How does the speed of the Internet depend on the modem. Basics of aDSL technology Telephone line capacity

ADSL technology

What lies behind this mysterious word:

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

How does ADSL work?

A phone or a regular 14.4 kbps modem 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, subject to weak interference, allows you to increase the speed to 33.6 kbps c. On the so-called digital exchanges, where the analog telephone signal is converted into a digital stream at the telephone exchange or node, the speed can be increased to 56.0 kbps. In practice, however, due to the imperfect quality of telephone lines, the real speed is less and rarely exceeds two dozen kilobits per second.
In conventional telephony, the so-called switched 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 the ISP's modem. The telephone channel is busy transmitting data, so you can not use the phone at this time.
The ADSL channel uses a higher frequency band. Even the lower limit of this range lies much higher than the frequencies used in the switched 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 digital network (for example, Ethernet LAN) 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 allows reaching a speed of 6 Mbit / s on a line of normal quality!
Unfortunately, not all telephone lines are suitable for an ADSL channel. Before connecting, the line must first be checked. The main obstacles are the twin line and the burglar alarm.
Connecting the ADSL modem directly to the telephone socket (without a splitter) is not recommended: 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 the simplest filters to separate low telephone and high ADSL frequencies. Filters are attached to the ADSL modem and are called splitter and microfilter. A splitter is a special tee, with one end connected to a telephone line, and with the other two to a telephone and a modem. The microfilter is connected at one end to the line, the other to the telephone - useful for connecting parallel telephones.

Modern world we cannot imagine without the Internet and computer networks. High-speed channels have entangled the world in a web - satellites, optical fiber, cables - the nerves and blood vessels of the worldwide information network. Giant speeds, gigantic traffic, high technologies... But at the same time, for many years, high-speed channels with a data transfer rate 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 transmission turned out to be very expensive, having not only a huge implementation cost, 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 Dial Up modems designed for use on analog telephone lines. Yes, and business, especially small ones, did not see the need to lay dedicated channels or install satellite Internet for themselves - expensive and inefficient. What to download at high speeds - news, prices, documents, kilobyte drivers? Over two decades of Dial Up access rules "last mile" - the very site through 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 owning high-speed data transmission channels. So an awkward picture turned out - between cities, countries and continents huge amounts 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, moreover, with a constant disconnectome.
For a long time, the possibilities of Dial Up modems suited many people. This technology, developed at the dawn of the computer age for analog telephone lines, has evolved extremely slowly and slowly - over the past 15 years, the data transfer rate has increased from 14400 Kbps to only 56000 Kbps. For many years it seemed that this speed was enough for almost everything - download an HTML web page, a text document, 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 a few hundred kilobytes - all this did not take much time and did not require high-speed connections. But life has 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 3D graphics accelerators and the explosive increase in the capacity of information storage devices, has also led to a dramatic increase in the volume of information sent. Computer evolution, which followed the principle of "bigger, higher, faster", has led to the fact that programs and files have grown to monstrous sizes. For example, a Word document that has now become a standard 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 from standard 128 Kbps has risen to 192 Kbps, which also significantly affects the size. Yes, compression algorithms that have been significantly improved lately help to some extent, but this is still not a panacea. Driver sizes have recently grown to gigantic sizes, for example, Detonator FX from nVidia takes about 10 megabytes (despite the fact that two years ago they occupied only 2 megabytes), and unified drivers for the nForce platform of the same company are already 25 megabytes, and this the trend captures an increasing number of manufacturers of computer hardware. But the main trouble that causes Dial Up modems to heat up, not giving them a minute of rest, is software patches or patches that correct errors in software. The widespread introduction of rapid development tools has led to the mass release of raw, unoptimized programs. And why optimize the program if the computer hardware is redundant anyway? Why engage in beta testing of the program, if there is an Internet network - just sell the raw program, then look at the list of the most common problems and errors that users themselves will make when contacting support and then release a patch, after it another, third, and so on ad infinitum . Involuntarily, I recall with nostalgia the times when the Internet was the lot of a handful of the elite, and programmers 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 came out much less frequently, but they worked as Swiss Watches. And now, sadly looking at, for example, the fourth (!) Microsoft patch for Windows 2000 with a size of 175 megabytes, you understand that Dial Up access will not drain this lump even in a week, and how much will this patch cost if hourly pay! But there is more 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 information technologies, which are practically not available to dialers.
All these gloomy thoughts lead to the idea that Dial Up Internet access has become obsolete and urgently needs to be replaced. What can replace moribund technologies? The classic ISDN (Integrated Services Digital Network) and the relatively new satellite Internet immediately come to mind. They come at once, but after much thought, 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 + payment for traffic). In principle, this type of access is possible when laying a home network, when several users share a high-speed channel for themselves, and then distribute it over apartment building through the local network. But as the further material of the article will show, ISDN has a powerful competitor, nullifying 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 see if the satellite of the provider providing this service in your area is visible and at what angle it is visible, it depends on what size 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). Data transfer from the user to the provider is carried out by phone, so the phone line is just 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 are, to put it mildly, 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 put a replacement into the same orbit), and subjective - remember the fiasco of the NTV + satellite Internet, 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 leased lines, but directly on a telephone copper cable. After all, a subscriber telephone line is nothing more than a cable for the network. Yes, the quality is terrible, but you can develop new data transfer technologies, convert everything to digital, modulate everything in a special way, correct errors that occur and get a broadband digital channel as a result. So it turns out that all hope for progress. And the dreams and hopes turned out to be not at all fruitless - a holy place does not happen empty, and progress does not stand still - they received 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.

