How Do PoE Switches Supply Power for PoE Powered Devices?

Updated on December 6, 2021

Driven by the demand for connection and monitoring of smart IoT devices, PoE (Power over Ethernet) network switch has progressed as an increasingly efficient medium for power and data delivery over the network. Then, to take PoE switch as an example to explore how PoE switch delivers power for the PDs and if Power over Ethernet damages the connected devices or not. The post offers detailed explanations of PoE switch power supply.

Definition of Power over Ethernet & Standards of PoE Switch

Power over Ethernet is the full name of PoE, which allows devices like IP phones, IP surveillance cameras, wireless LAN access points, and PoE lighting to receive power as well as data over the same Ethernet cable, which eliminates the need for separate data or power cables. For example, as a PSE (power sourcing equipment), PoE network switch supplies power to PDs (powered devices) by Ethernet cables based on different PoE standards, which are divided into PoE, PoE+ and PoE++. For more details you can refer to: Understanding PoE Standards and PoE Wattage.

Since the PoE++ technology is not mature enough, only a few vendors' products support it at present, such as FS S5860-24XB-U switch.

PoE Switch Power Supply Explaination

To understand the working principles of PoE switch power supply, let's take the PoE network switch and PoE IP camera as an example to see how the PoE switch power supply works. Next, we will analyze it from its working process, PoE supply mode, and PoE distance for powering.

PoE Switch Power Supply Working Process of PoE Switch

After connecting a PoE IP camera to a PoE Ethernet switch, the working process is as follows:

Detection of PDs: At the beginning, the PoE switch outputs a very small voltage at the port until it detects that the cable terminal connection is a PD that supports the IEEE802.3af standard. Generally, a 24.9kΩ resistor is selected in the flashlight equipment to confirm the load IEEE802.3AF power supply standard. It's worthwhile to mention that only active PoE network switches will perform this check, whereas passive PoE network switch or injector will not. The differences between active and passive PoE Switch will be further illustrated here: Active vs. Passive PoE Switch: Which Should We Choose?

Classification of PoE swich power supply capability: After detecting the PD, the PoE switch will apply a voltage of 15-20V to the PD and determine the specific level of the PD by measuring the current. The switch classifies the device as Class 0, 1, 2, 3, 4, 5, 6, 7, 8 types and supplies proper power when it detects a resistor.

Class PSE Output Power (W) PD Input Power (W)
0 15.4 0.44-12.94
1 4 0.44-3.84
2 7 3.84-6.49
3 15.4 6.49-12.95
4 30 12.95-25.50
5 45 40 (4-pair)
6 60 51 (4-pair)
7 75 62 (4-pair)
8 99 71.3 (4-pair)

Start to supply power: Completing the classification of Power over Ethernet, within a start-up period of a configurable time (generally less than 15μs), the PSE device starts to supply power to the PD device from a low voltage until it is then raised to the full 48V DC eventually.

Normal power supply: After the voltage is reached to 48V, the PoE switch provides stable and reliable 48V DC power to the PD.

Disconnection of PoE switch power supply: PoE switch will quickly (usually within 300-400ms) cuts off the power and re-enters the PD detection procedure when the following situations occur:

  • The PD is removed.

  • The power consumption of the PD is overloaded or short-circuited.

  • The total power consumed of the PDs is out of the power budget of the PoE powered switch.

In the mentioned scenarios, the switch and PDs are protected, which prevent damage to non-PoE devices that are inadvertently connected to the PoE ports after the PDs are removed.

PoE Switch Power Supply Mode

PoE switch power supply mode between the PSE and PDs can be divided into three different categories. Power over Ethernet switch, as a typical type of PSE, will be taken as an example to explain PoE supply mode to PDs here.

Mode A

The PoE switch supplies power to PDs by data pair 1-2 and pair 3-6. The pair 1-2 acts as the positive polarity, while the pair 3-6 acts as the negative polarity.

Mode B

The PoE switch delivers power to PDs via pair 4-5 and pair 7-8. In 10BASE-T and 100BASE-T, the two pairs are not used for transmitting data. So they are called spare pairs in 10/100M PoE powering. The pair 4-5 acts as the positive polarity, while pair 7-8 acts as the negative polarity.

The main difference between Mode A vs Mode B lies in the use of PINs. The following diagram will illustrate the difference visually:

Mode A vs Mode B Working Principle

PSE that relays power through the data pairs (Mode A) is referred to as "endspan," whereas PSE that relays power through the spare pairs (Mode B) is known as "midspan." Normally speaking, compliant PSE can support Mode A, Mode B, or both, and compliant PDs support both Mode A and Mode B, while compatible PDs typically support Mode B only. Here provide the working scenarios between switches and IP cameras based on the two different modes.

Mode A vs Mode B Working Scenario

4-pair Delivery

In this mode, power is delivered on four pairs. Pair 1-2 and pair 4-5 are the positive polarities, and pair 3-6 and pair 7-8 are the negative polarities.

The chart below presents the three modes in two different network situations:

10/100BASE-T Network 1000BASE-T Network
Pins at Switch PoE Mode A (Data & Mixed DC) PoE Mode B (DC on Spares) 4-pair PoE PoE Mode A (Bi-Data & DC) PoE Mode B (Bi-Data & DC) 4-pair PoE
Pin 1 Rx + & DC + Rx + Rx + & DC + TxRx A + & DC + TxRx A + TxRx A + & DC +
Pin 2 Rx - & DC + Rx - Rx - & DC + TxRx A - & DC + TxRx A - TxRx A - & DC +
Pin 3 Tx + & DC - Tx + Tx + & DC - TxRx B + & DC - TxRx B + TxRx B + & DC -
Pin 4 Unused DC + DC + TxRx C + TxRx C + & DC + TxRx C + & DC +
Pin 5 Unused DC + DC + TxRx C - TxRx C - & DC + TxRx C - & DC +
Pin 6 Tx - & DC - Tx - Tx - & DC - TxRx B - & DC - TxRx B - TxRx B - & DC -
Pin 7 Unused DC - DC - TxRx D + TxRx D + & DC - TxRx D + & DC -
Pin 8 Unused DC - DC - TxRx D - TxRx D - & DC - TxRx D - & DC -

Note that PoE power supply mode is decided by PSE. And both PoE switch and PoE injector can act as the PSE to send power and data to the PDs. PoE Ethernet switch, as an endspan (IEEE 802.3af refers to it as endpoint), often uses PoE mode A. PoE injector (also called midspan device) is an intermediary device between a non-PoE switch and PD. It only supports PoE mode B.

PoE Switch Power Supply Distance

PoE can transmit 100 meters from the PSE to the PDs. In fact, power is not the factor of limitation. However, due to the existent signal attenuation, Ethernet cabling standards limit the total length of cabling to 100 meters. Generally, to take a PoE switch as an example, 100 meters is the furthest distance for powering that a PoE switch can reach. However, a PoE extender can extend the reach of Ethernet data and PoE power up to 4000 feet (1219m).


PoE technology is becoming a vital part of the digital transformation journey. Knowing PoE switch power supply details will contribute to protecting PoE and non-PoE devices. Plus, getting familiar with common issues and solutions of PoE switch connection can avoid unwanted time and money waste when deploying PoE networks.

Related Articles:

Understanding IEEE 802.3bt High Power PoE (Hi-PoE)

FS S5860-24XB-U PoE Switch Guarantees High-reliability Power Supply


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