[TriLUG] IP Network Camera/PoE

[Joseph Mack NA3T]

On Wed, 16 Jan 2008, James Tuttle wrote:

> Hi,
>
> I'm buying a wired network camera (that runs Linux!) and 
> want to buy a Power over Ethernet adapter, but am not sure 
> how the adapters work.  I guess I'm not sure how the 
> camera gets power from the adapter.

I haven't used PoE so I don't know all the details. Here's a 
first pass.

All ethernet cables have 4 pairs of wires. Ethernet only 
uses 2 of these pairs (why people are selling 4-pair wires 
when they could sell cheaper, thinner and lighter 2-pair 
wires I have no idea). The other 2 pairs of wire are free to 
for whatever you want, provided that neither end gets a 
surprise when two random devices are connected. One 
convention is for one of the spare pairs to be used to 
supply power to low power devices at the other end. 
Presumably if you're applying power at one end, the voltage 
has to be agreed upon by the receiving end. Maybe there's an 
agreed upon voltage (I don't know).

I'd assumed that the device at the receiving end would pick 
the power directly off the ethernet cable (I still think 
this is right). It looks as if the device pair in the URL 
splices into the ethernet cable, supplying power at one end 
which only appears on the outgoing cable. The receiving end 
picks off the power, which is made available via small power 
plug, and the ethernet is passed on, with no power on the 
ethernet cable. This setup is for remoted devices which 
aren't PoE ready.

I expect what you want is power to be applied on the 
computer end - whether you need an external device to do 
this I don't know (do NICs/hubs/switches exist which apply 
power to ethernet cables?). Your camera being PoE ready will 
just pull the power directly off the ethernet cable.

Joe

-- 
Joseph Mack NA3T EME(B,D), FM05lw North Carolina
jmack (at) wm7d (dot) net - azimuthal equidistant map
generator at http://www.wm7d.net/azproj.shtml
Homepage http://www.austintek.com/ It's GNU/Linux!