ADSL - what is it?

Let's start with the name: ADSL stands for Asymmetric Digital Subscriber Line.
This standard is included in a whole group of high-speed data transfer technologies, under the general name xDSL, where x is the letter characterizing the channel speed, and DSL is the abbreviation we already know Digital Subscriber Line - a digital subscriber line. For the first time the name DSL sounded back in 1989, it was then that the very idea of ​​​​digital communications first arose 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 require the use of special cables, while ADSL can operate on a conventional copper pair, which is widely used in telephone networks. The development of ADSL technology began in the early 1990s. 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, to bury the copper subscriber line consisting of two wires to us my opinion is still premature. Its cross section is quite sufficient to ensure the passage of digital information over fairly considerable distances. Just imagine how many millions of kilometers of such a wire have been laid all over the Earth since the appearance of the first telephones! Yes, no one has canceled 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 DSL high technologies adapted to a copper pair on a subscriber telephone line, it has become possible to use these millions of kilometers of analog lines to organize cost-effective high-speed data transmission from a provider that owns a thick digital channel to the end user. The wire, once intended solely for providing analog telephone communications, with a flick of the wrist turns into a broadband digital channel, while retaining its original duties, since ADSL modem owners can use the subscriber line for traditional telephone communications at the same time as transferring digital information. This is achieved due to the fact that when using ADSL technology on a subscriber line to organize high-speed data transmission, information is transmitted in the form of digital signals with a much higher frequency modulation than that usually used for traditional analog telephone communications, which significantly expands the communication capabilities of existing telephone lines.

ADSL - how does it all work?

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

Data exchange between ADSL modems takes place on three widely spaced frequency modulations.

As can be seen from the figure, voice frequencies (1) are not involved in the reception / transmission of data at all, and are used exclusively for telephone communications. The data reception band (3) is clearly demarcated from the transmit band (2). Thus, three information channels are organized on each telephone line - an outgoing data transfer stream, an incoming data transfer stream and a conventional telephone communication channel. ADSL technology reserves a 4 kHz bandwidth 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 in the event of a power outage, telephone communication will not disappear anywhere, as it happens when using ISDN on a dedicated channel, which, of course, is an 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 improve the reliability of telephone communications, special filters are installed that extremely effectively separate the analog and digital components of the connection from each other, while not excluding joint simultaneous operation on one pair of wires.
ADSL technology is asymmetric, as are 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 uploads more information than transmits. Both the transmit and receive speeds of ADSL technology are significantly faster 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 an ordinary telephone cable, which is as ideal 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 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 subdivided into several more frequency bands - the so-called bandwidth splitting method, which allows you to transmit several signals on 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 reception and transmission ranges are divided into many low-speed channels, which provide data reception / transmission in parallel mode.
Oddly enough, but when considering the method of dividing the bandwidth, such a widespread class of programs as the Download manager comes to mind as an analogy - they use the method of splitting them into parts and simultaneously downloading all these parts to download files, which allows you to more efficiently 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 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 you to compress / decompress large amounts of information on the fly - this is one of the secrets of the speeds shown by ADSL.
Thirdly, ADSL uses an order of magnitude larger frequency range compared to ISDN, which allows you to create a much larger number of parallel information transmission channels. For ISDN technology, a frequency range of 100 kHz is standard, while ADSL uses a frequency range of about 1.5 MHz. Of course, long-distance telephone lines, especially domestic ones, weaken the receive / transmit 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 it still continues to be confidently 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 pick up a digital signal in the mess 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 anyway, compared to competitors, the cost of ADSL hardware for end users is much lower.

