It works fine.  The reason it's not common is because the jacks are
wired for future ethernet standards that may use the other two pairs.
 Some early 100Mb gear did use all four pairs.  I've heard that 1Gb gear
does use all four, but I don't remember where and could easily be wrong.

You can buy ethernet splitters that does exactly this.  It has two jacks
and one plug.  You buy two and put one one one end and one on the other
- it eliminates having to rewire the jacks in the office.

There are many devices out there that use the extra pairs for power as
well.  Used for ethernet cameras, remote powerless hubs, some 802.11
access points, etc.  Not terribly common, I suspect becuase there is no
standard and they are often incompatible from manufacturer to manufacturer.

-Adam

Russell McMahon wrote:

>Questions:    How reliably are Cat5e network cables when used
>                    to carry two 100 Mbps signals simultaneously using
>                   the two "spare" pairs?
>
>                 What are the affects of running other low speed or
>                 analogue circuits on the unused pairs?
>
>Discussion:        100 Mbps Cat5e network cables have 4 pairs in the sheath
>but use only two. There are two standard wiring versions  (A & B) (one uses
>green & orange pairs, other = ?). Only difference is which pairs are used. I
>expect that the cable design and the standard take account of the relative
>location of the pairs within the "quad" formed by the 4 pairs and attempt to
>maximise balance and minimise crosstalk.
>
>The version of the standard which I am using is wired
>green/grnwht/org/bluwht/blue/orgwht/brown/brnwhite
>I have noticed that flexible cables sold commercially almost invariably
>DON'T adhere to either standard. They are usually wired 'straight" eg
>grn/grnwhite/org/orgwhite/blue/bluwhite/brown./brnwhite. Assuming the NIC is
>connected in a manner which anticipates the cable being wired to the
>standard, this has the result of "splitting" the second pair used onto a
>separate leg of two physical pairs. This is an abominable thing to do and
>goes against every form of good practice known - even at audio frequencies,
>let alone at 100 Mbps. Despite this a 30 metre flexible cable seems to work
>OK at 100 Mbps !.
>Clearly there is some margin for error. I would expect severe impedance
>bumps at both ends of the cable and a degradation of the BER. The switch I
>am using gives a visible indication when it drops back to 10 Mbps and it is
>not doing so with these cables. A proper tester, which I have not got, or a
>TDR, which I also have not got, would give some indication of what was
>really happening.
>
>Some cables I have seen have only two pairs used and these are wired to pins
>1,2,3,6 which is what is required by the standard. Clearly the NICs ARE
>expecting this arrangement. This cable also works at 100 Mbps without
>"apparent" problems.
>
>This obvious tolerance to pathetic practice encourages me to wonder if I can
>RELIABLY run two 100 Mbps circuits on a single CAT5E cable. eg green &
>orange for one circuit (as per standard) and blue and brown for the other. I
>can of course easily try this (and will) but even if it works (as I'm sure
>it will) I will not have a good idea of the effect on BER without extensive
>testing.
>
>I have also heard of the other pairs being used for eg telephone circuits
>and I'm sure that would also work "well enough"
>I have two CAT5E cables going to required locations in my premises. These
>use flexible cable. Adopting various of the above abominable practices would
>allow greater capacity traded off against unknown degradation.
>
>SO -
>
>Has anyone got any practical experience or knowledge about the effects of
>running two x 100 Mbps circuits in one cable and/or other lower speed
>circuits?
>
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