[Make-wifi-fast] Re: [Bloat] Re: [Codel] [Rpm] Re: Re: [Cake] "Fi-Wi is a new forwarding plane for wireless" - Bob McMahon

[David Lang <david@lang.hm>]

Sebastian Moeller via Bloat wrote:

> the point is current WiFi sucks under staturating loads, period.

This is correct and I doubt that it can be fixed.

Wifi was designed for a different era. I remember renting a pcmcia card for a 
conference with a $800 deposit because that was the purchase price for the card. 
Wifi was expected to be something rare and the protocol reflects that.

In an environment where there are only a few statinos, the retry and then retry 
at a slower data rate do work, and the slower data rate is more likely to be 
deciphered if the problem is a weak signal or high noise level

but today you only see such environments in a lab, on a farm, or possibly a 
large estate.

In the more common conditions today where airtime is the limiting factor and 
hidden transmitters (where the receiving station can hear two stations that 
can't hear each other) are the rule rather than the exception, having to 
retransmit everything when you are stepped on because the receiving station 
can't validate anything it heard if it doesn't hear the complete transmission 
wastes airtime, and transmitting at a lower data rate will make it more likely 
that you get stepped on, not more likely that you will get your message through

this leads to performance falling off a cliff when the available airtime fills 
up rather than graceful degredation

If the next wifi standard could have checksums at various places during the 
broadcast, it would be possible for part of the data to get through, and get 
acked and normal tcp retries to get the rest transmitted later. Combine this 
with NOT slowing down when you have trouble getting through would help greatly 
reduce this cliff (as long as your signal strength is high enough, acks from the 
AP would have to report how they hear you)

but without coordination between the stations that can't hear each other, you 
can't fix the hidden transmitter problem.

you can somewhat mitigate it if you have lots of APs and you set the APs to only 
pay attemtion to signals that are pretty strong. If you tune this just exactly 
right you can make it so the stations at opposite sides of you can hear each 
other to avoid collisions. This tuning would be very difficult to get right.

the only theoretical answer to the problem is to have the AP manage transmission 
slots, but that doesn't work very well in practice (the management eats so much 
airtime that the 'dumber' design wins with higher throughput, up until total 
collapse.



getting cake to be able to adjust to changing bandwidth would help avoid driving 
the network to saturation, but being able to deploy more APs to use more 
channels at lower power is adding additional airtime, pushing the limit futher 
out. It's not the best solution, it just works in practice (most of the time)

David Lang