ADSL (Asymmetric Digital Subscriber Loop) is a transmission technique that, applied on the subscriber loops of the traditional telephone network, allows the transmission of data on them at high speed. To do this, it uses frequencies higher than those used in the telephone service and without interfering with them, thus allowing the simultaneous use of the loop for the telephone service and to access data services through ADSL.
The asymmetry that characterizes ADSL systems allows to offer a greater transmission capacity in the so-called "downstream" (from the telecommunications network to the user) than in the "upstream" (from the user to the network). This makes them especially suitable for applications such as Web-based Internet access, where the volume of information received by users is markedly greater than that sent.
ADSL is the most commonly used name to designate several standards of the xDSL (Digital Subscriber Line) family, which also include ADSL2 and ADSL2+, ADSL evolutions that offer higher speeds or range, VDSL and VDSL2, which allow very high speeds although restricted to very short length loops, and SDSL, which allows to have the same speed in the downward and upward direction.
While the basic ADSL standard does not allow speeds higher than 8 Mbit/s, the most widely deployed technology today is the so-called ADSL2+ that allows reaching up to 24 Mbit/s in ideal conditions. VDSL technology allows a maximum download speed of 52 Mbit/s and VDSL2 up to 100 Mbit/s, also in ideal conditions.
These speeds are the maximum achievable levels, which does not imply that users can have them on all lines or at all times. There will be lines that due to their physical characteristics, mainly the length, can never reach these maximum levels. This is why certain offers available in the market that advertise speeds “up to 20 Mbit/s” do not guarantee that all customers can have this maximum speed.
On the other hand, although the access line is dedicated for each user, the data networks that channel the traffic of sets of ADSL lines are dimensioned taking into account their shared resource character, so that as more users are connected the actual speed that each one enjoys tends to decrease.