<s>
A	O
sensor	B-Device
node	I-Device
(	O
also	O
known	O
as	O
a	O
mote	B-Device
in	O
North	O
America	O
)	O
,	O
consists	O
of	O
an	O
individual	O
node	B-Protocol
from	O
a	O
sensor	B-Architecture
network	I-Architecture
that	O
is	O
capable	O
of	O
performing	O
a	O
desired	O
action	O
such	O
as	O
gathering	O
,	O
processing	O
or	O
communicating	O
information	O
with	O
other	O
connected	O
nodes	O
in	O
a	O
network	O
.	O
</s>
<s>
Although	O
wireless	B-Architecture
sensor	I-Architecture
networks	I-Architecture
have	O
existed	O
for	O
decades	O
and	O
used	O
for	O
diverse	O
applications	O
such	O
as	O
earthquake	O
measurements	O
or	O
warfare	O
,	O
the	O
modern	O
development	O
of	O
small	O
sensor	B-Architecture
nodes	I-Architecture
dates	O
back	O
to	O
the	O
1998	O
Smartdust	B-Device
project	O
and	O
the	O
NASA	O
.	O
</s>
<s>
Sensor	B-Algorithm
Web	I-Algorithm
One	O
of	O
the	O
objectives	O
of	O
the	O
Smartdust	B-Device
project	O
was	O
to	O
create	O
autonomous	O
sensing	O
and	O
communication	O
within	O
a	O
cubic	O
millimeter	O
of	O
space	O
,	O
though	O
this	O
project	O
ended	O
early	O
on	O
,	O
it	O
led	O
to	O
many	O
more	O
research	O
projects	O
and	O
major	O
research	O
centres	O
such	O
as	O
The	O
Berkeley	O
NEST	O
and	O
CENS	O
.	O
</s>
<s>
The	O
researchers	O
involved	O
in	O
these	O
projects	O
coined	O
the	O
term	O
mote	B-Device
to	O
refer	O
to	O
a	O
sensor	B-Device
node	I-Device
.	O
</s>
<s>
The	O
equivalent	O
term	O
in	O
the	O
NASA	O
Sensor	B-Algorithm
Webs	I-Algorithm
Project	O
for	O
a	O
physical	O
sensor	B-Device
node	I-Device
is	O
pod	O
,	O
although	O
the	O
sensor	B-Device
node	I-Device
in	O
a	O
Sensor	B-Algorithm
Web	I-Algorithm
can	O
be	O
another	O
Sensor	B-Algorithm
Web	I-Algorithm
itself	O
.	O
</s>
<s>
Physical	O
sensor	B-Architecture
nodes	I-Architecture
have	O
been	O
able	O
to	O
increase	O
their	O
effectiveness	O
and	O
its	O
capability	O
in	O
conjunction	O
with	O
Moore	O
's	O
Law	O
.	O
</s>
<s>
The	O
chip	O
footprint	O
contains	O
more	O
complex	O
and	O
lower	O
powered	O
microcontrollers	B-Architecture
.	O
</s>
<s>
Thus	O
,	O
for	O
the	O
same	O
node	B-Protocol
footprint	O
,	O
more	O
silicon	O
capability	O
can	O
be	O
packed	O
into	O
it	O
.	O
</s>
<s>
Nowadays	O
,	O
motes	B-Device
focus	O
on	O
providing	O
the	O
longest	O
wireless	O
range	O
(	O
dozens	O
of	O
km	O
)	O
,	O
the	O
lowest	O
energy	O
consumption	O
(	O
a	O
few	O
uA	O
)	O
and	O
the	O
easiest	O
development	O
process	O
for	O
the	O
user	O
.	O
</s>
<s>
The	O
main	O
components	O
of	O
a	O
sensor	B-Device
node	I-Device
usually	O
involve	O
a	O
microcontroller	B-Architecture
,	O
transceiver	O
,	O
external	O
memory	O
,	O
power	O
source	O
and	O
one	O
or	O
more	O
sensors	B-Application
.	O
</s>
