of the high-frequency signal. The used frequency spectrum is narrowed, the maximum number of parallel channels is reduced, and the speed also drops accordingly. The table shows the change in the throughput of the channels for receiving and transmitting data when the distance to the provider changes.

In addition to distance, the data transfer rate is strongly 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 taps. On our telephone networks, traditionally of poor quality, with a wire cross-section of 0.5 sq. mm and an eternally distant provider, the most common connection speeds will be 128 Kbps - 1.5 Mbps for receiving data going to the user and 128 Kbps - 640 Kbps for sending data from the user at distances within 5 kilometers. However, with the improvement of telephone lines, the speed of ADSL will also increase.

to be continued...

Recorded by

For comparison, consider other technologies.

Dial Up modems, as you know, are limited by the maximum data rate of 56 Kbps, a speed that I, for example, never used on analog modems. For data transfer, their speed is maximum 44 Kbps for modems using the v.92 protocol, provided that the provider also supports this protocol. The typical upload speed is 33.6 Kbps.
The maximum speed of ISDN in dual-channel mode is 128 Kbps, or, as it is not difficult to calculate, at 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, as one of the channels is busy. The data is sent at the same rates.
Cable modems can provide data 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, the narrower the channel for each of the users. When using ADSL technology, the entire bandwidth of the channel belongs to the end user, making the connection speed more stable compared to cable modems.
And finally, the dedicated digital lines E1 and E3 can show the data transfer rate, in the synchronous mode, 2 Mbit / s and 34 Mbit / s, respectively. The performance is very good, but the prices for the wiring and maintenance of these lines are sky-high.

Glossary.

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

Analog signal- a continuous waveform, characterized by such concepts as frequency and amplitude. Analog signals with 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- digital signal, unlike analog intermittent (discrete), the signal value changes from minimum to maximum without transient states. The minimum value of the digital signal corresponds to the state "0", the maximum "1". Thus, the digital transmission of information uses a binary code, the most common in the environment of computers. A digital signal, unlike analog, 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 communication 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 certain frequency, intended for transmission over a subscriber line, over a special cable or, for wireless systems, over radio waves. The process of inverse transformation of a modulated signal is called demodulation.

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

Cable Modems- modems using cables from existing cable television networks. These networks are shared networks, that is, the data transfer rate strongly depends on the number of users simultaneously on the network. Therefore, although the maximum speed of cable modems reaches 30 Mbps, in practice it is rarely possible to get more than 1 Mbps.
P.S. If you do not understand any terms in the article, write, the glossary will be expanded.

ADSL Technology (by Jeff Newman)
ADSL (Asymmetric Digital Subscriber Line) technology is a form of xDSL technology that provides users with affordable broadband transmission between relatively close network nodes.
ADSL research and development has been spurred by investment from telephone companies, which, unlike conventional broadcast television, wanted to provide video on demand to users. Advances in the development of ADSL technology have made it suitable not only for digital television broadcasting, but also for many other high-speed interactive applications, such as Internet access, delivery of corporate information to remote offices and branch offices, and audio and video information on demand. Under the best operating conditions and acceptable distances, using ADSL technology, you can transfer data at speeds up to 6 Mbps in the forward direction (in some versions, up to 9 Mbps) and 1 Mbps in the opposite direction.

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

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 evaluated the advantages of their performance and disadvantages of ADSL technology.

As a result, when testing ADSL devices with a rather heavy load, no significant flaws were revealed, 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 start negotiations with telephone companies now.

No additional wiring needed.

The main advantage of ADSL technology is that it uses the ubiquitous twisted-pair copper wire that is used today. In addition, in this case there is no need for expensive upgrades of switches, laying of 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 the duration of the communication session and the degree of channel utilization), ADSL is a leased-line service.

Signals are transmitted over a pair of wires between two ADSL modems installed at the remote network node and at the local PBX. An ADSL network modem converts digital data from a computer or some other device into an analog signal suitable for transmission over twisted pair cables. For parity check, redundant bits are inserted into the transmitted digital sequence. This guarantees the reliability of information delivery 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 between them. In this case, the "remote" ADSL modem operating in the receive mode and the "central" transmitting ADSL modem can be connected with a copper wire without any additional intermediate elements between them. The connection of offices located at long distances from each other, provided that each of them is located relatively close to "its" PBX, is carried out using the trunk lines provided by the telephone companies.

