<s>
Alternate	O
mark	O
inversion	O
(	O
AMI	O
)	O
line	B-Protocol
codes	I-Protocol
are	O
modified	O
by	O
deliberate	O
insertion	O
of	O
bipolar	B-Protocol
violations	I-Protocol
.	O
</s>
<s>
The	O
clock	O
rate	O
of	O
an	O
incoming	O
T-carrier	O
is	O
extracted	O
from	O
its	O
bipolar	O
line	B-Protocol
code	I-Protocol
.	O
</s>
<s>
To	O
prevent	O
loss	O
of	O
synchronization	O
when	O
a	O
long	O
string	O
of	O
zeros	O
is	O
present	O
in	O
the	O
payload	O
,	O
deliberate	O
bipolar	B-Protocol
violations	I-Protocol
are	O
inserted	O
into	O
the	O
line	B-Protocol
code	I-Protocol
,	O
to	O
create	O
a	O
sufficient	O
number	O
of	O
transitions	O
to	O
maintain	O
synchronization	O
;	O
this	O
is	O
a	O
form	O
of	O
run	B-Protocol
length	I-Protocol
limited	I-Protocol
coding	I-Protocol
.	O
</s>
<s>
The	O
receive	O
terminal	O
equipment	O
recognizes	O
the	O
bipolar	B-Protocol
violations	I-Protocol
and	O
removes	O
from	O
the	O
user	O
data	O
the	O
marks	O
attributable	O
to	O
the	O
bipolar	B-Protocol
violations	I-Protocol
.	O
</s>
<s>
When	O
voice	O
signals	O
are	O
digitized	O
for	O
transmission	O
via	O
T-carrier	O
,	O
the	O
data	B-General_Concept
stream	I-General_Concept
always	O
includes	O
ample	O
1	O
bits	O
to	O
maintain	O
synchronization	O
.	O
</s>
<s>
(	O
To	O
help	O
this	O
,	O
the	O
μ-law	B-Algorithm
algorithm	I-Algorithm
for	O
digitizing	O
voice	O
signals	O
encodes	O
silence	O
as	O
a	O
continuous	O
stream	O
of	O
1	O
bits	O
.	O
)	O
</s>
<s>
However	O
,	O
when	O
used	O
for	O
the	O
transmission	O
of	O
digital	B-General_Concept
data	I-General_Concept
,	O
the	O
conventional	O
AMI	O
line	B-Protocol
code	I-Protocol
may	O
fail	O
to	O
have	O
sufficient	O
marks	O
to	O
permit	O
recovery	O
of	O
the	O
incoming	O
clock	O
,	O
and	O
synchronization	O
is	O
lost	O
.	O
</s>
<s>
The	O
exact	O
pattern	O
of	O
bipolar	B-Protocol
violations	I-Protocol
that	O
is	O
transmitted	O
in	O
any	O
given	O
case	O
depends	O
on	O
the	O
line	O
rate	O
(	O
i.e.	O
,	O
the	O
level	O
of	O
the	O
line	B-Protocol
code	I-Protocol
in	O
the	O
T-carrier	O
hierarchy	O
)	O
and	O
the	O
polarity	O
of	O
the	O
last	O
valid	O
mark	O
in	O
the	O
user	O
data	O
prior	O
to	O
the	O
unacceptably	O
long	O
string	O
of	O
zeros	O
.	O
</s>
<s>
In	O
the	O
descriptions	O
below	O
,	O
""	O
denotes	O
a	O
balancing	O
mark	O
with	O
the	O
opposite	O
polarity	O
to	O
that	O
of	O
the	O
preceding	O
mark	O
,	O
while	O
""	O
denotes	O
a	O
bipolar	B-Protocol
violation	I-Protocol
mark	O
,	O
which	O
has	O
the	O
same	O
polarity	O
as	O
the	O
preceding	O
mark	O
.	O
</s>
<s>
This	O
happens	O
automatically	O
for	O
balancing	O
(	O
)	O
marks	O
,	O
but	O
the	O
line	B-Protocol
code	I-Protocol
must	O
ensure	O
that	O
positive	O
and	O
negative	O
violation	O
marks	O
balance	O
each	O
other	O
.	O
</s>
<s>
The	O
first	O
technique	O
used	O
to	O
ensure	O
a	O
minimum	O
density	O
of	O
marks	O
was	O
zero	B-Protocol
code	I-Protocol
suppression	I-Protocol
a	O
form	O
of	O
bit	B-Protocol
stuffing	I-Protocol
,	O
which	O
set	O
the	O
least	O
significant	O
bit	O
of	O
each	O
8-bit	O
byte	O
transmitted	O
to	O
a	O
1	O
.	O
</s>
<s>
Increased	O
demand	O
for	O
bandwidth	O
,	O
and	O
compatibility	O
with	O
the	O
G.703	O
and	O
ISDN	B-Protocol
PRI	I-Protocol
standards	O
which	O
called	O
for	O
64,000	O
bits	O
per	O
second	O
,	O
led	O
to	O
this	O
system	O
being	O
superseded	O
by	O
B8ZS	O
.	O
</s>
<s>
Commonly	O
used	O
in	O
the	O
North	O
American	O
T1	O
(	O
Digital	B-Protocol
Signal	I-Protocol
1	I-Protocol
)	O
1.544	O
Mbit/s	O
line	B-Protocol
code	I-Protocol
,	O
bipolar	O
with	O
eight-zero	O
substitution	O
(	O
B8ZS	O
)	O
replaces	O
each	O
string	O
of	O
8	O
consecutive	O
zeros	O
with	O
the	O
special	O
pattern	O
""	O
.	O
</s>
<s>
At	O
the	O
North	O
American	O
T2	O
rate	O
(	O
6.312	O
Mbit/s	O
)	O
,	O
bipolar	B-Protocol
violations	I-Protocol
are	O
inserted	O
if	O
6	O
or	O
more	O
consecutive	O
zeros	O
occur	O
.	O
</s>
<s>
This	O
line	B-Protocol
code	I-Protocol
is	O
called	O
bipolar	O
with	O
six-zero	O
substitution	O
(	O
B6ZS	O
)	O
,	O
and	O
replaces	O
6	O
consecutive	O
zeros	O
with	O
the	O
pattern	O
""	O
.	O
</s>
<s>
At	O
the	O
North	O
American	O
T3	O
rate	O
(	O
44.736	O
Mbit/s	O
)	O
,	O
bipolar	B-Protocol
violations	I-Protocol
are	O
inserted	O
if	O
3	O
or	O
more	O
consecutive	O
zeros	O
occur	O
.	O
</s>
<s>
This	O
line	B-Protocol
code	I-Protocol
is	O
called	O
bipolar	O
with	O
three-zero	O
substitution	O
(	O
B3ZS	O
)	O
,	O
and	O
is	O
very	O
similar	O
to	O
HDB3	O
.	O
</s>
