Technology ADSL depends upon advanced digital signal processing and creative algorithms to squeeze so much information through twisted-pair telephone lines. On the inside, where all the transistors work, there is a miracle of modern technology. FDM assigns one band for upstream data and another band for downstream data. The downstream path is then divided by time division multiplexing into one or more high speed channels and one or more low speed channels. The upstream path is also multiplexed into corresponding low speed channels.
Echo cancellation uses bandwidth more efficiently, but at the expense of complexity and cost.
With either technique, ADSL splits off a 4 kHz region for POTS at the DC end of the band. The receiver then corrects errors that occur during transmission up to the limits implied by the code and the block length. Initial trials indicate that this level of correction will create effective error rates suitable for MPEG2 and other digital video compression schemes. Issue II, now under study by T1E1.4, will expand the standard to include a multiplexed interface at the premise end, protocols for configuration and network management, and other improvements. The ADSL Forum was formed in December of 1994 to promote the ADSL concept and facilitate development of ADSL system architectures, protocols, and interfaces for major ADSL applications.