wap54g "basic rates" setting, and prism hostap driver

Jouni Malinen jkmaline at cc.hut.fi
Wed Mar 22 22:05:53 EST 2006


On Wed, Mar 22, 2006 at 05:29:19PM -0500, Paul Fox wrote:

> i have a Linksys WAP54G (firmware 3.01), an NL-2511CD prism
> pcmcia card (firmware 1.8.4), and hostap drivers (either 0.3.9 or
> 0.4.7, same results).  i'm using WPA, but don't know that that
> matters.

> the WAP54G has a setting for "Basic Rates", which can
> have the values "ALL", "1-2 Mbps", and "Default".  if
> i select either "1-2 Mbps" or "Default", all is well.  if
> i select "ALL", i can no longer associate.

If that "ALL" includes 802.11g rates, setting basic rates to that will
prevent all 802.11b stations from associating with the AP.

>     Basic Rates.  This setting is not actually one rate of
>     transmission but a series of rates that are advertised to the
>     other wireless devices in your network, so they know at which
>     rates the Access Point can transmit.  At the Default setting,
>     the Access Point will advertise that it will automatically
>     select the best rate for transmission.  Other options are
>     1-2Mbps, for use with older wireless technology, and All,
>     when you wish to have all rates advertised.  The Basic Data
>     Rates are not the rates transmitted; the rates transmitted
>     can be configured through the Transmission Rates setting on
>     this screen.

This is confusing at best..  Basic rate sets are the rates that will be
used for special uses like broadcast (to be visible for all STAs) and
ACKs. There is another rate set, "Supported Rates", that would be much
closer to this description.

> why would the explicit advertisement of rates affect association?
> is there a requirement that a client support all advertised rates?

Yes, that is indeed exactly what is required for Basic Rates. Supported
Rates does not have such requirement.

> the more i think about this, the more i think that my observed
> behavior may be expected -- the AP is probably advertising
> rates up through 54, and the card doesn't like them all, so it
> doesn't associate.  or something like that.  but if that's the
> case, i'm wondering what the possible use of this particular
> negotiation is -- why wouldn't the AP and card just agree to the
> mutually supported subset of rates, rather than an all-or-nothing
> behavior.

Your 802.11b station is likely trying to associate, but the AP is
rejecting this since the station does not support all required options.
Usually 802.11g APs are configured to only include 802.11b rates as
basic rates to allow backwards compatibility. All 802.11g rates can be
in the supported rate set so that 802.11g stations can use them for
unicast frames. Including 802.11g rates in basic rate set may improve
multicast performance, so there may be valid use cases for that, too,
but at the cost of dropping backwards compatibility.

-- 
Jouni Malinen                                            PGP id EFC895FA



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