Intro Review, Virtual carrier Sense, Physical carrier Sense, Duty Cycle and spectrum, Air Time Usage, Multi-PSK, An interesting Site Survey, SLE’s
– All right, in here, I’m going to do very, very, very quick review– a very quick review on what we talked about last time. I talked about what I consider to be the most important thing for you to understand in Wi-Fi and how Wi-Fi works, and that is Wi-Fi is half duplex.
Now, right now, I’ve got my MacBook plugged into a switch downstairs with an ethernet cable. And ethernet gives us full duplex connectivity, which means we can send traffic in two directions at the same time. My MacBook can talk to the switch. The switch can talk to my MacBook at the exact same time. We don’t have to take turns talking.
It’s kind of like a two-lane highway, where traffic can travel north and south at the same time, or east and west, depending on the highway. Wi-Fi, on the other hand, is completely different. It is half duplex, which means only one thing can talk on a channel at a time.
For example, if we have a MacBook, if it wants to talk, the very first thing that it has to do is it has to perform a clear channel assessment and see if anything is talking. Once it checks the channel to see if anything is talking, if the channel is clear, there’s nothing going on, then it will go ahead and transmit its data to the device on the other end. Then, the device on the other end, whether that’s an access point– we could flip these two roles around. It doesn’t matter which icon is on which side of the screen here.
But once that receiving device– we can call it a station. Any device like this could be called a station. Once that station receives that frame, it’ll check it to make sure it understood the frame, and it’ll go, yep, check, I understood it. And it will reply with an acknowledgment. It’s this constant process of data, acknowledgment, data, acknowledgment, data, acknowledgment.
Now, if we back out, if we zoom out a little bit and look a little bit wider, broaden our scope a little bit here, then we have to think about what happens on the entire channel. So if we’ve got an access point and a handful of client devices, since Wi-Fi is half duplex, only one of these devices can talk on a channel at a time. So if this access point is transmitting, then everything else on the channel has to shut up and be quiet and wait for its turn to talk. One thing gets to talk on a channel at a time. That includes other access points, too, on the same channel.
If you have an AP and a bunch of client devices over here and they’re also on the same channel, they have to– if they’re within range, they’re going to have to take turns talking as well. On any given channel, at any given time, only one device can talk. One thing gets to talk on a channel of time. That’s the most important thing that I think that you need to know about Wi-Fi.
Basics 2.2
– Virtual carrier sense. We talked about this just a little bit– a little bit last time around, but I want to expand on it a little bit now. Virtual carrier sense happens whenever a device transmits. Whenever a device begins to transmit, it includes– you probably remember this from last time– but it includes what’s called a NAV timer.
NAV stands for network allocation vector. But the big thing to remember here is that it’s basically a timer that gets broadcast to everyone else on the channel, and everyone else on the channel basically all synchronizes their watches. They hear the device going, “I’m going to talk for 52 microseconds,” and everyone goes, synchronize watches and shut up. And they all shut up for 52 microseconds.
And so they all basically set this timer. They all set this little internal timer in their own heads. They don’t confirm that they’ve set the timer. They don’t do anything like that. They just set this timer, and everyone basically waits until that timer reaches zero until they will go ahead and transmit again.
As you can see, Wi-Fi, 802.11 is– it’s very cooperative. It’s all about devices cooperating. And when you see this stuff, it’s kind of amazing to me that it works at all. But it totally does. It absolutely does work. So that is virtual carrier sense.
Now, when a device sets that NAV timer, they can set as big of a number as they want. Basically, as far as I know, there is an upper limit. I think it’s around– I don’t know. It’s like 32,000 microseconds or something like that. If I remember correctly, it’s about a 1/3 of a second. I think it’s 0.3 seconds. Double check my math there. Double check that I’m correct on that. But we can set pretty long NAV timers if we need to, but these are going to get set every single time we transmit anything. It
Doesn’t matter what it is. If we transmit something, if it’s an 802.11 frame, if it’s a Wi-Fi frame, it’s going to include this NAV timer. It could be a request-to-send, a clear-to-send. It can be a beacon frame saying, hey, I’m Joel’s network. I’m here. It can be any of those frames. They’re going to set an NAV timer whenever they talk.
So with the combination of physical carrier sense and virtual carrier sense, that’s how we make sure that wireless devices don’t end up talking over each other. Cool. So somehow we went backwards. I don’t know how that happened.
Basics 2.3
– So what I want to talk about now just a little bit is I want to talk about the exact mechanisms for how this works. I did a bunch of hand-wavy, like, it listens on the channel to see if anything is talking. There’s a little bit more to it than that. The mechanism that we use– there’s actually two mechanisms that we use to see whether a channel is busy or not.
The first one is called physical carrier sense. And then the other one, which we’re going to cover in a minute, is called virtual carrier sense. And so there’s two mechanisms– two broad mechanisms that we use.
Now under that physical carrier sense thing, under that only, there are two subways that will check the channel. There’s two ways that we’ll look at the channel to see exactly what is going on. The first one is called Energy Detect. Energy Detect is where we just listen on the channel to see if anything is talking at all.
We just look to see, is there anything talking on our channel right now? It doesn’t even need to be Wi-Fi. It could be Wi-Fi. It could be a cordless phone. It could be a microwave oven. It could be a baby monitor.
It could be a Zigbee thing. It could be anything that uses the same spectrum. Anything that talks in that can potentially trip an Energy Detect. Now, you remember from our last discussion how Wi-Fi uses 2.4 and 5 gigahertz, and those are unlicensed spectrum, which means you don’t need to have a license to operate consumer electronics in those spaces.
You have to follow some basic rules, but they’re both the Wild, Wild West of spectrum. License spectrum– you have to buy a license. You get that spectrum. No one else is allowed to use it. The FCC will hit you with a huge fine if you use someone else’s license spectrum. But unlicensed spectrum, basically, anything goes here.
And so that’s why things like cordless phones and baby monitors and microwave ovens all use unlicensed spectrum– typically, 2.4 but we’re seeing more and more 5-gigahertz stuff. We see a lot of consumer electronics there, as well– things like drone cameras, stuff like that. That’s a lot of the things that we’re seeing down there now.
