Re: Is the data rate estimation for 5GHz channels overly pessimistic?

[James Prestwood <prestwoj@gmail.com>]

Hi Simonas,

On 10/24/23 7:26 AM, Denis Kenzior wrote:
> Hi James,
> 
> On 10/24/23 07:32, James Prestwood wrote:
>> Hi Denis/Simonas,
>>
>>>
>>> Now the question is, how do we make sure iwd is estimating the HE 
>>> rate if available?  Also, how do we tweak the estimation logic with 
>>> sensitivity numbers (obtained from a spec sheet, or through 
>>> experimentation) for specific hardware being used.
>>
>> Trying to catalog different hardware performance and act on it for 
>> ranking is an impossible task :)
> 
> We can actually have our own hwdb of sorts for this.  Hardware 
> sensitivity is a differentiator (think marketing), so should be fairly 
> easy to find.  Also, it doesn't have to be absolutely perfect, just 
> reflect the hw capability a bit more closely.

I shouldn't have said impossible, just quite an undertaking. It would 
rely near 100% on community support to add various cards, in addition to 
the "hwdb" framework being committed to the project. But we're open to 
contributions!

> 
>>
>> The only simple solution I can think of would be to add a user-option 
>> for some threshold RSSI in the rate calculation. If set and the RSSI 
>> is below the lowest of ht_vht_he_base_rssi just use the last index 
>> (-82) (and maybe force a 20MHz channel width?). This would at least 
>> let the rate logic return _something_, albeit maybe not accurate. But 
>> again, those RSSI thresholds were sorta made up anyways :)
> 
> They are not made up, they're direct from 802.11.  But again, they're 
> the _minimum_ specified sensitivity.  Hardware typically does better.

The rates/MCS table (ht_vht_rates) is from 802.11, but the mapping from 
RSSI to MCS index's (ht_vht_he_base_rssi) is not. The spec just has a 
formula for calculating the rate using a given MCS/channel width. IWD 
just chooses what it thinks the MCS will be for a given RSSI.

I don't remember exactly where I got those values from but this seems to 
match:

https://wlanprofessionals.com/mcs-table-and-how-to-use-it/

So ultimately its a bit "hand-wavy" but so far has served us pretty 
well. Whats happening in your case is the RSSI is below the lowest 
threshold so its not calculating a rate for HT/VHT/HE (at least I think 
thats whats happening). So my idea is we just force it to use the lowest 
MCS if some threshold is set. That will calculate _something_ at least 
rather than failing.

> 
>>
>> So you could set:
>> [Rank].LowSignalRateThreshold=-90
>>
>> Any RSSI between -82 and -90 would use -82 for the rank calculation. 
>> No idea how this would play out in practice, but its at least simple 
>> and not tied to any given hardware.
> 
> No, I was more thinking about:
> /* Added to the rssi value prior to looking up in the 
> ht_vht_he_base_rssi */
> SensitivityFudgeFactor=4
> /* Width penalty */
> SensitivityWidthPenalty=0 (3 today)
> /* NSS penalty */
> SensitivityNSSPenalty=3
> /* Same as SensitivityFudgeFactor, but for HE */
> SensitivityHEFudgeFactor=10
> 
> Regards,
> -Denis