🚀 go-pugleaf

In article <35DB3C67.1041534F@edwardjones.com>,
Daniel Erskine  <netsup75@edwardjones.com> wrote:

>  I wouldn't think this (above) would make me "_extremely_ lucky", just
>*VERY*
>cautious!  I NEVER connected the 50ohm 10b2 into the 75ohm CATV network! 
>Both
>"networks" are self-contained, or atleast the PC network is ;)
>  Based on your observation, YES combining both networks should not work at
>all!
>A friend of mine told me one of infamous "school-of-hard-knocks"
>war-stories
>about doing this exact same thing... Aaa-ack!

>Sorry, I did not clarify this better!


The basic issue with respect to mixing the 75 Ohm CATV cable with the 50
Ohm Ethernet cable is that because of the different impedances, there
will be signal reflections at the junction.  These reflections can cause
various problems, including such things as (apparent) collisions.

The phenomenon in question has nothing to do with whether or not there
actually is a TV signal on the CATV wire.  Rather, it's similar to the
effect when you look through a window:  the glass has a different index
of refraction from the air (or whatever medium you're looking through,
unless you're in some rather strange circumstances) -- actually, "glass"
is a very loose category of things with a considerable range of indices
of refraction.  Anyway, depending on ambient lighting &c., you'll often
see a reflection in the glass.

If you could "adjust" the index of refaction of the window or the
intervening medium (or both), you would find that the intensity of the
reflection would be minimal when the indices of refraction match.

Similarly, in an electrical circuit, when a signal gets to a point where
there's a (sudden) change in the circuit's impedance, part of the signal
will continue on through, but part will be reflected.

BTW, this is also why coax Ethernet uses terminating resistors -- the
end of the wire is an exceedingly high "impedance mismatch".  :-}  The
resistors' function is to absorb any signal that gets that far (and
there should be a fair amount of that!) to minimize the reflections at
the end.  And this also applies to other terminators, such as those on
SCSI chains....

So:  sometimes you can get "lucky" in spite of impedance mismatches; the
busier the circuit, the less lucky you're likely to be.

Hope this helps clarify what's going on,
david
-- 
David H. Wolfskill				david@dhw.vip.best.com
As a computing professional, I believe it would be unethical for me to
advise, recommend, or support the use (save possibly for personal
amusement) of any product that is or depends on any Microsoft product.