very good, thanks for the clairefication.
I was indeed missing something.
73
Colin, V A6BKX
----- Original Message ----- 
From: "Martin McCormick" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Friday, July 14, 2006 1:01 PM
Subject: Re: jacks on cat5


> 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
>
>