Near EndCrosstalk(NEXT) and Far End Crosstalk (FEXT)

Crosstalk is signal interference between pairs that may be caused by a pair picking up unwanted signals from either adjacent pairs of conductors or nearby cables. For example, this interference can result from the magnetic field that surrounds any current carrying conductor. The crosstalk interference can be intelligible or unintelligible depending on thecoupling modes.

Two types of crosstalk coupling losses are of interest to the telephone circuit designer, Near End Crosstalk(NEXT) and Far End Crosstalk (FEXT). Near End Crosstalk (NEXT) is measured by applying and measuring the signal transfer level at the same end.

Far End Crosstalk (FEXT) is measured by applying the disturbing signal on one pair at the near end and measuring the signal transfer level on the disturbed pair at the far end.

Crosstalk may be reduced by twisting the pair. Where there is more than one pair in a cable, crosstalk between them is reduced by twisting them at different rates. For higher frequency use, the twist rate must be increased. The difference in twist rates also becomes more important. Screening and shielding are also used to reduce crosstalk. Power-sum NEXT (PSNEXT)(or Power-sum FEXT (PSFEXT)) is actually a calculation, not a measurement. PSNEXT is derived from an algebraic summation of the individual NEXT effects on each pair by the other pairs. The signal to noise relationship may also be expressed as the attenuation to crosstalk ratio (ACR). NEXT, FEXT, PS-, ACR

ACR is the difference between the attenuated signal and the unwanted coupled signal and can be calculated as follows:
ACR = Minimum NEXT – Maximum Attenuation

source : general cable