How Can You Explain Asymmetrical DSL?

Asymmetrical DSL or ADSL is like a 56 K modem, which provides higher speed (the bit rate) in the down stream direction (from the internet to the resident) than in the upstream direction (from resident to the internet). This is the reason it is known as asymmetric. Unlike the asymmetry in 56 K modems, the designers of the ADSL specifically divided the available bandwidth of the local loop unevenly for the residential customer. The service is not suitable for the business customers who need a large bandwidth in the both directions. One interesting point is that ADSL uses existing local loop.

But a question arises here that how does ADSL reach a data rate that was never achieved with traditional modems? The answer is that twisted pair local loop is actually capable of handling bandwidths up to 1.1 MHz, but the filter installed at the end of the line by the telephone company limits the bandwidth to 4 KHz (sufficient for the voice communication). This was done to allow multiplexing of a large number of the voice channels. If the filter is removed, the entire 1.1 MHz is available for data and voice communications. The standard modulation technique is known as discrete multi-tone technique (DMT).