I Dream of Wi-Fi (802.11ay)
- Brad Wegner Sr
- May 3
- 4 min read
Introduction
I woke up this morning thinking about the concepts of Radio Frequency traveling at the Speed of Light (through air), Guard Intervals (GI) and Time Division Duplexing (TDD). I figured the best way to get this out of mind was to perform a little research and see what the world thinks.
One of the thoughts popping into my head at the early hour of the day was that the wireless bridges I've been experimenting with use TDD which is a form of time slicing I learned about back in my VOIP days. TDD requires a higher level of coordination than CSMA-CA and provides the network with more control. It was my theory that TDD would be significantly more effective at long distances than the CSMA-CA used in traditional Wi-Fi.
The Question
The question in my head this morning was, "given a long enough highly directional link, would my distance from my sender to my recipient (using traditional 802.11 standard equipment) exceed the configured Guard Interval and allow for issues with ISI among other things".
A little bit about distance
Thinking about this distance test led me down a path similar to what Susinder Rajan Gulasekaran was talking about in his presentation at WLPC back in February. In his presentation Susinder talks about difference in time as an inaccurate measure of distance, and while my understanding of this is, let's say -- rudimentary at best, it has helped me make a connection.
Going the Distance
Whether your medium is Fiber, Copper, Air, Water, etc. the elapsed time from when a wave is created or measured from can be calculated to a distance away from the point we are measuring from D=R/T. It just so happens that in our field we tend to measure relatively short distances at a very high rate, over a very small amounts of time. I did a little work this morning and thought I would make this available to anyone curious.
I prepared the formula below by solving for Time by multiplying the well know formula for distance by 1/Rate on each side. I then calculated the speed of light through air at 1 microsecond to work with smaller numbers. I'll include an editable here if anyone wants to play with it further.
Some Calculations
Writing out this table was a great exercise as I was able to quickly ascertain that my suspicions were correct. If I have a long enough PtP shot using highly directional equipment (perhaps with a booster or repeater inline) I have the potential to exceed the Guard Interval in my transmissions. This reinforced my thoughts that traditional off the shelf APs would not suffice for long PtP implementations.
Guard Intervals over the Generations of Wi-Fi
Wi-Fi Standard | Guard Interval (μs) | Purpose/Notes |
802.11a/g | 0.8 μs | Fixed GI; no support for shorter intervals |
802.11n (Wi-Fi 4) | 0.8 μs / 0.4 μs | Short GI improves throughput (~10%) |
802.11ac (Wi-Fi 5) | 0.8 μs / 0.4 μs | Short GI commonly used for better performance |
802.11ax (Wi-Fi 6) | 0.8 μs, 1.6 μs, 3.2 μs | Longer GIs support high-density/multipath environments |
802.11be (Wi-Fi 7) | Expected: 0.8 μs, 1.6 μs, 3.2 μs | Likely maintains flexible GI options like Wi-Fi 6 |
*Second Table was generated using ChatGPT
A Thought on Guard Intervals
As I understand them, Guard Intervals are a necessary evil in Wi-Fi. They prevent a radio from interfering with itself and others. When I started pursuing knowledge of Wi-Fi in earnest (802.11n era) the introduction of the Short Guard Interval was all the rage. We could click a button and increase throughput (sometimes at the expense of performance). Although I think I have a basic grasp on this science (tenuous at best, honestly) my understanding is that as we are building radios capable of achieving higher QAM (finer granularity in signal generation and demodulation) it may take the radios a bit longer to achieve that perfection when facing issues like multipath and delay spread, or when new features like OFDMA or MU-MIMO are introduced. The trade-off is a much higher throughput achieved through more complex symbols.
Conclusion
My experience with outdoor PtP is growing, but limited, and I have no practical experience yet with long 5GHz radio shots. When researching this topic, I was frankly shocked at the kind of distances that are available in commercially available 5GHz products (I saw up to 250km!), especially at what I would consider quite reasonable prices. If you're looking at building a PtP or PtMP project out, I would highly recommend going with gear designed for that purpose as opposed to taking your typical vendor's External Antenna model and rigging it with a highly direction or semi-directional antenna and hoping it works. There's top quality gear out there that can deliver.
Afterward
This morning I was initially thinking/theorizing some concepts on 60GHz PTP Radio Links and my desire to do a larger lab over the course of the Summer. I am envisioning a sort of Field Day, where I pack out a bunch of gear and a few canopies to work under and setup in a field or parking lot for a series of tests and labs. *(If you are in or near Northeast Ohio with some land and power, and maybe a few sleds or poles, I'd love to work with you)
While I was mocking up the design for a Point to Multi-Point topology in my head, one of the first things that came to mind was that I'd really like to test the distance limitations of the gear I have. I was lucky enough to get some top-notch 60GHz demo gear from Cambium and I am looking for more ways to prove this out as I am developing specific business use cases for the technology.
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