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

With the combined use of the line for data transmission and telephony, the latter will work without additional power supply, as is necessary in the case of ISDN. In the event of a power failure, conventional telephony will continue to operate with power 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 with a Plain Old Telephone Service (POTS) frequency splitter called a frequency splitter. These functionalities of ADSL give it a reputation for being a reliable technology. It is also harmless, since in the event of an accident it has no effect on the operation of telephony. ADSL seems like a pretty basic technology, in fact it is. Installing and running it is not difficult. Simply connect the device to the network and the telephone line, and leave the rest to the telephone company.

However, this technology has some quirks to consider when building and operating your network. For example, ADSL devices can be affected by certain physical factors inherent in signal transmission over a pair of wires. The most important of these is line attenuation. In addition, the reliability and throughput of the data link can be affected by significant electromagnetic interference on the cable, especially from the telephone company's own network.

Line coding types

ADSL modems use three types of linear coding, or modulation: Discrete Multitone (DMT), Carrierless Amplitude / Phase (CAP), and rarely used Quadrature Amplitude Modulation (QAM). Modulation is required for link establishment, signaling between two ADSL modems, rate matching, channel identification, and error correction.

DMT is considered the best modulation 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 the standard for line coding ADSL channels.

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

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

For testing, we used a 24-gauge copper pair with 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 function reliably over much longer distances. We also found that the actual range of most modems exceeds 4.6 km (26 dB). ADSL-modems based on DMT operated at the maximum possible distance in our conditions - 5.5 km - at speeds of 791 Kbps in the forward direction and 582 Kbps in the opposite direction (the measured signal attenuation in the line is 31 dB).

Both CAP-based ADSL modems operated at 4 Mbps forward and 422 Kbps reverse 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 the ones just described, we carried out tests in which we reproduced real conditions on the lines, for example, we checked the work with bridge taps, which are often used in telephony. A branch bridge is an open telephone line that branches out to the side of the main line. Typically, this additional line is not used and therefore does not create additional crosstalk on the main line, but significantly increases its attenuation. Therefore, it is surprising that some of the modems tested worked fine with a tap-off length of 1.5 km and a main line length of 3.7 km. With an increase in the length of the main line to 4.6 km, the reliability of signal transmission became lower than the permissible level only in the case of an increase in the length of the branch line to 300 m.

Electromagnetic interference

Near-End Crosstalk (NEXT; Far-End Crosstalk - FEXT) lines are forms of electromagnetic interference that distort the signal on the ADSL channel and thus adversely affect its decoding. This type of pickup can occur at either end of the connection if there is an unwanted line alongside the ADSL line, such as a T1 or other 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 the adjacent loaded T1 line on the ADSL data stream was minimal, and the ADSL and T1 signal quality did not deteriorate. This impact on the PBX is likely to increase if multiple T1 lines and multiple ADSL lines are interspersed with each other. When installing ADSL lines, the telephone company must consider this cross-linking.

Another noise 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 powerful electrical devices such as refrigerators and laser printers, or near powerful motors installed in an elevator shaft. The MCI engineers who tested the modems applied a 5V impulse voltage across the twisted pair parallel to our ADSL line, but the bit error rate remained at an acceptable level. In fact, this effect on modems could be neglected in our tests.

In our opinion, about a year is left before the widespread introduction of ADSL technology in public networks. In the meantime, it is under development and the possibility of its application is being evaluated. However, ADSL technology is already used in the networks of corporations and small towns. Many firms have started making products for ADSL. The wide bandwidth and noise immunity of the first ADSL modems we tested proved their high reliability. Now, when upgrading your network and increasing the number of users, ADSL technology can no longer be neglected.

What is ADSL (one more article)
ADSL (Asymmetric Digital Subscriber Line) is one of the high-speed data transmission technologies known as Digital Subscriber Line (DSL) 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 was first introduced, which would improve the technology for transmitting data 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 access to LAN and other networks).

