3.3 Crosstalk


Crosstalk occurs when there is more than one cable being used. The electromagnetic force of one cable may be sufficient to have an effect on another. Some cables are twisted to reduce the possibility of crosstalk.

There are two main types of crosstalk but both involve some form of unwanted noise between sender and receiver.

NEXT stands for near end crosstalk and happens at the sending end of the cable

FEXT stands far end crosstalk and happens at the receiving end of the cable

In badly made twisted pair cables crosstalk will occur when the cable at the jack is untwisted or exposed out of the outer jacket.

Several factors can contribute to near-end crosstalk. The most common cause is crossed pairs.

Near-end crosstalk can also be caused by twisted pairs that have become untwisted after being attached to cross-connect devices (eg: patch panels) that have patch cords that are untwisted, or by cables that have been pulled too tightly around sharp corners, causing pairs to change position inside the cable jacket. If you measure near-end crosstalk, you should do a visual check of the horizontal cabling, in order to rule out any of these possibilities.

If you find nothing, then split pairs have most likely caused the problem.

A cable tester measures for near-end crosstalk by measuring a series of frequencies up to 100 MHz. High numbers are good but low numbers indicate problems on the network.

Attenuation to Crosstalk Ratio (ACR)

This is the difference in decibels (dB) between the signal attenuation and near-end crosstalk (NEXT). The ACR is calculated to make sure that a signal transmission is stronger at the receiver than any interference due to crosstalk.

Attenuation and crosstalk must be minimised in order for acceptable signal transmission. Attenuation depends on the length and type of cable and as such cannot be changed. However, crosstalk can be minimised by making sure that the cables meet certain standards.

ACR is also referred to as headroom.