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POE (Power Over Ethernet) is the abbreviation of Power over Ethernet, which means that in the case of existing Ethernet Cat.5 cabling infrastructure without any changes, for some IP-based terminals (such as IP phones, WLAN access points (APs, network cameras, etc.) can also provide DC power for such devices while transmitting data signals. POE technology can ensure the normal operation of existing networks while ensuring the safety of existing structured cabling, and minimize costs.
PoE is also known as PoL (Power over LAN) or Active Ethernet, sometimes also referred to as Power over Ethernet. This is the use of existing standard Ethernet transmission cables for simultaneous data transmission and The latest standard specification of electrical power, and maintain compatibility with existing Ethernet systems and users. The IEEE802.3af standard is a new standard for POE based on Power over Ethernet (POE). It adds the relevant standard for direct power supply via the network cable based on IEEE 802.3. It is an extension of the existing Ethernet standard and is also the first one about power distribution. International standards. 
The IEEE began to develop this standard in 1999. The earliest vendors involved were 3Com, Intel, Power Dsine, Nortel, Mitel, and National Semiconductor. However, the shortcomings of the standard have always restricted the expansion of the market. Until June of 2003, the IEEE approved the 802.3af standard, which clearly stipulates power detection and control in remote systems, and provides routers, switches, and hubs with Ethernet cables to IP phones, security systems, and wireless LAN access points. The method of power supply for such equipment is stipulated. The development of IEEE802.3af includes the efforts of many company experts, which also makes the standard can be tested in various aspects.
A typical Power over Ethernet system. Keep the Ethernet switch device in the wiring closet, and use a power supply hub (Midspan HUB) to provide power to the LAN's twisted pair cable. At the end of the twisted pair, this power is used to drive phones, wireless access points, cameras, and other devices. To avoid power outages, use a UPS.
principle
The standard Cat 5 cable has four pairs of twisted pairs, but only two of them are used in 10MBASE-T and 100MBASE-T. IEEE802.3af allows two uses. When the idle pin is used, the pin 4 and pin 5 are positive and the pin 7 and pin 8 are negative.
When the data pin is used for power supply, the DC power supply is added to the midpoint of the transmission transformer, which does not affect the data transmission. In this way the pairs 1, 2 and the pairs 3, 6 can be of any polarity. 
The standard does not allow the application of both cases. The power supply device PSE can only provide one usage, but the power application device PD must be able to adapt to both situations at the same time. The standard specifies that the power supply is usually 48V, 13W. It is easier for the PD device to provide 48V to low voltage conversion, but at the same time there should be an insulation safety voltage of 1500V.
parameter
A complete POE system includes two parts: PSE (Power Sourcing Equipment) and PD (Powered Device). The PSE device is a device that provides power for Ethernet client devices and is also the administrator of the entire PoE Power over Ethernet process. The PD device is a PSE load that receives power, that is, client devices of a POE system, such as IP phones, network security cameras, APs, and PDAs or mobile phone chargers, and many other Ethernet devices (in fact, any power Devices that do not exceed 13W can get corresponding power from the RJ45 socket). Based on the IEEE 802.3af standard, the two establish information links regarding the connection status, device type, and power consumption level of the power-receiving device PD, and use this to provide power to the PD through the Ethernet through the PSE.
The main power supply parameters of the POE standard power supply system are:
1. The voltage is between 44-57V, typically 48V.
2. The maximum allowable current is 550mA and the maximum starting current is 500mA.
3. The typical operating current is 10-350mA and the overload detection current is 350-500mA.
4. Under no-load conditions, the maximum required current is 5mA.
5. Provide the PD device with 3.84~12.95W five levels of electric power request, the maximum is not more than 13W.
POE Technology Comprehensive Analysis and Detailed Explanation
Introduction