COLOR-CODE STANDARDS Last
updated: 5/20/2002
Again, please bear with me... Let's
start with simple pin-out diagrams of the two types of UTP
Ethernet cables and watch how committees can make a can of
worms out of them. Here are the diagrams:
Note that the TX (transmitter)
pins are connected to corresponding RX (receiver) pins, plus
to plus and minus to minus. And that you must use
a crossover cable to connect units with identical
interfaces. If you use a straight-through cable, one of
the two units must, in effect, perform the cross-over
function.
Two wire color-code standards apply:
EIA/TIA 568A and EIA/TIA 568B. The codes are
commonly depicted with RJ-45 jacks as follows:
If we apply the 568A color code and show all eight
wires, our pin-out looks like this:
Note that pins 4, 5, 7, and 8 and the blue
and brown pairs are not used in either standard. Quite
contrary to what you may read elsewhere, these pins and wires
are not used or required to implement 100BASE-TX
duplexing--they are just plain wasted.
However, the actual cables are not
physically that simple. In the diagrams, the orange pair
of wires are not adjacent. The blue pair is
upside-down. The right ends match RJ-45 jacks and the
left ends do not. If, for example, we invert the left
side of the 568A "straight"-thru cable to match a 568A
jack--put one 180° twist in the entire cable from
end-to-end--and twist together and rearrange the appropriate
pairs, we get the following can-of-worms:
This further emphasizes, I hope, the importance
of the word "twist" in making network cables which will
work. You cannot use an flat-untwisted telephone cable
for a network cable. Furthermore, you must use a pair of
twisted wires to connect a set of transmitter pins to their
corresponding receiver pins. You cannot use a wire from
one pair and another wire from a different pair.
Keeping the above principles in
mind, we can simplify the diagram for a 568A straight-thru
cable by untwisting the wires, except the 180° twist in
the entire cable, and bending the ends upward. Likewise,
if we exchange the green and orange pairs in the 568A diagram
we will get a simplified diagram for a 568B straight-thru
cable. If we cross the green and orange pairs in the
568A diagram we will arrive at a simplified diagram for a
crossover cable. All three are shown below.
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