Simple, but genius: Power over Ethernet

December 22nd, 2017 | #networking

When I started my first office job, I was wondering how my VoIP phone could work with only one network cable. I know my VoIP phone needs two things: network and power. How can they be transferred through one cable?! Well, my colleague explained to me that it worked, because of Power over Ethernet (PoE). Since then, I only knew that it works. Just a few minutes ago, I learned how PoE actually works. It is some of those things, you only look up when you cannot sleep—at least in my case. 🙃

As you may know, a standard CAT5 Ethernet cable has four twisted pairs. However, only two of these pairs are used for 10BASE-T and 100BASE-T—twisted-pair cables used for the physical layer of an Ethernet computer network. It are these specification that allow two options for using these cables for power. In this blogposts I will explain shortly explain these two methods.

Using the data pairs

It is possible to apply DC power to the center tap of the isolation transformer without upsetting the data tranfer, because ethernet pairs are transformer coupled at each end. The pairs can be of either polarity.

Using the spare pairs

The following image shows the pair on pins 4 and 5, which form the positive supply. The pair on pins 7 and 8 are forming the negative supply.

Note: TR and RX in the above images are abbreviations for, respecitvely, Transmit and Receive. (Transmit FROM x, and Receive TO x.)

Hat tip to those who came up with the—simple, but genious—idea of using four previously unused cables—and later—the data wires to supply DC power to devices. 🙌

© 2018 Tonny Garić | With from Nijmegen, The Netherlands