<s>
The	O
Hayes	B-Device
command	I-Device
set	I-Device
(	O
also	O
known	O
as	O
the	O
AT	B-Device
command	I-Device
set	I-Device
)	O
is	O
a	O
specific	O
command	B-Language
language	I-Language
originally	O
developed	O
by	O
Dennis	O
Hayes	O
for	O
the	O
Hayes	O
Smartmodem	O
300	O
baud	O
modem	B-Application
in	O
1981	O
.	O
</s>
<s>
The	O
vast	O
majority	O
of	O
dial-up	B-Application
modems	I-Application
use	O
the	O
Hayes	B-Device
command	I-Device
set	I-Device
in	O
numerous	O
variations	O
.	O
</s>
<s>
The	O
command	O
set	O
covered	O
only	O
those	O
operations	O
supported	O
by	O
the	O
earliest	O
300	O
bit/s	O
modems	B-Application
.	O
</s>
<s>
When	O
new	O
commands	O
were	O
required	O
to	O
control	O
additional	O
functionality	O
in	O
higher	O
speed	O
modems	B-Application
,	O
a	O
variety	O
of	O
one-off	O
standards	O
emerged	O
from	O
each	O
of	O
the	O
major	O
vendors	O
.	O
</s>
<s>
Many	O
of	O
these	O
were	O
re-standardized	O
on	O
the	O
Hayes	O
extensions	O
after	O
the	O
introduction	O
of	O
the	O
SupraFAXModem	B-Device
14400	I-Device
and	O
the	O
market	O
consolidation	O
that	O
followed	O
.	O
</s>
<s>
The	O
term	O
"	O
Hayes	B-Device
compatible	I-Device
"	O
was	O
and	O
as	O
of	O
2018	O
still	O
is	O
important	O
within	O
the	O
industry	O
.	O
</s>
<s>
Prior	O
to	O
the	O
introduction	O
of	O
the	O
Bulletin	B-Application
Board	I-Application
System	I-Application
(	O
BBS	O
)	O
,	O
modems	B-Application
typically	O
operated	O
on	O
direct-dial	O
telephone	O
lines	O
that	O
always	O
began	O
and	O
ended	O
with	O
a	O
known	O
modem	B-Application
at	O
each	O
end	O
.	O
</s>
<s>
The	O
modems	B-Application
operated	O
in	O
either	O
"	O
originate	O
"	O
or	O
"	O
answer	O
"	O
modes	O
,	O
manually	O
switching	O
between	O
two	O
sets	O
of	O
frequencies	O
for	O
data	O
transfer	O
.	O
</s>
<s>
Generally	O
,	O
the	O
user	O
placing	O
the	O
call	O
would	O
switch	O
their	O
modem	B-Application
to	O
"	O
originate	O
"	O
and	O
then	O
dial	O
the	O
number	O
by	O
hand	O
.	O
</s>
<s>
When	O
the	O
remote	O
modem	B-Application
answered	O
,	O
already	O
set	O
to	O
"	O
answer	O
"	O
mode	O
,	O
the	O
telephone	O
handset	O
was	O
switched	O
off	O
and	O
communications	O
continued	O
until	O
the	O
caller	O
manually	O
disconnected	O
.	O
</s>
<s>
To	O
fill	O
this	O
role	O
,	O
some	O
modems	B-Application
included	O
the	O
ability	O
to	O
pick	O
up	O
the	O
phone	O
automatically	O
when	O
it	O
was	O
in	O
answer	O
mode	O
,	O
and	O
clearing	O
the	O
line	O
when	O
the	O
other	O
user	O
manually	O
disconnected	O
.	O
</s>
<s>
This	O
was	O
normally	O
plugged	O
into	O
a	O
separate	O
input/output	B-General_Concept
port	O
on	O
the	O
computer	O
(	O
typically	O
an	O
RS-232	O
port	O
)	O
and	O
