<s>
Exponential	B-Algorithm
backoff	I-Algorithm
is	O
an	O
algorithm	O
that	O
uses	O
feedback	O
to	O
multiplicatively	O
decrease	O
the	O
rate	O
of	O
some	O
process	O
,	O
in	O
order	O
to	O
gradually	O
find	O
an	O
acceptable	O
rate	O
.	O
</s>
<s>
These	O
algorithms	O
find	O
usage	O
in	O
a	O
wide	O
range	O
of	O
systems	O
and	O
processes	O
,	O
with	O
radio	O
networks	O
and	O
computer	B-Architecture
networks	I-Architecture
being	O
particularly	O
notable	O
.	O
</s>
<s>
An	O
exponential	B-Algorithm
backoff	I-Algorithm
algorithm	I-Algorithm
is	O
a	O
form	O
of	O
closed-loop	O
control	O
system	O
that	O
reduces	O
the	O
rate	O
of	O
a	O
controlled	O
process	O
in	O
response	O
to	O
adverse	O
events	O
.	O
</s>
<s>
An	O
exponential	B-Algorithm
backoff	I-Algorithm
algorithm	I-Algorithm
where	O
is	O
referred	O
to	O
as	O
a	O
binary	O
exponential	B-Algorithm
backoff	I-Algorithm
algorithm	I-Algorithm
.	O
</s>
<s>
The	O
time	O
period	O
that	O
must	O
elapse	O
before	O
attempting	O
to	O
increase	O
the	O
rate	O
again	O
may	O
,	O
itself	O
,	O
be	O
determined	O
by	O
an	O
exponential	B-Algorithm
backoff	I-Algorithm
algorithm	I-Algorithm
.	O
</s>
<s>
Exponential	B-Algorithm
backoff	I-Algorithm
is	O
commonly	O
utilised	O
as	O
part	O
of	O
rate	O
limiting	O
mechanisms	O
in	O
computer	O
systems	O
such	O
as	O
web	O
services	O
,	O
to	O
help	O
enforce	O
fair	O
distribution	O
of	O
access	O
to	O
resources	O
and	O
prevent	O
network	B-Protocol
congestion	I-Protocol
.	O
</s>
<s>
A	O
benefit	O
of	O
utilising	O
an	O
exponential	B-Algorithm
backoff	I-Algorithm
algorithm	I-Algorithm
,	O
over	O
of	O
a	O
fixed	O
rate	O
limit	O
,	O
is	O
that	O
rate	O
limits	O
can	O
be	O
achieved	O
dynamically	O
without	O
providing	O
any	O
prior	O
information	O
to	O
the	O
client	O
.	O
</s>
<s>
For	O
example	O
,	O
in	O
SIP	B-Application
protocol	I-Application
over	O
unreliable	O
transport	O
(	O
such	O
as	O
UDP	B-Protocol
)	O
the	O
client	O
retransmits	O
requests	O
at	O
an	O
interval	O
that	O
starts	O
at	O
T1	O
seconds	O
(	O
usually	O
,	O
500	O
ms	O
,	O
which	O
is	O
the	O
estimate	O
of	O
the	O
round-trip	O
time	O
)	O
and	O
doubles	O
after	O
every	O
retransmission	B-Protocol
until	O
it	O
reaches	O
T2	O
seconds	O
(	O
which	O
defaults	O
to	O
4	O
s	O
)	O
.	O
</s>
<s>
This	O
results	O
in	O
retransmission	B-Protocol
intervals	O
of	O
500	O
ms	O
,	O
1	O
s	O
,	O
2	O
s	O
,	O
4	O
s	O
,	O
4	O
s	O
,	O
4	O
s	O
,	O
etc	O
.	O
</s>
<s>
Exponential	B-Algorithm
backoff	I-Algorithm
algorithms	I-Algorithm
can	O
be	O
used	O
to	O
avoid	O
network	O
collisions	O
