Rate Algorithm Ubiquity AirOS 3.4

 Rate Algorithm: defines data rate algorithm convergence:

Optimistic Algorithm is aggressive enough to move to a higher rate but yet tries to conservatively capture the fluctuations of the RSSI. It starts with the highest possible rate and then decreases till the rate can be supported while periodically transmitting packets at higher rates and computing the transmission time. The optimistic rate algorithm always looks to achieve highest throughput while sacrificing noise immunity and robustness.

Conservative Algorithm is less sensitive to individual packet failure as it is based on a function of number of successful and erroneous transmission/retransmission over a sampling period. It steps down to a lower rate after continuous packet failure and steps up after number of successful packets. The conservative rate algorithm provides the best case stability / robustness, but may compromise maximum throughput. It is recommended to select conservative rate algorithm when the signal strength is low due to noisy environment or link distance.

EWMA Algorithm is trying to move to a higher rate but is continuously monitoring the packet failure counters. The Exponential Weighted Moving Average (EWMA) Algorithm (also known as minstrel) is a hybrid of the Conservative and Optimistic Algorithm. It is the compromise for most of the wireless network use cases. 

sumber: http://www.ubnt.com/wiki/AirOS_3.4