What 'Power over Ethernet' is and why you might want to use it

By Rich Edmonds
published

Power over Ethernet (PoE) is a feature supported by devices that enable them to be powered by ethernet cabling. This technology cuts down on connections by combining data and power into a single cable.

Ethernet
Ethernet (Image credit: Windows Central)

Rasberry Pi 2

PoE is a great way to cut down on installation costs and improve reliability by removing a point of failure, the second cable for power. With devices that support this technology, it's possible to provide both data and power connectivity by plugging in a single network cable, making everything easier to maintain.

This means a unit like a security camera or access point could be installed where access to a power outlet is limited.One could plug an ethernet cable in and call it a day if the camera or AP supported PoE. It provides more flexibility and enables just about anyone to install electrical equipment.

What's required

All that's required to provide PoE is a compatible switch (opens in new tab), which will be able to detect what devices connected to the unit can support PoE. A wide range of switches are available with this functionality, from unmanaged user-friendly options to server rack units. What devices are commonly found to support Power over Ethernet?

  • Access points
  • IP cameras
  • VoIP phones

Should access to a PoE switch be non-existent, it's still possible to provide power over the data cabling by hooking up a PoE injector (opens in new tab) in-between the two devices to automatically provide controlled power.

And to connect everything up, all that's required are a few ethernet cables. Nothing special. Just your bog-standard RJ45 cat 6 used for connecting a device to the home router or access point. Upgrading to PoE isn't an expensive endeavor, nor does it require advanced technical knowledge.

It's worth noting that checking a device is PoE compatible before plugging in an ethernet cable to supply both data and power is a great way to help avoid damage to equipment. Should a device be not compatible, it's possible to connect it to the PoE switch by installing a picker (also known as a tap) somewhere along the cable to take away the live current. Splitters can be used to separate data and power into two separate connections that can be used with devices like the Rasberry Pi, for example.

Should you not really be into adding devices to your home network that can take advantage of PoE then you really don't have any need to research into it. However, those of you who back the Internet of Things and enjoy simplifying networked configurations, including cameras, APs and phones, it may be a cost-effective solution to consider.

Rich Edmonds
Rich Edmonds
Senior Editor, PC Build

Rich Edmonds was formerly a Senior Editor of PC hardware at Windows Central, covering everything related to PC components and NAS. He's been involved in technology for more than a decade and knows a thing or two about the magic inside a PC chassis. You can follow him on Twitter at @RichEdmonds.

