<s>
Fall	B-General_Concept
back	I-General_Concept
is	O
a	O
feature	O
of	O
a	O
modem	B-Application
protocol	O
in	O
data	O
communication	O
whereby	O
two	O
communicating	O
modems	B-Application
which	O
experience	O
data	O
corruption	O
(	O
due	O
to	O
line	O
noise	O
,	O
for	O
example	O
)	O
can	O
renegotiate	O
with	O
each	O
other	O
to	O
use	O
a	O
lower-speed	O
connection	O
.	O
</s>
<s>
Fall	B-General_Concept
forward	I-General_Concept
is	O
a	O
corresponding	O
feature	O
whereby	O
two	O
modems	B-Application
which	O
have	O
"	O
fallen	O
back	O
"	O
to	O
a	O
lower	O
speed	O
can	O
later	O
return	O
to	O
the	O
higher	O
speed	O
if	O
the	O
connection	O
improves	O
.	O
</s>
<s>
A	O
common	O
feature	O
modems	B-Application
was	O
the	O
concept	O
of	O
fallback	O
,	O
allowing	O
them	O
to	O
talk	O
to	O
less-capable	O
modems	B-Application
.	O
</s>
<s>
During	O
the	O
call	O
initiation	O
the	O
modem	B-Application
would	O
play	O
a	O
series	O
of	O
signals	O
into	O
the	O
line	O
and	O
wait	O
for	O
the	O
remote	O
modem	B-Application
to	O
"	O
answer	O
"	O
them	O
.	O
</s>
<s>
Thus	O
,	O
two	O
USR	O
modems	B-Application
would	O
be	O
able	O
to	O
connect	O
at	O
9600	O
bit/s	O
,	O
but	O
,	O
when	O
a	O
user	O
with	O
a	O
2400-bit/s	O
modem	B-Application
called	O
in	O
,	O
the	O
USR	O
would	O
"	O
fall	B-General_Concept
back	I-General_Concept
"	O
to	O
the	O
common	O
2400-bit/s	O
speed	O
.	O
</s>
<s>
This	O
would	O
also	O
happen	O
if	O
a	O
V.32	O
modem	B-Application
and	O
a	O
HST	O
modem	B-Application
were	O
connected	O
.	O
</s>
<s>
Because	O
they	O
used	O
a	O
different	O
standard	O
at	O
9600	O
bit/s	O
,	O
they	O
would	O
fall	B-General_Concept
back	I-General_Concept
to	O
their	O
highest	O
commonly	O
supported	O
standard	O
at	O
2400	O
bit/s	O
.	O
</s>
<s>
The	O
same	O
applies	O
to	O
V.32bis	O
and	O
14400	O
bit/s	O
HST	O
modem	B-Application
,	O
which	O
would	O
still	O
be	O
able	O
to	O
communicate	O
with	O
each	O
other	O
at	O
only	O
2400	O
bit/s	O
.	O
</s>
<s>
A	O
modem	B-Application
that	O
can	O
fall	B-General_Concept
back	I-General_Concept
in	O
order	O
to	O
communicate	O
with	O
an	O
older	O
,	O
slower	O
modem	B-Application
is	O
an	O
example	O
of	O
backward	B-General_Concept
compatibility	I-General_Concept
,	O
while	O
one	O
that	O
can	O
fall	B-General_Concept
back	I-General_Concept
in	O
order	O
to	O
communicate	O
when	O
the	O
line	O
becomes	O
noisy	O
is	O
an	O
example	O
of	O
graceful	B-General_Concept
degradation	I-General_Concept
.	O
</s>
<s>
Article	O
based	O
on	O
fall	B-General_Concept
back	I-General_Concept
and	O
fall	B-General_Concept
forward	I-General_Concept
at	O
FOLDOC	O
,	O
used	O
with	O
permission	O
.	O
</s>
