[Make-wifi-fast] the hidden station problem

[Henning Rogge]

On Thu, Apr 21, 2016 at 7:57 PM, David Lang <david at lang.hm> wrote:
> On Thu, 21 Apr 2016, Dave Taht wrote:
>
>> I was watching myself do then make-wifi-fast Q&A and henning mentioned
>> the hidden station problem and it's interaction with minstrel...
>>
>> https://www.youtube.com/watch?v=Rb-UnHDw02o
>>
>> Since we are doing up some better testbeds, I am curious as to what
>> might be a good (simplified) setup (bench or air) for it, and/or if
>> there has been a paper that shows the interaction problems with
>> minstrel in particular.
>
>
> the basic way to see this is to take two stations and move them far enough
> apart, or put shielding between them so that they cannot talk to each other.
>
> Then position a third station so that it can see both of the first two.

Yes, three stations would be the minimum, but I would suggest trying
it with 4 stations (all of them in IBSS/Adhoc-Mode).

With three stations you still have one who can see everyone, which
might change the effects.

As soon as you have the chain of 3/4 stations, just setup IP
forwarding routes and send traffic over the chain in one or both
directions.

> If you really turn the power down, you may be able to get away with them
> fairly near each other with a metal sheet next to one of them.

Easy to do in an office building with 5 GHz devices... often you
cannot even reach offices through two walls.

> You will see that you can talk to either of them quite nicely if the other
> is pretty idle, but if you have them both sending a lot of data at the same
> time, disaster strikes.

Yes.

> If you are writing a simulator, add a probability that a packet transmitted
> from an edge station to the central station doesn't get through. Ramp up
> this probability and watch what happens. A better simulator would scale the
> probability up based on the amount of airtime needed, so that as the sender
> slowes down, the probability goes up.

Not sure how well current simulators can handle this.

> This is one of the hardest problems for wifi to deal with. It manifests as
> massive amounts of lost packets when the first two are sending to the third
> one, and no amount of backoff helps. Slowing down the transmit rate just
> makes things worse as it takes longer to transmit each bundle and so it's
> more likely to be stepped on.

I think reporting the "used airtime" in the beacons would help. This
might be a first step to detect the presence of hidden stations
because your own view of the airtime is different than the one of your
neighbor.

It would also be possible to add a hash of all known neighbor mac
addresses to your beacon. This way your neighbors KNOW if there is the
potential of a hidden station.

> Reading up on Minstrel at
> https://wireless.wiki.kernel.org/en/developers/documentation/mac80211/ratecontrol/minstrel
> there is a comment
>
>> Inspection of the code in different rate algorithms left us bewildered.
>> Why did all the code bases we looked at contain the assumption that packets
>> sent at slow data rates are more likely to succeed than packets sent at
>> higher datarates? The physics behind this assumption baffled us. A slow data
>> rate packet has the highest possibility of being “shot down” by some other
>> node sending a packet.

> the answer to this is that the higher data rates require a better signal to
> noise ratio, and so if the problem is that the stations are too far apart,
> or there is a wall between them that makes the signal weaker, or that there
> is just a lot of low-volume noise in the area, the slower data rates are far
> more likely to be understandable than the faster data rates. Since Wifi was
> designed long before anyone imagined how common it would become (I remember
> when the pcmcia cards were >$1000 each rather than the current <$10 for a
> much faster USB adapter), they designed the protocol to fall back to lower
> rates if the packets don't get through.

Exactly... the "lower bits per airtime" should help the receiver to
decode the frames. QAM64 (for high data rates) needs a lot more SNR
(or SNIR, Signal to Noise and Interference Ratio) than QPSK.

> This works well if you are out in the boonies and trying for range. It fails
> horribly in very high density environments (this is why most conference wifi
> is worthless for example)
>
> This is why it's a good idea to disable the lowest data rates if you know
> that you don't need them.
>
> reading the minsrel page, it seems intuitively obvious to me that this
> random packet drop would really mess with their moving average and thus the
> decisions they end up making.

I wonder if it would make sense to try the best data-rate again for a
second time if you detect you are in a hidden station environment with
a "low amount of airtime available".

Henning