[Make-wifi-fast] Thoughts on tackling airtime fairness
moeller0 at gmx.de
Thu May 12 02:41:09 EDT 2016
> On May 12, 2016, at 01:25 , David Lang <david at lang.hm> wrote:
> On Wed, 11 May 2016, moeller0 wrote:
>>> On May 11, 2016, at 17:15 , Toke Høiland-Jørgensen <toke at toke.dk> wrote:
>>> David Lang <david at lang.hm> writes:
>>>> Take retransmissions as an example. They only happen because the
>>>> receiver didn't see them. If you were to get an aircap off the same
>>>> antenna as the receiver, you also wouldn't see them and therefor could
>>>> not account for them. In the real world, you are doing the aircap from
>>>> a different device, with a different antenna so what you see will be
>>>> even more different. Now think about the normal case where you have
>>>> two stations taking in two very different locations and one device to
>>>> do the aircap.
>>>> If we don't have anything else, aircaps are what we have to fall back
>>>> on, but we need to realize how much we can't see at that point.
>>> Hmm, hadn't thought of that. Damn. Well, guess we'll have to trust the
>>> driver (or make it trustworthy if we can’t).
>> This issue seems surmountable, just make sure the air-capping machine is a) located halfway between the other two hosts in question and b) has better antennas ;) . It should be possible to “degrade” the antenna quality of the two hosts to make sure the air-capper (sounds like a fancy whale species) has better RF visibility…
> better antennas on the aircap machine doesn't solve the problem for two reasons.
> 1. ‘better' antennas are almost never better in every direction, they tend to change the pattern (for omnidirectional antennas, they trade vertical sensitivity for better horizontal sensitivity), so your 'better' antenna may not hear interference from the floor above you.
Please read what I wrote again; the better “better antennas” requirement can be fulfilled by making the antennas of the to be tested device pair worse… that should take care of your concern pretty much. Plus locating the device halfway between the pair will also give it better RF visibility to signals from both…
> 2. your ‘better' antenna will pick up interference from stations further away which the receiving stations don't hear, so something they hear you may not be able to make out.
Which is totally fine to a degree as long as the aircap device will pick up all transmissions/retransmissions of the device pair under test. Also placing the air-capper at the midpoint will also ameliorate this concern.
> There’s a reason why RF engineering is half black magic and experience :-(
Oh, if you require perfect I am sure you are right, if we aim for good enough I have a hunch we can avoid the requirement for at least the “black” part of the magic component ;)
> David Lang
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