How Does POE Work?

Network cables, such as Cat 5e and Cat 6, comprise eight wires arranged as four twisted pairs. In 10 and 100BASE-T Ethernet, two of these pairs are used for sending information, and these are known as the data pairs. The other two pairs are unused and are referred to as the spare pairs (Gigabit Ethernet uses all four pairs).

 

Because electrical currents flow in a loop, two conductors are required to deliver power over a cable. 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.

 

Power over Ethernet is injected onto the cable at a voltage between 44 and 57 volts DC, and typically 48 volts is used. This relatively high voltage allows efficient power transfer along the cable, while still being low enough to be regarded as safe.

 

This voltage is safe for users, but it can still damage equipment that has not been designed to receive POE. Therefore, before a POE switch or midspan (known as a PSE, for power sourcing equipment) can enable power to a connected IP camera or other equipment (known as a PD, for powered device), it must perform a signature detection process.

 

Signature detection uses a lower voltage to detect a characteristic signature of IEEE-compatible PDs (a 25kOhm resistance). Once this signature has been detected, the PSE knows that higher voltages can be safely applied.

 

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. This means that PSEs with a limited total power budget can allocate it effectively.

 

The differences between power delivered by the PSE and power received by the PD account for power that is lost as heat in the cable. If a PD does not display a signature, it is class 0 and must be allocated the maximum 12.95 watts.

 

POE Plus equipment has a power class of 4. If a regular 802.3af POE source detects this class it will simply enable power as if it was a class 0 device. However, an 802.3at PSE will not only recognise the PD as a POE Plus device, it will also repeat the classification stage, as a signal to the PD that is connected to a power source with full POE Plus power available. (In theory the PD should also be able to request the extra power by communicating across the network link.) POE Plus PSEs can supply up to 30 watts and available device power is 25.5 watts.

 

The final stage after detection and classification of a newly connected device is to enable power: the 48V supply is connected to the cable by the PSE so the PD can operate. Once enabled, the PSE continues to monitor how much electrical current it is delivering to the PD, and will cut the power to the cable if too much, or not enough, power is drawn. This protects the PSE against overload, and ensures that POE is disconnected from the cable if the PD is unplugged.