[Make-wifi-fast] deep wifi

Bob McMahon bob.mcmahon at broadcom.com
Tue Aug 28 01:43:42 EDT 2018


I'm confused.  An AP sending a CTS is telling others not to transmit so the
AP can listen and successfully decode energy per the RTS originator
<https://warpproject.org/trac/wiki/802.11/wlan_exp/app_notes/dcf_with_hidden_nodes>.
I don't see this a denial of service, though a device blasting CTS might be
able to create a DoS attack - not sure.

Going back to the original question about ED only which I think hit the
issues Jonathan pointed out.

   - The software might be simpler, but the hardware would need to be
   overspecified to the point of making it unreasonably expensive for consumer
   devices.
   - Radio hardware generally has a significant TX/RX turnaround time,
   required for the RX deafening circuits to disengage.  Without those
   deafening circuits, the receivers would be damaged by the comparatively
   vast TX power in the antenna.
   - So in practice, it's easier to measure SNR at the receiver, or
   indirectly by observing packet loss by dint of missing acknowledgements
   returned to the transmitter.

Being a software guy, I hope it's ok to ask more "dumb" questions ;)

   1. Why would consumer "hardware" be unreasonably expensive?  Is it a mfg
   yield thing?  Not possible per the state of CMOS radio process technology?
    Just curious to what would drive the expense.
   2. Maybe indirect detection via packet loss is good enough - not sure.
   But still can't get rid of first try transmit's EDCA back offs even if when
   they aren't useful, e.g. ED only would have been sufficient?   Can a device
   (tx) know the state of the EDCA arbitrations and decide if backoffs are
   likely to be required or not?

Again thanks to all for the edifications.

Bob

On Mon, Aug 27, 2018 at 9:44 PM Dave Taht <dave.taht at gmail.com> wrote:

> On Mon, Aug 27, 2018 at 8:32 PM David Lang <david at lang.hm> wrote:
> >
> > Think of the DoS possibilities if you can tell other networks to not
> transmit.
>
> Actually, that's a thing. google for "ap suppression".
>
>
>
>
> > David Lang
> >
> >
> > > On Mon, Aug 27, 2018 at 6:56 PM Bob McMahon <bob.mcmahon at broadcom.com>
> wrote:
> > >>
> > >> Hmm, not sure I understand the distinction.   CTS per the AP informs
> those other transmitters to stay quiet per the CTS NAV.  I may be
> misunderstanding things.  Thanks for the continued discussions.  It helps
> to better thoroughly understand the issues.
> > >>
> > >> Bob
> > >>
> > >> On Mon, Aug 27, 2018, 6:52 PM David Lang <david at lang.hm> wrote:
> > >>>
> > >>> On Mon, 27 Aug 2018, Bob McMahon wrote:
> > >>>
> > >>>> I thought that RTS/CTS would handle the case of hidden nodes, i.e.
> a device
> > >>>> that fails to successfully transmit can resort to RTS/CTS to get the
> > >>>> receiver to reserve time for it.  Also, lack of a RX ack seems ok to
> > >>>> trigger MAC level retransmits.
> > >>>
> > >>> the problem isn't getting the receiver to reserve time for it, it's
> getting the
> > >>> other transmitter(s) to not step on it when it transmits. Those other
> > >>> transmitters may belong to different people, sharing a channel with
> your system
> > >>> and nothing else.
> > >>>
> > >>> David Lang
> > >>>
> > >>>> It seems the LBT bug is the collision avoidance overheads when it
> isn't
> > >>>> needed, i.e. no other energy would cause the RX PHY to fail its
> decode and
> > >>>> the EDCA backoffs had no benefit, stochastic or otherwise.
>  Optimizing
> > >>>> that out is said to be not possible from local information only and
> per
> > >>>> "shared" spectrum.
> > >>>>
> > >>>> Bob
> > >>>>
> > >>>>
> > >>>> On Mon, Aug 27, 2018 at 3:33 PM David Lang <david at lang.hm> wrote:
> > >>>>
> > >>>>> On Mon, 27 Aug 2018, Jonathan Morton wrote:
> > >>>>>
> > >>>>>> So in practice, it's easier to measure SNR at the receiver, or
> > >>>>> indirectly by
> > >>>>>> observing packet loss by dint of missing acknowledgements
> returned to
> > >>>>> the
> > >>>>>> transmitter.
> > >>>>>
> > >>>>> Also, there may be other transmitters that the recipient of the
> packets
> > >>>>> can hear
> > >>>>> that you cannot hear, so it's not possible to detect colliding
> > >>>>> transmissions
> > >>>>> directly in all cases.
> > >>>>>
> > >>>>> This is another trap that digital/wired people fall into that
> doesn't
> > >>>>> really
> > >>>>> apply in the analog/radio world.
> > >>>>>
> > >>>>> David Lang
> > >>>>>
> > >>>>
> > >>
> > >> _______________________________________________
> > >> Make-wifi-fast mailing list
> > >> Make-wifi-fast at lists.bufferbloat.net
> > >> https://lists.bufferbloat.net/listinfo/make-wifi-fast
> > >
> > >
> > >
> > >
>
>
>
> --
>
> Dave Täht
> CEO, TekLibre, LLC
> http://www.teklibre.com
> Tel: 1-669-226-2619
>
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