How Does Power over Ethernet Work?

There are two basic components in an IEEE 802.3af compliant PoE network:

• Power Sourcing Equipment (PSE)
  A device that supplies power

•  Powered Device (PD)
  A device that receives and utilizes the power

When the PSE connects to a network device, the PSE determines or “discovers” if the device is a PD or not. This prevents non-PoE enabled Ethernet equipment from receiving power, which could cause damage. The PSE applies two small current-limited voltage signals across the cable as it checks for the presence of a characteristic resistance. If resistance is detected, power is provided. A PD may also classify how much power it will require from the PSE. This feature supports the PSE by helping it supply power in an efficient way.

After the PSE has discovered a PD, it supplies 48 V and a maximum current of 350 mA. Voltage may be lost along the cable, depending on distance. However, a minimum of about 13 W is available to each PD, which is adequate power for numerous applications including VoIP telephones, WAPs, security cameras and building access systems.

Once the PSE begins to provide power, it continuously monitors the PD current draw. Once the PD current consumption drops below a minimum value, for example when the device is unplugged, the PSE discontinues supplying power and the discovery process begins again.

IEEE 802.3at
In September of 2005, the IEEE agreed to begin reviewing new PoE specifications in order to enhance current PoE guidelines into a next-generation standard (IEEE 802.3at), commonly referred to as PoE Plus.

The goal for the new standard (expected to be ratified in 2008) is to increase maximum power rating to the 30 to 60 watt range to provide greater power for evolving application applications such as motorized network cameras, IP telephony video phones, RFID readers and access-control systems, point-of-sale and information kiosks, and laptops.

Midspans Vs. Endspans

The IEEE 802.3af standard defines two different types of PSEs: endspan and midspan.

An endspan PSE - Integrates the power sourcing functionality with a network switch. Endspans available today look and function exactly the same as any other Ethernet switch, except they can provide PoE in addition to routing data. Since Ethernet data pairs use transformers coupled at each end of the link, DC power can easily be added to the center tap of the transformer without disrupting the data. In this mode of operation, an endspan injects both power and data on pin-pairs 3 and 6 and pin-pairs 1 and 2.

A midspan PSE - Fits in between the switch and the PD. It supplies power over the unused cable pin-pairs 4 and 5 and pin-pairs 7 and 8. Data is routed through the midspan device without modification, as shown in Figure 2. These devices are usually mounted adjacent to the Ethernet switch in an equipment rack. It is important to note that although the PSE can only use pin-pairs assigned from an endspan or a midspan, the PD must have the capability to accept power from both.