Help me to understand b-g protection in wifi

https://en.wikipedia.org/wiki/IEEE_802.11

See Control Frames.  Or do you mean Ruckus "Air Time Fairness"?

Airtime fairness, as currently implemented on Ruckus APs,
carves up airtime uniformly across client devices – the AP will take no more
than 4ms transmitting to any one client before moving on to the next.

Historically this prevents slow clients from dominating
airtime and ensures fast clients an equal opportunity.  

With 802.11ac, frame aggregation allows 11ac clients
to dominate airtime because the aggregated frames are so large that they can
take longer to transmit than legacy client frames. 

ATF 2.0 implements enhancement to balance 4ms client traffic as long as there are frames in the lower queue, else fast client higher rates.