Ad-hoc Networking

John Gilmore gnu at toad.com
Wed Apr 23 19:27:50 EDT 2008


> | The presence implementation only works on access points and on meshes --
> | but not on non-meshed, ad-hoc 802.11.  The vast majority of computers
> | with 802.11 don't have mesh, but they would benefit from being able to
> | "see" nearby laptops and share applications with them ...
>
> There is explicitly support for this idea in 802.11s.  It is called a
> "Mesh Access Point", or MAP, ...  A Mesh Access
> Point is connected to an 802.11s network, and also acts as an 802.11b
> Access Point whose clients can essentially join the mesh.

This isn't what I was talking about.  Forget about the mesh.  Ban it
from your mind.

Take an ordinary laptop, perhaps a Macintosh.

Take two such laptops.

Take them way out into the country so they can only communicate with
each other.

There are no access points.  No laptop is acting as an access point, either.

Can those two laptops communicate?  Under 802.11b/g, the answer is yes.
They are communicating in "ad-hoc" mode.  Each can transmit packets and
the other one gets the packets.  (This works for N laptops, not just two.)

The IETF "ZeroConf" protocols provide for self-assignment of IP
addresses in such a case.  (The same thing happens if you plug two
laptops together with a short Ethernet cable and no DHCP server.)
Once they self-assign IP addresses, the laptops can talk at an IP
level.  They can send unicast, multicast, and broadcast packets to
each other.  (If you look in the Mac Control Panel, you'll see this
reported as a "self-assigned IP address", with a warning that you
probably aren't connected to the global Internet, since most of the
time that was what you probably expected.)

Does the OLPC Presence Service work in such a case?

Currently and inexplicably, the XO laptop software provides no easy
way to shut off the troublesome Mesh and just live with the 802.11b/g,
(See e.g. http://dev.laptop.org/ticket/5144#comment:9 from 3 months ago).
So I don't know if the Presence Service works in such a case.  But I
would be surprised if it did, since we don't test it in such an
environment.

In ad-hoc mode, there is no forwarding of packets.  It isn't a mesh.
It's just point-to-point communication between two radios, without any
intermediaries.  This mode works in every 802.11 chip set.  So if our
presence and collab software worked in this mode, our collaborating
applications could collaborate "under a tree" using any mfr's laptop.

	John




More information about the Devel mailing list