So Energy Detect is, do I hear RF– that’s radio frequency– do I hear RF energy on my channel? Basically, any kind of noise at all. And that is about a negative 62-dBm threshold. If we hear it above negative 62-dBm, if we hear any kind of RF energy, we’re going to back off. We’re not going to transmit because we have detected energy on our channel.
Now negative 62– if you remember from last time when we talked about signal strength, that’s pretty dang loud. I mean, that is– that’s loud. So Energy Detect really only gets tripped if there’s something very close by that’s causing interference. And so, negative 62– you probably remember, I put that in the– yeah, that’s getting up there as far as signal strength goes category. And you can check that out in the previous video, if you don’t remember the specifics about that.
Now, the other type of physical carrier sense is Clear Channel Assessment. And this is the one– this is the term that I applied earlier and did the whole hand-wavy, yeah, this thing happens. Now we’re getting into the details of that.
With a Clear Channel Assessment, that is when we’re going to look to see if there’s any other Wi-Fi– if there’s any other Wi-Fi devices talking. So we’ll basically– Wi-Fi devices are essentially always listening. There’s a few caveats to that statement. But for the most part, they’re always listening.
And before they transmit, they will check to see if there’s any other Wi-Fi activity happening on the channel. They’ll check to see, can I demodulate any other Wi-Fi signals right now? And if we can– and that’s usually about a negative 82-dBm threshold– if we hear any other Wi-Fi devices on the channel, we’ll also back off.
So that’s physical carrier sense. We’re looking at the physical carrier– that’s the RF. We’re looking at the radio frequency activity to see exactly what is going on. So that’s physical carrier sets. Now I’m going to move on to the next slide, and we’re going to look at virtual carrier sense.
This is the second way that we make sure that we don’t have devices that are talking over each other. So let’s move. Hey, can I clear my drawings? Yes, I can. Cool. OK, so–
Basics 2.4
– The next thing that I want to talk about is I want to talk about just a little bit, how do we measure this stuff? How do we measure– how often devices are talking? How do we measure what our channels look like? Because as we talk about this stuff, you’re probably starting to get this idea that you’re probably starting to kind of understand this idea that each Wi-Fi channel has a specific amount of time available on the channel.
There is a certain amount of availability of a channel. When the channel gets full, that’s it. We’re out of time on the channel, and there’s no more time left. What that typically, when you’re looking at a channel, and you’re kind of evaluating the health of a channel from a how busy is that channel standpoint. If you see 10% something like that, if you see 20%, that’s a pretty healthy channel. Things are doing great.
If you’re doing voice over IP, once you hit about 50% busyness on that channel, once you kind of hit that point where it’s being used about 50% of the time, that’s where voice over IP applications tend to break down. That’s where things just don’t work as well. If you’re just doing general data traffic like web, email, stuff like that, typically, about 70%.
That’s kind of the breaking point for things where once you hit 70%, you’re going to have a bad time, right? Let’s see a question here. Sorry, I don’t have your full name in the chat here. But is the transmitting client that sets the duration of the NAV timer? Yes. It is the device that is currently transmitting.
And it doesn’t even have to be the client, it can actually be the AP. One thing that I typically do when I’m kind of assigning labels to devices is I will call this iPad. I’ll call it a station. I will call this access point also a station. I’ll call this iPhone. They’re all stations.
Basically, my terminology, my own personal terminology, I don’t know if everyone agrees with this. This is kind of Joel’s opinion. But I consider a station to be any 802.11 device that can transmit, and that includes a missed AP 43, and that includes an iPad, and that includes my cheapo Nokia 6.1 smartphone.
And so any of those devices can and will set a NAV timer whenever they transmit. And that also includes my old Linksys WRT54G. This will also set NAV timers whenever it talks. Don’t worry, more on this guy later. OK, so let’s get back to the excellent question by the way. And I really appreciate the questions like that.
If you have any questions at all, please feel free to keep them coming. I’m more than happy to answer those. So let’s talk about how we measure this stuff. Kind of one way that you can measure how often things are talking things are talking is with duty cycle.
To be honest, this is one that I feel like is misused pretty often, and so I’m going to try to lay out a bit of a definition here. And maybe you agree, maybe you don’t agree. That’s OK. Either way, it’s totally fine. But I just kind of want to give you my viewpoint on this.
First off, duty cycle is when we measure how often a device is transmitting versus not transmitting. How often is the device on versus not on? And you can actually apply this to more than an RF radio and an antenna. You can apply this to an LED if you want.
If you have an LED like on an Arduino, and it’s in there blinking on, off, on, off, on, off. That is a duty cycle of 50%. It’s on 50% of time. It’s off 50% of the time. That’s an LED just sitting there blinking. You remember on the old MacBooks like the polycarbonate ones, they had a white LED on the front.
And it would sit there, and it would breathe, right? It was a really cool effect where it’s kind of slowly get brighter and then it would get dimmer, right? I really like that. I thought that was really cool. I didn’t have a Mac back in those days. But I did think that was really cool.
What they’re doing there is they’re changing the duty cycle on that led. They’re doing pulse width modulation. They’re making it bright and then dim by just turning it on and off really, really quickly. That is changing the duty cycle of the LED to get that cool effect out of it.
So duty cycle is how often is a device transmitting versus not transmitting. Essentially, it’s a measurement of how much time, how much of a percentage a device spends talking, right? So over here, you’re going to see this image a couple of times, and I kind of want to explain what this is.
This image, this right here, this is kind of our typical way of representing a radio transceiver. This is basically a chipset of a Wi-Fi device. And then this triangle here, that’s the antenna. So that’s what we’re looking at here. So if we’ve got a device, and we’re looking at a certain time span here, and we see it turn on and then off and then on and then off and then on, the percentage of time that it spends on is basically the duty cycle of that device.
How often is that device talking. Now one thing that I kind of like to use duty cycle for is I like to use duty cycle to refer to a specific device, one device. How often is that device talking? I don’t like it. I don’t like using this term to represent what is the overall business of a channel, including all the devices on the channel.
