What is ADSL? description/uses/characteristics/parameters/functions/ADSL standards/history/advantages/disadvantages/speed table

The ADSL (from the abbreviations in  Asymmetric Digital Subscriber List that translates Asymmetric Digital Subscriber Line) is type of technology of digital data transmission and Internet access , which consists of the transmission through symmetrical pairs of telephone line copper. This is, a method of accessing the Internet through the telephone line (Telephone Network Switched, PSTN) that does not prevent the regular use of the line for calls.

The ADSL is a type of broadband connection , whose name comes from the fact that the capacity of download (from the network to the computer) and load (from the computer to the network) are not coincident, but the first is greater than the second. After all, most Internet users receive more information than they emit.

It can serve you: FTP

For the ADSL to work, it is necessary to have a telephone line and a modulation system that separates the frequencies of the voice and data , by installing discriminating filters (called splitters , micro filters or DSL filters) and an ADSL router. provided by the company that provides the service.

This type of copper urban wiring was traditionally implemented, but in the final decades of the 20th century it was gradually replaced by fiberglass, a material that offered better driving and performance, thus accommodating even better methods of digital information transmission, such as the cable modem or the Ethernet.

Picture in words of ADSL

Frequencies used in ADSL: the red area is the one used for voice in the switched telephone network (PSTN), the green one is the one used for data upload ( upstream ) and the blue one is the one used for data download ( downstream ) .

ADSL is a technology access to Internet of broadband , which implies a speed higher than a wired modem in data transfer because the modem uses voice band and thus prevents voice service while use and vice versa. This is achieved by modulating the data signals in a higher frequency band than that used in conventional telephone conversations (300 to 3400 Hz ), a function performed by the ADSL router . To avoid distortions in the transmitted signals, you need to install a filter (discriminator, DSL filter ) which is responsible for separating the conventional telephone signal from the modulated signals of the connection through ADSL.

This technology is called “asymmetric” because the download capabilities (from the network to the user) and data upload (in reverse) do not match. ADSL technology is designed so that the download or download capacity is greater than the upload capacity, which corresponds to the use of the Internet by most end users, who receive more information than they send (or download more of what goes up).

See also: Difference between DSL and ADSL with table

In an ADSL line, three communication channels are established :

  1. data sending channel,
  2. data reception channel, and,
  3. normal telephone service channel.

Obtaining these three channels can be done using two techniques: The first consists of using FDM to assign a band to both the upstream and downstream data channel, these two bands are in turn divided into sub channels using TDM, these channels can be high and low speed. The second technique is Echo Cancellation, used in very specific analog modems such as V.32 and V.34 ITU-T.

Telephone companies implement improved versions of this technology, such as ADSL2 and ADSL2 + , with the capacity to supply high-quality television and video through the telephone pair . It involves tough competition between telephone companies and cable operators , and the appearance of integrated voice , data and television offers , from the same line and within a company or several, that offers these three communication services through the same medium. : Triple play . The use of higher bandwidth for these services further limits the distance that the pair of wires can operate.

ADSL2 and ADSL2 + incorporate advanced physical resource management and modulation mechanisms, so that they not only increase the capacity of conventional ADSL from 8 Mbit / s to 12 and 24 Mbit / s respectively, but also introduce improvements to avoid interference or noise, and reduce the effects of attenuation, hence distances of up to 9 km citation needed ] .

ADSL is a technology that uses the copper pair and has less bandwidth than other technologies such as cable modem or Metro Ethernet citation required ] , whose urban cabling is made up of fiber optic wires instead of the copper pair implemented in its greatest part in the 1950s and 1960s.

See also: What is the ISP?

What is ADSL for?

As with other digital transmission systems, ADSL serves as a channel for the transmission of digital data and connection to the Internet .

This means that it is a virtually unlimited source of information and online services , with all that this implies. This method of connection was key in the massification of the Internet in homes and urban environments during the 1990s.

Must Read: What is digital television? (TV)

Characteristics of ADSL

In an ADSL connection , three independent transmission lines are established:

  • Data transmission channel . Where the user’s information will be sent to the Internet ( upload or upload).
  • Data reception channel . Where the information will be received from the Internet ( download or download). It will be the widest of the three channels.
  • Regular telephone service channel . Where the telephone calls will be transmitted.

