Digital Subscriber Line (DSL) is technology used for transmitting digital data over existing traditional voice-grade copper telephone cable concurrent with an analog voice signal. The voice signal is carried in lower frequencies while the data is carried in higher frequencies on the same copper twisted pair (local loop). DSL deployment, including subscriber and service provider equipment, is far less costly than installing new, fiber-optic cable along the same path.
DSL technologies have proliferated over recent decades into a family of variations, often summarized as xDSL. Two highly-popular flavors that are widely installed include very-high-bit-rate DSL (VDSL) and single-pair high-speed digital subscriber line (SHDSL), now standardized as G.SHDSL.bis. VDSL modem and G.SHDSL modem functionality today may be embedded a Voice-over-IP gateway, router or enterprise session border controller (eSBC), in which case the device is called an integrated access device (IAD). Recently DSL has been adapted to support Ethernet-in-the-First-Mile (EFM) applications.