So what exactly is ADSL? First of all, ADSL is a technology that transforms a twisted pair of telephone wires into a high-speed data transmission path. An ADSL line connects two ADSL modems that are connected to a telephone cable (see figure). At the same time, three information channels are organized - a "downward" data transmission stream, an "upward" data transmission stream and a regular telephone communication channel. The phone line is filtered using filters to ensure that your phone will work even if the ADSL connection fails.
ADSL is an asymmetric technology - the rate of the "downstream" data stream (that is, the data that is transmitted to the end-user) is higher than the rate of the "up" data stream (in turn, transmitted from the user to the network).
To compress a large amount of information transmitted over a twisted pair of telephone wires, ADSL technology uses digital signal processing and specially designed algorithms, advanced analog filters and analog-to-digital converters.
ADSL technology uses a technique to divide the bandwidth of a copper telephone line into multiple frequency bands (also called carriers). This allows multiple signals to be transmitted simultaneously over one line. With ADSL, different carriers carry different portions of the transmitted data at the same time. This is how ADSL can provide, for example, simultaneous high-speed data transmission, video transmission and fax transmission. And all this without interrupting the regular telephone connection, which uses the same telephone line.
The factors affecting the data transfer rate are the state of the subscriber line (i.e. wire diameter, presence of cable branches, etc.) and its length. Signal attenuation in the 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. ADSL currently provides downstream data rates up to 8 Mbps and upstream data rates up to 1.5 Mbps.

Do you need an ADSL line?

It's up to you, but in order for you to make the right decision, consider the benefits of ADSL.

First of all, the high data transfer rate.
In order to connect to the Internet or to a data network, you do not need to dial a phone number. ADSL creates a broadband data link using an existing telephone line. After installing ADSL modems, you get a permanently established connection. High-speed data link is always ready to work - whenever you need it.
ADSL technology allows full use of line resources. In ordinary telephone communication, about one hundredth of the capacity of the telephone line is used. ADSL technology removes 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 (up to approximately 4 kHz) is used, while the rest of the bandwidth is used for high-speed data transmission.
ADSL opens up completely new possibilities in those areas in which high-quality video signals are required to be transmitted in real time. These include, for example, video conferencing, distance learning and video on demand. ADSL technology makes it possible to provide services with a data transfer rate more than 100 times higher than the speed of the currently fastest analog modem (56 Kbps) and more than 70 times higher than the data transfer rate in ISDN (128 Kbps).
Don't forget about costs. ADSL technology is effective from an economic point of view, if only because it does not require the laying of special cables, but uses already existing two-wire copper telephone lines. That is, if you have a connected telephone at home or in your office, you do not need to lay additional wires to use ADSL.
The subscriber has the ability to flexibly increase the speed without changing equipment, depending on his needs.
Based on materials from the Verkhnevolzhsky branch of Centrotelecom.

ADSL and SDSL

Asymmetrical and balanced DSL lines

Individuals with limited 56.6 Kbps dial-up connectivity want access to broadband applications, while businesses with their expensive T-1 / E-1 Internet connections want to lower their costs. The best of technology allows you to solve problems with the hardware you have. Switch to Digital Subscriber Line (DSL) where possible.

DSL technology connects the user's premises to the service provider's Central Office (CO) over pre-existing copper telephone lines. If the lines meet the established requirements, then with the help of 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 is highly dependent 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 particular area. DSL variants generally follow one of two basic schemas, although they may differ in specific characteristics. Two main models - asymmetric (Asymmetric DSL, ADSL) and symmetric (Symmetric DSL, SDSL) digital subscriber lines - stood out in the early stages of technology development. In the asymmetric model, preference is given to the 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.

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

ABOUT ASYMMETRY

ADSL is making its way into the high-speed residential market where it competes with cable modems. Fully satisfying the appetites of home users in their "walks" on the WWW, ADSL provides data transfer rates from 384 Kbps to 7.1 Mbps in the main direction and from 128 Kbps to 1.54 Mbps in the opposite direction.

