There are several technologies that provide access to the internet. One of the best known is ADSL. Acronym for Assymmetric Digital Subscriber Line (see abbreviationfinder), this standard is quite popular because it takes advantage of the fixed telephony infrastructure, allowing fast connections at relatively low prices. In this text, you will know what ADSL is, understand how this standard works, as well as briefly learn about its variations, such as ADSL2, ADSL2 + and VDSL.
What is ADSL?
When the internet started to become popular, most people used dial-up connections, known as ” dial-up connections” in Brazil. For that, it was necessary to connect the computer to a modem and this, in turn, to a telephone line. Then, the user had to use a specific program to dial a provider’s number in order to establish the connection.
The problem is that dial-up connections offer many disadvantages: they are slow – by default, support up to 56 Kb / s (kilobits per second) -, leave the phone line busy, are subject to conventional charging per minute of use and can be unstable, causing that a new connection has to be established from time to time.
ADSL technology, which emerged in 1989, shows itself as a viable alternative because it also uses the infrastructure of conventional telephony (technically called POTS, from Plain Old Telephone Service), but it does so without leaving the line busy. In addition, the standard is capable of offering high data transfer speeds and is charged differently than telephone calls.
In fact, ADSL is not a single standard, but part of a “family” of technologies called DSL (Digital Subscriber Line) or just xDSL. Among the “sister” specifications are standards such as HDSL and VDSL, which will also be covered in this text.
How do ADSL services work?
When using a regular phone line, you know that if someone tries to call you at that moment, the person will receive a notice saying that your number is busy. Likewise, you cannot use this line to make a new call until you leave the ongoing call. So, how do DSL technologies manage to establish connections to the internet without leaving the line busy?
In fact, there is no “magic” in this. What happens is that when the phone is being used for a voice call, it uses only a small part of the line’s transmission capacity. What some DSL technologies do is to take advantage of the unused part.
This is because, normally, a voice call – the POTS part – uses a very low wave frequency, between 300 Hz and 4000 Hz. This is an interval that corresponds to a very small range of line capacity. The rest can then be used for applications that run at higher frequencies. This is where DSL technologies come into play.
The use of the free spectrum, that is, the part not used for POTS, is usually done with the FDM (Frequency Division Multiplexing) technique or with the Echo Cancellation technique (Echo Cancellation).
With FDM, part of the free spectrum is used for sending data (upstream) and part for receiving data (downstream), the latter being larger and divided into smaller, faster and smaller channels for better performance.
With Echo Cancellation, the parts for upstream and downstream overlap in the spectrum, but the use of echo cancellation, a procedure that removes the “distortion” of the signal during transmission from subtraction calculations, manages to “separate them”.
As DSL technologies are data transmission and reception systems that use analogue means, it is necessary to use modulation, a process that, in a nutshell, transforms data and voice into signals for traffic in radio frequency waves. For this purpose, ADSL essentially has two techniques: the oldest is called CAP (Carrierless Amplitude / Phase); the most current and most used commercially is called DMT (Discrete Multitone).
CAP modulation, which normally uses the FDM technique, “divides” the telephone line into three parts: one that corresponds to voice calls, another that is destined for sending data (upstream) and a third that is reserved for receiving data (downstream), the latter two being the internet connection itself.
By default, the voice range is from 0 to 4 KHz (4000 Hz), while the upload is between 25 and 160 KHz. The download range, in turn, occupies the largest range, starting at 240 KHz and reaching, at most, up to 1,550 KHz (normally going up to 1,100 KHz). Look at the image below to better understand:
At this point, you can already understand why the term ” Assymmetric ” in the name of ADSL: this type of technology is considered asymmetric because the download rate is commonly higher than the upload rate, since it is understood that most connections receive more data than send it.
But, as already informed, DMT modulation is the most used, being able to use both the FDM technique and the Echo Cancellation technique. In it, the telephone line is also used for both voice and data, obviously, with the difference that the frequency range – from 0 to 1,100 KHz, approximately – be divided into up to 256 channels, each with a width of 4 KHz and spacing between them of 4,3125 KHz. The initial part of this division – the first six channels, usually – is intended for voice calls, with a group also responsible for the upstream (generally, channels 6 to 30) and another, larger, for the downstream. However, with the use of Echo Cancellation, the upstream channels can also be used for downstream, obviously.
The DMT technique gained preference in the market for offering better performance and greater resistance to noise (interference) in the transmission. One reason for this is the fact that each channel is monitored. Channels that are, for some reason, operating with low quality, lose priority in signal transmission, working with fewer bits than better-off channels.
In an Internet access via ADSL, the telephone line is, in fact, only a means of communication formed by a pair of metallic wires. The connection itself ends up taking place thanks to the equipment used both on the client side (which requests the connection) and on the provider side (which establishes the connection).
On the client side, a device popularly known as an ADSL modem is connected to the wires of an existing telephone line. Technically, its name is ATU-R (ADSL Terminal Unit – Remote, something like “ADSL Terminal Unit – Remote”). This connection, in turn, is not infrequently intermediated by a microfilter called splitter, which has the function of creating a channel for the connection with the modem and another channel for communication with the telephone set.
