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What is Power over Ethernet?
PoE is a technology that integrates data and power on standard Ethernet
infrastructure, providing new options for power distribution. This allows
IP telephones, wireless LAN access points, IP surveillance cameras and
other embedded appliances to receive power as well as data over
existing Cat.5 (or above) Ethernet cabling. Formally approved as an
international standard, PoE is established as an economical, safe power
distribution method and is already deployed in corporations throughout
the world.
In simple terms, it uses the wires in standard Ethernet cabling to send
power to the operating device. The device can then take its power from
the Ethernet cable which means that a separate, local, power supply
close to the device is not needed.
Growth in PoE Markets
According to VDC research, shipments of PoE-Enabled switch ports
totaled approximately 47 million units in 2007. The research predicts
that the growth rate for PoE switch ports will be almost double the rate
of overall Ethernet port shipments to reach more than 130 million by
2012, which represents one quarter of all switch ports.
PoE Standard: IEEE 802.3af
The IEEE standard 802.3af describes the mechanism for Power over
Ethernet. The standard provides the capability to deliver both power and
data over standard Cat.5 (or above) Ethernet cabling.
The 802.3af specifcation provides 48 volts DC over two out of the four
available pairs on a Cat.5 Ethernet cable with a maximum current of
350mA. After taking into account some power loss over the cable run,
about 12.95W is available to the Powered Device.
The standard also permits usage not only with 10BASE-T and 100BASE-
TX, which only uses two of the four pairs in the cable, but also with
1000BASE-T (Gigabit Ethernet) which uses all four pairs for data
transmission. This is possible by utilizing the Phantom Powering method.
PoE Plus Standard: IEEE 802.3at
In September 2005, the IEEE 802.3at task forced to begin working on a
new standard known as PoE Plus, which would allow standard Ethernet
cables to supply up to 24W of power by using both twisted pairs. With
more power available, Power over Ethernet applications will be able
to accommodate self-powered devices including thin clients, P/T/Z
cameras, WiMAX transmitters, and video phones. The objectives for the
802.3at standard include the following:
- Adherence to relevant 802.3af power safety rules and limitations
- 802.3at PSE must be backwards compatible with 802.3af PD
- Maximum power within practical limits provided to PDs (at least 24W)
- Indication that an 802.3at PSE is required when connecting an 802.3at
PD to an 802.3af PSE
PoE Overview
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2007
2012
100
200
300
400
500
Worldwide Shipments of Switch Ports
(Millions)
PoE Enabled Switch Ports Non-PoE Enabled Switch Ports