OT: cat 5 cable, ethernet, connection jacks

[John Macdonald]

On Thu, Mar 24, 2005 at 12:19:13PM -0500, Lennart Sorensen wrote:
> 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.

One suggestion that I don't think anyone else has made is that
if the ASCII picture is correct you could replace the rj45 plug
in the middle with two rj45 plugs, one connected to the cable
that comes from the hub, the other to the cable that runs to
the room on the right.  Then, instead of connecting directly to
the computer in the middle room, connect them both to a switch
(or hub) - the cable from the originating hub going to the
uplink port (if the switch doesn't autosense).  Add a third
connection from the new switch to the computer and you're set.
This method means (1) you don't have to redo the existing
wiring, just one wall plug, and (2) you don't have to make
your own rj45 plugs (that takes a special crimping tool and
a small amount of experience - the wall socket just takes a
screwdriver and keeping track of which colour wires need to
be used).  If you use an autosensing switch, you won't need
to figure out which of the two cables is from the hub and
which goes on to the next room - they both would just plug
into random ports on the switch anyhow.


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