Understanding PoE Standards

PoE Standards Introduction

At present, PoE standard has three types: IEEE 802.3af, IEEE 802.3at and IEEE 802.3bt. Those PoE standards define the minimum power that PSE can source and the maximum power that the PD will expect to receive.

IEEE 802.3af

IEEE 802.3af is also known as standard PoE with supply voltage of 44-57V, and supply current of 10-350mA. In this standard, the maximum power output of a port is limited to 15.4W. However, some power will be lost on the Ethernet cable during the transmission. Thus, the minimum guaranteed power available at the PD is 12.95 watts per port. It can support VoIP phones, sensors and so on.

IEEE 802.3at

The updated IEEE 802.3at standard also named PoE+, which is backward-compatible with standard PoE. The supply voltage of PoE+ ranges from 50V to 57V, and the supply current can be 10-600mA. It provides up to 30W of power on each port of a PSE. Due to power loss, the minimum output power assured on each port is 25W. This type can support devices that require more power like LCD displays, biometric sensors and tablets.

IEEE 802.3bt

IEEE 802.3bt is the latest PoE standard that defines two types of powering/wattage standards – Type 3 and Type 4 in the table above. They will increase the maximum PoE power by delivering more power through two or more pairs of Ethernet cables. In Type 3 and Type 4 modes, PSEs will identify the PDs and set the power accordingly to the maximum PD power, resulting in a better power-delivery system.

Type 3

Type 3 is also known as PoE++, which can carry up to 60W for each PoE port (minimum power ensured on each PD port is 51W) over a single RJ45 cable to power devices like video conferencing system components.

Type 4

Type 4 is named higher-power PoE. It can supply maximum power output of 100W on each PoE port (minimum power ensured on each PD port is 71W), which is suitable for devices like laptops or TVs, etc. Both the two modes of IEEE 802.3bt are backward compatible with 802.3af and 802.3at. The following table concludes the specifications of the PoE standards. The following table concludes the specifications of the PoE standards.