Newsgroups: comp.dcom.cabling
Subject: Re: wire question on Cat5 or Cat5e cables
References:   <[email protected]>
"Watson A.Name - \"Watt Sun, the Dark Remover\""  writes:

> "Dmitri(Cabling-Design.com)" 
> wrote in message news:[email protected]...
> 
> [sbip]
> 
> > With a standard 100BASE-TX equipment it would not really matter if the
> > "missing" conductors are shorted or crossed somehow: they are usually
> ALL
> > connected to the ground plane of the Ethernet switch/hub.
> 
> I checked a 3Com NIC, and I found that the unused pairs were connected
> to ground as you said.  

The unused pairs on Etherner card could be, depending on the card 
wired on one of the following ways:
1. left floating freely (not connected to anything)
2. free wired wired toghether and connected to ground 
   (maybe though coupling capacitor)
3. wires are terminated together through Bob Smith Termination 
   (several resistors and then through capacitor to card ground).

Options 1 and 3 seem to be the most common ways this has been done. 
This is based on looking at the Ethernet designd on Internet and 
looking at some Ethernet equipment I have opened (hubs, switches, 
Ethernet cards etc..)

> The problem I see with this is that the newer
> Crisco switches may have POE, power over ethernet, for a VoIP phone,
> etc.  If that is turned on, the shorted pairs could draw excessive
> current.

The current POE standard (IEEE 802.3af) says that the equioment 
that feed power to the Ethernet wiring must first check that it 
is safe ty feed the power to the wiring. There is a specific 
identifiaction  protocol that identified that there is equipment 
on the other end of the cable that is designed to take standard POE 
power and the protocol even identifies some product features 
(what is the power rating of those equipment). 
The protocol is such that that it really needs to be a POE 
supporting equipment on the cable, before the power supply 
will be turned on. And there is also over current protection 
if short circuit happens afterwards. 
Also earlier pre standard commercial POE impementations 
(for example from Cisco and PowerDsine) to my knowledge 
has some form of identification system to make sure 
that power is not fed to places it should not be fed. 

The fact that the normally unused pair could be 
connected differently on different Ethernet equipment 
it is an issue with DIY Power-over-Ethernet that just 
simply feed the power to those free wires. 
Depending on the connection you could get a fried 
power supply or some fried Bob Smith Termination resistors 
on Ethernet card. In any case the power supply for 
this kind of tricks should be always so energy limited 
(either built in current limit or some fuses or similar) 
that short circuit on the wiring does not cause 
more current to flow on the wiring that it can safely handle. 
For example at equipment buit according POE standard 
(IEEE 802.3af) power supplying hub cutoff current limit is 
typically around 350mA.

Some more information on POE can be found at 
http://www.epanorama.net/links/tele_lan.html#poe



-- 
Tomi Engdahl (http://www.iki.fi/then/)
Take a look at my electronics web links and documents at 
http://www.epanorama.net/