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I see some APs allow setting the wireless mode to "G-only". I guess it means that it will set the basic rates to values higher than 11mbps. Is that right ?

I am checking an AP ( Linksys WAP54G ) with that G-only mode, and its beacons never show the "Use Protection bit" on even when I try to use some 802.11b to associate to that AP. I think even if the AP does not want to associate with 802.11b by settings the BSS rate to hight values, it still must set the Use Protection in order to prevent the interference with the 802.11b stations. Am I wrong somewhere ?

Thanks,

Vu

Hi Vu:

"G-only mode" is not defined by the IEEE 802.11 standard. A fair guess is that basic rates include non of the HR/DSSS rates of 1, 2, 5.5, and 11, and include only the OFDM rates that begin at 6. This means no HR/DSSS or DSSS clients may associate. If HR/DSSS rates are also supported then the access point and presumably its clients could use protection mechanisms such as CTS-to-self to share the RF space with HR/DSSS or DSSS clients of other nearby basic service sets.

Some say that "G-only mode" should also exclude the HR/DSSS rates from the access point's supported rates list. Such a configuration would prohibit the access point from using protection mechanisms even when the service set would benefit from them.

This topic has been controversial because some have observed that an access point that does not support HR/DSSS rates also does not use protection mechanisms and they have wrongly concluded that this configuration must not >>need<< protection mechanisms. This has led to sub-optimum configurations of ERP equipment that is actually more at risk of losing performance if an HR/DSSS system comes into range than if all HR/DSSS rates were supported.

I hope this helps. Thanks. /criss

Criss,

Thanks for the explanation.

I just made some tests and think they are interesting.

The protocol analyzer shows when the WAP54G is in G-only mode, it not only supports 1,2,5.5 and 11 but also 6,12 and 24 as basic rates.

Then I add basic-6, basic-12 and basic-24 to my 871W AP, all the 802.11b are lost the connection and the beacons of this AP shows no Protection bit as well as Non-ERP-Present. It also shows that the AP does see and undertands and replies to the Probe Request from the 802.11b .

Then I remove the basic rates for 802.11b from the AP. This time the AP still understands and responds to the Probe Req from 802.11b clients, but client cannot understand the response because this time the AP transmits the respond at 6 Mhz

Therefore I think for G-only APs, 802.11b may not cause noise, but it may be for G-only clients.

Vu

Vu Pham, quite interesting analysis, thanks for sharing. You are quite a help to us all.

Regards and keep it up :)