[NTLUG:Discuss] Switch Question

[Paul Ingendorf]

Connecting from one switch to another switch your fine.  The real problems
are usually experience using hubs which for 100Mb ethernet I would never use
any more because supposedly you can't from one hub to another without a
switch, never actually tried this to see how it breaks.  Electrical
isolation only prevents ground loop problems that were common with things
like 10 base 2.  Going from one building to another can offer other
electrical issues that will traverse the cable however.  This is why it is
still recommended you use fiber for ethernet from one building to another.
Also as far as I have ever known the limit to standard copper ethernet has
always been 100m or about 330ft.

In the end two of those killer wrt routers with maybe wifi-box or something
similar install will probably be sufficient for what you are looking to do
and has a much lower risk associated with it.  If you need more power to
make the jump from one building to the other 2 coffee cans and a length of
50Ohm wire will probably give you more than enough gain to make it plans for
these antenna can be found all over the net.

-----Original Message-----
From: discuss-bounces at ntlug.org [mailto:discuss-bounces at ntlug.org]On
Behalf Of Kipton Moravec
Sent: Sunday, November 14, 2004 7:54 AM
To: NTLUG Discussion List
Subject: Re: [NTLUG:Discuss] Switch Question


At 06:09 PM 11/13/04, you wrote:

The problem you are referring to is that the ground potential between two
buildings could be several volts if they share the same power-pole
transformer, or more if they have different power pole transformers.

This is a problem if you want to use RS-232 or RS-422 or RS-485 because
usually one of the lines is a ground, so you are matching ground to ground
through the relatively thin wire. Basically the whole building will want to
equalize through the small ground wire. Not a good idea.

The way around it is not to use RS-232, but an isolated RS-422 (full
duplex) or RS-485 (half duplex) if you are going with serial connections
and wire.  This works by using a little transformer to isolate and prevent
the grounds from connecting.  You end up with a "relative" system,
independent of the building's power.

However, Ethernet is already an isolated system. The people who designed
the standard were aware of the problem and the final component on all
Ethernet cards and interfaces is a little transformer to provide the
isolation, to prevent exactly what you are worried about.

Kip



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