Digital Communication


Digital Communication

802.3af: Power over LAN cable


IEEE 802.3af is a standard for powering a terminal with up to 12.95 W via a LAN cable. For this, however, it is important that the terminal is compatible with Power-over-Ethernetotherwise it may be damaged.

What is IEEE 802.3af?

In 2003, the Institute of Electrical and Electronics Engineers (IEEE) adopted a standard for the power supply of devices via a network cable : IEEE 802.3af. It works with PoE (Power over Ethernet) enabled terminals with a wattage of 15W at the start point and up to 12.95W at the terminal. It allows power supply and simultaneous data transmission via LAN cable. In addition to 802.3af, there are also the following standards: 802.3at (PoE+) and 802.3bt (PoE++ or 4PPoE), which make it possible to transmit even higher electrical loads. For single-pair Ethernet, the 802.3bu (PoDL) standard has been implemented.

How does the 802.3af standard work?

IEEE 802.3af requires a cable designed for data transmission in order to supply power to a terminal. This generally concerns devices that require little power such as surveillance cameras, VoIP phones, wireless access points (WAP), hubs or even small servers. The 802.3af standard is suitable for Ethernet (10BASE-T) and Fast Ethernet (100BASE-TX). The PoE then passes through a wire of pair 1.2 and another wire of pair 3.6, simultaneously with the data transmission. The other two pairs of wires 4.5 and 7.8 can instead be used for power supply. Two variants are therefore possible for the power supply:

  • Replacement pair power supply : in this case, only the two pairs of unused wires are used for the power supply. The other two pairs are reserved for data transmission.
  • Phantom power : in this variant, the unused pairs of wires, as well as those provided for data transmission, are all used for the power supply.

Direct current is then delivered with a voltage of up to 44 V. For this active power supply, networking switches are used. The passive power supply works thanks to interposed PoE injectors.

With the 802.3af standard, Ethernet cables are therefore no longer only used for data transmission, but also for power supply: a transmitter injects a positive voltage on one side and a negative voltage on the other side into each pair of conductive wires. A receiver then differentiates the two voltages and determines the data signal. By then raising the voltage to the potential required for power supply, the cable can also be used for current. For that, however, it must be designed for this type of voltage.

What are the technical characteristics of the IEEE 802.3af standard?

The IEEE 802.3af standard can deliver up to 175 milliamps per pair of wires. Thus, with two pairs of wires, we reach a total of 350mA, knowing that up to 400 mA is allowed when switching on. The output power can reach 15.4W per wire and the useful output power 12.95W at most. There is therefore a loss of energy, because the development of heat plays an important role during the transfer. Losses also occur at power supplies. The voltage of 802.3af is between 44 and 57 V, but generally stabilizes around 48 V.

What are the advantages of the 802.3af standard?

The main advantage of the IEEE 802.3af standard is that it can do without at least one cable. The 802.3af standard helps overcome difficulties in accessing devices or complicated cable connections. This method also eliminates the use of a battery and improves distribution on power outlets. In addition, it saves space: bulky power supplies are no longer necessary and devices no longer need to be placed in close proximity to the power supply.

What should you pay attention to with the 802.3af standard?

802.3af is a great way to run small, low-power devices in a space-saving and easy way using a single cable. This standard is therefore a good choice for the small devices mentioned above. However, this technique is not suitable for larger energy requirements. Power loss due to heat and distance must be taken into account. In addition, not all terminals are compatible with IEEE 802.3af. If you plug in an incompatible device, it may be damaged or even destroyed by direct voltage.

It is for this reason that the IEEE 802.3af standard has a terminal control mechanism. A low voltage classification current is therefore used to check the PoE compatibility of a terminal and to know which class it belongs to. This process is called “ Resistive Power Directory »: it measures, among other things, parameters such as resistance and capacitance. If the internal resistance is between 19 and 26.5 ohm and the capacitance is less than or equal to 10 farads, then an IEEE 802.3af compliant power supply is established.

To learn more about IEEE standards, you can find articles about them in the IONOS Digital Guide:

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