tec Tip Videos

Power over Ethernet

Nathan explains some basic factors involved in Power over Ethernet (PoE), including classes and the 802.3af standard.


Hello and thank you for joining us. My name is Nathan and today, we're going to be talking about Power over Ethernet or PoE for short.

I get a lot of questions about PoE and while it seems like a very basic topic, it's something that can easily get confusing especially when we're dealing with so many different options and varieties of choices with the different classes and things like that.

I want to break everything down for everybody and we'll start very simply with the 802.3af standard and this is something you've probably heard when you look at switches or you've considered switches in the past. That is comprised of three classes, one, two and three.

These here, these numbers, are the wattages that you can expect with each class. Class 1, about 4 watts. I do have a note down here in red that I guess I'll draw your attention to right away and I put, "Give or take 1 or 2 watts." In some cases, it may be more.

These are rough numbers, so again, math is my Kryptonite, so 7 watts on Class 2 and then 15 watts or you'll often see 15.4 watts on Class 3. Why does all of this matter? I would say the majority of devices today hover in the Class 2 to 3 range. That would be the most common.

Where you'll see things in like the 7 watt range are things like phones or IP cameras, things like that. Of course, there are examples that can go higher, but in Class 3, you'll see a lot of access points and things or even color phones that may require something in the 15 watt.

Now, what you'll notice here was even though we're talking Class 3, like I said, we're off a couple of watts, but 12.95 is probably the most common number I see referring to Class 3, but that's still within that range.

Now, more and more common is things like PoE plus which I'm sure a lot of folks, especially if you're looking at your newer switches, you're coming into contact more and more with PoE plus. What is that? That's anything up to about 30 watts in power. Big jump, we're talking double the number and this will be referred to as the 802.3at standard.

This is becoming or starting to become more common like I said. You're probably running into this more. Devices that use it, I don't run into a lot of devices at this point and do, but I know there are some out there. We do have some customers that do have some equipment that require this much power.

Then that actually folds into something that maybe you might not be very familiar with and that's Universal PoE or UPOE. It's a Cisco proprietary PoE standard that's boasting up to 60 watts. The reality is more like 51, but 61 watts we'll go with for the marketing number. It's really meant to be a doubling of PoE plus and it claims to still be compatible with the 802.3at standard.

Hopefully, this brings everything together and maybe we can look at an example. All right, so here's a real world example we can take a look at. This is a 3750G PS was my model switch for taking a look at these power numbers. The 3750G PS and this is actually a 48 port version. I didn't note that up here, I apologize.

370 watts is available for PoE from that power supply. How does that break down? If you put as much PoE as you can on this switch, that means that for 24 ports, you can do that at 15 watts, but for only having this much power. If you wanted PoE on all 48 ports simultaneously, the max would only be 7 watts.

Now, you'll often times hear people say, "Well, is that half PoE switch? Is it a full PoE switch?" These switches are full PoE, it's just how much is the available power. You could think of this 370 is like a pool of water that you're drawing from at any given moment. If we draw for all 48 ports, the most that we can move to spread around equally is 7 watts, but if we only want to draw up to 24, we can afford 15.

Can you mix and match the available wattage? Yes. I believe with Cisco devices, on Cisco's switches, you absolutely can, but you want to be mindful of the fact that you only have so much power to work with and that's really what your limitation is.

On the newer switches like 3750-X's and E's and any of those newer models, they have larger power supplies. You'll see things like an 1100 watt power supply that has a lot more available PoE power and then you can get that 15 watts on all 48 ports and then even jump in to the PoE plus standard for less than full ports.

I wanted to give you that example. I have it on pretty good authority that not heeding the loss of PoE was responsible for the disruption of Krypton. Paying attention to these rules earns us the green smiley face and I'd like to thank you for joining us and hopefully you learned something today.