<s>
X.21	B-Protocol
(	O
sometimes	O
referred	O
to	O
as	O
X21	B-Protocol
)	O
is	O
an	O
interface	O
specification	O
for	O
differential	O
communications	O
introduced	O
in	O
the	O
mid-1970s	O
by	O
the	O
ITU-T	O
.	O
X.21	B-Protocol
was	O
first	O
introduced	O
as	O
a	O
means	O
to	O
provide	O
a	O
digital	O
signaling	O
interface	O
for	O
telecommunications	O
between	O
carriers	O
and	O
customers	O
 '	O
equipment	O
.	O
</s>
<s>
This	O
includes	O
specifications	O
for	O
DTE/DCE	O
physical	O
interface	O
elements	O
,	O
alignment	O
of	O
call	O
control	O
characters	O
and	O
error	B-Error_Name
checking	I-Error_Name
,	O
elements	O
of	O
the	O
call	O
control	O
phase	O
for	O
circuit	B-Protocol
switching	I-Protocol
services	O
,	O
and	O
test	O
loops	O
.	O
</s>
<s>
When	O
X.21	B-Protocol
is	O
used	O
with	O
V.11	O
,	O
it	O
provides	O
synchronous	O
data	O
transmission	O
at	O
rates	O
from	O
600	O
bit/s	O
to	O
10	O
Mbit/s	O
.	O
</s>
<s>
There	O
is	O
also	O
a	O
variant	O
of	O
X.21	B-Protocol
that	O
is	O
only	O
used	O
in	O
select	O
legacy	O
applications	O
,	O
“	O
circuit	B-Protocol
switched	I-Protocol
X.21	B-Protocol
”	O
.	O
</s>
<s>
X.21	B-Protocol
normally	O
is	O
found	O
on	O
a	O
15-pin	O
D-sub	O
connector	O
and	O
is	O
capable	O
of	O
running	O
full-duplex	O
data	O
transmissions	O
.	O
</s>
<s>
X.21	B-Protocol
was	O
primarily	O
used	O
in	O
Europe	O
and	O
Japan	O
,	O
for	O
example	O
in	O
the	O
Scandinavian	O
DATEX	O
and	O
German	O
circuit-switched	B-Protocol
networks	I-Protocol
during	O
the	O
1980s	O
.	O
</s>
