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Power Over Ethernet, PoE
1 What is Power Over Ethernet?
Power Over Ethernet (or PoE) is a technology that integrates data, voice and power on standard
Ethernet infrastructure providing new options for power distribution. This allows IP telephones,
wireless LAN access points, surveillance cameras and other embedded appliances to receive power as
well as data over existing CAT5 cabling. Formally approved as an international standard, Power over
Ethernet is established as an economical, safe power distribution method and is already deployed in
corporations throughout the world.
In simple terms, it uses the unused wires in standard CAT5 LAN cabling to send power up the LAN
leads to the operating device. The device can then take it’s power from the LAN cable which precludes
the requirement for a separate, local, power supply close to the device. e.g. if the device is an outdoor
wireless bridging unit then you can use PoE to send the power and data up the connecting LAN cable
to the outdoor unit thus removing the requirement to route mains power to the box.
The IEEE standard 802.3af (often erroneously called 802.11af) describes a mechanism for Power over
Ethernet (PoE). The standard provides the capability to deliver both power (max 12.95W) and data
over standard ethernet Cat.3/Cat.5 cabling.
2 PoE: IEEE 802.3af
IEEE 802.3af provides 48 volts DC over two out of four available pairs on a Cat.3/Cat.5 cable with a
maximum current of 400 mA for a maximum load power of 19.20 watts, although, after counting
losses, only about 12.95 watts are available. Most switched power supplies will lose another 10-20% of
the available energy. A "phantom" technique is used so that the powered pairs may also carry data.
This permits its use not only with 10BASE-T and 100BASE-TX, which use only two of the four pairs
in the cable, but also with 1000BASE-T (Gigabit Ethernet), which uses all four pairs for data
transmission. This is possible because all versions of Ethernet over twisted pair cable specify
differential data transmission over each pair with transformer coupling; the DC supply and load
connections can be made to the transformer centre-taps at each end. Each pair thus operates in
"common mode" as one side of the DC supply, so two pairs are required to complete the circuit. The
polarity of the DC supply is unspecified; the powered device must operate with either polarity or pair
45+78 or 12+36 with the use of a bridge rectifier. However, the industry standards are for 48V to be
delivered as DC+ on data pins 1 and 2 and DC- data on pins 3 and 6 (called Alternative A) OR DC+ on
unused (on Base 10/100 ethernet) pins 4 and 5 and DC- on unused pins 7 and 8 (called Alternative B).
If you want to cover both bases, then you can deliver power on BOTH the data AND the unused pins
but the total power remains the same 
3 So how’s it done?
With PoE the LAN cable serves as the data/power interface between the two Ethernet elements (e.g.
one end might be an outdoor wireless bridge and the other end a switch in the office, on the end of the
LAN cable). There are two optional connection methods used with PoE, named Alternative A & B.
Most common PoE uses are with Alternative B so this is the one I shall discuss first of all. With
Alternative B the DC power, typically 48V, is transmitted over the ethernet wires connected to pins 4,5
and 7,8 which are not used for Base 10/100 networks:
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Alternative B Wiring
Thus, power is carried over the spare pairs (4/5 & 7/8). Each pair's wires are shorted to one another.
Note: Because a PoE system Alternative B uses the unused wires in a LAN cable it is vital that
your LAN cables are fully, 8 wire, populated. If you don’t have all 8 wires connected up in your
LAN connection then it won’t work!  It also means that Alternative B wiring is incompatible
with any system which actually uses the ‘unused pins 4,5,7,8 e.g. Gigabit networking.
In Alternative A wiring the DC power is superimposed onto the data pins 1,2 and 3,6. Since the same
pins are used for both data AND power then Alternative A wiring is compatibile with applications
where the pins 4,5 and 7,8 are employed for other purposes e.g. in Gigabit network cabling.
The table below shows the various wiring and power systems sometimes used for sending power over
ethernet cables:
Table 1. Summary Comparison of Various PoE Standards
Ethernet RJ-45 connector pin number
DC Load
Voltage Connector
IEEE 802.3af
48 V DC, RX, RX, TX,
spare spare
spare spare
using data
protected DC+ DC+ DCDCpairs
IEEE 802.3af
48 V DC,
using spare
RX RX TX DC+ DC+ TX DC- DCprotected
12 or 24 RX RX TX DC+ DC+ TX DC- DCV DC
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Industry Standard
for embedded PoE
– Alternative A
Industry Standard
for embedded PoE
– Alternative B
Alternative to
‘Alternative B’
wiring which uses a
lower DC voltage
Older Cisco
(OLD old
(NEW old
polarity is
New Cisco is IEEE
compliant using
48 V DC RX RX TX DC+ DC+ TX DC- DC(embedded)
Alternative B
D-Link PoE
5VDC @
48 V DC RX RX TX DC?? DC?? TX DC?? DC??
Adapter for non2.5A
5.5/2.5mm PoE products.
