Perle Systems Technical Note

A Background on Power Over Ethernet ( PoE )

IOLAN STS P and SDS P Terminal Servers
Similar in principle to the way that the wire pair that carries your POTS (Plain Old Telephone System) telephone signals into your house carries sufficient power for the headset, dial, and ringer systems for your telephone, PoE delivers DC power (up to about 13 watts ) over standard Ethernet cabling (CAT5 and better ) enough to power a growing number of commonly-used devices as well as IT equipment.
PoE has been around for a number of years, with proprietary (and often noninteroperable) implementations from various vendors to power wireless access points, VoIP phones and network cameras. In June 2003, however, the IEEE ratified its 802.3af Power over Ethernet standard, which has, in turn, spurred standard-compliant 802.3af products.
This enables organizations now to deploy equipment in locations that are difficult or too costly to have separate AC power installed such as ceilings, walls and kiosks.

Power is "injected" into the cabling at the wire-closet side. This can be done either through "end-span" LAN/WAN switches, which incorporate the power supply circuitry, or (if you don't want to replace existing LAN switches or only want to power some segments) "mid-span" power patch panels or injectors, which are connected in-between the switch and the segment. The 802.3af standard defines details such as which wire pairs are used and what its power needs are. The end (powered) devices have to support PoE. Many of the new PoE capable switches also have capability of power management where PoE devices connected to it can actually be power cycled. This is ideal for remote management applications where IT equipment such as servers, and routers may need to be power cycled in order to recover.

When an 802.3af  (PoE) powered device (PD) is turned on, the PD informs the Ethernet end-span or mid-span switch through an information exchange that it can receive power through the Ethernet cable. A fully complaint 802.3af PD can accept power over the data lines or unused pins on the Ethernet interface. Some PD’s claiming compliancy with 803.2af are in fact restricted to using mid-span power sources utilizing only the unused pairs.
To protect the device from input current rushes that may occur during power up, it is important that the device have inrush current protection. The PD with this protection begins with a low current draw stage to protect the power sourcing device ( PSD ), and then switches to a high current stage allowing the PD to draw its required power up a maximum 12.95 watts.

In summary, the benefits of PoE technology are;

• Only one set of wires to bring to your Ethernet appliance - simplifies installation and saves space.
• There is no need to pay for an expensive electrician, or delay your installation to meet the electrician's schedule - saves time and money.
• The appliance can be easily moved, to wherever you can lay a LAN cable - minimal disruption to the workplace.
• Safer - no mains voltages anywhere.
• A UPS can guarantee power to appliances even during a mains power failure.
• Appliances can be shut down or reset remotely - no need for a reset button or power switch.