PoE - How It Works

   
 

Xmultiple's Engineering Department


How Power Over Ethernet (PoE) Works

In the PoE scheme, two different types of devices are involved: Power Sourcing Equipment (PSE) and Powered Devices (PD).

PDs are PoE-enabled network end devices equipped to accept low voltage power transmitted over structured Ethernet CAT-5, CAT-5e, CAT-6 and CAT-7 cabling.

PSEs provide the DC power to PDs. PSEs source up to 12 watts at 48 volts to each PD. A PSE may be an endspan device or a midspan device. An endspan device typically is a network switch enabled to provide PoE power on each port. A midspan device is connected in-line to each end device and adds power to the line.

Power is carried over the cabling using two techniques: Alternative A and Alternative B

Alternative A implements a simplex, or ˇĄphantom feedingˇ¦ method for delivering power to the end device. Power is carried on the same conductors as data. CAT-5 cabling for standard 10BaseT and 100Base-TX Ethernet uses two data/signal pairs connected to pins 1 and 2 and pins 3 and 6 on RJ-45 connectors. Power sourcing equipment superimposes power onto these pins via the center tap of internal signal coupling transformers. In the powered device (PD) the power is derived from these lines using the reverse technique.

Figure 1: Power over Ethernet Alternative A

Alternative B carries power over spare wire pairs in the cable. The power sourcing equipment applies positive voltage to pins 4 and 5. Negative voltage is applied to pins 7 and 8.

Figure 2: Power over Ethernet Alternative B

 

More on Powered Devices

PDs are designed to accept PoE in either format (Alternative A or B) so that they can receive power regardless of which Alternative is implemented in the PSE. When operating in Alternative A mode PDs automatically adjust for polarity of the power supply voltage. This ensures that the device will operate even if a crossover cable is being used. (This sometimes happens if a Media Dependant Interface (MDI), such as a PC network card, is connected to the PD, which also is wired as a MDI.)

Figure 3: A Powered Device

PDs operate from 48 VDC (approximately) supplied by the PSE, which means they are classified as a Safety Extra Low Voltage (SELV) device. The maximum current supplied by the PSE is about 350 mA. With some loss in the CAT-5 cable, the PD is limited to about 12 watts of power. Maximum allowable cable resistance is 20 ohms.

 

More on Power Sourcing Equipment

The two types of PSE (endspan and midspan) are used in different situations. Endspan PSEs typically are used in new installations where a new switch or router is required. Midspan devices more often are added to an existing network to add PoE capabilities. Midspan devices are connected in-line, between the existing switch and the PD. They can be obtained as a single or multiple line units.

On initial power up Power Sourcing Equipment is designed to detect, and only supply power to the network device if it is identified as a PoE-enabled device. When first connected the PSE initiates a detection process that involves low probing voltages to sense the type of PD connected while avoiding damage to non-PoE network devices.

Some Power Sourcing Equipment may also detect the power class of the PD. PDs are assigned to one of the following five classes, depending on the amount of power they require.

Class
PD Power (W)
Class
0
0.44 to 12.95
Default
1
0.44 to 3.84
Optional
2
0.44 to 6.49
Optional
3
0.44 to 12.95
Optional
4
Future Use
Future Use

If a PD does not support classification it is assigned a default classification of 0, indicating the PSE should supply the maximum needed power. Classification of connected PDs allows the PSE to optimize its power distribution.

PSEs are able to detect several undesirable load conditions, including shorted communications cables, disconnected PDs and the connection of non-PoE devices. When a fault is detected the PSE will disconnect power from the communications line before damage can occur.


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