Mac Polarity
48 V DC RX RX TX DC?? DC?? TX DC?? DC??
AirPort PoE,
to ???
Variety of Options
same as
Available to Fit
RX RX TX DC+ DC+ TX DC- DCinput and others Most Brands of
12 or 24
same as
New York City
or 48 V RX RX TX DC+ DC+ TX DC- DCcoaxial
Wireless PoE
"Roll Your
or as reqd
In any PoE system, the device that inserts the power into the LAN cable is called the ‘injector’. The
feature/device at the product end which takes the power out of the LAN cable is called the ‘splitter’.
e.g. above shows a LAN cable, with data, coming from a PC. The injector takes in the data cable and
inserts the power into the ongoing LAN cable going to the wireless AP. The AP has a built in ‘splitter’
which extracts the power from the LAN cable and passes it internally onto the built in power circuits.
This is an AP with built in PoE capability.
For devices which don’t have built in PoE capability then you can, instead, get a separate splitter which
sits on the LAN cable in front of the device and extracts the power from the incoming LAN cable. The
separate splitter then normally has a short DC type power lead which is then plugged into the devices
power socket. e.g.
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So, in above, the splitter is connected to the LAN cable from the injector. Then, from the splitter, there
are two cables going to the AP: One is a short data only LAN cable, the other is a short DC power lead.
Generally the splitter does more than just split out the 48V from the LAN cable; it normally performs a
second function of ‘dropping’ the 48V to the native power voltage required by the device e.g. 12 or 5V.
4 The PoE Provision Process
While adding PoE support to networked devices is relatively painless, it should be realized that power
cannot simply be transferred over existing CAT-5 cables. Without proper preparation, doing so may
result in damage to devices that are not designed to support provision of power over their network
To try and reduce the potential for damage with misuse of PoE systems, the 802.3af standard
incorporates a series of management checks that the PoE systems must go through to determine if it’s
safe to put the power onto the LAN cable. In the beginning, only small voltage level is induced on the
injector port's output, until a valid end device is detected. Sometimes the injector may also do tests to
estimate the amount of power to be consumed by this end device. After a time controlled start-up, the
injector begins supplying the 48 VDC level to the device until it is physically or electrically
disconnected. Upon disconnection, voltage and power shut down. Several incidents such as
consumption overload, short circuit, out-of-power-budget and other scenarios, may terminate the
process in the middle, only to restart it from the beginning.
Thus, fully, 802.3af PoE devices do a lot more than simple power delivery: They also have built in
intelligence to properly control and manage the process outlined above. This high level of additional
intelligence does tend to mean that fully 802.3af PoE devices are a bit on the expensive side. Therefore,
most lower cost PoE systems remove some or all of this intelligence and are simple devices which
simply inject the 48V onto the pins 4/5 and 7/8 (Alternative B wiring).
5 Products in detail
Let’s look at some specific products so at to, hopefully, clear up any confusion still left 
First of all a 48V, PoE injector:
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This is a PoE injector. It consists of the injector body which has a LAN in port (data only) and a
Data+Power out port which has the data on pins 1,2,3,6 and then 48V on pins 4,5,7,8. Also included is
a separate mains powered 48V DC power supply which powers the injector part. In operation data
LAN cable from (for example) your switch goes in one side and the data+power LAN cable comes out
the other (and then to, for example, your outdoor AP). This device is not fully 802.3af compliant since
it doesn’t include the extra sensing and intelligence to determine if the Power Device (called the PD)
can accept power over the LAN cable. As such it’s important that the user verifies that the PD is
suitable. See the POE-EPE-5818af product below for an injector which includes the full 802.3af
The NET-POE12V is an IEEE 802.3af 48V Power Injector and Splitter (12V, 2.1Ø/5.5Ø). It’s intended
for end devices which don’t have any inbuilt PoE capability. The way it works is one part of the kit is a
normal PoE injector; this sits down near your switch (for example). The other half of the kit is the
splitter. The splitter sits next to your device to be powered (e.g. an IP camera). The LAN cable carrying
data+power comes from the injector and goes into the splitter. The splitter then has a short, data only,
LAN cable coming out which goes to the LAN data port of the device to be powered (e.g. IP camera).
