<s>
B	B-Protocol
channel	I-Protocol
(	O
bearer	O
)	O
is	O
a	O
telecommunications	O
term	O
which	O
refers	O
to	O
the	O
ISDN	B-Protocol
channel	O
in	O
which	O
the	O
primary	O
data	O
or	O
voice	O
communication	O
is	O
carried	O
.	O
</s>
<s>
The	O
term	O
is	O
applied	O
primarily	O
in	O
relation	O
to	O
the	O
ISDN	B-Protocol
access	O
interfaces	O
(	O
PRA	B-Protocol
or	I-Protocol
PRI	I-Protocol
and	O
BRA	B-Protocol
or	I-Protocol
BRI	I-Protocol
)	O
,	O
since	O
deeper	O
in	O
the	O
PSTN	O
network	O
an	O
ISDN	B-Protocol
bearer	O
channel	O
is	O
essentially	O
indistinguishable	O
from	O
any	O
other	O
bearer	O
channel	O
.	O
</s>
<s>
Apart	O
from	O
any	O
transmission	O
errors	O
,	O
the	O
purpose	O
of	O
the	O
network	O
is	O
to	O
carry	O
the	O
contents	O
of	O
the	O
B	B-Protocol
channel	I-Protocol
transparently	O
between	O
the	O
endpoints	O
of	O
the	O
call	O
.	O
</s>
<s>
International	O
voice	O
calls	O
may	O
include	O
A-law	B-Algorithm
to	O
μ-law	B-Algorithm
conversion	O
,	O
which	O
also	O
impairs	O
the	O
channel	O
when	O
used	O
for	O
data	O
communications	O
purposes	O
.	O
</s>
<s>
Long-distance	O
voice	O
calls	O
may	O
also	O
employ	O
some	O
form	O
of	O
voice	B-Application
compression	I-Application
,	O
typically	O
provided	O
by	O
equipment	O
such	O
as	O
DCME	B-Protocol
.	O
</s>
<s>
ISDN	B-Protocol
terminal	O
equipment	O
can	O
specify	O
the	O
required	O
Bearer	B-Protocol
Capability	I-Protocol
when	O
establishing	O
the	O
call	O
.	O
</s>
<s>
If	O
the	O
call	O
is	O
being	O
set	O
up	O
as	O
a	O
voice	O
call	O
between	O
modems	O
,	O
then	O
a	O
2100Hz	O
answer	B-Device
tone	I-Device
sent	O
in	O
the	O
backward	O
direction	O
during	O
the	O
initial	O
seconds	O
of	O
the	O
conversation	O
phase	O
of	O
the	O
call	O
will	O
result	O
in	O
any	O
echo	O
cancellers	O
or	O
echo	O
suppressors	O
being	O
disabled	O
for	O
the	O
remainder	O
of	O
the	O
call	O
.	O
</s>
