CWNP CWNA – PoE

1. PoE

So in this module, we’re going to have a discussion about power over ethernet and why that’s important to us in our wireless setup. First, I’ll give you a little bit of a history about poe that’s again the Power over ethernet. We’ll talk about the devices as an overview and what we can do to plan and deploy having power over ethernet.

2. Non –Standard

So as with most of our technologies, the initial poe products were proprietary solutions created by the different companies that recognized that there had to be a need for the technology. Now exactly what am I getting at here? So let’s take a voice over IP telephone. You might have noticed at your desk that there is not a cord that is in a power socket with a plug or a cable going into the power supply to turn on the phone. It is possible they do have that capability, but it’s not very easy when it comes to deploying phones to have that many extra power outlets. Well now when we start talking about putting access points up on the ceiling, that means you’re going to already have to string a cable for the wired connection, but you’d also have to string a cable for the actual power supply to plug in and up in the ceilings.

I don’t remember seeing too many places to plug electronics into. And so what we do is we supply power over that ethernet cable so that the device gets the electricity it needs to operate and is also at the same time connected to the wired network. Now, when we started having things being proprietary, that pretty much meant that we had to go with a solution all of one vendor, rather than being able to pick and choose those vendors who had products that we think looked better. So in 1999, the ieee started a process to create a Poe standard. But it took them about four years before that standard actually became a reality. And in that in between time, vendors continued to make their own proprietary poe types of devices.

3. 802.3af

Now, one of the first standards we saw was the 802 three af. So this is underneath ethernet power over Ethernet. Committee created that amendment as a standard, and it was officially referred to as just the eight two three amendment for data terminal equipment power via the media dependent interface.

That amendment standard was approved in 2003, and it defined how to provide power over Ethernet on your ten base. The ten megabit, the 100 megabit, and the gigabit ethernet, obviously, all of them twisted pair. We’re not talking about fiber cables here. We’re talking about copper wires.

4. Powered Device

So the thing we’re plugging in with power is going to be called the powered device and it’s either going to request or draw power from the psc or the power sourcing equipment. Now your pds should have the capability of accepting up to 57 volts from either data lines or the unused pairs of your Ethernet cables. And a lot of it also depends on what the app actual power needs are and how much power the pse can actually send. But that was like kind of the maximum that they had to be capable of dealing with.

Now I’m not going to take you into too much of the eight wires and try to give you an idea of which ones are positive or negative, but that’s what it’s showing you. Remember cat cables, ethernet cables are four twisted pairs. So there’s eight wires. And obviously in some deployments, some of the wires are sent to transmit and receive data where the other wires that are really unused are going to be the ones that help send the voltage. And to have voltage, of course, you need to have a positive side and a negative side. So that’s what you’re seeing here with this little list.

5. Discovery Part1

So in the past, in order for a device, a pse, to recognize that there was something out there that wanted power. And most often, it’s our switches that are providing the power through the actual rj 45 jack that we call interfaces across that cable to whatever that device was, if it was a phone, or if the device was going to eventually be an access point. Now, one of the things the switch has to know is whether or not you have a device that wants to have power and how much power does it want to have. The reason this is kind of a big deal is that if you plug the device in and it just suddenly started getting power, you might burn that device out, because it might have been designed to not have electrical current sent into it at all.

So they have to do some negotiations. We’ll talk a little bit more about it, but Cisco, through what they called the cisco discovery protocol, made it easy so that any one cisco device would announce not only the type of device it is, but what its power needs are. But that was good for cisco. It didn’t help with everybody else. There is an open standard for that same type of discovery called the lldp, the link layer discovery protocol. And like cdp, it’s the device basically saying, hey, look, I’m an access point, or I’m a phone, or whatever that device is. And then we’ll tell the psc, which is the switch again, how much power that it needs.

6. Discovery Part2

So in the old days, when I say the old days again, talking about the nineties, and it’s really sad to think of that as the old days, but in the old days, you’d have a business traveler who’d be going to a hotel. And, you know, in those mid 90s, we were so excited to be able to connect a phone line into the phone jack of the wall and to be able to do really fast high speed dial up. I mean, that’s what we did. We had one of the biggest companies there in that day was America online. So you’d sign up for an account and you tell them what city you’re in, they tell you what phone number to dial. Compared to today, that was horrible. But when you went to a hotel, most often in the hotels, they had digital phones, right? I mean, they had the really cool handset, and they had all these feature buttons that you could use to call the lobby or room service or, you know, get messages on your phone. And these digital phones needed to have electricity power.

