It's impossible without a spectrum analyzer or specialized sniffing software to see what's truly happening in these bands, but you can get a survey of what other Wi-Fi networks are present with Apple's built-in Wireless Diagnostics in OS X or the free iStumbler. Apple's program can be launched by holding down Option, selecting the Wi-Fi menu, and choosing Wireless Diagnostics. Then pick Scan from the Windows menu.
Split the 2.4 GHz and 5 GHz band. Nearly all modern 802.11n and 802.11ac routers, those using the latest two Wi-Fi specifications, accept connections simultaneously over both frequency bands using the same network name. However, you can create unique network names. This lets you force a computer or mobile device to connect to 5 GHz, which might be preferable to reduce interference or produce higher throughput.
In AirPort Utility, select a base station, click Edit, click the Wireless tab, and then click Wireless Options. Check the 5 GHz Network Name, and enter a unique name. Click Save and then Update, and the base station restarts. Now choose the 5 GHz network name from any devices you want to force onto the 5 GHz network, and remove the original network name.
In iOS, tap the i button next to the network you want to remove, and then tap Forget This Network. Confirm by tapping Forget. In OS X, in Network Preferences, select the Wi-Fi adapter, click Advanced, select the network in the Preferred Networks list, and then click the - (the minus sign) and confirm by clicking Remove. Click OK, and then click Apply.
You can also use AirPort Utility to see what band and at what speed any given device has connected. Click a base station, hover over a name in the Wireless Client lists, and you'll get a popover. The PHY label will say b/g/n for 2.4 GHz and a/n or a/n/ac for 5 GHz.
Make sure your ethernet cables aren't twisted or degraded. A bad or failing cable can still work — it's not an all or nothing proposition. But you'll see strangely low or inconsistent rates. Examine your cables between a broadband modem and a router, and if you have a spare, try swapping out cables to see if your problems disappear. This has solved some baffling consistency issues for me after all else has failed. (Also, you need what is called Category 5E or Category 6 cabling — Cat5E and Cat6 for short. Monoprice sells a 7-foot Cat6 ethernet cable for $1.67, for instance. Older "Cat5" will generally work, but it's not guaranteed to be up to the task, especially over longer runs.)
With multiple base stations, make sure they're all using the same security. In troubleshooting Aaron's mystery, I may have solved one of my own. We have three base stations in my modestly sized house in order to fill in poor coverage areas at opposite ends of our partly finished basement. The automatic handoff (roaming), that's supposed to work seamlessly, fails frequently.
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