To me, this one is all about a specific device, how often is that specific device talking. And one last thing I want to explain here is that this is what I call a layer 1 measurement. Now in the world of networking, I’m sure that you’re familiar with the OSI model. So I’m not going to get into the OSI model really, really, really, really in depth here.
But I’m sure that you remember that at layer 1 of the OSI model is the physical layer, right? Let’s see. Layer 1 is please do not throw sausage pizza away. That’s how I remember the OSI model. Now down here at the bottom of the OSI model. First off, we have layer 1 which is the physical layer. Then we have layer 2 or later two.
We have layer 2 which is the data link layer. Now down here at these two layers, this is basically where Wi-Fi lives. Applications like Google Chrome and all that other stuff, that’s all up here on these upper layers. I’m not going to get into those today.
But layer 1 and layer 2 is where Wi-Fi essentially lives. Now layer 1 refers to basically the RF. That’s basically the radio frequency activity that is transmitted between devices. That’s all. That’s all the radio waves. That’s all that stuff. That’s all happening at layer 1.
When we kind of move to layer 2 kind of conceptually, layer 2 is where instead of just hearing raw radio frequency activity. This is where we’re actually decoding that data and turning it into bits. And we’re actually looking to see what is happening, what kind of conversations are occurring there.
So layer 1, I see this as a layer 1 measurement. This is where we’re looking isn’t on or is it off. That’s essentially what we’re looking at here. And now we’re going to look at another layer 1 measurement here. The next one I want to look at is utilization.
Utilization is yet another layer 1 measurement because we’re still looking at raw radio frequency activity, right? Oh, man, my GIF stopped animating. There we go. This is looking at raw radio frequency activity to see what is happening. Basically, this is a measurement of how often raw radio frequency activity is heard in the spectrum.
Basically, how often do we hear something talk in the spectrum. Just realize really quick, I forgot to boot up my virtual machine which we’re going to very much need for a quick demonstration, I’m going to do here in a couple of minutes. So let me get that booting up really fast, so that can be running in the background.
Let it boot up. Are we booting up? And we are. Sweet. Excellent. OK, cool. So yeah. Back to the utilization discussion here. What we’re doing here is we’re using a special piece of hardware called a spectrum analyzer to get this data. Actually, I have one right here.
This is kind of the de facto spectrum analyzer for Wi-Fi engineering for a very, very long time. This is a little box called a Metageek Wi-Spy. It looks a lot like a Wi-Fi adapter but it’s not. This is a special piece of hardware called a spectrum analyzer that listens to raw radio frequency activity in the spectrum.
And these things are really cool to have around because you can do a couple of really, really neat things with them. First off, you can see non-Wi-Fi interference with this. Now since the spectrum analyzer here is not just Wi-Fi, it doesn’t just decode Wi-Fi like a Wi-Fi adapter does. It shows us everything that happens in the spectrum.
It doesn’t matter whether it’s Wi-Fi or not Wi-Fi or a microwave oven. It can be anything that chirps in the spectrum. We’re going to be able to see that with the spectrum analyzer. So just a quick preview of what we’re looking at right now.
Basically, the height of this graph tells us how loud something is. Notice the power graph here, right? Basically, how loud is it like for example, this spike right here is hitting about negative 70 dBm. This spike right here is peaking at about negative 55 dBm.
And so we can see exactly how loud those are. Hey, Garth, do you have a cool example of a microwave that you want to show? You want like a couple of minutes to get that ready. I’d be happy to pass this screen over to you if you want to show it.
Absolutely. Give me about two minutes.
I could talk for two hours if you need me to, so no rush.
– It’s funny Joel. Garth and I were just looking at that picture and talking about it.
– Sweet. Very cool. Yeah, I’ll pass the screen. Let me finish the kind of– I don’t know. Yeah, and then I’ll pass the screen over. Give me like three or four minutes, something like that. And I’ll check with you again before I put you on the spot.
So yeah. These all tell us the– like basically, how loud are things talking. If you see a spike that’s really, really tall like that, that’s something that’s really, really super loud. If you see a spike that’s really short kind of like that, it’s not very loud. It’s there, but it’s not talking very loudly at all.
The next thing that we can see is we can see what channel or what frequency it’s happening on. So for example, this spike down here, this is down below channel 1. And this spike up here, this is way up above channel 11. And so we can see where it’s happening on the spectrum analyzer.
The comparison that I like to make here is I like to compare this to a tool that I used to use back in the days. If anybody recognizes this one. I used to use a tool called Winamp back in the day. Anybody remember that? Put a note in the chat if you used Winamp back in the day.
But this was an MP3 playback tool that I used to use all the time. This is kind of back in the Napster days. Wonder if I can zoom it a little bit on this. Oh, yeah, there we go. So a whole bunch of people in the chat say they used Winamp.
So Winamp actually had– I’m going to turn it down here because I’m listening to the Llama whip in intro now. But Winamp actually had a spectrum analyzer built in. So this little box right here, that’s a spectrum analyzer. So the peaks tell us how loud things are at different slices of spectrum.
So if you see spikes down here, that means that low notes are being played. If you see spikes up here, that means that high notes are being played. You can see the amplitude. It’s all there. It’s a spectrum analyzer. It’s just for the audio spectrum instead of the radio spectrum.
And Garth, I’m not going to do the quote here on this because it’s going to go on the internet for anyone to hear. But thank you.
Those who are familiar with it probably remember.
– Oh, yeah. Go to webamp.org and just double click on DJ Mike Llama here. Just make sure your little kids running around and things like that. So it’s pretty great. But that is one of my favorite examples of a spectrum analyzer. That’s exactly what we’re doing with a spectrum analyzer that’s designed for Wi-Fi.
We’re just looking at what we’re hearing and showing it in the spectrum. By the way, what we’re looking at here, a couple of different things that are happening here. This curve shape in the center on channel 6, you kind of see how it’s centered on channel 6. That is 802.11b activity.
That’s a data rate of 1 or 2 or 5.5 or 11 megabits per second. They make curve shape in the spectrum. And then all of these little spikes that you see everywhere that are just happening all over the place, that is– I have for my Nintendo over here, I have a little 8BitDo controller that operates in 2.4 gigahertz. And it uses some proprietary frequency hopping thing.
