offtopic question about high density wifi
C. Scott Ananian
cscott at laptop.org
Tue Jan 26 00:41:26 EST 2010
On Mon, Jan 25, 2010 at 10:31 PM, <david at lang.hm> wrote:
> I've read through these, and they have a lot of useful info. I do have
> good RF experiance (and even some halfway decent tools for looking at
> things), but I didn't know what, if any limits there were on the number of
> clients other than what can be supported by the available airtime.
Lots of very subtle protocol limits, having to do with all sorts of
random mostly-timing related parameters where the 802.11 spec gives
implementers a lot of freedom to choose arbitrary values. *Plus* the
details of all your clients. There are all sorts of fancy algorithms
you can use to tune your AP, but all it takes is one bad client and
everything goes to hell.
And that's completely apart from the arbitrary software limits that
some access point manufacturers include, in order to differentiate
their "consumer" and "professional" product lines. *And* not
accounting for stupid software which decides to use the broadcast
features of 802.11, which chew up bandwidth 50x faster than normal
So, basically: theory is no substitute for experience. It's not
really the protocol that's the limit, it's the particular choices that
particular access point makes and the choices that "common" clients
and "common" software make. So your best bet is really to (a) find
someone who's done it before, and slavishly copy their setup
(variations that you think are trivial, like between firmware
revisions, may in fact be critical), or (b) find a company who's
invested the time and money to figure out all the variables and do the
real world testing, and fork over the $$$ for the "commercial quality"
or "pro grade" or whatever-they-call-it access point with a guarantee
about the number of clients it can support.
And a couple of accessible wired switches and wired internet kiosks
will go a long way toward mitigating your downside if it turns out
that your wireless totally melts down under load.
I've attempted a couple of mystery hunts
(http://en.wikipedia.org/wiki/Mystery_Hunt) with ~50 clients on one
access point and can vouch that that's above the workable capacity of
even consumer-grade access points. (We had a fancy commercial grade
access point this year with 10s of compact antennas and it did much
better.) As Chris noted, we did testing at OLPC and found that even
30 clients was pushing for most access points. At the time, I backed
that up with a literature search and could cite the various parts of
the 802.11 collision-avoidance algorithm which melted down in the 10s
of clients. I can't cite chapter and verse any more, sadly.
( http://cscott.net/ )
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