It is also common to use a simpler microfilter only when connecting the line and the telephone set, with the modem directly connected to the telephone network. In all cases, the idea is to avoid interference in and between voice and data channels, and may also offer protection against lightning strikes.
On the one hand, the modem must then be connected to a computer or network equipment, such as a Wi-Fi router, so that the equipment connected to them has access to the internet. On the other, the device is connected to the line to perform its function of organizing the data flow so that the transmission occurs within the frequencies established for upstream and downstream.
At the opposite end of the connection is a telephone exchange (or equivalent). In it, the signal for each telephone line is “separated” with the help of splitters, so that what is voice is sent to a PSTN (Public Switched Telephone Network) that handles this type of communication and what data is sent to a device called DSLAM (Digital Subscriber Line Access Multiplexer).
If the modem in the user’s home or office plays the role of ATU-R, DSLAM takes the opposite side, being the ATU-C (ADSL Terminal Unit – Central, something like “ADSL Terminal Unit – Central”). It is up to this equipment to concentrate the digital signals of several telephone lines (which serve a neighborhood, for example) as if they were one to connect them to an internet access link.
For this, each DSLAM needs to communicate with a BRAS (Broadband Remote Access Server). This equipment has among its functions to concentrate the connections coming from one or more DSLAMs and to allocate IP addresses for each line that is part of the network. Typically, communication between DSLAM and BRAS is via ATM (Asyncronous Transfer Mode) or Ethernet technology.
It is important to note that DSLAM does not divide the speed of access between the lines. In other words, adding one connection does not affect the speed of the others. On the other hand, each DSLAM – as well as each BRAS – has a limit on the number of connections, making it necessary to increase the network infrastructure to increase the number of users.
There is also a variation of this equipment called Mini-DSLAM that has a similar proposal, but serves a smaller number of connections. These devices are interesting because they cost less and avoid waste in places where there is little demand, such as a street or a residential condominium.
There is an important factor that can limit or even make it impossible to subscribe to an ADSL service: the physical distance between the user’s modem and the DSLAM, that is, between the ATU-R and the ATU-C. The further away one is from the other, the less quality and speed the connection will have.
By default, the maximum viable distance is approximately 6 kilometers, which may be slightly less depending on the characteristics of the region and the quality of the equipment. Anyway, the ideal is that this interval does not exceed 4 kilometers. That is why places with few inhabitants or very remote, such as rural areas, are often without access to the internet via ADSL.
This is because the signal that travels the line starts to degrade considerably after a certain point. In other words, the electromagnetic waves that travel through the wires lose power as they move away from the point of origin, making the connection unable to maintain a level of intensity sufficient to support high speeds over greater distances.
It may happen that a telephone operator offers an Internet access service in a given region as ADSL, but in fact uses a variation of this. Nothing more normal, after all, like other technologies, ADSL also undergoes revisions over time based on certain needs, such as meeting the growing demand for connections with greater speed. The main versions follow.
The specifications of what we can call “first ADSL” were ratified by the International Telecomunication Union (ITU) in 1999, receiving the ITU identification G.992.1 (G.DMT). There was also standardization by the American National Standards Institute (INSI) under the identification ANSI T1.413. Its main features include downstream of up to 8 Mb / s (megabits per second) and upstream of up to 1 Mb / s.
Later, there was a version called ITU G.992.2 (G.Lite), which was also known as ADSL Lite. As the name suggests, this is a simplified version of the technology. Its main feature is that, at least theoretically, it does not require splitters or the like. On the other hand, their download and upload rates are up to 1.5 Mb / s and 512 Kb / s, respectively.
ADSL2 and ADSL2 +
In 2002, the specifications of what became known as ADSL2 (ITU G.992.3 / ITU G.992.4) were ratified. This standard has practically the same principles as ADSL, but it has been optimized in several aspects: modulation, for example, is more efficient; in addition, ADSL2 is able to improve the data flow by working with headers for signaling with size of 4 KB, and in ADSL this parameter is fixed at 32 KB; there is also the possibility of using IMA (Inverse Multiplexing for ATM) technology), which allows for an increase in traffic capacity by adding one or more lines to the connection. These and other characteristics make this version able to reach up to 12 Mb / s in the downstream keeping the upstream in up to 1 Mb / s.
Other factors that contribute to the speed aspect are the use of a “channeling” technique, which allows the use of channels other than the connection for the exclusive use of an application (for example, for streaming, that is, video transmission and audio) and constant communication supervision, which adjusts transmission rates to avoid errors and data packet loss.
ADSL2 also stands out for having better power management, generating energy savings. In this sense, connections of the type can count on three modes of operation, being able to switch between them automatically:
- L0 (Full On) mode : in this, the connection works in its entirety;
- L2 (Low Power) mode : here, the energy level decreases, slowing down the transmission. Useful when the user is downloading data in small quantities;
- L3 mode (Idle) : this mode works as a kind of “rest” – the connection remains active, but does not transmit data.
