Because Power over Ethernet (PoE) delivers power over the same cable as data, it’s popular for powering devices such as VoIP phones, wireless access points, and security cameras. It often leads to significant savings by eliminating the need to install a separate power outlet.
Most PoE today is IEEE 802.3af PoE or the higher-powered IEEE 802.3at PoE+, which are very safe because power source equipment (PSE) doesn’t add power to the data line unless it detects a compatible powered device (PD) connected to the other end of the cable. This protects devices that do not support PoE. PSEs and PDs also negotiate power requirements, so a PD never receives too much power. Both PSEs and PDs have power supplies and regulators isolated from ground to minimize shock hazard.
There is a newer PoE being developed, IEEE 802.3bt PoE++, expected to be ratified in 2016-2017.
Because most PoE available today is standards-based 802.3af or 802.3at, it’s easy to forget that there are versions of PoE that are NOT standards based. Some of these non-standards-based versions of PoE feature power injectors that inject power without checking compatibility—and that can be very bad news for an innocent network device.
Non-standard PoE tends to fall into three categories: proprietary PoE, high-wattage proprietary PoE, and passive PoE.
Before the ratification of the 802.3af standard in 2003, PoE was a free-for-all with many vendors offering their own method of delivering power over data lines. Some vendors still offer their own proprietary PoE. These proprietary solutions offer varying degrees of communication between PSE and PD.
Many vendors offer high-wattage PoE solutions designed to deliver from 50 watts to 100 or even 200 watts per port. High-wattage proprietary PoE is often used with high-powered outdoor wireless radios.
Passive PoE injects power into an Ethernet cable on Pins 4 and 5 with negative return on Pins 7 and 8 and absolutely no communication between PSE and PD. This method was once a very common “home brew” method of transferring power over data cable and is still commonly used in telecomm applications.
There’s nothing wrong with PoE that’s not standards based—these power methods work as well as 802.3af/at PoE to power network devices. You do, however, need to be aware of what kind of Power over Ethernet you have and what it will work with. Good network documentation and labeling are the keys that enable you to know that, for instance, that power injector is a high-wattage proprietary injector that will fry the IP camera you’re about to connect. Proper documentation, which is good practice in any case, becomes absolutely vital when you have components that may damage other components.