Digital Subscriber Line (DSL) is a family of technologies that provides digital data transmission over the wires of a local telephone network. DSL originally stood for digital subscriber loop. In telecommunications marketing, the term Digital Subscriber Line is widely understood to mean Asymmetric Digital Subscriber Line (ADSL), the most commonly installed technical varieties of DSL. DSL service is delivered simultaneously with regular telephone on the same telephone line as it uses a higher frequency band that is separated by filtering.
The data throughput of consumer DSL services typically ranges from 384 kbit/s to 20 Mbits/s in the direction to the customer, depending on DSL technology, line conditions, and service-level implementation. Typically, the data throughput in the reverse direction, i.e. in the direction to the service provider is lower, hence the designation of asymmetric service, but the two are equal for the Symmetric Digital Subscriber Line (SDSL) service.
DSL uses a second, higher frequency band (greater than 25 kHz) above the low frequency regime (5 kHz and below) used by voice communications. On the customer premises, a DSL filter is installed on each outlet for telephone handsets to remove the high frequency band, eliminating interference with the operation of the telephone set, and enabling simultaneous use.
Implementation of Digital Subscriber Line technology originally was part of the Integrated Services Digital Network (ISDN) specification published in 1984 by the CCITT and ITU as part of Recommendation I.120, later reused as ISDN Digital Subscriber Line (IDSL). Engineers have developed higher-speed DSL facilities such as High bit rate Digital Subscriber Line (HDSL) and Symmetric Digital Subscriber Line (SDSL) to provision traditional Digital Signal 1 (DS1) services over standard copper pair facilities. Consumer-oriented Asymmetric Digital Subscriber Line (ADSL), first tested at Bellcore in 1988, was designed to operate on existing lines already conditioned for BRI ISDN services, which itself is a switched digital service (non-IP), though most incumbent local exchange carriers (ILECs) provision Rate-Adaptive Digital Subscriber Line (RADSL) to work on virtually any available copper pair facility—whether conditioned for BRI or not.
The development of DSL, like many other forms of communication, can be traced back to Claude Shannon's seminal 1948 paper: A Mathematical Theory of Communication. Employees at Bellcore (now Telcordia Technologies) developed ADSL in 1988 by placing wide-band digital signals above the existing baseband analog voice signal carried between telephone company central offices and customers on conventional twisted pair cabling facilities.
Contrary to its name, while a DSL circuit provides digital service, it is actually not a digital signal. The underlying technology of transport across DSL facilities uses high-frequency sinusoidal carrier wave modulation, which is an analog signal transmission. A DSL circuit terminates at each end in a modem which modulates patterns of bits into certain high-frequency impulses for transmission to the opposing modem. Signals received from the far-end modem are demodulated to yield a corresponding bit pattern that the modem retransmits, in digital form, to its interfaced equipment, such as a computer, router, switch, etc. Unlike traditional dial-up modems, which modulate bits into signals in the 300–3400 Hz baseband (voice service), DSL modems modulate frequencies from 4000 Hz to as high as 4 MHz. This frequency band separation enables DSL service and plain old telephone service (POTS) to coexist on the same copper pair facility. Generally, higher bit rate transmissions require a wider frequency band, though the ratio of bit rate to bandwidth are not linear due to significant innovations in digital signal processing and digital modulation methods.