<s>
Carrier-sense	O
multiple	O
access	O
with	O
collision	B-Protocol
avoidance	I-Protocol
(	O
CSMA/CA	O
)	O
in	O
computer	B-Architecture
networking	I-Architecture
,	O
is	O
a	O
network	O
multiple	O
access	O
method	O
in	O
which	O
carrier	O
sensing	O
is	O
used	O
,	O
but	O
nodes	O
attempt	O
to	O
avoid	O
collisions	O
by	O
beginning	O
transmission	O
only	O
after	O
the	O
channel	O
is	O
sensed	O
to	O
be	O
"	O
idle	O
"	O
.	O
</s>
<s>
CSMA/CA	O
is	O
unreliable	O
due	O
to	O
the	O
hidden	B-General_Concept
node	I-General_Concept
problem	I-General_Concept
.	O
</s>
<s>
CSMA/CA	O
is	O
a	O
protocol	O
that	O
operates	O
in	O
the	O
data	B-Protocol
link	I-Protocol
layer	I-Protocol
(	O
Layer	B-Protocol
2	I-Protocol
)	O
of	O
the	O
OSI	O
model	O
.	O
</s>
<s>
Collision	B-Protocol
avoidance	I-Protocol
is	O
used	O
to	O
improve	O
the	O
performance	O
of	O
the	O
CSMA	O
method	O
by	O
attempting	O
to	O
divide	O
the	O
channel	O
somewhat	O
equally	O
among	O
all	O
transmitting	O
nodes	O
within	O
the	O
collision	O
domain	O
.	O
</s>
<s>
Carrier	O
Sense	O
:	O
prior	O
to	O
transmitting	O
,	O
a	O
node	B-Protocol
first	O
listens	O
to	O
the	O
shared	O
medium	O
(	O
such	O
as	O
listening	O
for	O
wireless	O
signals	O
in	O
a	O
wireless	O
network	O
)	O
to	O
determine	O
whether	O
another	O
node	B-Protocol
is	O
transmitting	O
or	O
not	O
.	O
</s>
<s>
Note	O
that	O
the	O
hidden	B-General_Concept
node	I-General_Concept
problem	I-General_Concept
means	O
another	O
node	B-Protocol
may	O
be	O
transmitting	O
which	O
goes	O
undetected	O
at	O
this	O
stage	O
.	O
</s>
<s>
Collision	B-Protocol
Avoidance	I-Protocol
:	O
if	O
another	O
node	B-Protocol
was	O
heard	O
,	O
we	O
wait	O
for	O
a	O
period	O
of	O
time	O
(	O
usually	O
random	O
)	O
for	O
the	O
node	B-Protocol
to	O
stop	O
transmitting	O
before	O
listening	O
again	O
for	O
a	O
free	O
communications	O
channel	O
.	O
</s>
<s>
This	O
goes	O
some	O
way	O
to	O
alleviating	O
the	O
problem	O
of	O
hidden	B-General_Concept
nodes	I-General_Concept
because	O
,	O
for	O
instance	O
,	O
in	O
a	O
wireless	O
network	O
,	O
the	O
Access	O
Point	O
only	O
issues	O
a	O
Clear	O
to	O
Send	O
to	O
one	O
node	B-Protocol
at	O
a	O
time	O
.	O
</s>
<s>
Transmission	O
:	O
if	O
the	O
medium	O
was	O
identified	O
as	O
being	O
clear	O
or	O
the	O
node	B-Protocol
received	O
a	O
CTS	O
to	O
explicitly	O
indicate	O
it	O
can	O
send	O
,	O
it	O
sends	O
the	O
frame	O
in	O
its	O
entirety	O
.	O
</s>
<s>
Unlike	O
CSMA/CD	O
,	O
it	O
is	O
very	O
challenging	O
for	O
a	O
wireless	O
node	B-Protocol
to	O
listen	O
at	O
the	O
same	O
time	O
as	O
it	O
transmits	O
(	O
its	O
transmission	O
will	O
dwarf	O
any	O
attempt	O
to	O
listen	O
)	O
.	O
</s>
<s>
Continuing	O
the	O
wireless	O
example	O
,	O
the	O
node	B-Protocol
awaits	O
receipt	O
of	O
an	O
acknowledgement	O
packet	O
from	O
