Re: Gigabit network

[ian.bird@xxxxxxxxxxxxxxxxxxxxxxx]

I am in the process of building a POE PD (powered device) and have bought a
Sonicwall PoE injector
(http://www.sonicwall.com/products/wirelesssolutionenablers.html#poe)
as my
reference power supply. This is designed to meet the 802.3af standard and
does not mention Gigabit. It does use the spare pairs and also honours the
signature resistor reading of a true PD and will only supply power when it
is seen.

For my PD end of things the hot favourite is a Texas device
http://focus.ti.com/docs/prod/folders/print/ptb48540c.html
which is a
pretty integrated solution providing 12 volts (also available in 5 and 3.3)
at just under an amp from the injector. I will not be making any allowance
for Gigabit as the embedded chips I use are only capable of keeping up with
10Mbit. I will however probably use centre tapped transformers if I can get
them at a competitive price (one design instead of two moving forward).

>From what I can see the Dlink/Linksys stuff simply puts low voltage
(12v?)
onto the spare pairs and hopes to be able to get the same out the other
end. None of the brains/safety behind the 802.3af standard.

Whoever wanted it simple ;-))

Ian






Richard
Malcolm-Smith            To:       ukha_d@xxxxxxx
<rich@xxxxxxx>        cc:       (bcc: Ian Bird/CV/Novartis)
Subject:  Re: [ukha_d] Gigabit network
25/02/2005 18:59
Please respond to
ukha_d







ian.bird@xxxxxxx wrote:

> Secondly, power over Ethernet is becoming popular with new embedded
> gadgets supporting it. It uses the 'unused' pairs in a 10/100
interface
> for the power. In the Gigabit world there are no unused pairs and you
may

> find that the majority of these 'new' devices cannot exist on this
faster

> network. There are ways round this latter point at the design stage
but I

> have not seen any devices advertising this ability.

802.1af supports 2 modes of delivery, and only one is over the spare pairs.
The
other has centre tapped transfromers in the magnetics, and uses that to
send DC
over the 2 pairs while they are being used for signaling. Any 802.1af
complient
device should accept power both ways, but the only thing that can supply it
on
the same pairs as the ethernet signal.

This is why the add ons you see for linksys and dlink are not actually
802.11af
power over ethernet, they are only able to use the spare pairs.