Usually the WLAN throughput will drop as the interference increases. The signal generated by other ISM devices (from WLAN prospects, it is the interference) is relative high level and can be measured by a spectrum analyzer. Turn off the WLAN equipment and on the ISM devices, then measure the interference. Turn on the WLAN equipment and off the ISM devices, then measure the 11b/g signal. Now you get the S/N. Turn on both ISM and WLAN devices to measure the throughput. Adjust the tx power of the WLAN device with monitoring with a spectrum analyzer, you can get different S/N.