Re: [A bit OT] Diagnosing home network
Mark Fletcher wrote:
> On Sun, May 21, 2017 at 02:18:06PM -0000, Dan Purgert wrote:
>> Mark Fletcher wrote:
>> >
>> > My home network consists of 2 Debian machines, one Jessie and one
>> > Stretch, an LFS mini-ITX machine acting as my firewall, another LFS
>> > laptop that is connected only occasionally, a Windows 8.1 laptop, 3
>> > iPhones of varying ages, 2 iPads, 1 Android tablet device, a couple of
>> > other proprietary tablets and a Buffalo Linkstation that provides most
>> > of the connectivity.
>>
>> Quick google doesn't show any "Linkstation" devices with more than one
>> ethernet port (much less wifi). Do you perhaps mean an "Airstation"?
>> Could you provide the model number, so we can look it up?
>
> Oops. Quite right. It is an AirStation. Specifically, a WZR-1750-DHP2.
> When I google it everything I find is in Japanese, possibly this model
> was only ever sold in Japan.
Happens :).. "Japanese only" is gonna make it hard for me to read the
specs though.
>
>>
>> [...]
>> From there, you have to divide up the available throughput by number of
>> clients (i.e. a given wifi client's speed is 1/n, assuming that all
>> clients are using the same technology). So if your wifi was *perfect*
>> for 802.11g, and one client got 24 mbit -- 2 clients would average 12
>> mbit each, 3 would average 8, and so on.
>>
>
> If they were all talking at the same time, right? Just sitting there
> doing nothing shouldn't use up a _lot_ of bandwidth on ethernet,
> although doubtless keepalives and so on will use some.
More or less -- idle clients (on your WLAN) don't take up much
bandwidth. It's more the next bit (neighbors on the same channel)
that's more likely to cause you trouble.
>
>> In addition to "your clients", you have to contend with neighbors on the
>> same channels (who add to the 1/n throughput troubles). 5 GHz helps
>> here, as it's less likely that the clients in your home will see the 5
>> GHz signal from your neighbors, even if you are on the same channel.
>>
>> > [...]
>>
>> Also check your wifi channel usage -- 2.4 GHz should be on channel 1, 6,
>> or 11 (if you're somewhere where 13 is allowed, you could try that too).
>> If you use any other channel (2-5 or 7-10), you're going to be getting a
>> lot of interference (and throughput losses) from your neighbors.
>>
>>
> 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.
--
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