How powering over ethernet works with 802.3af

IEEE 802.3af is a standard for the power supply of a terminal device with up to 12.95 watts via a LAN cable. It’s important that the end device is compatible with Power over Ethernet, otherwise damage may occur.

What is IEEE 802.3af?

In 2003, the Institute of Electrical and Electronics Engineers (IEEE) adopted a standard for powering devices over a network cable. The standard for what’s known as Power over Ethernet (PoE) is called IEEE 802.3af or 802.3af. It has a wattage of 15 watts at source and up to 12.95 watts at the end device. It enables power supply and simultaneous data transmission via a single LAN cable. In addition to 802.3af, successor standards 802.3at (PoE+) and 802.3bt (PoE++ or 4PPoE) enable the transmission of greater electrical power. The 802.3bu (PoDL) standard is available for single pair ethernet.

How does 802.3af work?

IEEE 802.3af uses a cable intended for data transfer to supply power to an end device. This mostly concerns devices that require little power, such as surveillance cameras, IP telephones, wireless access points (WAP), hubs or smaller servers. 802.3af is suitable for Ethernet (10BASE-T) and Fast Ethernet (100BASE-TX). Only the two wire pairs 1/2 and 3/6 are used for data transfer. The other two wire pairs 4/5 and 7/8, however, can be used for the power supply. Two variants are possible for power supply:

  • Spare pair power supply: Both of the two unused wire pairs are used. The other two pairs are reserved for data transfer.
  • Phantom power supply: This power supply uses the unused wire pairs and the other two. The data transfer is superimposed for this.

The direct current is then fed in with a voltage of up to 44 volts. Network switches are used for active power supply. With passive power supply, PoE injectors are connected in between.

In ethernet cables used for the data transfer and power supply with 802.3af, a transmitter feeds a positive voltage from one side and a negative voltage from the other into each pair of wires. A receiver then differentiates the two voltages and determines the data signal from the difference. Raising the voltage to the required potential of the power supply, the cable can be used for power, provided it’s designed for this type of voltage.

What are the technical features of IEEE 802.3af?

With IEEE 802.3af, up to 175 milliamperes can be fed in per wire pair. Using two wire pairs, a total of 350 milliamperes can be achieved, with up to 400 milliamperes enabled during switch-on. The output power is up to 15.4 watts per wire and the standard power for use is a maximum of 12.95 watts. This means energy is lost because of heat development during the transfer. Losses occur at the power supplies. The voltage for 802.3af is between 44 and 57 volts, but usually settles at 48 volts.

What are the advantages of 802.3af?

The main advantage of IEEE 802.3af is the elimination of at least one cable. 802.3af is a good solution for devices that are difficult to access or those with cable connections that are difficult to reach. It also eliminates the need for a battery and improves the occupancy of power outlets. In addition, the devices no longer have to be placed in the direct vicinity of the power supply and clunky power supplies are a thing of the past.

What to look out for with 802.3af?

802.3af is a great way to operate small devices with low power requirements to save space using a single cable. This makes it a good choice for small devices. However, the technology is not suitable for greater power requirements. Power loss due to heat and distance must be taken into account when placing devices. Not every end device is compatible with IEEE 802.3af. If you connect the wrong device, it can be damaged or even destroyed by the DC voltage.

To this end, 802.3af includes a test option that determines whether a device is compatible or not when the power supply is active. It uses a low-voltage classification current to check which class an end device belongs to and whether it’s PoE-compatible. This process is called Resistive Power Directory and measures parameters such as resistance and capacitance. If the internal resistance is between 19 and 26.5 ohms and has a capacity of 10 farads or less, the power is supplied in accordance with IEEE 802.3af.


Learn more about network standards in our Digital Guide:

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