Crosstalk; Noise; Other Considerations - Cabletron Systems 100BASE-FX Cabling Manual

Enterasys 100base-fx transceivers: user guide
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Crosstalk

Crosstalk is electrical interference between wires. Crosstalk occurs when a cable
strand absorbs signals from other wires that it is adjacent to. Excessive crosstalk
can be caused by a break in the insulation or shielding that separates wires from
one another in a bundle.
Ethernet UTP cables should be checked for Near-End Crosstalk, or NEXT, at
installation. The allowable amount of NEXT for a UTP cable is dependent upon
the type of cable used in the installation.
25-Pair Cable
The acceptable amount of NEXT between pairs in a 25-pair cable is at least 60 dB
for a 10 MHz link.
Four-Pair Cable
The acceptable amount of NEXT between pairs in a four-pair cable is not less than
60 dB for a 10 MHz link.

Noise

As "noise" is not a readily quantified and tested aspect of installed cables, there
are no hard and fast rules for the amount of acceptable cable noise on a 10BASE-T
segment. If a cable that meets all other requirements for 10BASE-T operation is
experiencing an unusual number of errors, the introduction of noise may be a
problem.
If you suspect noise of causing signal degradation, examine the cable or cables in
question. If they are near possible sources of outside noise, such as lighting
fixtures, electric motors, or transformers, reroute the cable.

Other Considerations

UTP cabling, due to the small gauge of the wires it is constructed out of, is
susceptible to changes in attenuation due to heat. In an installation that exceeds
the control temperature of 20 C (68 F), the attenuation of PVC jacketed UTP
cabling that is within the 11.5 dB limitations may fall outside the acceptable range.
In installations where UTP cables are expected to be subjected to temperatures of
40 C (104 F) or greater, the use of plenum-jacketed cabling is recommended. The
thicker insulating jacket of a plenum-rated cable reduces the susceptibility of that
cable to heat-induced changes in attenuation characteristics.
10BASE-T
Ethernet Network Requirements
5-3

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