PoE Power Supply: How Does PoE Switch Deliver Power for PoE Devices?

Posted on October 14, 2019
July 6, 2020
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Driven by the need to connect and monitor smart IoT devices, PoE (Power over Ethernet) network switch has progressed as an increasingly efficient medium for power and data delivery over the network. It 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. Then, how does PoE switch deliver power for the devices, will PoE power damage the connected devices? Here offers detailed explanations of the PoE power supply.

PoE Power Supply Definition & Standards of PoE Switch

PoE switch facilitates installation and management in scenarios such as home applications, WLAN construction, security protection, and so on. The PoE network switch acts as a PSE (power sourcing equipment) that supplies power to PDs (powered devices) via Ethernet cables based on different PoE standards. The following table lists the existing PoE standards and corresponding PoE power supply values.

IEEE 802.3af IEEE 802.3at IEEE 802.3bt
Category PoE PoE+ PoE++
Type Type 1 Type 2 Type 3 Type 4
Power available at PD 12.95W 25.5W 51W 71W
Max. power delivered by PSE 15.4W 30.0W 60W 100W
Numbers of power line pair 2 2 4
Supported cables Cat3 or better Cat5 or better Cat5 or better

Note that, PoE and PoE+ are the two common standards that most PoE devices on the market can support, while PoE++ IEEE 802.3bt standard is the newest one released for smart building and IoT. Since the PoE++ technology is not mature enough, only a few vendors' products support it at present, such as Cisco or Huawei switch.

PoE Switch Power Supply Explained

To understand the working principles of PoE power supply, let's take the PoE network switch as an example to see its working process, PoE supply mode, and PoE distance for powering.

PoE Power Supply Working Process of PoE Switch

After connecting a PD to a PoE Ethernet switch, it works in the following procedures:

PoE switch supply process.jpg

Detection of PDs: This is the first step for the PoE switch to check whether the device is a true PD or not. The PoE powered switch sends a detection voltage pulse to the PD and measures the current. If it detects a valid resistance with a specified value, then the device connected to the PoE switch is confirmed as a true PD. 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 power supply capability: This is the second step for the switch to measure how much power the PD needs. 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: After determining the power over Ethernet class, the PoE switch will start to provide a low voltage to PD in less than 15 microseconds, which 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 power supply: PoE switch 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.

Therefore, it can protect the switch and PDs in the mentioned scenarios or prevent damage to non-PoE devices that are inadvertently connected to the PoE ports after the PDs are removed.

PoE Power Supply Mode of PoE switch

PoE 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.

Alternative A (PoE 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.

Alternative B (PoE 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:

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.

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 Distance for Powering

PoE can transmit 100 meters from the PoE switch to the PDs. Power is not the factor of limitation. In fact, it is the Ethernet cabling standards that limit the total length of cabling to 100 meters due to the existent signal attenuation. Generally, 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 (1219 m).

Troubleshooting PoE Power Supply

Once the PoE system is out of power, the following steps are recommended for diagnosing the issue.


  • Make sure the powered device supports PoE. Remember that not all networking devices support PoE technology. Before connecting it to a PoE switch, check the device supports PoE or not.

  • Make sure the PoE wattage of the powered device does not exceed the maximum power of one port on PoE switch. For example, there is a PoE network switch that only supports IEEE 802.3af, which means the switch can supply maximum power output of 15.4W for each port. Therefore, a powered device that requires 16W or more power can not be connected to the switch, otherwise, the powered device may be damaged by the unstable power or out of power.

  • Make sure the total wattage of all the connected PDs required does not exceed the amount of power that the switch can supply. For example, here is a 24-port PoE switch with a maximum power consumption of 400W. This means this switch can simultaneously connect 24 devices (15.4W×24=369.6<400W) with PoE standard. And it can support up to 13 devices (30W×13=390W<400W) with PoE+ standard.

  • Check the compatibility of power modes between the PSE and PD. For example, if a PD only supports PoE mode B power delivery, while a PoE switch is based on PoE mode A, as a result, the PD and PoE switch can not work together.

Conclusion

PoE technology is becoming a vital part of the digital transformation journey. Knowing PoE power supply details like the specific PoE class and PoE wattage 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.