And so as it goes bounce, bounce, bounce, bounce, bounce all over the place here. That’s my little 8BidDo controller. So now I would like to put Garth Humphrey on the spot who is one of our sales engineers here at MIST. Garth, I’m going to pass the screen share over to you. You should be able to share your screen.
GARTH HUMPHREY: Part of that is walking around with a handheld or a portable spectrum analyzer. In this case, we’re looking at the sidekick spectrum analysis tool. And so we’re walking around this customer site, and we’ve heard off and on occasionally during break times.
And sporadically throughout the day, 2.4 was just terrible and would actually completely disconnect all clients. And so I’m walking around, walking around the site, and I see a microwave over on the corner. And that’s typical in an office space for using microwaves. And some cases you see entire walls of microwaves.
And often, you’re pretty lucky that there are modern microwaves and have an appropriate seal around the door. In this case, that is not what we saw. And so you can see the incredibly high duty cycle, as well as transmit power from channel 4 and 1/2, 5, all the way to channel 14 at an incredibly high duty cycle.
So when this microwave was turned on, all 2.4 clients were essentially kicked off the network. And unbelievable noise and we’re not able to connect or maintain a connection during this time.
JOEL: So Garth, so do you think that’s because it was tripping all their energy detect, and those clients quickly just got dissatisfied with how busy the channel was and they said, well this is stupid. I’m going to disassociate. I’m going to find another AP to connect to on a different channel, because this channel is just completely crammed.
That’s absolutely right. That’s it. I’m out.
Nice. That’s awesome.
: So that’s exactly what we see here.
Question in the chat is can we point out how to read duty cycle? So in this case, I’m going to make a little bit of a statement here, Garth, and you could say whether you agree or disagree. Any other way is fine. But with a spectrum analyzer, you don’t get to see– I think some tools incorrectly label it as duty cycle.
I think that the two terms get interchanged a lot. I think the correct term here is probably utilization. And so the question in the chat is how do you read duty cycle? Well, you don’t get to read duty cycle, because we’re not looking at something for very specific device that’s transmitting.
But what we can see, Garth, maybe you can switch– are we looking at a screenshot right now? Oh, it’s a screenshot, isn’t it?
Yeah. OK, no worries. But basically, there is an option in how where you can flip that to have it show like statistics for each channel, and it will actually show you, it’ll actually show you how busy a channel is. How much utilization there is on a channel.
I tell you what, let me show you. Garth, any other comments or anything before I steal the screen back? Or is that what you want to say? I think that’s what you want to say. Cool. Thank you, Garth. Much appreciate it. And if you have any more interesting stories like that, I am 100% down from one of those. I think it’s great.
Totally talking about me. Take it away, man. Thanks.
: OK, cool. Sorry. Did you have anything else to say or you all good?
No, all good. Thank you.
All right. Sweet.
PI do think later on he has another story that I’d like him to chat about. It’s a cool one. It doesn’t need to do it now, but the lottery machine won, Garth.
Absolutely. Yeah, let me find that.
I thought that was so cool, you found that.
All right.
Let me find my material, and I’ll jump on as well.
– Awesome. To tell you what, Garth, I’ll give you a couple of minutes to track that down. I’ll show some stuff here in another spectrum analysis tool, and then we can talk about what you found.
Awesome. Thanks.
– As a wireless network engineer, I feel like I’m very backwards as far as most people they see interference and they go, oh, great. I have interference, right? Me, I see interference I’m like, yes, it’s so cool. You know like I get all excited about it.
And that was totally the case in this scenario too, Joel. This is not something you see very often, because of how they’ve modernized microwaves to. And this is high power 2.4. I mean, this is the kind of frequency that cooks food.
– Nice.
Right. So when you actually see this stuff in a live environment, you’re like, wow, this is– I don’t see this very often, right?
– Yeah. Classic, I love it. Yeah, whenever I find something weird in the spectrum, I always get excited about it. So cool. Well what I want to show now is this is another spectrum analysis tool. This is channelized by my good friends at Metageek. This works with the Wi-Spy that I showed you earlier.
These are actually made here where I live in Idaho. These are made with real 100% Idaho grown potatoes. I think they’re made like four or five miles from my house. They’re right here in town. But this is a recording of basically some spectrum analysis activity.
So to frame it up a little bit, not only does this do spectrum analysis activity, but it also includes Wi-Fi scanning as well. So there’s some networks there. I’m actually just going to enable two of them just to kind of show you where they are.
So there’s one network right here on channel 1 called 802.11b. And one thing I like about this tool is that actually assigns a curve shape to that to say– and this certificate thing, this is really bugging me. It actually assigns a curve shape here to say, hey, that’s kind of the shape that we can expect to see since it’s an 802.11b network.
Any signatures that come off of that will have a curve shape. I like that. I think that’s really cool. And then there’s another one here called 802.11g that’s up here on channel 11. There’s a bunch of other ones, but I’m going to leave them hidden for now just so that they don’t get in our way.
And so the boxes and the curve shape here, these are coming from a Wi-Fi scanner. This is a Wi-Fi adapter going hey, I know there’s a network on channel 11. I know its signal strength is negative 30. And so then the tool just draws a box here to say– I tell you what, let’s try clicking yes. Does that work? Will that go away? We’ll see.
It’ll draw a box here to say, hey, this is where the network is. And so then what we’re able to do is correlate the Wi-Fi scanning data and the spectrum analysis data together, which I think is really cool. I like that a lot. I like it how does that. Wi-Fi explorer is another tool that does that. Lots and lots of tools do that at this point.
So then what I’m going to do is this is a recording that was taken. What happened here is a throughput test was performed on the 802.11g network, and then there was some dead air where nothing happened. And then a throughput test was done on channel 1 and vise versa. We just went back and forth.
So what I’m going to do is I’ve got a time span here. I can actually change to look at different parts of the recording kind of like a DVR. It’s really neat. And I’m going to hit Play here. And we’re just going to play this back to look and see what happens on this channel.
