mesh network vs broadcast/multicast question

[Michail Bletsas]

The key point to remember in order to derive answers to these questions is 
that our mesh network operates at layer-2
For all practical purposes, there aren't any differences between broadcast 
and multicast frames and every node maintains a table of recently 
forwarded broadcast frames so that they are not broadcasted multiple 
times.

One can limit the radious of the (layer-2) neighborhood be means of 
controlling the Mesh TTL field.

M.





Dan Williams <dcbw at redhat.com> 
06/26/2007 10:50 AM

To
Alexander Larsson <alexl at redhat.com>
cc
olpc-devel <devel at lists.laptop.org>, mbletsas at laptop.org
Subject
Re: mesh network vs broadcast/multicast question






On Tue, 2007-06-26 at 15:22 +0200, Alexander Larsson wrote:
> Hi dan, I have some questions on the mesh network for the updates work.
> 
> What is the broadcast domain for a laptop on the mesh? (I.E. how far are
> broadcast messages sent)? Is it just the set of nodes reachable from
> your machine, or are broadcasts forwarded by the mesh?

Michail?

> This affects how avahi works, as avahi only uses local broadcasts.
> 
> Another question is on multicasts. How many hops do multicast packets
> live on the mesh? And are there any other limits than nr of hops to
> avoid multicast loops? (Otherwise its likely that nodes see multicast
> packages many times, especially in dense networks.)

Michail?

> I read on the olpc wiki about the mesh using three different channels.
> My understanding on this is that these pretty much generate three
> separate mesh networks that are routed between by the school server, and
> that laptops can end up on any channel. 

Right.  We have only one radio in the laptops, and therefore we can only
tune to one channel at a time.  However, having all laptops on one
channel would be detrimental from a bandwidth perspective, since all
laptops share the bandwidth of the channel, which isn't large (54Mbps
but that's certainly not seen in the real-world).

> Does this mean that two laptops next to each other can be on two
> completely different networks? This means broadcasts (and by extension

Yes.  But if they are at school, then you'll be able to see that person
because you'll both be connected to the school server.  We only
(ideally) run into problems when one or the other laptop can't connect
to the school server, and they are on different channels.

> Avahi) cannot reach from one laptop to the other. Is this true? And are
> we doing something to work around this (like letting users manually
> switch channels)?

We are letting users manually "search" for friends, which is a fairly
lengthy process that involves jumping to each channel, searching for
friends and activities using mDNS, then to the next channel, etc.
You'll be warned if you do something that will make you jump to a
different channel, and therefore break stuff.

Dan