Connection speed vs. signal/noise levels

Hello!

Could someone explain to me how the actual connection speed is choosen? Does 
it depend on the signal/noise levels or are there also other factors that 
influence the speed setting?

Sometimes I notice that speed is reduced from 54 to 11MBit/s or below, while 
the signal and noise levels are rather constant and at good values. So I'm 
wondering what other reasons there might be why the card would reduce the 
speed if signal quality is good?

thanks,
Max