asterisk users - Jan 2004 - [A-bit-OT] Power Over Ethernet Discovery process

Hi,

Since someone asked, here's how POE standard does discovery
process for a POE device. of course is a passive detection...
but that's why you don't have POE always-on on a POE enabled
switch port....

you can find more info in article area of 
http://www.poweroverethernet.com

and full specs @ http://www.ieee802.org/3/af/index.html

You will find a resistance value in the quote below.
The value is 19k to 26.5k for PSE detection signature, with a mid
value of 22.75K

<quote>
The Discovery Process

Power Over Ethernet PSEs are responsible for ensuring that conventional
Ethernet equipment is not damaged by the unexpected application of 48
Volts. The PSEs must determine that a Power Over Ethernet compliant
device is present before the 48V is applied. This is done by the
"discovery process". A relatively low voltage, current limited, is
applied to the CAT-5 cable periodically. A compliant device is required
to have a certain DC resistance between its twisted pairs. If the device
presents this resistance then power can be applied, but if it does not
then power is not applied.

The PSE is responsible for monitoring the Powered Device, to check that
it is continuing to draw power within certain limits. If it does not
(when it is unplugged, for example) then the PSE must remove the power
to that cable and return to the discovery stage again.

The Powered Device may optionally support a classification mechanism, by
which it can signal how much power it will require from the PSE. This
allows for better management of what may be a limited power source
within the PSE. 
</quote>

-- 
Brancaleoni Matteo <mbrancaleoni@espia.it>
Espia - Emmegi Srl