This separation allows a differentiated handling of data and transmission pulses, and can even be improved in the versions known as ADSL2 and ADSL2 +, in which a channel for the transmission of subscription television (cable TV) and video is also received. of high quality (HD).

Read: Difference Between DSL Modem And Cable Modem In Tabular Form

Advantages of ADSL

ADSL: Each subscriber receives an independent and non-transferable service directly to their home.
  • It does not occupy the telephone line . This represents an advantage when installing the service in homes and companies, since it allows the autonomy of both services, something impossible with the dialing method.
  • Use the traditional infrastructure . It does not require new and extensive investments or large works, because it takes advantage of traditional copper wiring.
  • It exceeds the dial-up connection . It offers much better speed / price ratio than the dialing methods, which in addition to occupying the telephone line are usually at 56 kps, while the ADSL can reach 4mbps (70 times faster).
  • It allows central and customized circuits . Since it uses telephone cables, each subscriber receives an independent and non-transferable service directly to his home or office.

Disadvantages of ADSL

  • It does not operate on any telephone line . It is not enough to have a copper wire telephone connection to use the ADSL, since this technology has a limit of 3km of line length from the emission center.
  • It demands quality of copper . In countries where this material is expensive or scarce, the service may not be as economical as in others.
  • It depends on external factors . The quality of the service is subject to the quality of the cables, the distance from the power station and fluctuations in the service flow.
  • It is very inferior to optical fiber . The fiber optic connections far exceed the ADSL stability and speed.

Must Read: Difference between modem and router with table

Development history of ADSL

The history of the development of ADSL technology starts in the second half of the eighties, when the search for technology was carried out to provide interactive television . Bellcore is a pioneer of the xDSL family of technologies . In 1987, it introduced the specification of the first technology from the xDSL family and launched it on the US telephone networks . The company soon fell apart, and the technology lay low.

In the mid -1990s, the xDSL family was supplemented by an asymmetric modification of the digital subscriber line – ADSL. In the following years, chipsets were created and improved to carry out data transmission via ADSL. The pace of development was slow because DSL was originally designed for video-on-demand systems. The systems themselves did not get widespread, and the ADSL technology got a second wind thanks to the development of Internet networks .

With the advent of the first ADSL modems, providers saw the promise of this technology and began to use it to provide access to the network. For the need to install equipment on each PBX, the costs of building and maintaining the network were noticeably higher than in the case of classic dial-up access, when all the provider’s modems were installed on one PBX , however, compared to the cost of other methods of providing high-speed Internet access, DSL technology turned out to be very cheap.

In large cities, ADSL is being supplanted by faster access technologies Ethernet ( ETTH ), GPON ( FTTH ) and DOCSIS (data over cable television standard). The reason for this is the limited bandwidth in ADSL networks (up to 24 Mbit / s in ADSL2 +), especially the “upstream” from the subscriber (up to 1.4 Mbit / s), while EuroDOCSIS 2.0 provides a data transfer rate of 50 ↓ / 27 ↑ Mbit / s, Fast Ethernet  – up to 100 Mbit / s, Gigabit Ethernet  – up to 1 Gbit / s, 10 Gbit / s EPON – up to 10 Gbit / s.

Despite the emergence of faster data transmission methods, ADSL technology is still the leader in the broadband data transmission market. In a number of European countries, ADSL is the de facto standard for providing the population with a fairly fast and inexpensive Internet. So, in Finland, where every inhabitant of the country has been guaranteed access to the Internet since June 2010, most houses are connected using ADSL technology , and British Telecom has provided the ability to connect services using ADSL technology to 99% of UK buildings .

How ADSL works?

ADSL technology is a variant of DSL in which the available bandwidth of the channel is asymmetrically distributed between outgoing and incoming traffic – for most users, incoming traffic is much more significant than outgoing traffic, so providing most of the bandwidth for it is quite justified (the exceptions to the rule are peer-to-peer networks , video calls and emailwhere the volume and speed of outgoing traffic are important). An ordinary telephone line uses the frequency band 0.3 … 3.4 kHz for voice transmission. In order not to interfere with the use of the telephone network for its intended purpose, the lower limit of the frequency range in ADSL is at the level of 26 kHz. The upper limit, based on the requirements for the data transfer rate and the capabilities of the telephone cable, is 1.1 MHz. This bandwidth is divided into two parts – frequencies from 26 kHz to 138 kHz are allocated to the outgoing data stream, and frequencies from 138 kHz to 1.1 MHz are allocated to the incoming data stream. The frequency band from 26 kHz to 1.1 MHz was not chosen by chance. In this range, the attenuation coefficient is almost independent of frequency.

