<s>
4B3T	B-Protocol
,	O
which	O
stands	O
for	O
4	O
(	O
four	O
)	O
binary	O
3	O
(	O
three	O
)	O
ternary	B-Protocol
,	O
is	O
a	O
line	B-Protocol
encoding	I-Protocol
scheme	O
used	O
for	O
ISDN	B-Protocol
PRI	O
interface	O
.	O
</s>
<s>
4B3T	B-Protocol
represents	O
four	O
binary	O
bits	O
using	O
three	O
pulses	O
.	O
</s>
<s>
4B3T	B-Protocol
uses	O
a	O
paired	B-Protocol
disparity	I-Protocol
code	I-Protocol
to	O
achieve	O
an	O
overall	O
zero	O
DC	O
bias	O
:	O
six	O
triplets	O
are	O
used	O
which	O
have	O
no	O
DC	O
component	O
(	O
0+−	O
,	O
0−+	O
,	O
+0−	O
,	O
−0+	O
,	O
+−0	O
,	O
−+0	O
)	O
,	O
and	O
the	O
remaining	O
20	O
are	O
grouped	O
into	O
10	O
pairs	O
with	O
differing	O
disparity	O
(	O
e.g.	O
</s>
<s>
This	O
mapping	O
from	O
4	O
bits	O
to	O
three	O
ternary	B-Protocol
states	O
is	O
given	O
in	O
a	O
table	O
known	O
as	O
Modified	O
Monitoring	O
State	O
43	O
(	O
MMS43	B-Protocol
)	O
.	O
</s>
<s>
A	O
competing	O
encoding	O
technique	O
,	O
used	O
for	O
the	O
ISDN	B-Protocol
basic	B-Protocol
rate	I-Protocol
interface	I-Protocol
where	O
4B3T	B-Protocol
is	O
not	O
used	O
,	O
is	O
2B1Q	B-Protocol
.	O
</s>
<s>
The	O
sync	O
sequence	O
used	O
is	O
the	O
11-symbol	O
Barker	B-Protocol
code	I-Protocol
,	O
+++−−−+−−+−	O
or	O
its	O
reverse	O
,	O
−+−−+−−−+++	O
.	O
</s>
<s>
Decoding	O
is	O
simpler	O
,	O
as	O
the	O
decoder	O
does	O
not	O
need	O
to	O
keep	O
track	O
of	O
the	O
encoder	O
state	O
,	O
although	O
doing	O
so	O
allows	O
greater	O
error	B-Error_Name
detection	I-Error_Name
.	O
</s>
