Comment by niobe
12 hours ago
An impressive attempt to summarise Wi-Fi which is a very deep topic. However I think the executive summary already missed the most critical thing about Wi-Fi:
only 1 transmitter at a time per channel - across all WLANs, yours and your neighbours, with no deterministic way to avoid collisions.
It's a shared medium and it's not even half duplex, unlike the dedicated full duplex you would typically get with an ethernet cable to a switch port.
The fact that Wi-Fi achieves what it does with this limitation, and how it co-ordinates the dance of multiple unknown clients using the same medium - and in the presence of other RF technologies to boot - is indeed an incredible technology story, but this achilles heel is the single most defining thing about Wi-Fi performance.
> only 1 transmitter at a time per channel - across all WLANs, yours and your neighbours, with no deterministic way to avoid collisions.
Not with newer standards:
> Orthogonal Frequency-Division Multiple Access (OFDMA) is a multi-user wireless transmission technology that divides a single Wi-Fi or cellular channel into smaller subcarriers called Resource Units (RUs), allowing multiple devices to transmit data simultaneously.
[…]
> Instead of one device occupying the entire channel (as in OFDM), OFDMA allows parallel transmissions. As a result, network congestion decreases significantly.
* https://www.netcomlearning.com/blog/what-is-ofdma
* https://airheads.hpe.com/blogs/antar1/2020/10/19/why-is-ofdm...
> In addition, the 802.11ax standard defines the smallest subchannel as a resource unit (RU), which includes at least 26 subcarriers and uniquely identifies a user. The resources of the entire channel are divided into small RUs with fixed sizes. In this mode, user data is carried on each RU. Therefore, on the total time-frequency resources, multiple users may simultaneously send data in each time segment, as shown in the following figure.
* https://info.support.huawei.com/info-finder/encyclopedia/en/...
> only 1 transmitter at a time per channel - across all WLANs, yours and your neighbours, with no deterministic way to avoid collisions.
That’s not correct. You and your neighbor can use the same channel at the same time. On your network, the transmissions of the other network appear will appear as noise. As long as the other devices are far enough away, however, your devices will still be able to make out their own signal.
Only if the difference in signal power is high (>40 dB). This is like saying the issue isn't an issue in cases it doesn't exist.
This is a common misconception.. you and your neighbour can configure the same channel, you cannot successfully transmit at the same time on the same channel within range. Nor can you and your own AP successfully transmit at the same time on the same channel.
When you and your neighbour _appear_ to be transmitting at the same time, each adapter is actually spending most of it's time waiting for a clear medium and for various backoff timers to expire before attempting to transmit.
"Appear as noise" is not defined for Wi-Fi adapters. There is only "I received a frame addressed to me and acknowledged it" or "I sent a frame and either did or didn't get an acknowledgement back from the receiver". Receivers do not know why they didn't receive a frame, or, if they received a corrupted frame, why it was corrupted. They just wait for a retransmit. Senders ordinarily wait a certain time to receive an acknowledgement, and if they don't, the start the transmit wait cycle again. But they often then reduce the data rate to increase the odds of a successful transmission.
I'm glossing over some complexity here, because there's a sender and receiver to consider, and each has a different view of the RF environment, but the point is always correct when all transmitters and receivers (lets say the 2 APs and each has 1 client) are in audible range of each other. And this is most of the time. Note that "audible range" (where the signal is such that the medium is deemed as busy by the adapter) is much larger than the "usable range" (where data can be transmitted at reasonable speeds). So transmitters create interference in a much larger area than they actually operate in.
That means your neighbour transmitting at 6Mbps to his AP will indeed degrade the performance of your client who wants to transmit at 600Mbps because your client has to wait ~100 times longer for a clear medium.
> There is only "I received a frame addressed to me and acknowledged it" or "I sent a frame and either did or didn't get an acknowledgement back from the receiver". Receivers do not know why they didn't receive a frame, or, if they received a corrupted frame, why it was corrupted.