How fast is ASDL technology?

Everything is known in comparison, it is impossible to evaluate the speed of technology without comparing it with others. But before that, you need to take into account a few 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 considered. Another feature of ADSL technology is the use of high-frequency signal modulation and the use of several lower-speed channels lying in the same field of receive and transmit frequencies for the 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 frequency spectrum used narrows, the maximum number of parallel channels decreases, and the speed decreases accordingly. The table shows the change in the bandwidth of the channels for receiving and transmitting data when the distance to the provider changes.

In addition to the distance, the data transfer rate is greatly affected 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 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 in the range of 5 kilometers. However, with the improvement of telephone lines, the speed of ADSL will also increase.

to be continued...

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For comparison, consider other technologies.

Dial up modems, as you know, are limited to a 56Kbps data rate limit, a rate that I, for example, have never found 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 speed of sending data is 33.6 Kbps.
The maximum speed of ISDN in dual-channel mode is 128 Kbps, or as it is not difficult to calculate, 64 Kbps per channel. If the user calls 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. 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, the narrower the channel for each user. 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.
Finally, dedicated digital lines E1 and E3 can show data rates, in synchronous mode, of 2 Mbps and 34 Mbps, respectively. The indicators are very good, but the prices for wiring and maintenance of these lines are exorbitant.

Glossary.

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

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

digital signal- digital signal, in contrast to analog intermittent (discrete), the signal value 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, the digital transmission of information uses a binary code, the most common among computers. A digital signal, unlike an analog signal, 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 corrupted 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 the 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 is highly dependent 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 any terms in the article are not clear to you, write, the glossary will be expanded.

ADSL Technology (by Jeff Newman)
ADSL (Asymmetric Digital Subscriber Line) technology is a type of xDSL technology that provides users with an affordable broadband transmission medium between relatively close network nodes.
ADSL research and development was spurred on by investments from telephone companies, which, unlike conventional broadcast television, wanted to provide 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 many other high-speed interactive applications, such as Internet access, delivery of corporate information to remote offices and branches, and audio and video information on demand. At best conditions operation and acceptable distances using ADSL technology, you can transfer data at speeds up to 6 Mbps in the forward direction (according to some versions, up to 9 Mbps) and 1 Mbps in the reverse direction.

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

Network Computing magazine employees 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 benefits of their work 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 of any technology decreases as it is implemented, it makes sense to start negotiations with telephone companies now.

Additional wiring is not needed.

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

Signals are transmitted over a pair of wires between two ADSL modems installed at a remote network node and at a local PBX. An ADSL network modem converts digital data from a computer or other device into an analog signal suitable for transmission over twisted pair. For parity, 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 necessary to bring the signal to the telephone exchange. For example, if branch offices are located within a small town, pairs of wires are used between them. In this case, the "remote" ADSL modem operating in receive mode and the "central" transmitting ADSL modem can be connected by a copper wire without any additional intermediate elements between them. The connection of offices spaced over long distances from one another, provided that each of them is located relatively close to "its" automatic telephone exchange, 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 at the same time. We were able to select the best transmission frequency for each specific application (for data, speech and video). Depending on the encoding method used in a particular implementation of ADSL, the signal quality is affected by the length of the connection and electromagnetic interference.

With the combined use of a data line 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 using the 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 with a frequency splitter 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, because in the event of an accident it does not have any effect on the operation of telephony. ADSL seems like a fairly rudimentary technology, and in fact it is. Installing and running it is not difficult. Simply connect the device to the network and telephone 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 some of the physical factors inherent in signaling over a pair of wires. The most important of these is line attenuation. In addition, the reliability and throughput of the data transmission channel can be affected by significant electromagnetic interference on the cable, especially from the network of the telephone company itself.

