<s>
Selective	O
Repeat	O
ARQ/Selective	O
Reject	O
ARQ	O
is	O
a	O
specific	O
instance	O
of	O
the	O
automatic	B-Error_Name
repeat	I-Error_Name
request	I-Error_Name
(	O
ARQ	O
)	O
protocol	O
used	O
to	O
manage	O
sequence	B-Protocol
numbers	I-Protocol
and	O
retransmissions	O
in	O
reliable	B-Protocol
communications	I-Protocol
.	O
</s>
<s>
Selective	B-Error_Name
Repeat	I-Error_Name
is	O
part	O
of	O
the	O
automatic	B-Error_Name
repeat	I-Error_Name
request	I-Error_Name
(	O
ARQ	O
)	O
.	O
</s>
<s>
With	O
selective	B-Error_Name
repeat	I-Error_Name
,	O
the	O
sender	O
sends	O
a	O
number	O
of	O
frames	B-Protocol
specified	O
by	O
a	O
window	O
size	O
even	O
without	O
the	O
need	O
to	O
wait	O
for	O
individual	O
ACK	B-Protocol
from	O
the	O
receiver	O
as	O
in	O
Go-Back-N	B-Error_Name
ARQ	I-Error_Name
.	O
</s>
<s>
The	O
receiver	O
may	O
selectively	O
reject	O
a	O
single	O
frame	B-Protocol
,	O
which	O
may	O
be	O
retransmitted	O
alone	O
;	O
this	O
contrasts	O
with	O
other	O
forms	O
of	O
ARQ	O
,	O
which	O
must	O
send	O
every	O
frame	B-Protocol
from	O
that	O
point	O
again	O
.	O
</s>
<s>
The	O
receiver	O
accepts	O
out-of-order	O
frames	B-Protocol
and	O
buffers	O
them	O
.	O
</s>
<s>
The	O
sender	O
individually	O
retransmits	O
frames	B-Protocol
that	O
have	O
timed	O
out	O
.	O
</s>
<s>
It	O
may	O
be	O
used	O
as	O
a	O
protocol	O
for	O
the	O
delivery	O
and	O
acknowledgement	B-Protocol
of	O
message	O
units	O
,	O
or	O
it	O
may	O
be	O
used	O
as	O
a	O
protocol	O
for	O
the	O
delivery	O
of	O
subdivided	O
message	O
sub-units	O
.	O
</s>
<s>
When	O
used	O
as	O
the	O
protocol	O
for	O
the	O
delivery	O
of	O
messages	O
,	O
the	O
sending	O
process	O
continues	O
to	O
send	O
a	O
number	O
of	O
frames	B-Protocol
specified	O
by	O
a	O
window	O
size	O
even	O
after	O
a	O
frame	B-Protocol
loss	O
.	O
</s>
<s>
Unlike	O
Go-Back-N	B-Error_Name
ARQ	I-Error_Name
,	O
the	O
receiving	O
process	O
will	O
continue	O
to	O
accept	O
and	O
acknowledge	B-Protocol
frames	B-Protocol
sent	O
after	O
an	O
initial	O
error	O
;	O
this	O
is	O
the	O
general	O
case	O
of	O
the	O
sliding	O
window	O
protocol	O
with	O
both	O
transmit	O
and	O
receive	O
window	O
sizes	O
greater	O
than	O
1	O
.	O
</s>
<s>
The	O
receiver	O
process	O
keeps	O
track	O
of	O
the	O
sequence	B-Protocol
number	I-Protocol
of	O
the	O
earliest	O
frame	B-Protocol
it	O
has	O
not	O
received	O
,	O
and	O
sends	O
that	O
number	O
with	O
every	O
acknowledgement	B-Protocol
(	O
ACK	B-Protocol
)	O
it	O
sends	O
.	O
</s>
<s>
If	O
a	O
frame	B-Protocol
from	O
the	O
sender	O
does	O
not	O
reach	O
the	O
receiver	O
,	O
the	O
sender	O
continues	O
to	O
send	O
subsequent	O
frames	B-Protocol
until	O
it	O
has	O
emptied	O
its	O
window	O
.	O
</s>
<s>
The	O
receiver	O
continues	O
to	O
fill	O
its	O
receiving	O
window	O
with	O
the	O
subsequent	O
frames	B-Protocol
,	O
replying	O
each	O
time	O
with	O
an	O
ACK	B-Protocol
containing	O
the	O
sequence	B-Protocol
number	I-Protocol
of	O
the	O
earliest	O
missing	O
frame	B-Protocol
.	O
</s>
<s>
Once	O
the	O
sender	O
has	O
sent	O
all	O
the	O
frames	B-Protocol
in	O
its	O
