offtopic question about high density wifi

[C. Scott Ananian]

On Tue, Jan 26, 2010 at 11:54 AM,  <david at lang.hm> wrote:
> On Tue, 26 Jan 2010, C. Scott Ananian wrote:
>
>> 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.
>
> right, this is the 'simple' issue of RF congestion
>
>> 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.
>
> this is the issue I am most worried about. the RF issues I can measure (and
> to the extent they are outside my control, like the clients, I just have to
> live with), but finding that an access point only allows 64 clients and then
> rejects everything after that is not something I know how to predict ahead
> of time.

In case I wasn't clear, this should be the least of your concerns.

Be grateful if the access point limits itself to the number of clients
it can actually handle -- at least those 64 will get reasonable
access.  Your more common problem will the access points which *don't*
limit themselves, and you run into the subtle protocol issues I
mentioned and everything melts down for everyone.

I'm *not* talking about RF congestion.  The protocol melts down far
before your bandwidth gets saturated.

>> 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.
>
> part of the reason I was asked to step in on this is that the last time they
> ran their own wireless, they went out and purchased a bunch of 'pro grade'
> access points, and things did not work (I think I've discovered that they
> limit connections to 64 nodes)

Right.  Find someone who's done it, and copy their setup.  Don't just
go out and read the box for something and blindly expect it will work.

> part of the problem is that, on the RF side, things depend so much on your
> exact environment (will the hotel turn off their wireless, what is the
> impact of the wireless from the 5 hotels adjacent to you, how much do the
> walls of this particular building block/bounce the signal, etc). no
> professional is going to tell you how to set things up without doing a bunch
> of (expensive) testing.

There's a reason for that.  You think you can do without the testing why?

> useing what other people use in terms of equipment can help, if you have the
> budget to buy all new equipment

Operating a 1,000+ node wifi network will be expensive.  Perhaps you
should start by talking to the budget folk for your event.

> I very much don't expect to get it 'right' this year, but I am hoping to
> avoid outright collapse (well, at least beyond the predictable 'people are
> trying to use more bandwidth than you have' issues)

Then my advice is: lots and lots of wired kiosks and switches.  Assume
they will be the only available internet.

> the problem with the 'lots of directional antennas' approach is that you hit
> the point of diminishing returns because while the directional antennas make
> the access point only see a few of the clients, the clients are all seeing
> each other, and so you end up with the hidden node problem.

You seem to be assuming a lot about how this piece of wifi gear worked.
  --scott

-- 
                         ( http://cscott.net/ )