Rainer Brockerhoff wrote:

I had some difficulty in getting the built-in Airport Extreme (802.11g) card to interoperate with my SMC Barricade 7004AWBR wireless router. I tried several things – setting the MTU to 1492, changing from fixed IP to DHCP, downgrading the Airport software from 3.1 to 3.04, and trying out different WEP encryption options. I’ve had no time to regress some of these things to see which specific one was the culprit, but it’s working now.

I took some time to reinstall all of these things and it still worked. Finally I realized I had switched from Channel 11 (recommended to reduce interference by mobile phones) to Channel 1 at some point in the process.

Testing most of the channels revealed an interesting pattern. Channels 1 to 3 work OK. Channels 4 and 5 work either intermittently or very slowly. Channels 6 to 11 don’t work at all! (Channels 12 to 14, supported on the SMC, apparently aren’t supported by Airport at all.) I’ll try to check this out on the Apple support boards – several people were complaining about similar problems, but I saw no reference to channel incompatibility.