say you have a D-LINK 524 Airplus
then you probably have 2 or 3, or more computers around your home, all connected via wifi .
you are downloading your favorite movie from the net, when someone decides to send a large file between 2 other computers in your network.
effect: one or several of your computers will disconnect from the wifi, or at least they will be dropped for a few minutes.

I found documentation on the net, about fragmentation, threshold, beacontimes, etc, but it's all useless.
So, fooling around I found out that the Fragmentation field, set by default to 2436 , is the "optimistic" field.
In any computer network connection you have something called MTU, the MTU is the size of the default packet when sent across the connection.
And this number is usually at 1500
So, I lowered my fragmentation size to 1700 (say to leave some room for headers, etc), and voilla, the shit works !