Power over Ethernet

In the Entertainment Technology world, powered Ethernet is probably the most useful development to take place over the last few years.

Powered Ethernet allows electronics to be powered via the Cat5e cable. This is a huge plus for Ethernet installations as there is no need to install a separate power cable.

There are two evolutionary standards for Powered Ethernet:

  • IEEE 802.3af-2003
  • IEEE 802.3at-2009

Powered Ethernet is usually abbreviated to PoE, meaning ‘Power over Ethernet’. The original PoE standard (IEEE802.3af-2003) provided a total maximum power of 15.4W per device (actually only guaranteed at 12.95W once cable volt drop is allowed for). The more recent IEEE802.3at-2009 version provides 25.5W and is generally called ‘PoE+’ or ‘PoE Plus’.

A PoE+ Ethernet switch is guaranteed to power a PoE device. However as a PoE Ethernet switch has a lower power rating it may not be able to power all PoE+ devices.

The development was originally driven by IP phones – for obvious reasons, IP phone manufacturers do not want to supply a separate power supply for every phone. Happily, the Entertainment Technology industry is riding the back of this technology wave.
 

How does it work?

The simple answer is that 48V DC power is supplied via the Cat5e cable. Sadly, it is not quite as simple as one may have hoped…

The 10BaseT and 100BaseTX protocols provide a really simple option for powered Ethernet. They both only use four of the eight wires. That leaves four wires for power.

Unfortunately 100BaseT4 and Gigabit (1000BaseT) confuse the issue by using all eight wires for data.

The Powered Ethernet task group have solved the problem by providing two interoperable power schemes.

The first scheme, called Scheme B, uses the spare pairs to supply power. The second scheme, Scheme A, ‘injects’ power into the data wires.

Most PoE Ethernet Switches support both schemes and automatically select the correct one for a given powered device.