History of PoE Technology

PoE technology appeared early, but there were no standards to follow for early PoE technology. The IEEE 802.3 working group standardized it in 2003 and gradually developed it into 802.3af, 802.3at, and the latest 802.3bt.

Let's take a look at the development history of PoE technology.

Stage 1: IEEE 802.3af（PoE）

IEEE 802.3af standard which was released in 2003, also known as standard PoE, operates at a voltage range of 44-57V and supplies a current of 10-350mA. In this standard, the maximum power output of a port is limited to 15.4W. IEEE 802.3af can support various devices such as VoIP phones and sensors.

Stage 2: IEEE 802.3at（PoE+）

The updated IEEE 802.3at PoE standard which was released in 2009, known as PoE plus (PoE+), is backward-compatible with standard PoE. The supply voltage of PoE+ ranges from 50V to 57V, and the supply current can be 10-600mA. PoE+ provides up to 30W of power on each port. IEEE 802.3at can support devices that require more power like LCD displays, biometric sensors, and tablets.

Stage 3: IEEE 802.3bt

The latest IEEE 802.3bt standard which was released in 2018 further expands the power capabilities of 802.3at. It is also known as PoE++. The standard introduces two additional power types: up to 51 W delivered power (Type 3) and up to 71.3W delivered power (Type 4). Each pair of twisted pairs needs to handle a current of up to 600mA (Type 3) or 960mA (Type 4). Additionally, support for 2.5GBASE-T, 5GBASE-T and 10GBASE-T is included.

With higher power, developers can easily add more features and upgrade existing products to meet the power supply needs of high-power terminals in systems such as monitoring, access control, information dissemination, parking lots, and even laptops and televisions.

The comparison of three PoE power supply technology standards is as follows:

PoE Power Supply Process Description

PoE power supply flowchart

1. Detection: The PSE device emits a voltage pulse of 2-10V at the port to detect whether the PD connected to its cable terminal is a standard supported receiving device. Only when PD is detected as a standard device, the next step will be continued.

2. Classification: Due to the wide variety of PD types and power requirements, it is necessary to detect the power level of the opposite PD device after the power supply device correctly detects the receiving device.

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

3. Power up: When the PSE detects which level of PD is connected to the cable end, it is ready to output a DC voltage of 44-57V to the PD.

4. Start power supply: PSE provides stable and reliable DC voltage for PD, and outputs corresponding levels of power based on PD classification results.

5. Disconnect: If the connecting cable to the PD is unplugged or the user turns off the PoE power supply function of the switch port by software, the PSE will quickly (usually within 30-40ms) stop supplying power to the PD. At any time during the entire process of PSE supplying power to PD, if there is a short circuit in the PD equipment, the power consumed during classification exceeds the corresponding power provided by PSE, or the power consumed exceeds the rated power, then the entire power supply process will be interrupted and the first detection process will be restarted.

PoE Line Sequence Description

Taking the PoE standard as an example, the standard defines two types of power supply line sequences, and PSE can adapt to PD using either power supply line sequence.

Power supply by idle line: Use unused idle line pairs (4/5, 7/8) in Ethernet cables to transmit DC power. This solution carries certain risks, as some network cables may not necessarily connect all 8 lines and only use 1/2, 3/6, so there is no possibility of idle lines supplying power.

Power supply by signal line: DC power (1/2, 3/6) is transmitted simultaneously on the core wire used for data transmission, using a different frequency from Ethernet data signals to achieve multiplexing of transmission data and current.

Transforming PoE power supply into a two core communication technology application, our company has multiple PoE Extenders that can transport PoE up to about 500meters, opening up a wider range for network expansion.