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

C. Scott Ananian cscott at laptop.org
Mon May 12 14:07:13 EDT 2008

On Fri, May 9, 2008 at 8:47 PM, Marcus Leech <mleech at nortel.com> wrote:

> I was playing in packet-radio circles long before 802.11 was even a
> gleam in anyones eye :-)

Yes, my original response above was really meant for others on devel,
since I've found myself trying to explain wired-vs-wireless issues
often recently.

> We had to deal with hidden-terminal issues, non-uniform propagation,
> etc.  The purpose of the
>  experiment I proposed above (measuring the ratio between a Cerebro
> network "equilibrating"
>  over both a wired and a wireless network).  Yes, there will be
> differences, but if they're
>  *large* compared to the raw bandwidth ratios, then something isn't
> working right, particularly
>  if all the Xos are in the same room.

I guess the question is, what is "working right"?  One could argue
that broadcast according to the 802.11a/b/g/s standard is not "working
right", if "reliability and broadcast speed comparable to wired
ethernet" is the criterion.

>  You shouldn't have
> hidden-terminal issues.  Yes,
>  there will be laptops that are in the "null" of another laptops
> radiation pattern, but
>  in terms of absolute received power, even being in a "null" (unless
> it's a *very* deep null
>  indeed) shouldn't dehance the SNR so as to not be able to coherently
> receive bits.  The
>  other thing that I wonder about is the collision behaviour in real
> life of an 802.11 network.
>  I understand that the network uses a Collision Avoidance (CA) scheme,
> but I wonder how
>  effective it is in real life.  Back in my packet-radio days, we moved
> from a pure
>  CSMA scheme to one that used P-persistant CSMA, with static
> determination of P
>  values.  This vastly improved overall throughput, and made collisions
> more rare (not
>  zero, but a lot better).
> 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:

which is trying to deal with low-level path discovery requests, which
also use the broadcast mechanism.

 ( http://cscott.net/ )

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