And so even though it was an RJ eleven jack that you saw in the wall, it was designed for that actual phone to supply it with electricity. And so what business people would do is they would unplug the phone, plug their laptop in, and that electrical current would go in there and immediately fry, literally. In some cases, you could smell the smoke from the power, the electricity being sent in there and destroying your modem card. And a laptop in those tastes were so expensive that it was just a ruin for your day. All right? One solution eventually came is that the people that made the digital phones, realizing this problem, put an extra phone line jack in so you could hook up your modem to the phone, and then from the phone go through their digital network to be able to make your phone call. Didn’t help you at the time.

So what do we do to make sure this doesn’t happen now? Because now I’m talking about a switch. And every time you plug an ethernet cable into the switch, is it just going to immediately burn out your network card because it’s supplying electricity when it wasn’t expecting it? And the answer is no. What it’s going to do is going to search for a power device by using a very small DC current detection signal. And I’m not an electrician, but I’ll just tell you that the idea was that if I have this like an IP phone that does want power, if resistance, and I believe I’m saying this right, if resistance to that current was found, then the idea was it was something that was wanting to have power. It was designed for electrical current.

So once the device was identified, then the switch would begin to provide power, but it would not provide all the power that it could at one time. What it would do is it would slowly power this up to get enough electricity for any device to be able to say, hey, I want 15. 4 watts. And once that communication was done, then the PSE was able to supply it exactly the amount of power that it needed. If the device didn’t respond to the vc current, then the psc would not use or send any electrical current through that wire. And that was the big deal. As I said was trying to prevent damage to different devices.

7. Endpoint

Now the endpoint PSE generally today is an ethernet switch that is doing electricity and data at the same time. They don’t have to be switches, remember? We have a little problem. These switches are often somewhere, maybe in what you might call a closet or a server room. And they may be a long distance away from the device that needs power. And even though it sounds cool, here’s an electrical current going to my phone.

By the time the cable actually gets to my phone, I might have exceeded the 100 meters limitation for attenuation, which means the phone’s not going to have electricity even if it wants it. And so that became a problem. The other thing is the equipment you have needed to have adequate power supplies to be able to provide electrical current over the ports, rather than having a power supply that was able to run the switch all by itself, but nothing extra.

8. Midspan

So that’s where the midspan comes in. So if I start with my switch, I’m going to make my switches a little bit smaller. And I know that at some point this switch is going to be connecting to a phone or an access point. But there are a long ways away. Then the switch here does not have to be a psc. It could just be a regular switch. And that case cable after I hit that 100 meters. What do we need to do? We need to find a way to boost that signal strength again. And we might then buy a separate device. It doesn’t even have to be a switch. It could just be something that’s in line that provides electrical current over ethernet so that we can then get the power and have that power closer to the device. So we didn’t run out of that or have that problem with the length. So there might be mid span equipment that is there to give you the poe to your networks. And this is also nice because maybe this is an older switch, but it’s still doing a great job. And you don’t want to go out and buy new equipment for something that’s working adequately. So you could buy something much more inexpensive, something that’s new like the midspan equipment. And then all you have to do is make sure it’s in between the ethernet switch and the power device.

9. MDI

So whether it’s this midspan device or it’s a switch, you do have to make sure that it has what they call a medium dependent interface to be able to carry the current. In other words, the switch ports were designed to be able to transmit electrical current to the powered device.

And again, remember, 100 meters, 328ft. That’s still the limitation for anything we do with copper cable. And so that’s why you might then have a cable connected to the midspan and from the midspan out to your powered device, like your access points.

10. PoE Part1

So we have a couple of options of how we can deliver power over ethernet. The first option is the easiest one and that is to have a switch that does inline power. And then of course you can, as you see here, plug right into that device. You have a switch without inline power that is going through a patch panel or another switch that is closer to the power device to be able to provide power. So, you know, so I need data connectivity all the way across, but I only need electricity for the last connection to the power device. So it doesn’t have to be an electrical current going through switch after switch after switch. Or like I said, you could have a switch with no power. Option three and go through a power injector something that does have to be plugged into the wall. Obviously, I mean, the electricity comes from somewhere and then that can be plugged into the access point or the phone or whatever else is out there.

