DSL (digital subscriber line) technologies, often grouped under the term DSL, connect a computer to the Internet. DSL uses existing copper pair phone line wiring in conjunction with special hardware on the switch and user ends of the line. This special hardware allows for a continuous digital connection over the phone lines.
Since the connection is digital, DSL technology doesn't have a digital-to-analog conversion like traditional modems. It eludes voice audio spectrum frequency boundaries because it can use frequencies above the voice audio spectrum. This means you can use your phone while maintaining your Internet connection.
These different frequencies allow DSL to encode more data, and allow Internet connection speeds of up to 50 times faster than standard modems, and up to 12 times faster than an ISDN connection. Additionally, since DSL is not a bus technology, it offers more consistent bandwidth than cable modems in which multiple users share very high bandwidth media. However, distance limitations can affect the transmission rates or can be too great, rendering DSL infeasible. The condition of your existing wiring can also affect transmission rates.
The most commonly available DSL technology is ADSL, or asymmetric DSL. It is asymmetric in that it is designed to accommodate typical consumer Internet use, with much more data flowing toward the user (multimedia and text) than from the user (mostly keystrokes and mouse behavior). The downstream rate (receiving rate) varies from 1.5 to 9 Mbps; the upstream rate (sending rate) varies from 16 to 640 Kbps. These speeds depend greatly on the distance to the telephone company's central office.