POE (Power over Ethernet)

最後更新: 2018-12-18



POE: A single cable to provide both data connection and electric power to devices (802.3)

PoE 一共有兩種制式, 分別是

  • PoE      # 舊
  • PoE+    # 新

某些 switch 會同時支援這兩種制式

PD: powered device

PSE: power sourcing equipment




# class from 0-4 based on how much power they require.

Classification follows the signature detection stage, and is an optional process.  

If a PD displays a classification signature, it lets the PSE know how much power it requires to operate, as one of three power classes.

Class 1    

Max device power: 3.84W

Class 2

Max device power: 6.49W

Class 3

Max device power: 12.95W

Class 0

devices have not received a proper class designation from the manufacturer

If a PD does not display a signature, it is class 0 and must be allocated the maximum 12.95 watts

even if the Class 0 device actually uses very little power.


802.3af (POE)


mode A & mode B

mode A

POE treats each pair as a single conductor,

and can use either the two data pairs or the two spare pairs to carry electrical current.

 * Need 2 pairs

# Power & Data

Pin 1,2  # +
Pin 3,6  # -


Power is transmitted on the data conductors by applying a common voltage to each pair.

mode B

 * Need 4 pairs

DC +
Pin 4 - Blue solid
Pin 5 - White/blue stripe

DC −
Pin 7 - White/brown stripe
Pin 8 - Brown solid


易 troubleshooting


Passive PoE


In passive PoE, no negotiation takes place between the two devices,

but instead the known cable layout is used from Standard 802.3af, mode B.

 * The power is "always on", so it's possible to burn out devices if they're not prepared for electrified Ethernet wires

Data + Power => Ingector --- Cable(1,2,3,6 || [+]4,5 [-]7,8) ---> Splitter => Data + Power

Active PoE

refers to any type of PoE that negotiates the correct voltage between the switch and the PoE-powered device.


802.3at (PoE+)


PoE+ PSEs can provide almost twice as much power over a single Ethernet cable.