Power over Ethernet (POE), Powering with Data on the Same Cable

2024-09-13 | By Will Siffer

In this post, I am excited to break down Power over Ethernet (PoE) technology, so hopefully, you don't have to go down as large of a rabbit hole as I did to get some technical answers. Recently, I purchased a security camera system and found that it supported PoE, and I was excited I only had one cable to run. The thing is, a question started nagging at me as soon as they were set up - how do they get enough power over a cable that is no different than the cables we already use for wired ethernet connections?!

What is Power over Ethernet anyway?

PoE was originally a standard developed in the 1990s but has since quickly developed into a technology that is widely used. Inside each ethernet cable are sets of twisted pairs of wires.

Typically, one of them is for transmit, one is for receive, and the other two can be used for either. Original PoE systems used those last two pairs to transmit power, but the power was more limited than it is today. Because of PoE technology, we can run only one cable, which can power a device and connect that device to the internet! In my case, that meant that I could power and run data for my cameras over 1 ethernet cable, saving me time and complexity in running wiring through my walls. There are countless other use cases for PoE standards, and you have likely seen them in your daily life!

Security Cameras
WiFi Access points in your work or school
Wired Phones
Employee time clock systems
Point of Sale (POS) kiosks at malls or inside fast-food restaurants
Intelligent lighting systems
Wall clocks
And much more!

Okay, but how does it work?

One thing that really confused me was how devices could use the same ethernet protocol with the same cable and suddenly be able to get power enough to run their internal circuitry. After some digging, I discovered that the answer is within how the ethernet signal works itself: the differential pair.

Power over Ethernet (POE), Powering with Data on the Same Cable

Consider that sample ethernet signal above. Depending on which signal is high and which is low, the deciphered value is either a 0 or a 1. This is a simplified version of how the ethernet protocol works in your house or workplace, only it happens on all of the twisted pairs of wires. But how can we add power to this same signal pair without disrupting the signal that is transmitted?

Power over Ethernet (POE), Powering with Data on the Same Cable

Consider the same signal but notice how I stretched the signal so the peaks are taller and the troughs are even lower. If the device on the other end of this signal accepts the increased voltage, it can extract that extra power and use it for running the onboard electronics while still being able to understand communication from the source device! This is a very oversimplified explanation but, hopefully, this can give you a basic idea of what is going on here.

What this also means though is that only special devices that are pre-designated with PoE capability can be used for a PoE system. If a PoE camera is expecting that offset and only gets the basic transmission signal, it won't have enough power to run. Likewise, if a device is not PoE capable and it is plugged into a PoE supply device, it will not have the onboard circuitry to draw that excess power, and power will still need to be run to the device separately.

How powerful is PoE anyway?

There are actually four different levels of PoE that can be used to power different devices. Each has its own IEEE standard and max power that it can provide.

Standard PoE uses IEEE 802.3af and can provide up to 12.95W of max power to a device with 2 energized pairs. This is enough power to supply for static IP cameras (like mine), VoIP phones, and wireless access points.
PoE+ is based on IEEE 802.3at and can provide up to 25.5W of max power to a device with 2 energized pairs. This extra power can now support alarm systems, PTZ cameras (these can move), and video phone systems.
POE++ (Type 3) is based on IEEE 802.3bt Type 3 and can provide up to 51W of max power to a device with 4 pairs of energized wires. These systems can support multi-radio access points and video conferencing equipment.
POE++ (Type 4) is based on IEEE 802.3bt Type 4 and can provide up to 71.3W of max power to a device with 4 pairs of energized wires. These systems can support the most power-hungry devices like laptops and flat-screen TV's!

What equipment do I need?

If you have a PoE switch that can supply PoE signals and you have a PoE device, you are all set! Obviously, if you do not have a PoE device you likely do not have the need for a PoE system but there may be a situation in which a network switch does not supply a PoE-type connection when you need it for a device. Fortunately, DigiKey stocks PoE injectors that can add power to an ethernet signal and make that ethernet run a PoE connection! These just need a connection to both the device and a power connection.

Want to experiment?

If you are a maker, you may want to experiment with PoE. I added an evaluation board to the end of this blog that allows for experimenting with PoE, and it supports many of the IEEE standards I mentioned above. This would be a fun way to practice and make your own PoE device!

Closing Remarks

I hope that this guide served you well in explaining the different levels of PoE and how it works. This was a technology I was hoping to understand better, and in making this, I definitely have learned a lot, so I hope you had the chance to learn something new as well. Until next time, my name is Will, and I hope you learned a new skill today.