programmed	O
separately	O
from	O
the	O
modem	B-Application
itself	O
.	O
</s>
<s>
This	O
method	O
of	O
operation	O
worked	O
satisfactorily	O
in	O
the	O
1960s	O
and	O
early	O
1970s	O
,	O
when	O
modems	B-Application
were	O
generally	O
used	O
to	O
connect	O
dumb	O
devices	O
like	O
computer	B-General_Concept
terminals	I-General_Concept
(	O
dialling	O
out	O
)	O
with	O
smart	O
mainframe	B-Architecture
computers	I-Architecture
(	O
answering	O
)	O
.	O
</s>
<s>
However	O
,	O
the	O
microcomputer	O
revolution	O
of	O
the	O
1970s	O
led	O
to	O
the	O
introduction	O
of	O
low-cost	O
modems	B-Application
and	O
the	O
idea	O
of	O
a	O
semi-dedicated	O
point-to-point	O
link	O
was	O
no	O
longer	O
appropriate	O
.	O
</s>
<s>
The	O
computer	B-General_Concept
industry	I-General_Concept
needed	O
a	O
way	O
to	O
tell	O
the	O
modem	B-Application
what	O
number	O
to	O
dial	O
through	O
software	O
.	O
</s>
<s>
Another	O
solution	O
would	O
have	O
been	O
to	O
use	O
a	O
separate	O
set	O
of	O
"	O
command	O
pins	O
"	O
dedicated	O
to	O
sending	O
and	O
receiving	O
commands	O
,	O
another	O
could	O
have	O
used	O
a	O
signal	O
pin	O
indicating	O
that	O
the	O
modem	B-Application
should	O
interpret	O
incoming	O
data	O
as	O
a	O
command	O
.	O
</s>
<s>
Instead	O
,	O
the	O
modem	B-Application
itself	O
could	O
be	O
switched	O
between	O
one	O
of	O
two	O
modes	O
:	O
</s>
<s>
data	B-Device
mode	I-Device
in	O
which	O
the	O
modem	B-Application
sends	O
the	O
data	O
to	O
the	O
remote	O
modem	B-Application
.	O
</s>
<s>
(	O
A	O
modem	B-Application
in	O
data	B-Device
mode	I-Device
treats	O
everything	O
it	O
receives	O
from	O
the	O
computer	O
as	O
data	O
and	O
sends	O
it	O
across	O
the	O
phone	O
line	O
)	O
.	O
</s>
<s>
command	B-Device
mode	I-Device
in	O
which	O
data	O
is	O
interpreted	O
as	O
commands	O
to	O
the	O
local	B-General_Concept
modem	B-Application
(	O
commands	O
the	O
local	B-General_Concept
modem	B-Application
should	O
execute	O
)	O
.	O
</s>
<s>
To	O
switch	O
from	O
data	B-Device
mode	I-Device
to	O
command	B-Device
mode	I-Device
,	O
sessions	O
sent	O
an	O
escape	O
sequence	O
string	O
of	O
three	O
plus	O
signs	O
("+++"	O
)	O
followed	O
by	O
a	O
pause	O
of	O
about	O
a	O
second	O
.	O
</s>
<s>
In	O
order	O
to	O
avoid	O
licensing	O
Hayes	O
's	O
patent	O
,	O
some	O
manufacturers	O
implemented	O
the	O
escape	O
sequence	O
without	O
the	O
time	O
guard	O
interval	O
(	O
Time	B-Device
Independent	I-Device
Escape	I-Device
Sequence	I-Device
(	O
TIES	O
)	O
)	O
.	O
</s>
<s>
This	O
had	O
a	O
major	O
denial	O
of	O
service	O
security	O
implication	O
in	O
that	O
it	O
would	O
lead	O
to	O
the	O
modem	B-Application
hanging	O
up	O
the	O
connection	O
should	O
the	O
computer	O
ever	O
