In the AirPort Admin Utility, set base station density to "High." After doing that, you should be able to set the Multicast rate to 11mbps.
In networks with base stations that are close together, setting base station density to High achieves higher transfer rates by forcing a client to look for a new base station when the signal of its current base station goes below 11 megabits per second (Mbps).
Originally posted by kendall
move me please to the correct forum