f you were to put together a list
of the hottest technologies in today’s
networking industry, WiFi (wireless networking) and
VoIP (Voice over Internet Protocol) would be near the
top. Recently, several major corporations announced
plans for a global rollout of IP phones. By the same to-
ken, WiFi hotspots have cropped up everywhere.
In addition to being current industry buzzwords,
WiFi and VoIP also share another common attri-
bute. They both require Ethernet and power to func-
tion. In June 2003, the Institute of Electrical
and Electronics Engineers (IEEE—
www.ieee.org) released IEEE
802.3af, also known as the
Power over Ethernet (PoE)
standard, which was estab-
lished to provide a universal
way to provide power and Ethernet-based data to a re-
mote powered device over standard Ethernet cabling,
such as Category 5e or Category 6.
Before 802.3af, vendors and consumers of IP phones
were faced with two options: Provide local power
through an AC (alternating current) electrical outlet,
or use non-standards-based proprietary solutions to
provide inline powering. Both choices were equally
unappealing.
The release of 802.3af has enabled technologies such
as VoIP and WiFi, and has paved the way for literally
hundreds of new product ideas that require the com-
bination of both data and power. Examples include
radio-frequency identi cation (RFID) scanners, IP-

JOHN SCHMIDT is a product manager with ADC (www.adc.com).
Before deploying a midspan powering device like this one from
ADC, make sure it complies with the IEEE 802.3af Power over
Ethernet speci cations and the TIA Category cabling speci ca-
tions that apply to your cabling system.
Five keys to
successful PoE deployment
design
I
Reprinted from the February 2005 edition of CABLING INSTALLATION & MAINTENANCE
Copyright 2005 by PennWell Corporation
1. Provide sufcient power to the remote powered device.
This may sound simple, but in practice, it is difcult. The
IEEE 802.3af standard identies four possible power clas-
sications. At maximum, the powered remote device can
draw up to 12.95 watts of power. Factoring loss through
the length of the cable, this means that, at maximum, the
power-sourcing equipment must have the ability to pro-
vide 15.4 watts of power to each port. In a 24-port Eth-
ernet switch or midspan device, this means that approxi-
mately 370 watts of power must be available to supply the
necessary power to each port. For the Ethernet switches,
additional power above and beyond that required for PoE
must be available for its switching functions. With this in
mind, pay close attention to the size of the power supply
used in each device. Midspan devices should have in excess
of 370 watts available and Ethernet switches should have
signicantly more, depending on how much power their
switching functions require. A rule of thumb is to check
the power-supply size used on a non-PoE-enabled switch

ernet with a midspan device, how does that affect your per-
formance? Pay close attention to the manufacturer’s speci-
cations. Look for Category 5e and 6 compliance. Midspans
should be treated just like an-
other patch panel in your chan-
nel; if they are of an unkown
Category or one lower than the
rest of your cabling, you may
be signicantly disregarding
the performance of your infra-
structure. Also, remember that
per TIA standards, only four
connectors can exist between the switch or hub and the
network interface card (NIC). A midspan device should be
counted and treated as one of these connection points. Peo-
ple commonly ask how far they can run Power over Ethernet.
The answer is 100 meters from the switch or hub to the NIC,
regardless of where the power is injected. The limitation is
not the power; it’s the Ethernet cabling standards that limit
the total length of cabling to 100 meters.
5. Use the most cost-effective PoE method for your network.
The business motivation behind deploying IP-based tech-
nologies like WiFi and VoIP is to decrease networking costs.
A signicant benet of PoE is that it runs on your existing
infrastructure. If you have recently purchased or upgraded
your Ethernet switches, but they are not compliant to the
IEEE 802.3af standard, before you run out and buy all new
Ethernet switches (which you probably won’t do), and be-
fore you abandon or delay your project (which you don’t
want to do), consider using midspan devices to inject power.