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.