11. PoE Part2

So when we’re thinking about power, we don’t have at least still today, that many devices that need to actually have power over ethernet. Yes, the proliferation of voice over IP is getting big. So I could see needing a lot of options to supply power to those phones access points. Of course, that’s our big focus throughout this course, need to have electrical power. And as I said, they don’t have to be connected to a switch. But the big thing we have to do is determine where those devices are and how many pscs, because I told you, you don’t have to go all the way to the server room floor and have a switch that does power over ethernet.

Just whatever that device, that pd, the power device, whatever it’s connected to as far as the copper cable, when you follow it from that phone or that access point, the object it connects to should be the thing that is providing the power service. Now, most everything takes 15. 4 watts, unless it’s something they call like a poe Plus, which are now devices that are going to ask for up to 30 watts of power. That could even be a video camera of some sort or so many more devices now that we’re plugging in, and I hope that I made an argument why this is important.

The reason I made the argument is not every office has an electrical outlet for every single device. So it would be impractical to plug them in your phones and access points, or just impractical because of where you place them. So power over ethernet is really kind of cool, but I think I made mention that when you look at the switch, it too is plugged into the wall somewhere and is getting electrical current. And those are called the power supplies.

So the power supply is responsible to provide enough electricity for the switch to do its job of switching. But if you’re going to do power over ethernet, then your power supply has to have extra power to be able to provide electrical current. Now, when these switches first started coming out, if we had a 24 port switch, it wasn’t uncommon for only eight of those ports to be able to send power. Meaning the power supply had enough for eight ports, but not for all 24 ports. As time has moved on, it’s more common that every port can do power whether you want to or not. But if you did the math, at 15. 4 watts times a 24 port switch, you’re at almost 370 extra watts of power that has to come out of that power supply.

12. PoE Part3

Now, one of the things we have either through the command line or a web GUI like you’re seeing here, is that the device can show you how it’s distributing power. For example, here, it tells me on this ethernet port that it’s supplying 5. 5 watts of power. It tells me that it’s a standardsbased powered device. The class we’re not going to get into as far as the different classes of devices. And you have the ability, if you really wanted to, you could say yes to those ports that you want to supply power. Remember, I said not all switches could supply power to every port. And then you could say no for those ports that you know, do not need to supply electricity. And then you could leave the other ones in a searching mode where they can make the yes no decision based on their discovery of a powered device. You.

13. PoE Part4

I don’t know if this statement is completely true. I think it really kind of depends on which generation you grew up in. But for me, at least as a child, I knew that even if the power went out somehow the phone still worked. Why? Because the telephone company was doing power over the cable, the four wire cable that was connected to the phone. And so it still had the power to be able to connect and make phone calls. Often those telephone lines were running separate from well, always running separate from the actual electric lines because the electric lines would have just blown everything up. But sometimes they’re even strong underground as opposed to on telephone poles, even though we call them telephone poles.

So we had the ability to have that kind of as a backup service. And the same thing is true here another reason why that we don’t want you to have these separate AC adapters to plug in all these devices. It’s not just because there’s not enough outlets or not just because it would be inconvenient to even find a way to do it. It’s that because your equipment, assuming is connected to a Ups and maybe a generator, that means your networking equipment, the switches, the routers and everything else are still running. And so I still have that same level of service I was expecting as a kid, that if the power went out, that I could pick up the phone. I could still be able to make phone calls.

And that’s true now, right as we have voiceover IP phones, or even our voice over wifi telephones that are replacing the traditional phone systems, we want to have that same type of continuance or continuous service. And I think it becomes a safety issue as well, because often the reason power has gone out might have also resulted in somebody needing to make some sort of call to emergency services. So again, make sure that you have that uninterruptable power source, the Ups, and even though those don’t run very long, that’s where if you’re a large corporation or large company, you’ve also invested in having the backup generators.

14. Module 17 Review

So in this module, we talked about the history of power over ethernet. Took kind of an overview of the different devices and some of the options you have as far as how you plan and deploy power over ethernet.