Roman Kleinerman; Daniel Feldman (May 2011), Power over Ethernet (PoE): An Energy-Efficient Alternative (PDF), Marvell, archived (PDF) from the original on 2016-04-16 , retrieved 2016-08-31

Outdoor point to point microwave and millimeter wave radios and some Free Space Optics (FSO) units usually featuring proprietary PoE Mode A has two alternate configurations (MDI and MDI-X), using the same pairs but with different polarities. In mode A, pins 1 and 2 (pair #2 in T568B wiring) form one side of the 48V DC, and pins 3 and 6 (pair #3 in T568B) form the other side. These are the same two pairs used for data transmission in 10BASE-T and 100BASE-TX, allowing the provision of both power and data over only two pairs in such networks. The free polarity allows PoE to accommodate for crossover cables, patch cables and Auto MDI-X. The RJ45 Ethernet side runs at 100Mbit and would be perfect for older Raspberry Pi models such as the 1/2 where they only have 100Mbit ports anyway. You’d be limiting yourself a little with this on the Pi 3 and 4 but depending on what you’re using it for, you may not mind. The POE-173S and 802.3bt type-4 PoE++ switches use cables that meet the IEEE 802.3bt standard. Instead of delivering power over 2-pair twisted UTP cable – be it end-span (Pins 1, 2, 3 and 6) or mid-span (Pins 4, 5, 7 and 8), they are able to transmit ultra PoE power and data by using all the four pairs of standard Cat. 5e/6 Ethernet cabling.IEEE 802.3at capable devices are also referred to as Type 2. An 802.3at PSE may also use LLDP communication to signal 802.3at capability. [38] Power levels available [39] [40] Class

This is then later "abused" in PoE. If you put a DC voltage across two pairs, this will not couple into the electronics on either side, whether they know about the DC or not, as long as if they know, they handle it properly. This device has two input ports and one output port. The first input is for connecting normal Ethernet cable and another input is for connecting DC power supply, while the output is used for connecting PoE Ethernet cable. A PoE switch is a network switch with the ability to provide power over Ethernet from each interface while still being able to forward frames. Simply put, it works like a port for connecting multiple PoE devices to a single PC. Cisco Universal Power Over Ethernet - Unleash the Power of your Network White Paper". Cisco Systems. 2014-07-11. Archived from the original on 2017-11-28.I’ve just bought 5 of them to replace the older v2 where the fans are noisy and for the extra power. Of course, you can't just "short" the signal by ripping the DC off of one of the wires without considering the signal. In that case the signal would get flattened or destroyed altogether. Till now we assumed there are free lines available in the Ethernet cable which can be used for PoE without much difficulty.