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Fig.5.
Experimental performance of the best asynchronous evolved CA as a function
of the configuration density for three values of the lattice size.
is faulty functioning of the rules, or of the cells, or both. In this section we explore
in some detail the behavior of evolved CAs subject to the second type of noise.
This kind of considerations will be important in the future, when it is likely
that self-organizing computational systems composed of an enormous number of
parts will be used in order to design systems that are partially or totally tolerant
to suchfaults. Of course, te comparatively small and simple systems studied
here are only toys with respect to real future computing machines. Nonetheless,
their study is certainly a worthy first step. It should be noted that we will not try
to correct the errors, which is an important but very complicated issue. Rather,
we will focus on the self-recovering capabilities of the systems under study.
We will study two kinds of perturbations and their effect on the density task:
the first is
probabilistic updating
and the second is
intermittent
faults. They are
defined as follows:
-
probabilistic updating: a CA rule may yield the incorrect output bit with
probability
p
f
, and thus the probability of correct functioning will be (1
−p
f
).
Futhermore, we assume that errors are uncorrelated.
-
intermittent faults: at time
t
a given cell has a certain probability of being
inactive, that is of keeping its current state.
For probabilistic updating usually two initially identical copies of the system
are used. One evolves undisturbed with
p
f
= 0, while the second is submitted
to a non-zero probability of fault (see e.g. [9] and references therein, where the
case of synchronous, non-uniform CAs is examined). Figure 6 depicts the typical
behavior of the best evolved synchronous rule under noise. We see that even for
relatively low (
p
f
=0
.
001) values of the fault probability the CA does not work
correctly. For higher values (
p
f
=0
.
01) the CA is so perturbed that it cannot
