What is PoE?

PoE, short for Power over Ethernet, is a technology that allows for both data and electrical power to be sent over an ethernet cable. This means devices can be powered by a singular cable, reducing cost and increasing the simplicity of a network.

 

Definitions

 

PSE (Power Source Equipment) refers to devices that can supply DC power to other devices through Ethernet cable. PSEs can automatically search for devices, classify them, and supply power.

 

PD (Powered Device) refers to devices that receive power from a PSE. Powered Devices can be categorized into standard devices and nonstandard devices. Standard PDs are devices compatible with the IEEE802.3af/at standard. Standard PDs can be powered by the standard PSEs whereas nonstandard PDs require a PSE that can output the appropriate power.

 

PoE Standards

 

IEEE802.3af defines the maximum output power from a single port as 15.4W. PDs under this standard receive a supplied power of 12.95W.

 

IEEE802.3at improved the standard set by IEEE802.3af and increased the maximum power per port to 30W (supplied power of 25.50W). This standard is intended for devices with higher power requirements such as video phones, PTZ devices, etc.

 

Please see the comparison table below.

  

 

PoE Power Supply modes

 

There are two power supply mode based on the IEEE802.3af/at standard.

 

Alternative A supplies power using the data line pairs 1/2 and 3/6 of the Ethernet cable.

Alternative B supplies power using the spare line pairs 4/5 and 7/8 of the Ethernet cable.

 

Alternative A

As shown below, in Alternative A the PSE supplies power using date line pairs 1/2 and 3/6. The polarity of the line pairs can be positive or negative, which is decided by the designation of chip vendors.

 

 

Alternative B

As shown below, in Alternative B the spare line pairs 4/5 and 7/8 are free lines. Pair 4/5 pair is the positive pole while the 7/8 pair is the negative pole.

 

Notes:

According to the IEEE 802.3af/at protocol, standard PSEs can support either Alternative A or B, while standard PDs must support both modes.

At present, all of TP-Link’s PoE switches are standard PSE and designed based on Alternative A. The line pair 1/2 is the passive pole while the line pair 3/6 is the positive pole.

 

Common issues with PoE devices

Q1: Why do my PDs fail to work with a PoE switch? The PoE LED on the ports do not light up.

 

1. Make sure the powered device (PD) is a standard device based on IEEE802.3af/at protocol.

2. Ensure you are using a Cat5 or higher ethernet cable and that the length of the cable does not exceed 100m.

3. Check that your specific switch model has PoE ports.

4. Confirm the power supply mode with the PD’s vendor. TP-Link’s PoE switches cannot support devices that use the Alternative B protocol. If the PD supports Alternative A, check if the line pair 1/2 is passive pole and the line pair 3/6 is positive pole. If this is the case, you will need to use a crossover cable instead of a normal straight-through cable.

 

If support from TP-Link is needed, please ensure to provide the above answers in your ticket submission.

 

Q2: Why is my LED indicator flickering on my PoE switch?

 

A: One potential cause is that the total output power of the PSE exceeds or approaches the maximum output power. You can check the status via PoE MAX light. If the PoE MAX light is on or blinking, it means the power is insufficient.

 

Another potential cause is the link-loss detection mechanism used by the switch and your powered device (PD) not being compatible. All TP-Link PoE switches only support DC link-loss detection mode. If your PD only supports AC link-loss detection mode, it will not be compatible with the switch.

 

Q3: Why can my PD work with other vendor’s PSEs but cannot work with TP-Link’s PoE switches?

 

A: We should first make sure the powered device (PD) is a standard device and confirm the power supply mode, polarities and link-loss detection mode it supports.

 

Some devices can only work with the products produced by the same manufacturer. However, some nonstandard devices may be able to cooperate with nonstandard devices if the parameters are matched. It is highly recommended to check the protocols supported by the PD before purchasing.

 

Notes:  

Although some vendors also claim their PDs are compatible with IEEE802.3af/at, they may have included some non-standard or proprietary technology. This may cause incompatibility with other vendor PSEs. These PDs may also have problems when work with other vendors’ standard PSEs.

Please refer to the User Guide of your device for assistance interpreting the meaning of the LEDs on the switch.