Re: Scanning and channel types.

[Ben Greear <greearb@candelatech.com>]

On 02/06/2011 11:59 AM, Felix Fietkau wrote:
> On 2011-02-06 8:54 PM, Ben Greear wrote:
>> On 02/06/2011 11:42 AM, Felix Fietkau wrote:
>>> On 2011-02-06 8:01 PM, Ben Greear wrote:
>>>> Current code always sets the channel type to NO_HT when scanning.
>>>>
>>>>    From what I can tell, we should be able to send NO_HT packets on
>>>> any channel type, and for passive scanning, it should not matter
>>>> at all what channel-type we are using.
>>>>
>>>> I tested relaxing scanning to use the current channel type
>>>> when scanning on the operating channel, and it seems to
>>>> work.
>>>>
>>>> Does anyone see any problems with this approach?
>>> One thing you should make sure is that once you're done associating to
>>> an NO_HT or HT20 AP (and you have no other interfaces to consider), the
>>> channel mode must not be HT40 - otherwise it could reduce throughput.
>>
>> That is currently handled correctly by the ieee80211_set_channel_type
>> method, as far as I can tell...
>>
>> Regardless of that, in my multi-vif testing, I see lower throughput when
>> using HT40- than using HT20 (between 128 ath9k vif machine and 1 VAP ath9k machine).
>> The VIFS all claim 300Mbps rate.
>> I haven't looked into this any further at this point...
> Well, with HT40 you take a hit from not just the busy time of the
> primary channel, but also from the busy time of the extension channel.
> If you have other wifi activity on the extension channel, then it's
> normal that this would reduce throughput.

I think you perfectly described the problem.  I had another system that
had a bunch of STAs configured and was making all sorts of noise.
With it powered down, latency & throughput looks quite nice
with 300Mbps HT40- (128 stas, 20kbps per sta tx and rx to other stas).

Thanks,
Ben

-- 
Ben Greear <greearb@candelatech.com>
Candela Technologies Inc  http://www.candelatech.com