Telephone companies installed loading coils to compensate for this effect, blocking out frequencies above 4 kHz, eliminating the bandwidth for DSL services. Splices and bridged taps also are a source of delay and attenuation, and these are extensively found in the phone lines. Two other factors going against ADSL are crosstalk and impulse noise. I have already explained the effect impulse noise has on error correction in another section of this project. Signal crosstalk increases with frequency.
Crosstalk noise is generally stationary while impulse noise is random.
Thus, it is difficult to model.