Digital Subscriber Line, or DSL, is a technology for bringing high-bandwidth connectivity to homes and small businesses over ordinary copper telephone lines. xDSL refers to the family of digital subscriber line technologies, such as ADSL, HDSL, and RADSL. Connection speeds for DSL typically range from 1.544 Mbps to 512 Kbps downstream and around 128 Kbps upstream. In addition, a DSL line allows for one line to carry both voice and data signals, and for the data part of the line to be continuously connected. DSL achieves higher data transfer rates by utilizing more of the available bandwidth spectrum. Ordinary telephone service only makes use of the 0 - 3400 Hz frequency range, which accounts for the 56 Kbps speed limit on standard analog modems. DSL eludes the 3400 Hz frequency boundary by outmoding the digital-to-analog conversion that modems perform and connecting both ends digitally. Hence, larger bandwidth is available, allowing higher transfer rates.
DSL use of the existing twisted pair infrastructure makes it cheaper to install than other services which require additional cable to be laid. And unlike cable modems, DSL is not a bus technology, so the bandwidth available to the end user is more consistent. However, despite all of its positive attributes, DSL is not without flaw. For instance, in order to be eligible for DSL, the end user must be geographically within a certain distance from the central telephone office, otherwise the signal degradation is too great and DSL is unfeasible (for ADSL that distance is two miles.) In addition, numerous standards still exist for DSL, hardware is still comparatively pricey, and service is available only in limited areas. Despite these drawbacks, DSL is still a faster alternative to analog modems and ISDN, and will rival cable modems as far as actual bandwidth offerings.