Hi everybody. Welcome. My name is Tyler McMinn with Aruba, a Hewlett Packard Enterprise company and enterprise company and this is Aruba Mobility Essentials video course. We're nearing the end of part 1, this is our ninth video on MIMO and co-channel interference and you're probably wondering what MIMO is, I'm about to explain it. Sit back and let us jump on in. MIMO stands for Multiple Input, Multiple Output and it refers to your antenna usage. In the old days, we would use radios which is two antennas, an A antenna and a B antenna, and as you send your signal to a host, if the signal was good, you start with that antenna, you then use the other one. But if the signal as a wave was bouncing off the ceiling and cause a multipath issues where it would phase shift or it would cause the signal to be received out of phase by your user. You might decide, well, let's shift over to the B side and see if that avoids that multipath issue where maybe it's bouncing off the ceiling and missing the user and therefore you're getting a much better result. That was known as Single In Single Out as a strategy for deploying radios and that was fine in the 802.11a, 802.11b, 802.11g, but along came 802.11n. With 802.11n and beyond ac and ax, the use of multiple antennas and multipathing as an advantage came about. Without getting too much into this, what the big advantage here is that you get better signal strength with multiple antennas and aggregating or bundling the received signal shrink together, you get a cleaner signal which means better bandwidth but most importantly, you support something known as spatial streams. What spatial streams is, is a technology where you send half of your data or let's say you're sending your data normally, whatever, 100 percent of your data on one antenna, and then you wouldn't use the other. With MIMO, what you can do is send 100 percent information on one, 100 percent additional information on another antenna, and you could double your throughput with supporting two spatial streams, or three spatial streams you could triple your throughput. The number of spatial streams that's supported is dictated by the lowest number of antennas between the access point and the receiving client or station. If my access point has four antennas running four by four and it supports three spatial streams, the receiver in that client device, maybe a two by two supporting two spatial streams. That means I can only really do two spatial streams because of this two transmit, two receive antennas that my client has. If a client has only one antenna, then it's only going to be able to support one spatial stream. This is something that just keep in mind, with n, you can only send to one client device at a time. With ac and ax, one of the big advantages of these guys is they support multiple user MIMO or MU-MIMO, let's call it MU-MIMO. Multi-user means that you could send two spatial streams to this guy, and a third spatial stream at the same time to another client. This was amazing because it was our first entrance into being able to send and receive two multiple clients at the same time. Now it's not full duplex, it's not a switch with 48 ports and they can all send or receive at the same time with no collisions. It's much, much more limited but we're looking at ways to expand that with Wi-Fi 6 as well to try and get even more clients up and running at the same time. It's pretty cool stuff. Other than free space path loss and absorption, assuming you have a clean signal, you are in a good data modulation data rate, all topics we talked about in previous videos. There is still the issue of collisions and what we would call co-channel interference. Collisions themselves, we mentioned the CSMA CA collision avoidance mechanism where you're registering with the AP to find out if you can send or not. That registration results in a clear channel assessment of whether the channel is busy or not. The device can hear if other users are on that channel, but it's constantly counting down to see if it itself is available to send. What you don't want to have is two devices counting down to zero at the exact same time, they both go to send and boom, you have yourself a collision. Registering with the access point is a good way to make sure that you don't hear anything, but you may be so far away from the AP that these other devices don't reach you, so while you're not hearing anything, this guy is actually sending, you might say, "All the channels clear." This registration process avoids that. Remember Wi-Fi is shared its half duplex communication. The more devices that are on an AP, the more they'll have to wait. What's the solution to this? The first step you might want to do to minimize this is by looking at your design. What did you do when you laid out your access points? With just one AP you might have the signal strength to cover all of these users. But now everybody is sharing their connection with this one channel on this one radio. The problem with that is it's a bad experience for everybody. It's like sharing a pizza with 100 of your closest friends. You're not going to get a very thick slice of time in that regard. The first thing and probably one of the best pieces of advice is to consider using more APs, introducing more radios on separate channels. By doing this, there's no risk of co-channel interference, at least well from yourself obviously, which is what co-channel interference is and you've got a nice spread on the amount of devices. If I have 100 users spread across four APs, that means I've got an average of 25 users per radio. In that case, that's a pretty good number to work with and you're going to get some pretty decent data rates. If you're sharing one gigabit, that means you're going to get an average of 40 megabits per user. That's a pretty decent experience for users on that AP. If the average user is peaking at about 25 megabits to 50 megabits per user, that's not bad. Generally, we try and keep the number around 20-30 users. Less is better. In the illustration, each AP is on a different channel, so no co-channel interference between these radio-frequencies cells of coverage. Each cell can have a transmission happening at the same time. Therefore, you can have this user transmitting, and this user transmitting, and this user transmitting, and that user transmitting all at the same time without any fancy dancy technology or whatever. Everything's working great. What changed in this illustration? Do you guys see it? It's down here. They had channel 36, 40, and 52, but then they went back to channel 36. Now, this is a pretty good design because you are going to want to reuse channels at some point. You want to put them as far away physically from its neighboring on the same channel. Every 60 seconds or so, these APs will send these neighbor messages to make sure that their power levels aren't so strong that they're actually overlapping with each other. They'll actually tune their power level down to what you see here to a small cell of coverage to make sure that they're not overlapping on the same channel. This way they're filling in any holes while avoiding co-channel interference. You could have some device here that's broadcasting so widely that it's reaching this AP, but it's also causing interference with devices over on this access point. But generally, the more separation you have the better, so it's unlikely. Radios from one cell cannot hear radios from the other since the two cells are far apart. Co-channel interference is not likely to occur in this example. Each RF cell can have a transmission happening at the same time. You still got these four same simultaneous transmissions going on without risk of interfering with each other or minimal risk of interfering with each other. What's wrong with this one? You guys see it? Well, they went to channel 36 on both sides. This is what happens when people get involved and they say, "Hey, I know how to do this. " and they make a mistake, which people do. I make mistakes all the time. Now you've got basically one area of coverage that any user that's broadcasting here is going to interfere with the other users that are going to see that frame. You're not just backing off from the 25 users here, but you're backing off from these additional 25 users. You might as well have just put all 50 users in one AP. It's a very contentious environment the world of radio. Proper channel assignment, allowing dynamic channel assignment, or automatic channel assignment is usually the way to avoid these kinds of issues. In co-channel interference, I can't really stress this enough, this is pretty much the number one issue in most network designs that they have to overcome. More channels would solve this issue. By going to channel 56 or whatever that would have solved this and avoided this issue altogether. I hope you guys enjoyed this video. I'll see you in the next one where we are going to talk about some basic planning concepts you want to touch on. I've already hit on a lot of these in our previous videos. But I'm going to tie it all together for you and we're going to close out our part 1 discussion for Aruba mobility essentials.