try	O
to	O
transmit	O
the	O
byte	O
sequence	O
"	O
+++ATH0	B-Device
"	O
in	O
data	B-Device
mode	I-Device
.	O
</s>
<s>
For	O
any	O
computer	O
connected	O
to	O
the	O
Internet	O
through	O
such	O
a	O
modem	B-Application
,	O
this	O
could	O
be	O
easily	O
exploited	O
by	O
sending	O
it	O
a	O
ping	O
of	O
death	O
request	O
containing	O
the	O
sequence	O
"	O
+++ATH0	B-Device
"	O
in	O
the	O
payload	O
.	O
</s>
<s>
The	O
computer	O
operating	O
system	O
would	O
automatically	O
try	O
to	O
reply	O
the	O
sender	O
with	O
the	O
same	O
payload	O
,	O
immediately	O
disconnecting	O
itself	O
from	O
the	O
Internet	O
,	O
as	O
the	O
modem	B-Application
would	O
interpret	O
the	O
ICMP	B-Protocol
packet	O
's	O
data	O
payload	O
as	O
a	O
Hayes	B-Device
command	I-Device
.	O
</s>
<s>
The	O
Hayes	B-Device
command	I-Device
set	I-Device
includes	O
commands	O
for	O
various	O
phone-line	O
operations	O
such	O
as	O
dialing	O
and	O
hanging-up	O
.	O
</s>
<s>
It	O
also	O
includes	O
various	O
controls	O
to	O
set	O
up	O
the	O
modem	B-Application
,	O
including	O
a	O
set	O
of	O
register	B-General_Concept
commands	O
which	O
allowed	O
the	O
user	O
to	O
directly	O
set	O
the	O
various	O
memory	O
locations	O
in	O
the	O
original	O
Hayes	O
modem	B-Application
.	O
</s>
<s>
The	O
command	O
set	O
was	O
copied	O
largely	O
verbatim	O
,	O
including	O
the	O
meaning	O
of	O
the	O
registers	O
,	O
by	O
almost	O
all	O
early	O
300	O
baud	O
modem	B-Application
manufacturers	O
,	O
of	O
which	O
there	O
were	O
quite	O
a	O
few	O
.	O
</s>
<s>
Essentially	O
by	O
accident	O
,	O
this	O
allowed	O
users	O
of	O
existing	O
1200	O
baud	O
modems	B-Application
to	O
use	O
the	O
new	O
Hayes	O
2400	O
models	O
without	O
changing	O
their	O
software	O
.	O
</s>
<s>
Years	O
later	O
,	O
the	O
Telecommunications	O
Industry	O
Association	O
(	O
TIA	O
)	O
/Electronic	O
Industries	O
Alliance	O
(	O
EIA	O
)	O
raised	O
the	O
2400-baud	O
command	O
set	O
into	O
a	O
formal	O
standard	O
with	O
the	O
title	O
Data	O
Transmission	O
Systems	O
and	O
Equipment	O
–	O
Serial	O
Asynchronous	O
Automatic	O
Dialing	O
and	O
Control	O
,	O
TIA/EIA	B-Device
-602	I-Device
.	O
</s>
<s>
However	O
,	O
Hayes	O
Communications	O
were	O
slow	O
to	O
release	O
modems	B-Application
supporting	O
higher	O
speeds	O
or	O
compression	O
,	O
and	O
three	O
other	O
companies	O
led	O
the	O
way	O
here	O
—	O
Microcom	O
,	O
U.S.	O
Robotics	O
and	O
Telebit	O
.	O
</s>
<s>
Things	O
became	O
simpler	O
again	O
during	O
the	O
widespread	O
introduction	O
of	O
14.4	O
and	O
28.8	O
kbit/s	O
modems	B-Application
in	O
the	O
early	O
1990s	O
.	O
</s>
<s>
Only	O
one	O
other	O
command	O
set	O
has	O
remained	O
popular	O
,	O
the	O
US	O
Robotics	O
set	O
from	O
their	O
