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What Is Defined in the POE Standard?

IEEE 802.3AF defines a POE technology in detail, aiming to provide 15.4 W DC power supply (minimum 44 VDC and 350 mA) for each device. Due to the loss in the cable, only 12.95 W can be provided for electrical equipment.

This technology uses standard RJ 45 connectors and Cat5 (or even Cat3) cables, which can handle tens of watts of power. After the Ethernet network is installed for communication, it can also be used for power supply, thus saving materials, labor, installation time and continuous maintenance costs.

Since a typical 10 to 100 Mbps physical layer only needs two of the four pairs of stranded wires in a Cat5 cables, the power can be transmitted on the unused wires of the Ethernet cables. By applying a common-mode voltage to each pair of stranded wires, power can also be transmitted on the data conductors of the cables. Since industrial Ethernet network uses differential signal transmission, it will not interfere with the data transmission of the cable.

POE 802.3 AF defines two types of POE equipment: power supply equipment (PSE) and powered device (PD). The PSE obtains power from its own conventional power supply, and then it manages the power transmitted to the PD through the Ethernet cable network. PD can get the power it needs through RJ 45 connector, so it does not need built-in power supply. POE can supply power to PD on typical Ethernet cables with a running length of 100 meters. PD devices include original VoIP phones and wireless access points, security cameras, cash register (POS) terminals, temperature control systems, and even onboard entertainment systems.

The IEEE POE standard also specifies the signal transmission between PSE and PD. This signal transmission enables PSE to detect devices that meet the specifications, thus avoiding damage to non-POE devices connected to the network. The PSE and PD coordinate the required or available power. PSE applies a DC voltage of 2.8 to 10 V across the wire in order to detect PD. The PSE then determines whether there is a connected PD by measuring the loop current. PD should have a resistive load of 19 to 27 kΩ and be marked with a parallel capacitor that is no more than 120 nF.

Although POE can provide PD with about 13 W power, some devices may benefit from higher power (for example, cameras with pan, tilt and zoom functions). In order to meet the needs of these products, the second standard IEEE 802.3AT hit the market in 2009. This technology, also known as "POE+", can provide up to 25.5 W of direct current for the PD and 50 to 57 VDC for the PSE. The current of POE 802.3 AT increased to 600 mA, while the current of early technology was 350 mA.

The 802.3AT standard only uses Cat5 cable (there are eight wires inside it compared with four wires of Cat3), which reduces the probability of possible impedance and power dissipation. In addition, 802.3AT also provides more functions for network administrators, such as providing new remote power diagnosis, status report and PD power management functions (including remote power cycle of embedded devices).

At last, POE 802.3AT provides dynamic power distribution, optimized power distribution and good power utilization, improving system efficiency and reducing costs.

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