Power over Ethernet (PoE)

What is PoE?

Power over Ethernet is a technology that transmits power over a network cable, which simultaneously transmits data and supplies power to IP end devices (e.g., IP phones, APs, IP cameras, etc.) over a network cable with the help of an existing Ethernet network. PoE is also called Power over LAN (PoL) or Active Ethernet. To promote the standardisation and development of Power over Ethernet (PoE) technology, and to address the issue of compatibility between different manufacturers of power supply and receiving equipment, the IEEE standards committee has released three PoE standards: IEEE 802.3af standard, IEEE 802.3at standard, IEEE 802.3bt standard.

Detailed Explanation of PoE Standard

Earlier, we mentioned that the IEEE Standards Committee, to solve the problem of adaptability between different manufacturers of power supply and powered devices, will have released three PoE standards: IEEE 802.3af standard, IEEE 802.3at standard, and IEEE 802.3bt standard. So, what is the difference between these three standards?

In June 2003, the IEEE 802.3 Working Group developed the IEEE 802.3af standard as an extension of the Ethernet standard, with detailed provisions for network-powered power, transmission and reception. For example, the IEEE 802.3af standard specifies that PSE devices must provide up to 15.4W of DC power per port.

Due to some power dissipation in the cable, only 12.95W was available for the receiving device. In October 2009, to correspond to the demand for high-power terminals, the IEEE 802.3at standard was born. The IEEE 802.3at standard provides up to 25.5W of power to meet the new demand based on compatibility with the 802.3af standard.

In September 2018, in order to further increase the PoE power supply power as well as optimize the standard, the IEEE standards committee's released the IEEE 802.3bt standard. The IEEE 802.3bt standard further improves the power supply capacity, with Type 3 providing up to 51 W of power supply power, and Type 4 providing up to 71.3 W. The IEEE 802.3bt standard also includes an update to the 2.5 W standard, which is compatible with the 802.3af standard, providing up to 25.5 W of power. Support for 2.5GBASE-T, 5GBASE-T, and 10GBASE-T is also included, expanding the use of applications such as high-performance wireless access points and surveillance cameras.

The power supply technology corresponding to the IEEE 802.3af standard is generally referred to as PoE power supply, the power supply technology corresponding to the IEEE 802.3at standard is referred to as PoE+ power supply, and the power supply technology corresponding to the IEEE 802.3bt standard is referred to as PoE++ power supply, or also known as 4PPoE. 

Why Do We Need PoE?

As IP telephony, network video surveillance, and wireless Ethernet devices become more widespread, the need to provide power support through the Ethernet itself is becoming increasingly pressing. In most cases, end devices require DC power, and end devices are usually installed in high ceilings or outdoors, where it is difficult to find suitable power outlets nearby, and even if there are outlets, it is challenging to locate the AC/DC converters required by the end devices. In addition, in many large-scale LAN applications, administrators need to manage multiple terminal devices at the same time. These devices need a unified power supply and unified management due to the limitations of the power supply location, which significantly inconveniences the power supply management. Power over Ethernet is just the right solution to this problem.

PoE is a wired Power over Ethernet technology, the use of network cable for data transmission at the same time with the ability to DC power supply, an effective solution to the centralized power supply of IP phones, wireless APs, portable device chargers, card machines, cameras, data collection and other terminals. PoE power supply has the advantages of reliable, simple connection, standardized:

Reliable: A PoE device can supply power to multiple terminal devices at the same time, realizing centralized power supply and power backup at the same time.

Simple connection: Terminal equipment does not need an external power supply, only a network cable.

Standard: Conforms to international standards and uses a globally unified RJ45 power connector, which ensures docking with equipment from different manufacturers.

How Does PoE Work?

As shown in the figure below, the PoE power supply system includes the following two equipment roles:

PSE (Power-sourcing Equipment): PoE devices supply power to powered devices via Ethernet, providing detection, analysis, and intelligent power management, e.g., PoE switches.

PD: Powered devices such as wireless APs, portable chargers, card swipers, cameras, etc. Depending on whether it conforms to the IEEE standard, PD is divided into standard PD and non-standard PD.

The following is a description of the PoE power supply process.

Detection

The PSE device sends a voltage pulse of 2~10V at the port to detect whether the PD connected at the end of its cable is a standard supported powered device. It will proceed to the next step only when it detects that the PD is a standard device.

PD Classification

Since many types of PDs require different power supplies, after the power supply device has correctly detected the powered device, the power class of the PD device at the opposite end should be detected.

After detecting the PD, PSE will classify the PD device, PSE applies 15.5V~20.5V detection voltage (current limited to less than 100mA), the PD device will connect a classification resistor in series to the line to identify its power, and PSE determines which classification the PD device belongs to by testing the magnitude of the characteristic current returned.

Power up

When the PSE detects that a standard PD is connected to the end of the cable and has classified the PD, it starts to supply power to the PD, outputting 44~57V DC.

Power supply

PSE provides stable and reliable DC voltage for PD and outputs the corresponding power level according to the PD classification result.

Disconnection

The PSE will quickly (usually within 30-40ms) stop supplying power to the PD if the connecting cable to the PD is unplugged or if the user disables the PoE power function of the switch port from the software. At any time during the whole process of PSE powering the PD, if the PD device is short-circuited, consumes more power than the PSE can provide, consumes more power than the rated power, etc., the whole power supply process will be interrupted and restarted from the first step of the detection process.