asterisk users - Jan 2004 - Power Over Ethernet for *any* ethernet switch(or hub); product idea

PoE, or 802.3af, uses a device detection routine to determine if the
connected device needs power.

The process, in greatly simplified terms, is as follows:
1.  Detect link state
2.  Send a pulse of a known frequency and intensity over the
	TX/RX pairs
3.  Listen for reflection.
	3a.  No reflection- provide power
	3b.  Reflection- no power

Devices that comply with 802.3af have filters designed into the
TX/RX paths to block the detection pulses, thereby identifing
themselves as able to use PoE.  The detection process is passive
on the device, since if it has no power it cannot 'signal' that
it needs power.

The process is repeated several times a second to ensure that a PoE
is not unplugged and a non-PoE is plugged into it's place and damaged.

Issues with midspans devices:  The 24 port models are usually 12 port
in reality.  12 in and 12 out.  Sure there are 24 ports, but you are
only going to power 12 devices.  So in a larger environment they quickly
get expensive.

To make the whole situation more interesting the Cisco phones support
not
only 802.3af, but Cisco's own spin on inline power, which is similar in
design to 802.3af.

Dan

-----Original Message-----
From: Steven Critchfield [mailto:critch@basesys.com] 
Sent: Tuesday, January 20, 2004 7:55 AM
To: asterisk-users@lists.digium.com
Subject: Re: [Asterisk-Users] Power Over Ethernet for *any* ethernet
switch(or hub); product idea


On Tue, 2004-01-20 at 08:02, Matteo Brancaleoni wrote:
> Hi.
> 
> > The POEI simply connects the four ethernet signals on each of its 
> > "inputs"
> > (pins 1,2,3,6 on each) to the same pins on its corresponding
"outputs". 
> > Additionally, it supplies -48VDC (maybe selectable if there are
other 
> > voltage needs) on the appropriate pins (also maybe selectable if
different 
> > vendors use different wiring conventions for POE) of its
"outputs".
> 
> and probably you're going to fry something on your lan.
> POE isn't simple power on the right pins, but is
> a sort of "protocol". Really, on POE enabled devices
> (or injectors) you won't measure the DC with a tester,
> simply because POE on port X is enabled after a request
> by the device on that port. this is for mantaining compatibity with 
> non POE devices. so you will need also something that detects the 
> power request on each port and enables it.
How does a non powered device request power?
-- 
Steven Critchfield  <critch@basesys.com>

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