Over the years I’ve come to admire 802.3af standard power-over-Ethernet (aka P.O.E.), even for small- or home-office applications. What follows is an introduction to the topic, and some novel ideas about its use in possibly unexpected applications.
IEEE 802.3af Power-over-Ethernet is the industry standard approach to delivering DC power to network attached devices. Given a P.O.E.-capable switch, or a P.O.E. inserter, DC power is delivered over the same Ethernet connection that provides connectivity. Thus one wire is all that’s required to a distant device on the network.
Why Is This Great?
Delivering power over the network means that it’s hugely more convenient to locate devices in hard to reach places. Consider surveillance cameras, which are almost always located in hard to reach places. WiFi access points also benefit, since they can be located at the optimal place for RF performance. No worry about ensuring that there’s AC power at that location.
Further, devices powered by the network can be isolated from utility power issues. A single, suitably capable, UPS in the wiring closet can sustain all the attached devices in the event of interruption to utility power. In my office that means riding over the switching bumps that occur when the utility works to balance its load during the hottest of the Texas summer.
In larger installations it allows the network to be sustained during the period when utility power is lost, but before the standby generator has yet fired up.
Types of Power Over Ethernet
There are actually several types of P.O.E. In the bad old days, before the standards processes were completed, there was an ad hoc form of P.O.E. This approach was found in early IP phones, like Polycom’s SoundPoint IP500, circa 2001.
This approach hijacks otherwise unused wires in the Cat 5 cable to deliver power. This is sometimes referred to as “passive” POE since it doesn’t require any logical interaction between the connected devices to negotiate the connection.
Since it wasn’t standardized, various manufacturers implementations differed. They might vary in how much voltage or current provided, or simple polarity of presented power.
Curiously, even though there are now standards for “active” P.O.E., some manufacturers (yeah, I’m talking about you Ubiquiti) continue to offer products based upon passive POE. It allows them to save a small amount manufacturing each device, which can add up when many devices are deployed. In an installation where all of the devices are from the same company, like a wireless ISP deploying WiFi APs, the cost savings can be considerable.
The 2003 Standard: IEEE 802.3af
In 2003 it became easier to leverage P.O.E. on a network with a mix of devices. This standard, now widely deployed, provides up to 14.5 watts to each device. That’s a minimum of 44 vdc and 350 mA.
Devices declare themselves to the upstream power source, making it possible to intelligently manage the total power available from the switch. If the switch can’t handle another device it simply won’t provide power to that port.
The 2009 Standard: IEEE 802.3at
There a newer 802.3at standard (aka POE+ or PoE Plus) that provides up to 25.5 watts per device. This not only accommodates more power hungry devices, it allows a network device, itself powered by the network, to also pass power onward to another item.
For example, the HP NJ2000G Intellijack (below, left – descendant of the visionary 3com Netjack) is a five-port managed switch that receives power from the upstream network, while also providing 802.3af power to two of its Ethernet jacks.
The NETGEAR ProSAFE GS105PE (above right) is a similar, but more affordable device, perhaps more better suited to SOHO installation.
Power From Where Exactly?
If you’ve no experience with P.O.E. you might ask,”Where does the power come from?” It most typically comes from the network switch. The power supply in an enterprise class P.O.E.-capable network switch is much beefier to be able to power all the various network attached devices.
There are smaller, SOHO switches where some portion of the ports are POE-capable. In the past I’ve used Netgear’s GS308PE, which is a 8-port unmanaged switch. In that case, only four of the eight ports provide power.
You’ll notice that P.O.E. is much more common in larger, managed switches. Also, P.O.E.-capable gigabit switches remain quite costly compared to 10/100 “Fast Ethernet” models.
There are times when it may not make sense to invest in a costly, P.O.E.-capable switch. Yet, you may still want to take advantage of P.O.E. for a handful of devices. In these cases you can add a P.O.E. inserter (aka injector) inline with the network powered device(s).
Later I used similar small injectors with various other devices. For example, the Polycom SoundStation IP7000 conference phone must be powered over the network. Their power supply kit is really a small P.O.E.-injector.
The similarly excellent Revo Labs FLX UC 1000 also requires power via the network. Our newly installed Ubiquiti UniFi AC Pro WiFi access point requires P.O.E. It came with a single port injector in the box.
There’s nothing wrong with my core network switch. An older 24-port gigabit model from Netgear, it’s not fancy, but it gets the job done. It’s difficult to justify the expense of new, P.O.E.-capable gigabit switch. I’m loathed to replace it with a more affordable, if POE-capable, 10/100 Mbit model.
The solution is to add a multiport mid-span P.O.E. injector. I have my eye on a 12 port model (pictured below) from WiFi-Texas.
These devices allow you to inexpensively add P.O.E. capability to an existing installation without replacing the switch. They can be had in configurations from 4 to 24 ports. Some support older “passive” P.O.E. and the 802.3af standard.
They come with different power supplies, accommodating differing power requirements. I’d likely get the 120 watt version which would readily handle all of the devices powered via my network.
Power For What?
As I mentioned, P.O.E. is commonly used for IP phones, IP surveillance cameras and WiFi access points. It can certainly be used for other things.
Algo Solutions make network powered pubic address speakers, which are really just an extension of the telephony application. Fourteen watts can be plenty loud when used to drive an efficient class D power amplifier.
With a little ingenuity you can use P.O.E. for anything that requires a modest amount of DC power and would benefit from the reliability of Ethernet connectivity.
A P.O.E. splitter makes it possible to extract power from the network, to power a device that isn’t natively P.O.E.-capable.
Here are a few ways that I’ve put it to use around my home office:
I use it to power a Logitech Squeezebox streaming music player. I use a simple POE splitter to extract power from the Ethernet and provide it to the coaxial DC jack on the Squeezebox.
This is made possible by a $21 Amcrest P.O.E. splitter. This splitter has adjustable DC voltage and a selection interchangeable of coaxial DC connections.
2. Raspberry Pi Pseudo-Squeezebox
In another location, I use P.O.E. to sustain a Raspberry Pi that emulates a Squeezebox.
Instead, I use a very cheap and simple P.O.E. splitter for the Raspberry Pi that delivers 5 vdc via a micro usb connector. While I’d prefer a more mechanically stable power connection, it works fine as long as the entire assembly is securely mounted. Best of all, it allows me to use the standard HiFiBerry case, which is quite affordable.
3. An Analog Telephone Adapter (ATA)
There was recently a thread over at the DSL Reports VoIP Tech Forum about choosing a new ATA. It’s not clear to me if any ATAs support P.O.E. Given their target market, and price targets, it seems unlikely. There’s no reason that an ATA couldn’t be powered using a P.O.E. splitter.
That would allow it to stay operational, presumably on UPS power, during a brief loss of utility power. It would allow a line-powered phone, like a Princess or Trimline, to keep working more like it was connected to a POTS line.
Further, it would eliminate a wall-wart type power supply, which is surely a noble effort.
4. An Android Tablet
I’m considering a little project that would have an Android tablet located in a weather tight case as part of a remotely located instrumentation package. It would be running a real-time sound level (SPL db A) monitoring app.
If the tablet was a Nexus 7 in its dock, I can provide power via USB and get the video output via micro-HDMI. Thus the tablet could be powered using a P.O.E. splitter not unlike the one I use with the Raspberry Pi.
The output of the tablet, once encoded into an H264 stream, would be recorded to a network video recorder like just another surveillance camera.
As time ticks past I keep finding more, and more novel, ways to leverage power-over-Ethernet. Each time I do some aspect of our operation becomes a little more reliable. It’s a strategy that you might consider for yourself.