Data rates if slow stations present

[Tobias Gunkel]

Hello,

I have a general question regarding the behavior of an AP if slow stations are present. 
Imagine there are two stations - one 802.11n capable station and one that is capable of only 802.11b. 
Now the 802.11n station connects to an AP and is able to transfer data with the lets say 150 MBit/s.
After a while the 11b only station connects to the AP but as the SNR is very bad it can only transfer at a 1 MBit/s rate.

What happens now with the data-rate of the 11n station? Will this station switch to 11b two or is an AP able to manage stations in different modes at the "same" time? At least the 11b station will occupy the channel for some time so that the 11n station will have less time-slots for data transfers so the data rate will drop at least 1 MBit/s (probably more because the 11b station needs more time for transferring 1 MBit than the 11n station).

Is the behavior of the AP standardized? I already had a look into the 802.11 standard but could not find a description of the behavior of AP stations (there is only a very short Annex N: AP functional description). 
What is the behavior of hostAP in this case?

Thanks in advance for answers or pointers to other sites, standards, ...

Regards,
Tobias Gunkel