<s>
In	O
wireless	B-General_Concept
networking	I-General_Concept
,	O
the	O
hidden	B-General_Concept
node	I-General_Concept
problem	I-General_Concept
or	O
hidden	B-General_Concept
terminal	I-General_Concept
problem	I-General_Concept
occurs	O
when	O
a	O
node	B-Protocol
can	O
communicate	O
with	O
a	O
wireless	B-Device
access	I-Device
point	I-Device
(	O
AP	O
)	O
,	O
but	O
cannot	O
directly	O
communicate	O
with	O
other	O
nodes	O
that	O
are	O
communicating	O
with	O
that	O
AP	O
.	O
</s>
<s>
This	O
leads	O
to	O
difficulties	O
in	O
medium	B-Protocol
access	I-Protocol
control	I-Protocol
sublayer	I-Protocol
since	O
multiple	O
nodes	O
can	O
send	O
data	B-Protocol
packets	I-Protocol
to	O
the	O
AP	O
simultaneously	O
,	O
which	O
creates	O
interference	O
at	O
the	O
AP	O
resulting	O
in	O
no	O
packet	B-Protocol
getting	O
through	O
.	O
</s>
<s>
Although	O
some	O
loss	O
of	O
packets	B-Protocol
is	O
normal	O
in	O
wireless	B-General_Concept
networking	I-General_Concept
,	O
and	O
the	O
higher	O
layers	O
will	O
resend	O
them	O
,	O
if	O
one	O
of	O
the	O
nodes	O
is	O
transferring	O
a	O
lot	O
of	O
large	O
packets	B-Protocol
over	O
a	O
long	O
period	O
,	O
the	O
other	O
node	B-Protocol
may	O
get	O
very	O
little	O
goodput	O
.	O
</s>
<s>
Practical	O
protocol	O
solutions	O
exist	O
to	O
the	O
hidden	B-General_Concept
node	I-General_Concept
problem	I-General_Concept
.	O
</s>
<s>
For	O
example	O
,	O
Request	O
To	O
Send/Clear	O
To	O
Send	O
(	O
RTS/CTS	O
)	O
mechanisms	O
where	O
nodes	O
send	O
short	O
packets	B-Protocol
to	O
request	O
permission	O
of	O
the	O
access	B-Device
point	I-Device
to	O
send	O
longer	O
data	B-Protocol
packets	I-Protocol
.	O
</s>
<s>
However	O
,	O
the	O
mechanism	O
introduces	O
latency	B-Protocol
,	O
and	O
the	O
overhead	O
can	O
often	O
be	O
greater	O
than	O
the	O
cost	O
,	O
particularly	O
for	O
short	O
data	B-Protocol
packets	I-Protocol
.	O
</s>
<s>
Hidden	B-General_Concept
nodes	I-General_Concept
in	O
a	O
wireless	B-General_Concept
network	I-General_Concept
are	O
nodes	O
that	O
are	O
out	O
of	O
range	O
of	O
other	O
nodes	O
or	O
a	O
collection	O
of	O
nodes	O
.	O
</s>
<s>
Consider	O
a	O
physical	O
star	B-Architecture
topology	I-Architecture
with	O
an	O
access	B-Device
point	I-Device
with	O
many	O
nodes	O
surrounding	O
it	O
in	O
a	O
circular	O
fashion	O
:	O
each	O
node	B-Protocol
is	O
within	O
communication	O
range	O
of	O
the	O
AP	O
,	O
but	O
the	O
nodes	O
cannot	O
communicate	O
with	O
each	O
other	O
.	O
</s>
<s>
For	O
example	O
,	O
in	O
a	O
wireless	B-General_Concept
network	I-General_Concept
,	O
it	O
is	O
likely	O
that	O
the	O
node	B-Protocol
at	O
the	O
far	O
edge	O
of	O
the	O
access	B-Device
point	I-Device
's	O
range	O
,	O
which	O
is	O
known	O
as	O
A	O
,	O
can	O
see	O
the	O
access	B-Device
point	I-Device
,	O
but	O
it	O
is	O
unlikely	O
that	O
the	O
same	O
node	B-Protocol
can	O
communicate	O
with	O
a	O
node	B-Protocol
on	O
the	O
opposite	O
end	O
of	O
the	O
access	B-Device
point	I-Device
's	O
range	O
,	O
C	O
.	O
These	O
nodes	O
are	O
known	O
as	O
hidden	O
.	O
</s>
<s>
The	O
problem	O
is	O
when	O
nodes	O
A	O
and	O
C	O
start	O
to	O
send	O
packets	B-Protocol
simultaneously	O
to	O
the	O
access	B-Device
