<s>
Automatic	B-Error_Name
repeat	I-Error_Name
request	I-Error_Name
(	O
ARQ	O
)	O
,	O
also	O
known	O
as	O
automatic	O
repeat	O
query	O
,	O
is	O
an	O
error-control	B-Error_Name
method	O
for	O
data	O
transmission	O
that	O
uses	O
acknowledgements	B-Protocol
(	O
messages	O
sent	O
by	O
the	O
receiver	O
indicating	O
that	O
it	O
has	O
correctly	O
received	O
a	O
message	O
)	O
and	O
timeouts	O
(	O
specified	O
periods	O
of	O
time	O
allowed	O
to	O
elapse	O
before	O
an	O
acknowledgment	B-Protocol
is	O
to	O
be	O
received	O
)	O
to	O
achieve	O
reliable	B-Protocol
data	I-Protocol
transmission	I-Protocol
over	O
an	O
unreliable	O
communication	O
channel	O
.	O
</s>
<s>
If	O
the	O
sender	O
does	O
not	O
receive	O
an	O
acknowledgment	B-Protocol
before	O
the	O
timeout	O
,	O
it	O
re-transmits	B-Protocol
the	O
message	O
until	O
it	O
receives	O
an	O
acknowledgment	B-Protocol
or	O
exceeds	O
a	O
predefined	O
number	O
of	O
retransmissions	B-Protocol
.	O
</s>
<s>
Variations	O
of	O
ARQ	B-Error_Name
protocols	I-Error_Name
include	O
Stop-and-wait	B-Error_Name
ARQ	I-Error_Name
,	O
Go-Back-N	B-Error_Name
ARQ	I-Error_Name
,	O
and	O
Selective	B-Error_Name
Repeat	I-Error_Name
ARQ	I-Error_Name
.	O
</s>
<s>
These	O
protocols	O
reside	O
in	O
the	O
data	B-Protocol
link	I-Protocol
or	O
transport	B-Architecture
layers	I-Architecture
(	O
layers	O
2	O
and	O
4	O
)	O
of	O
the	O
OSI	O
model	O
.	O
</s>
<s>
The	O
Transmission	B-Protocol
Control	I-Protocol
Protocol	I-Protocol
uses	O
a	O
variant	O
of	O
Go-Back-N	B-Error_Name
ARQ	I-Error_Name
to	O
ensure	O
reliable	O
transmission	O
of	O
data	O
over	O
the	O
Internet	B-Protocol
Protocol	I-Protocol
,	O
which	O
does	O
not	O
provide	O
guaranteed	O
delivery	O
of	O
packets	O
;	O
with	O
Selective	O
Acknowledgement	B-Protocol
(	O
SACK	O
)	O
,	O
it	O
uses	O
Selective	B-Error_Name
Repeat	I-Error_Name
ARQ	I-Error_Name
.	O
</s>
<s>
IEEE	O
802.11	O
wireless	O
networking	O
uses	O
ARQ	O
retransmissions	B-Protocol
at	O
the	O
data-link	B-Protocol
layer	I-Protocol
.	O
</s>
<s>
ITU-T	B-General_Concept
G.hn	I-General_Concept
uses	O
hybrid	B-Error_Name
ARQ	I-Error_Name
,	O
a	O
mixture	O
of	O
high-rate	O
forward	B-Error_Name
error	I-Error_Name
correction	I-Error_Name
(	O
FEC	O
)	O
and	O
ARQ	O
.	O
</s>
<s>
It	O
is	O
a	O
high-speed	O
local	B-General_Concept
area	I-General_Concept
network	I-General_Concept
standard	O
that	O
can	O
operate	O
at	O
data	O
rates	O
up	O
to	O
1Gbit/s	O
over	O
existing	O
home	O
wiring	O
(	O
power	B-Device
lines	I-Device
,	O
phone	O
lines	O
and	O
coaxial	B-Protocol
cables	I-Protocol
)	O
.	O
</s>
<s>
G.hn	B-General_Concept
uses	O
CRC-32C	O
for	O
Error	B-Error_Name
Detection	I-Error_Name
,	O
LDPC	B-Error_Name
for	O
FEC	O
and	O
selective	B-Error_Name
repeat	I-Error_Name
for	O
ARQ	O
.	O
</s>
<s>
ARQ	O
systems	O
were	O
widely	O
used	O
on	O
shortwave	O
radio	O
to	O
ensure	O
reliable	B-Protocol
delivery	I-Protocol
of	O
data	O
such	O
as	O
for	O
telegrams	O
.	O
</s>
<s>
These	O
systems	O
came	O
in	O
forms	O
called	O
ARQ-E	B-Device
and	O
ARQ-M	B-Device
,	O
which	O
also	O
included	O
the	O
ability	O
to	O
multiplex	O
two	O
or	O
four	O
channels	O
.	O
</s>
<s>
In	O
these	O
high	O
throughput	O
environments	O
negative	O
acknowledgements	B-Protocol
are	O
used	O
to	O
drive	O
down	O
overheads	O
.	O
</s>
