ADSL

ADSL (Asymmetric Digital Subscriber Line) is a technology for transmitting digital information at a high bandwidth on existing phone lines to homes and businesses. Unlike regular dialup phone service, ADSL provides continuously-available, "always on" connection. ADSL is asymmetric in that it uses most of the channel to transmit downstream to the user and only a small part to receive information from the user. ADSL simultaneously accommodates analog (voice) information on the same line. ADSL is generally offered at downstream data rates from 512 Kbps to about 6 Mbps. A form of ADSL, known as Universal ADSL or G.Lite, has been approved as a standard by the ITU-TS. 

It is a type of DSL broadband communications technology used for connecting to the Internet. ADSL allows more data to be sent over existing copper telephone lines on plain old telephone services (POTS), when compared to traditional modem lines. A special filter, called a microfilter, is installed on a subscriber's telephone line to allow both ADSL and regular voice (telephone) services to be used at the same time. ADSL requires a special ADSL modem and subscribers must be in close geographical locations to the provider's central office to receive ADSL service. Typically this distance is within a radius of 2 to 2.5 miles. ADSL supports data rates of from 1.5 to 9 Mbps when receiving data (known as the downstream rate) and from 16 to 640 Kbps when sending data (known as the upstream rate). ADSL is designed to support the typical home user who frequently downloads large amounts of data from Web sites and P2P networks but upload relatively less often. ADSL works by allocating a majority of the available phone line frequencies for communication of downstream traffic. ADSL was specifically designed to exploit the one-way nature of most multimedia communication in which large amounts of information flow toward the user and only a small amount of interactive control information is returned. Several experiments with ADSL to real users began in 1996. In 1998, wide-scale installations began in several parts of the U.S. In 2000 and beyond, ADSL and other forms of DSL are expected to become generally available in urban areas. With ADSL (and other forms of DSL), telephone companies are competing with cable companies and their cable modem services. 

Benefits that ASDL can provide:

• Provides the ability to talk on the phone while surfing through the Internet, because, as noted above, voice and data work in separate bands, which implies a separate channel.
•  Use existing infrastructure (the basic telephone network). This is advantageous both for the operators who do not face large costs for the implementation of this technology to users, since the cost and time it takes to keep available the service is less than if the operator had to undertaking works to build new infrastructure.
• ADSL users have access to the Internet, not having to establish this connection by dialing or signaling to the network.
• Provides connection speeds much higher than the one made by dial-up Internet. This is the most interesting to users.

This is possible because they have point to point, so that the line between the PBX and the user is not shared, which also ensures dedicated bandwidth to each user, and increases the quality of service. 

Advantages of ADSL:

• You can leave your Internet connection open and still use the phone line for voice calls.
• The speed is much higher than a regular modem
• DSL doesn't necessarily require new wiring; it can use the phone line you already have.
• The company that offers DSL will usually provide the modem as part of the installation.

Disadvantages of ADSL:

• A DSL connection works better when you are closer to the provider's central office. The farther away you get from the central office, the weaker the signal becomes.
• The connection is faster for receiving data than it is for sending data over the Internet.
• The service is not available everywhere.

In other respects, ADSL possesses all of the characteristics one associates with DSL, including "high-speed" service, an "always on" combination of voice and data support, and availability and performance that is limited by physical distance. ADSL is technically capable of up to 6 Mbps (roughly 6000 Kbps), but ADSL customers in practice obtain 2 Mbps or lower for downloads and up to 512 Kbps for uploads. 

Sources:

http://searchnetworking.techtarget.com/definition/ADSL

http://compnetworking.about.com/od/dsldigitalsubscriberline/g/bldef_adsl.htm

http://www.webopedia.com/TERM/A/ADSL.html

http://computer.howstuffworks.com/dsl.htm

http://freepressreleases.eu/advantages-and-disadvantages-of-adsl-technology/