On Tue, 26 Jan 2010, C. Scott Ananian wrote:
On Tue, Jan 26, 2010 at 11:54 AM, <[email protected]> wrote:
On Tue, 26 Jan 2010, C. Scott Ananian wrote:
On Mon, Jan 25, 2010 at 10:31 PM, <[email protected]> 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
it.
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
happening?
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
used.
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
_______________________________________________
Devel mailing list
[email protected]
http://lists.laptop.org/listinfo/devel