popular	O
line	O
of	O
modems	B-Application
.	O
</s>
<s>
The	O
following	O
text	O
lists	O
part	O
of	O
the	O
Hayes	B-Device
command	I-Device
set	I-Device
,	O
also	O
called	O
the	O
AT	B-Device
commands	I-Device
:	O
"	O
AT	O
"	O
meaning	O
'	O
attention	O
 '	O
.	O
</s>
<s>
The	O
Hayes	B-Device
command	I-Device
set	I-Device
can	O
subdivide	O
into	O
four	O
groups	O
:	O
</s>
<s>
proprietary	O
command	O
set	O
–	O
Usually	O
starting	O
either	O
with	O
a	O
backslash	O
(	O
“	O
\”	O
)	O
or	O
with	O
a	O
percent	O
sign	O
(	O
“	O
%	O
”	O
)	O
;	O
these	O
commands	O
vary	O
widely	O
among	O
modem	B-Application
manufacturers	O
.	O
</s>
<s>
register	B-General_Concept
commands	O
–	O
Sr	O
=	O
n	O
where	O
r	O
is	O
the	O
number	O
of	O
the	O
register	B-General_Concept
to	O
be	O
changed	O
,	O
and	O
n	O
is	O
the	O
new	O
value	O
that	O
is	O
assigned	O
.	O
</s>
<s>
A	O
register	B-General_Concept
represents	O
a	O
specific	O
physical	O
location	O
in	O
memory	O
.	O
</s>
<s>
Modems	B-Application
have	O
small	O
amounts	O
of	O
memory	O
on	O
board	O
.	O
</s>
<s>
The	O
fourth	O
set	O
of	O
commands	O
serves	O
for	O
entering	O
values	O
into	O
a	O
particular	O
register	B-General_Concept
(	O
memory	O
location	O
)	O
.	O
</s>
<s>
For	O
example	O
,	O
S7	O
=	O
60	O
instructs	O
the	O
modem	B-Application
to	O
"	O
Set	O
register	B-General_Concept
#7	O
to	O
the	O
value	O
60	O
"	O
.	O
</s>
<s>
Registers	O
usually	O
control	O
aspects	O
of	O
the	O
modem	B-Application
operation	O
(	O
e.g.	O
</s>
<s>
When	O
in	O
data	B-Device
mode	I-Device
,	O
an	O
escape	O
sequence	O
can	O
return	O
the	O
modem	B-Application
to	O
command	B-Device
mode	I-Device
.	O
</s>
<s>
Carriage	O
return	O
character	O
,	O
is	O
the	O
command	O
line	O
and	O
result	O
code	O
terminator	O
character	O
,	O
which	O
value	O
,	O
in	O
decimal	O
ASCII	O
between	O
0	O
and	O
255	O
,	O
is	O
specified	O
in	O
register	B-General_Concept
S3	O
.	O
</s>
<s>
Its	O
value	O
,	O
in	O
decimal	O
ASCII	O
between	O
0	O
and	O
255	O
,	O
is	O
specified	O
in	O
register	B-General_Concept
S4	O
.	O
</s>
<s>
When	O
the	O
subparameter	O
is	O
not	O
given	O
in	O
AT	B-Device
commands	I-Device
which	O
have	O
a	O
Read	O
command	O
,	O
the	O
new	O
value	O
equals	O
its	O
previous	O
value	O
.	O
</s>
<s>
In	O
AT	B-Device
commands	I-Device
which	O
do	O
not	O
store	O
the	O
values	O
of	O
any	O
of	O
their	O
subparameters	O
,	O
and	O
so	O
have	O
not	O
a	O
Read	O
command	O
,	O
which	O
are	O
called	O
action	O
type	O
commands	O
,	O
the	O
action	O
should	O
be	O
done	O
on	O
the	O
basis	O
of	O
the	O
recommended	O
default	O
setting	O
of	O
the	O
subparameter	O
.	O
</s>
<s>
A	O
string	O
can	O
contain	O
many	O
Hayes	B-Device
commands	I-Device
placed	O
