<s>
In	O
telecommunications	O
,	O
echo	B-Error_Name
is	O
the	O
local	O
display	O
of	O
data	O
,	O
either	O
initially	O
as	O
it	O
is	O
locally	O
sourced	O
and	O
sent	O
,	O
or	O
finally	O
as	O
a	O
copy	O
of	O
it	O
is	O
received	O
back	O
from	O
a	O
remote	O
destination	O
.	O
</s>
<s>
Local	B-Error_Name
echo	I-Error_Name
is	O
where	O
the	O
local	O
sending	O
equipment	O
displays	O
the	O
outgoing	O
sent	O
data	O
.	O
</s>
<s>
Remote	B-Error_Name
echo	I-Error_Name
is	O
where	O
the	O
display	O
is	O
a	O
return	O
copy	O
of	O
data	O
as	O
received	O
remotely	O
.	O
</s>
<s>
Both	O
are	O
used	O
together	O
in	O
a	O
computed	O
form	O
of	O
error	B-Error_Name
detection	I-Error_Name
to	O
ensure	O
that	O
data	O
received	O
at	O
the	O
remote	O
destination	O
of	O
a	O
telecommunication	O
are	O
the	O
same	O
as	O
data	O
sent	O
from	O
the	O
local	O
source	O
(	O
a/k/a	O
echoplex	O
,	O
echo	B-Error_Name
check	I-Error_Name
,	O
or	O
loop	B-Error_Name
check	I-Error_Name
)	O
.	O
</s>
<s>
When	O
(	O
two	O
)	O
modems	B-Application
communicate	O
in	O
echoplex	B-Error_Name
mode	I-Error_Name
the	O
remote	O
modem	B-Application
echoes	B-Error_Name
whatever	O
it	O
receives	O
from	O
the	O
local	O
modem	B-Application
.	O
</s>
<s>
A	O
displayed	O
'	O
echo	B-Error_Name
 '	O
is	O
independent	O
of	O
'	O
duplex	O
 '	O
(	O
or	O
any	O
)	O
telecommunications	O
transmission	O
protocol	O
.	O
</s>
<s>
Probably	O
from	O
technical	O
ignorance	O
,	O
"	O
half-duplex	O
"	O
and	O
"	O
full-duplex	O
"	O
are	O
used	O
as	O
slang	O
for	O
'	O
local	B-Error_Name
echo	I-Error_Name
 '	O
(	O
a/k/a	O
echo	B-Error_Name
on	O
)	O
and	O
'	O
remote	B-Error_Name
echo	I-Error_Name
 '	O
,	O
respectively	O
,	O
as	O
typically	O
they	O
accompany	O
one	O
another	O
.	O
</s>
<s>
Typically	O
'	O
local	B-Error_Name
echo	I-Error_Name
 '	O
accompanies	O
half-duplex	O
transmission	O
,	O
which	O
effectively	O
doubles	O
channel	O
bandwidth	O
by	O
not	O
repeating	O
(	O
echoing	O
)	O
data	O
back	O
from	O
its	O
destination	O
(	O
remote	O
)	O
,	O
as	O
is	O
reserved-for	O
with	O
'	O
full	O
duplex	O
 '	O
(	O
which	O
has	O
only	O
half	O
of	O
the	O
bandwidth	O
of	O
'	O
half	O
duplex	O
 '	O
)	O
.	O
</s>
<s>
Half-duplex	O
can	O
be	O
set	O
to	O
'	O
echo	B-Error_Name
off	O
 '	O
for	O
no	O
echo	B-Error_Name
at	O
all	O
.	O
</s>
<s>
One	O
example	O
of	O
'	O
local	B-Error_Name
echo	I-Error_Name
 '	O
used	O
together	O
with	O
'	O
remote	B-Error_Name
echo	I-Error_Name
 '	O
(	O
requires	O
full-duplex	O
)	O
is	O
for	O
error	B-Error_Name
checking	I-Error_Name
pairs	O
of	O
data	O
characters	O
or	O
chunks	O
(	O
echoplex	O
)	O
ensuring	O
their	O
duplicity	O
(	O
or	O
else	O
its	O
just	O
an	O
extraneous	O
annoyance	O
)	O
.	O
</s>
<s>
Similarly	O
,	O
for	O
another	O
example	O
,	O
in	O
the	O
case	O
of	O
the	O
TELNET	B-Protocol
communications	O
protocol	O
a	O
local	B-Error_Name
echo	I-Error_Name
protocol	O
operates	O
