<s>
In	O
computer	B-General_Concept
science	I-General_Concept
,	O
a	O
communicating	B-Application
finite-state	I-Application
machine	I-Application
is	O
a	O
finite	B-Architecture
state	I-Architecture
machine	I-Architecture
labeled	O
with	O
"	O
receive	O
"	O
and	O
"	O
send	O
"	O
operations	O
over	O
some	O
alphabet	O
of	O
channels	O
.	O
</s>
<s>
They	O
were	O
introduced	O
by	O
Brand	O
and	O
Zafiropulo	O
,	O
and	O
can	O
be	O
used	O
as	O
a	O
model	O
of	O
concurrent	B-Operating_System
processes	O
like	O
Petri	B-Operating_System
nets	I-Operating_System
.	O
</s>
<s>
Communicating	O
finite	B-Architecture
state	I-Architecture
machines	I-Architecture
are	O
used	O
frequently	O
for	O
modeling	O
a	O
communication	O
protocol	O
since	O
they	O
make	O
it	O
possible	O
to	O
detect	O
major	O
protocol	O
design	O
errors	O
,	O
including	O
boundedness	O
,	O
deadlocks	O
,	O
and	O
unspecified	O
receptions	O
.	O
</s>
<s>
The	O
advantage	O
of	O
communicating	O
finite	B-Architecture
state	I-Architecture
machines	I-Architecture
is	O
that	O
they	O
make	O
it	O
possible	O
to	O
decide	O
many	O
properties	O
in	O
communication	O
protocols	O
,	O
beyond	O
the	O
level	O
of	O
just	O
detecting	O
such	O
properties	O
.	O
</s>
<s>
Communicating	O
finite	B-Architecture
state	I-Architecture
machines	I-Architecture
can	O
be	O
more	O
powerful	O
than	O
finite	B-Architecture
state	I-Architecture
machines	I-Architecture
in	O
situations	O
where	O
the	O
propagation	O
delay	O
is	O
not	O
negligible	O
(	O
so	O
that	O
several	O
messages	O
can	O
be	O
in	O
transit	O
at	O
one	O
time	O
)	O
and	O
in	O
situations	O
where	O
it	O
is	O
natural	O
to	O
describe	O
the	O
protocol	O
parties	O
and	O
the	O
communication	O
medium	O
as	O
separate	O
entities	O
.	O
</s>
<s>
Hierarchical	O
state	B-Architecture
machines	I-Architecture
are	O
finite	B-Architecture
state	I-Architecture
machines	I-Architecture
whose	O
states	O
themselves	O
can	O
be	O
other	O
machines	O
.	O
</s>
<s>
Since	O
a	O
communicating	O
finite	B-Architecture
state	I-Architecture
machine	I-Architecture
is	O
characterized	O
by	O
concurrency	B-Operating_System
,	O
the	O
most	O
notable	O
trait	O
in	O
a	O
communicating	O
hierarchical	O
state	B-Architecture
machine	I-Architecture
is	O
the	O
coexistence	O
of	O
hierarchy	O
and	O
concurrency	B-Operating_System
.	O
</s>
<s>
However	O
,	O
it	O
was	O
proved	O
that	O
the	O
coexistence	O
of	O
hierarchy	O
and	O
concurrency	B-Operating_System
intrinsically	O
costs	O
language	O
inclusion	O
,	O
language	O
equivalence	O
,	O
and	O
all	O
of	O
universality	O
.	O
</s>
<s>
It	O
has	O
been	O
proved	O
with	O
the	O
introduction	O
of	O
the	O
concept	O
itself	O
that	O
when	O
two	O
finite	B-Architecture
state	I-Architecture
machines	I-Architecture
communicate	O
with	O
only	O
one	O
type	O
of	O
messages	O
,	O
boundedness	O
,	O
deadlocks	O
,	O
and	O
unspecified	O
reception	O
state	O
can	O
be	O
decided	O
and	O
identified	O
while	O
such	O
is	O
not	O
the	O
case	O
when	O
the	O
machines	O
communicate	O
with	O
two	O
or	O
more	O
types	O
of	O
messages	O
.	O
</s>
<s>
Later	O
,	O
it	O
has	O
been	O
further	O
proved	O
that	O
when	O
only	O
one	O
finite	B-Architecture
state	I-Architecture
machine	I-Architecture
communicates	O
with	O
single	O
type	O
of	O
message	O
while	O
the	O
communication	O
of	O
its	O
partner	O
is	O
unconstrained	O
,	O
we	O
can	O
still	O
decide	O
and	O
identify	O
boundedness	O
,	O
deadlocks	O
,	O
and	O
unspecified	O
reception	O
state	O
.	O
</s>
<s>
It	O
has	O
been	O
further	O
proved	O
that	O
when	O
the	O
message	O
priority	O
relation	O
is	O
empty	O
,	O
boundedness	O
,	O
deadlocks	O
and	O
unspecified	O
reception	O
state	O
can	O
be	O
decided	O
even	O
under	O
the	O
condition	O
in	O
which	O
there	O
are	O
two	O
or	O
more	O
types	O
of	O
messages	O
in	O
the	O
communication	O
between	O
finite	B-Architecture
state	I-Architecture
machines	I-Architecture
.	O
</s>
<s>
A	O
channel	O
system	O
is	O
essentially	O
a	O
version	O
of	O
communicating	B-Application
finite-state	I-Application
machine	I-Application
in	O
which	O
the	O
machine	O
is	O
not	O
divided	O
into	O
distinct	O
process	O
.	O
</s>