<s>
Sensors	B-Application
are	O
used	O
by	O
wireless	B-Architecture
sensor	I-Architecture
nodes	O
to	O
capture	O
data	O
from	O
their	O
environment	O
.	O
</s>
<s>
Sensors	B-Application
measure	O
physical	O
data	O
of	O
the	O
parameter	O
to	O
be	O
monitored	O
and	O
have	O
specific	O
characteristics	O
such	O
as	O
accuracy	O
,	O
sensitivity	O
etc	O
.	O
</s>
<s>
The	O
continual	O
analog	O
signal	O
produced	O
by	O
the	O
sensors	B-Application
is	O
digitized	O
by	O
an	O
analog-to-digital	O
converter	O
and	O
sent	O
to	O
controllers	O
for	O
further	O
processing	O
.	O
</s>
<s>
Some	O
sensors	B-Application
contain	O
the	O
necessary	O
electronics	O
to	O
convert	O
the	O
raw	O
signals	O
into	O
readings	O
which	O
can	O
be	O
retrieved	O
via	O
a	O
digital	O
link	O
(	O
e.g.	O
</s>
<s>
Most	O
sensor	B-Architecture
nodes	I-Architecture
are	O
small	O
in	O
size	O
,	O
consume	O
little	O
energy	O
,	O
operate	O
in	O
high	O
volumetric	O
densities	O
,	O
be	O
autonomous	O
and	O
operate	O
unattended	O
,	O
and	O
be	O
adaptive	O
to	O
the	O
environment	O
.	O
</s>
<s>
As	O
wireless	B-Architecture
sensor	I-Architecture
nodes	O
are	O
typically	O
very	O
small	O
electronic	O
devices	O
,	O
they	O
can	O
only	O
be	O
equipped	O
with	O
a	O
limited	O
power	O
source	O
of	O
less	O
than	O
0.5-2	O
ampere-hour	O
and	O
1.2-3.7	O
volts	O
.	O
</s>
<s>
Sensors	B-Application
are	O
classified	O
into	O
three	O
categories	O
:	O
passive	O
,	O
omnidirectional	O
sensors	B-Application
;	O
passive	O
,	O
narrow-beam	O
sensors	B-Application
;	O
and	O
active	O
sensors	B-Application
.	O
</s>
<s>
Passive	O
sensors	B-Application
sense	O
the	O
data	O
without	O
actually	O
manipulating	O
the	O
environment	O
by	O
active	O
probing	O
.	O
</s>
<s>
Active	O
sensors	B-Application
actively	O
probe	O
the	O
environment	O
,	O
for	O
example	O
,	O
a	O
sonar	O
or	O
radar	O
sensor	O
,	O
and	O
they	O
require	O
continuous	O
energy	O
from	O
a	O
power	O
source	O
.	O
</s>
<s>
Narrow-beam	O
sensors	B-Application
have	O
a	O
well-defined	O
notion	O
of	O
direction	O
of	O
measurement	O
,	O
similar	O
to	O
a	O
camera	O
.	O
</s>
<s>
Omnidirectional	O
sensors	B-Application
have	O
no	O
notion	O
of	O
direction	O
involved	O
in	O
their	O
measurements	O
.	O
</s>
<s>
Most	O
theoretical	O
work	O
on	O
WSNs	B-Architecture
assumes	O
the	O
use	O
of	O
passive	O
,	O
omnidirectional	O
sensors	B-Application
.	O
</s>
<s>
Each	O
sensor	B-Device
node	I-Device
has	O
a	O
certain	O
area	O
of	O
coverage	O
for	O
which	O
it	O
can	O
reliably	O
and	O
accurately	O
report	O
the	O
particular	O
quantity	O
that	O
it	O
is	O
observing	O
.	O
</s>
<s>
Several	O
sources	O
of	O
power	O
consumption	O
in	O
sensors	B-Application
are	O
:	O
signal	O
sampling	O
and	O
