Power over Ethernet, PoE
- an overview of Power over Ethernet, PoE also called Power over LAN, PoL,
or Power on LAN, PoL, and the related IEEE 802.3af standard.
Powering network devices can sometime present problems,
especially if they are located remotely. On solution to this is to supply the
power over an Ethernet LAN cable. This Power over Ethernet is now being used for
a wide variety of applications including powering IP telephone, wireless LAN
access points, webcams, Ethernet hubs and switches and many more devices.
In view of the fact that it is necessary to ensure that any
items that use power over Ethernet can interoperate satisfactorily, the IEE
standard for Ethernet defines how this may be achieved. It is defined within
Clause 33 of IEEE 802.3-2005 - IEEE 802.3 being the overall standard for
Ethernet. It is therefore always advisable to ensure that any power over
Ethernet devices conform to this specification rather than any ad-hoc solutions
that may be possible. Often the Power over Ethernet standard is quoted as being
the 802.3af standard.
PoE Development
With Ethernet now an established standard, one of the
limtations of Ethernet related equipment was that it required power and this was
not always easily available. As a result some manufacturers started to offer
solutions whereby power could be supplied over the Ethernet cables themselves.
To prevent a variety of incompatible Power over Ethernet, PoE, solutions
appearing on the market, and the resulting confusion, the IEEE began their
standardisation process in 1999.
A variety of companies were involved in the development of
the IEEE standard. The result was the IEEE802.3af standard that was approved for
release on 12 June 2003. Although some products were released before this date
and may not fully conform to the standard, most products available today will
conform to it, especially if they quote compliance with 802.3af.
PoE overview
The standard allows for a supply of 48 volts with a maximum
current of 400 milliamps to be provided over two of the available four pairs
used on Cat 3 or Cat 5 cable. While this sounds very useful with a maximum
available power of 19.2 watts, the losses in the system normally reduce this to
just under 13 watts.
The standard Cat 5 cable has sets of twisted pair cable, and
the IEEE standard allows for either to be used for 10Base-T and 100Base-T
systems. The standard allows for two options for Power over Ethernet: one uses
the spare twisted pairs, while the second option uses the wires carrying the
data. Only one option may be used and not both.
When using the spare twisted pairs for the supply, the pair
on pins 4 and 5 connected together and normally used for the positive supply.
The pair connected to pins 7 and 8 of the connector are connected for the
negative supply. While this is the standard polarity, the specification actually
allows for either polarity to be used.
When the pairs used for carrying the data are employed it is
it is possible to apply DC power to the center tap of the isolation transformer
that are used to terminate the data wires without disrupting the data transfer.
In this mode of operation the pair on pins 3 and 6 and the pair on pins 1 and 2
can be of either polarity.
As the supply reaching the powered device can be of either
polarity a full wave rectifier (bridge rectifier) is used to ensure that the
device consuming the power receives the correct polarity power.
Within the 802.3af standard two types of device are
described:
- Power Sourcing Equipment (PSE) This is the equipment that
supplies power to the Ethernet cable.
- Powered Devices (PD) This is equipment that interfaces to the
Ethernet cable and is powered by supply on the cable. These equipments may
range from switches and hibs to other items including webcams, etc.
Power Sourcing Equipment
This needs to provide a number of functions apart from simply
supplying the power over the Ethernet system. The PSE obviously needs to ensure
that no damage is possible to any equipment that may be present on the Ethernet
system. The PSE first looks for devices that comply with the IEEE 802.3af
specification. This is achieved by applying a small current-limited voltage to
the cable. The PSE then checks for the presence of a 25k ohm resistor in the
remote device. If this load or resistor is detected, then the 48V is applied to
the cable, but it is still current-limited to prevent damage to cables and
equipment under fault conditions.
The PSE will continue to supply power until the Powered
Device (PD) is removed, or the PD stops drawing its minimum current.
Powered Device (PD)
The powered device must be able to operate within the
confines of the Power over Ethernet specification. It recieves a nominal 48
volts from the cable, and must be able to accept power from either option, i.e.
either over the spare or data cables. Additionally the 48 volts supplied is too
high for operating the electronics to be powered, and accordingly an isolated
DC-DC converter is used to transform the 48V to a lower voltage. This also
enables 1500V isolation to be provided for safety reasons.
PoE Summary
Power over Ethernet is a particularly valuable means of
remotely supplying and controlling equipment that may be connected to an
Ethernet network or system. It enables units to be powered in situations where
it may not be convenient to run in a new power supply for the unit. While there
are limitations to the power that can be supplied, the intention is that only
small units are likely to need powering in this way. Larger units can be powered
using more conventional means.
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