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The EIA/TIA 568 standard for LAN cables only uses 4
of the wires in the eight-wire cable. The other 4 wires
in the cable may have faults, but these will not affect
the operation of the LAN. The CNT 10 L tests all of the
wires in the LAN cable, and identifies the faults. Even
though these wires may not being used in the LAN
system.
Not all LAN cables are wired straight through Crossover
cables used on LANs are purposely miswired, with their
Receive and Transmit wires cross over. The CNT 10 L
will test this as a bad cable, but it may work just fine as
Crossover cable.
7.6.2 The CNT 10 L says my cable is good, but it
does not work on my LAN
: Many cable testers like
the CNT 10 L only perform continuity style tests (open,
short. crossed, etc). Ethernet LAN cables are const-
ructed in a special way. The 8 wires inside are grouped
into 4 pairs of 2 wires each. Not only must the 8 wires
connect form end to end of the cable, the pairs in the
cable must connect to specific pins of RJ-45 plugs on
to the ends of cable, ignoring the pairing(as described
in ElA/TIA 568), and the cable will test ok. But when
the cable is tried on the LAN, it does not work. This is
because the Iack of the proper pairing causes exces-
sive cross talk in the cable. Preventing the LAN from
working. This type of cable fault is sometimes call a split
pair or double split pair, to detect this type of fault, a
more sophisticated tester capable of performing a NEXT
test (Near End Cross Talk) must be used.
Notes:
Cross talk increases with the length of the cable. A LAN
system will tolerate a certain amount of cross talk. A
short cable (10ft or less) that is improperly paired may
work just fine. However, Ionger cables, paired in exactly
the same way, may not work. This explains why an
installer can make short Jumper cables that work (al-
though they are improperly paired), but when he installs
RJ-45‘s in exactly the same way on a Ionger cable, the
cable does not work.
7.6.3 The CNT 10 L says my phone cable is bad, but
my
phone works oK.
Most single line telephones only use 2 wires in a
modular cable. The modular cable, which terminates in
RJ-11connectors, could have as may as 6 wires in it.
The unused wires may have faults, which the
CNT 10 L identifies, but these faults may have no effect
an the working 2-wire telephone circuit.
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