This frequency division allows you to talk on the phone without interrupting the communication on the same line. Of course, situations are possible when either the high-frequency signal of the ADSL modem negatively affects the electronics of a modern phone, or the phone, due to any peculiarities of its circuitry, introduces extraneous high-frequency noise into the line or greatly changes its frequency response in the high-frequency region; to combat this directly to the telephone network subscriber in the flat low-pass filter set ( frequency divider , Engl.  Splitter), which passes only the low-frequency component of the signal to ordinary telephones and eliminates the possible influence of telephones on the line. Such filters do not require additional power supply, so the voice channel remains operational when the power supply is disconnected and in the event of a malfunction of the ADSL equipment.

Transmission to the subscriber is carried out at speeds up to 10 Mbit / s, although today there are devices that transmit data at speeds up to 25 Mbit / s ( VDSL ), however, this speed is not defined in the standard. In ADSL systems, 25% of the total speed is allocated for overhead information, in contrast to ADSL2 , where the number of overhead bits in a frame can vary from 5.12% to 25%. The maximum line speed depends on a number of factors such as line length, cross-section and cable resistivity. Also, a significant contribution to the increase in speed is made by the fact that twisted pair is recommended for the ADSL line (not TRP ) and shielded, and if it is a multi-pair cable, then in compliance with the direction and pitch of the twist.

Separation of transmitted and received data

With ADSL, data is transmitted over a common pair in full duplex form. In order to separate the transmitted and received data stream, there are two methods: frequency division multiplexing ( Engl.  Frequency Division Multiplexing , FDM) and echo cancellation ( Engl.  Echo Cancellation , EC)

Frequency division multiplexing

When using this mechanism, the low-speed data channel is located immediately after the frequency band used for the transmission of analog telephony. The high speed channel of received data is located at higher frequencies. The bandwidth depends on the number of bits transmitted by one signal.


  • Echo Cancellation (EC) can improve performance by 2 dB, but is more difficult to implement.
  • The benefits of EC grow with higher speed technologies such as ISDN or 384 kbps video telephony. In these cases, FDM requires the allocation of higher frequencies for the high-speed channel of the received data, which leads to an increase in attenuation and a reduction in the maximum transmission distance.
  • Combining two channels in the same frequency range, when using EC, leads to the appearance of an intrinsic NEXT effect, which is absent when using FDM.
  • The ADSL standard provides for interoperability between different equipment using both the FDM and EC mechanisms, the choice of a specific mechanism is determined when the connection is established.

Communication line parameters

The subscriber telephone line , when used for ADSL technology, must have the following parameters:

Primary parameters

  • Loop resistance – no more than 900 Ohm
  • Insulation resistance between residential and ground – more than 100 megohms, between conductors – not less than 100 megohms
  • Loop capacity – no more than 300 nF
  • Capacitive asymmetry – no more than 10 nF, or no more than 5%
  • Operation with an increase in the loop and low isolation is possible on condition of high-quality station equipment at a reduced data transfer rate.

Secondary parameters

Attenuation (Line Attenuation):

  • up to 20 dB is an excellent line;
  • from 20 dB to 40 dB – working line;
  • from 40 dB to 50 dB – malfunctions are possible;
  • from 50 dB to 60 dB – synchronization periodically disappears;
  • from 60 dB and more – the equipment will not work.