Line coding types

AT ADSL modems three types of linear coding, or modulation, are used: discrete multitone modulation (Discrete Multitone - DMT), amplitude-phase modulation without a carrier (Carrierless Amplitude / Phase - CAP) and rarely used quadrature amplitude modulation (Quadrature Amplitude Modulation - QAM). Modulation is required for connection establishment, signaling between two ADSL modems, rate negotiation, channel identification, and error correction.

DMT modulation is considered the best, as 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 ADSL line coding.

However, many do not agree that DMT modulation is better than CAP, so we decided to test both of them. And although the modems used in our tests were early implementations, they all worked perfectly. As a result, we were convinced of the following: DMT-based ADSL modems are indeed more stable during 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 line coding method between modems (for example, from your office to the service provider's PBX). If these devices are used in packet-switched networks, such as the Internet, it is not your business to worry about possible conflicts between network nodes.

For testing, we used a copper pair with 24 gauge wire, which has a signal attenuation of 2-3 dB per 300 m. According to the specification, the length of the ADSL line should not exceed 3.7 km (attenuation is 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). DMT-based ADSL modems operated at the maximum distance possible in our conditions - 5.5 km - at speeds of 791 Kbps in the forward direction and 582 Kbps in the reverse direction (measured signal attenuation in the line is 31 dB).

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

In addition to the ones just described, we conducted 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 spur bridge is an open telephone line that branches away from the main line. As a rule, this additional line is not used and, therefore, does not create additional crosstalk in the main line, but significantly increases the attenuation in it. Therefore, it is surprising that some of the modems tested worked fine with a spur 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 acceptable level only if the branch length is increased to 300 m.

Electromagnetic interference

Electromagnetic interference at the near and far ends (Near-End Crosstalk - NEXT; Far-End Crosstalk - FEXT) lines are forms of electromagnetic interference that distort the signal in the ADSL channel and thus adversely affect its decoding. This type of interference can occur at either end of the connection if there is any line carrying spurious signals near the ADSL line, 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 ADSL data stream was minimal, and the signaling quality of the ADSL and T1 lines was not degraded. 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 lines, the telephone company must take into account this line interference.

Another interference that occurs during signal transmission over an ADSL line is amplitude modulation noise (Amplitude Modulation - AM). It is similar to the noise that occurs on a line passing near powerful electrical appliances such as refrigerators and laser printers, or near powerful motors installed in an elevator shaft. The MCI engineers testing the modems applied up to 5 volts of pulsed voltage to the twisted-pair cable running parallel to our ADSL line, but the bit error rate remained at acceptable level. In fact, such an impact on modems in our tests could be neglected.

In our opinion, about a year is left before the widespread introduction of ADSL technology in public networks. It is currently under development and the possibility of its application is being evaluated. However, ADSL technology is already being used in the networks of corporations and small towns. Many firms have begun to produce products for ADSL. The high bandwidth and noise immunity of the first versions of ADSL modems that participated in our tests confirmed their high reliability. Now, when you modernize your network and increase the number of users, you can no longer neglect ADSL technology.

What is ADSL (another article)
ADSL (Asymmetric Digital Subscriber Line) is one of the high-speed data transfer technologies known as DSL (Digital Subscriber Line) technologies and collectively referred to as xDSL.
The name DSL technologies originated in 1989, when the idea first appeared to use analog-to-digital conversion at the subscriber's end of the line, 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 allows you to turn 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). In this case, three information channels are organized - a "downward" data transfer stream, an "upward" data transfer stream and a conventional telephone communication channel. The telephone communication channel is allocated with the help of filters, which guarantees the operation of your phone even if the ADSL connection fails.
ADSL is an asymmetric technology - the speed of the "downstream" data stream (that is, the data that is transmitted towards the end user) is higher than the rate of the "upstream" data stream (in turn transmitted from the user to the network side).
ADSL technology uses digital signal processing and specially designed algorithms, advanced analog filters and analog-to-digital converters to compress the large amount of information transmitted over twisted-pair telephone wires.
ADSL technology uses a method of dividing the bandwidth of a copper telephone line into multiple frequency bands (also called carriers). This allows multiple signals to be transmitted simultaneously on a single line. With 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 signal transmission and fax transmission. And all this without interrupting the normal telephone connection, which uses the same telephone line.
Factors affecting the data transfer rate are the condition of the subscriber line (ie, the diameter of the wires, the presence of cable outlets, etc.) and its length. The signal attenuation in the line increases with increasing line length and 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 speeds up to 8 Mbps and upstream speeds 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, let's look at the benefits of ADSL.

