[Tfug] PoE supplies/injectors

[johngalt1]

>----- Original Message ----- 
>From: "Zack Williams" <zdwzdw>
>To: "Tucson Free Unix Group" <tfug at tfug.org>
>Sent: Sunday, February 14, 2010 12:19 AM
>Subject: Re: [Tfug] PoE supplies/injectors
>

>> 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.

As I say it, in the olden days there were not an abundance 
of devices that would use the power. So why bother?

>> Or, is it more typical for the switch to be replaced to
>> provide PoE to *all* drops?

Depends on the application. If a PoE phone intallation is 
going in, the answer is yes.

>Usually I've seen a small dedicated PoE switch installed in 
>addition
>to any existing switch.
>
>> Are there any "smart" injectors (midpoint or endpoint) 
>> where
>> "something" can control the delivery of power *to* the 
>> PD's?

Yes. See below.

>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...
>
>> 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?
>

Maybe that's why they developed IEEE 802.3af and 802.3at

http://en.wikipedia.org/wiki/PoE#IEEE_802.3af_.2F_802.3at_Power_over_Ethernet

A problem is that older stuff especially has spotty 
compliance to said specs. But really, that is not the sort 
of thing one can anticipate and design for, IMO.














- Zack

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