Overlapping BSS/NAV Issue

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Jun 22nd, 2010
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Hi All,


Just a theoretical question here. Both networks in clear with no encryption or authentication.


If I had an AP that was on my location and it was for corporate data traffic for my employees - on Channel 6 - SSID Corporate


Then


There is a coffee shop AP next door again on channel 6 and both APs are very close together - SSID Coffeeshop


The medium is shared, so all my wireless employees would receive the coffee shop traffic and the coffee shop devices would receive my corporate data traffic. Is that correct?


Also, if a coffee shop packet is transmitted into the RF, and I receive it, my corporate data traffic would receive the packet, set the NAV on my corporate client and defer sending until the medium is free again, is this correct?


Many thx indeed,

Ken

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Elliott Shawd Wed, 06/23/2010 - 18:20
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You can't receive wireless traffic that isn't destined for your client unless you put your wireless card into "promiscuous mode." The only thing your wireless network could be suffering from is interference. When you have two things on the same radio frequency, in the same proximity, there will be interference which can degrade your wireless signal in all sorts of different ways. If your wireless is set up as you say it is where you are on the same channel 6 as a neighboring coffee shop, I would change the channel of your access point to either 1 or 11 as soon as possible. The separation between each 1, 6, and 11 is large enough to where they do not interfere with one another. Keep in mind, that if you have more than 1 access point in your office, you cannot just change the channel of just one of the access points...

kfarrington Thu, 06/24/2010 - 00:56
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Hi Elliott,


Im kinda confused, so please bear with me mate please


The RF cells are overlapping (co-channel interference).  (Please see jpeg image)


Question 1a

I was under the impression that the RF medium was the same as ethernet medium.  All traffic is flooded out to the medium, then every STA in that medium receices that frame, then inspects the mac layer address and if it does not contain a destination MAC layer address of itself, the STA simply drops it.


Question 1b

So with the situation of two networks operating in the same frequency, the coffee shop user sends a frame into the air (lets just say is is simply browsing the web and does a HTTP request and sends it to his default gateway),  all STAs and APs on corporate network and coffee shop network receive this packet,  the coffe shop AP is the default gateway so accepts it, all other devices drop the frame.


Question 2

So if both parts to question 1 are correct,  when the coffee shop user sends that frame, I would assume all STAs and APs on coffee shop network and corporate network, that operate in that frequency, will receive that frame, look at the duration field in the frame and set their NAV field to defer sending for the NAV.


Could we please clarify this point.


Many thx indeed for the great help,


Ken

Elliott Shawd Tue, 06/29/2010 - 08:11
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I don't know enough about RF to give you a definite answer, but I'm guessing you are on the right track.


I'm wondering what your point is in figuring this out. Yes, your wireless adapter maybe intercept packets not destined for it and discard them, but the client never sees them. In order to actually see them, your card has to be in promiscuous mode.


Regardless of what you are trying to do though, you should change your channel to either 1 or 11. You are suffering from co-channel interference even if you don't notice it. Your wireless network would be much healthier on one of those two proposed channels.

George Stefanick Wed, 06/23/2010 - 20:32
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Great question. you want to insure you have about 25 db different between same channel access points.


So if your next to your ap and it reads 40 dbm your next ap on channel should be at 75 dbm or higher

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