So first off, this square shape right here, if you remember from our last discussion, that’s OFDM. That is orthogonal frequency-division multiplexing. Don’t worry about remembering what that means. But what I want you to remember is that if you see a flat tabletop shape, that is a faster data rate like 6, 9, 12, 18, 24, 36, 48, 54 megabits per second all the way up into the 802.11n data rates.
If we were looking at 5 gigahertz, that would include 802.11ac and 802.11ax. They would all kind of make this flat tabletop shape in the spectrum. And so then if we go to the channels table and how has an identical channels table like this, and we go look at channel 11, we can see that channel 11 is being used 75% of the time, 78% of the time.
And so 78% of the time, the spectrum analyzer heard RF on this channel versus not heard RF on this channel. And so you can use that as a pretty effective way to quickly look and see how busy is your channel, how often is your channel being used. Now that transmission you can see this view here in the middle is called a waterfall view.
It kind of shows us what’s happening over time, and we can see that has now stopped. Whatever that transmission was it’s now ended. And as it rolls off of our time span that we’re looking at right now, it’ll go down to essentially zero. So down to 7%, down to 3%, and then it basically just falls off the radar and it’s done.
Now here’s the important thing to keep in mind with a tool like this. We’re always looking at a time span of some kind. In this case, we are looking at a 30 minute time span. Or not 30 minutes rather, 30 second time span. So this, that is 30 seconds worth of data.
This is 30 seconds worth of data. We’re showing 30 seconds worth of data up here. This is 30 seconds worth of data. It’s all tied together. It’s all 30 seconds. But we can change that if we want to. We could grab these time spans, and we could grab a big old bunch of time and look like, OK, well over 6 minutes and 18 seconds. How much utilization did we see on all these channels?
OK, so then before we leave this application and go look at the next thing that Garth has for us, I’m going to play back what happened on the 802.11b network. So we’ll go look up here at channel 1. Channel 1’s utilization was hanging out at about 4%.
And now as we start to fill up our time span with this 802.11b activity, so remember, this is that slower older modulation scheme then it’s going to make a curve shape in the spectrum instead of a flat tabletop shape. This is a data rate of 1 or 2 or 5.5 or 11 megabits per second. The technical name for this type of modulation is direct sequence spread spectrum.
Don’t worry about remembering that. Maybe now you’ve heard it. So if somebody says it, you’ll remember what that is. But the big thing to keep in mind is that g and n rates make a flat tabletop shape, and b rates make a curve shape in the spectrum. But check out our utilization.
We’re hitting 87%. I think at one point, we hit 89% utilization on that channel. That channel is slammed right now. It’s completely occupied. It’s completely occupied with activity right now. So that’s something that a spectrum analyzer can show you, which I think is really, really cool.
And I think it kind of helps solidify in your mind what is going on a Wi-Fi channel at any given time. Cool. So let’s talk about the next way that we can measure. And Garth can just let me know when you’re ready to go here. I’m happy to stop at any time, but no rush.
So airtime or airtime usage is another way to measure how busy a channel is. Now remember earlier, we were looking at layer 1. We were looking at raw radio frequency activity, how often do we hear raw radio frequency activity on the channel versus how often do we not hear raw radio frequency activity on the channel.
Airtime usage goes up a layer. It goes from layer 1 where we’re just looking at the raw radio just how often is stuff talking. Where we go up to layer 2, now we’re going to actually look at the packets that are being transmitted between devices, and we’re going to look– look at those conversations, and see how much time 802.11 devices are reserving to talk.
Whenever they transmit, how much time are they asking for on the channel. And so the way that we measure this is with packet capture and with packet analysis. Now this is a screenshot from a packet analyzer called Wireshark, and I’m sure that most of you have used Wireshark before.
You can use Wireshark to capture packets for both wired and wireless. You can use it for even capturing Bluetooth stuff with the right hardware. You can capture Zigbee stuff. I’ve seen people capture cell phone packets. There’s all kinds of interesting things that you can do with Wireshark.
But as a wireless network engineer, I primarily use it to look at wireless 802.11 captures. So what we’ve got here when we look at a packet analyzer is first off, we’ve got our packet number. Wireshark looks at the packets. It just gives them a number. So that way, it’s easy to go, oh, yeah, that happened down at packet number 28 or whatever, right?
We can get there and look and see what was going on that channel. Next is the transmitter and the receiver. Who transmitted it, and who is it destined for? Who is supposed to be receiving it? And so for example, we could see at this Microsoft device transmitted to this MAC address, right?
Oh, no. I’m not sharing my screen again. And I haven’t been for a while. Thank you for pointing that out. Whoops, a whole bunch of stuff I was drawing all over the screen and nobody could see it.
Basics 2.5
– Yeah. So this is a screenshot of– this is a screenshot of Wireshark right here. And so Wireshark will show us basically any frames that– if we do a wireless packet capture, it will show us those packets, and so we can see exactly what happened during that packet capture. And I’ll show you what a packet capture looks like here in a little while.
So again, just to sum up all the things that I missed there. The number here tells us basically just which number in the packet capture we’re looking at as far as frames go. We could say, hey, frame 29 is where the problem occurred. It’s just nice to have something to tie that number to.
The transmitter says, hey, who is transmitting this? And the receiver says, who was supposed to receive this? But this is the column that I really want you to pay attention to for a second here, the Duration column. This duration column is essentially our NAV timer.
When a device transmits a frame, it always includes this duration field which basically tells everyone how long they need to shut up for, how long they need to be quiet for. And notice how different some of these are. Like for example, this little clear to send is only 28 microseconds. Whereas, this beacon is just a total chunk or this thing’s like 348 microseconds, right?
And so you’ll see different durations for different frames depending on how much time they need to reserve. And you remember our discussion last time about data rate, how slower data rates take longer to transmit? Check out this frame right here transmitted by this Apple device.
So I’m not sure where it was coming from. I don’t recognize that Mac address, but it was going to go to this Apple device. That might even be my MacBook. Let me look and see. I don’t know s that’s my MacBook or not. My MacBook is something AB. No, that’s not my MacBook. But either way, it was transmitted at 300 megabits per second. And so it only used 42 microseconds of time on the air.
