OT: cat 5 cable, ethernet, connection jacks

[Lennart Sorensen]

On Thu, Mar 24, 2005 at 12:13:26PM -0500, Matt Price wrote:
> Hope it's ok if I ask a networking question here... I'm planning to
> lay a bunch of ethernet cable in a room.  I have these fancy Leviton
> RJ45 female jacks and am running the cable through wolding, so having
> many cables come out of a hub is a royal pain.  The computers in the
> room are arrayed along a straight line.  Is it at all possible to use
> the RJ45 jack as a simple junction connecting two pieces of cat 5
> cable?  This would look something like this:
> 
> 
> ----		   ___________			__________
> |hub|--------------|rj45 plug|-----------------|rj45 plug|
> -----		  ------------			---------     
> 		      |				  |
> 		      |				  |
> 		      |				  |
> 		 Computer		       Computer	       
> 
> I imagine this is impossible, or people would do it all the time
> instead of using hubs...  anyway, if someone can explain to me at
> least why it doesn't work, that'd be a help.

Well with RJ45 plugs (used for twisted pair ethernet) each connection
uses either 2 or 4 pairs of wire out of the 4 pairs in the cable.  How
many depends on the speed and type of the link.  10 and 100TX use 2
pairs, 100T4 uses 4 pairs, and Gigabit uses 4 pairs.

So if you don't ever want to support gigabit then you could
theoretically conenct the second connection to the two unused pairs in
the cable, although you would probably cause extra signal noise
problems, and of course would need to split the cable to two plugs at
the hub (is it a hub?  anyone still use hubs when switches are almost
free?  Don't you like full duplex?).  I believe pin 1&2 is the first
pair, and 3&6 is the second pair for normal ethernet.  4&5 is the pair
originally reserved for phone use, but that too causes lots of noise,
and 7&8 is the last pair.

Lennart Sorensen
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