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3. Power over Ethernet Overview
What is PoE?
The PoE is an abbreviation of Power over Ethernet; the PoE technology means
a system to pass electrical power safely, along with 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 category 3 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 networking 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 15.4 W of DC power
(minimum 44 V DC and 350mA) to each device. Only 12.95 W 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 25.5 W 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 is tap the unused wire pairs 4/5 and 7/8 to carry power; the other four is for
data transmit.
End-Span
End-Span device is directly connecting with powered 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 Po wered 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 CAT-5 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 continuous output
power per cable in IEEE 802.3af is 15.40 W. A later specification, IEEE 802.3at,
offers 25.50 W. When the device is a switch, it is commonly called an End-span
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