together	O
,	O
so	O
as	O
to	O
optimally	O
prepare	O
the	O
modem	B-Application
to	O
dial	O
out	O
or	O
answer	O
,	O
e.g.	O
</s>
<s>
Most	O
modem	B-Application
software	O
supported	O
a	O
user	O
supplied	O
initialization	O
string	O
,	O
which	O
was	O
typically	O
a	O
long	O
concatenated	O
AT	B-Device
command	I-Device
which	O
was	O
sent	O
to	O
the	O
modem	B-Application
upon	O
launch	O
.	O
</s>
<s>
The	O
following	O
represents	O
two	O
computers	O
,	O
computer	O
A	O
and	O
computer	O
B	O
,	O
both	O
with	O
modems	B-Application
attached	O
,	O
and	O
the	O
user	O
controlling	O
the	O
modems	B-Application
with	O
terminal-emulator	B-Application
software	O
.	O
</s>
<s>
Terminal-emulator	B-Application
software	O
typically	O
allows	O
the	O
user	O
to	O
send	O
Hayes	B-Device
commands	I-Device
directly	O
to	O
the	O
modem	B-Application
,	O
and	O
to	O
see	O
the	O
responses	O
.	O
</s>
<s>
In	O
this	O
example	O
,	O
the	O
user	O
of	O
computer	O
A	O
makes	O
the	O
modem	B-Application
dial	O
the	O
phone	O
number	O
of	O
modem	B-Application
B	O
at	O
phone	O
number	O
(	O
212	O
)	O
555-0100	O
(	O
long	O
distance	O
)	O
.	O
</s>
<s>
Modem	B-Application
A	O
Modem	B-Application
B	O
Comment	O
ATDT12125550100	O
User	O
at	O
modem	B-Application
A	O
issues	O
a	O
dial	O
command	O
:	O
AT-Get	O
the	O
modem	B-Application
's	O
ATtention	O
;	O
D-Dial	O
;	O
T-Touch-Tone	O
;	O
12125550100-Call	O
this	O
number	O
RING	O
Modem	B-Application
A	O
begins	O
dialing	O
.	O
</s>
<s>
Modem	B-Application
B	O
's	O
phone-line	O
rings	O
,	O
and	O
the	O
modem	B-Application
reports	O
the	O
fact	O
.	O
</s>
<s>
ATA	O
Computer	O
at	O
modem	B-Application
B	O
issues	O
answer	O
command	O
.	O
</s>
<s>
CONNECT	O
CONNECT	O
The	O
modems	B-Application
connect	O
,	O
and	O
both	O
modems	B-Application
report	O
"	O
connect	O
"	O
.	O
</s>
<s>
(	O
In	O
practice	O
,	O
most	O
modems	B-Application
report	O
more	O
information	O
after	O
the	O
word	O
CONNECT	O
—	O
specifying	O
the	O
speed	O
of	O
the	O
connection	O
.	O
)	O
</s>
<s>
Also	O
,	O
at	O
this	O
time	O
,	O
both	O
modems	B-Application
will	O
raise	O
the	O
DCD	O
,	O
or	O
Data	O
Carrier	O
Detect	O
signal	O
,	O
on	O
the	O
serial	O
port	O
.	O
</s>
<s>
abcdef	O
abcdef	O
When	O
the	O
modems	B-Application
are	O
connected	O
,	O
any	O
characters	O
typed	O
at	O
either	O
side	O
will	O
appear	O
on	O
the	O
other	O
side	O
.	O
</s>
<s>
The	O
characters	O
pass	O
through	O
the	O
modem	B-Application
and	O
appear	O
on	O
computer	O
B	O
's	O
screen	O
.	O
</s>
<s>
(	O
User	O
A	O
may	O
not	O
see	O
his	O
own	O
typed	O
characters	O
—	O
depending	O
on	O
the	O
terminal	B-Application
software	I-Application
's	O
local	B-Error_Name
echo	I-Error_Name
setting	O
)	O
.	O
