[AusNOG] Group WiFi

Trent Lloyd lathiat at bur.st
Wed Oct 31 21:15:46 EST 2007

Hi Stephen,

On 31/10/2007, at 4:18 PM, Stephen Baxter wrote:

> Hey Guys,
> After some feedback here on doing wireless for a conference. What
> wireless access points work best together ? Is it just as simple as
> running some WRTG54 on the same SSID in the same space and they all  
> play
> happy together or are some products and how they are deployed better
> than others.
> I need to get wireless access for 130+ people for a couple of days. If
> you want to know why then please go to http://2207.ausnog.net :-)    
> Any
> advice appreciated.

This is actually no small task.  Delivering reliable wireless to a  
mass of people is very difficult, but there are a lot of things that  
can give you a good head start

1) Channel Spacing

While 802.11 has channels 1 through 11 (and sometimes 12, 13), they  
actually overlap.  That means if you use channel 1 and channel 2, they  
will actually cause interference with each other.

In order not to overlap, you need use the channels 1, 6 and 11.  Any  
closer together (i.e. 1, 5) and they will interfere.  This really  
isn't a problem on a small scale but on the big scale with lots of  
radio noise, it makes a big difference.

The trick is to try and space your access points apart such that any 2  
APs on the same channel are as far away from each other as possible.  
[as a general rule, anyway]

2) More is not necessarily better

Just having more APs isn't necessarily better, as you can actually  
cause more noise - but you can't just have 3 either.  You need to  
strike a balance, but putting 50 APs in a small are is a bad idea

3) Provide wired access

This might seem obvious to some, but some conferences such as  
Linux.conf.au last year actually denied wired access and setup only  
wireless.  This is a bad idea because you want to get as many people  
off your wireless as possible, but there are also other concerns like  
tripping over cables and the likes.  But if you have a desked area,  
setting up some switches is a fantastic idea.

I've also seen a bunch of couches around a central area with a switch  
and power in the middle but exiting directly behind your chair so you  
dont have to walk over other peoples cables.   This also seemed to  
work well.

4) Turn the AP power down

If you have quite a few APs, it can help to turn the transmit power  
down so it only goes a short distance and interferes with other APs a  
short distance away less. But careful not to create holes in your  

WRT54s are a good example of an AP that will let you do this.

5) Consider a few 802.11a or 802.11n APs on 5.4GHz

Not the majority, but some laptops (and in fact alot more of the newer  
laptops) support 802.11a @ 5.4GHz, or 802.11n @ 5.4GHz - even if only  
10% of your users can make use of this, thats still 10-20 people that  
will get off your 2.4GHz band and can make a big difference.

This works better at more technical conferences, because the more  
knowledgable users might force their machines onto 5.4 in an attempt  
to get better wireless quality.

So if you have a couple of these, and deploy 1 in your big busy areas,  
this can help also.

6) Consider rate limiting

Consider rate limiting per-ip to stop any 1 person hammering it.  This  
may not be necessary but can help.

7) Multicast/Broadcast blocking
Also consider blocking broadcast and multicast traffic between APs  
(usually easily done by your switch in the middle).  I hate to suggest  
this being one of the authors of Avahi which uses multicast... (and  
this jacked me off at linux.conf.au when they did it).. buut - this  
cuts down your basic rate traffic which is also very helpful.

Hope this helps! You don't necessarily have to do all of this, but the  
_most_ import is number 1.  If you have any questions please let me  

Also interested in hearing others feedback on either what -else-, or  
any points about anything I said.


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