Ethernet cables deliver power to devices with low energy requirements, eliminating the need for a dedicated power supply.
How does PoE work?
For a PoE configuration, Power Source Equipment (PSE) detect the presence of a powered device and provides an electrical current that is conducted along the data cable. The powered device operates using the power received via the Ethernet data cable; no connection to an additional power source (e.g., an AC wall socket) is required. PoE works by enabling electrical currents to hitch a ride along the twisted pair of wires in an Ethernet cable. Only two of the four twisted pairs in an Ethernet cable are used for data in 10Base-T or 100Base-TX Ethernet, leaving room for power to be transmitted on the two unused pairs. This is a popular technique because it keeps data and power flow separate, making troubleshooting easier.
For 1000BASE-T, unlike 10BASE-T and 100BASE-TX, all four twisted pairs are used. This allows for simultaneous transmission in both directions and is accomplished through the use of adaptive equalization and pulse amplitude modulation.
Power can also be transmitted using the wires that conduct data, though it’s a bit trickier: A common-mode voltage is applied to two pairs of wires, as if they were a DC power supply, and is then extracted at the other end using the center tap (contact) of the standard Ethernet pulse transformer.
What devices can be powered by PoE?
The IEEE 802.3af standard defines three power levels for PoE: 15.4W, 7.0W and 4.0W. Power is dissipated in the cable, so the maximum power available to a powered device is 12.95W. As for voltage, a DC-to-DC converter in the powered device reduces the voltage to the level its components need.
Powered and non-powered devices can be connected to a multiport PoE switch: the switch performs a sequence of tests at each port before applying power. If no power is requested during the sequence of tests, only data traffic is enabled on that Ethernet port.
PoE compatibility is now built into low-power devices – so new equipment is often PoE-ready right out of the box. This has made it even easier to make reliable and scalable power safely available for devices throughout the entire network.
The latest PoE standards – 802.3at for PoE+ and PoE++
Although 802.3af works well for network devices that need less than 13 watts, many devices need a bit more. For those, there is now 802.3at, or PoE+. This standard almost doubles the amount of power available – up to 25.5 watts. This standard allows more network equipment to be PoE powered, including multichannel wireless access points and IP cameras with heater/blowers. 802.3at can exist along with the 802.3af standard.
The latest standard being developed and expected to be ratified in early 2017 is the IEEE 802.3bt, which is the IEEE standard for 60W and known as PoE++. In the meantime, over 60 watts have been achieved by doubling the 802.3at PoE+ capability.