Another question is, is the maximum NAV configurable on the AP? Nope. As far as I know, that is not configurable by the AP or the client device. It is a set value basically in the standard, where they can use anywhere between 0 and a 32,000 microsecond. Double check me on that, but I believe it’s about a 32,000 microsecond now that they can set, which I think translates to about 0.3 seconds. Again, double check me on that. I’m not good with math and remembering specific numbers and stuff like that.
Hey, Garth, how you doing over there? You get the stuff together yet, or are you still working on it? Maybe he’s still working on it.
I am still working on it. I’ve got a couple of screenshots, but I’ve got a recorded spectrum.
– No rush.
Replay. I can bring up, so give me one second.
– You’re good. Just throw something in the chat. Just throw some random characters in the chat or something. I’ll keep an eye on it so that you can–
erfect.
– –whenever you’re ready. So let’s talk about things that consume. Now that we’ve kind of talked about the difference between– remember duty cycle is how often is a specific transmitter on versus off. Utilization is very similar, but utilization is when we’re looking at the whole channel. How often is that whole channel being used from a layer one perspective? How often do we hear RF versus not hear RF on the channel?
And then airtime is how often do devices– how much time do devices reserve in the spectrum? And so how much time do we have left on that channel?
So let’s talk about the different things that consume airtime on a wireless network. Now the first one is more devices. You remember last time we talked about half duplex Wi-Fi, we talked about how if you put multiple APs on the same channel, they have to take turns talking, if we put more devices on the same channel, that means smaller and smaller opportunities for everyone to talk.
I like to think of a channel as like a dinner table. If you have dinner with a couple of friends, maybe you have dinner with three friends or something like that, since there’s only three people at the table, there’s plenty of time for everyone to be a part of the conversation, right? Everyone can get in and everyone can have an opportunity to talk.
But when you have a large table, maybe you have 13, 14, 15, 16 people at a dinner table, maybe everybody comes over for a holiday dinner or something like that, everyone’s opportunities to talk get smaller and smaller and smaller the more people you add to it table. Same is true for a Wi-Fi channel. The more devices you put on a channel, the smaller and smaller everyone’s opportunities to talk.
Next is poor coverage. You remember last time, we talked about how devices that have to fall back to a slower data rate, they will talk slower as well. If we’re talking at a nice fast 300 megabits per second, that means we can get on the air and off the air really, really fast. If we have to fall back 250 megabits per second, well, we’re still pretty good. We’re talking nice and quick, but certainly not as fast as 300 megabits per second.
If we have to fall all the way down to maybe the slowest 802.11n data rate, which is 6.5 megabits per second, now it’s going to take us a really long time to say the same amount of information. So poor coverage will cause devices to talk slower as well.
Next, our legacy data rates. If we allow legacy data rates to be used, then devices can fall back to those slower data rates. So you probably remember that 802.11b devices are 1– or 802.11b data rates are 1, 2, 5.5, 11 megabits per second. 802.11g data rates are 6, 9, 12, 18, 24, 36, 48, and 54 megabits per second. There might be an error or two in there, but I think that’s right.
The 802.11n data rates are– there’s tons, and the 802.11ac data rates, there are tons. And then you get the 802.11ax, and it’s just insane amounts of data rates that are available. I’m not going to memorize those. No, thanks.
But if we turn off some of these legacy data rates, we can actually force devices to talk faster and conserve more airtime. And I’m going show you how to do that in the Mist dashboard here in just a couple minutes.
The last thing we can do is we can have too many virtual SSIDs. Now, you probably remember on the last slide, you probably noticed some of these little beacon frames in here. For example, here’s a beacon right here– all the dark purple ones, these are all beacons.
Basically, a beacon is a network announcement. It says, hey, I’m Joel’s Network. I’m here. I support these data rates. I support this type of encryption. You can connect to me. That’s what that is. And beacons happen about 10 times a second. That’s how often we try to beacon.
Here’s the thing about beacons, though. You’ve probably heard of virtual SSIDs, where we can have multiple SSIDs that are all broadcast from the same access point. And virtual SSIDs are really nice because you could have, for example, the guest network and then the staff network. And then the guest network goes on to a VLAN that goes to a captive portal, and then just goes right out to the internet, right? We just have free internet access for our guests.
But then the staff VLAN goes to the internal network, where we’ve got our file servers and things like that are all in a VLAN or all in a subnet that are all in the internal network. It’s a really nice way to separate out traffic. The problem is, is that every SSID has to have its own beacon.
And so if we only have one SSID, we just go, hey, I’m Joel’s Network 10 times a second. We just do that 10 times a second, and we don’t use up a ton of airtime. If we use virtual SSIDs, if we have Joel’s Network and then Joel’s Guest Network, and then Joel’s Voice Over IP network, and then Joel’s IoT Network, each one of those SSIDs requires its own set of beacons.
And that’s a major part problem, because now we’re having to go, hey, I’m Joel’s Network, I’m here. Hey, I’m Joel’s Guest Network, I’m here, too. Hey, I’m Joel’s Voice Over IP network, I am also here. Hey, I’m Joel’s IoT. You see what’s going on there? We’re consuming a ton of airtime with virtual SSIDs.
So typically, we recommend no more than about four– and again this is just Joel’s opinions. Garth, Jean, if you guys want to put your own numbers in the chat or something like that, go for it. But I typically say try to keep it under four virtual SSIDs if you can to conserve airtime. There’s other ways that we can split up traffic between different VLANs and all that.
So a question in the chat. Is it still the same if you hide the SSID from broadcasting? Yes, it is. When you hide an SSID, all we do is we still transmit our beacon. Our beacon is still there, but inside that beacon, there’s a field that says what that SSID is. For example, my SSID here at home is Joel’s Network, right? There’s a field in there it says SSID, and it’s just Joel’s Network.
For a hidden SSID, the beacon is identical. It’s exactly the same. We say SSID, and then the name of it, it’s just nothing. That’s what the name of it is. So it still consumes airtime. It’s still there. We can totally still see it, we just don’t know what the name is. That’s all you get when you hide an SSID.
