How Power over Ethernet (PoE) works  

How Power over Ethernet (PoE) works

Power over the network cable, otherwise known as Power over Ethernet, offers many advantages and applications. Here you can find out how the technology works and which standards have to be met, along with what you can use it for.

What is PoE?

Power over Ethernet means the power supply of various network-enabled devices is via the LAN cable. The cable supplies both data and power. The clear advantage of this technique is that you don’t need to install power cabling, avoiding cable clutter, saving space and reducing installation costs.

Important requirements for Power over Ethernet

In order for a device to be able to draw power and data at the same time via the data network, additional power must be fed into the data line – ideally in the network distributor. This makes this method particularly suitable for devices such as IP phones, switches, wireless access points and surveillance cameras.

Another point to consider before installation is the temperature of the cable. With more current flowing through the LAN cable, more heat is generated by the resistor. This can affect the data transfer. Therefore, you should determine the load that the cable must withstand and how to control the heat of the cable.

In addition, the LAN cable cannot be too long because longer lines result in a voltage drop. Therefore, when purchasing the cable, it is important to pay close attention to the core cross section, which should not exceed an AWG value of 24 (CAT5).

How does it work?

There are several ways to transfer power over Ethernet cables:

  1. In a typical CAT5 cable, only two of the four signal pairs are used. Mode B occupies the two unused pairs with the data transmission. Mode A transports the data on the same wire pairs as 10/100 Mbit/s Ethernet, similar to the power of transistor microphones via decoupled DC-free data lines.
  2. With Gigabit Ethernet and faster connections, all four pairs are needed for data transfer. In this case, both alternatives conduct electricity via line pairs, which are also needed for the data transfer.


In addition, the following standards are available:

  • The IEEE 802.3af 2003 standard supplies devices with up to 15.4 W (44 V and 350 mA minimum) on each port. However, only 12.95 W are guaranteed, because a certain amount is lost in the transmission.
  • The newer IEEE 802.3at 2009 standard (also known as PoE +) provides up to 25.5 W. This standard prohibits power transmission across all four signal pairs.

Another standard, which is particularly important in industrial environments, such as in vehicles or production machines, is the IEEE 802.3bu (PoDL) for single-pair lines. There are ten levels from 5 to 50 W.

The newest standard, the IEEE 802.3bt (4PPoE or PoE ++) provides even larger capacities for the power supply – up to 55 W (level 3) and 100 W (level 4). Thus, each line pair would have to withstand a voltage of up to 600 or 960 mA. So, a whole workplace could be supplied with a computer, screen and telephone over the LAN cable. The launch of this standard is expected later this year.

Performance classes

Depending on the application and the device being used, different performance classes can be selected. Here is an overview:

StandardClassClassificationMax. Power supply (PSE)Max. Withdrawal rate (PD)Ethernet Type
IEEE 802.3af00-4 mA15,4 W0,44-12,95 W10/100 Base-T
IEEE 802.3af19-12 mA4,0 W
0,44-3,84 W10/100 Base-T
IEEE 802.3af
217-20 mA7,0 W3,84-6,49 W10/100 Base-T
IEEE 802.3af326-30 mA15,4 W6,49-12,95 W10/100 Base-T
IEEE 802.3at436-44 mA25,5 W12,95-21,90 W10/100 Base-T


The application areas are diverse and range from products for the private sector to office equipment and industrial use. The most common applications are VoIP phones or IP cameras. The PoE technology is particularly suitable for WiFi access points, network routers, VoIP telephones, network switches and IP cameras.

In the future, the technology will be particularly useful in smart homes or smart offices, for automated light control or in LED switches, for example.

In industrial environments, the technology could be used for sensors, counters or controllers, among others.


Image: Fotolia, # 66328739, zariam74



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