[Server-devel] XS on the XO 1.5

James Cameron quozl at laptop.org
Mon Aug 15 22:58:34 EDT 2011


On Mon, Aug 15, 2011 at 07:10:55PM -0700, Sameer Verma wrote:
> I was thinking (once again) about the possibility of running XS on a
> XO 1.5. On the XO-1 the built-in radio runs in the 802.11s mesh mode
> and serves out IPs via DHCP. Given that the 1.5 does not do 802.11s
> mesh, 

That's not true.  The XO-1.5 can do 802.11s mesh, if the right software
is loaded.  See http://wiki.laptop.org/go/Thinfirm_1.5

However, the XO-1.5 builds from OLPC do not load the right software.
That's probably what you meant.

> can't the radio instead simply work in adhoc mode and serve the
> same purpose? I am not sure what the limitations would be of adhoc vs
> mesh mode in terms of number of clients.

Worth testing, I think.

It would depend on the layout of the clients with respect to the server,
and therefore impractical to predict.

mesh would support more clients than ad-hoc if the clients could be
spread out.  Yet not at the same latency.

ad-hoc would support more clients than mesh if the clients were
concentrated.

Imagine three scenarios;

1.  a classroom, all clients can hear each other and the server, over
RF, or

2.  two classrooms with no separation, some clients cannot hear each
other but they can hear the server, or

3.  two classrooms with some separation, some clients cannot hear each
other or the server, but they can reach the server via other clients
over mesh,

ad-hoc would work fine in the first scenario, less well in the second
scenario, and not at all in the third scenario.

mesh would work fine in the first scenario, less well in the second
scenario, and even less well in the third scenario.

The effect of increasing the number of clients in the first and second
scenario is greater contention for the airtime around whichever node has
the adhoc beacon duty, but it should still work, albeit less well.

The effect of increasing the number of clients in the third scenario, is
to increase contention for the nodes that serve as relays.

Lastly, an effect of separate beacon operating areas (imagine one
beacon per classroom) is to increase the noise level experienced by the
other operating area ... because there will be transmissions from the
other classroom that occur at the same time as transmissions in the
first classroom.  This can mean that a laptop at one end of a classroom
can no longer reach another in the same classroom because of the higher
noise.

-- 
James Cameron
http://quozl.linux.org.au/


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