conversion	O
of	O
physical	O
signals	O
to	O
electrical	O
ones	O
,	O
signal	O
conditioning	O
,	O
and	O
analog-to-digital	O
conversion	O
.	O
</s>
<s>
Spatial	O
density	O
of	O
sensor	B-Architecture
nodes	I-Architecture
in	O
the	O
field	O
may	O
be	O
as	O
high	O
as	O
20	O
nodes	O
per	O
cubic	O
meter	O
.	O
</s>
<s>
The	O
controller	O
performs	O
tasks	O
,	O
processes	O
data	O
and	O
controls	O
the	O
functionality	O
of	O
other	O
components	O
in	O
the	O
sensor	B-Device
node	I-Device
.	O
</s>
<s>
While	O
the	O
most	O
common	O
controller	O
is	O
a	O
microcontroller	B-Architecture
,	O
other	O
alternatives	O
that	O
can	O
be	O
used	O
as	O
a	O
controller	O
are	O
:	O
a	O
general	O
purpose	O
desktop	B-Device
microprocessor	B-Architecture
,	O
digital	B-Architecture
signal	I-Architecture
processors	I-Architecture
,	O
FPGAs	B-Architecture
and	O
ASICs	O
.	O
</s>
<s>
A	O
microcontroller	B-Architecture
is	O
often	O
used	O
in	O
many	O
sensor	B-Architecture
nodes	I-Architecture
due	O
to	O
its	O
low	O
cost	O
,	O
flexibility	O
to	O
connect	O
to	O
other	O
devices	O
(	O
or	O
nodes	O
in	O
a	O
network	O
)	O
,	O
ease	O
of	O
programming	O
,	O
and	O
low	O
power	O
consumption	O
.	O
</s>
<s>
A	O
general	O
purpose	O
microprocessor	B-Architecture
generally	O
has	O
a	O
higher	O
power	O
consumption	O
than	O
a	O
microcontroller	B-Architecture
,	O
making	O
it	O
an	O
undesirable	O
choice	O
for	O
a	O
sensor	B-Device
node	I-Device
.	O
</s>
<s>
Digital	B-Architecture
Signal	I-Architecture
Processors	I-Architecture
may	O
be	O
chosen	O
for	O
broadband	O
wireless	O
communication	O
applications	O
,	O
but	O
in	O
Wireless	B-Architecture
Sensor	I-Architecture
Networks	I-Architecture
the	O
wireless	O
communication	O
is	O
often	O
modest	O
:	O
i.e.	O
,	O
simpler	O
,	O
easier	O
to	O
process	O
modulation	B-Application
and	O
the	O
signal	O
processing	O
tasks	O
of	O
actual	O
sensing	O
of	O
data	O
is	O
less	O
complicated	O
.	O
</s>
<s>
Therefore	O
,	O
the	O
advantages	O
of	O
DSPs	O
are	O
not	O
usually	O
of	O
much	O
importance	O
to	O
wireless	B-Architecture
sensor	I-Architecture
nodes	O
.	O
</s>
<s>
FPGAs	B-Architecture
can	O
be	O
reprogrammed	O
and	O
reconfigured	O
according	O
to	O
requirements	O
,	O
but	O
this	O
takes	O
more	O
time	O
and	O
energy	O
than	O
desired	O
.	O
</s>
<s>
Sensor	B-Architecture
nodes	I-Architecture
often	O
make	O
use	O
of	O
ISM	B-Algorithm
band	I-Algorithm
,	O
which	O
gives	O
free	O
radio	O
,	O
spectrum	O
allocation	O
and	O
global	O
availability	O
.	O
</s>
<s>
Infrared	O
,	O
like	O
lasers	O
,	O
needs	O
no	O
antenna	O
but	O
it	O
is	O
limited	O
in	O
its	O
broadcasting	B-Operating_System
capacity	O
.	O
</s>
<s>
Radio	O
frequency-based	O
communication	O
is	O
the	O
most	O
