Length of Ethernet lines
From: Rich Lobdill (richardlaxon.com)
Date: Wed, 6 Jun 2001 05:58:06 -0600 (MDT)
Peter requested info about how critical is the 100m length limit on
Ethernet.

Here at Tierra Nueva we had dedicated conduit running from the CH to all of
the houses. I knew we were making some long runs but in the mix of
priorities during build, some pie-in-the-sky network (no DSL then) didn't
garner a whole lot of excitement.

We've now installed the system and have learned some things about long runs.
I would guess our longest runs are more close to 200m (we've wired it all
with CAT5e 'extended' which is . We are successful with these runs but we
cannot get them to run at 100BaseT. They run fine at 10BaseT which right now
is way fine for the DSL speeds (we don't do any file sharing across our
network).

Things to look out for:

If you have an Auto Switching hub or switch which will run 10 or 100BaseT
you have to manually set the LAN card at the other end of the line to force
it to 10BaseT. However, you only get this option on PCs. The Mac's we have
here are mostly IMac's and I see no way to manually force them to 10BaseT.
So what happens at start up is the Mac and the hub communicate at 10MHz and
ask each other if they can run at 100MHz if both say yes they crank it up to
100MHz and communications fail. It's not like a modem protocol which tests
each speed for valid data an then settles on the fastest speed.

So for Mac's on a long run we have a separate fixed 10BaseT hub which then
forces the Mac to work at 10MHz and the problem is solved.

We do have one PC on a long run which has a Linksys NIC and we seem to have
had problems with that one running at 10MHz. In that case we have placed a
cheap 4 channel hub right near the computer to sort of act like a repeater
and that seems to have solved the problem.

Also there was a bit of misinformation on the number of wires in a CAT5
cable used for Ethernet. The maximum number of wires used for one connection
is four. That is pairs 1-2 and 3-6 (usually w/org-org and w/grn-grn). This
is for 100BaseT. If you are only using 10BaseT you can use only pair 1-2.

The rooms in our houses were wired with home runs from a given room to the
phone box outside. There are a number of installations where we piggy backed
on the phone line CAT5 -- the phones use two pair for line1/line2 and we
used the unused two pair for a 100BaseT connection. The wall jack in
question would end up with two jacks, one for phone one for LAN.


Our network is definitely done on the cheap, there was no convincing
technical evangelist in the group but even with its short comings people
light up when they see that first web page download and have no monthly ISP
charge and they can talk on the phone at the same time.

Rich Lobdill


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