In 2003, the ADSL2 + specifications (ITU G.992.5) emerged, which, by using a higher frequency range, going up to 2,200 KHz, can reach up to 24 Mb / s downstream, maintaining, again, the upstream at up to 1 Mb / s, there is also a variation (ADSL2 + M – ITU G.992.5 Annex M) that increases the latter to up to 3 Mb / s. The problem here is that high speeds are only achieved over distances up to 1.5 kilometers. Furthermore, the maximum speed drops considerably, reaching around 4 Mb / s over distances of approximately 3.5 kilometers.
Acronym for Rate Adaptative Digital Subscriber Line, RADSL is a lesser- known version of ADSL. Its main feature is the fact that the modem is able to adjust download and upload rates automatically, based on criteria such as distance from the exchange and the quality of transmission at certain times.
By default, your limits are also 8 Mb / s downstream and 1 Mb / s upstream. Its automatic adjustment capacity makes its use more common in connections that are not close to the telephone exchange, as long as this distance is within a radius of, at most, 6 kilometers, as in ADSL.
Other types of DSL
As you already know, ADSL is not a single standard, but part of a fairly large set of DSL technologies. The following are some of the others.
It can be said that ADSL, also considering its variations, is the most well-known DSL technology on the market. However, this does not mean that it was the first to emerge: before it, around the end of the 1980s, the HDSL (High Bit Rate digital Subscriber Line) appeared, whose identification is ITU G.991.1.
This is a specification that emerged as an option for data transmission on digital telephone lines of types T1 and E1, the latter being a European standard also used in Brazil. Among its main features are the full duplex operation mode (sending and receiving data at the same time) and symmetrical transmission: 784 Kb / s for downstream and another 784 Kb / s for upstream, totaling just over 1.5 Mb / s. On E1 lines, this total can reach 2 Mb / s. In all cases, the maximum distance considered is approximately 5 kilometers.
HDSL technology ended up not becoming as popular as ADSL because it requires the use of two pairs of twisted wires (which can reach three pairs on E1 lines), that is, two telephone lines simultaneously, and also because it does not allow voice calls during Internet connection, since the frequency bands used for this purpose are used during data transmission.
Subsequently, a variation called HDSL2 appeared that maintains practically the same maximum speed rates, but uses only one pair of wires.
The SDSL (Symmetric Digital Subscriber Line) is very similar to HDSL: typically transmits data symmetrically, have maximum rates of 1.5 Mb / s (T1) or 2 Mb / s (T2) and does not allow the use of voice and data at the same time. On the other hand, it has the advantage of requiring only one pair of wires, facilitating its implementation considerably. In addition, the maximum distance for type connections tends to be slightly less than for HDSL.
Acronym for Single Pair High Speed Digital Subscriber Line, the SHDSL was ratified in 2001, receiving the identification ITU G.991.2. It is a technology that also works in a symmetrical way: if you use only one pair of wires, connections of the type can have a speed of up to 2.3 Mb / s; with two pairs, this limit increases to up to 4.6 Mb / s.
With SHDSL, it is also not possible to use the line for voice and data applications at the same time.
VDSL and VDSL2
Acronym for Very High Bit Rate Digital Subscriber Line, VSDL (identified as ITU G.993.1) is a technology recognized in 2004 that resembles ADSL, working as well as this, asymmetrically, using the DMT technique more often and allowing use of voice and data simultaneously.
Its differential lies in its ability to achieve considerably higher transfer rates: up to 52 Mb / s in the downstream and up to 16 Mb / s in the upstream. The technology can also be adjusted to work symmetrically.
In 2006, it was the turn of VDSL2 to be ratified, receiving the identification ITU G.993.2. This version can reach up to 100 Mb / s in the downstream and 30 Mb / s in the upstream. Such high speeds even allow both VDSL and VDSL2 to be used to offer voice communication, internet access and TV subscription (IPTV) through a single telephone line.
Such high rates are possible thanks to a combination of factors, such as the use of higher frequencies on the telephone line – up to 12 MHz on VDSL and up to 30 MHz on VSDL2 – and the combination with another technology, such as optical fiber. In the latter case, the operator’s network structure is almost completely covered by this technology, leaving only the last few hundred meters to the user’s location connected by the traditional pair of wires.
Speaking of meters, the main disadvantage of the VDSL and VDSL2 connections is precisely in the aspect of the distance between the switch and the user’s location: normally, the supported limit is up to 1.5 kilometers, with the VDSL2 only being able to have its speed reached in its totality within a maximum radius of just over 300 meters.
DSL technologies are able to take advantage of the telephony infrastructure and, because of that, allow reasonable subscription values, which is why they are widely used in various parts of the world. These characteristics make it especially interesting in locations that do not have other access modes, such as internet via cable TV, for example.
Because of this, we will see DSL standards for a long time on the market, even with the emergence of technologies that can offer better speeds and coverage, such as 4G LTE networks. Versions such as ADSL2 + and VDSL2 are the operators’ bets to meet the demand for ever higher data transmission rates.