</s>
<s>
+++	O
The	O
person	O
at	O
computer	O
B	O
issues	O
the	O
modem	B-Application
escape	O
command	O
.	O
</s>
<s>
(	O
Alternately	O
,	O
and	O
more	O
commonly	O
,	O
the	O
computer	O
B	O
could	O
drop	O
the	O
DTR	O
,	O
or	O
Data	B-General_Concept
Terminal	I-General_Concept
Ready	O
signal	O
,	O
to	O
achieve	O
a	O
hangup	O
,	O
without	O
needing	O
to	O
use	O
+++	O
or	O
ATH	O
.	O
)	O
</s>
<s>
OK	O
The	O
modem	B-Application
acknowledges	O
it	O
.	O
</s>
<s>
NO	B-Device
CARRIER	I-Device
OK	O
Both	O
modems	B-Application
report	O
that	O
the	O
connection	O
has	O
ended	O
.	O
</s>
<s>
Modem	B-Application
B	O
responds	O
"	O
OK	O
"	O
as	O
the	O
expected	O
result	O
of	O
the	O
command	O
;	O
modem	B-Application
A	O
says	O
NO	B-Device
CARRIER	I-Device
to	O
report	O
that	O
the	O
remote	O
side	O
interrupted	O
the	O
connection	O
.	O
</s>
<s>
The	O
modems	B-Application
on	O
both	O
sides	O
drop	O
their	O
DCD	O
signals	O
as	O
well	O
.	O
</s>
<s>
While	O
the	O
original	O
Hayes	B-Device
command	I-Device
set	I-Device
represented	O
a	O
huge	O
leap	O
forward	O
in	O
modem-based	O
communications	O
,	O
with	O
time	O
many	O
problems	O
set	O
in	O
,	O
almost	O
none	O
of	O
them	O
due	O
to	O
Hayes	O
per	O
se	O
:	O
</s>
<s>
Due	O
to	O
the	O
lack	O
of	O
a	O
written	O
standard	O
,	O
other	O
modem	B-Application
manufacturers	O
just	O
copied	O
the	O
external	O
visible	O
commands	O
and	O
(	O
roughly	O
)	O
the	O
basic	O
actions	O
.	O
</s>
<s>
This	O
led	O
to	O
a	O
wide	O
variety	O
of	O
subtle	O
differences	O
in	O
how	O
modems	B-Application
changed	O
from	O
state	O
to	O
state	O
,	O
and	O
how	O
they	O
handled	O
error	O
conditions	O
,	O
hangups	O
,	O
and	O
timeouts	O
.	O
</s>
<s>
Each	O
manufacturer	O
tended	O
to	O
add	O
new	O
commands	O
to	O
handle	O
emerging	O
needs	O
,	O
often	O
incompatible	O
with	O
other	O
modems	B-Application
.	O
</s>
<s>
For	O
example	O
,	O
setting	O
up	O
hardware	O
or	O
software	O
handshaking	O
often	O
required	O
many	O
different	O
commands	O
for	O
different	O
modems	B-Application
.	O
</s>
<s>
This	O
undermined	O
the	O
handy	O
universality	O
of	O
the	O
basic	O
Hayes	B-Device
command	I-Device
set	I-Device
.	O
</s>
<s>
Many	O
Hayes	B-Device
compatible	I-Device
modems	B-Application
had	O
serious	O
quirks	O
that	O
made	O
them	O
effectively	O
incompatible	O
.	O
</s>
<s>
For	O
example	O
,	O
many	O
modems	B-Application
required	O
a	O
pause	O
of	O
several	O
seconds	O
after	O
receiving	O
the	O
"	O
AT	O
Z	O
"	O
reset	O
command	O
.	O
</s>
<s>
Some	O
modems	B-Application
required	O
spaces	O
between	O
commands	O
,	O
while	O
others	O
did	O
not	O
.	O
</s>
<s>
As	O
a	O
result	O
of	O
all	O
this	O
,	O
eventually	O
many	O
communications	O
programs	O
