<s>
In	O
telecommunication	O
,	O
4B5B	B-Protocol
is	O
a	O
form	O
of	O
data	O
communications	O
line	B-Protocol
code	I-Protocol
.	O
</s>
<s>
4B5B	B-Protocol
maps	O
groups	O
of	O
4	O
bits	O
of	O
data	O
onto	O
groups	O
of	O
5	O
bits	O
for	O
transmission	O
.	O
</s>
<s>
An	O
alternative	O
to	O
using	O
4B5B	B-Protocol
coding	O
is	O
to	O
use	O
a	O
scrambler	O
.	O
</s>
<s>
Some	O
systems	O
use	O
scramblers	O
in	O
conjunction	O
with	O
4B5B	B-Protocol
coding	O
to	O
assure	O
DC	O
balance	O
and	O
improve	O
electromagnetic	O
compatibility	O
.	O
</s>
<s>
4B5B	B-Protocol
was	O
popularized	O
by	O
Fiber	B-Protocol
Distributed	I-Protocol
Data	I-Protocol
Interface	I-Protocol
(	O
FDDI	B-Protocol
)	O
in	O
the	O
mid-1980s	O
.	O
</s>
<s>
It	O
was	O
adopted	O
for	O
digital	O
audio	O
transmission	O
by	O
MADI	O
in	O
1989	O
.	O
and	O
by	O
Fast	B-Protocol
Ethernet	I-Protocol
in	O
1995	O
.	O
</s>
<s>
The	O
name	O
4B5B	B-Protocol
is	O
generally	O
taken	O
to	O
mean	O
the	O
FDDI	B-Protocol
version	O
.	O
</s>
<s>
Other	O
4-to-5-bit	O
codes	O
have	O
been	O
used	O
for	O
magnetic	O
recording	O
and	O
are	O
known	O
as	O
group	B-Device
coded	I-Device
recording	I-Device
(	O
GCR	B-Device
)	O
,	O
but	O
those	O
are	O
(	O
0	O
,	O
2	O
)	O
run-length	B-Protocol
limited	I-Protocol
codes	I-Protocol
,	O
with	O
at	O
most	O
two	O
consecutive	O
zeros	O
.	O
</s>
<s>
4B5B	B-Protocol
allows	O
up	O
to	O
three	O
consecutive	O
zeros	O
(	O
a	O
(	O
0	O
,	O
3	O
)	O
RLL	B-Protocol
code	I-Protocol
)	O
,	O
providing	O
a	O
greater	O
variety	O
of	O
control	O
codes	O
.	O
</s>
<s>
On	O
optical	O
fiber	O
,	O
the	O
4B5B	B-Protocol
output	O
is	O
NRZI-encoded	O
.	O
</s>
<s>
FDDI	B-Protocol
over	O
copper	O
(	O
CDDI	B-Protocol
)	O
uses	O
MLT-3	B-Protocol
encoding	I-Protocol
instead	O
,	O
as	O
does	O
100BASE-TX	B-Protocol
Fast	B-Protocol
Ethernet	I-Protocol
.	O
</s>
<s>
The	O
4B5B	B-Protocol
encoding	O
is	O
also	O
used	O
for	O
USB	O
Power	O
Delivery	O
communication	O
,	O
where	O
it	O
is	O
sent	O
over	O
the	O
USB-C	O
CC	O
pin	O
(	O
further	O
encoded	O
using	O
biphase	B-Protocol
mark	I-Protocol
code	I-Protocol
)	O
or	O
the	O
USB-A/B	O
power	O
lines	O
(	O
further	O
encoded	O
using	O
frequency-shift	O
keying	O
)	O
.	O
</s>
<s>
4B5B	B-Protocol
codes	O
are	O
designed	O
to	O
produce	O
at	O
least	O
two	O
transitions	O
per	O
5	O
bits	O
of	O
output	O
code	O
regardless	O
of	O
input	O
data	O
.	O
</s>
<s>
4B5B	B-Protocol
solves	O
this	O
problem	O
by	O
assigning	O
the	O
4-bit	O
block	O
a	O
5-bit	O
code	O
,	O
in	O
this	O
case	O
,	O
11110	O
.	O
</s>
