On Fri, May 26, 2017 at 10:09:08AM -0000, Dan Purgert wrote: > Mark Fletcher wrote: > > On Sun, May 21, 2017 at 02:18:06PM -0000, Dan Purgert wrote: > >> Mark Fletcher wrote: > >> > > >> > > Channel selection is automatic -- shouldn't it pick the clearest one? > > Also I am curious as to why selecting 1, 6, 11 or 13 if available is > > better and less likely to result in interference? > > "automatic" is a nice way of saying "braindead" in many instances. > Nearly all gear (until you're spending $1500+ for a single AP) is only > "auto" when it boots up ... and 9 times out of 10, it'll pull some > stupid channel like 3. > > The reason for channels 1, 6, or 11 is that > (a) they're universal channels globally > (b) they're the only three (2.4 GHz) channels that don't overlap > > For the "standard" channels (1-11), they are 20 MHz wide, and center > frequencies are spaced 5 MHz apart. This means that channels 1 and 2 > (for example) overlap their spectrum use about 3/4. In turn, this > raises the noise floor on both WLANs, leading to garbled packets / > re-transmissions / other slowdowns -- all of which get mitigated by > using channels 1,6,11 simply because they don't overlap. > > On the other side - with "everyone" using 1,6,11 - if two APs (or client > devices) on the same channel are able to "hear" each other, they'll both > employ their collision avoidance routines to share the channel - even > when they're on different WLANs. It's just simple checks along the > lines of > > 1. Is anyone transmitting right now? > * If yes, wait til they're done, plus random milliseconds, then > goto2 > * If no, wait random milliseconds, then goto2 > 2. Start transmitting > > > It's not perfect, and sometimes you get "hidden node" issues (where say > your laptop and mine can't hear each other transmit, but our respective > APs can - they'll then send us the "shutup, someone else is transmitting" > signal), but instead of both WLANs trying to shout over each other and > simply generate noise - which near on always slows everything down - > they share. Sure, sharing the channel means some degree of slowdown, > but it's generally not nearly as pronounced as those caused by > interference. >
Fascinating. And makes lots of sense -- thanks! I've entirely left channel selection up to the AP so gawd alone knows what channel state I am in. I will do battle with the Japanese-language WUI to find out... Actually maybe I will have more luck interrogating the client side for that... Mark