Troubleshooting LAN Wiring - LAN Wiring - page 424

take will cause most performance-critical applications to fail. Remember that the
category of performance is limited by the lowest category or component or cable
used in a link. In addition, of course, you must maintain the distance below the
limit, route the wire away from potential interference sources, and maintain the
minimum twist of the pairs.
Many older cabling systems can support Category 3 operation. Cable that is 4
pairs and has a minimum of 2 to 3 twists per foot should operate, although not nec-
essarily at the distance limit. But cable that barely meets Category 3 cannot be used
for Category 5e or higher. Multipair cables of 25 or more pairs should be limited to
6 circuits (12 pairs) to minimize the effects of crosstalk that occurs on such cables.
Analog telephone circuits can usually be accommodated on pairs in the same cable
sheath, as their frequency of operation is much lower than LAN frequencies.
However, you must use caution when placing digital telephone signals, including
ISDN, in the same cable jacket as LAN data, as the two may very well interfere with
each other. Many modern phone systems are digital systems, and fall into this area
of caution. The best rule is to use only one application per cable, even with analog
systems. A later upgrade of your phone equipment might inadvertently shut down
your LAN.
Excessive Untwist. The amount of untwisted cable that is allowable when operat-
ing at Category 3 is so great that it is easy to view the Category 5e/6 requirements
with skepticism. After all, how could you go from a generous allowance of two or
more inches of loosely twisted wire to a ridiculous requirement of less than 1/2 in
(13 mm)? Well, the answer is easily found when you consider that the maximum fre-
quency of “interest” of the two categories jumps from 10 to 100 (or to 250) MHz.
Obviously, the Category 5e/6 requirements are much more stringent because they
have to be to assure cable link performance at this level.
Good cabling practice is to limit the amount of untwist to only that required
to terminate the wires. This is good practice, whether you are terminating Category
6 or Category 3. Another good practice is to strip back only enough of the outer
jacket as is needed to terminate the pairs.
This maintains the cable twist and the positioning of the individual pairs in
relation to each other. The better connectors and cable terminations have additional
cable management devices to secure the wire in place and to minimize stress and
bending of the wire.
Wrong Impedance. Throughout this topic we have talked about UTP cabling with
100-ohm impedance and STP cabling with 150-ohm impedance. Any mixing of
these two types of wire in the same cable link will result in an impedance mismatch