First of all, high data transfer rate.
You do not need to dial a phone number to connect to the Internet or a data network. ADSL creates a broadband data link using an already existing telephone line. After installing ADSL modems, you get a permanently established connection. The high-speed data link is always ready to go - whenever you need it.
ADSL technology allows full use of line resources. Conventional telephony uses about one hundredth of the capacity of a telephone line. ADSL technology eliminates this "flaw" and uses the remaining 99% for high-speed data transmission. In this case, different frequency bands are used for different functions. For telephone (voice) communication, the lowest frequency region of the entire line bandwidth (up to approximately 4 kHz) is used, and the rest of the band is used for high-speed data transmission.
ADSL opens up completely new possibilities in those areas in which it is necessary to transmit a high-quality video signal in real time. These include, for example, videoconferencing, distance learning and video-on-demand. ADSL technology allows you to provide services that are more than 100 times faster than the fastest analog modem (56 Kbps) and more than 70 times faster than ISDN (128 Kbps).
We should not forget about the costs. ADSL technology is efficient from an economic point of view, if only because it does not require the laying of special cables, but uses existing two-wire copper telephone lines. That is, if you have a connected telephone at your home or 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 Upper Volga branch of Centrotelecom.

ADSL and SDSL

Asymmetric and balanced DSL lines

Private users limited by 56.6Kbps dial-up connectivity want access to broadband applications and commercial organizations, with their expensive T-1/E-1 Internet connections, would like to keep their costs down. The best of technology allows you to solve problems with existing equipment. Where possible, you should switch to digital subscriber lines (Digital Subscriber Line, DSL).

DSL technology allows the user's premises to be connected to the central office (Central Office, CO) of the service provider via pre-existing copper telephone lines. If the lines meet the established requirements, then with the help of DSL modems the transfer rate can be increased from the mentioned 56.6 Kbps to 1.54 Mbps or more. However, the main disadvantage of DSL lines is that the ability to use them depends largely on the distance to the service provider's node.

DSL is not one technology for all occasions, it has many varieties, although some of them may not be available in a particular area. DSL variants usually follow one of two basic schemes, although they may differ in specific characteristics. Two main models - asymmetric (Asymmetric DSL, ADSL) and symmetrical (Symmetric DSL, SDSL) digital subscriber line - stood out in the early stages of technology development. In the asymmetric model, data flow is preferred in the forward direction (from the provider to the subscriber), while in the symmetric model, the flow rate in both directions is the same.

Private users 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 technology is actively penetrating the high-speed connection market 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 rates 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. US ADSL costs typically range from $40 to $200 per month, depending on expected data rates and service level guarantees. Cable modem-based 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 transmits data at frequencies from 26 to 1100 kHz, while the same copper cable can transmit analog voice in the range from 0 to 3.4 kHz. Symmetric DSL (SDSL) occupies the entire bandwidth of the data line and is not compatible with analog voice signals.

The carrier line is able to support ADSL along with analog voice by allocating digital signals to frequencies outside the frequency spectrum for conventional telephone signals (see Figure 1), which requires the installation of a divider. The divider uses a low-pass filter to separate 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 maximum ADSL speeds, splitters must be installed both at the user's premises and at the central site; they do not require power and therefore will not interfere with "life-saving" voice service in the event of a power loss.

Determining ADSL speeds is more of an art than a science, although speed reductions occur at fairly predictable intervals. Providers provide the best possible service, with results highly dependent on distance from the central hub. Usually "best possible" means that the ISPs guarantee 50% throughput. Attenuation and interference such as crosstalk become significant on links longer than 3 km, and at distances greater than 5.5 km they can make the lines unsuitable for data transmission.

At distances up to 3.5 km from the central node, ADSL speeds can reach 7.1 Mbps in the forward direction 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, "Actually, 7.1 Mbps is impossible to achieve, even under laboratory conditions." 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 significantly - up to 384 Kbps in the forward direction of the flow and up to 128 Kbps - in the reverse direction.

Service contracts for ADSL services may contain a clause for the user to opt out of connecting to home networks or Web servers. However, DSL technology alone does not prevent home connectivity. local networks. 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 sufficient 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 for splitting the spectrum of available frequencies into 256 channels in the range from 26 to 1100 kHz, 4.3125 kHz each.

