Most Ethernet systems exist in current network infrastructures are without PoE Power over Ethernet support, but more and more terminal device are PoE enabled as electrical power and data  pass through only a single Ethernet cable, so in order to make the most of existing network infrastructures without adding extra more hassle and costs, a Power Source Equipment (PSE) as the mid-span in Power over Ethernet network system, it's also known as "PoE Injector" or "PoE Adapter" which is a kind of power controller that is necessary to be deployed between Ethernet switch and end terminal PoE enabled or PoE disabled devices in a PoE system. As PoE injector adds DC power on current Ethernet cable to feed the end point devices such as IP camera, VoIP phone, Access Control, AP, RFID reader and other equipment that not matter PoE compatible or not.

There are two different power injectors over Ethernet according to compliance with IEEE802.3 or not, we mostly call them passive PoE injector and active PoE injector. The active injectors are IEEE802.3af and IEEE802.3at compliant that always go through powered device (PD) discovery, classification, current limit, and load disconnect detection these five steps before supplying power to the terminal PD. Instead, passive power injectors feed power to the end point device directly without advanced detection. Active PoE injectors all  deliver 48-57Vdc power supply, and passive power injectors deliver DC 12V, 24V, 48V or other voltage value.