[olug] RJ45s

[Rob Townley]

On 8/24/07, Luke -Jr <luke at dashjr.org> wrote:
>
> Reading up on 802.3af (PoE), it appears to be intentionally designed to be
> compatible with MDI-X.
> In particular, Wikipedia says:
>         The polarity of the DC supply is unspecified; the powered device
> must operate
>          with either polarity on pair 45+78 or 12+36 with the use of a
> bridge
>          rectifier.
>
> Pairs 45+78 are the same in both T568a and T568b. Pairs 12+36 are flipped
> in a
> MDI-X arrangement. Thus, if the PoE provider used pairs 12+36, the
> polarity
> would be reversed and a 802.3af compliant device must tolerate it. If it
> uses
> pairs 45+78, it looks the same as a regular MDI cable.


Yes, the design goal is to be compatible with MD-X, but from my experience
with supposedly 802.3af compatible hardware, i would not count on the
manufacturer to implement this part correctly.  The conversation from the
ieee website posted earlier shows there it just adds more confusion.  I wish
they would have spent more time reserving a wire for grounding than making
it compatible with crossover cables.