WLAN Performance parameters - configure AP or Client?

Tried googling. Couldn't find it. Maybe that's just me though - someone smarter than me, please try.

If I wanna change certain WLAN performance parameters say, RTS threshold or fragmentation, do I wanna fuck around with the AP or with the Client PC's (all of them?!?!) or Both?

Also, how do I find out if I have a connection going on.. if these parameters are what I want them to be?

Could the AP and the client be communicating with each other while both of them using different parameters? So I'd need to configure both of them….

Would make sense now that I think about it…

But still.. ???