Re: [A bit OT] Diagnosing home network
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
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