Power Over Ethernet (PoE) Tutorial

Overview

For any device you need power and communications. Normally you would power a device off a power point which meant for remote devices you would to run both power and data cables. This has safety and cost issues that can significantly push the costs beyond the cost of the equipment itself.

With the advent of VOIP, security cameras and other systems users started implementing ways of providing power to them using the data cable. This meant that just one cable needed to be installed. In those days there were no standards for implementing power over ethernet.

The early network devices only supported 10BASE-T(10mbs) or 100BASE-T(100mbs). They also used CAT5 data cable which consists of 4 pairs of wires. For 10BASE-T and 100BASE-T only 2 of the 4 pairs is required. This left 2 pairs which could be used for injecting power over the same cable. There is a limit to the amount of current that can be carried but this is sufficient for low power devices. For the latest 1000BASE-T (Gigabit) interfaces all 4 pairs are in use, so you need to use 802.3aX compatible equipment to implement Power over Ethernet.

Some devices only use 12V but it was common to find 24V being sent down the cable. Depending on the length of the cable there is a voltage drop. For the full length of 100 metres a drop of 1 to 3V is not unusual depending on the quality of the cable. The latest versions of PoE that are now covered by the IEEE 802.3aX standard support up to 50V. Most telecommunications services standardise on 48V in there exchanges.

For the rest of this tutorial we look at methods of implementing Power over Ethernet using both standard based and non standard techniques.

PoE Standards

Earlier on manufactures implemented there own ways of sending power over ethernet. They used different voltages and signalling techniques that were not compatible with other manufacturers equipment. To overcome this the following standard was implemented.

The first standard IEEE 802.3af-2003 (POE) was released around 2003 and this provided power of approx 15.4 W of DC power (minimum 44 V DC and 350 mA) to a device. Later on a revised version of this standard IEEE 802.3at-2009 (POE+ or POE Plus) was released. This standard provided up to 25 W of DC power (50 to 57V DC at 600mA) per device.

The standard also implented other things such as power management that is controlled through the ethernet interface. We will not cover any of this in the tutorial as it does really apply for the devices we are using. The other issues with these standards is the 40 to 50V dc power that is used. This creates issues for devices such as Arduino's, Raspberry or other devices that really on need 9 to 12V. Converting from 48V to 9 to 12V can be tricky and pretty expensive for the humble Arduino or alternative IoT device.

Ways to Implement Power Over Ethernet

In this tutorial we are going to look at the following methods with application to the Arduino or similar IoT devices

  • Direct Inline Injection into Ethernet Cable (homemade version)
  • Direct Inline Injection using third party devices
  • IEEE 802.3aX Standard POE

Before we look at each method we will have a look at the cable pinouts of the data cable and what pairs are used / unused.

Category 5 Cable Pinouts


Direct Inline Injection into Ethernet Cable (homemade version) (Not 802.3aX complaint)


Direct Inline Injection using third party devices (Not 802.3aX complaint)


IEEE 802.3aX Standard POE