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Power over Ethernet Demystified

  • 10 August 2016
  • Author: Dan Santee
  • Number of views: 2138
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Power over Ethernet Demystified

When I purchased my first Ubiquiti UniFi wireless access point, it came with a Power over Ethernet (PoE) injector to supply power. Now that I have four of them, I'm looking at converting my primary Ethernet switch over to PoE so I can remove the injectors and have the switch power them directly. This gives the advantage of allowing me to use one uninterruptable power supply to power them, plus reduces the complexity of cabling being used. Also, I'm looking at some IP cameras and home automation equipment which can be powered via PoE, so I want to set myself up for success later on.

When I began looking at PoE switches, I figured it'd be pretty simple: find a PoE switch with the number of ports I want and buy it. Alas, it's not so - there are different PoE standards, and not all PoE switches are created equally.

There are three primary types of PoE in use today: Passive (12-24v) PoE, 802.2af PoE, and 802.3at PoE+. The first (and most important, in my opinion) is that even though it's labeled as PoE, Passive PoE is not compatible with any kind of PoE switch. If you use any Passive PoE devices (such as the Ubiquiti UAP-AC-LITE) you must use the included injector - a PoE switch is of higher voltage and will almost certainly damage the device. You have been warned.

The other two, PoE and PoE+, are very similar - PoE+ delivers more power per port (better voltage, plus 25.5w instead of 15.4 for PoE) and is more efficient. Anything that works with PoE (802.11af) will work with PoE+ (802.11at), but not the other way around. For the most part, consumer grade equipment is going to work fine with PoE, but be sure to look at what you're thinking of putting in to make sure you get the standard you need.

The second wrinkle is that most large PoE switches are not 100% PoE. For example, the NETGEAR JGS524PE ProSAFE 24-port gigabit PoE switch actually has only 12 PoE ports. They make that pretty clear in the description, but many switch manufacturers don't. Many switches marketed as 48-port PoE but only have 12 or 24 PoE ports, and the rest are just plain Ethernet ports.

The last thing to keep in mind is the amount of power your devices will require. Since PoE has a maximum draw of 15.5 watts, it'd be easy to think that 8 ports of PoE would provide 124 watts of power - but that's not the case. All PoE switches have a maximum current draw (the "power budget") which can't be exceeded by the PoE devices plugged into it. The Netgear 24-port switch mentioned above, for example, has 12 PoE ports but only a 100 watt power budget. That means that you can only use 100 total watts, regardless of how many PoE ports are in use. The UAP-AC-PRO that I use have a maximum draw of 9 watts, so I could only plug 11 of them into the Netgear, even though it has 12 ports. Higher power budgets increase the price of the switch faster than the number of ports, so it's an important factor to consider.

Hopefully, this has helped clear PoE up a little bit - I know I had a hard time figuring out some of these things on my own. If you're using PoE or PoE+ (but not Passive PoE) devices, a PoE switch is a great way to reduce clutter, simplify cable management and eliminate power injectors. Just be sure that you're getting the correct standard, number of ports and power budget you'll need for your installation and you'll be up and running in no time!

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