Specifications

34
Black Box Guide to Structured Cabling
Section NameSection Name
Other Standards
What is PoE?
Your twisted-pair Ethernet structured cabling system has
another role to play—providing electrical power to low-wattage
electrical devices.
Power over Ethernet (PoE) was ratified by the IEEE as the
802.3af-2003 standard. It defines the specifications for low-level
power delivery—roughly 13 watts at 48 VDC—over four-pair CAT3
or CAT5 twisted-pair data cables to PoE devices. PoE is forward and
backward compatible with other Ethernet protocols.
When planning your structured cabling system, consider PoE
for powering widely distributed devices, such as VoIP telephones,
wireless access points, IP cameras, video kiosks, and more.
Advantages of PoE.
Savings. Because PoE operates across the UTP data cable, it
eliminates the need to run both data and power lines to each
network device. It also reduces the need for conduit, electrical
wiring, and outlets.
Flexibility. PoE enables you to locate devices without worrying
about the nearest electrical outlet. This is especially important for
devices such as wireless access points or surveillance cameras that
are often located in hard-to-reach areas.
Reliability. PoE increases reliability because it enables you to
power remote devices using a centralized power source backed up
by an uninterruptible power supply (UPS). So equipment, such as
VoIP phones, will continue to work even if the power goes out.
How does PoE work?
Ethernet cable consists of four twisted pairs. PoE sends power
over these pairs to PoE-enabled devices. In one method, two pairs
are used to transmit data, and the remaining two pairs are used
for power. In the other method, power and data are sent over
the same pair.
When the same pair is used for both power and data, the power
and data transmissions don’t interfere with each other. Because
electricity and data function at opposite ends of the frequency
spectrum, they can travel over the same cable.
PoE devices.
There are two types of PoE devices: Power Sourcing Equipment
(PSE) and Powered Devices (PD).
PSEs provide power to PDs over the Ethernet cable.
The PD is the device that receives the power.
When connected to a network, a PSE discovers a PD and supplies
48 V and a maximum current of 350 mA. A minimum of about
13 W is sent to each PD. That’s enough to power PDs such as VoIP
telephones, wireless access points, security cameras, building access
systems, etc.
There are two types of PSE devices: end-span and mid-span.
An end-span device is often a PoE-enabled network switch
that supplies power directly to the cable from each port.
A mid-span device is inserted between a non-PoE switch and the
network, and supplies power to the PD. It offers the advantage of
being able to add PoE to a network while using existing non-PoE
equipment. There are also powered patch panels, which combine
a patch panel and mid-span device.
A power injector, a pre-standard PSE, supplies power to a specific
point on the network while the other network segments remain
without power.
IEEE 802.3at: PoE Plus.
In 2005, the IEEE began working on IEEE 802.3at, PoE Plus, a
revision to the 802.3af standard. It’s expected to be ratified in 2008.
The major enhancements include:
• Operation over CAT5 and higher cable.
• Increasing power over all four pairs to a minimum
of 30 watts up to 60 watts.
• Backwards compatibility with 802.3af.
• An active indication when an 802.3at PD is connected
to an 802.3af PSE.
• Research mid-spans for 1000BASE-T.
• Research mid-spans and end-spans for 10-GbE.
PC
IP phones
Ethernet switch
Mid-span Power
Sourcing Device
(PSE)
Powered Devices (PDs)
Power over Ethernet (PoE)
PoE Network