Hi everybody, welcome back. My name is Tyler McMinn with Aruba and this is our Aruba mobility essentials course part one. We are on video number seven with RF power the topic. Today, in the previous video, we covered signal to noise ratio.. As well as what are some of the causes of a bad signal like free path loss free space path loss or absorption. In this video, we're going to hit the rules of three and 10. I know you guys have been waiting for it. So let's get started. [MUSIC] Alright, so RF power with aruba mobility essentials, what is it? Well, we want to be able to measure the strength of a signal rather than just using those little bars on your phone or on your laptop. Instead, we want to actually look at the actual measured values. And so the way that we measure radio frequency power is either in milliwatts. Or in dBms spoiler alert, we are going to want to use dBms. And you'll see why here in just a second. So what is milliwatts? What are dBms? Well, a milliwatt is just simply the amount of power if you take a 60 watt light bulb that's 60 watts or 60,000 Milliwatts of energy. We use way less than that way, way less than that when we are talking about what's being broadcasted off of these APS. The dBm is a measure of change relative to that starting 1 milliwatt standard that we're going to use. So think of the 1 milliwatt as an initial building brick, and why they're going to stack them up or we're going to chop that brick up into slices. But we always start with one milliwatt to equal the equivalent of 0 dBm or 0 milliwatt decibels of change. So if I start with 1 milliwatt and I end with 1 milliwatt then I basically said, I am not changed at all. You knew me when I was 1 milliwatt, you know me when I am 1 milliwatt, no change. Sounds great. So, when we do have changes, for example, if we boost the power from 1 milliwatt to 100 milliwatts. What's the equivalent dBm expression of that? Well, it has to do with the rules of three and the rules of 10. The rules of three state that if you increase your decibels by three, you've effectively doubled your power. If you lose three decibels, you cut the power in half and if you take one milliwatt 0 equals 1, then what is minus 3 dBms? It's half a milliwatt. If I have one milliwatt and I add 3 decibels by adding an antenna or something like that I've increased the power. Then I've effectively gone from 1 to 2 milliwatts. What's the reverse then if I go from 1 milliwatt to 100 milliwatts, what does that equal in dBMs. Well, there's another rule that keep in mind here to make this easier. And that's known as the rules of 10 rules, the three rules of 10. So the rules of 10. If you increase by 10, you're going to actually get 10 times the power if you subtract 10 dBm. So you're going to end up with one 10th of power. And that's going to be shown here where if I start with one milliwatt. And I go to 100 milliwatts of power. What's the equivalent decibels of change there? Well, 1 milliwatt equals 0 dBms, what would be the equivalent dBms. If you have a hundred milliwatts to go from 1 to 100 how many times did I multiply this by 10. Or did I double it if you went from 1 to 10, then you would have had to multiply by 10 once if you go from 10 to 100 you'd have to multiply by 10 again. So we multiply by 10 twice in other words we added. 10 dB's and we did that again over here if you add up those dB's 10 plus 10 what you get is 20 dB's. The answer here is you can describe 100 milliwatts of power as 20 dB's if you have a signal level of say minus 60. On your receive signal strength indicator, that minus 60 directly correlates to your milliwatt decibel it is your milliwatt decibels signal. So we call this a received signal strength indicator here, but it's actually a measurement of milliwatt decibels. If we were to describe that in milliwatts, we would have to say it's .000001 and that's true it is. But instead we use a milliwatt decibel measurement versus the same amount of energy described in milliwatts. Only because it's easier to say minus 60 and Is this a 0.010? Right, I said that right. So due to free space path loss signal will attenuate quickly and what they're showing here is the source starting off at 20 dBm. With 100 milliwatts signal, one meter away, that's going to be minus 20. At 0.01, effectively, they wrote a 0.0098, whatever but 0.01 you go 10 meters away, it drops to minus 40. You go 20 meters away, it goes to -46. What's the difference between 40 and 46? Doesn't seem like that big of a deal. But you actually lost 3 dBs and another 3 dBs to go from 40 to -46. So from -40 to -46, that's a difference of -3, -3, which is the equivalent of ½ your power. Again, two times which is 1/4th. The amount of signal you get here is 4 times as strong as the amount of signal you get at -46. Is another way to think of it. So yeah, that's a huge amount of loss. And I didn't notice this when I first started into the world of wireless. I didn't realize that a difference between -60 and -70 was really that big of a deal. But this is actually one tenth the power When you're at -70, then you were at -60 or -50. To go from -70 and gain to -50. You've increased your throughput 100 times it's 100 times as strong. To go from -70 to -50. So yeah, these are huge differences in the strength of your signal. As we describe them out, milliwatts represents the state of logarithmically. Whereas dBms, we can represent in a much more linear fashion through this use of the rules of three rules in 10. Get a general feel for what the milliwatt equivalent would be. Most cases though, most your devices are going to represent all this in milliwatt decibels. So you don't really even need to worry about that milliwatts side. It's just there if you need to do that conversion. So because milliwatt decibels are so much easier to work with, as generally what we do. Anyway, I hope you guys like this description, it gets a little math-y in this. We're going to pull back from the math, get in to channels in the next video. But in this video, we were able to describe RF power, how that calculation takes place and understand that the milliwatt decibel. Is kind of the measurement method or the representation that we like the most, just because it's easier to wrap our heads around. So hopefully have a little more appreciation for those little signals on your device there. And what a difference can make being closer to your AP versus further away. Anyway, that's it for this video. I'll see you guys in the next one. Thank you
