3
How does it work?
A standard Cat.5 Ethernet cable has four twisted pairs, but only
two
of these are used for 10BASE-T and 100BASE-TX. The specifcation allows
two options for using these cables for power.
Power through the “spare pair”
The twisted pair on pins 4/5 is connected to form the positive electric
power supply, while the pair on pins 7/8 is connected to form the
negative supply. Each pair can accommodate either polarity.
Power through the “data pair”
Also known as phantom powering, the power is delivered along the data
pairs on pins 1/2 and 3/6 to the device without disturbing data transfer.
This method is applied in 1000BASE-T (Gigabit Ethernet) as all four pairs
are used for data transmission.
The specifcation does not allow both sets of wires to be used to send
power, a choice must be made. The Power Sourcing Equipment (PSE)
applies power to either set of wires. The Powered Device (PD) must be
able to accept power from both options.
An obvious requirement of the specification is to prevent damage to
existing Ethernet equipment. A “discovery process” runs from the PSE
which examines the Ethernet cables looking for devices that comply
with the specification. It does this by applying a small current-limited
voltage to the cable and checks for the presence of a 25k ohm resistor in
the remote device. Only if the resistor is present the full 48V applied, but
this is still current-limited to prevent damage to cables and equipment
in fault conditions.
The PD must continue to draw a minimum current. If it does not (for
example, when the device is unplugged) then the PSE removes the
power and the discovery process begins again.
As an optional extension to the discovery process, a PD may indicate to
the PSE its maximum power requirements. The following table shows the
diferent PD classes and the PSE power output for each corresponding
PD power range.
Class Usage Min. PSE Output Power Available for PD
1 Default
15.4W
0.44W – 12.95W
2 Optional 4.0W
0.44W – 3.84W
3 Optional 7.0W
3.84W – 6.49W
4 Reserved Treat as Class 0 Reserved
for Future
for Future Use
Use
The PSE may optionally provide a level of system management, using for
example, the Simple Network Management Protocol (SNMP). This allows
to manage of actions such as devices to be powered off at night, or
remotely reset.
Why PoE?
As Ethernet becomes more common in server rooms and network
backbones of most organizations, PoE becomes a much more attractive
solution to deploy network devices without changing your existing
infrastructure.
Cost Efective
Using one cable for both data and power to devices signifcantly reduce
the power line installation cost for electrical wiring, conduits, and
outlets throughout the buildings. It also reduces future maintenance
costs, bringing cost elimination when using PoE technology for large
installations.
Flexibility
PoE is standards based, so interoperability across vendors is guaranteed.
This means that PoE can be used to provide maximum flexibility for
device installation when it is difcult to provide a power source at hard
to reach locations.
Also, even greater fexibility can be provided with the use of PoE Splitters,
PoE technology can also be applied to devices weren’t designed for PoE.
Reliability
Using just one cable instead of separate cables for data and power
improves overall network reliability and deployment fexibility.
Safety
PoE is a safe power solution. To avoid damaging devices or accidental
contact, there are numerous safety procedures in the PoE specifcation
which includes Over-current, Under-current and fault protection.
Advanced Management
Managed PoE switches provide network administrators with additional
monitoring and control capabilities. Users can remotely enable and
disable the power output from the switch to powered devices. This can
be used to troubleshoot and reset devices by powering them on and of,
or to schedule the times that power is provided to the devices to control
when they will be turned on.
Power Sourcing
Equipment (PSE)
Powered Device
(PD)
1
2
1
2
TX
RX
SIGNAL PAIR
4
5
4
5
SPARE PAIR
7
8
7
8
SPARE PAIR
3
6
3
6
TX
RX
SIGNAL PAIR
48V
+/-
+/-
DC/DC
Converter