Hello.
If we believe Einstein's theory of simplicity, the main indicator of understanding a subject is the ability to explain it as simply as possible. In this post, I will try to explain in the simplest and most detailed way the action of just one component of the new standard, which for some reason even the Wi-Fi Alliance considers unworthy of mention in the infographic about the new features of Wi-Fi 6, although it is, as we will soon find out together, very important and noteworthy. This is neither very deep nor comprehensive (because it's hard to digest such a big topic, even in parts), but I hope we will all gain something new and interesting from my verbal exercises.
That very 802.11ax that we have been waiting for day after day for at least the last two years carries a lot of new and amazing features. Anyone wanting to talk about it always faces a choice: either to make overview jumps over a pile of acronyms, trying not to get bogged down in the complex mechanisms under the hood of each of them, or to wrap up a one-hour report on something, the most enjoyable for the author. I dare to go even further: most of my note will even be dedicated not to the new!
So, for more than twenty years now, part of the wireless data transmission networks has been built on a pile of standards from the 802.11 family, and as any respectable speaker would, I should restore the timeline of the entire chain of events that have gifted the world with billions of compatible devices — but as a respectful author, I will take the risk of not doing so. However, there are some things worth reminding each other about.
All iterations of Wi-Fi prioritized reliability, but not the maximization of throughput. This is evident from the medium access mechanism (CSMA/CA), which is not the most optimal in terms of squeezing the last kilobits per second from the transmission medium (more about the imperfections of the world in general and Wi-Fi in particular can be read in the article by my former colleague. ), but incredibly resilient in practically any conditions. Indeed, you can break almost all the principles of Wi-Fi network design — and data transmission will still occur in such a network! The mechanism that allows Wi-Fi network clients to send and/or receive their data packets is focused on ensuring what is referred to in English with the difficult-to-translate techie term, robustness. The entire layer of improvements atop modulation increases and data frame aggregation (not exactly, but let it be!) continues to function even after the two fundamental principles of 802.11 that ensure this unmatched reliability.
- “When one speaks, the others remain silent”;
- “Everything except the data is communicated slowly and clearly.”
The second point inflicts much greater damage to the network's bandwidth than it may seem at first glance. Here’s a neat graphic illustrating one data packet sent over a Wi-Fi network:
Let's break down what this means for ordinary people who don't know how many pages are in the 802.11-2016 standard. The data transmission speed that the system writes in the properties of the wireless network, and which marketers of any manufacturer depict on the boxes of access points (you've surely seen it — 1.7 Gb/s! 2.4 Gb/s! 9000 Gb/s!), is not only peak and maximum under 100% transmission time utilization, but it's also the speed at which only the blue part of this pretty graph will be sent. Everything else will be transmitted at a speed referred to in English as the management rate (and the same in Russian, because translating such expressions risks further misunderstanding among engineers), which is lower not just by several times, but by HUNDREDS of times. For example, without any additional settings, a network on 802.11ac, capable of operating with clients at a channel speed of 1300 Mb/s, transmits all control information (everything that isn't blue on our increasingly terrifying graph) at a management rate of 6 Mb/s. Over two hundred times slower!
A logical question is: what day, sorry, what month did such a detrimental idea even make it into the standard used by billions of devices worldwide? The logical answer is compatibility, compatibility, compatibility! The network at the latest access point must ensure operation for ten- and even fifteen-year-old devices, and all the 'non-blue' bits carry the information that slow, older devices will hear, understand correctly, and not try to pawn off their own during super-fast data chunks. Robustness requires sacrifices!
Now I am ready to provide anyone interested with an indispensable tool to marvel at the potentially lost megabits aimlessly drifting in modern Wi-Fi — the tool that has become a must-learn among concerned engineering circles. The WiFi AirTime Calculator by Norwegian enthusiast of 802.11 Gjermund Raaen. It is available at — the result of his work looks something like this:
Line 1 — the time spent transmitting a 1512-byte data packet by an 802.11n device on a 20 MHz channel.