the	O
Access	O
Point	O
to	O
indicate	O
the	O
packet	O
was	O
received	O
and	O
checksummed	O
correctly	O
.	O
</s>
<s>
If	O
such	O
acknowledgement	O
does	O
not	O
arrive	O
in	O
a	O
timely	O
manner	O
,	O
it	O
assumes	O
the	O
packet	O
collided	O
with	O
some	O
other	O
transmission	O
,	O
causing	O
the	O
node	B-Protocol
to	O
enter	O
a	O
period	O
of	O
binary	O
exponential	O
backoff	O
prior	O
to	O
attempting	O
to	O
re-transmit	O
.	O
</s>
<s>
Although	O
CSMA/CA	O
has	O
been	O
used	O
in	O
a	O
variety	O
of	O
wired	O
communication	O
systems	O
,	O
it	O
is	O
particularly	O
beneficial	O
in	O
a	O
wireless	B-Architecture
LAN	I-Architecture
due	O
to	O
a	O
common	O
problem	O
of	O
multiple	O
stations	O
being	O
able	O
to	O
see	O
the	O
Access	O
Point	O
,	O
but	O
not	O
each	O
other	O
.	O
</s>
<s>
This	O
is	O
the	O
so-called	O
'	O
hidden	B-General_Concept
node	I-General_Concept
 '	O
,	O
or	O
'	O
hidden	O
station	O
 '	O
problem	O
.	O
</s>
<s>
Devices	O
utilizing	O
802.11	O
based	O
standards	O
can	O
enjoy	O
the	O
benefits	O
of	O
collision	B-Protocol
avoidance	I-Protocol
(	O
RTS	O
/	O
CTS	O
handshake	O
,	O
also	O
Point	O
coordination	O
function	O
)	O
,	O
although	O
they	O
do	O
not	O
do	O
so	O
by	O
default	O
.	O
</s>
<s>
Implementation	O
of	O
RTS/CTS	O
helps	O
to	O
partially	O
solve	O
the	O
hidden	B-General_Concept
node	I-General_Concept
problem	I-General_Concept
that	O
is	O
often	O
found	O
in	O
wireless	O
networking	O
.	O
</s>
<s>
Studies	O
show	O
that	O
under	O
ideal	O
propagation	O
conditions	O
(	O
simulations	O
)	O
,	O
direct-sequence	B-Device
spread	I-Device
spectrum	I-Device
(	O
DSSS	B-Device
)	O
provides	O
the	O
highest	O
throughput	O
for	O
all	O
nodes	O
on	O
a	O
network	O
when	O
used	O
in	O
conjunction	O
with	O
CSMA/CA	O
and	O
the	O
IEEE	O
802.11	O
RTS/CTS	O
exchange	O
under	O
light	O
network	O
load	O
conditions	O
.	O
</s>
<s>
Frequency	B-Protocol
hopping	I-Protocol
spread	I-Protocol
spectrum	I-Protocol
(	O
FHSS	B-Protocol
)	O
follows	O
distantly	O
behind	O
DSSS	B-Device
with	O
regard	O
to	O
throughput	O
with	O
a	O
greater	O
throughput	O
once	O
network	O
load	O
becomes	O
substantially	O
heavy	O
.	O
</s>
<s>
Apple	O
's	O
LocalTalk	B-Device
implemented	O
CSMA/CA	O
on	O
an	O
electrical	O
bus	O
using	O
a	O
three-byte	O
jamming	O
signal	O
.	O
</s>
<s>
802.11	O
RTS/CTS	O
implements	O
virtual	O
carrier	O
sensing	O
using	O
short	O
request	O
to	O
send	O
and	O
clear	O
to	O
send	O
messages	O
for	O
WLANs	B-Architecture
(	O
802.11	O
mainly	O
relies	O
on	O
physical	O
carrier	O
sensing	O
though	O
)	O
.	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
CSMA/CA	O
as	O
a	O
channel	O
access	O
method	O
for	O
flows	O
that	O
do	O
not	O
require	O
guaranteed	O
quality	O
of	O
service	O
,	O
specifically	O
the	O
CSMA/CARP	B-Protocol
variant	O
.	O
</s>
