offtopic question about high density wifi

david at david at
Tue Jan 26 14:21:22 EST 2010

On Tue, 26 Jan 2010, C. Scott Ananian wrote:

> On Tue, Jan 26, 2010 at 11:54 AM,  <david at> wrote:
>> On Tue, 26 Jan 2010, C. Scott Ananian wrote:
>>> On Mon, Jan 25, 2010 at 10:31 PM,  <david at> 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.

yes and no, if the RF stuff isn't done right the rest can't possibly work.

> 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.

this depends on your definition of 'reasonable access'

if every user gets 128k of bandwidth, but they are all able to use the 
system, I would consider it a success.

if half the users get 56M of bandwidth to the access point and the other 
half can't connect to the access point at all, it is clearly a failure.

there aren't local resources to be accessed (or at least, not many of 
them), almost all the access will be out to the Internet, and there is 
just not that much bandwidth to go around, one heavy user could saturate 

> 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.

this I don't understand, if it's not the RF congestion things that are 
running into problems (backoff time, etc), where are the problems 

>>> 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?

well, I have to either do it with minimal testing to not have wireless 
access at all. I don't like it, but I am stepping in at the last minute.

That is why I am asking questions

I am wanting to do whatever testing that I can ahead of time. Unlike the 
last 7 years, this year someone (me) went down to the event location ahead 
of time and did some data gathering to find what existing access points 
are around and what the effect of the building structure on signals is.

I have people telling me that I should put 1-2 access points per room 
based on problems they have had in past years with the access points 
refusing connections. I know that this cannot work just from an RF point 
of view.

I can buy a handful of new access points (but I have to decide what this 
week). If I buy high-end commercial devices I can buy 3-4, if I can use 
cheaper devices I may be able to buy a dozen or so. I expect that if I can 
get enough equipment to get coverage on the 5GHz band I can lighten the 
load on the 2.4GHz band significantly, but I can only get 5GHz equipment 
if I can figure out the real limits on what 2.4GHz equipment can sanely be 

other than deploying stuff for the event and then figuring out what failed 
to try and do better next year, is there a way to find out the limits for 
a particular piece of equipment?

>> 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 am, but it boils down to a choice between buying network equipment and 
paying for the expertise, or paying for speaker transportation (this is a 
large community event, at least the majority of the speakers are not being 
paied) Unfortunantly budgets were decided long before I got involved.

>> 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.

you can't put kiosks in conferance rooms

>> 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.

I am making the following assumptions

1. it is running on the 3 b/g channels
2. you can't change the clients
3. 802.11 doesn't effectivly do cooperation between devices for air time.

there is a lot that you can do to shape and control the RF footprint of 
your infrastructure, but you have no control over the footprint of the 
clients, if you put clients on the same frequency close enough togeather 
that they hear each other, but are in the footprint of different parts of 
your infrastructure, you end up with the same type of problems that you 
would have if you make the footprint of the infrastructure so large that 
you have lots of clients that can't hear each other.

David Lang

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