Line 2 — the time spent transmitting the same packet by a device with the same antenna formula, but operating under the 802.11ac standard on an 80 MHz channel.
How is it that 'spoiled' four times more air is used, the maximum modulation has complicated from 64QAM to 256QAM, and the channel speed is higher by SIX times (433 Mbps instead of 72 Mbps), yet only a 25% gain in air time is achieved?
Compatibility and the two principles of 802.11, remember?
Well, how can we fix such injustice and wastefulness — we might ask ourselves, as perhaps every IEEE working group asked itself when starting to create the standard? A few logical paths come to mind:
- Accelerate data transmission in the 'green' portion of the graph. This is done with each standard's release because large numbers look nice on boxes. In practice, as we just noted, it provides a finite gain — even if we speed up the channel speed to a hundred thousand million gigabits in a nanosecond, all the other parts of the graph won't disappear. That's why I recommend skipping paragraphs mentioning megabits per second in all discussions about new 802.11 standards.
- Accelerate all other parts of the schedule. Indeed, if we at least double the speed at which everything 'non-green' (or 'non-blue', if you're still looking at the previous picture) is transmitted, we will achieve just under 50% increase in actual bandwidth — albeit at the cost of losing compatibility with devices and a number of nuances that you'll learn about when you prepare for the exam to earn the proud title of CWNA 🙂 Spoiler alert: this won't always be possible, so think carefully and understand where it will lead. In fact, this violates one of the two principles of 802.11, so you need to be very cautious with it!
- Glue together several frames like this with green parts. The longer the green part, the more effective the channel speed increase works. Yes, this is a viable strategy that originated back in 802.11n and is one of the cornerstones of its revolutionary nature. The problem is that, firstly, many applications couldn't care less about this aggregation (for example, that resource-hungry Voice over Wi-Fi), and secondly, a number of devices also didn't care at all (I once tried to capture at least a few of such aggregated frames on a real network of the company I work for, but out of >500k 'captured' frames, exactly zero were aggregated. Most likely, the problem lies in my data collection methodology, but I'm open to discussing it with anyone interested in a personal conversation!).
- To violate the first of the two principles of 802.11 by starting to speak while someone else is talking. And this is where 802.11ax comes to the rescue.
How wonderful, I've finally reached Wi-Fi 6 in my discussion! If you're still reading this, you are either obligated to for some reason, or you are genuinely interested. So, although 802.11ax inherits a significant portion of previous developments in the entire 802.11 family (and not only, by the way — some great features actually appeared in 802.16, also known as WiMAX), there are still some new and original elements in it. Typically, these words are accompanied by an image available on the Wi-Fi Alliance's website:
As I mentioned from the very beginning, we can only comprehensively discuss one of these key points within a single readable article, namely, none of those listed in the image (surprise!). I'm sure you've already read a million brief descriptions of each of these eight key elements, so I'll continue my long-winded tale about what follows from OFDMA — about multi-user access (MU-access control), which, as we can see, didn't make it onto the infographic at all. And that's quite a shame!
