Devices on roaming network not connecting to nearest access point
I have an extensive home network with several Airport Express (APX) devices (current firmware) identically configured (SSID, Automatic channel, 802.11n b/g/ compatible), connected to a wired backbone which connects to my router. I have several mobile devices, such as iPhones and iPads, and they all connect fine to a given APX. However, when I move from one room to another (becoming closer to another APX), the device I'm carrying doesn't reconnect to the closer APX (based on viewing the APX's wireless client list under Airport Utility), even after several hours. If I toggle the wireless mode on the mobile device off and back on, the device will associate with the closest APX. But without this toggling, it appears to remain connected to the APX with which it was originally associated. I don't know if this is normal behavior of if I have an incorrect configuration. If this is normal, I wonder if the signal must drop below a certain threshold before the device will try to reassociate with another access point?
Any thoughts or suggestions?
Thank you.