And a common misconception in the world of wireless is that hiding in SSID is a good security measure. It’s not. It’s actually slightly worse from a security perspective. And so I don’t recommend using hidden SSIDs. But there are some corner cases like maybe people are bugging you for the Wi-Fi all the time, maybe you have a clinic or something like that, and the staff is constantly getting asked, hey, can I use your Wi-Fi? Hiding the SSID that might be a corner case there, where maybe you could hide it to make things a little bit easier.
And Garth says that four is the maximum number of virtual SSIDs that he likes to use as well. So speaking of Garth, are you ready?
I have just a couple of comments on that. And by the way, I learned a new word actually on this slide when you were talking about the beacon frame there. And you said that’s a total junker. I’m going to use that. I thought that was really cool.
But anyway, regarding SSIDs, I too, I typically try to recommend three to four max, and even less than that, if I can. And sometimes what customers will say is like, hey, how do I do that? I have these different IoT type devices. Maybe they have printers, or sensors, or thermostats, or whatever it is that’s connecting, and they want to connect them securely, but they don’t know how to do that. So the way that we can do that here at Mist is really cool.
Basics 2.6
– So we could actually have a single SSID. And then, we have a feature called MultiPSK And what that means is–
– Oh thank you. Thank you. Thank you, Joel. I don’t know– by the way, I hope I’m not stealing a thunder because I think this is important, right.
– Not at all.
– Talking about SSIDs because customers will default to, hey, we’re just going to create a new SSID for the printers. We’re going to create a new SSID for this. We’re going to create a new SSID for that. And eventually, it gets to the point where they just have really bad Wi-Fi.
And so, we have a technology called MultiPSK and what we can do is that we can have one SSID. And we could have a personal pre-shared key for every client device. So maybe that’s what you can use for a guest network, for example. Or you could actually use a pre-shared key for a group of devices. For example, the printers will have their own pre-shared key. The thermostats will have their own.
And here’s the really cool thing that we could do is that we could actually have policies for those. So you can have a single SSID. Clients can have their own PSK. And we can actually have policies to actually help separate those guys in terms of what traffic they can get to on the network, what VLAN they go into on the backend, et cetera. Very, very powerful. It’s a nice differentiator that we have from a Mist perspective. And it helps customers have better Wi-Fi. That’s one thing.
The other thing that we can do is that you can create a single SSID. And then, you can do things like a enterprise type security. You’re– it’s your corporate SSID. You got a 802.1X. You’re authenticating with RADIUS servers, all that fun stuff.
And what we can do is that you can still have a single SSID. And depending on how the users are getting authenticated into the network, like for example, maybe somebody is in the accounting group and as they get authenticated the accounting group is in VLAN10. Or the somebody in the marketing department is in VLAN20 and they get authenticated that way. So what we can do is that we have a feature called– it’s a common feature out there in the industry. It’s called dynamic VLANs. But with a single SSID, you can actually put clients in VLANs on the backend.
So sorry, Joel. I know you’re making some notes. If you have any other color you want to add to that, please. Because it’s an important thing that– to cover.
– No. That is– that’s exactly– Yeah. Thank you for covering that. I really appreciate that, because that– yes. That’s a couple of excellent, excellent ways to keep from basically doing what I’ve done at my house where I have 10 SSIDs here. And maybe just for fun let’s go look at like look at the office. Let’s see. Oh yeah. Here’s live demo.
Do you think we have enough SSIDs on our– yeah. Not good. But this is for fun. This is not for production stuff.
– Yeah. Yeah. Hey. Hey, Joel. I– go back to live demo. Actually, go back to the True Mist Office. Just, I want to show something here. And I don’t know if you’re going to go there– go to Network and Radio– or Radio Management
– Let’s see. Yeah. Radio Management.
– There you go. And then, go down a little bit. And see where it says current radio values? Where’s the one that says Near Abhi’s desk? I’m laughing, because you see the little arrow on the right hand side? You can click on one of those.
– Yeah.
– See that little– Yeah. Yeah. Yeah.
– This is– so channel occupancy is– correct me if I’m wrong, but channel occupancy is how many– is how busy that channel is. Right? Or is that how many APs that are– no, it’s external APs versus internal APs. So this is APs that are not part of our network on channel 11. 10% of– 10% of this is our APs. And then this is non Wi-Fi.
– Well he’s running a private mesh network. That’s how he gets his wireless internet.
– I was going to make a comment. Abhi’s on the– I don’t know how he’s even listening, how he’s connected. But anyway.
– Like based on the Mist dashboard, his Wi-Fi should just not work, right.
– Yeah.
– Yeah. Yeah. Let’s see. A question that maybe you guys can pitch in on to is, where do you apply policies for separate traffic, the AP or the switch with multiple PSKs?
I think that depends on the– I’m going to say that depends on the mechanism that you use. Let’s go back to Joel’s house here. I don’t know. Jean, you want to speak? Do you have anything to say there? How do you apply policies to separate the traffic? I believe it’s going to be mostly in the Mist dashboard. Unless, you are doing WPA2 with EAP. And then, that’s kind of done on the RADIUS side if I remember correctly.
– Yeah. What we could do is that I’ll– in a little while here, I’ll find a good example. And I’ll bring it up to show everybody here a little bit.
– Well, maybe we can check back in with our good friend Garth and see if he’s ready to go. And Garth, do you have your stuff ready to go yet? How are you doing?
– We have no, Garth.
– That mute button is tricky.
– There he is.
– It is. Isn’t it?
– You got your stuff or do you need some more time?
– I do. It– just real quickly, I couldn’t find the Spectrum recording. But I will show you the device that I found and the problems that it was causing.
Basics 2.7
– So this is another example of the kind of fun stuff that we can find during site surveys or even troubleshooting on the dashboard. So I was walking around this grocery store and noticed that there was a legacy technology being broadcast that was essentially RF visible throughout the entire floor plan of this grocery store. And that is an old school modulation called frequency hopping spread spectrum which–
– Whoa. Is it this?
– –works on every single channel. It is. And not only was it powered at 100 milliwatts, but that is a 9-dBi gain antenna.
– Sweet. All kinds of–
– All sorts of fun RF blasting through this place. So after this– hey, look at that antenna right there. I’m getting real close. I got a 26 dB reading standing about 10 feet from it. So I’m thinking, this is quite overpowered. Found the device, pulled up quick, and noticed that even on the very front of it, it says right there, frequency hopping spread spectrum. And it was just blowing up RF throughout the entire store.
