[Soekris] Max distance for PoE

Henry Spencer henry at spsystems.net
Sun Nov 30 02:36:04 UTC 2003

On Sat, 29 Nov 2003, Christopher R. Hertel wrote:
> > Now, I wanna find out about Power over fiber :)
> Power over IP.  That's the ticket.

Believe it or not, it's been done, more or less.

A number of years ago, Bell Labs did an experimental phone which tried to
combine the use of a fiber link with a useful property of most ordinary
phones:  they work even when the power company is out to lunch.  (I.e.,
when the phone company's emergency generators are running but the
customer's wall-socket power is out.)

It actually succeeded, too.  It was digital, of course, and given the
realities of using a single fiber, it was half-duplex, switching rapidly
back and forth to give seemingly-continuous bidirectional communication. 
But the protocol was highly asymmetrical:  the phone-company switch sent
most of the time, with the phone getting to send only in occasional short
bursts.  The switch's transmitter put quite a bit of optical power into
the fiber, and at the phone, most of it went to a small high-efficiency
solar cell. 

That plus ultra-low-power electronics sufficed for voice communications;
the only trick was running a respectably loud ringer.  For that they had
to sweat a bit, including a high-efficiency custom ringer design and a
relatively short ring.  But they did make it work. 

This may be a little much to ask from Soeren. :-)

                                                          Henry Spencer
                                                       henry at spsystems.net

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