How much Mb ?

hi there.

I have two RB411 with R52 cards antenna 23dbi distance 4Km and the singal is -56 Point to Point

How much Mb can transmit here ?

under good condidtions you should get full 54mbps sync
with 35Mbps mbps TCP throughput in one direction
or roughly 17mbps tcp throughput full duplex.

thanks