<s>
Segmentation	B-Protocol
and	I-Protocol
reassembly	I-Protocol
(	O
SAR	O
)	O
is	O
the	O
process	O
used	O
to	O
fragment	O
and	O
reassemble	O
variable	O
length	O
packets	B-Protocol
into	O
fixed	O
length	O
cells	O
so	O
as	O
to	O
allow	O
them	O
to	O
be	O
transported	O
across	O
Asynchronous	O
Transfer	O
Mode	O
(	O
ATM	O
)	O
networks	O
or	O
other	O
cell	O
based	O
infrastructures	O
.	O
</s>
<s>
Since	O
ATM	O
's	O
payload	O
is	O
only	O
48	O
bytes	O
,	O
nearly	O
every	O
packet	B-Protocol
from	O
any	O
other	O
protocol	O
has	O
to	O
be	O
processed	O
in	O
this	O
way	O
.	O
</s>
<s>
The	O
process	O
is	O
conceptually	O
simple	O
:	O
an	O
incoming	O
packet	B-Protocol
from	O
another	O
protocol	O
to	O
be	O
transmitted	O
across	O
the	O
ATM	O
network	O
is	O
chopped	O
up	O
into	O
segments	O
that	O
fit	O
into	O
48-byte	O
chunks	O
carried	O
as	O
ATM	O
cell	O
payloads	O
.	O
</s>
<s>
At	O
the	O
far	O
end	O
,	O
these	O
chunks	O
are	O
fitted	O
back	O
together	O
to	O
reconstitute	O
the	O
original	O
packet	B-Protocol
.	O
</s>
<s>
The	O
process	O
is	O
analogous	O
to	O
the	O
fragmentation	B-Protocol
of	I-Protocol
IP	I-Protocol
packets	I-Protocol
on	O
reaching	O
an	O
interface	O
with	O
a	O
maximum	B-Protocol
transmission	I-Protocol
unit	I-Protocol
(	O
MTU	B-Protocol
)	O
less	O
than	O
the	O
packet	B-Protocol
size	O
and	O
the	O
subsequent	O
reassembly	O
of	O
the	O
original	O
packet	B-Protocol
once	O
the	O
fragments	O
have	O
reached	O
the	O
original	O
packet	B-Protocol
's	O
destination	O
.	O
</s>
<s>
There	O
are	O
several	O
different	O
schemes	O
,	O
referred	O
to	O
as	O
ATM	B-Protocol
adaptation	I-Protocol
layers	I-Protocol
(	O
AAL	B-Protocol
)	O
.	O
</s>
<s>
AAL1	B-Protocol
–	O
Constant	O
bitrate	O
,	O
circuit	O
emulation	O
(	O
T1	O
,	O
E1	O
,	O
etc	O
.	O
)	O
</s>
<s>
AAL2	B-Protocol
–	O
Variable	O
bitrate	O
synchronous	O
traffic	O
,	O
eous	O
traffic	O
,	O
e.g.	O
</s>
