ADSL stands for Asymmetric Digital Subscriber Line and is a technology used mainly in home and home offices on existing copper phone lines. It is called asymmetric because the technology dedicates most of its transmission to downloading digital information to the user and very little to uploading data from the consumer. ADSL has been used since 1996 and competes with Ethernet cable technology. Although ADSL is used mainly for residential use, it is also used for aspects of business that require download redundancy at the cheapest cost on existing copper lines, or satellite branching in rural areas.

ADSL is more popular than SDSL (Symmetric Digital Subscriber Line) because most people tend to download digital information more than upload information. ADSL allows an always on connection with the ability to transmit voice while data is being downloaded at speeds up to 6 megabits per second. Voice transmission does not need much line allocation and is not data dependent.

ADSL transmission speed and quality is dependent on the distance to the carrier's central office as well as the amount of total customers using the copper line. Customers have to contend with distance and fluctuating users counts. Businesses tend to use ADSL when no other options are available, or as an inexpensive backup to their main data transmission technologies.