relevant	O
that	O
fits	O
most	O
of	O
the	O
WSN	B-Architecture
applications	O
.	O
</s>
<s>
WSNs	B-Architecture
tend	O
to	O
use	O
license-free	O
communication	O
frequencies	O
:	O
173	O
,	O
433	O
,	O
868	O
,	O
and	O
915	O
MHz	O
;	O
and	O
2.4	B-Algorithm
GHz	I-Algorithm
.	O
</s>
<s>
Current	O
generation	O
transceivers	O
have	O
built-in	O
state	B-Architecture
machines	I-Architecture
that	O
perform	O
some	O
operations	O
automatically	O
.	O
</s>
<s>
From	O
an	O
energy	O
perspective	O
,	O
the	O
most	O
relevant	O
kinds	O
of	O
memory	O
are	O
the	O
on-chip	O
memory	O
of	O
a	O
microcontroller	B-Architecture
and	O
Flash	B-Device
memory	I-Device
—	O
off-chip	O
RAM	B-Architecture
is	O
rarely	O
,	O
if	O
ever	O
,	O
used	O
.	O
</s>
<s>
Flash	B-Device
memories	I-Device
are	O
used	O
due	O
to	O
their	O
cost	O
and	O
storage	O
capacity	O
.	O
</s>
<s>
A	O
wireless	B-Architecture
sensor	I-Architecture
node	I-Device
is	O
a	O
popular	O
solution	O
when	O
it	O
is	O
difficult	O
or	O
impossible	O
to	O
run	O
a	O
mains	O
supply	O
to	O
the	O
sensor	B-Device
node	I-Device
.	O
</s>
<s>
However	O
,	O
since	O
the	O
wireless	B-Architecture
sensor	I-Architecture
node	B-Protocol
is	O
often	O
placed	O
in	O
a	O
hard-to-reach	O
location	O
,	O
changing	O
the	O
battery	O
regularly	O
can	O
be	O
costly	O
and	O
inconvenient	O
.	O
</s>
<s>
An	O
important	O
aspect	O
in	O
the	O
development	O
of	O
a	O
wireless	B-Architecture
sensor	I-Architecture
node	B-Protocol
is	O
ensuring	O
that	O
there	O
is	O
always	O
adequate	O
energy	O
available	O
to	O
power	O
the	O
system	O
.	O
</s>
<s>
The	O
sensor	B-Device
node	I-Device
consumes	O
power	O
for	O
sensing	O
,	O
communicating	O
and	O
data	O
processing	O
.	O
</s>
<s>
Batteries	O
,	O
both	O
rechargeable	O
and	O
non-rechargeable	O
,	O
are	O
the	O
main	O
source	O
of	O
power	O
supply	O
for	O
sensor	B-Architecture
nodes	I-Architecture
.	O
</s>
<s>
They	O
are	O
also	O
classified	O
according	O
to	O
electrochemical	O
material	O
used	O
for	O
the	O
electrodes	O
such	O
as	O
NiCd	O
(	O
nickel-cadmium	O
)	O
,	O
NiZn	O
(	O
nickel-zinc	O
)	O
,	O
NiMH	B-Device
(	O
nickel-metal	B-Device
hydride	I-Device
)	O
,	O
and	O
lithium-ion	O
.	O
</s>
<s>
Current	O
sensors	B-Application
are	O
able	O
to	O
renew	O
their	O
energy	O
from	O
solar	O
sources	O
,	O
Radio	O
Frequency(RF )	O
,	O
temperature	O
differences	O
,	O
or	O
vibration	B-Algorithm
.	O
</s>
<s>
DPM	O
conserves	O
power	O
by	O
shutting	O
down	O
parts	O
of	O
the	O
sensor	B-Device
node	I-Device
which	O
are	O
not	O
currently	O
used	O
or	O
active	O
.	O
</s>
<s>
A	O
DVS	O
scheme	O
varies	O
the	O
power	O
levels	O
within	O
the	O
sensor	B-Device
node	I-Device
depending	O
on	O
the	O
non-deterministic	O
workload	O
.	O
</s>
