<s>
Two-binary	B-Protocol
,	I-Protocol
one-quaternary	I-Protocol
(	O
2B1Q	B-Protocol
)	O
is	O
a	O
line	B-Protocol
code	I-Protocol
used	O
in	O
the	O
U	B-Protocol
interface	I-Protocol
of	O
the	O
Integrated	B-Protocol
Services	I-Protocol
Digital	I-Protocol
Network	I-Protocol
(	O
ISDN	B-Protocol
)	O
Basic	B-Protocol
Rate	I-Protocol
Interface	I-Protocol
(	O
BRI	O
)	O
and	O
the	O
high-bit-rate	O
digital	O
subscriber	O
line	O
(	O
HDSL	O
)	O
.	O
</s>
<s>
2B1Q	B-Protocol
is	O
a	O
four-level	O
pulse-amplitude	B-Device
modulation	I-Device
(	O
PAM-4	O
)	O
scheme	O
without	O
redundancy	O
,	O
mapping	O
two	O
bits	O
(	O
2B	O
)	O
into	O
one	O
quaternary	O
symbol	O
(	O
1Q	O
)	O
.	O
</s>
<s>
A	O
competing	O
encoding	O
technique	O
in	O
the	O
ISDN	B-Protocol
basic	B-Protocol
rate	I-Protocol
U	B-Protocol
interface	I-Protocol
,	O
mainly	O
used	O
in	O
Europe	O
,	O
is	O
4B3T	B-Protocol
.	O
</s>
<s>
To	O
minimize	O
error	B-General_Concept
propagation	I-General_Concept
,	O
bit	O
pairs	O
(	O
dibits	O
)	O
are	O
assigned	O
to	O
voltage	O
levels	O
according	O
to	O
a	O
Gray	B-Device
code	I-Device
,	O
as	O
follows	O
:	O
</s>
<s>
2B1Q	B-Protocol
code	O
is	O
not	O
DC-balanced	O
.	O
</s>
