"Mesh" Dreams = OLSR

Richard A. Smith richard at laptop.org
Tue Aug 24 12:40:50 EDT 2010

On 08/24/2010 12:11 PM, L. Aaron Kaplan wrote:

>>>> The largest of our mesh problems did not have to do with
>>>> scalability on sheer number of nodes but rather scalability in
>>>> density.  Is there any information available on how these networks
>>>> perform when there are 50 - 100 of them next all in the same room
>>>> or in adjacent rooms?
>>> Yes! And the answer is very very simple: turn down the txpower!
>>> ;-)))
>> Can you provide me with a pointer to the numbers?  Whats the maximum number of nodes can you have operated in a given area and what sort of network traffic tests did you run?

> Well, the community wireless networks are not very much about very dense settings. We try to cover large areas with external (outdoor) antennas but still have very many nodes in one single mesh covering a whole city or so. See the attached current map of the Funkfeuer.at network.

Yes.  This is my point.  Comparison of our scenarios to those scenarios 
is not really a valid comparison.

> BUT!! Because we don't have a mesh with 100s of laptops in one room, does not mean, we don't know physics ;-)

> Since you asked if I know an example where there are many laptops in one room:
> One example that I know that worked brilliantly well with many wireless devices in one room was the RIPE meeting in Amsterdam. There they regularly have many small APs below the desks in the meeting room and these are turned down very much in "volume" (txpower).
> The effect is that they only cover a small area ( remember, power decreases by the square of the distance).
> So this is a way to avoid a lot of noise of many laptops in a small room.

Yes. I'm not disagreeing with any of the above. I'm just asserting that 
OLPC has limited development resources. Before we try to allocate any of 
these resources on a mesh implementation there needs to be _clear_ 
indication that the said mesh implementation can work in place of APs in 
a RF dense environment.  Not necessarily better than APs because not 
having to purchase/manage the APs is a win but if its worse then the 
decision metric becomes less clear.

> Another feature that you IMHO should look at is 802.11n devices (and of course also turn down the "volume" there!). These offer higher bandwidths in addition to actually using the multipath effects.
> When you have many many laptops in one room and everybody "screams"/sends very loud then you have lots of "echos" (multipath fading) bouncing off the walls etc. 802.11n thrives off these multipath effects.

We have and n is not an option for us yet and of course won't ever be an 
option for XO-1 unless they use some sort of external adapter.  1.5 has 
a replaceable wlan adapter but someone would have to produce a SDIO 
module for it first.

Richard A. Smith  <richard at laptop.org>
One Laptop per Child

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