had	O
to	O
give	O
up	O
any	O
sense	O
of	O
being	O
able	O
to	O
talk	O
to	O
all	O
"	O
Hayes-compatible	O
"	O
modems	B-Application
,	O
and	O
instead	O
the	O
programs	O
had	O
to	O
try	O
to	O
determine	O
the	O
modem	B-Application
type	O
from	O
its	O
responses	O
,	O
or	O
provide	O
the	O
user	O
with	O
some	O
option	O
whereby	O
they	O
could	O
enter	O
whatever	O
special	O
commands	O
it	O
took	O
to	O
coerce	O
their	O
particular	O
modem	B-Application
into	O
acting	O
properly	O
.	O
</s>
<s>
The	O
Hayes	B-Device
command	I-Device
set	I-Device
facilitated	O
automatic	O
baud	O
rate	O
detection	O
as	O
"	O
A	O
"	O
and	O
"	O
T	O
"	O
happen	O
to	O
have	O
bit	O
patterns	O
that	O
are	O
very	O
regular	O
;	O
"	O
A	O
"	O
is	O
"	O
100	O
0001	O
"	O
and	O
so	O
has	O
a	O
1	O
bit	O
at	O
the	O
start	O
and	O
end	O
and	O
"	O
T	O
"	O
is	O
"	O
101	O
0100	O
"	O
which	O
has	O
a	O
pattern	O
with	O
(	O
nearly	O
)	O
every	O
other	O
bit	O
set	O
.	O
</s>
<s>
The	O
following	O
commands	O
are	O
understood	O
by	O
virtually	O
all	O
modems	B-Application
supporting	O
an	O
AT	B-Device
command	I-Device
set	I-Device
,	O
whether	O
old	O
or	O
new	O
.	O
</s>
<s>
;	O
–	O
Remain	O
in	O
command	B-Device
mode	I-Device
after	O
dialing	O
.	O
</s>
<s>
Each	O
number	O
(	O
0	O
to	O
9	O
,	O
and	O
sometimes	O
10	O
and	O
above	O
)	O
returns	O
one	O
line	O
of	O
modem-specific	O
information	O
,	O
or	O
the	O
word	O
ERROR	O
if	O
the	O
line	O
is	O
not	O
defined	O
.	O
</s>
<s>
Today	O
,	O
Windows	O
uses	O
this	O
for	O
Plug-and-play	B-Device
detection	O
of	O
specific	O
modem	B-Application
types	O
.	O
</s>
<s>
Supported	O
only	O
by	O
some	O
modems	B-Application
with	O
speakers	O
.	O
</s>
<s>
Modems	B-Application
lacking	O
speakers	O
,	O
or	O
with	O
physical	O
volume	O
controls	O
,	O
or	O
ones	O
whose	O
sound	O
output	O
is	O
piped	O
through	O
the	O
sound	O
card	O
will	O
not	O
support	O
this	O
command	O
.	O
</s>
<s>
|	O
Returns	O
the	O
modem	B-Application
back	O
to	O
the	O
normal	O
connected	O
state	O
after	O
being	O
interrupted	O
by	O
the	O
"	O
+++	O
"	O
escape	O
code	O
.	O
</s>
<s>
and	O
=	O
r	O
are	O
actually	O
three	O
separate	O
commands	O
,	O
and	O
can	O
be	O
given	O
in	O
separate	O
AT	B-Device
commands	I-Device
.	O
</s>
<s>
|	O
Select	O
register	B-General_Concept
n	O
as	O
the	O
current	O
register	B-General_Concept
,	O
and	O
query	O
its	O
value	O
.	O
</s>
<s>
on	O
its	O
own	O
will	O
query	O
whichever	O
register	B-General_Concept
was	O
most	O
recently	O
selected	O
.	O
</s>
<s>
|	O
Select	O
register	B-General_Concept
n	O
as	O
the	O
current	O
register	B-General_Concept
,	O
and	O
store	O
r	O
in	O
it	O
.	O
</s>
<s>
Using	O
=	O
r	O
on	O
its	O
own	O
will	O
store	O
into	O
whichever	O
