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Comment by niobe

11 hours ago

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.

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niobe

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rayiner  10 hours ago

> 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.