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I run a wireless network WPA/TKIP.

My router is a Billion 7402G (it's getting old).

I have a Macbook Pro that I use daily for work and is more or less constantly connected.

The router is located in the living room, my office is downstairs and to the right of the router, I have excellent signal strength in the office.

Whenever a Windows machine (Windows 7 or Vista, not sure about XP) joins the network my Macbook loses connectivity to the outside world. I have no internal shares so am not certain whether it remains on the network. If I turn Airport off, then back on again, it can see the network but cannot join it, unless I walk upstairs physically closer to the router, where it begins to work again.

Once the Windows machine is off, everything works again.

The router is set to DHCP however all machines in the home are assigned IP based on MAC address, I was hoping that would resolve the issue but seemingly has failed to do so.

This occurs with my Windows 7 laptop (IP assigned via MAC address) and any guest's Windows laptop that are full DHCP.

Why does a Windows machine connecting to the network result in my Macbook losing connectivity?

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Does this also happen any time you connect a non-windows computer, like another mac? It shouldn't be possible for a given computer to cause another computer to be unable to join the network based on the operating system. From your description it sounds like you may be having interference problems, but it's hard to tell; more testing would be helpful. – nhinkle Nov 11 '10 at 0:41
Just fired up the Ubuntu laptop - everything fine. Brought the windows laptop down to the office and started that there, again everything fine. Took the windows laptop upstairs, probably equidistant to the router, everything fine. I will attempt to start the windows laptop without the Ubuntu laptop on in case that is having some sort of additional impact. – tojofo Nov 11 '10 at 1:22
Starting the Win laptop in the office without the Ubuntu laptop running has no impact on the Macbook, back to trying different locations upstairs. – tojofo Nov 11 '10 at 1:57
Starting the Win laptop in it's usual resting place just now hasn't impacted the Macbook. I'm starting to think it might be related to the first boot of the Win laptop for the day. I'll leave it for a while now and repeat tomorrow. – tojofo Nov 11 '10 at 2:14

If it FOR SURE happens only with windows joins the network, there was an issue with windows appending garbage to ssids that affected kismet for a while. It could be the driver on you macbook is trying to parse that and failing.

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