  Crosstalk and Near End Cross Talk
Of all the characteristics of LAN cable operation, crosstalk has the greatest effect on
cable performance. 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 Fluke DSP-100 test tool used by Missouri State University measures 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 crosstalk value is computed as the difference in amplitude
between the test signal and the crosstalk signal when measured from the same end of the
cable. This difference is called near-end crosstalk (NEXT) and is expressed in
decibels. 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 at the near end of a cable. To verify proper cable performance,
you should measure NEXT from both ends of the cable.
Minimizing Crosstalk
Crosstalk problems are minimized by twisting together the two wires in each cable pair.
Twisting the two wires together causes the electromagnetic fields around the wires
to cancel out, leaving virtually no external field to transmit signals to nearby cable
pairs.
You can avoid problems with crosstalk by observing the following precautions during
installation:
- When untwisting cable pairs to install connectors or make connections at punch-down
blocks, make the untwisted sections as short as possible.
- Be attentive when making wiring connections. Wiring errors that cause split pairs
create severe crosstalk problems.
- Do not make sharp bends or kinks in the cable. The radius of bends in a cable
should be larger than 1 inch (2.54 cm).
- Handle LAN cable carefully during installation. Do not step on the cable or pinch
it with tight cable ties.
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