POE Standard Details For 802.3af, 802.3at, 802.3bt

- Nov 11, 2019-

Recently, when I received a customer, I encountered an engineer who was doing program-controlled exchange. I was very interested in the way that HRUI used PoE system to supply power to the AP. It also gave a special example of the role of each line when the telephone line was used to power the fixed telephone. I will give you a talk about the PoE system and 802.3af (PoE), 802.3at (PoE+), 802.3bt (PoE++) standards.

PoE system configuration and power supply characteristics parameters A complete PoE system includes two parts: power supply equipment (PSE, PowerSourcing Equipment) and power receiving equipment (PD, PowerDevice). The HongRui POE switch is a PSE device, and the wireless AP or the Internet of Things LoRa gateway is a PD device. Based on the IEEE802.3af standard, the two establish information about the connection status, device type, and power consumption level of the PD of the powered device, and use this as the power supply to the PD through the Ethernet.



The standard five types of network cables have four pairs of twisted pairs. IEEE802.3af allows two line order power supply methods: one is to transmit current on the 4, 5, 7, and 8 pairs, and it is specified that 4 and 5 are positive, 7 8 is the negative electrode. Another type of power supply is to transmit power on the 1, 2, 3, and 6 lines, the polarity is arbitrary, 1, 2 is the positive pole, 3, 6 is the negative pole or 1, 2 is the negative pole, and 3 and 6 are the positive poles. The HRUI POE switch adopts the second power supply line sequence, and both the power supply line sequence of the wireless AP or the Internet of Things LoRa gateway are supported.

The working process of IEEE802.3af:

1. Detection: Before the PSE starts to supply power to the powered device, it first outputs a low voltage to detect whether the powered device (PD) complies with the IEEE802.3af standard.

2. Grading: When the PSE detects the requirements, the output voltage will be further increased to classify the powered device. If the powered device does not respond to the grading confirmation current at this time, the PSE will default to the level of the powered device. It is supplied with an output power of 15.4W.

3. Power supply: After confirming the classification, the PSE will output 48V DC power to the powered device and confirm that the powered device does not exceed the power requirement of 15.4W.

4. Maintenance: Update real-time power, perform open circuit detection and single port overload detection. When the powered device is overloaded or shorted, the PSE stops supplying power and enters the detection phase again.


IEEE802.3af main power supply parameters:

The DC voltage is between 44 and 57V, with a typical value of 48V. Typical operating current is 10 to 350 mA, typical output power is 15.4W. The overload detection current is 350 to 500 mA. Under no-load conditions, the maximum required current is 5 mA. Provides three class level electrical power requests for the PD device from 3.84 to 12.95W.

Classification parameters of IEEE802.3af:

The maximum working power required by the Class0 device is 0~12.95W.

Class1 devices require a maximum operating power of 0 to 3.84W;

The Class 2 device requires a working power ranging from 3.85W to 6.49W.

Class3 devices have a power range of 6.5 to 12.95W.

Why is there an IEEE802.3at standard?

Since the IEEE802.3af standard limits PoE power consumption on powered devices (PDs) to 12.95W, this limits the range of applications for Ethernet cable power. To overcome the PoE's power budget constraints, a new standard, IEEE802.3at (also known as PoE+), is defined as a Class 4 device with power requirements above 12.95W, which extends the power level to 25W or higher.

Compared with 802.3af, IEEE802.3at can output more than 2 times of power, and the output power of each port can be more than 30W. The PD of the powered device can be up to 29.95W, and the PSE will provide more than 30W DC power. The PD responds with a Class 4 current response, telling the PSE if it can provide it with a higher power specified by 802.3at.

IEEE802.3at (PoE+) main power supply parameters:

The DC voltage is between 50 and 57V, with a typical value of 50V. Typical operating current is 10 to 600 mA, and typical output power is 30W. The powered device PD supports the classification of Class 4.

IEEE802.3bt (PoE++):

The 802.3bt specification introduces four new high-power PD classes, bringing the total number of single feature categories to nine. Class 5~8 is new to the PoE standard and translates to a PD power level of 40.0W to 71W.

POE Class

PD Powered

POE Class

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802.3bt is backward compatible with 802.3at and 802.3af. A lower power 802.3at or 802.3af PD can be connected to a higher power 802.3bt PSE without any problems. When a higher power 802.3bt PD is connected to a lower power 802.3at or 802.3af PSE, the PD only needs to be able to operate in its respective lower power state, which is called "downgrade."

PoE protocols supported by various network cables: