[svlug] How do I make a cross-over patch cable?

[Ray Olszewski]

At 02:46 AM 9/20/98 -0700, Rick Moen wrote:
>Quoting Ray Olszewski (ray at comarre.com):
>
>> Why? (That is ... why is splitting a bad idea?)
>
>Probably cross-talk.  In any event, results have been poor.

Raffi said about the same thing. And from elsewhere, I have seen this (no
splitting) as conventional wisdom ... but in those cases it comes from
people who will make more money if they install more wires (sort of like
Seth's second-hand report), so I take it with at least a grain of salt.

But what experience is this based on? (When the answers are qualified by
"probably" and "I guess," this seems a fair question to ask.) I thought the
reason the pairs in a Cat 5 cable were twisted differently was to control
crosstalk. Why not go as far as to use 2 cables per connection, running the
orange and green pairs in entirely separate cables -- won't this reduce
crosstalk even further? I'm not seriously making this last suggestion, of
course, just trying to highlight my confusion here -- why is crosstalk
*between* two connections a more serious problem than crosstalk between the
two pairs *within* a connection?

A bit over 5 years ago, when I was the network manager for a school, we
wired the whole place with split cables. (Plenum-rated Cat 5 was very
expensive back then, and this trick saved thousands of dollars, a big
fraction of our wiring budget. We figured we'd reterminate if necessary when
we upgraded to 100 mbits, since reterminating is so much cheaper than
pulling wire.) Over the 3 years that I was there, only 1 wire failed, and
that one completely (even when 1 of the 2 connections was dark); I'm told
that since then, no others have failed, and performance was always good
(this only at 10 mbits, though).

So ... is splitting *really* a bad idea, or are the problems only
theoretical? Is it a bigger problem for 100 mbits than for 10? And why is
Cat 5 cable always manufactured this way -- with 2 pair that get used, 2
that stay dark, in ordinary use ... doesn't this seem like a waste?
------------------------------------"Never tell me the odds!"---
Ray Olszewski                                        -- Han Solo
762 Garland Drive
Palo Alto, CA  94303-3603
650.321.3561 voice                               ray at comarre.com
650.322.1209 fax                 http://www.comarre.com/ray.html
----------------------------------------------------------------


--
echo "unsubscribe svlug" | mail majordomo at svlug.org
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ to unsubscribe