Cabletron Systems 100BASE-FX, Cabling Manual

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100BASE-TX 7-3
Fast Ethernet Network Requirements
Delay
The maximum propagation delay allowable on a 100BASE-TX segment is
1 microsecond (
µ
s). If a Fast Ethernet signal is unable to traverse the entire length
of an installed UTP cable run within 1
µ
s, Out of Window (OOW) errors will
occur due to excessive delays between transmission of signals and notification of
collisions. This propagation delay requirement limits UTP cabling to a total
maximum length of 100 m (328 ft).
Crosstalk
Fast Ethernet UTP cables should be checked for Near-End Crosstalk, or NEXT, at
installation. The acceptable amount of NEXT between pairs in a four-pair cable is
not less than 27 dB for a 100 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
100BASE-TX segment. If a cable that meets all other requirements for
100BASE-TX 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 in a UTP cable, it 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 dB
limitations may fall outside the acceptable range of attenuation. 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.
The IEEE 802.3 100BASE-TX specification requires that all 100BASE-TX devices
support UTP cables up to 100 m (328 ft) in length. This requirement does not
factor in losses due to connectors, patch panels, punchdown blocks, or other cable
management hardware, which introduce additional loss.
For each connector or other intrusive cable management device in the total link,
subtract 12 m (39.4 ft) from the total allowable link length for purposes of
estimation.