[Asterisk-Users] Re: Power Over Ethernet for *any* ethernet switch (or hub); product idea

[Stephen R. Besch]

Steven Critchfield wrote:

>>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?

As far as I know, it doesn't. The POE source somehow monitors the line 
(using impedance, etc) to determine if there is anything connected to 
the pairs used to supply power. Since netcards and net connected 
equipment are not supposed to use the "power" pairs, this should work in 
most cases. If the termination just leaves them unconnected the 
impedance is infinity and power will not be applied. Conversely, if the 
termination are all grounded, impedance is (near) 0 and power will not 
be applied.  For some range of impedances in between, the far end is 
assumed to require power and power will be applied.

Stephen R. Besch