At a past job, I was shown another good reason for NOT trying to squeeze two
links over one cable.  Some CAT5 cable manufacturers 'cheat' on the 4-5 and
7-8 pairs.  Since they are not used in typical 10baseT and 100baseT
ethernet, those pairs are not twisted to CAT5 specs.  The cabling person who
told me this then demonstrated, by stripping some cable we had laying
around.  Sure enough, the 1-2 and 3-6 pairs were nice and tightly wound,
while the other two were nowhere near the same.

Steve Miller
Beechglen Develoment, Inc.
Cincinnati, Ohio

Jeff Kell wrote:
>
> ... Pinout-wise
> both 10Mb and 100Mb only use 1-2 and 3-6 (green and orange in your
> example).  4-5 and 7-8 are typically unused, though in some instances
> they can be used to "squeeze" two links over one cable (with suitable
> wiring accomodations on either end), but not recommended as the coming
> Gigabit over copper standard requires all 4 pairs and you will have to
> redo your cable infrastructure.

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