<s>
In	O
telecommunication	O
,	O
a	O
n-entity	B-Protocol
is	O
an	O
active	O
element	O
in	O
the	O
n-th	O
layer	O
of	O
the	O
Open	B-Operating_System
Systems	I-Operating_System
Interconnection--Reference	O
Model	O
(	O
OSI-RM	O
)	O
that	O
(	O
a	O
)	O
interacts	O
directly	O
with	O
elements	O
,	O
i.e.	O
,	O
entities	O
,	O
of	O
the	O
layer	O
immediately	O
above	O
or	O
below	O
the	O
n-th	O
layer	O
,	O
(	O
b	O
)	O
is	O
defined	O
by	O
a	O
unique	O
set	O
of	O
rules	O
,	O
i.e.	O
,	O
syntax	B-Application
,	O
and	O
information	O
formats	O
,	O
including	O
data	O
and	O
control	O
formats	O
,	O
and	O
(	O
c	O
)	O
performs	O
a	O
defined	O
set	O
of	O
functions	O
.	O
</s>
<s>
In	O
an	O
existing	O
layered	O
open	B-Operating_System
system	I-Operating_System
,	O
the	O
n	O
may	O
refer	O
to	O
any	O
given	O
layer	O
in	O
the	O
system	O
.	O
</s>
<s>
Layers	O
are	O
conventionally	O
numbered	O
from	O
the	O
lowest	O
,	O
i.e.	O
,	O
the	O
physical	B-Application
layer	I-Application
,	O
to	O
the	O
highest	O
,	O
so	O
that	O
the	O
-th	O
layer	O
is	O
above	O
the	O
n-th	O
layer	O
and	O
the	O
-th	O
layer	O
is	O
below	O
.	O
</s>