point	I-Device
B	O
.	O
</s>
<s>
Because	O
the	O
nodes	O
A	O
and	O
C	O
cannot	O
receive	O
each	O
other	O
's	O
signals	O
,	O
so	O
they	O
cannot	O
detect	O
the	O
collision	O
before	O
or	O
while	O
transmitting	O
,	O
carrier-sense	O
multiple	O
access	O
with	O
collision	O
detection	O
(	O
CSMA/CD	O
)	O
does	O
not	O
work	O
,	O
and	O
collisions	O
occur	O
,	O
which	O
then	O
corrupt	O
the	O
data	O
received	O
by	O
the	O
access	B-Device
point	I-Device
.	O
</s>
<s>
To	O
overcome	O
the	O
hidden	B-General_Concept
node	I-General_Concept
problem	I-General_Concept
,	O
request-to-send/clear	O
-to-send	O
(	O
RTS/CTS	O
)	O
handshaking	O
(	O
IEEE	O
802.11	O
RTS/CTS	O
)	O
is	O
implemented	O
at	O
the	O
Access	B-Device
Point	I-Device
in	O
conjunction	O
with	O
the	O
Carrier	O
sense	O
multiple	O
access	O
with	O
collision	O
avoidance	O
(	O
CSMA/CA	O
)	O
scheme	O
.	O
</s>
<s>
The	O
same	O
problem	O
exists	O
in	O
a	O
mobile	B-General_Concept
ad	I-General_Concept
hoc	I-General_Concept
network	I-General_Concept
(	O
MANET	B-General_Concept
)	O
.	O
</s>
<s>
IEEE	O
802.11	O
uses	O
802.11	O
RTS/CTS	O
acknowledgment	O
and	O
handshake	O
packets	B-Protocol
to	O
partly	O
overcome	O
the	O
hidden	B-General_Concept
node	I-General_Concept
problem	I-General_Concept
.	O
</s>
<s>
The	O
other	O
methods	O
that	O
can	O
be	O
employed	O
to	O
solve	O
hidden	B-General_Concept
node	I-General_Concept
problem	I-General_Concept
are	O
:	O
</s>
<s>
Increasing	O
the	O
transmission	O
power	O
of	O
the	O
nodes	O
can	O
solve	O
the	O
hidden	B-General_Concept
node	I-General_Concept
problem	I-General_Concept
by	O
allowing	O
the	O
cell	O
around	O
each	O
node	B-Protocol
to	O
increase	O
in	O
size	O
,	O
encompassing	O
all	O
of	O
the	O
other	O
nodes	O
.	O
</s>
<s>
This	O
configuration	O
enables	O
the	O
non-hidden	O
nodes	O
to	O
detect	O
,	O
or	O
hear	O
,	O
the	O
hidden	B-General_Concept
node	I-General_Concept
.	O
</s>
<s>
If	O
the	O
non-hidden	O
nodes	O
can	O
hear	O
the	O
hidden	B-General_Concept
node	I-General_Concept
,	O
the	O
hidden	B-General_Concept
node	I-General_Concept
is	O
no	O
longer	O
hidden	O
.	O
</s>
<s>
Because	O
wireless	B-Architecture
LANs	I-Architecture
use	O
the	O
CSMA/CA	O
protocol	O
,	O
nodes	O
will	O
wait	O
their	O
turn	O
before	O
communicating	O
with	O
the	O
access	B-Device
point	I-Device
.	O
</s>
<s>
In	O
the	O
typical	O
case	O
of	O
a	O
WiFi	O
network	O
,	O
increasing	O
transmission	O
power	O
on	O
the	O
access	B-Device
point	I-Device
only	O
will	O
not	O
solve	O
the	O
problem	O
because	O
typically	O
the	O
hidden	B-General_Concept
nodes	I-General_Concept
are	O
the	O
clients	O
(	O
e.g.	O
</s>
<s>
laptops	O
,	O
mobile	O
devices	O
)	O
,	O
not	O
the	O
access	B-Device
point	I-Device
itself	O
,	O
and	O
the	O
clients	O
will	O
still	O
not	O
be	O
able	O
to	O
hear	O
each	O
other	O
.	O
</s>
<s>
Increasing	O
transmission	O
power	O
on	O
the	O
access	B-Device
point	I-Device
is	O
actually	O
likely	O
to	O
make	O
the	O
problem	O
worse	O
,	O
because	O
it	O
will	O
put	O
new	O
clients	O
in	O
range	O
of	O
the	O
access	B-Device
point	I-Device
and	O
thus	O
add	O
new	O
nodes	O
to	O
the	O
network	O
that	O
are	O
hidden	O
from	O
other	O
clients	O
.	O
</s>
<s>
Since	O
