<s>
ARQ-M	B-Device
,	O
short	O
for	O
Automatic	O
Repeat	O
reQuest	O
,	O
Multiplex	O
,	O
is	O
a	O
radio	O
telegraphy	O
protocol	O
used	O
to	O
reliably	O
forward	O
telex	B-Application
messages	O
over	O
partially	O
reliable	O
radio	O
links	O
.	O
</s>
<s>
It	O
is	O
a	O
low-speed	O
system	O
designed	O
to	O
match	O
the	O
performance	O
of	O
landline	O
telex	B-Application
systems	O
and	O
allow	O
those	O
messages	O
to	O
be	O
forwarded	O
over	O
long	O
distances	O
using	O
shortwave	O
radios	O
.	O
</s>
<s>
The	O
first	O
ARQ-M	B-Device
link	O
was	O
built	O
in	O
the	O
Netherlands	O
,	O
and	O
began	O
exchanging	O
messages	O
with	O
a	O
counterpart	O
in	O
New	O
York	O
in	O
1947	O
.	O
</s>
<s>
ARQ-M	B-Device
is	O
similar	O
in	O
concept	O
to	O
ARQ-E	B-Device
,	O
but	O
ARQ-E	B-Device
has	O
no	O
multiplex	O
capability	O
and	O
uses	O
a	O
different	O
7-bit	O
alphabet	O
.	O
</s>
<s>
The	O
telex	B-Application
system	O
,	O
which	O
developed	O
out	O
of	O
the	O
telegraph	O
system	O
,	O
is	O
based	O
on	O
defined	O
electric	O
current	O
levels	O
that	O
are	O
interpreted	O
as	O
a	O
mark	O
or	O
space	O
signal	O
.	O
</s>
<s>
To	O
forward	O
telex	B-Application
messages	O
successfully	O
over	O
radio	O
links	O
,	O
some	O
form	O
of	O
error	B-Error_Name
correction	I-Error_Name
should	O
be	O
applied	O
.	O
</s>
<s>
ARQ-M	B-Device
was	O
developed	O
to	O
automatically	O
deal	O
with	O
errors	O
.	O
</s>
<s>
The	O
automatic	O
repeat	O
request	O
system	O
was	O
invented	O
by	O
Hendrik	O
van	O
Duuren	O
of	O
the	O
Netherlands	O
in	O
the	O
1940s	O
and	O
so	O
it	O
became	O
known	O
as	O
the	O
Van	O
Duuren	O
automatic	O
error	B-Error_Name
correction	I-Error_Name
system	O
.	O
</s>
<s>
TEX	O
was	O
an	O
early	O
name	O
for	O
TELEX	B-Application
which	O
is	O
a	O
system	O
to	O
allow	O
automatic	O
routing	O
of	O
private	O
messages	O
from	O
a	O
sender	O
to	O
a	O
receiver	O
on	O
the	O
TELEX	B-Application
network	O
.	O
</s>
<s>
Prior	O
to	O
the	O
introduction	O
or	O
ARQ-M	B-Device
there	O
were	O
so	O
many	O
errors	O
in	O
the	O
messages	O
received	O
that	O
staff	O
from	O
the	O
telecommunications	O
companies	O
had	O
to	O
manually	O
check	O
messages	O
and	O
conform	O
correct	O
receipt	O
.	O
</s>
<s>
Once	O
ARQ-M	B-Device
was	O
introduced	O
,	O
messages	O
could	O
be	O
automatically	O
and	O
reliably	O
sent	O
over	O
long	O
distances	O
via	O
shortwave	O
radio	O
,	O
and	O
so	O
was	O
introduced	O
into	O
automatic	O
systems	O
.	O
</s>
<s>
By	O
1959	O
most	O
radio	O
circuits	O
for	O
Telex	B-Application
were	O
using	O
ARQ	O
.	O
</s>
<s>
Radio	O
Corporation	O
of	O
America	O
,	O
who	O
were	O
the	O
first	O
in	O
USA	O
to	O
use	O
ARQ-M	B-Device
,	O
named	O
their	O
ARQ	O
equipment	O
"	O
automatic	O
error	O
reduction	O
and	O
correction	O
equipment	O
"	O
.	O
</s>
<s>
By	O
1990	O
use	O
of	O
ARQ-M	B-Device
had	O
reduced	O
.	O
</s>
<s>
ARQ-M	B-Device
