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I know B only AP won't work with 802.11g client.
Would somebody tell me why?
B only AP basic rate 1 2 5.5 11
No basic rate in G only or BG mixed client,
I cannot tell the difference between mixed mode and g only client(in probe request).
Mixed mode client works with B only AP.
Why G only client cannot work with B only AP?

thanks a lot.

An 802.11b access point should work with an 802.11g client. 802.11g is backwards compatible with 802.11b. The 802.11g client should be able to behave exactly as an 802.11b client would, unless it was prohibited from doing so by a setting in the client utilities or drivers or whatever.

If you have a "G only" client, then the client will only transmit using OFDM, not the DSSS modulation used by 802.11b. Because the 802.11b AP cannot understand OFDM modulation, the 802.11g client cannot talk to the 802.11b AP.

The difference between "mixed mode" and "G-only" in APs is usually whether the AP will allow 802.11b clients to associate. In "G-only", the AP rejects associations from 802.11b clients. In "mixed mode", it accepts them.

The difference between "mixed mode" and "G-only" in clients is usually whether the client will allow itself to pretend to be an 802.11b client and transmit DSSS instead of OFDM.

Hi Yang:

I find the terms "B-only", "G-only", and "mixed mode" configurations to be more confusing than helpful. Few can accurately define a "G-only" station or explain its disadvantages.

Here is my definition: A "G-only" station is an IEEE 802.11 ERP station which has been configured not to transmit ERP modulations that are also HR/DSSS or DSSS modulations.

This is accomplished in an access point station by configuring the basic rates and supported rates lists. A client station accepts the basic rates list of its BSS and may or may not allow its supported rates list to be configured.

An ERP station is described in IEEE 802.11 clause 19, brought to us by the IEEE 802.11g amendment, thus popularly called by the letter G. An ERP station can transmit and receive all HR/DSSS and DSSS modulations, plus higher data rate modulations peculiar to ERP (and borrowed from OFDM). An ERP station is backwardly compatible with its nonERP cousins.

A HR/DSSS station is described in IEEE 802.11 clause 18, brought to us by the IEEE 802.11b amendment, thus popularly called by the letter B. A HR/DSSS station can transmit and receive all DSSS modulations. A HR/DSSS station is backwardly compatible with its DSSS cousins.

A DSSS station is described in IEEE 802.11 clause 15, brought to us by the IEEE 802.11-1999 base document, thus popularly called, uh, nothing.

If an ERP station is configured to be "G-only", there are two immediate consequences. First a "G-only" ERP station has lower range, since it cannot utilize the lower data rate modulations it shares with its nonERP cousins. Second a "G-only" ERP station cannot, and does not, use the CTS-to-self protection mechanism designed to protect it from nearby nonERP stations.

A normal ERP station begins using protection as soon as it detects a nearby nonERP station in its own or an over-lapping BSS, reducing its maximum ERP throughput. A "G-only" ERP station continues to use its higher data rates exclusively, and accepts having to retransmit all its frames corrupted by overlapping nonERP transmissions. The result is often a lower throughput, but still higher than when using protection. This is the advantage that has "G-only" advocates cheering. However, if nonERP stations are especially active, "G-only" throughput can be reduced to a level lower than when using protection.

"G-only" advocates disallow nonERP association by eliminating all nonERP rates from both the basic rates and the supported rates lists of their access points. The better way to disallow nonERP association is to include in the basic rates list at least one ERP rate (modulation) that is not also a nonERP rate.

My advice is to eliminate nonERP stations as best as one can but never disable ERP protection mechanisms or limit ERP range. Or avoid the ERP/nonERP issues entirely and use clause 17 OFDM stations (also known as 802.11a)!

I hope this helps. Thanks. /criss

thanks a lot.
I am not sure "G only" client will only use transmit OFDM, no DSSS.
G only client's 802.11 manamement packet will be transmitted in Data Rate "1". In sniffer, probe request and association request are transmitted in 1. So I guess maybe G only also use DSSS.
I am confused about how "G only" client refuse to associate to B only AP. Can I find the secret in sniffer ?
thanks for your help sincerely

Hi Yang:

Your question goes directly to the next stage of the "G-only" story. The short answer is that an ERP client station only becomes "G-only" after associating to an ERP access point that has been configured to be "G-only".

Above I wrote:

"Here is my definition: A "G-only" station is an IEEE 802.11 ERP station which has been configured not to transmit ERP modulations that are also HR/DSSS or DSSS modulations.

