Well for one, if you have one ap and you want to configure N on the 2.4ghz and also support G, then that would be okay. If you ave more than one 1252 and you want to configure N on both using the 2.4ghz, then you will run into interference issues. If more than one ap, you should run N on your 5ghz radio.
802.11n is setup by setting the 5ghz to a channel like 36 and then configure 40mhz channel above, which will use the next adjacent channel which is 40. Or you can use 40 and configure using 40mhz channel below which it will use 36. You have to set the channel and power before you can set the 40mhz channel width.
-Scott
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