If I have a roaming network (BSA) and all of the clients are 802.11n compatible. By setting the mode to 802.11n only (instead of having two of three AP on mixed mode 802.11b/g/n) will this improve the speed/throughput?
from your inquiry I assume we are speaking about 2.4 GHz network. 802.11n only (aka Greenfield) uses different physical layer preamble, which is not understandable by legacy 802.11b/g nodes. This means that devices working in 802.11n and 802.11b/g/n (aka HT-mixed) cannot hear each other.
WiFi is a shared media and just one device can send the data at one time moment in a single direction, other nodes just listen for a next time moment.
Then, if we want to compare the performance, the main parameter is a modulation/data rate.
If you have 802.11n fast stations connected to AP in HT-mixed mode, the throughput should be high and very similar to that where AP would be in Greenfield mode. But practically very important factor
is an environment/interference. 2.4 GHz networks are usually crowded (just 80 MHz of spectrum available) and most likely there will be some legacy devices, which will cause direct interference to 802.11n conversation. To have better coexistence and speed AP should use IEEE 802.11b/g/n in 2.4 GHz and that is a default at Peplink/Pepwave devices.
Final answer: setting IEEE mode on AP from 802.11b/g/n to 802.11n (when stations 11n only) won’t improve the performance. It may be ever worse depending on environment.
If you could indicate next time what products you have or plan to use we would be more specific
Thanks and Regards