<s>
Tandem	B-General_Concept
signaling	I-General_Concept
is	O
the	O
two-step	O
conversion	O
of	O
a	O
digital	O
signal	O
into	O
an	O
analog	O
signal	O
,	O
followed	O
by	O
the	O
reverse	O
conversion	O
.	O
</s>
<s>
While	O
there	O
is	O
a	O
universal	O
standard	O
for	O
analog	O
signal	O
transmission	O
,	O
digital	O
signals	O
may	O
employ	O
a	O
variety	O
of	O
sampling	O
rates	O
and	O
encoding	O
methods	O
(	O
codecs	B-General_Concept
)	O
.	O
</s>
<s>
When	O
speech	O
is	O
being	O
transmitted	O
,	O
as	O
in	O
audio	O
telephone	O
calls	O
,	O
tandem	B-General_Concept
signaling	I-General_Concept
may	O
produce	O
unintelligible	O
results	O
.	O
</s>
<s>
Tandem	B-General_Concept
signaling	I-General_Concept
is	O
an	O
important	O
evaluation	O
criterion	O
in	O
the	O
evaluation	O
of	O
speech	B-Application
coders	I-Application
.	O
</s>
<s>
Mobile	O
telephones	O
use	O
digital	O
signals	O
,	O
while	O
landlines	B-Application
terminate	O
in	O
analog	O
signal	O
systems	O
.	O
</s>
<s>
If	O
one	O
phone	O
uses	O
a	O
high-bitrate	O
codec	B-General_Concept
,	O
and	O
the	O
other	O
a	O
low-bitrate	O
,	O
the	O
user	O
with	O
the	O
high	O
bitrate	O
will	O
be	O
able	O
to	O
hear	O
the	O
user	O
with	O
the	O
low	O
bitrate	O
,	O
but	O
possibly	O
not	O
the	O
reverse	O
.	O
</s>
<s>
Even	O
if	O
all	O
phones	O
used	O
the	O
same	O
audio	O
codec	B-General_Concept
,	O
packet	O
retransmission	O
and	O
re-encoding	O
are	O
still	O
required	O
.	O
</s>
<s>
When	O
a	O
user	O
makes	O
a	O
call	O
to	O
a	O
POTS	O
phone	O
,	O
the	O
audio	O
is	O
digitized	O
into	O
packets	O
that	O
are	O
transmitted	O
from	O
the	O
mobile	O
device	O
and	O
are	O
then	O
decoded	O
into	O
64	O
kbit/s	O
PCM	B-Algorithm
.	O
</s>
