Re: [Make-wifi-fast] perverse powersave bug with sta/ap mode

[dpreed@reed.com]

Interesting stuff.  A deeper problem with WiFi-type protocols is that the very idea of "multicast" on the PHY level (air interface) is flawed, based on a model of propagation that assumes that every station can be easily addressed simultaneously, at the same bitrate, etc. Multicast is seductive to designers who ignore the realities of propagation and channel coding issues, because they think it works one way, but the reality is quite different.

So just as years were wasted in the RTP and media streaming world on router/switch layer multicast (thought to be easy and more efficient), my personal opinion is that any wireless protocol that tries to solve problems with multicast at the PHY layer is a fragile, brittle design that will waste years of effort trying to make the horse dance on its forelegs.

THe list of issues is enormous, but the most obvious ones are a) equalization, b) inability to use MIMO, and c) PHY layer acknowledgment complexity.

The usual argument is that in some special case circumstance, using multicast is "optimal".  But how much better is that "optimal" than the non-multicast general solution, and how does that "optimization" make the normal operation worse, in common conditions?

Whenever someone says that a "cross layer optimization" or a complicated special case added into a robust design is "optimal", I check that my wallet is still in my pocket.  Because "optimal" is a magic word often used to distract one's attention from what really matters.

So "multicast" considered harmful is my view.


On Tuesday, April 26, 2016 7:27pm, "Aaron Wood" <woody77@gmail.com> said:

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> Has anyone modeled what the multicast to multiple-unicast efficiency
> threshold is?  The point where you go from it being more efficient to send
> multicast traffic to individual STAs instead of sending a monstrous (in
> time) multicast-rate packet?
> 
> 2, 5, 10 STAs?
> 
> The per-STA-queue work should make that relatively easy, by allowing the
> packet to be dumped into each STA's queue...
> 
> -Aaron
>