Some line encoding techniques used by DSL.
2B1Q — 2 Binary 1 Quaternary; for ISDN DSL (IDSL); sends two bits of data to every one cycle of an analogue waveform. Uses frequencies of up to 80,000 Hz; same encoding is used on ISDN lines.
QAM — Quadrature Amplitude and Phase Modulation; used in modems for more than 20 years.
CAP — Carrierless Amplitude and Phase modulation. Similar to QAM.
DMT — Discrete MultiTone; patented by AT&T Bell Labs more than 20 years ago.