window	O
,	O
it	O
re-sends	O
the	O
frame	B-Protocol
number	O
given	O
by	O
the	O
ACKs	B-Protocol
,	O
and	O
then	O
continues	O
where	O
it	O
left	O
off	O
.	O
</s>
<s>
The	O
size	O
of	O
the	O
sending	O
and	O
receiving	O
windows	O
must	O
be	O
equal	O
,	O
and	O
half	O
the	O
maximum	O
sequence	B-Protocol
number	I-Protocol
(	O
assuming	O
that	O
sequence	B-Protocol
numbers	I-Protocol
are	O
numbered	O
from	O
0	O
to	O
n−1	O
)	O
to	O
avoid	O
miscommunication	O
in	O
all	O
cases	O
of	O
packets	O
being	O
dropped	O
.	O
</s>
<s>
To	O
understand	O
this	O
,	O
consider	O
the	O
case	O
when	O
all	O
ACKs	B-Protocol
are	O
destroyed	O
.	O
</s>
<s>
If	O
the	O
receiving	O
window	O
is	O
larger	O
than	O
half	O
the	O
maximum	O
sequence	B-Protocol
number	I-Protocol
,	O
some	O
,	O
possibly	O
even	O
all	O
,	O
of	O
the	O
packets	O
that	O
are	O
present	O
after	O
timeouts	O
are	O
duplicates	O
that	O
are	O
not	O
recognized	O
as	O
such	O
.	O
</s>
<s>
In	O
non-continuous	O
channels	O
where	O
messages	O
may	O
be	O
variable	O
in	O
length	O
,	O
standard	O
ARQ	O
or	O
Hybrid	O
ARQ	B-Error_Name
protocols	I-Error_Name
may	O
treat	O
the	O
message	O
as	O
a	O
single	O
unit	O
.	O
</s>
<s>
Alternately	O
selective	O
retransmission	O
may	O
be	O
employed	O
in	O
conjunction	O
with	O
the	O
basic	O
ARQ	O
mechanism	O
where	O
the	O
message	O
is	O
first	O
subdivided	O
into	O
sub-blocks	O
(	O
typically	O
of	O
fixed	O
length	O
)	O
in	O
a	O
process	O
called	O
packet	B-Protocol
segmentation	I-Protocol
.	O
</s>
<s>
While	O
in	O
standard	O
ARQ	O
the	O
message	O
as	O
a	O
whole	O
is	O
either	O
acknowledged	O
(	O
ACKed	O
)	O
or	O
negatively	O
acknowledged	O
(	O
NAKed	O
)	O
,	O
in	O
ARQ	O
with	O
selective	O
transmission	O
the	O
ACK	B-Protocol
response	O
would	O
additionally	O
carry	O
a	O
bit	O
flag	O
indicating	O
the	O
identity	O
of	O
each	O
sub-block	O
successfully	O
received	O
.	O
</s>
<s>
Selective	B-Error_Name
Repeat	I-Error_Name
is	O
implemented	O
in	O
UDP	O
transmission	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
extension	O
,	O
it	O
may	O
also	O
use	O
Selective	B-Error_Name
Repeat	I-Error_Name
ARQ	I-Error_Name
.	O
</s>
<s>
The	O
ITU-T	B-General_Concept
G.hn	I-General_Concept
standard	O
,	O
which	O
provides	O
a	O
way	O
to	O
create	O
a	O
high-speed	O
(	O
up	O
to	O
1	O
Gigabit/s	O
)	O
Local	B-General_Concept
area	I-General_Concept
network	I-General_Concept
using	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
uses	O
Selective	B-Error_Name
Repeat	I-Error_Name
ARQ	I-Error_Name
to	O
ensure	O
reliable	O
transmission	O
over	O
noisy	O
media	O
.	O
</s>
<s>
G.hn	B-General_Concept
employs	O
packet	B-Protocol
segmentation	I-Protocol
to	O
sub-divide	O
messages	O
into	O
smaller	O
units	O
,	O
to	O
increase	O
the	O
probability	O
that	O
each	O
one	O
is	O
received	O
correctly	O
.	O
</s>
<s>
The	O
STANAG	O
5066	O
PROFILE	O
FOR	O
HF	O
RADIO	O
DATA	O
COMMUNICATIONS	O
uses	O
Selective	B-Error_Name
Repeat	I-Error_Name
ARQ	I-Error_Name
,	O
with	O
a	O
maximum	O
window	O
size	O
of	O
128	O
protocol-data	O
units	O
(	O
PDUs	O
)	O
.	O
</s>