.	O
</s>
<s>
In	O
a	O
point-to-multipoint	B-General_Concept
or	O
multiplexed	B-Architecture
network	O
,	O
multiple	O
senders	O
communicate	O
over	O
a	O
single	O
shared	O
channel	O
.	O
</s>
<s>
Each	O
sender	O
can	O
then	O
back	O
off	O
before	O
attempting	O
to	O
retransmit	B-Protocol
the	O
same	O
message	O
again	O
.	O
</s>
<s>
A	O
deterministic	B-General_Concept
exponential	B-Algorithm
backoff	I-Algorithm
algorithm	I-Algorithm
is	O
unsuitable	O
for	O
this	O
use	O
case	O
,	O
since	O
each	O
sender	O
would	O
back	O
off	O
for	O
the	O
same	O
time	O
period	O
,	O
leading	O
them	O
to	O
retransmit	B-Protocol
simultaneously	O
and	O
cause	O
another	O
collision	O
.	O
</s>
<s>
Instead	O
,	O
for	O
purposes	O
of	O
collision	O
avoidance	O
,	O
the	O
time	O
between	O
retransmissions	B-Protocol
is	O
randomized	B-General_Concept
and	O
the	O
exponential	B-Algorithm
backoff	I-Algorithm
algorithm	I-Algorithm
sets	O
the	O
range	O
of	O
delay	O
values	O
that	O
are	O
possible	O
.	O
</s>
<s>
The	O
time	O
delay	O
is	O
usually	O
measured	O
in	O
slots	B-Protocol
,	O
which	O
are	O
fixed-length	O
periods	O
(	O
or	O
"	O
slices	O
"	O
)	O
of	O
time	O
on	O
the	O
network	O
.	O
</s>
<s>
In	O
a	O
binary	O
exponential	B-Algorithm
backoff	I-Algorithm
algorithm	I-Algorithm
(	O
i.e.	O
</s>
<s>
one	O
where	O
)	O
,	O
after	O
collisions	O
,	O
each	O
retransmission	B-Protocol
is	O
delayed	O
by	O
a	O
random	O
number	O
of	O
slot	B-Protocol
times	I-Protocol
between	O
and	O
.	O
</s>
<s>
After	O
the	O
first	O
collision	O
,	O
each	O
sender	O
will	O
wait	O
0	O
or	O
1	O
slot	B-Protocol
times	I-Protocol
.	O
</s>
<s>
After	O
the	O
second	O
collision	O
,	O
the	O
senders	O
will	O
wait	O
anywhere	O
from	O
0	O
to	O
3	O
slot	B-Protocol
times	I-Protocol
(	O
inclusive	O
)	O
.	O
</s>
<s>
After	O
the	O
third	O
collision	O
,	O
the	O
senders	O
will	O
wait	O
anywhere	O
from	O
0	O
to	O
7	O
slot	B-Protocol
times	I-Protocol
(	O
inclusive	O
)	O
,	O
and	O
so	O
forth	O
.	O
</s>
<s>
As	O
the	O
number	O
of	O
retransmission	B-Protocol
attempts	O
increases	O
,	O
the	O
number	O
of	O
possibilities	O
for	O
delay	O
increases	O
exponentially	O
.	O
</s>
<s>
Exponential	B-Algorithm
backoff	I-Algorithm
is	O
utilised	O
during	O
retransmission	B-Protocol
of	O
frames	B-Protocol
in	O
carrier-sense	O
multiple	O
access	O
with	O
collision	O
avoidance	O
(	O
CSMA/CA	O
)	O
and	O
carrier-sense	O
multiple	O
access	O
with	O
collision	O
detection	O
(	O
CSMA/CD	O
)	O
networks	O
,	O
where	O
this	O
algorithm	O
is	O
part	O
of	O
the	O
channel	B-Protocol
access	I-Protocol
