65-node simple mesh test (and counting... ;-)

Ricardo Carrano carrano at laptop.org
Tue May 13 12:40:03 EDT 2008

On Tue, May 13, 2008 at 11:59 AM, John Watlington <wad at laptop.org> wrote:

> On May 12, 2008, at 3:31 PM, Ricardo Carrano wrote:
> > > How does the collision model/scheme change between AP mode and
> > > ad-hoc/mesh modes?
> >
> > As far as I can tell, it doesn't.  802.11s is interoperable with
> > 802.11abg, which means that the same media access algorithms are used.
> >  At least part of our problem might be in the synchronized transmits
> > occurring in our present 802.11s implementation of broadcast, which
> > are probably killing whatever CA scheme 802.11abg dictate.  See:
> >  http://wiki.laptop.org/go/
> > Path_Discovery_Mechanism:Sanity#Question_.232_-_Does_PDM_traffic_self-interfere.3F
> >
> > which is trying to deal with low-level path discovery requests, which
> > also use the broadcast mechanism.
> >  --scott
> >
> > Yes, it is the same 802.11 DCF for both scenarios (infra and mesh).
> >
> > I would like to add to this discussion that sparse and dense mesh are
> > too completely different animals. Most of the problems that we are trying to
> > address now, are associated to the latter.
> >
> Certainly an interesting question.  But is your answer really true ?
> I'd argue that very little, if any, testing has been done
> of the "large, yet sparse" mesh.   Certainly none by OLPC.
> Our problems certainly come from "large" meshes (more than
> 10-15 laptops in the mesh).   What is your definition of sparse ?

My definition, since I never found o good one in the literature. In a sparse
mesh the number of active neighbors is smaller than your retry limit. So, in
a sparse mesh your frame will never be discarded without being transmitted
at least one time. In a dense mesh, there is a chance that a frame will
reach the retry limit without being ever transmitted. So, 10 XOs in a room
is a dense mesh (in that definition).

I've done some tests in sparse topologies  early in 2007. but I doubt they
have any lasting values, after so many changes and fixes. Back there we were
particularly interested in the hidden node problem and checked to see if
RTS/CTS would help us (by the way it does not seem to help in multihop
scenarios http://www.midiacom.uff.br/~schara/publications/SBrT2007.pdf)
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