The asymmetric model fits well with the way of working on the Internet: in the forward direction, large volumes of multimedia and texts are transmitted, while in the opposite direction, the level of traffic is negligible. ADSL costs in the US typically range from $ 40 to $ 200 per month, depending on estimated data rates and service level guarantees. Cable modem services are often cheaper, around $ 40 a month, but the lines are shared by 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 an analog speech signal in the range from 0 to 3.4 kHz. Symmetrical DSL (SDSL) occupies the entire frequency range of the data line and is not compatible with analog voice signals.

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

Determining ADSL rates is an art rather than a science, although rates decline at fairly predictable intervals. ISPs provide the best possible service, with results highly dependent on distance from the central site. Typically, "best possible" means that providers guarantee 50% throughput. Attenuation and interference such as crosstalk become significant on lines over 3 km, and at distances over 5.5 km, they can render the lines unusable for data transmission.

At distances up to 3.5 km from the central hub, ADSL speeds can reach 7.1 Mbps in the forward direction of the flow and 1.5 Mbps in the direction from the subscriber to the CO. However, DSL Reports editor Nick Braak believes that the upper limit is unattainable in practice. Braak states: "In fact, 7.1 Mbps is not achievable, even in a laboratory setting." At distances over 3.5 km, the ADSL speed is reduced to 1.5 Mbps in the forward direction and to 384 Kbps from the subscriber to the CO; As the length of the subscriber line approaches 5.5 km, the speed drops even more - to 384 Kbps in the forward direction of the stream and to 128 Kbps in the opposite direction.

Service contracts for ADSL services may include a clause that the user refuses to connect to home networks or Web servers. However, DSL technology alone does not interfere with home LAN connectivity. For example, even if an ISP provides a customer with a single IP address, Network Address Translation (NAT) can be used by multiple users to share that single IP address.

One DSL connection is enough for a home with many computers. Some DSL modems have a built-in DSL hub as well as specialized devices called "resident 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 a division of the spectrum of available frequencies into 256 channels in the range from 26 to 1100 kHz, at 4.3125 kHz each.

CONNECTING COPPER LINE TO ATU-R

So we have a hub, twisted-pair copper cable, and a remote site. What to connect to what?

A so-called remote transmitting unit (ADSL Transmission Unit-Remote, ATU-R) is installed at the customer's site. Originally referring to ADSL only, ATU-R now refers to a remote device for any DSL service. In addition to providing DSL modem functionality, some ATU-Rs can provide bridging, routing, and time division multiplexing (TDM) functions. On the other side of the copper line, at the central site, is the 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 site, the DSLAM aggregates data traffic from multiple DSL lines and feeds it into the service provider's backbone, and the backbone already delivers it to all destinations on the network. Typically, the DSLAM connects to the ATM network over PVCs with ISPs and other networks.

G.LITE: ADSL WITHOUT DIVIDER

A modified version of ADSL known as G.lite eliminates the need to install a splitter at the customer 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 as ADSL, G.lite supports maximum speeds of 1.5 Mbps forward and 384 Kbps reverse.

ITU Recommendations G.992.1, also known as G.dmt, were first published in 1999, together with G992.2, or G.lite. G.lite equipment appeared on the market in 1999 and was cheaper than ADSL, mainly because the provider's technicians did not need to visit the customer for installation and troubleshooting. Service providers find it difficult to justify spending hundreds of dollars per fixed line with a $ 49 subscription, so any cost-saving modification is greeted with tremendous enthusiasm by the market.

DSL FOR BUSINESS

Businesses have very different needs from home users, making SDSL a natural choice for office applications.

Backward corporate bandwidth can be quickly depleted by heavy traffic on the Web server and large volumes of PDFs, PowerPoint presentations, and other documents sent by employees. Outgoing traffic can equal or even exceed incoming traffic. Delivering in both directions speeds in the order of 1.5 Mbps in North America and 2.048 Mbps in Europe, ADSL links resemble T-1 / E-1 connections, the dominant architectural component of corporate networks around the world.

