The limiting criteria for loop design are attenuation, noise, and crosstalk. To overcome loss and extend reach, phone companies opted to reduce the series resistance of the line by using larger gauge wire. They also increased the series inductance of the line with loading coils, and used analog electronic amplifiers to provide compensating gain to the transmission line. The most common sources of extraneous electrical signals are the adjacent electrical power lines and equipment. Noise can also be caused by broadcast TV, radio stations, cellular radio systems, and microwave radio systems.
Crosstalk occurs when energy is coupled between two adjacent cable pairs.
Crosstalk introduces bit errors and distortion on digital signals. This phenomenon occurs because practical transmission lines are laid in binder groups such as paired cables. When part of the energy in one transmission line is coupled with an adjacent line appearing as noise, crosstalk can also happen. There are additional issues to consider. To better understand the difficult environment the DSL transceivers must operate in, an overview of some digital transmission techniques is covered next.