PoE is a technology that allows for simultaneous transmission of data and power through a single class 5 twisted pair as a standard Ethernet connection.
PoE can provide the conventional 48V DC voltage for the traditional class five twisted pair. There are two kinds of devices using PoE, one is power supply equipment (PSE), which is mainly midspan adapter and Ethernet switch, and is powered over Ethernet Poe injector
(and network connection); The other is called electrical devices (PD), which can be any device, such as wireless access points, VOIP phones, security cameras and other digital devices.
The 802.3af standard specifies two modes of operation: in TypeA mode, power and data are transmitted through the same data pair, such as 10/100Base-T ethernet; In TypeB mode, the copyright of engineering network is controlled, and the power is transmitted through unused data. Choosing the TypeA mode or the TypeB mode depends on the PSE device, not the PD device. In addition, the standard also shows that PSE devices can use either mode, but not both.
At present, there are a lot of non-standard powered over Ethernet Poe injector. For example, before using the PSE device to supply power to the PD device, it is necessary to check whether the PSE device meets the standard. If the PD device does not meet the standard, the PSE device will automatically stop the power supply. Therefore, when the power supply of Ethernet is not used properly, this feature (integrated in each device that meets the 802.3af standard) can prevent the failure of equipment that does not meet the standard.
The PoE device has a 25 kOhm resistor that can be used as a "signature" for the PoE. When a signature resistor is detected by a switch or other powered over Ethernet Poe injector, it automatically detects the device connected to the resistor and supplies power to the device after the automatic detection is successful. When the resistor is not connected to any device, the PSE device will turn off the PoE power supply.