COLOR-CODE STANDARDS
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 (the view is from the front of the jacks):
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.
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