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

Polychronis Ypodimatopoulos ypod at mit.edu
Fri May 9 18:04:28 EDT 2008

On Fri, May 9, 2008 at 10:57 AM, Marcus Leech <mleech at nortel.com> wrote:
>> I'd be *very* interested to compare the distribution on a wired network.
>> It seems to me that given
>>  the broadcast model, everybody should see everybody else in much
>> shorter time than the 55 seconds
>>  shown in the outlying cluster on that graph.

Marcus, this is indeed an interesting idea. However it has a significant 
problem: wiring up more than 60 XOs onto a switch requires equipment, 
time and space that OLPC cannot presently provide. Such a testbed though 
is absolutely necessary not only as a proof of concept for your 
suggestion, but also for doing large scale mesh network testing in general.

> The common, but erroneous, assumption is often made that a wireless
> network is just like a wired network, but with the wires removed.

So very true!

> On a wireless network, broadcasts are successfully received with much
> lower probability.  RF is mysterious and magical, and all sorts of
> connection asymmetries, near-field effects, and radiation lobe
> patterns conspire to make it unlikely that *everyone* can hear you
> equally at once -- and then you get into remote collisions and other
> mechanisms that make you unaware that not everyone heard you.  And
> there is not 'ack' mechanism for 802.11 broadcast.

All these are true also, but I think we're mystifying things a little 
bit here. The wireless medium is unpredictable mainly because its 
properties are also a function of time (a non-issue in wired networks), 
but at least (thank God!) it [the wireless medium] does not discriminate 
between broadcast and unicast frames! Adding an ack scheme to broadcasts 
should yield equal (or even better due to lowered speed) reliability 
using broadcast frames. Even without the ack scheme, I noticed that, on 
average, some 95% of the data transmitted over broadcast are 
successfully received on all nodes. We are throwing this away by 
discarding it on our wireless interfaces.


Polychronis Ypodimatopoulos
Graduate student
Viral Communications
MIT Media Lab
Tel: +1 (617) 459-6058

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