Subject: Re: [Roll] Some results on reducing control overhead using Trickle'ssuppression mechanism

From: Omprakash Gnawali <[email-masked]>
Date: Sat, 1 May 2010 11:24:14 -0700

On Fri, Apr 30, 2010 at 9:28 AM, Pascal Thubert (pthubert) <
[email-masked]> wrote:

> Hi Phil
>
>
>
>
>
> In the image above, there’s a value (close to K=10 in this case) where the
> suppression is quasi linear. This looks like a local optimal so it attracts
> my curiosity.
>
> I’m wondering what the turn of concavity observed there means for the
> operation in that network, and which network properties influence that
> value.
>

The network does not have uniform density. On Tutornet, there are more nodes
along the hallways of the third floor than in the rooms. Then, not all the
rooms have nodes. Depending on the threshold, and depending on the area of
the network, CTP suppressed more or less beacons. For example, with k=3 or
5, there were a number of nodes in the dense part of the network that
suppressed a large number of beacons. With a large threshold, there are few
nodes with a large number of one-hop neighbors, so there are few nodes that
suppressed a large fraction of beacons. In fact, with a large threshold, a
significant fraction of the nodes did not suppress beacons.

- om_p

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From: Omprakash Gnawali <[email-masked]>
Date: Sat, 1 May 2010 11:39:48 -0700

On Fri, Apr 30, 2010 at 8:27 AM, Matteo Paris <[email-masked]> wrote:
>
> Hi Om, thanks for this work.  I have some questions.
>
> The two graphs seem to use different data sets.  For example, looking at the
> left bar graph for motelab for k = 10, it looks like about 10% suppression
> as compared with k = infinity.  But looking at the CDF plot, it looks like
> 50% suppression for k = 10.  The tutornet data does not fit either, and the
> CDF graph does not indicate whether it is Motelab or Tutornet data.  The CDF
> graph counts "beacons suppressed", whereas the bar graph counts "control
> packets sent". Are there control packets that are not beacons?  I would like
> to see both graphs for the same data set.

Recall that beacon suppression mechanism does not suppress beacons
that are sent due to nodes requesting routes (PULL bit in CTP) and
datapath inconsitency. So, although we suppressed 50% of the would-be
beacons, that does not necessarily transate to 50% reduction in
control overhead. This explains the apparent discrepancy between the
two plots. Sorry, I should have included this in the explanation. The
CDF on the right is for Tutornet. I have noted this on the page now.

> To get a true picture of the impact of beacon suppression on delivery
> ratios, the experiment must be done on many different networks with
> different densities and topologies.  In particular, networks with nonuniform
> density have a higher risk that beacon suppression will partition them.  We
> can't conclude from only two data points that "beacon suppression can be
> used to reduce control overhead without any negative impact on routing
> performance".

As I explained in my earlier email, this network has non-uniform
density. I found that suppression mechanism suppresses beacons to
different extent depending on the density. If we pick a threshold that
is not too aggressive, it does not partition the sparse parts of the
network (because it does not suppress the beacons) while reducing
control overhead on the denser parts of the network.

It is hard to vary the physical density on the testbeds. I did another
series of experiment to at least emulate change in density -- run
experiments with different transmit power but the same redundancy
threshold of 10. These are presented as the second set of results on
this page:
http://stuff.stanford.edu/~om_p/ctp/beaconsuppression.html

We can see that the suppression becomes more impactful at higher
density. At lower density, it does not suppress many beacons, which is
exactly what we want.

- om_p

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From: Omprakash Gnawali <[email-masked]>
Date: Mon, 10 May 2010 18:09:32 -0700

> density. At lower density, it does not suppress many beacons, which is

I performed a new set of experiments exploring beacon suppression on
channel 16 (overlapping WiFi). We expect the result to be noisier than
channel 26, on which there is no WiFi traffic. The results are under
the title "The Impact of WiFi interference on Beacon Suppression" at
this link below:

- om_p