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State machine efficiency

Hi all!

From end-user point of view, the IP-Phone should
have the same behavior as any other telephone set.

Especially the end-user should be able to use this
phone set in the same conditions as any other one.
For instance, when a mains failure occurs, classicaly
the  PBX is still working and end-users are still able
to use their phone sets even when these ones have a
local power supply.
In order to offer the same service with IP-Phone,
I think discovery mecanisms and state machines have to
take into account this aspect.
What does it mean?
Clearly, I'm thinking that looking after the terminal
disconnection by pooling the current delivered by
the PSE is not  the best way. Indeed if the terminal is
locally powered by a power supply providing a voltage
just a little bit higher than the voltage delivered by PSE
implies that PSE has not still to deliver any current and
then PSE will decide switching off the power.
Then when the mains failure occurs the terminal is not
powered anymore.