How
Power Over Ethernet (PoE) Works
In
the PoE scheme, two different types of devices
are involved: Power Sourcing Equipment (PSE)
and Powered Devices (PD).
PDs
are PoE-enabled network end devices equipped
to accept low voltage power transmitted over
structured Ethernet CAT-5, CAT-5e, CAT-6 and
CAT-7 cabling.
PSEs
provide the DC power to PDs. PSEs source up
to 12 watts at 48 volts to each PD. A PSE
may be an endspan device or a midspan device.
An endspan device typically is a network switch
enabled to provide PoE power on each port.
A midspan device is connected in-line to each
end device and adds power to the line.
Power
is carried over the cabling using two techniques:
Alternative A and Alternative B
Alternative
A implements a simplex, or ˇĄphantom feedingˇ¦
method for delivering power to the end device.
Power is carried on the same conductors as
data. CAT-5 cabling for standard 10BaseT and
100Base-TX Ethernet uses two data/signal pairs
connected to pins 1 and 2 and pins 3 and 6
on RJ-45 connectors. Power sourcing equipment
superimposes power onto these pins via the
center tap of internal signal coupling transformers.
In the powered device (PD) the power is derived
from these lines using the reverse technique.
Figure
1: Power over Ethernet Alternative A
Alternative
B carries power over spare wire pairs in the
cable. The power sourcing equipment applies
positive voltage to pins 4 and 5. Negative
voltage is applied to pins 7 and 8.
Figure
2: Power over Ethernet Alternative B
More
on Powered Devices
PDs
are designed to accept PoE in either format
(Alternative A or B) so that they can receive
power regardless of which Alternative is implemented
in the PSE. When operating in Alternative
A mode PDs automatically adjust for polarity
of the power supply voltage. This ensures
that the device will operate even if a crossover
cable is being used. (This sometimes happens
if a Media Dependant Interface (MDI), such
as a PC network card, is connected to the
PD, which also is wired as a MDI.)
Figure
3: A Powered Device
PDs
operate from 48 VDC (approximately) supplied
by the PSE, which means they are classified
as a Safety Extra Low Voltage (SELV) device.
The maximum current supplied by the PSE is
about 350 mA. With some loss in the CAT-5
cable, the PD is limited to about 12 watts
of power. Maximum allowable cable resistance
is 20 ohms.
More
on Power Sourcing Equipment
The
two types of PSE (endspan and midspan) are
used in different situations. Endspan PSEs
typically are used in new installations where
a new switch or router is required. Midspan
devices more often are added to an existing
network to add PoE capabilities. Midspan devices
are connected in-line, between the existing
switch and the PD. They can be obtained as
a single or multiple line units.
On
initial power up Power Sourcing Equipment
is designed to detect, and only supply power
to the network device if it is identified
as a PoE-enabled device. When first connected
the PSE initiates a detection process that
involves low probing voltages to sense the
type of PD connected while avoiding damage
to non-PoE network devices.
Some
Power Sourcing Equipment may also detect the
power class of the PD. PDs are assigned to
one of the following five classes, depending
on the amount of power they require.
Class |
PD
Power (W) |
Class |
0 |
|
Default
|
1 |
0.44
to 3.84 |
Optional
|
2 |
0.44
to 6.49 |
Optional
|
3 |
0.44
to 12.95 |
Optional
|
4 |
Future
Use |
Future
Use |
If
a PD does not support classification it is
assigned a default classification of 0, indicating
the PSE should supply the maximum needed power.
Classification of connected PDs allows the
PSE to optimize its power distribution.
PSEs
are able to detect several undesirable load
conditions, including shorted communications
cables, disconnected PDs and the connection
of non-PoE devices. When a fault is detected
the PSE will disconnect power from the communications
line before damage can occur.
