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Basics of Power over Ethernet (PoE)

Power over Ethernet - or short 'PoE' - allows you to supply DC power for another device over the ethernet network cable. The most common Power Source Equipment (PSE) types are switches and routers (endspan), but you could just as well put a PoE-injector (midspan) between a standard switch and the Powered Device (PD). Especially in corporate environments, PoE devices are growing in popularity, and just to list some examples of PDs: VoIP hardware, wireless access points, access control terminals, security cameras, and many more.

The main advantage is that you only need one cable for data and power for each device, don't need an extra power outlet at the location of the device, and can control the power supply over the PSE interface. For example, this is great for access points mounted under the ceiling where 'simply unplug it' is not an option.

On the other side, devices that provide PoE functionality are regularly more expensive, are getting warmer due to the power supply, and consume more electricity. That said, if the switch is dead, so are the connected PoE devices. This makes the use of an USV almost inevitable, especially if you power critical infrastructure with it.

PoE generally requires Cat5+ cables and has a normal working distance of 100m. An extender can be used to increase the distance.

The usage of PoE over a connection should not have any effect on the transfer or latency of the data connection. That said, cheap hardware can still do, and I had 3 cases in which turning off PoE explicitly on a switch port helped to solve a problem with disconnecting a non-PoE device. I still blame the printers.

Specification #

The following standards were created by the Institute of Electrical and Electronics Engineers (IEEE), and the following overview should give you a quick insight of the differences of the common standards.

Important: 802.3at/PoE+ and 802.3bt/PoE++ are backward compatible, as long as the PSE supports the higher standard (802.3at PSE supports 802.3af PD, but it does not work the other way around).

IEEE 802.3af-2003:
known as PoE
Type 1
max power delivered by PSE 15,4W / max power available at PD 12,95W
power management classes 1-3
supported cabling Cat3 and Cat5+
supported modes are A and B
IEEE 802.3at-2009:
known as PoE+ or **PoE Plus **
Type 2
max power delivered by PSE 30W / max power available at PD 25,5W
power management classes 1-4
supported cabling Cat5+
supported modes are A and B
IEEE 802.3bt-2018:
known as PoE++ or 4PPoE
Type 3
max power delivered by PSE 60W / max power available at PD 51W
power management classes 1-6
supported cabling Cat5+
supported modes are A,B and 4PPoE
Type 4
max power delivered by PS 100W / max power available at PD 71,3W
power management classes 1-8
supported cabling Cat5+
supported mode is only 4PPoE (as all 4 pairs are required)

UPoE/UPoE+ are Cisco proprietary and I won't go into detail. I think it is still worth mentioning.

Active PoE / Passive PoE #

Active and passive PoE are not inter-compatible and PSE and PD must support the same type.

PSE with active PoE does a handshake with the PD to determine how much power the PD requires and only after this handshake power will be sent to PD. Furthermore, active PoE connection often will be monitored and the PSE can turn off the power if there are any risks. Active PoE is more expensive but more common, reliable, and secure.

Side note: the above-mentioned standards are active PoE. Passive PoE has no standards.

Passive PoE - or 'Always-On PoE' - does not require any handshake and sends the configured power immediately. This means you need to know the requirements of the PD; otherwise, you could easily destroy your hardware. It was often used before the IEEE standards and is less expensive, but it is not recommended anymore since most modern PDs support active PoE.

Side note: some passive PoE PSEs can have a shorter distance and be limited to 100Mb/s.

Power management classes #

Power management classes prevent the over-powering of PDs.

Class - power at PD:
Class 0 - 0W - 12.95W (default)
Class 1 - 0W - 3.84W (802.3af,802.3at,802.3bt)
Class 2 - 3,84W - 6,49W (802.3af,802.3at,802.3bt)
Class 3 - 6,49W - 12,95W (802.3af,802.3at,802.3bt)
Class 4 - 12,95W - 25,5W (802.3at,802.3bt)
Class 5 - 40W (802.3bt Type 3+4)
Class 6 - 51W (802.3bt Type 3+4)
Class 7 - 62W (802.3bt Type 4)
Class 8 - 71,3W (802.3bt Type 4)

Modes #

There are three modes available. The following modes determine what pairs the power will be delivered to the PD. Mode A provides the power over the same pairs that are used for the data transfer (T568A pairs #1 + #2, T568B pairs #2 + #3) and Mode B delivers the power over the spare pairs (T568A + T568B pairs #3 + #4). 4PPoE stands for 4-pairs Power over Ethernet - and as the name implies - uses all four pairs to deliver the power to the PD.

The PSE decides what mode will be used and PD have to support at least mode A and B by the IEEE standard.

Compatible vs compliant

'Compliant' means that the required standards are met by the PD, and 'compatible' means, that it can work with a standard, but don't have to. 'Compatible' often means mode B only, but this depends on the PD.

That said, I don't think that this is the case in 100%. I've seen multiple devices that are 'compliant', but are marked 'compatible'. I've read about this multiple times and I thought it would be worth mentioning.


E-Mail hellofoo@ittafoovern.comcom


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