mac80211 initial rate: a question

mac80211 initial rate: a question

[Larry Finger]

In routine rate_control_simple_rate_init in net/mac80211/rc80211_simple.c, there is a comment that says

         /* TODO: what is a good starting rate for STA? About middle? Maybe not
          * the lowest or the highest rate.. Could consider using RSSI from
          * previous packets? Need to have IEEE 802.1X auth succeed immediately
          * after assoc.. */

After that the code goes on to set the rate equal to the last one in the rate table, which might be 
the highest rate. Although that rate might be OK for most of the devices using mac80211, it 
certainly is not for bcm43xx, which does not have the ability to transmit or receive at speeds much 
above 1 Mbs. In a private patch circulated only on the bcm43xx mailing list, I changed this routine 
to set an initial rate of 1 Mbs, which certainly helps bcm43xx authenticate and communicate with the 
DHCP server.

If this low rate were to be set for all drivers, how rapidly would the rate-control algorithm 
respond? Would this cause a serious performance degradation?

Larry

Re: mac80211 initial rate: a question

[Tomas Winkler]

What about making the initial rate to be configurable in rc80211_simple.c ?


On 6/1/07, Larry Finger <Larry.Finger@lwfinger.net> wrote:
> In routine rate_control_simple_rate_init in net/mac80211/rc80211_simple.c, there is a comment that says
>
>          /* TODO: what is a good starting rate for STA? About middle? Maybe not
>           * the lowest or the highest rate.. Could consider using RSSI from
>           * previous packets? Need to have IEEE 802.1X auth succeed immediately
>           * after assoc.. */
>
> After that the code goes on to set the rate equal to the last one in the rate table, which might be
> the highest rate. Although that rate might be OK for most of the devices using mac80211, it
> certainly is not for bcm43xx, which does not have the ability to transmit or receive at speeds much
> above 1 Mbs. In a private patch circulated only on the bcm43xx mailing list, I changed this routine
> to set an initial rate of 1 Mbs, which certainly helps bcm43xx authenticate and communicate with the
> DHCP server.
>
> If this low rate were to be set for all drivers, how rapidly would the rate-control algorithm
> respond? Would this cause a serious performance degradation?
>
> Larry
> -
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Re: mac80211 initial rate: a question

[Johannes Berg]

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On Fri, 2007-06-01 at 11:14 -0500, Larry Finger wrote:

>          /* TODO: what is a good starting rate for STA? About middle? Maybe not
>           * the lowest or the highest rate.. Could consider using RSSI from
>           * previous packets? Need to have IEEE 802.1X auth succeed immediately
>           * after assoc.. */
> 
> After that the code goes on to set the rate equal to the last one in the rate table, which might be 
> the highest rate. 

Yeah, I think that's very wrong.

> Although that rate might be OK for most of the devices using mac80211,

Even then, if the AP is just in range it might not receive transmissions
at 54M, depending on the fallback mechanism that might or might not be
ok.

> it 
> certainly is not for bcm43xx, which does not have the ability to transmit or receive at speeds much 
> above 1 Mbs. In a private patch circulated only on the bcm43xx mailing list, I changed this routine 
> to set an initial rate of 1 Mbs, which certainly helps bcm43xx authenticate and communicate with the 
> DHCP server.

The thing about 802.1X is that you have a handshake with a few packets
that need to go through or you need to restart, which is the wpa problem
people are seeing currently with bcm43xx, but my brother also sees with
madwifi/atheros in our living room upstairs (router is downstairs here)

> If this low rate were to be set for all drivers, how rapidly would the rate-control algorithm 
> respond? Would this cause a serious performance degradation?

This, unfortunately, I don't know. I'd think the algorithm would adjust
fast enough for it not to be an issue.

johannes

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