<s>
The	O
Primary	B-Protocol
Rate	I-Protocol
Interface	I-Protocol
(	O
PRI	O
)	O
is	O
a	O
telecommunications	O
interface	O
standard	O
used	O
on	O
an	O
Integrated	B-Protocol
Services	I-Protocol
Digital	I-Protocol
Network	I-Protocol
(	O
ISDN	B-Protocol
)	O
for	O
carrying	O
multiple	O
DS0	O
voice	O
and	O
data	O
transmissions	O
between	O
the	O
network	O
and	O
a	O
user	O
.	O
</s>
<s>
The	O
E1	O
carrier	O
provides	O
30	O
B	O
-	O
and	O
one	O
D-channel	B-Protocol
for	O
a	O
bandwidth	O
of	O
2.048	O
Mbit/s	O
.	O
</s>
<s>
The	O
first	O
timeslot	O
on	O
the	O
E1	O
is	O
used	O
for	O
synchronization	O
purposes	O
and	O
is	O
not	O
considered	O
to	O
be	O
a	O
B	O
-	O
or	O
D-channel	B-Protocol
.	O
</s>
<s>
The	O
D-channel	B-Protocol
typically	O
uses	O
timeslot	O
16	O
on	O
an	O
E1	O
,	O
while	O
it	O
is	O
timeslot	O
24	O
for	O
a	O
T1	O
.	O
</s>
<s>
The	O
Integrated	B-Protocol
Services	I-Protocol
Digital	I-Protocol
Network	I-Protocol
(	O
ISDN	B-Protocol
)	O
prescribes	O
two	O
levels	O
of	O
service	O
:	O
</s>
<s>
Basic	B-Protocol
Rate	I-Protocol
Interface	I-Protocol
(	O
BRI	O
)	O
:	O
one	O
16-kbit/s	O
D	B-Protocol
channel	I-Protocol
with	O
two	O
64-kbit/s	O
B	B-Protocol
channels	I-Protocol
,	O
intended	O
for	O
small	O
enterprises	O
and	O
residential	O
service	O
.	O
</s>
<s>
Primary	B-Protocol
Rate	I-Protocol
Interface	I-Protocol
(	O
PRI	O
)	O
:	O
one	O
64-kbit/s	O
D	B-Protocol
channel	I-Protocol
with	O
23	O
(	O
1.544	O
Mbit/s	O
T1	O
,	O
a.k.a.	O
</s>
<s>
"	O
30B	O
+	O
D	O
"	O
)	O
64-kbit/s	O
B	B-Protocol
channels	I-Protocol
,	O
intended	O
for	O
large	O
organizations	O
.	O
</s>
<s>
The	O
D-channel	B-Protocol
carries	O
control	O
and	O
signaling	O
information	O
.	O
</s>
<s>
A	O
dual	O
T1-PRI	O
could	O
have	O
24	O
+	O
23	O
=	O
47	O
B-channels	O
and	O
1	O
D-channel	B-Protocol
(	O
often	O
called	O
"	O
47B	O
+	O
D	O
"	O
)	O
,	O
but	O
more	O
commonly	O
has	O
46	O
B-channels	O
and	O
2	O
D-channels	B-Protocol
thus	O
providing	O
a	O
backup	O
signaling	O
channel	O
.	O
</s>
<s>
When	O
configuring	O
multiple	O
T1	O
's	O
as	O
ISDN-PRI	O
'	O
s	O
,	O
it	O
's	O
possible	O
to	O
use	O
NFAS	O
(	O
non-facility	O
associated	O
signaling	O
)	O
to	O
enable	O
one	O
or	O
two	O
D-channels	B-Protocol
to	O
support	O
additional	O
B-channels	O
on	O
separate	O
T1	O
circuits	O
.	O
</s>
<s>
The	O
Primary	B-Protocol
Rate	I-Protocol
Interface	I-Protocol
channels	O
are	O
typically	O
used	O
by	O
medium	O
to	O
large	O
enterprises	O
with	O
digital	O
private	B-Device
branch	I-Device
exchange	I-Device
(	O
PBX	O
)	O
telephone	O
systems	O
to	O
provide	O
digital	O
access	O
to	O
the	O
public	O
switched	O
telephone	O
network	O
(	O
PSTN	O
)	O
.	O
</s>
<s>
PRI	O
channels	O
and	O
direct	B-Device
inward	I-Device
dialing	I-Device
are	O
also	O
common	O
as	O
a	O
means	O
of	O
delivering	O
inbound	O
calls	O
to	O
voice	B-Application
over	I-Application
IP	I-Application
gateways	O
from	O
the	O
PSTN	O
.	O
</s>
