<s>
The	O
S	B-Protocol
interface	I-Protocol
or	O
S	O
reference	O
point	O
,	O
also	O
known	O
as	O
S0	O
,	O
is	O
a	O
user	O
–	O
network	O
interface	O
reference	O
point	O
in	O
an	O
ISDN	B-Protocol
BRI	I-Protocol
environment	O
,	O
characterized	O
by	O
a	O
four-wire	O
circuit	O
using	O
144kbit/s	O
(	O
2	O
bearer	O
and	O
1	O
signaling	O
channel	O
;	O
2B+D	B-Protocol
)	O
user	O
rate	O
.	O
</s>
<s>
The	O
S	B-Protocol
interface	I-Protocol
is	O
the	O
connection	O
between	O
ISDN	B-Protocol
terminal	O
equipment	O
(	O
TE	O
)	O
or	O
terminal	O
adapters	O
(	O
TAs	O
)	O
and	O
an	O
NT1	O
(	O
network	O
terminator	O
,	O
type	O
1	O
.	O
)	O
</s>
<s>
Not	O
all	O
TE	O
or	O
TAs	O
connect	O
externally	O
to	O
an	O
S	B-Protocol
interface	I-Protocol
,	O
but	O
instead	O
integrate	O
an	O
NT1	O
so	O
they	O
can	O
connect	O
directly	O
to	O
a	O
U	B-Protocol
interface	I-Protocol
(	O
local	O
loop	O
from	O
central	O
office	O
.	O
)	O
</s>
<s>
Contrast	O
to	O
the	O
T	B-Protocol
interface	I-Protocol
,	O
which	O
connects	O
between	O
an	O
NT2	O
(	O
PBX	O
or	O
other	O
local	O
switching	O
device	O
)	O
and	O
NT1	O
.	O
</s>
<s>
However	O
,	O
the	O
S	B-Protocol
interface	I-Protocol
is	O
electrically	O
equivalent	O
to	O
the	O
T	B-Protocol
interface	I-Protocol
,	O
and	O
the	O
two	O
are	O
jointly	O
referred	O
to	O
as	O
the	O
S/T	O
interface	O
.	O
</s>
<s>
The	O
S	B-Protocol
interface	I-Protocol
operates	O
at	O
4000	O
48-bit	O
frames	O
per	O
second	O
;	O
i.e.	O
,	O
192kbit/s	O
,	O
with	O
a	O
user	O
portion	O
of	O
36bits	O
per	O
frame	B-Protocol
;	O
i.e.	O
,	O
144kbit/s	O
.	O
</s>