Also coming out from the splitter is a short DC power lead which plugs into the normal DC power
socket on your device. This takes the place of the normal mains powered power supply the device
would normally use.
To support a range of different devices this kit is available with DC 5V output (naturally, for end
devices that need 5V) or with DC 12V output (for devices needed 12V). The NET-POE kits can
typically be used to power Ethernet devices such as IP telephone, Web Camera and Wireless AP. The
POE kits include features like Short Circuit Protection, Over Load Protection and High Voltage
Protection to prevent Ethernet devices from circuit shortage resulted from wrong pin installation or
cable damage. Please note the maximum power for PoE is only 12W so, for the 12V kit, ensure your
device doesn’t need more than 1Amp. For the 5V kit the limit is 2Amp.
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These two products are simple units which simply allow any power to put onto or take off the normal,
Alternative B, PoE power pins (4,5,7,8). Each device has a data only plus a data+power LAN port.
Each device also has a short DC power lead: One with a 5.5mm DC socket and the other with the
equivalent plug. Each part can be used separately or as a pair. Their application is as a simple, very low
cost injector (typically the POEADAPTSKT unit) or as a basic splitter (the POEADAPTPLG unit). A
separate power supply should be added to the ‘injector’ end (POEADAPTSKT) so as to inject power
onto the normal power pins (4,5,7,8). For ‘normal’ PoE applications your would use a 48VDC power
supply like this….
so the DC plug from the power supply attaches to the DC socket on the POEADAPTSKT unit. You
also, just like any PoE injector connect your data only LAN cable and also your data+power LAN
cable. The other end of the data+power LAN cable could be then connected to a PoE compliant product
(outdoor AP or similar). Please note, however, these devices are very simple units which simply allow
for basic power transfer applications on LAN cables: They do not include the extra management or
intelligence present in fully 802.3af units. As such, if your PoE compliant product demands these extra
functions then it will probably NOT work!
Another application of these products is to use them as a pair (injector and splitter) in combination with
the normal DC power supply supplied with the product you intend to power. e.g. if you have a wireless
AP that is supplied with a 12V DC power supply and you want to place the AP in a location where it’s
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hard to route a mains supply to then, down at the switch, connect the 12V power supply to the
POEADAPTSKT unit. Also connect a short LAN cable from this to your switch (or whatever). Then
run a full (8 wires) LAN cable from the data+power socket on the POEADAPTSKT up to where the
AP is located. Connect the LAN cable to the data+power socket on the POEADAPTPLG unit. Then
run a short LAN cable from the data LAN socket on the POEADAPTPLG to the normal LAN
connection on the AP. Then, take the short DC power plug from the POEADAPTPLG and connect to
the power socket on the AP. In this way, 12V is injected onto the connecting LAN cable and then split
out at the AP end.
Note: The amount of power transmission loss in the RJ45 plug and LAN cable is (for the same
power) inversely proportional to the voltage you are sending i.e. the power lost in the cable at
12V compaired with 48V is 17 times greater at the lower voltage! In effect, this means that, what
started as 12V, when you injected it into the LAN cable can, over a short distance, start to drop
off by quite a lot. Where as ‘normal’ 48V PoE typically quotes a maximum connecting LAN cable
length of 100m, when you change this to 12V you should expect that maximum cable length to
drop to less than 10m! If you use a 5V power source then the effective cable length could be
substantially lower; maybe just a few metres. You would need to test, in your particular
application, if your powered device operates correctly despite this power and voltage drop on the
This is a fully 802.3af compliant 48V PoE injector using the Alternative B system (uses the Base
10/100 LAN unused wires). In operation it’s very similar to the NET-POEINJECT48 product (detailed
above) in that it delivers the 48V DC power using the ‘spare’ wires however, unlike the POEINJECT48
product, it’s fully compliant to the 802.3af standard and as such correctly applies the pre-power
delivery tests to ensure that the PD (Powered Device) is capable of accepting 48V power over the LAN
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