So if you have one device that is strong enough to be visible over a 15,000 square foot floor plan, you probably want to reconsider what that antenna is doing.
– Nice. That is awesome. Now, it wasn’t old school 802.11 was it? It was just something else.
– It was. It was 802.11 and–
– Wow.
– –for the California lottery system so that they could pull into the parking lot and record data without having to enter the store.
– Wow. Just for a bit of background on what’s going on there, when 802.11 was originally introduced way back in the day, there were basically two variants. Oh, I did 802.11 twice.
So there was frequency hopping spread spectrum, FHSS. You remember the little Nintendo controller I showed you. That’s another form of frequency hopping. Boom. Boom. Boom. Boom. Boom, hopping all around in the spectrum. And then, there was direct sequence spread spectrum. Those were the kind of two competing modulation schemes with the original standard.
FHSS died. It went away. It wasn’t carried forward. There’s no backwards compatibility for it. And so, I’ve never actually seen one in the wild before. So that’s really, really cool. Awesome. Any other comments, Garth? Sweet.
– All square. Thanks, Joel.
– Awesome. Thank you for showing that. That is awesome. I love seeing stories like that of just the random fun things that you find. Cool. So let me double check and make sure– so a couple of things I want to show really quick before we move on. A couple–
Basics 2.8
– So in the Mist dashboard, this is one of the things that we do completely differently from other vendors. I think, it’s really, really powerful. A lot of vendors out there– and we’ll take a break here in just a couple of minutes, but I kind of want to go over this stuff really, really quickly before we move on.
Other vendors tend to, and I don’t want to make this like a them and us sort of thing, but a lot of vendors tend to look at basically what is the health of the access point, how are the APs do it. And I think that’s fine and all. In my opinion, it’s not necessarily a bad strategy, but what we do differently that I think is really, really cool is we focus on what is the health of the client device, how is the actual client device doing.
And there’s basically this page in the Mist dashboard called the– it’s called Monitor for wireless network. And this basically gives us service level expectations or SLEs, service level expectations. So what these are is these are a whole bunch of measurements that we take. We basically track the state of all the client devices on the network. There is one for APs in here by the way.
But we look at the state of all the clients on the network, and we determine how is the client experience, how are the clients doing. And just a couple that I want to point out really quick that can be helpful for this discussion are coverage and capacity. So these are basically looking to see, are we meeting a certain requirement or not?
So if we look at Coverage here, and we click on Settings, and we come actually down here to Coverage in the Settings, you can actually basically set your service level expectation. Here, we’re saying we want to see negative 66 dBm or better. If we see negative 66 dBm or better from our client devices, then we’re going to say, that’s good, we pass that requirement, right?
If we see less than negative 66 dBm from clients, then we say, that client fails. We did not pass our requirement from that client device. So if you look here at this service level expectation, 67% of the time, we are achieving our negative 66 dBm goal, right?
Now, the other one that plays into this here is also is capacity. How busy are all of our channels? How much Wi-Fi interference do we have? how many clients do we have on each access point? How much non-Wi-Fi interference do we have here? So let’s go take a look at– we’ll go take a look at capacity.
And right now, we’re saying we want no more than 80% capacity consumed on each access point on each channel. So we have basically a 20% overhead that we’re working with here. Do I have that right? Maybe I have that backwards. But either way, this sets how busy is our environment and how much activity are we going to allow on that.
So I just want to show that really quick. This is really nice to be able to look and see how are things looking over time and how much availability do we have on our network. Remember, like I said earlier, if you’re doing something like voice over IP, if you look at a channel and the channel is just consistently more than 50%, you’re going to have a bad time. If you’re doing general data usage like web, email, stuff like that, you can get away with about 70%. And 70% is where things are going to break down.
David asks, is there an easy way to filter out one specific client from those charts, i.e., if you have one device in a fringe coverage area? Yes, you can actually. So basically, what you can do is we can click on the– we can click on any of these subclassifiers to drill down into capacity and see what is going on. And then what we can do is we can drill down into Affected Items.
So let me go over to our production office here. So we’ll go up here to Monitor and go to Service Levels. We’re looking at Wireless Network. And I’m going to look at– instead of just today, I’m going to look over the past seven days. How have things been over the last seven days?
So then we’ll come down here to Coverage, and we’ll go look at those subclassifiers. So let’s just take a look at Weak Signal. So we’ll look at Weak Signal, then we’ll go through Affected Items. And then it’s going to bring in over the past seven days, we can see who has been affected by this, how often are they failing to meet our coverage requirement.
So for example Raymond’s XS. Comment?
Yeah, Joel. It’s Jean. See the one below that, the TS131? Look at the overall impact.
– Yeah.
So that’s obviously one client that you could actually pinpoint right away that is having a high overall impact on that SLE.
– Yeah, absolutely. Fun to look at access points to like which access points, which access points routinely basically see clients with a weak signal strength. And so R2D2 by Lobby TV, that one is heavily affected by that. So yeah, you can get in there and very, very specifically look and see what’s going on.
OK, cool. So what I want to do now–
Hey, Joel?
Yeah.
JEAN: Hey, Joel. Real quick while you’re in there. Sorry. Can you just click on the Coverage SLE over there on the left?
Let’s see.
I want to show one other thing. If you already showed this, my apologies. I didn’t catch it. And click on Location. And it shows you a map of the location with the APs, and then it will also show you which APs graphically that are having coverage issues.
– So R2D2 is the one we were looking at a minute ago. That’s this guy right here. If I hover my mouse right here, you see a little shadow pops up there, a little label. And so if you had a network where– maybe on one end of the building, you had a couple of APs that had coverage issues, then I would say, OK, we might need to go maybe do a site survey there, maybe go add a couple more APs to fill in a coverage gap there. Because in that specific area, we are seeing clients that are having coverage problems and adding another AP might fix that.
And so yeah, this is a really, really cool view. Thanks for bringing this one up, Jean. Any other comments or anything before we take a quick break?