HP ProCurve 2520 Planning And Implementation Manual page 12

Introduction
Overview
Since the original introduction of PoE, the IEEE has initiated a new project
called 802.3at which is commonly referred to as PoE+. This project enhances
PoE in a couple of very important ways. First, it provides up to 30W of power
to a Powered Device (PD), 25.5 watts to the device and 4.5 for line loss, and
allows this power to also run on cabling designed for 1000BASE-T. Secondly,
it provides a new mechanism for communicating power capability and
requirements using the 802.1ab Link Layer Discovery Protocol (LLDP). This
protocol addition allows PoE+ switches to deliver power more efficiently and
thereby provide power to more devices for a given power supply capacity. The
new standard is going to be a superset of the 802.3af because it provides all
the same functionality, and more. The table below shows the capabilities of
802.3af versus 802.3at.
In order for 802.3at to provide higher power, Class D (Cat5e) or better cables
are required. 802.3at also increases the minimum output voltage of the Power
Source Equipment (PSE) from 44 volts to 50 volts. For this reason, you may
note that ProCurve PoE+ devices use a 54 volt power supply.
N o t e
The detection and classification functions ensure that if two PoE sources are
attached together, power will not be improperly applied.
1-2
Power to PD Physical
<25.5W 802.3at
<12.95W 802.3af
802.3at
Classification Discovery
Logical
802.3at
802.3at