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Chapter 6
PoE overview
What is PoE?
PoE is an abbreviation for Power over Ethernet. PoE technology permits a system to
pass data and electrical power safely on an Ethernet UTP cable. The IEEE standard for
PoE technology requires a category 5 cable or higher for high power PoE levels, but
can operate with a 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 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 VDC and 350 mA) to each device. Only 12.95 W is assured to be
available at the powered device as some power dissipates 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 standards define two types of source equipment:
Mid-Span – A mid-span device is placed between a legacy switch and the powered
device (PD). Mid-span taps the unused wire pairs 4/5 and 7/8 to carry power. The other
four pairs are for data transmission.
End-Span – An end-span device connects directly to the PD. End-span taps the 1/2
and 3/6 wire pairs.
PoE system architecture
The PoE specification typically requires two devices: the Powered Source Equipment
(PSE) and the PD. The PSE is either an end-span or a mid-span, while the PD is a
PoE-enabled terminal such as an IP phone, Wireless LAN, etc. Power can be delivered
over data pairs or spare pairs of standard CAT-5 cabling.
Powered Source Equipment (PSE)
A 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
ES2402-24P-2C / ES2402-16P-2C / ES2402-8P-2C User Manual
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