is	O
a	O
duplex	O
radio	O
communications	O
protocol	O
used	O
to	O
ensure	O
that	O
data	O
is	O
transmitted	O
correctly	O
over	O
a	O
partially	O
unreliable	O
radio	O
path	O
.	O
</s>
<s>
Data	O
is	O
encoded	O
using	O
7	O
unit	O
binary	O
codes	O
sent	O
using	O
a	O
seven	O
bit	O
error	B-Error_Name
detecting	I-Error_Name
code	I-Error_Name
called	O
Van	O
Duuren	O
code	O
or	O
CCITT	O
Telegraph	O
Alphabet	O
No	O
.	O
</s>
<s>
The	O
data	O
is	O
sent	O
in	O
two	O
channels	O
labelled	O
A	O
and	O
B	O
,	O
or	O
four	O
channels	O
labeled	O
A	O
,	O
B	O
,	O
C	O
and	O
D	O
.	O
Data	O
from	O
the	O
different	O
channels	O
are	O
interleaved	O
using	O
a	O
time	B-Protocol
division	I-Protocol
multiplex	I-Protocol
system	O
.	O
</s>
<s>
The	O
two	O
channel	O
system	O
is	O
called	O
ARQ-M2	B-Device
,	O
the	O
four	O
channel	O
system	O
is	O
called	O
ARQ-M4	B-Device
.	O
</s>
<s>
These	O
rates	O
allowed	O
running	O
at	O
the	O
same	O
speed	O
as	O
50baud	O
Telex	B-Application
landline	O
systems	O
.	O
</s>
<s>
Two	O
performance	O
measures	O
give	O
quality	O
characteristics	O
of	O
an	O
ARQ-M	B-Device
link	O
.	O
</s>
<s>
If	O
the	O
unprotected	O
5	O
unit	O
code	O
had	O
an	O
error	O
rate	O
of	O
1%	O
,	O
the	O
ARQ-M	B-Device
protected	O
code	O
error	O
rate	O
is	O
0.0025	O
%	O
.	O
</s>
<s>
The	O
alphabet	O
used	O
in	O
the	O
ARQ-M	B-Device
protocol	O
is	O
based	O
on	O
the	O
CCITT	O
Telegraph	O
Alphabet	O
No	O
.	O
</s>
<s>
The	O
ARQ-M	B-Device
alphabet	O
being	O
synchronous	O
always	O
has	O
to	O
send	O
data	O
and	O
does	O
not	O
have	O
gaps	O
between	O
characters	O
.	O
</s>
<s>
A	O
prolonged	O
start	O
signal	O
can	O
be	O
used	O
for	O
signalling	O
,	O
for	O
example	O
to	O
select	O
a	O
destination	O
for	O
a	O
telex	B-Application
connection	O
.	O
</s>
<s>
The	O
ARQ-M	B-Device
characters	O
are	O
remapped	O
to	O
a	O
seven	O
bit	O
system	O
so	O
that	O
there	O
are	O
always	O
three	O
bits	O
on	O
and	O
four	O
bits	O
off	O
.	O
</s>
<s>
This	O
differs	O
from	O
the	O
alphabet	O
used	O
in	O
the	O
ARQ-E	B-Device
single	O
channel	O
system	O
.	O
</s>
<s>
These	O
are	O
combined	O
in	O
a	O
time	B-Protocol
division	I-Protocol
multiplex	I-Protocol
method	O
with	O
characters	O
in	O
the	O
order	O
A1	O
B4	O
A2	O
B1	O
A3	O
B2	O
A4	O
B3	O
resulting	O
in	O
a	O
marking	O
pattern	O
↓↓↑↑↑↓↑↓	O
(	O
44333434	O
one	O
bits	O
)	O
to	O
match	O
the	O
first	O
character	O
from	O
channel	O
A	O
at	O
the	O
start	O
.	O
</s>
<s>
ARQ-M	B-Device
is	O
recommended	O
by	O
CCIR	O
342-2	O
(	O
now	O
called	O
ITU-R	O
F.342-2	O
)	O
and	O
earlier	O
CCIR	O
242	O
.	O
</s>
<s>
ARQ-M	B-Device
was	O
standardised	O
by	O
the	O
Comité	O
consultatif	O
international	O
pour	O
la	O
radio	O
.	O
</s>
<s>
The	O
XII	O
Plenary	O
session	O
in	O
New	O
Delhi	O
in	O
1970	O
approved	O
recommendation	O
342-2	O
Automatic	O
error-correcting	B-Error_Name
system	O
for	O
telegraph	O
signals	O
