<s>
While	O
the	O
seven-layer	O
OSI	O
model	O
is	O
often	O
used	O
as	O
a	O
reference	O
for	O
teaching	O
and	O
documentation	O
,	O
the	O
protocols	O
originally	O
conceived	O
for	O
the	O
model	O
did	O
not	O
gain	O
popularity	O
,	O
and	O
only	O
X.400	B-Protocol
,	O
X.500	B-Protocol
,	O
and	O
IS-IS	B-Protocol
have	O
achieved	O
lasting	O
impact	O
.	O
</s>
<s>
The	O
goal	O
of	O
an	O
open-standard	O
protocol	B-Protocol
suite	I-Protocol
instead	O
has	O
been	O
met	O
by	O
the	O
Internet	B-Protocol
protocol	I-Protocol
suite	I-Protocol
,	O
maintained	O
by	O
the	O
Internet	O
Engineering	O
Task	O
Force	O
(	O
IETF	O
)	O
.	O
</s>
<s>
The	O
OSI	O
protocol	B-Protocol
stack	I-Protocol
is	O
structured	O
into	O
seven	O
conceptual	O
layers	O
.	O
</s>
<s>
Each	O
layer	O
adds	O
encapsulation	O
information	O
(	O
header	B-General_Concept
)	O
to	O
the	O
incoming	O
information	O
before	O
it	O
is	O
passed	O
to	O
the	O
lower	O
layer	O
.	O
</s>
<s>
Headers	B-General_Concept
generally	O
include	O
address	O
of	O
source	O
and	O
destination	O
,	O
error	O
control	O
information	O
,	O
protocol	O
identification	O
and	O
protocol	O
parameters	O
such	O
as	O
flow	O
control	O
options	O
and	O
sequence	O
numbers	O
.	O
</s>
<s>
It	O
can	O
be	O
twisted	O
pair	O
,	O
coaxial	O
cable	O
,	O
optical	B-Architecture
fiber	I-Architecture
,	O
wireless	O
,	O
or	O
other	O
transmission	O
media	O
.	O
</s>
<s>
The	O
data	B-Protocol
link	I-Protocol
layer	I-Protocol
packages	O
raw	O
bits	O
from	O
the	O
physical	O
layer	O
into	O
frames	B-Protocol
(	O
logical	O
,	O
structured	O
packets	O
for	O
data	O
)	O
.	O
</s>
<s>
This	O
layer	O
is	O
responsible	O
for	O
transferring	O
frames	B-Protocol
from	O
one	O
host	O
to	O
another	O
.	O
</s>
<s>
This	O
layer	O
further	O
consists	O
of	O
two	O
sublayers	O
:	O
MAC	B-Protocol
and	O
LLC	O
.	O
</s>
<s>
Connectionless	B-Protocol
Network	I-Protocol
Service	I-Protocol
(	O
CLNS	B-Protocol
)	O
–	O
.	O
</s>
<s>
Connectionless	O
Network	O
Protocol	O
(	O
CLNP	B-Protocol
)	O
–	O
.	O
</s>
<s>
Connection-Oriented	B-Protocol
Network	I-Protocol
Service	I-Protocol
(	O
CONS	B-Protocol
)	O
–	O
.	O
</s>
<s>
Connection-Oriented	O
Network	O
Protocol	O
(	O
X.25	B-Protocol
)	O
–	O
.	O
</s>
<s>
This	O
is	O
the	O
use	O
of	O
the	O
X.25	B-Protocol
protocol	O
to	O
provide	O
the	O
CONS	B-Protocol
.	O
</s>
<s>
Intermediate	B-Protocol
System	I-Protocol
to	I-Protocol
Intermediate	I-Protocol
System	I-Protocol
Intra-domain	O
Routing	O
Protocol	O
(	O
IS-IS	B-Protocol
)	O
-	O
ISO/IEC	O
10589	O
(	O
reprinted	O
in	O
RFC	O
1142	O
)	O
,	O
later	O
adapted	O
for	O
the	O
TCP/IP	B-Protocol
model	I-Protocol
.	O
</s>
<s>
The	O
OSI	O
model	O
made	O
this	O
layer	O
responsible	O
for	O
graceful	O
close	O
of	O
sessions	O
,	O
which	O
is	O
a	O
property	O
of	O
the	O
Transmission	O
Control	O
Protocol	O
,	O
and	O
also	O
for	O
session	O
checkpointing	O
and	O
recovery	O
,	O
which	O
is	O
not	O
usually	O
used	O
in	O
the	O
Internet	B-Protocol
Protocol	I-Protocol
Suite	I-Protocol
.	O
</s>
<s>
Association	B-Protocol
Control	I-Protocol
Service	I-Protocol
Element	I-Protocol
(	O
ACSE	O
)	O
–	O
,	O
,	O
.	O
</s>