The following are the norms for operating attenuation and a chain length of 1 km:

  • ADSL and ADSL2 systems (maximum frequency 1.024 MHz)
    • CCI cable (d0 = 0.4 mm) ≤27 dB
    • CCI cable (d0 = 0.5 mm) ≤21 dB
  • ADSL2 + system (maximum frequency 2.048 MHz)
    • CCI cable (d0 = 0.4 mm) ≤35.5 dB
    • CCI cable (d0 = 0.5 mm) ≤28.5 dB
  • VDSL system (maximum frequency 4.096 MHz)
    • Twisted pair cable (d0 = 0.5 mm) ≤42.5 dB
  • VDSL2 system (maximum frequency 12 MHz)
    • Twisted pair cable (d0 = 0.5 mm) ≤60.3 dB

Noise Level (RMS Noise Energy (dB re 1 mW @ 600 ohm load)):

  • -65 dBm to -51 dBm is an excellent line;
  • -50 dBm to -36 dBm is a good line;
  • from -35 dBm to -20 dBm – bad line;
  • from −20 dBm and more – the equipment cannot operate.

S / N ratio (Signal-to-Noise Ratio ( SNR ), noise margin (Noise Margin)):

  • 6 dB and below – bad line, there are synchronization problems;
  • 7-10 dB – failures are possible;
  • 11-20 dB is a good line, no problems with synchronization;
  • 20-28 dB is a very good line;
  • 29 dB and above is an excellent line.

For ADSL lines, it is recommended to use twisted pair (and not ” noodles “), otherwise the bandwidth of the data transmission channel is reduced.

ADSL standards

Name of the standard Common name Downstream speed, Mbps Upstream speed, Mbps Approved in
ANSI T1.413-1998 Issue 2 ADSL 8.160 Mbps 1.216 Mbps 1998
ITU G.992.1 ADSL ( G.DMT ) 8 Mbps 1.3 Mbps 1999-07
ITU G.992.1 Annex A ADSL over POTS 12 Mbps 1.3 Mbps
ITU G.992.1 Annex B ADSL over ISDN 12 Mbps 1.3 Mbps
ITU G.992.2 ADSL Lite ( G.Lite ) 1.5 Mbps 0.5 Mbps 1999-07
ITU G.992.3 ADSL2 12 Mbps 1.216 Mbps 2002-07
ITU G.992.3 Annex A ADSL2 over POTS 12 Mbps 1.216 Mbps
ITU G.992.3 Annex B ADSL2 over ISDN 12 Mbps 1.216 Mbps
ITU G.992.3 Annex J ADSL2 12 Mbps 3.5 Mbps
ITU G.992.3 Annex L RE-ADSL2 5 Mbps 0.8 Mbps
ITU G.992.3 Annex M ADSL2 ( ) 12 Mbps 3.5 Mbps
ITU G.992.4 Splitterless ADSL2 1.5 Mbps 0.5 Mbps 2002-07
ITU G.992.5 ADSL2 + 24 Mbps 1.216 Mbps 2003-05
ITU G.992.5 Annex A ADSL2 + over POTS 24 Mbps 1.216 Mbps
ITU G.992.5 Annex B ADSL2 + over ISDN 24 Mbps 1.216 Mbps
ITU G.992.5 Annex M ADSL2 + 24 Mbps 3.5 Mbps
ITU G.992.5 Annex L RE-ADSL2 + 24 Mbps 1.5 Mbps

ADSL equipment

  • ADSL modem
  • ADSL filter

Comparative table of speeds

Name Common name Maximum reception
(Mbit / s)
Maximum transmission
(Mbit / s)
ANSI T1.413-1998 Issue 2 ADSL 8 1.0
ITU G.992.1 ADSL (G.DMT) 8 1.3
ITU G.992.1 Annex A ADSL over POTS 8 1.3
ITU G.992.1 Annex B ADSL over ISDN 8 1.8
ITU G.992.2 ADSL Lite (G.Lite) 1.5 0.5
ITU G.992.3 ADSL2 12 1.0
ITU G.992.3 Annex J ADSL2 13 3.15
ITU G.992.3 Annex L RE-ADSL2 5 0.8
ITU G.992.4 splitterless ADSL2 1.5 0.5
ITU G.992.5 ADSL2 + 24 1.0
ITU G.992.5 Annex M ADSL2 + M 24 3.5

What is VDSL?

The VDSL or VHDSL (of the abbreviations in English Very High-bit-rate Digital Suscriber Line or Line of Subscriber Digital of very high rate of Transfer) is a type of connection to Internet by Broadband of very superior performance to the ADSL , representing its evolution or superior step.

This technology consists of the use of four channels for the transmission of data : two for the download and two for the load, substantially increasing the transmission power of the service.

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