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How Do PoE Switches Supply Power for PoE Powered Devices?

Updated on Dec 6, 2021 by
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Driven by the demand for connection and monitoring of smart IoT devices, the PoE switch has progressed as an increasingly efficient medium for power and data delivery over the network. This article will explain the working principles and working mode of the PoE switch power supply, the limited distance, and the maximum voltages of the PoE switch supply.

The Devices in PoE Network

In the PoE network, there are two kinds of devices, PSE (power sourcing equipment) and PD (powered devices). PSE supplies power to PD. The PSE device is a more typical PoE switch, and the PD device generally has IP phones, IP surveillance cameras, wireless LAN access points, PoE lighting, etc.

PoE Switch Power Supply Working Process

To understand the working principles of the 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. 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 minimal 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 switches or injectors will not. The differences between active and passive PoE switches will be further illustrated here: Active vs. Passive PoE Switch: Which Should We Choose?

Classification of PoE Switch 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, and 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 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) cut 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 by the PDs is out of the power budget of the PoE-powered switch.

In the mentioned scenarios, the switch and PDs are protected, which prevents 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.

Mode A

The PoE switch supplies power to PDs by data pair 1-2 and pair 3-6. The pair 1-2 act as the positive polarity, while the pair 3-6 act 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 act as positive polarity, while pair 7-8 act as negative polarity.

The main difference between Mode A and 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 Scenario

4-pair Delivery

In this mode, power is delivered in 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 the PoE power supply mode is decided by PSE. And the 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 a factor of limitation. However, due to the existent signal attenuation, Ethernet cabling standards limit the total length of the 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. The PoE transmission distance varies with different network cables. In the case of Cat5e, Cat6, Cat6a, and Cat7, the maximum transmission distance is 100m. While Cat8 can reach 25/40 Base-T transmission speed, the upper limit of transmission distance is only 30m.

PoE Switch Supply Voltages and Types

PoE switches are divided into four types according to the IEEE 802.3 standard. Each type also has a different supply voltage. The following table will detail the supply voltage for each type.

  PoE PoE+ PoE++
IEEE Standard IEEE 802.3af IEEE 802.3at IEEE 802.3bt
PoE Type Type 1 Type 2 Type 3 Type 4
Switch Port Power
Max. Power Per Port 15.4W 30W 60W 100W
Port Voltage Range 44-57V 50-57V 50-57V 52-57V
Powered Device Power
Max Power to Device 12.95W 25.5W 51W 71W
Voltage Range to Device 37-57V 42.5-57V 42.5-57V 41.1-57V

If you want more details, you can click the article: Understanding PoE Standards and PoE Wattage.

Conclusion

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. In addition, getting familiar with common issues and solutions for PoE switch connections can avoid unwanted time and money waste when deploying PoE networks.

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