I bought recently a 2 TB Time Capsule machine (dual band) for backing up my network devices and also to extend my wifi-network on the top floor.
This discussion might help.
On my ground floor I have an internet connected dsl wifi-router (a Fritzbox) from my provider with the possibility of wifi on 2.4 GHz or 5 GHz or both. I have both and distinguished them by adding 2.4-GHz and 5-GHz to the ssid names.
On my second floor I have a Time Capsule, connected to the groundfloor machine (by ethernet wire).
I had configured the Time Capsule's wifi first as a new network with different ssid names than the machine on the ground floor, so that I could notice to which of the different wifi networks a device would connect. But then my devices did not (re)connect automatically to the stronger signal, though the were known. They kept as long as possible connected to signal of the machine below.
But when I changed the ssid names on the Time Capsule exactly to the ssid names below with the same passwords, the switch works automatically and perfect.
You can test it by walking with connected device (iPhone) from the ground floor to the second floor. Halfway I saw my wifi reception getting lower but more upstairs it took the stronger signal from the Time Capsule automatically.
I see a lot of discussions about extending wifi and using a Time Machine, so this experience might help.
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