What I discovered — and what solved a problem that plagued me for almost two years — is that if you have different security methods that use the same network password, roaming works erratically rather than failing.
Far ago (about five-plus years ago), there were still plenty of bits of hardware and computers that couldn't use the slightly newer WPA2 Personal encryption protocol. Plain old WPA had been designed to be backward compatible with hardware that had shipped, including every computer released by Apple, since 1999. But WPA2 required newer hardware, typically only working on base stations and clients from 2003 and later.
Apple solved this problem in its base stations by offering mixed WPA/WPA2 Personal encryption. Select this, and your newer devices would connect using WPA2, while older ones weren't left out. (If you find that pre-2004 Macs won't join a Wi-Fi network and don't provide an error, that's why: the network is WPA2-only and the Mac doesn't understand why it's being rejected.)
In my case, I had one base station that used an ancient configuration file that I'd kept transitioning over the years as I moved from one model to another of AirPort, even as I added two more base stations in the house connected via ethernet that I configured as passthrough. Macs and iOS devices sometimes lost a connection in one room of the house or strained to reach a base station several rooms away rather than the nearest one!
In helping to troubleshoot a wily Wi-Fi problem weeks ago, I went into the innards of my base station to check my own settings, and, lo, there was terribleness.
If you launch AirPort Utility, select your base station, and click Edit, and then select the Wireless tab, you can see what security option is set. As you can see in the accompanying figure, while WPA/WPA2 Personal is an option in the Wireless Security menu, choosing it was my downfall because I had just WPA2 Personal enabled on two other base stations.
When I switched that main base station to WPA2 Personal, all my roaming difficulties went away. If you're seeing similar strangeness, check that all your base stations use absolutely identical security settings.
Ask Mac 911
We're always looking for problems to solve! Email us at firstname.lastname@example.org or tweet them at me (if brief) @glennf. Mac 911 can't provide direct email responses or answers for every question. For that, turn to AppleCare, an Apple Store Genius bar, or the Apple Support Communities.
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