[Tinyos-devel] Re: CTP vs LQI

[Omprakash Gnawali]

On Dec 20, 2007 3:37 PM, Philip Levis <pal at cs.stanford.edu> wrote:
>
> On Dec 20, 2007, at 3:22 PM, Omprakash Gnawali wrote:
>
>
> > I added four sets of graph to see if goodcost is roughly the same
> > over time:
> > http://enl.usc.edu/~om_p/net2/post-hotnets/figures.html
> >
> > The first set is called "cumpktcount(t) with goodcost(pkt)> 20 (ctp)
> > goodcost(pkt) > 8 (lqi)".
> > This graph plots the cumulative pkt count with "high" cost over time.
> > There are three
> > groups of links - the groups correspond to the experiments with
> > different durations. The first
> > six experiments (a1..lqi2) were about 1300 seconds long, the next set
> > (lqi3..ctpa5) were
> > about 3700 seconds long, and the last set (aa8..ca8) were about 4050
> > seconds long. If
> > the high cost pkts mostly occur in the beginning, we expect the
> > distribution to start out
> > steep and flatten later. If these occurrences are uniform, we expect
> > the distributions to be
> > linear. Two groups of the distributions are linear suggesting that the
> > high cost pkts are random
> > events. The middle group (in the graph) which actually correspond to
> > the last set of data that
> > goes to 4050 shows relatively more high cost pkts in the beginning
> > compared to later
> > in the experiment: 50% of the high cost pkts in the first 1/4 of
> > the experiment.
> >
> > We can look at the data in more detail using the next three sets of
> > data that are titled
> > goodcost(t), cumgoodcost(seq), and  cumgoodcost(t), which are at the
> > bottom of the page.
> > These three sets plot goodcost for all (high as well as low cost)
> > pkts. High cost pkts
> > are seen jumping out of the mass of colors randomly throughout the
> > experiment in goodcost(t).
> > The cumgoodcost(seq) seems mostly linear - which says that there is
> > no systemic
> > improvement or degradation of tree quality over time.
>
> I don't think cumulative cost is a good way to look at the data: it's
> hard to see slight slope changes. I'm interested in cumpktcount(t)
> with goodcost(pkt) > 20 (ctp), goodcost(pkt) > 8 (lqi): if you loop
> at aa9, then 40% of its high cost packets are in the first 500
> seconds, and 80% are in the first half of the experiment. If you were
> to take the derivative of this -- the rate at which high cost packets
> appear -- then it is decreasing and approaching zero. Of course,
> chances are it will not reach zero, but...

Isn't derivative just goodcost(t)? You can look at the goodcost(t) set of
graphs if you are interested in the derivative. I have organized the graphs
into different pages; you can see the links at the bottom of the page.

I added two new sets of graph to understand if the high THL packets occur
in the beginning of the experiment or throughout the experiment. The graph
is called "Cumpktcount(t) with thl(pkt) > 10". This looks similar to
the high cost pkt arrival time distribution graph but not quite - look at
the aa8 and aa9 lines on the high thl and high goodcost graphs - they are
different. Graphs that plot the derivate of the high thl distribution
are called THL(t) and the link is at the bottom.

>
> Why are there so many experiments with alpha=8 but not 9? It seems
> like 9 outperforms significantly.

Alpha of 8 was the default on my local copy of 4bitle and earlier the focus
wasn't so much the difference between 8 and 9 but it was convincing ourselves
that it was not very bright to set an alpha of 2. Now we are trying to get the
best ctp, it makes sense to do most experiments with an alpha of 9.

>
> To tease apart the routing efficiency and recovery costs, it might be
> useful to look at things like 50th percentile costs. It LQI is just
> dropping the packets that are hard to deliver, then it does have
> lower goodcost (we need a better name) but as you've noted the wasted
> transmissions before drops lead its cost to be equivalent. The Mirage
> results suggest 4b can find better routes in stable networks.
>
> Phil
>

We need a statistical definition of "high" cost and "high" thl packets and
might employ a technique similar to the one you mention above.

- om_p