Introduction to Power over Ethernet (PoE)

It is an IEEE802.3af Standard defined in 2003 for supplying low voltage power to networked devices through the communications line. It was defined as Data Terminal Equipment (DTE) Power via Media Dependant Interface (MDI). But it is now a day most commonly referred as Power over Ethernet (POE).

The basic idea of Power over Ethernet (PoE) is, for providing uninterrupted network the end devices must be powered up. For this we have to place all the end devices in the places where AC power output is available. Or AC power line needs to be laid up to end devices. So it is a limitation for placing network end devices as per requirement. PoE has come over to solve this limitation and give the network end devices power without additional power line but through the communication line that carries data. 

PoE increased the mobility and reliability of network end devices, also reduce cost and improve safety aspect. The installation process also got simplified with PoE.

It uses two techniques to carry power to network end devices.

a.       Alternative A

b.      Alternative B

With Alternative A method power is carried through same cable with data and with Alternative B method power is carried through unused cables.

These techniques are implemented with two types of devices. They are PSE and PD. PD stands for Powered Device that receives power through Ethernet and PSE stands for Power Sourcing Equipment. The PD operating voltage is 48VDC which is supplied by PSE. So they can be categorized as SELV circuit. SELV stands for safety extra low voltage as its voltage not exceed from 60VDC. The maximum current supplied by PSE is 350mA. And it provides up to 12W at 48VDC to each PD.

Endspan and midspan these two types of PSE system normally used. Generally endspan PSE is used in new installation and midspan PSE used to add PoE capability in network.