Thursday, 28 November 2013

Power Over Ethernet - be careful!

I have recently learned a whole lot about CAT5 cables, ethernet and wi-fi since I've started working for a new company.  The reason the title says "be careful" is because you could fry your equipment if you inject it with to much power when it actually requires a lower amount of power


There are different kinds of POE and this post will explain the differences between the passive PoE (Power Over Ethernet), the 802.3at IEEE standard and the 802.3af standard.

Some background info
The picture on the left is an extract from +IEEE 's website and it shows some of the other 802 standards.  Here you can see that 802.3 deals with ethernet.

Power over Ethernet allows devices to be powered up by the same cable that performs the communication between it, and the rest of the network it is connected to.  This can be achieved because communication only uses four of the eight strands in a standard LAN/CAT5e cable (see picture below).

Strands 1 and2, 3 and 6 are used for data communication, but, depending on the design of the (the device that is powered up via PoE) and the power source equipment (the device providing the power), power can be provided through these same strands or through the unused strands (4, 5, 7, 8).  Power can even be provided over all eight strands in some high-power applications.
powered device

Different types of PoE

Basically, passive POE is just called that because power is being injected into the PD (powered device) over an Ethernet (CAT5) cable, but it is in fact, not standard POE.  It can be 12V, 19V or 24V DC (direct current) at somewhere between 500mA and 2A.

Equipment designed to adhere to the IEEE 802.3af-2003 standard provides 15.4W at 44Vdc - 57Vdc at the PSE (power source equipment) and 12.9W at 37V - 57Vdc at the PD.

Equipment designed to adhere to the IEEE 802.3at-2009 standard (aka POE plus) provides 34.2W at the PSE, but some vendors have claimed to be able to provide up to 51W of 802.3at compliant power over a single CAT5 cable.  At the PSE the voltage will range between 50V and 57V while it will range between 42V and 57V at the PD.

Here is a table comparing the 802.3at with 802.3af with each other compliments of Wikipedia:
802.3at vs 802.3af
An easy way to remember which one is which, is to remember that 802.3af comes before 802.3at in the alphabet (because of the F and the T) and so is the one that was developed earlier and so provides less power.

How to identify equipment 
The manufacturer of the equipment you are using should clearly state what power input is needed for the device you want to power up.  If you come across equipment that is already installed and the requirements information is not available, you can get PoE testers at fairly cheap prices, which will be able to show whether power is on or not and what the voltage is.

That is all for now, and as always, I will keep on expanding my knowledge, and I hope you do to.
Thank you for reading my blog.
I hope you learn something new every day, even after you think you know everything.