Interference, which causes data bit errors, can come from outside the cable or within the cable. We might say from the “enemy without” or the “enemy within”. Generally, the former is easier to get rid of than the latter.
The enemy without:
Induced electrical “spikes” or transients from electrical power cables particularly those carrying heavy switching currents.
Induced/radiated electrical “spikes” or transients from electric motors, fluorescent lights, photocopiers, air-conditioning motors, electric arc welders, electrical transformers especially under fault (unbalanced) conditions.
Induced transients from lightning or surge suppression discharge currents.
Generally be controlled by:
- Sufficient physical separation.
- Shielding of the interference source or shielding of the cable.
The enemy within:
Near End and Far End Crosstalk – which is data signal coupling from the disturbing pair into the disturbed pair.
Secondary data signals within a cable pair caused by a portion of the original signal reflecting from impedance discontinuities and interfering with the original line data transmission.
Difficult to eliminate but controlled by:
- Cable manufacture (different twist rates for each pair accurately implemented).
- Careful cable installation to eliminate physical stress to the cable which can cause impedance discontinuities