That's not correct. WiFi is "listen before talk." Radios listen to the channel, trying to decode preambles from other networks, before transmitting. In that process, they can detect other signals well below the threshold where they'll consider the medium in use (the CCA threshold). If you have an otherwise clean channel, the noise floor might be -95 dBm. Radios typically can decode the preambles 3-4 dB above the noise floor. Conventionally, the WiFi standards set the CCA threshold at -82 dBm. So the radio can "hear" a lot of signals that won't cause it to trigger collision avoidance. More recent standards allow using a CCA threshold as high as -62 dBM under certain circumstances to facilitate spatial reuse: https://arista.my.site.com/AristaCommunity/s/article/Spatial....
Also, what the Wifi standards do is less aggressive than what radios could do. The CCA thresholds are set to facilitate orderly use of the spectrum--they're not physical limits. To receive a transmission, you just need sufficient signal-to-noise ratio. An adjacent network transmission raises the noise floor, but if your radio is close enough to your AP, you might still have sufficient SNR.
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The multi access story is improving, though.
OFDMA on wifi7/802.11be: https://blogs.cisco.com/networking/wi-fi-7-mru-ofdma-turning...
It is not even switched on in some early version of WiFi 7 router and receivers.
As a general rule of thumb, the best version of WiFi x will only come with WiFi x+1. So for all the problems to be solved and ironed out on OFDMA it will be WiFi 8 then. And for all the promises of Ultra-High Reliability, it will have to be WIFI 9.
WiFi is clearly moving more towards like 4G and 5G with every version. I just hope someday that it really is good enough where there are many people using it at the same time.
> It is not even switched on in some early version of WiFi 7 router and receivers.
OFDMA was first used with Wifi 6:
* https://blogs.cisco.com/networking/wi-fi-6-ofdma-resource-un...
* https://www.litepoint.com/blog/wi-fi-6-ofdma/
Yes, and before that MU-MIMO is also an improvement to the problem. Still only 1 transmitter at a time, but multiple receivers.
Well the newer WiFi standards on 6Ghz support a lot more channels. Not a perfect work around by any means but it does significantly reduce congestion.
Yes, that helps quiet a lot in practice because in most places there's limited "frequency-domain" capacity (i.e. free channels) but plenty of "time-domain" capacity, (i.e. free air-time). So even if you are sharing a channel with 4 other APs and their users, everybody may subjectively feel the network is fast. When chopping up the time domain into nanoseconds there's just a lot of idle time available, even if clients are pulling down files at 600Mbps.
But at a fundamental level, the channel space (~60 across all bands best case) is extremely limited but the potential growth in transmitters is unbounded. It's like a linear hack to an exponential problem. It seems to work at first, but under very high load conditions performance still degrades ever faster until it falls off a cliff. Then there's all sorts of complex dynamic behaviour like the hidden node problem to add to this, but it all boils down to needing air-time and SNR.
> But at a fundamental level, the channel space (~60 across all bands best case) is extremely limited but the potential growth in transmitters is unbounded.
You’re overlooking the spatial dimension: https://en.wikipedia.org/wiki/Spatial_multiplexing
Yeah 6Ghz freq doesn't have DFS channels which remove a lot of usable channels for 5Ghz. Unfortunately it'll be a while until most devices support 6Ghz.
> Unfortunately it'll be a while until most devices support 6Ghz.
Per this May 2025 Juniper presentation, half of their deployed APs have 6 GHZ enabled, and at least 20%—but as much as 50% depending on the environment—of clients have 6 GHz:
* https://www.youtube.com/watch?v=sV-3gA0OP9s
Corporate environments (where client hardware is more standardize) has higher 6 GHz adoption, BYOD (universities) environments have lower adoption.
So I'm not sure how you define "a while" as, but it's probably already the majority at most workplaces, and will be for personal stuff with-in a year or so.