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

As a viable alternative to T-1 / E-1, SDSL has attracted the attention of Competitive Local Exchange Carriers (CLECs) as a means of providing value-added services. In general, SDSL services are usually distributed by the CLEC, however ILECs usually use HDSL to implement the T-1 service. Under optimal conditions, SDSL can rival T-1 / E-1 in data transfer rates and is three times faster than 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 speed; in addition, the 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 is better suited to the needs of organizations than ADSL to the needs of residential users. While cable modem service providers entice private users with lower prices than ADSL, SDSL offers the same transfer rates as T-1 / E-1 at substantially less money. The standard price range for the T-1 is $ 500 to $ 1,500, depending on distance, while the equivalent SDSL range is $ 170 to $ 450. The lower the cost of SDSL services, the lower the guaranteed data transfer rate.

LET'S CLEAR

Signal quality is affected by many changing factors, many of which are not exclusive to DSL. However, some devices in the past that made our life in switched networks easier today hinder the use of digital subscriber lines.

Cross-talk. Electrical energy radiated from the bundles of wires converging at the service provider's central site produces interference known as Near-End Crosstalk (NEXT). When signals pass between channels of different cables, the capacitance of the line drops. Near end means that the interference is coming from an adjacent pair of cables in the same area.

Separating DSL and T-1 / E-1 lines significantly reduces the negative impact of crosstalk, but there is no guarantee that a service provider will decide to implement this very principle.

EXT has a counterpart, Far-End Crosstalk (FEXT), which is sourced from the other cable pair, at the far end of the line. As far as DSL is concerned, FEXT's impact on such lines is significantly lower than NEXT.

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

Low impedance wiring is able to minimize signal attenuation, but any particular provider may find the costs unreasonable. Thick wires have less resistance than thin wires, but they are more expensive. The most popular cables are 24 (approximately 0.5 mm) and 26 (approximately 0.4 mm); the lower attenuation of the 24 caliber makes it suitable for use over longer distances.

Load inductors. In a time when public switched telephone networks (PSTN) carried only voice calls, inductors helped lengthen telephone lines — a laudable goal. The problem today is that they adversely affect the functioning of DSL.

The fact that load inductors cut frequencies above 3.4 kHz to improve the transmission of frequencies in the voice range makes them mutually incompatible with DSL. Potential DSL subscribers will not be able to receive DSL service as long as the inductors remain on the copper cable runs.

Shunted branches. If the telephone company is not going to completely shut off the unused section of wiring, it will cut it down by installing a shunt branch. This practice did not particularly bother anyone until the rapid growth in demand for DSL began. Shunts greatly affect the suitability of a line for DSL support and often simply need to be removed before a DSL line can be qualified for use.

Echo cancellation. The echo canceller allows a signal to be transmitted simultaneously in only one direction. The devices block potential echoes, but make two-way communication impossible. To disable the 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 in DSL adoption. If the subscriber line uses fiber, then this route is not suitable for DSL. Fiber optics support digital transmission, but DSL lines were designed exclusively for analog copper wiring. Local communication lines in the future will be based on a hybrid fiber / twisted pair approach, with small copper sections to the nearest fiber node.

SUPERIOR SPEECH

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 transmitting digital data at higher frequencies, but VoDSL is following an alternative course. VoDSL converts speech from analog to digital and transmits it as part of its digital load.

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

to be continued...

Choices based solely on price can be frustrating in the end. The lower the monthly fee, the less available 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 do with cable modems, which is definitely a plus in terms of security. However, both technologies can be at risk of intrusion and denial of service attacks due to persistent connections and fixed IP addresses.

If data transmission systems could ever turn into living organisms, then copper "twisted pair" would be the most tenacious of them. The Last Mile is a large and growing market that is particularly sensitive to affordable high-bandwidth technologies.

Free, unlimited, broadband is not possible for everyone in our life, but if you are going to purchase DSL services, then you are heading in the right direction.

Speed ​​and modulation.
ADSL connection speed.

First:
That a unit of information is a byte, in one byte there are 8 bits. 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 set speed, the greater the likelihood of communication 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 simultaneously watch IP-TV and have an Internet connection, then keep in mind that for the Internet, the channel width will decrease by the above amount. In addition, if for some reason you need to download information simultaneously in 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 the modulations.
Question: What are the characteristics of modulations?
Answer:
G.dmt is an asymmetric DSL modulation based on DMT technology, which provides data transfer rates up to 8 Mbit / s towards the user, and up to 1.544 Mbit / s towards the user.

