[GLLUG] Want to learn PoE

Chris Bell chrisbell at chrisbell.org.uk
Thu Oct 24 10:14:08 UTC 2019

On Thursday, 24 October 2019 09:31:25 BST nickmount91 via GLLUG wrote:
> Hi everyone,
> I have some Cisco CP-7911G VoIP Phones that I want to use on a fibre
> broadband connection. Some of them have PSUs, but some don't. Can anyone
> point me towards resources where I can learn about Power over Ethernet,
> please?
> regards
> Nick
> Sent with [ProtonMail](https://protonmail.com) Secure Email.
Hello Nick,
If they are using 10/100 base ethernet they will only be using two of the four 
pairs, and PoE could be supplied via the other two pairs, which is what I do 
with CCTV cameras, I just separate the unused pairs and provide close to 48v 
DC correct polarity via self re-setting fuses and let them take whatever 
current they require via their own switchmode regulator.
However I assume they are using gigabit ethernet.
The full procedure is fairly complex, with the PoE sender chatting to the 
device and increasing the output on demand over up to all pairs, so the 
easiest method is to use a commercial PoE inserter or a PoE enabled bridge/
switch. Expect non-PoE ethernet devices to link transformer centre-taps to 0v, 
and short PoE.
They are listed at millsltd.com and cpc.farnell.com
Chris Bell
Website http://chrisbell.org.uk
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