Cabletron Systems 100BASE-FX, Cabling Manual

Page: 67 / 192

Full-Duplex 10BASE-T 6-3
Full-Duplex Ethernet Network Requirements
Crosstalk
Crosstalk is electrical interference between wires. Crosstalk occurs when a cable
strand absorbs signals from adjacent wires. 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 that noise is 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
Due to the small gauge of the wires which make up UTP cabling, it is susceptible
to changes in attenuation due to heat. If the temperature at the installation site
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.