<s>
NO	B-Device
CARRIER	I-Device
(	O
capitalized	O
)	O
is	O
a	O
text	O
message	O
transmitted	O
from	O
a	O
modem	B-Application
to	O
its	O
attached	O
device	O
(	O
typically	O
a	O
computer	O
)	O
,	O
indicating	O
the	O
modem	B-Application
is	O
not	O
(	O
or	O
no	O
longer	O
)	O
connected	O
to	O
a	O
remote	O
system	O
.	O
</s>
<s>
NO	B-Device
CARRIER	I-Device
is	O
a	O
response	O
message	O
that	O
is	O
defined	O
in	O
the	O
Hayes	B-Device
command	I-Device
set	I-Device
.	O
</s>
<s>
Due	O
to	O
the	O
popularity	O
of	O
Hayes	O
modems	B-Application
during	O
the	O
heyday	O
of	O
dial-up	O
connectivity	O
,	O
most	O
other	O
modem	B-Application
manufacturers	O
supported	O
the	O
Hayes	B-Device
command	I-Device
set	I-Device
.	O
</s>
<s>
For	O
this	O
reason	O
,	O
the	O
NO	B-Device
CARRIER	I-Device
message	O
was	O
ubiquitously	O
understood	O
to	O
mean	O
that	O
one	O
was	O
no	O
longer	O
connected	O
to	O
a	O
remote	O
system	O
.	O
</s>
<s>
A	O
carrier	O
tone	O
is	O
an	O
audio	O
carrier	O
signal	O
used	O
by	O
two	O
modems	B-Application
to	O
suppress	O
echo	O
cancellation	O
and	O
establish	O
a	O
baseline	O
frequency	O
for	O
communication	O
.	O
</s>
<s>
When	O
the	O
answering	O
modem	B-Application
detects	O
a	O
ringtone	O
on	O
the	O
phone	O
line	O
,	O
it	O
picks	O
up	O
that	O
line	O
and	O
starts	O
transmitting	O
a	O
carrier	O
tone	O
.	O
</s>
<s>
If	O
it	O
does	O
not	O
receive	O
data	O
from	O
the	O
calling	O
modem	B-Application
within	O
a	O
set	O
amount	O
of	O
time	O
,	O
it	O
disconnects	O
the	O
line	O
.	O
</s>
<s>
The	O
calling	O
modem	B-Application
waits	O
for	O
the	O
tone	O
after	O
it	O
dials	O
the	O
phone	O
line	O
before	O
it	O
initiates	O
data	O
transmission	O
.	O
</s>
<s>
If	O
it	O
does	O
not	O
receive	O
a	O
carrier	O
tone	O
within	O
a	O
set	O
amount	O
of	O
time	O
,	O
it	O
will	O
disconnect	O
the	O
phone	O
line	O
and	O
issues	O
the	O
NO	B-Device
CARRIER	I-Device
message	O
.	O
</s>
<s>
The	O
actual	O
data	O
is	O
transmitted	O
from	O
the	O
answering	O
modem	B-Application
to	O
the	O
calling	O
modem	B-Application
via	O
modulation	B-Application
of	O
the	O
carrier	O
.	O
</s>
<s>
The	O
NO	B-Device
CARRIER	I-Device
message	O
is	O
issued	O
by	O
a	O
modem	B-Application
for	O
any	O
of	O
the	O
following	O
reasons	O
:	O
</s>
<s>
A	O
dial	O
(	O
ATD	O
)	O
or	O
answer	O
(	O
ATA	O
)	O
command	O
did	O
not	O
result	O
in	O
a	O
successful	O
connection	O
to	O
another	O
modem	B-Application
,	O
and	O
the	O
reason	O
was	O
n't	O
that	O
the	O
line	O
was	O
BUSY	O
(	O
a	O
separately	O
defined	O
message	O
)	O
.	O
</s>
<s>
A	O
previously	O
established	O
data	O
connection	O
has	O
ended	O
(	O
either	O
at	O
the	O
attached	O
computer	O
's	O
command	O
,	O
or	O
as	O
a	O
result	O
of	O
being	O
disconnected	O
from	O
the	O
remote	O
end	O
)	O
,	O
and	O
the	O
modem	B-Application
has	O
now	O
gone	O
from	O
the	O
data	B-Device
mode	I-Device
to	O
the	O
command	B-Device
mode	I-Device
.	O
</s>
<s>
As	O
modems	B-Application
have	O
progressed	O
to	O
pack	O
more	O
bits	O
per	O
second	O
over	O
a	O
telephone	O
line	O
,	O
the	O
specific	O
implementation	O
involves	O
modulating	O
on	O
other	O
than	O
the	O
audio-frequency	O
carrier	O
(	O
particularly	O
on	O
digital	O
modems	B-Application
that	O
connect	O
via	O
ISDN	B-Protocol
or	O
cellular	O
networks	O
)	O
,	O
the	O
message	O
NO	B-Device
CARRIER	I-Device
remains	O
consistent	O
for	O
the	O
sake	O
of	O
compatibility	O
.	O
</s>
<s>
Linux	B-Application
's	O
network	B-Protocol
stack	I-Protocol
uses	O
the	O
NO	B-Device
CARRIER	I-Device
status	O
for	O
a	O
network	B-Protocol
interface	I-Protocol
that	O
is	O
turned	O
on	O
(	O
"	O
up	O
"	O
)	O
but	O
cannot	O
be	O
connected	O
because	O
the	O
physical	B-Application
layer	I-Application
is	O
not	O
operating	O
properly	O
,	O
e.g.	O
</s>
<s>
The	O
NO	B-Device
CARRIER	I-Device
message	O
can	O
be	O
used	O
at	O
the	O
end	O
of	O
a	O
sentence	O
for	O
humorous	O
purposes	O
in	O
Internet	O
messages	O
and	O
forum	O
posts	O
,	O
signifying	O
that	O
the	O
person	O
typing	O
the	O
message	O
was	O
suddenly	O
cut	O
off	O
.	O
</s>
<s>
The	O
NO	B-Device
CARRIER	I-Device
message	O
is	O
usually	O
preceded	O
by	O
a	O
string	O
of	O
garbage	O
characters	O
,	O
as	O
a	O
parody	O
of	O
what	O
used	O
to	O
frequently	O
happen	O
to	O
users	O
of	O
dialup	O
BBSes	O
whose	O
telephones	O
suddenly	O
dropped	O
signal	O
in	O
the	O
middle	O
of	O
a	O
message	O
.	O
</s>
<s>
(	O
)	O
This	O
joke	O
anachronistically	O
continues	O
to	O
be	O
seen	O
on	O
the	O
Internet	O
despite	O
the	O
waning	O
use	O
of	O
modems	B-Application
in	O
favor	O
of	O
broadband	B-General_Concept
connectivity	O
.	O
</s>
