| Ethernet RJ-45 jacks use wires 1, 2, 3 and 6 of the eight
available. The diagram at right shows the "standard" wiring scheme
used by electricians when they install RJ-45 jacks. This
scheme is T568B.
The pattern of the colored pairs rises from the fact that
RJ-11, plain old telephone system (POTS), jacks have ony 4
wires, and that they are paired inner and outer, with the
inner pair being the primary telephone line. The outer pair
may be used for a second telphone line in many home and
small-business situations.
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| Because one historically assumes that the center, or first
pair is POTS, ethernet uses the second and third pairs. The
second pair is the orange pair, outside the center, for
historical reasons. The third pair is the green pair, at the
bottom in the RJ-45 plug diagram.
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| Having made this explanation, one may now see why the back of
the RJ-45 jack has the pairs ordered blue, orange, green,
brown. The pairs of interest in ethernet are the green (1 & 2)
and orange (3 & 6) pairs.
It should be noted that even if the colors are different in
your ethernet cable, or if only the necessary two pairs are
wired, the numbering and connection pattern are still the
same. The conductors in an ethernet wire are counted left to
right, with the plug facing away from you and the plastic
spring-clip down.
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