<s>
CRC-based	B-Protocol
framing	I-Protocol
is	O
a	O
kind	O
of	O
frame	O
synchronization	O
used	O
in	O
Asynchronous	O
Transfer	O
Mode	O
(	O
ATM	O
)	O
and	O
other	O
similar	O
protocols	O
.	O
</s>
<s>
The	O
concept	O
of	O
CRC-based	B-Protocol
framing	I-Protocol
was	O
developed	O
by	O
StrataCom	O
,	O
Inc	O
.	O
in	O
order	O
to	O
improve	O
the	O
efficiency	O
of	O
a	O
pre-standard	O
Asynchronous	O
Transfer	O
Mode	O
(	O
ATM	O
)	O
link	O
protocol	O
.	O
</s>
<s>
An	O
advanced	O
version	O
of	O
CRC-based	B-Protocol
framing	I-Protocol
was	O
used	O
in	O
the	O
ITU-T	O
SG15	O
G.7041	B-Protocol
Generic	B-Protocol
Framing	I-Protocol
Procedure	I-Protocol
(	O
GFP	B-Protocol
)	O
,	O
which	O
itself	O
is	O
used	O
in	O
several	O
packet	O
link	O
protocols	O
.	O
</s>
<s>
The	O
method	O
of	O
CRC-Based	B-Protocol
framing	I-Protocol
re-uses	O
the	O
header	O
cyclic	O
redundancy	O
check	O
(	O
CRC	O
)	O
,	O
which	O
is	O
present	O
in	O
ATM	O
and	O
other	O
similar	O
protocols	O
,	O
to	O
provide	O
framing	O
on	O
the	O
link	O
with	O
no	O
additional	O
overhead	O
.	O
</s>
<s>
For	O
a	O
tutorial	O
and	O
an	O
example	O
of	O
computing	O
the	O
CRC	O
see	O
mathematics	B-Algorithm
of	I-Algorithm
cyclic	I-Algorithm
redundancy	I-Algorithm
checks	I-Algorithm
.	O
</s>
<s>
A	O
receiver	O
utilizing	O
CRC-based	B-Protocol
framing	I-Protocol
bit-shifts	O
along	O
the	O
received	O
bit	O
stream	O
until	O
it	O
finds	O
a	O
bit	O
position	O
where	O
the	O
header	O
CRC	O
is	O
correct	O
for	O
a	O
number	O
of	O
times	O
.	O
</s>
<s>
In	O
links	O
where	O
there	O
is	O
already	O
a	O
byte	B-Application
lock	O
mechanism	O
present	O
such	O
as	O
within	O
an	O
E-carrier	O
or	O
SDH	B-Protocol
frame	O
,	O
the	O
receiver	O
need	O
only	O
byte-shift	O
(	O
rather	O
than	O
of	O
bit-shifting	O
)	O
along	O
the	O
receive	O
data	O
stream	O
to	O
find	O
lock	O
.	O
</s>
<s>
An	O
advanced	O
,	O
variable	O
frame	O
size	O
version	O
of	O
CRC-Based	B-Protocol
framing	I-Protocol
is	O
used	O
in	O
ITU-T	O
SG15	O
G.7041	B-Protocol
GFP	B-Protocol
links	O
where	O
it	O
is	O
known	O
as	O
Length/HEC	O
-based	O
framing	O
.	O
</s>
<s>
The	O
receiver	O
looks	O
for	O
a	O
position	O
in	O
the	O
receive	O
data	O
stream	O
following	O
the	O
rules	O
that	O
the	O
header	O
CRC/HEC	O
is	O
correct	O
and	O
the	O
byte	B-Application
offset	O
correctly	O
points	O
to	O
the	O
next	O
valid	O
header	O
CRC/HEC	O
.	O
</s>
<s>
The	O
IPX	O
used	O
24	O
byte	B-Application
cells	O
instead	O
of	O
ATM	O
's	O
53	O
byte	B-Application
cells	O
,	O
and	O
the	O
field	O
definitions	O
were	O
slightly	O
different	O
,	O
but	O
the	O
basic	O
idea	O
of	O
using	O
short	O
,	O
fixed	O
length	O
cells	O
was	O
identical	O
.	O
</s>
<s>
T1	O
is	O
a	O
time-division	B-Protocol
multiplexing	I-Protocol
(	O
TDM	O
)	O
protocol	O
with	O
24	O
byte	B-Application
payloads	O
carried	O
in	O
a	O
193	O
bit	O
frame	O
.	O
</s>
<s>
A	O
receiver	O
finds	O
this	O
special	O
pattern	O
by	O
sequentially	O
looking	O
for	O
the	O
bit	O
position	O
in	O
the	O
receive	O
data	O
where	O
a	O
bit	O
from	O
this	O
pattern	O
shows	O
up	O
every	O
193rd	O
byte	B-Application
.	O
</s>
<s>
This	O
device	O
found	O
the	O
193	O
bit	O
long	O
TDM	O
frame	O
and	O
put	O
out	O
the	O
24	O
bytes	B-Application
in	O
a	O
form	O
that	O
could	O
be	O
used	O
effectively	O
.	O
</s>
<s>
When	O
it	O
came	O
time	O
to	O
produce	O
a	O
European	O
product	O
,	O
the	O
benefit	O
of	O
using	O
24	O
byte	B-Application
frames	O
became	O
a	O
liability	O
.	O
</s>
<s>
The	O
European	O
E-carrier	O
(	O
E1	O
)	O
format	O
has	O
a	O
32	O
byte	B-Application
frame	O
of	O
which	O
30	O
bytes	B-Application
could	O
carry	O
data	O
.	O
</s>
<s>
The	O
development	O
team	O
's	O
first	O
proposal	O
used	O
the	O
HDLC	O
protocol	O
to	O
encapsulate	O
a	O
sequence	O
of	O
24	O
byte	B-Application
cells	O
into	O
a	O
byte	B-Application
stream	O
collected	O
from	O
the	O
30	O
byte	B-Application
E1	O
payloads	O
.	O
</s>
<s>
A	O
circuit	O
was	O
designed	O
which	O
examined	O
the	O
incoming	O
byte	B-Application
stream	O
emerging	O
from	O
the	O
E1	O
framer	O
device	O
and	O
found	O
a	O
byte	B-Application
position	O
for	O
which	O
the	O
header	O
CRC	O
value	O
was	O
consistently	O
correct	O
.	O
</s>