CONNECTING THE COPPER LINE TO THE ATU-R

So, we have a central site, a copper twisted pair cable 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 a remote device for any DSL service. In addition to providing DSL modem functionality, some ATU-Rs may perform bridging, routing, and time multiplexing (TDM) functions. On the other side of the copper line, at the central node, is the ADSL Transmission Unit-Central Office (ATU-C), which coordinates the link from the CO side.

A DSL provider multiplexes a plurality of DSL subscriber lines into one high-speed backbone network using a DSL Access Multiplexer (DSLAM). While at the central site, the DSLAM aggregates data traffic from multiple DSL lines and feeds it to the service provider's backbone, and the backbone already delivers it to all destinations in the network. Typically, the DSLAM connects to the ATM network through 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's premises.

The bandwidth 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 being introduced by remote control.

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

The ITU G.992.1 recommendations, also known as G.dmt, were first published in 1999, along with G992.2, or G.lite. G.lite equipment entered the market in 1999 and cost less 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 hard for service providers to justify spending hundreds of dollars on a single fixed connection with a $49 subscription fee, so any cost-reducing modification is greeted with extreme enthusiasm by the market.

DSL FOR BUSINESS

Businesses have very different needs than home users, so a balanced SDSL line becomes the natural choice for office applications.

Corporate bandwidth for data flow in the reverse direction can quickly become exhausted due to heavy traffic of the Web server and the transfer of large volumes of PDF by employees, PowerPoint presentations and other documents. Outgoing traffic can equal or even exceed incoming traffic. Providing speeds in the order of 1.5 Mbps in North America and 2.048 Mbps in Europe in both directions, ADSL links resemble T-1/E-1 connections, the dominant architectural component corporate networks worldwide.

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

As a viable alternative to T-1/E-1, SDSL has attracted the attention of Competitive Local Exchange Carriers (CLEC) as a means of providing additional services. In general, SDSL services typically distribute CLECs, however, ILECs typically use HDSL to implement the T-1 service. Under optimal conditions, SDSL can compete with T-1/E-1 in terms of data transfer speeds and has three times the speeds of ISDN (128 Kbps) at maximum distances. Figure 2 shows speed versus distance for SDSL: the longer the distance, the slower 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 lines are better suited to the needs of organizations than ADSL to the needs of residential users. While cable modem service providers lure private users with lower prices than ADSL, SDSL offers the same transmission 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 for the equivalent SDSL range, $170 to $450. The lower the cost of SDSL services, the lower the guaranteed data rate.

LET'S CLEAR

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

Crosstalk. Radiated by bundles of wires converging at the central node of the service provider Electric Energy generates interference known as near-end crosstalk (NEXT). When signals move between channels of different cables, the capacitance of the line drops. "Near end" means that the interference comes from an adjacent pair of cables in the same area.

Separating DSL and T-1/E-1 lines greatly reduces Negative influence crosstalk, but there is no guarantee that the service provider will decide to apply this particular implementation principle.

EXT has a counterpart, Far-End Crosstalk (FEXT), which is sourced from another pair of cables at the far end of the line. As far as DSL is concerned, the degree of influence on such lines of FEXT is significantly lower than that of NEXT.

Linear attenuation. The signal strength drops as it propagates through the 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-resistance wiring can minimize signal attenuation, but any particular provider may find the required expense unjustified. Thick wires have less resistance than thin wires, but they are more expensive. The most popular cables are 24 gauge (approx. 0.5 mm) and 26 gauge (approx. 0.4 mm); the lower damping of the caliber 24 makes it suitable for longer ranges.

load inductors. At a time when the public switched telephone network (PSTN) carried only voice calls, inductors helped increase the length of telephone lines - a very laudable goal. The problem today is that they have a negative effect on the functioning of the DSL.

The fact that load inductors cut frequencies above 3.4 kHz to improve voice band transmission 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 sections.

Shunted branches. If the telephone company is not going to completely turn off an unused section of wiring, they shorten it by installing a bypass. This practice did not particularly bother anyone until the rapid growth in demand for DSL began. Shunts greatly affect the suitability of a DSL link and often just need to be removed so that a DSL link 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 communication impossible. To disable the echo canceller, modems can send a 2.1 kHz response signal at the start of a connection.

