You vary the PSD level of the noise to simulate the number of disturbers. The noise you use to simulate the disturbers must be Gaussian white noise that has a crest factor of 5. Connect your noise source to the test loop, which must have an impedance greater than 4 k W . In addition to simulating disturbers, you must also simulate several configurations of twisted-pair lines. They also define the locations of bridge taps in the loops.
Bridge taps are unterminated connections to a subscriber loop.
Those unterminated wires cause reflections and can distort signals. Figure 2 shows two subscriber loops with bridge taps used for testing. While performing a crosstalk test, set your noise source to the appropriate level for the type of crosstalk you are simulating. The process of establishing the connection between the two ADSL modems is called training. The difference between your noise level and the reference level is called a margin.