Allied Telesis AlliedWare Plus Operating System Version 5.4.4C (x310-26FT,x310-26FP,x310-50FT,x310-50FP) - Manual (Page 587)

Power over Ethernet Introduction
Software Reference for x310 Series Switches
C613-50046-01 REV A AlliedWare Plus TM Operating System - Version 5.4.4C 22.3
PoE (all standards)
The general objective that is common to all PoE methods described in this document is to
distribute both data and cable over the same cabling that is used for transmitting Ethernet
based data. This eliminates the need for having one set of cables and outlets for data, and
another set for power. Also, because the voltage and power requirements are much lower
than for mains powered devices, the cabling and installation costs are significantly
reduced.
Power Sourcing Equipment (PSE) such as an Ethernet LAN switch or router, supplies
power to the cable together with the data. Powered Devices (PDs) such as Wireless Access
Points or an IP Phones, receive power and data over this same cabling. The PSE employs
various methods of power classification (depending on the standard) for detecting
compatible PDs from non-compatible devices and will only provide the maximum power
limit to compatible PDs, based on their PoE device class. The PSE continuously monitors
the PDs and stops providing power when it is no longer requested or it detects an
overload or short circuit condition on a port.
PoE (IEEE 802.3af )
The IEEE 802.3at-2003 standard specifies how power is distributed along with data on
twisted pair Ethernet LAN cables. The standard specifies that the PSE is able to supply up
to 15.4 watts (W) of power (at a nominal 48 VDC), with the full 100 m cable length, is then
able to utilize 12.95 W. The difference between these power levels (15.4 -12.98) allows for
power loss within the cabling. This figure is approximate and will vary with the cable
length and quality. The IEEE 802.3af physical layer classification is a static power allocation
based on power bands for power management.
Enhanced PoE
Enhanced was developed prior to IEEE802.3at standard to provide more power the PDs
than was currently offered by the old 802.3af standard. Enhanced PoE supplied between
15.4 W and 20 W per port at 48 VDC. Typically, these PD would be used for applications
such as building security and video surveillance. Note that IEEE 802.3at standard PoE+ PDs
that require 56 VDC cannot use Enhanced PoE PSEs instead of IEEE 802.3at standard PoE+
PSEs. Enhanced PoE PSEs cannot replace IEEE 802.3at standard PSEs when using any PoE+
PDs.