Better WiFi

The wireless on campus is rather spotty, at best. In certain rooms, I think my Macbook is picking up the access point on the floor below when in reality there is one with MUCH better strength next door.

Since they all have the same SID, how does my laptop decide which access point to connect to and is there anything I can do to better the decision making process?

It's weird, when I'm in this classroom the signal is almost non-extant and packet loss is very high; but I can move two doors down on the other side of the room and the signal is great. There is nothing unusual in any of these rooms that would cause this behavior, so it must be selecting the access point from downstairs.

-Chri