DSL is a type of signal transmission technology implemented on analog copper phonelines using s "type of" synchronous signalling sematic modulation scheme and is an alternative to ISDN lines or Coaxial "Digital Cable" internet. This DSL is actually xDSL, depending on the type of mudulation scheme used. There are 3 types commonly used, but there are more. The three common are:
1) ADSL, asynchronous
2) SDSL, synchronous
3) LDSL, LiteDSL

ISPs usually provide aDSL service to consumers. DSL works on the principle of a modulated wave delivered by localized DSLAMS, separated by filters if the carrier signal is insufficient. In standard dial up modems, digital data is converted to analog signal, (modulated) then at the destination it is turned back into it's original form (de-modulated). This process uses most or all of the signal carrier that conversations travel on. In DSL technology, these two processes are replaced by a mudulated wave, in which the digital data, is a constant stream (download and upload combined) of an analog waveform. This boasts speed without the loss of phoneline usage.