Power over Ethernet / PoE / IEEE 803.11af

small butt plug

In standard cat-5 cabling for 10Mbit or 100Mbit ethernet only 4 of the 8 wires are actually used. Power over ethernet is a technique to use the unused pairs: (4,5(+) and 8,7 (- )) to carry DC power to the device.

Some wireless access point and single board computers, like the Soekris 4521, contain the circuitery to be powered directly in this manner.; In other cases you need to split the power off the ethernet cabling and feed it into the equipment the normal way.

For more information see these links and pages:

- Normal PoE Device

- PoE injector/splitter

- PoE injector/splitter for the Lucent

- A simple and very illegal hack to construct a PoeHub on the cheap.


Table for 10/100 Mbit ethernet wiring and Power over Ethernet as used by most vendors:



wire colour (According to T-568B and in Europe)

US modern T-568A


Ethernet Tx+




Ethernet Tx-




Ethernet Rx+




PoE - higher voltage (+)




PoE - higher voltage (+)


blue -white


Ethernet Rx-




PoE - lower voltage (-)




PoE - lower voltage (-)



More information and plug/header pictures are here and here


- The 803.11af standard does not yet exist and is still in draft format.

- Polarity Confusion: Often in a telco world you'll have GND (0 volt) and -48 Volt. So in the above diagram you can have 0 Volt on 7,8 and 12 Volt on 4,5. Or alternatively 0 Volt on 4,5 and -12 volt on 7,8 (i.e. MINUS 12 volts). For this reason we've used (+) and (-); where (+) is the higher of the two voltages.

- Expect a drop of a few volts; about 1 volt per 100 meters at 5 watt at room temperature; about double that at freezing -15 deg C. (Note when using all four spare wires; not when just using wire 4 and 8).

- Cisco and some telco's have the polarity swapped; i.e 7,8 is higher than 4,5.

- The Soekris 4211, Intel, Ayaya, Orinoco, Wavelan, 3Com and Symbol use 4,5 higher than 7,8

This is what has been suggested for the IEEE 803.11af standard.

- Injectors commonly found in (European) idsn networks are wired correctly; but their powersupply is typically AC rather than DC.


The net4511 and net4521 of Soekris support Power Over Ethernet. The 4501 does not.

The net4511 and net4521 have a switch mode power supply and takes11V to 56V. Higher PoE higher voltages mean lower currents which mean lower cable loss (proportional to currrent squared). Expect around 4Watt with the CPU running and 2Watt with the CPU in autohalt. The 802.3af spec say 48V.

The net4501 does not support PoE - and takes 7V (5.5V for latest revision) to 20V on the normal power jack. You can of course use use a splitter as described in the NYC document. But note the smaller voltage range. The PCI 5V is generated by a linear regulator, so lowest voltage should be using if higher current from the 5V is needed.

All boards have reverse polarity protection on external power connectors. The Soekris adheres to the 7,8 (-) and 4,5 (+) standard. Note that the second ethernet port has the usual resistor termination of unused pairs, so they cannot withstand DC for any prolonged period of time. An IEEE 802.3af compliant powersupply is supposed to test this before applying full power. The 3Com 3CNJPSE seems to do that, while a regular wallmount transformer will not.

See Sorens Message 3E2C5474.8030206@soekris.com for the authoritative details on the above. :)


Although the information containted herein is under this license and provided AS-IS with no warranty whatsoever - please do contact dirkx@webweaving.org if you have *any* addition or find any faults in the above. As it may prevent some one else their equipment from blowing up.

PowerOverEthernet (last edited 2010-06-29 23:31:18 by 119)