[Tfug] PoE supplies/injectors

[Bexley Hall]

Hi Zack,

> > I'm curious as to the type of *typical* support provided PoE.
> > I assume legacy installations use midpoint injectors on
> > *select* drops (to eliminate the cost of replacing existing
> > switches)?
> 
> Correct.
> 
> > Or, is it more typical for the switch to be replaced to
> > provide PoE to *all* drops?
> 
> Usually I've seen a small dedicated PoE switch installed in
> addition to any existing switch.

Suggesting that the "cost adder" for PoE is significant enough
that you don't *just* "replace the switch".  I.e., you wouldn't
hesitate to replace an old hub with a new switch -- but *would*
hesitate before replacing a switch with a PoE switch (?)

> > Are there any "smart" injectors (midpoint or endpoint) where
> > "something" can control the delivery of power *to* the PD's?
> 
> Some switches have a per-port PoE setting you can turn on
> and off.  If
> the switch or power injector had a CLI you could probably
> script it so that specific MAC addresses would get power, but...

Exactly.  More to the point, to be able to cycle power to 
individual PD's at will -- not just a "static" setting.

> > I.e., so that I can selectively apply/remove power from
> > individual drops?  And, ideally sense/control the class
> > negotiated by the PD (i.e., so I can *manage* power instead
> > of just providing it)?
> 
> In terms of being plug and play, and full autosense this is a
> chicken/egg problem.  How does the switch known that the device
> connected to it if said device has to be powered in order
> for it to inform the switch that it needs power?

You're assuming the device needs to tell the "controller" (and
not the other way around).

E.g., if you had an AP on the end of a drop, you could
(remotely) decide when and if that AP should be powered.

Or, in the event of a power failure, the injector might
be told to shed certain loads to maximize up-time on
the standby power source.  E.g., in a VoIP installation,
you could opt to only provide power to "every other" station
set to effectively double your up-time on backup.  (this
sort of thing would be relatively easy to configure as
the installer could just specify which drops need to be
available at all times -- like the attendant's station,
the security guard's office, etc.)

The situation that you describe can also be addressed:  by
supplying a lower power (class) to the PD -- just enough
for the network interface to be functional so that it
could signal the "remote" controller that it wants to be
fully powered.  The remote can then tell the injector
to allow the PD to negotiate a "higher" class.  (AFAICT,
this requires cycling power to the PD using "conventional"
PoE signalling  :< )