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A station 11b associates to AP which support both b and g mode.:

It means that AP will adjust performance follow 11b data rate and all existing 11G station associated will be reduced throughput, ?

and it also affect to transmitting between 2 11G stations ?

thanks

Yes. In an 802.11b/g environment, if a b client associates to the AP the throughput with automatically drop to 50%. When a AP detects an B Client, it will enable protection mode (802.11G protection mechanism) . It sets the use protection and non-erp client flags to 1 within the beacon. Also, if an AP hears another AP beacon with the use protection flag set to 1, that AP will also set it's use protection flag to 1 even though there arent b clients in the area. When this occurs, the non-erp client will be 0 and use protection will be 1.

Thanks simpon clarified

One question about this: is this per AP or per SSID? I've got 5 SSID's on my AP's and only one has clients that are b. Does the whole network slow down? I'm guessing yes, because it is the same medium and you wouldn't be able to segment one SSID from the other.

David

Good question. It's per AP. That's why you have to avoid Co-channel and Adjacent cell interference. If your access point hears another b client or any other beacon with a b client (or use protection = 1) it will affect your network regardless if it's a part of your network or not. That's why a good site survey is a good idea.

If your APs are close together or on max power, an ap on channel 1 and channel 6 can hear each other(by the side lobes) and that will decrease throughput.