Is that Wifi co-channel interference?

Question

I've read many subjects about WiFi interference but I'm still not 100% satisfied about the problem I encountered.

Well, I have my own Internet connection and my own WiFi 2.4Ghz router on channel 11 (approx. -45db). when I test my connection with SpeedTest or another speed test website, I get the full speed provided by my ISP.

I also reach my neighbour's WiFi 2.4Ghz connection which is also set on channel 11 (approx. -55db). I have access to this connection (my neighbour is my cousin). Again, when I test this connection, I get a very good result.

(There are also about 10 more WiFi networks set on different channels within range in my room)

BUT, I don't know why, last night I had the strange idea to test both connections at the same time with 2 laptops. And the results I got were not good. I almost lost 50% bandwidth on both connections !

So, I'm wondering, is it "normal" ? Is it what we call "co-channel interference" ?

Thank you for your help =)

Answer 1

I'm going to make up some numbers here. Lets say your ISP provides you with a 50Mb/s line and your cousin has the same. The WiFi connection only supports 60Mb/s of data on a given channel (or channel range, but that may be a bit outside of this).

This means when you use your connection alone the WiFi bandwidth of 60Mb/s is easily enough for the 50Mb/s of data your ISP is sending you.

But if you and your cousin use both connections at once that WiFi bandwidth is split so you only get 30Mb/s of it each.

This article explains a bit about channel ranges and why we use 1, 6 and 11. Short version, if you and your cousin used channels 6 and 11 you wouldn't interfere. Using 10 and 11 you would, almost as much as both using 11. http://www.metageek.com/training/resources/why-channels-1-6-11.html

Answer 2

That is not really interference at all, that's Wi-Fi carrier sense doing exactly what it's supposed to do. Out of the four devices operating on channel 11, only one can transmit at a time. The other three devices wait for the current transmission to end before transmitting. The effect is that total available bandwidth is mostly evenly shared between the two networks. You get a slower connection that is still reliable and works at full range with no packet loss.

Try the experiment again, but this time with your router set to channel 8 or 9. Try to have the laptops as far from the access points as the access points are from each other. Channels 11 and 8 or 9 will conflict and the total available bandwidth will drop way down below half. The packet speed will likely drop down to the slowest 1 or 2mbps DSSS mode which is more tolerant to interference from an overlapping channel. The connection should get very unreliable from packet loss as you move farther away. If the laptops are moved very close to their respective access points, then the interference from the other network won't be strong enough to interfere, the carrier sense will be effectively disabled due to the other network being two or three channels away, and both laptops will have full speed available.

Answer 3

yeah I tried changing the channel of my own network from channel 11 to 1 and it's better. My own network doesn't seem to be impacted anymore but my neighbour's one still is. So there's still a kind of "interference" even with 9 channels separating them. WiFi is really hard to understand :(