on	O
top	O
of	O
a	O
full-duplex	O
underlying	O
protocol	O
.	O
</s>
<s>
The	O
TCP	B-Protocol
connection	O
over	O
which	O
the	O
TELNET	B-Protocol
protocol	O
is	O
layered	O
provides	O
a	O
full-duplex	O
connection	O
,	O
with	O
no	O
echo	B-Error_Name
,	O
across	O
which	O
data	O
may	O
be	O
sent	O
in	O
either	O
direction	O
simultaneously	O
.	O
</s>
<s>
Whereas	O
the	O
Network	O
Virtual	O
Terminal	O
that	O
the	O
TELNET	B-Protocol
protocol	O
itself	O
incorporates	O
is	O
a	O
half-duplex	O
device	O
with	O
(	O
by	O
default	O
)	O
local	B-Error_Name
echo	I-Error_Name
.	O
</s>
<s>
Others	O
include	O
modems	B-Application
,	O
some	O
form	O
of	O
intervening	O
communications	O
processor	O
,	O
or	O
even	O
the	O
host	O
system	O
itself	O
.	O
</s>
<s>
For	O
several	O
common	O
computer	B-General_Concept
operating	I-General_Concept
systems	I-General_Concept
,	O
it	O
is	O
the	O
host	O
system	O
itself	O
that	O
performs	O
the	O
echoing	O
,	O
if	O
appropriate	O
(	O
which	O
it	O
is	O
n't	O
for	O
,	O
say	O
,	O
entry	O
of	O
a	O
user	O
password	O
when	O
a	O
terminal	O
first	O
connects	O
and	O
a	O
user	O
is	O
prompted	O
to	O
log	O
in	O
)	O
.	O
</s>
<s>
On	O
OpenVMS	B-Operating_System
,	O
for	O
example	O
,	O
echoing	O
is	O
performed	O
as	O
necessary	O
by	O
the	O
host	O
system	O
.	O
</s>
<s>
Similarly	O
,	O
on	O
Unix-like	B-Operating_System
systems	I-Operating_System
,	O
local	B-Error_Name
echo	I-Error_Name
is	O
performed	O
by	O
the	O
operating	B-General_Concept
system	I-General_Concept
kernel	O
's	O
terminal	O
device	O
driver	O
,	O
according	O
to	O
the	O
state	O
of	O
a	O
device	O
control	O
flag	O
,	O
maintained	O
in	O
software	O
and	O
alterable	O
by	O
applications	O
programs	O
via	O
an	O
ioctl( )	O
system	O
call	O
.	O
</s>
<s>
The	O
actual	O
terminals	O
and	O
modems	B-Application
connected	O
to	O
such	O
systems	O
should	O
have	O
their	O
local	B-Error_Name
echo	I-Error_Name
facilities	O
switched	O
off	O
(	O
so	O
that	O
they	O
operate	O
in	O
no	O
echo	B-Error_Name
mode	O
)	O
,	O
lest	O
passwords	O
be	O
locally	O
echoed	O
at	O
password	O
prompts	O
,	O
and	O
all	O
other	O
input	O
appear	O
echoed	O
twice	O
.	O
</s>
<s>
This	O
is	O
as	O
true	O
for	O
terminal	B-Application
emulator	I-Application
programs	O
,	O
such	O
as	O
C-Kermit	B-Protocol
,	O
running	O
on	O
a	O
computer	O
as	O
it	O
is	O
for	O
real	O
terminals	O
.	O
</s>
<s>
Most	O
terminal	B-Application
emulator	I-Application
programs	O
have	O
the	O
ability	O
to	O
perform	O
echo	B-Error_Name
locally	O
(	O
which	O
sometimes	O
they	O
misname	O
"	O
half-duplex	O
"	O
)	O
:	O
</s>
<s>
In	O
the	O
C-Kermit	B-Protocol
terminal	B-Application
emulator	I-Application
program	O
,	O
local	B-Error_Name
echo	I-Error_Name
is	O
controlled	O
by	O
the	O
SET	O
TERMINAL	O
ECHO	B-Error_Name
command	O
,	O
which	O
can	O
be	O
either	O
SET	O
TERMINAL	O
ECHO	B-Error_Name
LOCAL	O
(	O
which	O
enables	O
local	O
