[Asterisk-Users] [OT] Old Building Needs a New Telephone System

[Bownes, Robert]

My standard answer to POE questions....Mostly stolen from or repeated in
a Network Computing issue about 2 months ago.


PoE factoids:

PoE uses the spare pairs *or* the data pairs (which one to use is
automatically detected) in an ethernet (10 or 10/100) cable to carry
-48V dc from the power sourcing equipment (PSE) in an endpoint switch
(or midspan hub) to the powered device (PD) appliance at the other end
of the cable. Clearly, use of the spare pairs requires that they be
connected all the way from PSE to PD, which may not be the case in some
legacy installations.

The PoE power limit is 13W per PSE port. A new standard is being
discussed which will raise this to about 25W. But don't expect it for a
few years and it's primary use is security cameras requiring
pan/tilt/zoom.

Newer ethernet switches include the PSE function internally, but Midspan
Hubs can also be used to insert PoE power in legacy installations.
Legacy PDs can also be powered by PoE 'splitters' or 'taps', which pull
the power from the ethernet and deliver it to the PD via a short cable.

PoE appliances include:

	Phones
	Cameras
	RF ID readers
	Displays
	Wireless Access Points
	Musical instruments
	

The PoE standard is IEEE 802.3af. It was approved about a year ago.
There are previous, proprietary PoE schemes from a number of vendors.

PoE's -48 V dc is designated as Safety Extra-Low Voltage (SELV). SELV
(safety extra low voltage) is a secondary circuit which is designed and
protected so that under normal and single-fault conditions, the voltage
between any two accessible parts does not exceed a safe value (42.2 V
peak or 60 V DC). It is lower than standard telephone network voltage
(TNV).

> -----Original Message-----
> From: Kevin P. Fleming [mailto:kpfleming at starnetworks.us] 
> Sent: Monday, November 08, 2004 2:14 PM
> To: Asterisk Users Mailing List - Non-Commercial Discussion
> Subject: Re: [Asterisk-Users] [OT] Old Building Needs a New 
> Telephone System
> 
> Tim Donahue wrote:
> > First, I will admit that I have not worked with PoE before so I'm 
> > asking this for my own benefit as well as the OP's benefit. 
>  Doesn't 
> > PoE require at lest 3 pairs to be availible?  I know that 
> pins 1, 2, 
> > 3, and
> > 6 get used for ethernet communications and doesn't the power get 
> > transmitted over pins 4 and 5?
> 
> A PoE-enabled connection needs all four pairs. Two pairs for 
> Tx/Rx, one pair for power, one pair for ground.
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