20 Comments
  • Why showing a Raspberry Pi as the header background? Unfortunately the Pi do not support PoE... probably their biggest missed oportunity for small networked devices.
  • PoE made it a breeze in hooking up cameras and access points in my house.  Just make sure you plan it all out regarding the total power draw and PoE switch used.
  • When selecting the devices we should keep in mind that there is a newer version called PoE+.
  • Difference is more power.  15 watts (PoE IEEE802.3AF)  25 watts (PoE+ IEEE802.3AT)
  • How much power can you get through this? Is it possible in theory to power something like TV / Shield box / Xbone or is it limited to low power devices?
  • Here's a link to the Wiki - https://en.wikipedia.org/wiki/Power_over_Ethernet It's not high powered. 
  • Devices can negotiate their power requirements, PoE has a maximum of 15.4W per port (plus a global max budget for all ports), and PoE+ supports up to 25.5W. Available power at the device is a bit lower because of dissipation inside the cable, which depends on the length and quality of the cable.
    ​PoE is typically enough for cameras and phones, while PoE+ can be necessary for PoE-powered downlink switches and Wi-Fi access points with more than two radios (typically MiMo ones). Neither is enough to power a large TV, and the dissipation in the cable would be too important to make higher power practical. ​This can however already reduce the number of power adapters and provide more efficient power (switches power supplies are typically better than discrete individual power supplies). For example I have 2 phones, 2 cameras, 2 cable-TV tuners (HDHomeRun), and a Philips Hue bridge all powered directly from my appartment switch. I just had to use external PoE splitters for the TV tuners and the Hue bridge.
  • PoE can be a bit more tricky...
    802.3af (the most common 15.4W PoE) only allow the use of two pairs for power. It can either use the same pair as the data pair in 10/100 connexions, named alternative 'A', or the other pair, typically unused in 10/100 connexions, named alternative 'B'. Spreading the power over all 4 pairs to reduce dissipation in the cable is explicitely not allowed.
    ​
    High end switches such as Catalyst supports both alternatives and can power any equipment just fine.
    Power injectors, on the other hand, typically support only one of these alternatives, and if faced with a device that supports only the other alternative, they will simply not work at all. To make things worse, they typically only specify 802.3af compatibility, but not which alternative. This can also be an issue if you have 10/100 wirering that only has two pairs connected, such as thinner 100Mbps ethernet cables, cheaper installations of pre-gigabit wall outlets, or when using passive splitters on both ends to connect two devices, such as PC and phone, to a single outlet. In such cases, only alternative 'A' can work as the pair required for alternative 'B' isn't connected. 25.5W PoE+ (802.3at) uses all 4 pairs for power, so there is no alternatives incompatibilities like with the 15.4W PoE, but you will still face issues if only two pairs are connected, even if the data link is 10 or 100Mbps. All four pairs must be connected regardless of the speed for PoE+ to work. ​Finally, each device negotiate their power budget with the switch. Badly behaving devices, and most power splitters, will simply always request the highest power possible and exhaust the power budget of the switch unnecessarily. Other devices might then be denied PoE. ​Finally, PoE should never be an issue for devices not designed for PoE, you don't have to disable PoE on some ports or have dedicated non-PoE ports for non-PoE devices. The device has to expose a specific resistivity when connected for the switch to start providing power. The ethernet twisted pair standard also requires electromagnetic coupling, so the transceiver is effectively electrically isolated from the cable. If there is no PoE pickup before the coupling in the device, the power won't reach the device. (an exception would be some Raspberry Pi that were not properly coupled).
  • POE gives you sooooooo much flexibility in access point placement, instead of being isolated to areas next to plug sockets. You can stick them on a ceiling and run just one cable. I'm aware they are used in enterprise settings but rarely in a domestic household.
  • Someone should've told me it was for enterprise scenarios, as I have two PoE switches with two Cisco WAP371's all networking my house. It all works brilliantly
  • Hows the coverage on the WAP371's. I was looking at the draytek vigor 902, it only needs 802.3af so the switch is much cheaper.
  • I think it's fantastic. I have the WAP's approximately 15 metres apart and they cover my entire house (parts of my side & backyard too). I used to have them positioned 11m apart - which was essentially direct line-of-sight from my back bedroom/study, down a corridor, and into my main loungeroom (slightly off to the side). That worked well generally, except there was a dead spot in my second loungeroom. So I ended up moving it into that room and it completely covers the house now - even though it's further away from the other WAP and it's no longer line-of-sight (down the same corridor, but then turns right, goes through the kitchen, into the 2nd lounge). I have them paired with a TP-Link TL-SG2210P switch each, which also connects nearby equipment (PC's in my study, older xbox/playstation in my second lounge).
  • @schlubadub. Great info thanks! Have you transitioned completely to AC or still have N devices?
  • On the networking side I have, as I upgraded all of my aging G routers & access points... the WAP371's are the only wireless I have in my house. I actually never owned an N access point, so the upgrade was long overdue! The connecting devices (phones, tablets, laptops, consoles etc) vary in age, so they typically use G or N. Any consoles in range of the switches will connect via cable, whether they have wireless or not. I was contemplating sticking an old wireless G router in my shed, for the times I'm out there working on something and want to listen to music without 4G. Either that or a powerline adapter to network it that way... although it might be cheaper to just buy a radio haha
  • G to AC, indeed long over due lol. I have various lan set ups in mind so only specific APs work. Last question I promise! Lol.
    What would you say is the total coverage space is with both points?
  • It covers pretty much my whole house + patio area which is around 190 sqm. There is some reception outside that range, but it depends where I am e.g. it's better in the backyard as it's closer to one of the AP's through the rear wall. Bear in mind that my house is single storey, with double-brick walls, and steel door & window frames. It could possibly work much better in a house with thinner walls or less steel throughout... or in an open-plan office without much physical interference. I don't know how much reception I would get if I had a second storey (I'd have to sit on my roof to test, and I'm not that keen). 
  • Wouldn't recommend sitting on the roof testing, the pigeons would be the least of your worries lol. Thanks for the awesome info, helps alot :) 👍👍.
  • oh my... didn't know this till today!!! orz
  • This is a good tilted article, but it needs more pictures to demonstrate
  • I had heard about PoE and PoE plus but didnt think it was anything more than a lab experiment. Had no idea that it has become such a big and exciting new technology. Will definitely read more now !