echoing	O
within	O
the	O
terminal	B-Application
emulator	I-Application
program	O
itself	O
)	O
or	O
SET	O
TERMINAL	O
ECHO	B-Error_Name
REMOTE	O
(	O
where	O
disables	O
local	O
echoing	O
,	O
leaving	O
that	O
up	O
to	O
another	O
device	O
in	O
the	O
communications	O
channel	O
—	O
be	O
that	O
the	O
modem	B-Application
or	O
the	O
remote	O
host	O
system	O
—	O
to	O
perform	O
as	O
appropriate	O
)	O
.	O
</s>
<s>
In	O
ProComm	B-Protocol
it	O
is	O
the	O
combination	O
,	O
which	O
is	O
a	O
hot	O
key	O
that	O
may	O
be	O
used	O
at	O
any	O
time	O
to	O
toggle	O
local	B-Error_Name
echo	I-Error_Name
on	O
and	O
off	O
.	O
</s>
<s>
In	O
the	O
Terminal	B-Application
program	I-Application
that	O
came	O
with	O
Microsoft	B-Application
Windows	I-Application
3.1	I-Application
,	O
local	B-Error_Name
echo	I-Error_Name
is	O
controlled	O
by	O
a	O
checkbox	O
in	O
the	O
"	O
Terminal	O
Preferences	O
"	O
dialogue	O
box	O
accessed	O
from	O
the	O
menu	O
of	O
the	O
terminal	B-Application
program	I-Application
's	O
window	O
.	O
</s>
<s>
The	O
Hayes	O
commands	O
that	O
control	O
local	B-Error_Name
echo	I-Error_Name
(	O
in	O
command	O
mode	O
)	O
are	O
for	O
off	O
and	O
for	O
on	O
.	O
</s>
<s>
For	O
local	B-Error_Name
echo	I-Error_Name
(	O
in	O
data	O
mode	O
)	O
,	O
the	O
commands	O
are	O
and	O
respectively	O
.	O
</s>
<s>
Some	O
host	O
systems	O
perform	O
local	B-Error_Name
echo	I-Error_Name
themselves	O
,	O
in	O
their	O
device	O
drivers	O
and	O
so	O
forth	O
.	O
</s>
<s>
In	O
Unix	O
and	O
POSIX-compatible	O
systems	O
,	O
local	B-Error_Name
echo	I-Error_Name
is	O
a	O
flag	O
in	O
the	O
POSIX	B-Application
terminal	I-Application
interface	I-Application
,	O
settable	O
programmatically	O
with	O
the	O
tcsetattr( )	O
function	O
.	O
</s>
<s>
The	O
echoing	O
is	O
performed	O
by	O
the	O
operating	B-General_Concept
system	I-General_Concept
's	O
terminal	O
device	O
(	O
in	O
some	O
way	O
that	O
is	O
not	O
specified	O
by	O
the	O
POSIX	O
standard	O
)	O
.	O
</s>
<s>
The	O
standard	O
utility	O
program	O
that	O
alters	O
this	O
flag	O
programmatically	O
is	O
the	O
stty	O
command	O
,	O
using	O
which	O
the	O
flag	O
may	O
be	O
altered	O
from	O
shell	B-Language
scripts	I-Language
or	O
an	O
interactive	O
shell	O
.	O
</s>
<s>
The	O
command	O
to	O
turn	O
local	B-Error_Name
echo	I-Error_Name
(	O
by	O
the	O
host	O
system	O
)	O
on	O
is	O
stty	O
echo	B-Error_Name
and	O
the	O
command	O
to	O
turn	O
it	O
off	O
is	O
stty	O
-echo	O
.	O
</s>
<s>
On	O
OpenVMS	B-Operating_System
systems	O
,	O
the	O
operating	B-General_Concept
system	I-General_Concept
's	O
terminal	O
driver	O
normally	O
performs	O
echoing	O
.	O
</s>
<s>
The	O
terminal	O
characteristic	O
that	O
controls	O
whether	O
it	O
does	O
this	O
is	O
the	O
ECHO	B-Error_Name
characteristic	O
,	O
settable	O
with	O
the	O
DCL	O
command	O
SET	O
TERMINAL	O
/ECHO	O
and	O
unsettable	O
with	O
SET	O
TERMINAL	O
/NOECHO	O
.	O
</s>
