[OLPC-AU] limits on ad-hoc connections

[James Cameron]

On Thu, Feb 09, 2012 at 03:11:42PM +1100, Sridhar Dhanapalan wrote:
> Hmm I am thinking that my understanding of the ad-hoc implementation
> might be incorrect.
> 
> I was under the assumption that one XO acts as the ad-hoc host, and
> the others connect to it. That made me wonder whether that host could
> limit how many clients connect to it.

No, that's not how ad-hoc works.  I'll simplify and translate for you.

In an 802.11 wireless ad-hoc network, each node has the duty and right
to be the beacon, especially if there is no other beacon heard.

The beacon is used for timing the transmissions, so that they occur in
empty time.  Transmissions that occur simultaneously would interfere
with each other, and the receivers would be more likely to miss them.

Always, the first node to begin an ad-hoc network begins by being the
beacon.

If a node cannot hear a beacon, then after a very short while it will
try to become the beacon.  In effect, they compete for the job, in a
psuedo-random fashion.

This is implemented in the wireless device firmware, not in the host,
not in the CPU, not in the kernel, not in the user-space networking
tools.

If a cluster of XOs that have formed an ad-hoc network, are slowly
spread out physically, then eventually the responsibility for beacon
should tend to be in the centre of the cluster.

If a cluster of XOs is split in two, and the two groups slowly moved
apart from each other, then eventually two beacons will be active and
there will be two networks.

References:

http://en.wikipedia.org/wiki/Timing_Synchronization_Function_%28TSF%29
http://en.wikipedia.org/wiki/Beacon_frame
http://en.wikipedia.org/wiki/Traffic_indication_map

(ad-hoc is IBSS)

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