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What is an Asymmetric Digital Subscriber Line?

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  • Written By: John Lister
  • Edited By: Bronwyn Harris
  • Last Modified Date: 01 December 2016
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    Conjecture Corporation
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An asymmetric digital subscriber line, or ADSL, is a means of transmitting internet data to and from a customer. It is a form of digital subscriber line, which uses an ordinary telephone line to carry information. Along with cable, an ADSL is one of the two most common ways of supplying broadband internet services.

The first word of the phrase "asymmetric digital subscriber line" refers to the fact that information can be received at a faster rate than it can be transmitted. For most internet users, this fits very efficiently with their pattern of use. While many people regularly view very detailed webpages and download audio or video files, the only large amounts of information they usually send is attachments to e-mails or uploads through filesharing services. ADSL allows the average user to get the benefits of fast downloads without needlessly paying for the benefits of fast uploads.

Surprisingly, the asymmetric digital subscriber line system didn’t develop as a response to these needs. It’s a result of the fact that the biggest source of interference to internet data is near phone exchanges rather than near the customer’s home. This means the data passing through the exchange towards the customer – in other words, downloads – needs to be transmitted at a faster rate to get past this interference.

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Other than increased speed, there are several advantages to ADSL. One is that it can be used at the same time as a voice call made on the same line. This is because the two types of data are transmitted on different frequencies. However, users must install a filter at the main phone socket which separates the two sets of data. Another advantage is that computers can begin accessing the internet almost instantly as there is no need to dial-up an internet firm and make a connection first.

ADSL is not just the name of the technology, but is also used to refer to a specific technical standard. This includes a download rate of at least 8 Mbits per second and an upload rate of at least 1 Mbit per second. It’s important to note that both of these figures are theoretical figures used for comparing services. In practice customers will get lower results because of physical limitations such as interference on lines, or the distance between the phone exchange and the home.

There are many other standards based on the asymmetric digital subscriber line system. The most common include ADSL2 and ADSL2+. The former has a theoretical download speed of 12 Mbits per second and an upload speed of either 1 Mbit per second or 3.5 Mbits per second depending on the precise system used. The latter has the same upload speed but doubles the theoretical download speed to 24 MBits per second.

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