G wireless throttling down for B clients

Unanswered Question
Mar 11th, 2008

I heard Cisco came out with away were the G clients could still run on ofdm and not be knocked over to dsss when a B client join the wireless network. My question is what does Cisco call this and what version did it come out in.

I have this problem too.
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ivillegas Mon, 03/17/2008 - 12:27

If the B clients join a G network data rate would reduec. In order for G to have maximum performance it is better not allow G clients. This can be accomplished allowing only G data rates on access point.

dennischolmes Tue, 03/18/2008 - 16:28

Sorry guys, this is a misconception. Part of the 802.11 standard states that the .g access point must be backwards compatible with the .b standard. To achieve this the AP switches to compatiblity or protected mode. This reduces the overall speed of the .g AP. You can tell your WLC that an SSID is G only but that will not stop protected mode from being enforced.Its part of the IEEE standard.

thefranmaneds Tue, 07/29/2008 - 13:37

This is what my Cisco Rep told me and he asked the local Cisco wireless guy who confirmed it. Of course he didn't give me any deals on the what it is called or the version.

We are having issues at sites with mixed B only APs next to G/B APs. I was hoping to figured out if it may be a code issue.

neilhall Wed, 08/20/2008 - 07:38

Does anyone know if the throttling down of G is specific to the AP or specific to the SSID?

For example, if I have a secure G SSID that I can ensure will never have a B client on it, but I also have a public B/G SSID on the same AP, will the secure G SSID users be slowed down when a B client connects to the public SSID or will only users on the Public ssid be effected?

In case it matters, this is in an LWAPP environment.

Thanks.

thefranmaneds Wed, 11/10/2010 - 06:17

2 years later I found the answer... from the cisco books.  The G will client will run at G speeds even if there is a B client.

Now that the cell knows about the 802.11b clients and the protection mechanism has been
invoked, the way that data is sent within the cell changes. The protection mechanism requires clients associated at the access point to use request to send/clear to send
(RTS/CTS).  Any client wishing to transmit to the access point must send the access point
an RTS frame, which is done using the lowest mandatory 802.11b data rate. The access
point will respond with a CTS frame, sent at its lowest mandatory 802.11b data rate, ensuring all associated clients hear the signal.  The CTS frame includes the MAC address of
the client who is allowed to transmit; implicit in this CTS frame is a command for all other
stations to remain silent. Figure 6-3 illustrates this process.
So if client A sends an RTS frame the access point will respond with a CTS frame specifying client A’s MAC address;  all other clients now know, even if they can’t hear the original
RTS frame due to distance, that they have to wait. Client A now sends its frame. The preamble will be sent at the lowest mandatory data rate used in the cell, ensuring as many
clients as possible hear it. The preamble includes the duration field, letting the other
clients know how long they have to remain quiet. Assuming the preamble is sent using
802.11b data rates, the 802.11b client will also know how long it has to remain silent. The
remainder of the frame can then be sent at the best 802.11g data rate the client and access
point can support. Even if the 802.11b client can’t understand the formatting of the frame

once client A switches to 802.11g speeds, it still knows it has to remain quiet.

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