DSL
Digital subscriber line technology was originally implemented as part of the ISDN specification. Joe Lechleider at Bellcore (now Telcordia Technologies) developed ADSL in 1988 by placing wideband digital signals above the existing baseband analog voice signal carried between telephone company central offices and customers on conventional twisted pair cabling.
DSL service was first provided over a dedicated "dry loop", but when the FCC required ILECs to lease their lines to competing providers, shared-line DSL became common. This allows a single pair to carry data (via a DSLAM) and analog voice (via a circuit switched telephone switch) at the same time. Inline low-pass filter/splitters keep the high frequency DSL signals out of the user's telephones. Although DSL avoids the voice frequency band, the nonlinear elements in the phone would otherwise generate audible intermodulation products and impair the operation of the data modem.