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Date:      Mon, 19 Jul 1999 00:38:49 -0700
From:      Tim Baird <tim@storm.digital-rain.com>
To:        Doug <Doug@gorean.org>
Cc:        freebsd-hackers@freebsd.org
Subject:   Re: poor ethernet performance?
Message-ID:  <3.0.2.32.19990719003849.00981d20@storm.digital-rain.com>
In-Reply-To: <37921FB4.B01C6F86@gorean.org>
References:  <3.0.2.32.19990716231622.007e2100@storm.digital-rain.com> <3.0.2.32.19990717015406.0095f670@storm.digital-rain.com>

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At 11:40 AM 18/07/99 -0700, you wrote:
>Tim Baird wrote:
>
>> I hope everyone is benefitting by these simple facts....
>
>	*chuckle* "Simple facts.." You sound like my physics professor. I for one
>am benefitting very much from the discussion. I got hired at my current job
>as a software person, but I have a background in hardware so I try and make
>it into the NOC every excuse I get (promotability, don't you know). It
>always helps if sound like I have a vague idea what I'm talking about. :)  

I'll take that as a compliment ;)

>
>	I just made up my first ethernet cables the other day, and learned an
>interesting tidbit that I'm sure is beyond elementary to most of you, but
>may benefit someone else. What I was told is that the reason Cat 5 cable is
>so much more efficient is that each of the 4 pairs of wire is twisted at a
>different rate. This helps reduce the possibility of frequency
>synchronization for the EM fields the pairs create. 

Your description (what you were told) here is incorrect....the number of
twists in a cable had **NO** effect on the spectral content of the
cunducted signal or resulting radiated/induced signal.....to do so would
require the conductors to have a nonlinear conduction characteristic which
they most assuredly do not (for all practical purposes).   The design of
the cable is such that adjacent pairs have as little effectively parallel
length as possible.  Obviously, the currents in the wires share the same
axis, so the magnetic coupling is only reduced by the fact that interfering
magnetic fields will tend to induce a common mode current in adjacent
pairs...particulary since both conductors in the receiving cable pair --on
average-- are exposed equally, the idea in "randomizing" the twists is to
reduce the capacitive coupling as much as possible.  Capacitive coupling is
a more localized effect, and thrives when conductors share a common plane
in close proximity...this is why capacitors are designed as two metal
plates very close together..the electric field between the plates
(conductors) is much higher than if they were perpendicular...or not nicely
parallel.

I hope this clarifies the situation :)




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