<s>
ATM	B-Protocol
Adaptation	I-Protocol
Layer	I-Protocol
2	I-Protocol
(	O
AAL2	B-Protocol
)	O
is	O
an	O
Asynchronous	O
Transfer	O
Mode	O
(	O
ATM	O
)	O
adaptation	B-Protocol
layer	I-Protocol
,	O
used	O
primarily	O
in	O
telecommunications	O
;	O
for	O
example	O
,	O
it	O
is	O
used	O
for	O
the	O
Iu	O
interfaces	O
in	O
the	O
Universal	B-Protocol
Mobile	I-Protocol
Telecommunications	I-Protocol
System	I-Protocol
,	O
and	O
is	O
also	O
used	O
for	O
transporting	O
digital	O
voice	O
.	O
</s>
<s>
The	O
standard	O
specifications	O
related	O
to	O
AAL2	B-Protocol
are	O
ITU	O
standards	O
I.363.2	O
and	O
I366.1	O
.	O
</s>
<s>
AAL2	B-Protocol
is	O
a	O
variable-bitrate	B-General_Concept
connection-oriented	B-Protocol
low-latency	O
service	O
originally	O
intended	O
to	O
adapt	O
voice	O
for	O
transmission	O
over	O
ATM	O
.	O
</s>
<s>
Like	O
other	O
ATM	B-Protocol
adaptation	I-Protocol
layers	I-Protocol
,	O
AAL2	B-Protocol
defines	O
segmentation	O
and	O
reassembly	O
of	O
higher-layer	O
packets	O
into	O
ATM	O
cells	O
,	O
in	O
this	O
case	O
packets	O
of	O
data	O
containing	O
voice	O
and	O
control	O
information	O
.	O
</s>
<s>
AAL2	B-Protocol
is	O
further	O
separated	O
into	O
two	O
sub-layers	O
that	O
help	O
with	O
the	O
mapping	O
from	O
upper-layer	O
services	O
to	O
ATM	O
cells	O
.	O
</s>
<s>
The	O
AAL2	B-Protocol
protocol	O
improves	O
on	O
other	O
ATM	B-Protocol
Adaptation	I-Protocol
Layers	I-Protocol
,	O
by	O
packing	O
lots	O
of	O
small	O
packets	O
efficiently	O
into	O
one	O
standard-sized	O
ATM	O
cell	O
of	O
53	O
bytes	O
.	O
</s>
<s>
The	O
transport	O
networks	O
for	O
ATM	O
are	O
well	O
standardized	O
fiber	B-Architecture
optic	I-Architecture
(	O
SDH/Sonet	O
,	O
i.e.	O
</s>
<s>
E1/T1/JT1	O
or	O
higher	O
bandwidth	O
fixed	O
lines	O
)	O
based	O
synchronous	O
networks	O
with	O
built-in	O
redundancy	O
and	O
OAM-related	O
network	O
features	O
which	O
Ethernet	O
networks	O
never	O
had	O
originally	O
(	O
in	O
order	O
to	O
keep	O
things	O
simple	O
)	O
but	O
are	O
sorely	O
missed	O
in	O
metro	B-Protocol
Ethernet	I-Protocol
standard	O
networks	O
.	O
</s>
<s>
AAL2	B-Protocol
is	O
one	O
example	O
of	O
a	O
useful	O
benefit	O
of	O
ATM	O
,	O
as	O
a	O
general	O
standard	O
for	O
Layer	B-Protocol
2	I-Protocol
protocols	O
.	O
</s>
<s>
ATM/AAL2	O
'	O
s	O
efficient	O
handling	O
of	O
small	O
packets	O
contrasts	O
with	O
Ethernet	O
's	O
minimum	O
payload	O
of	O
46	O
bytes	O
vs	O
the	O
1-byte	O
minimum	O
size	O
for	O
an	O
AAL2	B-Protocol
CPS	O
packet	O
.	O
</s>
<s>
AAL2	B-Protocol
is	O
the	O
standard	O
layer	B-Protocol
2	I-Protocol
protocol	O
used	O
in	O
all	O
Iu	O
interfaces	O
,	O
i.e.	O
</s>
<s>
the	O
interfaces	O
between	O
UMTS	B-Protocol
base	O
stations	O
and	O
UMTS	B-Protocol
Radio	O
Network	O
Controllers	O
(	O
RNCs	O
)	O
(	O
Iu-B	O
)	O
,	O
inter-RNCs	O
(	O
Iu-R	O
)	O
,	O
UMTS	B-Protocol
RNCs	O
and	O
UMTS	B-Protocol
Serving	O
GPRS	O
Support	O
Nodes	O
(	O
SGSNs	O
)	O
(	O
Iu-PS	O
)	O
,	O
and	O
UMTS	B-Protocol
RNCs	O
and	O
media	O
gateways	O
(	O
MGWs	O
)	O
(	O
Iu-CS	O
)	O
.	O
</s>
<s>
The	O
basic	O
component	O
of	O
AAL2	B-Protocol
is	O
the	O
CPS	O
packet	O
.	O
</s>
<s>
The	O
Parity	B-Error_Name
bit	I-Error_Name
is	O
used	O
to	O
detect	O
error	O
in	O
the	O
OSF	O
and	O
SN	O
fields	O
.	O
</s>
<s>
One	O
common	O
adaptation	O
of	O
AAL2	B-Protocol
,	O
AAL2u	O
,	O
does	O
n't	O
use	O
the	O
STF	O
field	O
at	O
all	O
.	O
</s>
<s>
that	O
needs	O
to	O
be	O
done	O
for	O
standard	O
AAL2	B-Protocol
either	O
is	O
too	O
strenuous	O
,	O
is	O
unsupported	O
,	O
or	O
requires	O
too	O
much	O
overhead	O
(	O
i.e.	O
</s>
<s>
Most	O
computer	O
chips	O
do	O
not	O
support	O
AAL2	B-Protocol
,	O
so	O
stripping	O
this	O
layer	O
away	O
makes	O
it	O
easier	O
to	O
interwork	O
between	O
the	O
ATM	O
interface	O
and	O
the	O
rest	O
of	O
the	O
network	O
.	O
</s>
<s>
The	O
following	O
is	O
diagram	O
of	O
the	O
AAL2	B-Protocol
ATM	O
cell	O
:	O
</s>
<s>
The	O
AAL2	B-Protocol
channel	O
is	O
a	O
bi-directional	O
channel	O
and	O
the	O
same	O
channel	O
identification	O
value	O
is	O
used	O
for	O
both	O
directions	O
.	O
</s>
<s>
This	O
is	O
Header	O
Error	B-Error_Name
Control	I-Error_Name
and	O
checks	O
for	O
errors	O
in	O
the	O
CID	O
,	O
LI	O
and	O
UUI	O
fields	O
.	O
</s>
