<s>
Non-Facility	O
Associated	O
Signaling	O
or	O
NFAS	O
is	O
a	O
Primary	B-Protocol
Rate	I-Protocol
Interface	I-Protocol
configuration	O
whereby	O
multiple	O
T1	O
carriers	O
share	O
a	O
signaling	O
channel	O
(	O
or	O
D	B-Protocol
channel	I-Protocol
)	O
.	O
</s>
<s>
When	O
a	O
T1	O
circuit	O
is	O
used	O
to	O
carry	O
Primary	B-Protocol
Rate	I-Protocol
ISDN	B-Protocol
one	O
of	O
the	O
timeslots	O
is	O
used	O
to	O
carry	O
the	O
D	B-Protocol
channel	I-Protocol
.	O
</s>
<s>
A	O
single	O
Primary	B-Protocol
Rate	I-Protocol
ISDN	B-Protocol
circuit	O
is	O
thus	O
sometimes	O
described	O
as	O
23B	O
+	O
D	O
.	O
There	O
are	O
23	O
bearer	O
channels	O
carrying	O
voice	O
or	O
data	O
,	O
and	O
one	O
D	B-Protocol
channel	I-Protocol
carrying	O
the	O
Common	O
Channel	O
Signaling	O
.	O
</s>
<s>
In	O
an	O
NFAS	O
configuration	O
,	O
multiple	O
T1	O
circuits	O
share	O
a	O
single	O
D	B-Protocol
channel	I-Protocol
,	O
with	O
an	O
upper	O
limit	O
of	O
20	O
T1	O
circuits	O
in	O
a	O
single	O
NFAS	O
configuration	O
.	O
</s>
<s>
A	O
full	O
NFAS	O
configuration	O
can	O
then	O
be	O
described	O
as	O
479B	O
+	O
D	O
.	O
There	O
is	O
one	O
problem	O
;	O
a	O
failure	O
on	O
the	O
T1	O
trunk	O
carrying	O
the	O
D	B-Protocol
channel	I-Protocol
will	O
also	O
affect	O
all	O
19	O
other	O
trunks	O
.	O
</s>
<s>
The	O
solution	O
is	O
D	B-Protocol
channel	I-Protocol
backup	O
where	O
a	O
second	O
D	B-Protocol
channel	I-Protocol
is	O
configured	O
on	O
another	O
trunk	O
.	O
</s>
<s>
In	O
the	O
event	O
of	O
failure	O
the	O
backup	O
D	B-Protocol
channel	I-Protocol
takes	O
over	O
the	O
signaling	O
.	O
</s>
<s>
Customers	O
ordering	O
a	O
Primary	B-Protocol
Rate	I-Protocol
ISDN	B-Protocol
service	O
will	O
be	O
charged	O
for	O
each	O
signaling	O
channel	O
.	O
</s>
<s>
North	O
American	O
switches	O
such	O
as	O
the	O
Lucent	O
5ESS	B-Device
,	O
and	O
the	O
Nortel	O
DMS-100	B-Protocol
did	O
not	O
handle	O
common	O
channel	O
signaling	O
such	O
as	O
ISDN	B-Protocol
on	O
the	O
same	O
line	O
card	O
that	O
terminated	O
the	O
T1	O
circuit	O
.	O
</s>
<s>
So	O
,	O
the	O
telephone	O
company	O
needs	O
to	O
buy	O
and	O
maintain	O
a	O
separate	O
signaling	O
card	O
for	O
every	O
D	B-Protocol
channel	I-Protocol
.	O
</s>
<s>
A	O
Primary	B-Protocol
Rate	I-Protocol
ISDN	B-Protocol
configurations	O
uses	O
E1	O
carriers	O
,	O
where	O
each	O
carrier	O
has	O
32	O
timeslots	O
.	O
</s>
