Once those are fixed, it is physically impossible to exceed the fundamental limit that renowned Bell Labs scientist Claude E. Shannon spelled out in a 1948 paper . Shannon's formula tells us that such a modem is very close to the channel's theoretical capacity, about 35 kbps, a feat that has taken 30 years of modem circuit development. Modems operating at 56 kbps achieve their rates by taking advantage of digital connections that circumvent some sources of noise in transmissions toward the end user. But these bit rates are far from the maximum possible on a twisted pair alone. One process that limits bandwidth and signal strength is the steady attenuation of the signal as it travels down the line, with the higher frequencies being affected more severely.
Greater capacity is therefore available if the lines are kept short.
He also proposed multilevel coding of the signal to further enhance performance. This transmission technique was dubbed the digital subscriber line. The technique divides an overall bandwidth of about 1 MHz into 256 subchannels of about 4 kHz each. In essence, it creates 256 virtual modems operating simultaneously over the same line. Originally, the Discrete Multitone approach was intended for sending entertainment video over telephone wires.