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

ADSL - modulation provides data transfer rate up to 8 Mbps towards the user, and up to 768 Kbps towards the user.

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

ADSL2 +

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

A rather amusing situation has developed in our country. When there was a boom in ADSL providers all over the world and almost 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 strongly limited by the high-speed capabilities of xDSL networks, and in Russia ETTH is already available in all major cities. Thus, we kind of stepped over one stage of network development (ADSL providers developed in parallel with ETTH, but there was no obvious dominance) and ended up among the leaders. Wow, at least in something! But that's not what we're going to discuss today. 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 for the Internet provider market.

General concepts

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

The ADSL channel is asymmetric, that is, the upstream (from user to provider) and downstream (in the opposite direction) streams are not equivalent. Moreover, the equipment is different on both sides. From the user side it is a modem, and from 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 +), in fact there are 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 the operation of ADSL technology on various types of telephone lines. For example, Annex A uses bandwidths ranging from 25 kHz to 1107 kHz, while Annex B operating frequencies start at 149 kHz. The first was designed for data transmission over public switched 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 burglar 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, shorter initialization time, and improved DSP performance. Let's sort it out in order.

As you know, ADSL uses Orthogonal Frequency Division Multiplexing (OFDM) Quadrature Amplitude Modulation (QAM). Without going into technical details, on the fingers, the situation is something like this: the available bandwidth (fits into the frequency range of 25-1107 kHz) is divided into channels (25 for transmission and 224 for reception); a portion of the signal is transmitted on each of the channels, 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 reverse order.

QAM, depending on the quality of the lines, encodes words of different depths and sends them at a time to the channel. For example, the QAM-64 algorithm used in ADSL2 uses 64 states to send an 8-bit word at a time. Moreover, in ADSL, the so-called equalizing mechanism is used - 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 higher 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 redesign of the modulation and coding algorithms made it possible to work more efficiently on the same communication lines.

To improve performance over long distances, the developers also reduced redundancy, which was previously a fixed 32 kbps. Now this value can change depending on the state of the physical environment from 4 to 32 kbps. And although this is not so critical at high speeds, at a great distance, when it becomes possible to use only low bitrates, this in some way increases the throughput.

ADSL2 +

It would seem that so many changes in ADSL2 in comparison with the first ADSL made it possible to increase the speed by only 1.5 times. What was it invented in ADSL2 + to increase the downlink bandwidth by 2 times in comparison with ADSL2 and 3 times in comparison with ADSL? Everything is banal and simple - the frequency range has expanded to 2.2 MHz, which made a two-fold increase in speed real.

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

Instead of a conclusion

What would you like to say in the end? The benefits of the new standards are, in fact, more than obvious. From the point of view of an ordinary user, this is an increase in the speed threshold, which "pulled up" the speed of ADSL to the level of cable networks. Purely nominally, both are capable of providing HD content transmission. But as practice shows, where high-quality ETTH has reached, ADSL and cable operators are gradually starting to lose ground, feeling at ease only in the absence of serious competition. It would seem, why do we need such high speeds, because in many regions of our country a massive transition from dial-up access to broadband is just beginning? According to some forecasts, by 2010 the prices for traffic will decrease by 3-4 times. And if the speed of the incoming channel (ADSL2 + - 24 Mbit / s) has a significant margin, then the low speed of the return channel (ADSL - 1 Mbit / s, ADSL2 + - 3.5 Mbit / s) very strongly limits ADSL users. For example, one of the main advantages of ETTH networks - internal resources - is technically possible to implement in ADSL, however, the relatively low upload speed is a serious obstacle for 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 Mbps.

Undoubtedly, ADSL has a future, and its "overclocked" versions will allow you to use fast Internet without hindrance for a couple of years for sure. And what will happen next? Wait and see.

Glossary

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

Symbol- signal state per unit of 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 data block 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 the message of a sequence of characters that allows you to write it more concisely, using the same characters using coding. Redundancy increases the reliability of information transmission.