CCNA Power over Ethernet



Chapter 1 – Introduction

Got a quick and to-the-point video for you today, and it’s all about power.


Imagine that you have a few desk phones, security cameras and wireless access points to connect to your network. These devices need power, but you don’t want to add an extra wall socket next to each of them – It takes more space, and adds more messy cabling.


The alternative is to use Power over Ethernet to power devices through the network. That’s where we have power and data using the same network cable.

Any device that we can power up over the network is called a Powered Device or PD. They generally only need a few watts of power, so we’re only talking about smaller devices, like this phone, not servers or workstations.


When we connect this phone to the network, we can see that it automatically starts booting up. No need for any extra power adapters.

Chapter 2 – The PoE Standards

Cisco originally introduced powered interfaces over 20 years ago. It was called Cisco Inline Power, and it could deliver up to 7 watts of power per port. The maximum port speed for this standard was 100-Meg.


This was a Cisco only technology, so if you wanted it, you needed a Cisco switch. It was a pretty good idea, so since then the IEEE have released several PoE standards that any vendor can use.


The original PoE standard was 802.3af, which is now known as Type-1 PoE. It uses two powered wires, and provides 15.4 watts of power per port. Type-1 PoE is still used quite commonly, as it’s very good for powering small appliances like desk phones, and small wireless access points.


You probably know that we measure power in watts. In any power solution, there is a little power loss as power runs over the cable. It’s no different with network cables. So type-1 PoE supplies 15.4 watts at the port, but only guarantees 12.95 watts to the powered device.


Just to explain the terminology a bit… Any device that supplies power is called a PSE, or Power Sourcing Equipment. Any device that consumes power is called a PD, or Powered Device.

There are some other devices that simply need more power to run. These include 4-in-1 security cameras, tablets, LCD screens, and others. So PoE+, was developed. This supplies 30 watts of power per port.


But why stop there? We now have two additional PoE++ standards, called type-3 and type-4,  that can deliver 60 and 100 watts per port. Cisco have their own versions of these called UPoE and UPoE+. Most notably, these standards power up two pairs of wires in the network cable.


These higher powered standards are good for teleconferencing solutions, kiosk terminals, small switches, laptops, and small TV screens.


Each of these standards is a Power Class. Powered Devices can change their power class over time if needed. For example, a device may only need type-1 to power up. Later on, it might need to turn on a screen, so it can signal the switch and change to type-3 or type-4. 

So, the next question is, does it matter what type of cabling we use? A bit.


It obviously has to be copper cabling. You can’t supply power over fibre. Also, it should be no longer than 100 metres long, and ideally it should be CAT5e or better.

Here’s the big question. Is PoE safe?


Well, it’s a much smaller amount of power than you would have in your average wall socket. Also, it is DC power, not AC, which makes it much safer at small power levels. 


Additionally, the IEEE standard says that a PSE will only supply power to a device that requests it. So if you plug in a device that doesn’t need PoE, it won’t get fried.


So while it’s relatively safe, it’s still power, so use appropriate caution. Don’t go licking your network cables or anything like that. Although to be fair, if you’re the kind of person that licks power cables, you’re probably in the wrong industry.


The quiz questions are pretty easy today. I’m sure you’ll have no troubles working this out.

Chapter 3 – Supplying Power

We’ve been talking about using switches as Power Sourcing Equipment. This is the nicest solution, but there are other options.


For example, this PoE injector. This is a separate device that combines the network signal from the switch, with power from some other power source. So if you have a switch that doesn’t support PoE, this may be a suitable alternative.


One common use of this is to power outdoor Wireless Access Points, which are too far away from the switch for regular PoE.

Similar to an injector, you can get a PoE splitter. This is where we have something providing PoE power to a device that doesn’t natively support PoE power.


For example, a Raspberry Pi. It has a power source that’s separate to the network interface. So, the PoE splitter will separate the power from the data, allowing you to plug in both separately. The advantage is that you still don’t need a wall socket to power your devices.

Please remember that there isn’t a bottomless pit of power available to you. Each switch has a power budget.


The power budget is the maximum amount of power that it can supply with PoE. For example, one switch I have used in the past is a small 12 port switch with a power budget of 100 watts. That means I could enable PoE on only 6 ports, or PoE+ on up to 3 ports, or some combination that’s less than 100 watts.


So, keep this in mind when buying a new switch. Generally, you can get a data sheet, like the one shown here, from the switch vendor which gives you this information.

The interesting take away is how the budget is calculated. Let’s say you have a desk phone connected to an interface. You enable PoE on the interface, which can supply up to 15.4W. However, a simple desk phone is unlikely to draw all 15 watts. It may draw only 5 watts.


When calculating your power budget, work with the amount of power that the interface can supply. What I mean is, assume that this phone will consume all 15.4 watts of power.


So if your switch had a budget of 100 watts, you wouldn’t assume that you could run 20 phones at 5 watts each. You need to assume that you can only have 6 phones at 15.4 watts each. This is because the connected device may start drawing more power than you expect, up to the limit on that port.


Also, when looking at switch data sheets, they may provide two values; The total power the switch consumes, and the PoE budget. For example, a switch may consume up to 830 watts of power. Remember though, that it needs to consume power itself, just to run. So while it may consume up to 830 watts of power, 720 watts may be for PoE, while 110 watts is for the switch itself.


Some switches allow you to add a second power supply. Depending on the model, adding a second power supply will increase your power budget. So, our 830 watt switch may then consume up to 1550 watts of power, still 110 watts for the switch itself, but now 1440 watts of PoE budget.


And here are the last two quiz questions, I hope the quizzes have helped you, and you’ve learned something from them!

End Scene

We’ve reached the end of our section on layer-2 technology and switching. In the next section we’re going to look at layer-3 and routing, starting with a basic review on IP addresses and the routing table. Please join me there!