register	B-General_Concept
was	O
most	O
recently	O
selected	O
.	O
</s>
<s>
CONNECT	O
,	O
BUSY	O
,	O
NO	B-Device
CARRIER	I-Device
etc	O
.	O
)	O
</s>
<s>
|	O
Reset	O
modem	B-Application
to	O
stored	O
configuration	O
,	O
and	O
usually	O
also	O
physically	O
power-cycles	O
the	O
modem	B-Application
(	O
during	O
which	O
it	O
is	O
unresponsive	O
)	O
.	O
</s>
<s>
Although	O
not	O
part	O
of	O
the	O
command	O
set	O
,	O
a	O
tilde	O
character	O
~	O
is	O
commonly	O
used	O
in	O
modem	B-Application
command	O
sequences	O
.	O
</s>
<s>
The	O
~'''	O
is	O
not	O
sent	O
to	O
the	O
modem	B-Application
.	O
</s>
<s>
It	O
was	O
renamed	O
V.250	O
in	O
1998	O
with	O
an	O
annex	O
that	O
was	O
not	O
concerning	O
the	O
Hayes	B-Device
command	I-Device
set	I-Device
renamed	O
as	O
V.251	O
''	O
.	O
</s>
<s>
A	O
V.250	O
compliant	O
modem	B-Application
implements	O
the	O
A	O
,	O
D	O
,	O
E	O
,	O
H	O
,	O
I	O
,	O
L	O
,	O
M	O
,	O
N	O
,	O
O	O
,	O
P	O
,	O
Q	O
,	O
T	O
,	O
V	O
,	O
X	O
,	O
Z	O
,	O
&	O
C	O
,	O
&	O
D	O
,	O
and	O
&	O
F	O
commands	O
in	O
the	O
way	O
specified	O
by	O
the	O
standard	O
.	O
</s>
<s>
Modem	B-Application
manufacturers	O
are	O
free	O
to	O
implement	O
other	O
commands	O
and	O
S-registers	O
as	O
they	O
see	O
fit	O
,	O
and	O
may	O
add	O
options	O
to	O
standard	O
commands	O
.	O
</s>
<s>
The	O
ETSI	O
GSM	O
07.07	O
(	O
3GPP	O
TS	O
27.007	O
)	O
specifies	O
AT	O
style	O
commands	O
for	O
controlling	O
a	O
GSM	O
phone	O
or	O
modem	B-Application
.	O
</s>
<s>
GSM/3G	O
modems	B-Application
typically	O
support	O
the	O
ETSI	O
GSM	O
07.07/3GPP	O
TS	O
27.007	O
AT	B-Device
command	I-Device
set	I-Device
extensions	O
,	O
although	O
how	O
many	O
commands	O
are	O
implemented	O
varies	O
.	O
</s>
<s>
Most	O
USB	O
modem	B-Application
vendors	O
,	O
such	O
as	O
Huawei	O
,	O
Sierra	O
Wireless	O
,	O
Option	O
,	O
have	O
also	O
defined	O
proprietary	O
extensions	O
for	O
radio	O
mode	O
selection	O
(	O
GSM/3G	O
preference	O
)	O
or	O
similar	O
.	O
</s>
<s>
Some	O
recent	O
high	O
speed	O
modems	B-Application
provide	O
a	O
virtual	O
Ethernet	O
interface	O
instead	O
of	O
using	O
a	O
Point-to-Point	B-Protocol
Protocol	I-Protocol
(	O
PPP	O
)	O
for	O
the	O
data	O
connection	O
because	O
of	O
performance	O
reasons	O
(	O
PPP	B-Protocol
connection	I-Protocol
is	O
only	O
used	O
between	O
the	O
computer	O
and	O
the	O
modem	B-Application
,	O
not	O
over	O
network	O
)	O
.	O
</s>
<s>
The	O
set-up	O
requires	O
vendor-specific	O
AT	B-Device
command	I-Device
extensions	O
.	O
</s>
<s>
Modems	B-Application
with	O
voice	O
or	O
answering-machine	O
capabilities	O
support	O
a	O
superset	O
of	O
these	O
commands	O
to	O
enable	O
digital	O
audio	O
playback	O
and	O
recording	O
.	O
</s>
