Why is our WLAN experiencing throughput dropping even where signal strength is excellent?

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Checking for interference is the right place to start. Signal strength is not the only indication of interference -- you want to look at Noise (background energy) and SNR (the ratio between Signal and Noise). You also want to look for excessive retransmissions and collisions. The tool that you are using, AirMagnet Surveyor, can plot co-channel interference and channel noise, based on surveyed measurements. If you have already gone through this process, have verified good SNR, and found no co-channel interference, it's time to consider other possibilities.

Because you are installing an 802.11g WLAN and have been able to overcome problems by changing channel, I have a hunch that you are experiencing 802.11g degradation due to the presence of 802.11b stations. 802.11g APs use a protection mechanism to reduce collisions when operating in the vicinity of 802.11b stations on the same channel. If protection is not enabled, collisions cause throughput to drop significantly. On the other hand, when protection is used, the mere presence of 802.11b stations increases WLAN overhead, causing throughput to drop. To learn more about protection, its impact on 802.11g WLANs, and what you can do to minimize impact, read this WLAN Advisor Tip.