Multi-user access is essential; without it, dividing the channel into subcarriers makes no sense. Why bother examining different pieces of the spectrum if there isn't a mechanism to allow clients on the new Wi-Fi 6 network to break one of the previously immutable rules and start communicating simultaneously? Of course, such a mechanism had to emerge — and reduce the impact of the 'long' problem compared to control information. How? Very simply: let the 'slow', control part be transmitted just like before, while the 'fast' part, which carries the actual data, is sent simultaneously from several (or multiple) devices on command! It looks something like this:
It seems complicated, but it's quite easy to explain in essence: the access point, using a special frame that is understandable to all (even non-Wi-Fi 6!) devices, announces that it is ready to transmit data simultaneously to STA1 and STA2. Since the 'header' of this frame is completely comprehensible even to very old clients, they correctly conclude that the air will be occupied for a certain amount of time for transmitting information to other clients in the network and start counting down the time until the end of this period (as they always do in Wi-Fi). Meanwhile, devices STA1 and STA2 understand that data will be transmitted to them in a new way, simultaneously, with each on their own piece of the channel, and respond to the access point at the same time, and then synchronize to confirm the receipt of the frame (each with their own data!), and the medium is free again. 'Bottom-up', it works in a very similar manner:
The main and most noticeable difference is that the access point in this situation informs stations that can communicate simultaneously when to start transmission, using a special frame called a Trigger. This is essentially a new 'trigger' for the entire multiple simultaneous access mechanism, which, in my humble opinion, is one of the most important innovations 'under the hood' of the new standard. It is in this mechanism that clients receive a 'schedule' for how to divide a single frequency channel among themselves; it is here that clients simultaneously inform the access point that they have received their data portions and were able to process them. In this, the access point notifies everyone who can 'talk' at the same time about the beginning of data transmission — it is also where the access point initiates sending the required data. The new Trigger frame mechanism essentially allows for more efficient use of the airwaves — and that efficiency increases with the number of clients who can utilize it and correctly interpret the signals!
Now, let's summarize the main points that follow from this long discussion and claim to be TL;DR:
- Access points of the new 802.11ax standard, even relying on just one of its many innovations, will begin to increase the overall throughput of the entire network starting from the second compatible client device! As soon as there are at least two clients that can communicate simultaneously, then, all else being equal (I have no reason to believe that drivers for client radio modules will be written any better than before, which means that the aggregation of 'useful' packet parts and many other client-dependent functions will still operate 'on average across the board' and won't be that great), they will ALREADY increase the average throughput. So, if you're considering a new Wi-Fi network — it's worth looking at the newest and best access points right away, because even if there aren't many clients available for them now — this situation won't last long.
- All the tricks and techniques that a good wireless engineer has in their arsenal today will remain relevant for a long time — the access mechanism has updated, breaking long-standing principles that lasted over 20 years, but it still prioritizes compatibility. It is still necessary to cut out the 'slow' management rates (and it's still essential to understand why and when), and proper planning of the physical layer is crucial because no mechanism at the data link layer will work if there are issues at the physical layer. Now there’s just the opportunity to do even better.
- Almost all decisions in Wi-Fi 6 are made by the access point. As we can see, it manages client access to the medium, grouping devices together in 'periods' of simultaneous operation. Moving slightly further away — the operation of TWT is also entirely on the shoulders of the access point. Now the AP must not only 'broadcast the network' and keep traffic in queues but also track all clients, planning how to optimally group them based on their bandwidth and traffic needs, their battery levels, and much more — I call this process 'orchestration.' The algorithms that the access point will use to make all these decisions are not regulated, meaning that the true quality and structural approach of manufacturers will emerge specifically in the development of orchestration algorithms. The more accurately access points predict client needs, the better and more evenly they can group them into multiple access groups — consequently, the more efficiently the resources of the spectrum will be utilized and the higher the overall bandwidth of such an access point will be. The algorithm is the final frontier!
- The transition from Wi-Fi 5 to Wi-Fi 6 is as revolutionary in its essence and importance as the shift from 802.11g to 802.11n. Back then, we got multi-channel support and payload aggregation — now we gain simultaneous access to the medium, and finally functional MU-MIMO and Beamforming (for one, as we know, these are nearly the same; secondly, discussing why MU-MIMO was invented in 802.11ac but couldn't be made to work is a topic for a separate long article 🙂 ). Both 802.11n and Wi-Fi 6 operate in both frequency bands (2.4 GHz and 5 GHz), unlike their 'intermediate' predecessors — truly, 'six is the new four'!
A bit about the origins of this article
The article was written for a contest organized by Huawei (originally published ). Much of it was based on my own presentation at the 'Wireless' conference held in 2019 in St. Petersburg (you can watch the recording of the talk , just keep in mind — the sound quality is, frankly, not great, despite the video’s origin from St. Petersburg!).
Source: habr.com
