Where should I place my Airport Express to boost my signal?
When people are designing their Airport networks, they often assume that if they have a low signal they can use a second Airport base station as a WDS remote base station in that location to boost the signal.
If you have a low signal then the Airport Express in the location of a low signal can only "boost" that low signal it can not make it better. so you don't actually gain any benefit and will actually have a more degraded signal as WDS effectively halves the bandwidth, so though your client's signal strength will be high the actual strength of the signal will be effectively half what it would be if they were wirelessly connected direct to the main base station.
In order to effectively boost a signal, you need to place the remote Airport Express at a location where it is getting a decent signal from the main base station (which will be between the main base station and the remote location (in this example named office)) and then can "boost" the signal to the office.
The first diagram shows using a single base station, the client is getting a low signal in the office.
The second diagram shows placing the second WDS remote base station in the "wrong" location, though the client is getting a strong signal, as the Airport Express is getting a very low signal from the main base station to begin with, the "strong" signal is in fact much weaker than if the client connected direct to the main base station.
In the third diagram, shows placing the second WDS remote base station in a "better" location, though the client is getting a weaker signal than diagram two, as the Airport Express is getting a much stronger signal from the main base station to begin with, the "weaker" signal is in fact much stronger than if the client connected to a WDS remote station in its location.