transmitted	O
over	O
radio	O
circuits	O
.	O
</s>
<s>
The	O
CCITT	O
produced	O
recommendation	O
S.13	O
Use	O
on	O
radio	O
circuits	O
of	O
7-unit	O
synchronous	O
systems	O
giving	O
error	B-Error_Name
correction	I-Error_Name
by	O
automatic	O
repetition	O
.	O
</s>
<s>
Alternative	O
names	O
for	O
ARQ-M	B-Device
are	O
TDM	O
,	O
TDM-242	B-Device
,	O
TDM-342	B-Device
or	O
96-TDM	B-Device
.	O
</s>
<s>
ARQ-M2	B-Device
is	O
also	O
known	O
as	O
TDM-2	O
,	O
or	O
ARQ-28	B-Device
,	O
and	O
the	O
ARQ-M4	B-Device
variant	O
is	O
known	O
as	O
TDM-4	O
,	O
or	O
ARQ-56	B-Device
.	O
</s>
<s>
Hasler	O
from	O
Berne	O
in	O
Switzerland	O
made	O
a	O
four	O
channel	O
TOR	O
(	O
Telex	B-Application
on	O
Radio	O
)	O
system	O
.	O
</s>
<s>
The	O
equipment	O
for	O
the	O
ARQ-M	B-Device
systems	O
contained	O
a	O
receive	O
side	O
and	O
a	O
transmit	O
side	O
.	O
</s>
<s>
Next	O
the	O
seven	O
bit	O
character	O
goes	O
to	O
an	O
output	O
shift	B-General_Concept
register	I-General_Concept
,	O
and	O
then	O
to	O
a	O
polarity	O
inverter	O
.	O
</s>
<s>
On	O
the	O
receive	O
section	O
the	O
signal	O
from	O
the	O
radio	O
receiver	O
comes	O
to	O
a	O
tone	O
demodulator	O
,	O
then	O
a	O
polarity	O
inverter	O
,	O
and	O
then	O
to	O
an	O
error	B-Error_Name
checking	I-Error_Name
circuit	O
,	O
and	O
at	O
the	O
same	O
time	O
to	O
an	O
input	O
shift	B-General_Concept
register	I-General_Concept
that	O
converts	O
from	O
serial	O
to	O
parallel	O
.	O
</s>
<s>
Because	O
the	O
telex	B-Application
network	O
does	O
not	O
give	O
feedback	O
to	O
a	O
sender	O
about	O
when	O
a	O
message	O
is	O
delivered	O
,	O
and	O
a	O
shortwave	O
ARQ	O
link	O
may	O
or	O
may	O
not	O
be	O
available	O
,	O
systems	O
with	O
ARQ	O
links	O
set	O
them	O
up	O
to	O
be	O
store	O
and	O
forward	O
half	O
duplex	O
systems	O
.	O
</s>
<s>
This	O
means	O
that	O
a	O
telex	B-Application
sender	O
transmits	O
their	O
telegram	B-Application
in	O
full	O
to	O
the	O
message	O
centre	O
that	O
then	O
stores	O
and	O
forwards	O
the	O
message	O
over	O
the	O
shortwave	O
link	O
when	O
it	O
is	O
available	O
.	O
</s>
<s>
An	O
international	O
telex	B-Application
exchange	O
would	O
have	O
signalling	O
conversion	O
facilities	O
.	O
</s>
<s>
For	O
radio	O
spectrum	O
monitors	O
,	O
ARQ-M	B-Device
is	O
classified	O
as	O
a	O
digital	O
mode	O
,	O
and	O
radio	O
stations	O
that	O
use	O
it	O
will	O
be	O
termed	O
utility	O
stations	O
.	O
</s>
<s>
Two	O
stations	O
communicating	O
with	O
ARQ-M	B-Device
will	O
use	O
two	O
different	O
frequencies	O
as	O
data	O
is	O
continuously	O
transmitted	O
in	O
each	O
direction	O
.	O
</s>
<s>
Software	O
is	O
available	O
to	O
receive	O
and	O
decode	O
utility	O
transmissions	O
that	O
use	O
ARQ-M	B-Device
including	O
Go2MONITOR	O
,	O
and	O
there	O
is	O
also	O
hardware	O
such	O
as	O
the	O
Code300-A	O
from	O
Hoka	O
Electronic	O
,	O
and	O
Wavecom	O
M	O
4010	O
.	O
</s>
