Colin McDonald writes:
>is there some sort of capacitence/inductence issue at stake then?
>Since, if there isn't, then you really could connect pin one to pin one and
>so on...once you hook the cable up, the device you are connecting decides
>which pins to use...unless there is some sort of interaction going on
>between the wires in the twisted pairs?

	Yes, there are issues.  The pairs are twisted such that
each pair doesn't introduce a magnetic field in to the
neighboring pairs so if you made a pair by taking one lead of one
pair and another lead from another pair, your continuity checker
would work and any DC voltages would certainly go through the
cable, but signals that are fluctuating at 100 megabits or even
10 megabits per second will couple very nicely to neighboring
pairs making a nice little transformer out of the wiring in the
cable.  This is surely not what you wanted and it may show up as
sporadic performance or no performance at all when used as an
Ethernet cable.

	Ethernet cards have transformers in them that pass
signals in and out of the cable so the cable acts like a balanced
feed line.  You might get away with using the wrong colored pairs
on the wrong sets of pins, but if you've got to keep the pairs
straight, it's better to at least get one end right and then you
can use your favorite cable testing tricks to tell you which
wires in the other end need to be hooked up.

	I guess to be accurate, I should say that an Ethernet
cable is four balanced transmission lines.  Some equipment
supplies DC power over the cable also to energize things like
wireless access points, etc.  I really don't know much about the
details of that except that we are installing some of that type
of Ethernet service on our campus.  It is called POE or Power
Over Ethernet.  I don't know what currents are involved, but it
is fairly small and low-voltage.

	Anyway, you do need to keep the pairs straight to
minimize crosstalk between the four pairs.

Martin McCormick WB5AGZ  Stillwater, OK 
Systems Engineer
OSU Information Technology Department Network Operations Group