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4. Power over Ethernet Overview
What is PoE?
PoE is an abbreviation of Power over Ethernet. The PoE technology means a
system safely transmits both power and data on Ethernet UTP cable. The IEEE
standard for PoE technology requires Category 5 cable or higher for high power
PoE levels, but can operate with Cat3 cable for low power levels. Power is supplied
in common mode over two or more of the differential pairs of wires found in the
Ethernet cables and comes from a power supply within a PoE-enabled network
device such as an Ethernet switch or can be injected into a cable run with a mid-
span power supply.
The original IEEE 802.3af-2003 PoE standard provides up to 15.4W of DC power
(minimum 44V DC and 350mA) to each device. Only 12.95W is assured to be
available at the powered device as some power is dissipated in the cable.
The updated IEEE 802.3at-2009 PoE standard, also known as PoE+ or PoE plus,
provides up to 25.5W of power. The 2009 standard prohibits a powered device
from using all four pairs for power.
The 802.3af/802.3at define two types of source equipment: mid-span and end-
span.
Mid-span
Mid-span device is placed between legacy switch and the powered device. Mid-span
taps the unused wire pairs 4/5 and 7/8 to carry power; the other four are for data
transmit.
End-span
End-span device is directly connected with power device. End-span could also tap
the wire 1/2 and 3/6.
PoE System Architecture
The specification of PoE typically requires two devices: the
Powered Source
Equipment (PSE)
and the
Powered Device (PD)
. The PSE is either an end-span
or a mid-span, while the PD is a PoE-enabled terminal, such as IP phones, wireless
LAN, etc. Power can be delivered over data pairs or spare pairs of standard Cat5
cabling.
Powered Source Equipment (PSE)
Power sourcing equipment (PSE) is a device such as a switch that provides
(sources) power on the Ethernet cable. The maximum allowed for continuous
output power per cable in IEEE 802.3af is 15.4W. A later speci-fication, IEEE
802.3at, offers 25.50W. When the device is a switch, it is commonly called an
end-span (although IEEE 802.3af refers to it as endpoint). Otherwise, if it is an
