<s>
The	O
Bell	B-Device
202	I-Device
modem	I-Device
was	O
an	O
early	O
(	O
1976	O
)	O
modem	B-Application
standard	O
developed	O
by	O
the	O
Bell	O
System	O
.	O
</s>
<s>
These	O
signalling	O
protocols	O
,	O
also	O
used	O
in	O
third-party	O
modems	B-Application
,	O
are	O
referred	O
to	O
generically	O
as	O
Bell	B-Device
202	I-Device
modulation	O
,	O
and	O
any	O
device	O
employing	O
it	O
as	O
Bell-202-compatible	O
.	O
</s>
<s>
Bell	B-Device
202	I-Device
AFSK	O
uses	O
a	O
1200Hz	O
tone	O
for	O
mark	O
(	O
typically	O
a	O
binary	O
1	O
)	O
and	O
2200Hz	O
for	O
space	O
(	O
typically	O
a	O
binary	O
0	O
)	O
.	O
</s>
<s>
In	O
North	O
America	O
,	O
Bell	B-Device
202	I-Device
AFSK	O
modulation	O
is	O
used	O
to	O
transmit	O
Caller	O
ID	O
information	O
over	O
POTS	O
lines	O
in	O
the	O
public	O
telephone	O
network	O
.	O
</s>
<s>
In	O
addition	O
,	O
Bell	B-Device
202	I-Device
is	O
the	O
basis	O
for	O
the	O
most	O
commonly	O
used	O
physical	O
layer	O
for	O
the	O
HART	B-Protocol
Communication	I-Protocol
Protocol	I-Protocol
-	O
a	O
communication	B-Protocol
protocol	I-Protocol
widely	O
used	O
in	O
the	O
process	O
industries	O
.	O
</s>
<s>
Surplus	O
Bell	B-Device
202	I-Device
modems	I-Device
were	O
used	O
by	O
amateur	O
radio	O
operators	O
to	O
construct	O
the	O
first	O
packet	B-Application
radio	I-Application
stations	O
,	O
despite	O
its	O
low	O
signalling	O
speed	O
.	O
</s>
<s>
A	O
modified	O
Bell	B-Device
202	I-Device
AFSK	O
modulation	O
,	O
a	O
common	O
physical	O
layer	O
for	O
AX.25	B-Protocol
,	O
remains	O
the	O
standard	O
for	O
amateur	O
VHF	B-Algorithm
operation	O
in	O
most	O
areas	O
.	O
</s>
<s>
Notably	O
,	O
Automatic	B-Application
Packet	I-Application
Reporting	I-Application
System	I-Application
(	O
APRS	O
)	O
transmissions	O
are	O
encoded	O
this	O
way	O
on	O
VHF	B-Algorithm
.	O
</s>
<s>
On	O
HF	B-Algorithm
,	O
APRS	O
uses	O
Bell	O
103	O
modulation	O
.	O
</s>
<s>
The	O
Bell	B-Device
202	I-Device
standard	O
was	O
adopted	O
around	O
1980	O
as	O
the	O
communications	O
standard	O
for	O
subsea	O
oil	O
and	O
gas	O
production	O
control	O
systems	O
,	O
pioneered	O
by	O
the	O
then	O
FSSL	O
(	O
Ferranti	O
Subsea	O
Systems	O
Ltd	O
.	O
)	O
Controls	O
,	O
a	O
spin-out	O
company	O
from	O
the	O
former	O
TRW	O
-	O
Ferranti	O
joint	O
venture	O
in	O
the	O
UK	O
.	O
</s>
<s>
The	O
202	O
standard	O
permitted	O
useful	O
techniques	O
such	O
as	O
multi-dropping	O
of	O
slave	O
modems	B-Application
to	O
allow	O
multiple	O
nodes	O
to	O
be	O
connected	O
to	O
the	O
host	O
via	O
a	O
single	O
modem	B-Application
channel	O
.	O
</s>
<s>
This	O
has	O
been	O
enhanced	O
through	O
the	O
use	O
of	O
Manchester	B-Protocol
encoding	I-Protocol
over	O
the	O
FSK	O
link	O
,	O
to	O
provide	O
simple	O
Modulo-2	O
RZ	O
(	O
return	O
to	O
zero	O
)	O
bit	O
error	O
detection	O
and	O
suppression	O
improvement	O
over	O
these	O
long	O
distances	O
.	O
</s>
