[Starlink] SatNetLab: A call to arms for the next global> Internet testbed

[Ben Greear]

On 7/13/21 11:13 AM, David Lang wrote:
> On Tue, 13 Jul 2021, Ben Greear wrote:
> 
>> On 7/13/21 11:01 AM, David Lang wrote:
>>> On Tue, 13 Jul 2021, Rodney W. Grimes wrote:
>>>
>>>> It wasnt suggested "lowering the bit rate", it was suggested to make the
>>>> packets smaller, which actually does address the hidden transmitter problem
>>>> to some degree as it *would* reduce your air time occupancy, but the damn
>>>> wifi LL aggregation gets in your way cause it blows them back up.  When I
>>>> am having to deal/use wifi in a hidden transmitter prone situation I always
>>>> crank down the Fragmentation Threshold setting from the default of 2346 bytes
>>>> to the often the minimum of 256 with good results.
>>>
>>> The problem is that with wifi at modern data rates, you have a header at a low data rate and then data at a much higher data rate (in extreme cases, a >50x 
>>> difference), so the amount of data that you send has a pretty minor difference in the airtime used. So you really do want to send a large amount of data per 
>>> transmission to minimize the overhead
>>>
>>> IT's not quite as bad if you have disabled 802.11b speeds on the entire network as that raises the header/housekeeping transmissions from 1Mb/s to 11Mb/s
>>
>> The quiesce period waiting for medium access also takes some time, so that is another reason to try to put lots of frames on air in the same tx operation...
> 
> yep, mentally I lump that into the header/housekeeping functionality as that's all fixed-time no matter how much data you are transmitting.

Not exactly fixed though?  With a few transmitters trying to get on medium, air-time may theoretically improve
a bit for first few transmitters due to random backoff timers finding clear air after shorter over-all quiesce
period, but I would image it gets pretty bad with several hundred radios detecting collisions and increasing
their backoff before accessing medium again?

> 
>> David, I'm curious about the rate-ctrl aspect of this.  Have you found any implementations of rate-ctrl that try harder to decrease amsdu groupings and/or 
>> keep MCS higher (maybe based on RSSI?) in a congested environment to deal better with hidden node problems?
> 
> I have not experimented with that. I help run the network at the SCALE conference each year (3500 geeks with their gear over ~100k sq ft of conference center 
> with >100 APs running openwrt), and I'm open to suggestions for monitoring/tweaking the network stack, as long as it's easy to revert to default if we run into 
> grief.

Mucking with rate-ctrl typically involves hacking firmware in modern APs.  Impossible for most due to lack of
source, and tricky stuff even for those with source.  And if you do it wrong, you can completely ruin a
network.

Maybe something like MTK AX chipset will someday offer a host-based rate-ctrl where experimenters could put some
effort in this direction.  I don't know of any other chipset that would have any chance of user-based rate-ctrl.

Thanks,
Ben

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