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i have read many documents about wifi planning design, i wondered that why do we need to take into account the concurrent users in the air? for all i know, 802.11 clearly define that only one packet can be in the air at a time. so under the csma/ca conditions, there are no concurrent transmission. so how did they(cisco and aruba) think about?

thanks in advance.

I am not totally sure of the question but I will take a stab at some of it.

First, 802.11 uses csma/ca (Collision avoidance). That means that collisions do happen and the more stations you have the better the chance of it.

Wireless is like a public water fountain, only 1 person can use it at a time on a given channel. If you have 50 people trying to use it, there is going to be a wait.

I have seen access points stating that they can handle 100 users. I am not talking about some nice array like Xirrus uses, just a standard 802.11n access point.

That many users could not be doing much when connected to 1 access point. Think about it, maybe 300Mbps throughput, you get about half that so 150Mbps throughput. Stick 100 people on there fighting for that and you get very little throughput. Not to mention if it is set up wrong, you may have some guy at 2Mbps trying to download something and killing it for everybody. That would be like waiting on the super slow guy at the water fountain.

In a perfect wireless environment there should be very few concurrent transmissions, but that isn't the real world.

May I ask what do you mean about Aruba and Cisco? Can you please elaborate.

Hope that actually answered something hehe.

Neal,

I really like the 'water fountain' analogy--helps illustrate the half-duplex communication of 802.11.   I may use it in a future presentation.

Good explanation and illustration.

qq,

There used to be a really good pdf on arbitration on this website.   But I can't find it now.

The link shown on this page,  http://www.cwnp.com/hot-off-the-press-arbitration-whitepaper , only gets a 404-Not Found.

If someone can restore it, it should answer your questions.

Below link works.

http://www.cwnp.com/wp-content/uploads/pdf/802.11_arbitration.pdf

Thanks.

That's it !

yes, thanks, your explanation is very clear, i had trouble to understand the airtime before.

thanks everybody.