"This is accomplished in an access point station by configuring the basic rates and supported rates lists. A client station accepts the basic rates list of its BSS and may or may not allow its supported rates list to be configured."

While theoretically a client station configuration utility could expose its supported rates list for configuration, I have not found one that did. Granted my field experience is small, but I asked several experts with vast field experience and still have found no report of an ERP client utility that allowed its supported rates list to be modified. Thus there appears to be no way to >directly< configure an ERP client station to not use nonERP modulations. Instead, an ERP client station follows the lead of its access point.

Many access point configuration utilities expose both basic rates list and supported rates list for configuration. Essentially basic rates are those that each station in the BSS >must< be capable of transmitting, and supported rates are those that each station in the BSS >may< use for transmitting. Unselect all the nonERP rates and, voila, you have yourself a "G-only" BSS with reduced range and no way to protect ERP transmissions from nonERP neighbors. Cool. (silly facial expression here)

There is another way that an ERP client follows the lead of its ERP access point. An ERP access point sets a "use protection" bit in its beacons under several circumstances involving nonERP stations operating in the vicinity. This causes itself and all its ERP clients to begin using protection mechanisms with attendant increased overhead and reduced throughput. An ERP access point with all its nonERP rates disabled never sets this bit because protection mechanisms require the use of nonERP rates and those have been disallowed throughout the BSS. Consequently all ERP stations in the BSS simply soldier on in the face of withering retransmissions when the nonERP stations in the vicinity get busy (and stay busy doing their own nonERP retransmissions; at least this is an equal opportunity meltdown!).

So called "G-Only" operation is appealing, until it is thoroughly understood. It is not part of the IEEE standard. It is not a selection button in product utilities (at least I hope not). Avoid it and help others to do the same.

I hope this helps. Thanks. /criss

Actually there are quite a few products providing "G-Only" "buttons" or configuration modes. All this does is turn off support for the .11b data rates so only .11g clients can associate in that environment. It's not really a RF Engineer/Enterprise WLAN-type setting, it's more of a SOHO setting put there as a marketing tool (just my opinion). You'll see boxes marked: "New G-Only Capability! Optimized for 54Mbps WLANs!" Of course, we've all been able to do this ever since .11g was released, it just wasn't as easy as clicking the "G-Only" button.

Hi Joel:

Thanks for this update.

It was a year ago I did my searching.

Is this button in the access point configuration, client configuration, or both?

Can you confirm that customers who press this button leave their ERP stations without protection in the face of nearby nonERP traffic?

Thanks. /criss

I'll pick on Linksys to answer this question since I like their (our) products so much.

In the Linksys user manual for the WRT54G Wireless Broadband Router/AP, there is a section that states:

From the Network Mode drop-down menu, select the wireless standards running on your network. If you have both 802.11g and 802.11b devices in your network, keep the default setting, Mixed Mode. If you have only 802.11g devices, select G-Only. If you have only 802.11b devices, select B-Only. If you want to disable your wireless network, select Disable.

In the Linksys user manual for the WPC54G B/G client card, it states:

Network Mode - Select Mixed Mode, and both Wireless-G and Wireless-B computers will be allowed on the network, but the speed will be reduced. Select G-Only Mode for maximum speed, but no Wireless-B users will be allowed on the network.

UPDATE: The client card setting above is only available in the Ad Hoc mode configuration and is not available when the client is connecting to an Infrastructure WLAN, i.e. it's basically an "AP-type" of setting since the client card would be communicating only with other client cards also in Ad Hoc mode.

I haven't done a trace on the g-only settings so no, I can't confirm the ERP vs nonERP traffic.

Joel

Hi Joel:

Cisco/Linksys is doing a very good thing if they have defined "G-Only" to mean no more than "have to be ERP to associate". This is accomplished by adding at least one ERP rate that is not also a nonERP rate to the basic rates list for the BSS. I hope this is what the Linksys button does.

Others use "G-Only" to mean "no nonERP rates used in this BSS". This disables ERP protection and reduces ERP range.

Here is an opinion from Jim Geier:
http://www.wireless-nets.com/blogger/2005/03/improve-performance-with-g-only.html

If "G-Only" only adds to basic rates, I am for it.
If "G-Only" subtracts from supported rates, I am against it.

So which is Linksys doing?

Thanks. /criss

No idea.