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

SPEECH OVERLAY

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.

The savvy user will prefer to have an auto-switching redundancy in the form of standard V.90 or ISDN switched technology, if possible, even when they finally get DSL service. DSL lines may occasionally become unusable.

Choosing based solely on price can end up being disappointing. 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 as you, as is the case with cable modems, which is certainly 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 the copper "twisted pair" would be the most tenacious of them. " last mile» is a large and growing market, particularly sensitive to affordable technologies with high supported bandwidth.

Free, unlimited, broadband access for everyone is not possible in our lives, but if you are going to purchase DSL services then 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 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 watch IP-TV and have an Internet connection at the same time, then 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 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 at 062 (this is done right away!).

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 transfer rates up to 8 Mbps in the direction of the user, and up to 1.544 Mbps in the direction away from the user.

G.lite is a modulation based on DMT technology, which provides data transfer rates up to 1.5 Mbps towards the user, and up to 384 Kbps in the direction away from the user. "

ADSL - modulation provides a data transfer rate in the direction of the user up to 8 Mbps, and in the direction from the user up to 768 Kbps.

T1.413 is a discrete asymmetric multitone 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 might have seemed to many that ADSL technology was changing the world. Makes available fantastic speeds hitherto unseen by 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 around 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 severely limited by the speed capabilities of xDSL networks, and in Russia ETTH is already available in all major cities. Thus, we sort 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. You have to, 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 in the Internet provider 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 speed data transmission over existing telephone lines. Despite the fact that ADSL is far from being the “fastest” technology from the xDSL family, it is precisely this technology that has become most widespread in the world due to the optimal combination of speed and range.

The ADSL channel is asymmetric, 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, this is a modem, and on the provider side, it is a DSLAM (ADSL switch).

While only three versions of ADSL (ADSL, ADSL2, and ADSL2+) are widely known, there are actually many more specifications. I propose to take a look at the table, which presents all the major ADSL standards. By and large, the specifications differ in operating frequencies and are needed to enable the operation of ADSL technology on various types of telephone lines. For example, Annex A uses a frequency band ranging from 25 kHz to 1107 kHz, while Annex B's operating frequencies start at 149 kHz. The first was designed to transmit data over public telephone networks (PSTN or POTS, in English), and the second was designed 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: an improved modulation mechanism, reduced overhead in transmitted frames, more efficient coding, reduced initialization time, and improved DSP performance. Let's take it in order.

As is known, ADSL uses Quadrature Amplitude Modulation (QAM) with Orthogonal Frequency Multiplexing (OFDM). 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 through each of the channels, which is modulated using QAM; further, the signals are multiplexed using the 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 various 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 processing of modulation and coding algorithms made it possible to more efficient work on the same communication lines.

To improve performance over long distances, the developers also reduced the redundancy, which was previously fixed at 32 kbps. Now this value can vary depending on the state of the physical medium from 4 to 32 kbps. And although this is not so critical at high speeds, at great 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 made it possible to increase the speed by only 1.5 times. What did they come up with in ADSL2+ to increase the throughput of the direct channel (downlink) by 2 times compared to ADSL2 and 3 times compared to ADSL? Everything is trite 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 possibility of combining ports (port bonding). Thus, by combining two lines into one logical channel, you will get a throughput of 48/7 Mbps. This, of course, is a rarity, but if there are two telephone numbers in the apartment, this is quite real. Or, alternatively, you can get a double increase in speed 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 advantages 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 ADSL speed to the level of cable networks. Purely nominally, both of them are capable of transmitting HD content. But as practice shows, where high-quality ETTH has reached, ADSL and cable companies 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 traffic prices will drop by 3-4 times. And if the speed of the incoming channel (ADSL2+ - 24 Mbps) has a significant margin, then the low speed of the reverse channel (ADSL - 1 Mbps, ADSL2+ - 3.5 Mbps) severely 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 Mbps.

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

Glossary

Modulation– change in the parameters (phase and/or amplitude) of the modulated oscillation (high-frequency) under the influence of a control (low-frequency) signal.
Quadrature Amplitude Modulation (QAM) - with this type of modulation, information in the signal is encoded by changing both its phase and amplitude, which allows you 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 a frame, containing service information and data, and ending with a sequence indicating the end of a 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 encoding. Redundancy increases the reliability of information transmission.