method	O
used	O
to	O
send	O
data	O
on	O
these	O
networks	O
.	O
</s>
<s>
In	O
Ethernet	O
networks	O
,	O
the	O
algorithm	O
is	O
commonly	O
used	O
to	O
schedule	O
retransmissions	B-Protocol
after	O
collisions	O
.	O
</s>
<s>
The	O
retransmission	B-Protocol
is	O
delayed	O
by	O
an	O
amount	O
of	O
time	O
derived	O
from	O
the	O
slot	B-Protocol
time	I-Protocol
(	O
for	O
example	O
,	O
the	O
time	O
it	O
takes	O
to	O
send	O
512	O
bits	O
;	O
i.e.	O
,	O
512	O
bit-times	B-Protocol
)	O
and	O
the	O
number	O
of	O
attempts	O
to	O
retransmit	B-Protocol
.	O
</s>
<s>
An	O
exponential	B-Algorithm
backoff	I-Algorithm
algorithm	I-Algorithm
is	O
therefore	O
used	O
.	O
</s>
<s>
The	O
value	O
51.2	O
μs	O
is	O
used	O
as	O
an	O
example	O
here	O
because	O
it	O
is	O
the	O
slot	B-Protocol
time	I-Protocol
for	O
a	O
10	O
Mbit/s	O
Ethernet	O
line	O
.	O
</s>
<s>
Resend	B-Protocol
a	O
frame	O
after	O
either	O
0	O
seconds	O
or	O
51.2	O
μs	O
,	O
chosen	O
at	O
random	O
.	O
</s>
<s>
If	O
that	O
fails	O
,	O
resend	B-Protocol
the	O
frame	O
after	O
either	O
0	O
s	O
,	O
51.2	O
μs	O
,	O
102.4	O
μs	O
,	O
or	O
153.6	O
μs	O
.	O
</s>
<s>
If	O
that	O
still	O
fails	O
,	O
resend	B-Protocol
the	O
frame	O
after	O
,	O
where	O
is	O
a	O
random	O
integer	O
between	O
0	O
and	O
.	O
</s>
<s>
For	O
further	O
failures	O
,	O
after	O
the	O
cth	O
failed	O
attempt	O
,	O
resend	B-Protocol
the	O
frame	O
after	O
,	O
where	O
is	O
a	O
random	O
integer	O
between	O
0	O
and	O
.	O
</s>
<s>
In	O
a	O
randomized	B-General_Concept
system	O
this	O
may	O
occur	O
by	O
chance	O
,	O
leading	O
to	O
unpredictable	O
latency	O
;	O
longer	O
delays	O
due	O
to	O
unbounded	O
increases	O
in	O
are	O
exponentially	O
less	O
probable	O
,	O
but	O
they	O
are	O
effectively	O
inevitable	O
on	O
a	O
busy	O
network	O
due	O
to	O
the	O
law	O
of	O
large	O
numbers	O
.	O
</s>
<s>
For	O
example	O
,	O
if	O
the	O
ceiling	O
is	O
set	O
at	O
in	O
a	O
truncated	O
binary	O
exponential	B-Algorithm
backoff	I-Algorithm
algorithm	I-Algorithm
,	O
(	O
as	O
it	O
is	O
in	O
the	O
IEEE	O
802.3	O
CSMA/CD	O
standard	O
)	O
,	O
then	O
the	O
maximum	O
delay	O
is	O
1023	O
slot	B-Protocol
times	I-Protocol
,	O
i.e.	O
</s>
<s>
At	O
the	O
same	O
time	O
,	O
increasing	O
the	O
limit	O
also	O
exponentially	O
increases	O
the	O
range	O
of	O
possible	O
latency	O
times	O
for	O
a	O
transmission	O
,	O
leading	O
to	O
less	O
deterministic	B-General_Concept
performance	O
and	O
an	O
increase	O
in	O
the	O
average	O
latency	O
.	O
</s>
<s>
which	O
is	O
,	O
for	O
the	O
example	O
,	O
slots	B-Protocol
.	O
</s>