nodes	O
using	O
directional	O
antennas	O
are	O
nearly	O
invisible	O
to	O
nodes	O
that	O
are	O
not	O
positioned	O
in	O
the	O
direction	O
the	O
antenna	O
is	O
aimed	O
at	O
,	O
directional	O
antennas	O
should	O
be	O
used	O
only	O
for	O
very	O
small	O
networks	O
(	O
e.g.	O
,	O
dedicated	O
point-to-point	B-Architecture
connections	O
)	O
.	O
</s>
<s>
Increasing	O
the	O
power	O
on	O
mobile	O
nodes	O
may	O
not	O
work	O
if	O
,	O
for	O
example	O
,	O
the	O
reason	O
one	O
node	B-Protocol
is	O
hidden	O
is	O
that	O
there	O
is	O
a	O
concrete	O
or	O
steel	O
wall	O
preventing	O
communication	O
with	O
other	O
nodes	O
.	O
</s>
<s>
It	O
is	O
doubtful	O
that	O
one	O
would	O
be	O
able	O
to	O
remove	O
such	O
an	O
obstacle	O
,	O
but	O
removal	O
of	O
the	O
obstacle	O
is	O
another	O
method	O
of	O
remedy	O
for	O
the	O
hidden	B-General_Concept
node	I-General_Concept
problem	I-General_Concept
.	O
</s>
<s>
Another	O
method	O
of	O
solving	O
the	O
hidden	B-General_Concept
node	I-General_Concept
problem	I-General_Concept
is	O
moving	O
the	O
nodes	O
so	O
that	O
they	O
can	O
all	O
hear	O
each	O
other	O
.	O
</s>
<s>
If	O
it	O
is	O
found	O
that	O
the	O
hidden	B-General_Concept
node	I-General_Concept
problem	I-General_Concept
is	O
the	O
result	O
of	O
a	O
user	O
moving	O
his	O
computer	O
to	O
an	O
area	O
that	O
is	O
hidden	O
from	O
the	O
other	O
wireless	O
nodes	O
,	O
it	O
may	O
be	O
necessary	O
to	O
have	O
that	O
user	O
move	O
again	O
.	O
</s>
<s>
The	O
alternative	O
to	O
forcing	O
users	O
to	O
move	O
is	O
extending	O
the	O
wireless	B-Architecture
LAN	I-Architecture
to	O
add	O
proper	O
coverage	O
to	O
the	O
hidden	O
area	O
,	O
perhaps	O
using	O
additional	O
access	O
points	O
.	O
</s>
<s>
There	O
are	O
several	O
software	O
implementations	O
of	O
additional	O
protocols	O
that	O
essentially	O
implement	O
a	O
polling	B-General_Concept
or	O
token	B-Protocol
passing	I-Protocol
strategy	O
.	O
</s>
<s>
Then	O
,	O
a	O
master	O
(	O
typically	O
the	O
access	B-Device
point	I-Device
)	O
dynamically	O
polls	O
clients	O
for	O
data	O
.	O
</s>
<s>
This	O
eliminates	O
the	O
hidden	B-General_Concept
node	I-General_Concept
problem	I-General_Concept
at	O
the	O
cost	O
of	O
increased	O
latency	B-Protocol
and	O
less	O
maximum	O
throughput	O
.	O
</s>
<s>
Clients	O
that	O
wish	O
to	O
send	O
data	O
send	O
an	O
RTS	O
frame	O
,	O
the	O
access	B-Device
point	I-Device
then	O
sends	O
a	O
CTS	O
frame	O
when	O
it	O
is	O
ready	O
for	O
that	O
particular	O
node	B-Protocol
.	O
</s>
<s>
For	O
short	O
packets	B-Protocol
the	O
overhead	O
is	O
quite	O
large	O
,	O
so	O
short	O
packets	B-Protocol
do	O
not	O
usually	O
use	O
it	O
,	O
the	O
minimum	O
size	O
is	O
generally	O
configurable	O
.	O
</s>
<s>
With	O
cellular	O
networks	O
the	O
hidden	B-General_Concept
node	I-General_Concept
problem	I-General_Concept
has	O
practical	O
solutions	O
by	O
time	O
domain	O
multiplexing	O
for	O
each	O
given	O
client	O
for	O
a	O
mast	O
,	O
and	O
using	O
spatially	O
diverse	O
transmitters	O
,	O
so	O
that	O
each	O
node	B-Protocol
is	O
potentially	O
served	O
by	O
any	O
of	O
three	O
masts	O
to	O
greatly	O
minimise	O
issues	O
with	O
obstacles	O
interfering	O
with	O
radio	O
propagation	O
.	O
</s>
