Basic Cable Testing
Crosstalk
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Crosstalk
Crosstalk is undesirable signal transmission from one cable pair to another nearby
pair. Like electrical noise from outside sources, crosstalk can cause
communication problems in networks. Of all the characteristics of LAN cable
operation, crosstalk has the greatest effect on network performance.
The test tool makes two crosstalk measurements: NEXT and ELFEXT.
NEXT
The test tool measures NEXT (near-end crosstalk) by applying a test signal to one
cable pair and measuring the amplitude of the crosstalk signals received by the
other cable pairs. The NEXT value, expressed in decibels, is computed as the
difference in amplitude between the test signal and the crosstalk signal measured
at the same end of the cable. Higher NEXT values correspond to less crosstalk and
better cable performance.
All signals transmitted through a cable are affected by attenuation. Because of
attenuation, crosstalk occurring at the far end of a cable contributes less to NEXT
than crosstalk occurring closer to the signal source. To verify proper cable
performance, NEXT is measured from both ends of the cable.
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